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REVIEW ARTICLE

Remote thermometry with thermographic phosphors: Instrumentationand applications

S. W. Allisona)Engineering Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-7280

G. T. Gilliesb)Department of Mechanical, Aerospace, and Nuclear Engineering, University of Virginia, Charlottesville,Virginia 22901

~Received 12 July 1996; accepted for publication 8 April 1997!

The temperature-dependent characteristics of fluorescence of several rare-earth-doped ceramicphosphors has made these materials the focus of a major effort in the field of noncontactthermometry over the past few decades. These ‘‘thermographic phosphors,’’ e.g., Y2O3:Eu, havebeen used for remote measurements of the temperatures of both static and moving surfaces, andhave performed many other tasks that standard sensors~thermocouples, thermistors, etc.! cannot.The range of usefulness of this class of materials extends from cryogenic temperatures to thoseapproaching 2000 °C. The instrumentation needed for this type of thermometry has followed manydifferent lines of development, and this evolution has produced a wide variety of both field- andlaboratory-grade systems that are now described in the literature. In general, the technique offershigh sensitivity ('0.05 °C), robustness~e.g., stability of the sensor sample in harsh environments!,and NIST traceability. In addition, such systems have been successfully adapted to make remotelysensed measurements of pressure, heat flux, shear stress, and strain. In this review, we summarizethe physical mechanisms that form the basis for the technique, and then catalog and discuss theinstrumentation-related aspects of several different remote thermometry systems that employthermographic phosphors as the sensors. ©1997 American Institute of Physics.@S0034-6748~97!01407-X#

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I. INTRODUCTION

The measurement of temperature is a virtually ubiqtous requirement in all of the experimental and/or applfields of science, engineering, and medicine. Moreover,accelerating proliferation of technology in these areas hasto an ever-increasing variety of situations that requiknowledge of the temperature of some specimen, comnent, system or process. As a result, new approaches tomeasurement of temperature are often called for, and thisled to the development of novel instrumentation systeaimed at meeting these needs.

One thermometric technique that is adaptable toneeds of a wide variety of situations is based on fluorescmaterials, many of them phosphors. The thermal dependof phosphor fluorescence may be exploited to provide fononcontact, emissivity-independent, optical alternativeother more conventional techniques, e.g., those emplopyrometry, thermocouples, or thermistors. In fact, as dcussed below, there are certain situations in which the adtages fluorescence-based thermometry has over other mods make it the only useful approach.

The general goal of this article is to review the metho

a!Electronic mail: [email protected]!Also with Dept. of Biomedical Engineering, Health Sciences Center, Uversity of Virginia, Charlottesville, VA 22908 and Div. of NeurosurgerMedical College of Virginia, Virginia Commonwealth University, Richmond, VA 23298-0631.

Rev. Sci. Instrum. 68 (7), July 1997 0034-6748/97/68(7)/2615/3

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of thermometry that are based on fluorescence, with empsis on those employing a special class of materials knowthermographic phosphors. We shall concentrate on that cof techniques which is noncontact in nature, but extenddiscussion to include the relevant features of contact-badevices when appropriate. The latter class of instrumentaconsists largely of phosphor-tipped fiberoptic probes, thesign principles and performance characteristics of whhave been described in detail by Grattan and Zhang.1

We begin with an historical background that outlines tnature of phosphors and describes the evolution of phospbased thermometry from the 1940s to the 1980s, whenfirst commercial instruments appeared. Section II alsocludes some discussion of the various technical advanthat have driven the developmental work in this area ansurvey of the physical principles underlying the prototypicfluorescence thermometer. Following this, in Sec. III tpracticalities of selecting a phosphor and bonding it ontosurface of interest are addressed. Section IV presents ascription of the instrumentation needed to formulate theneric phosphor thermometry system and it then goes odiscuss the pertinent calibration methodologies. Thereawe catalog the characteristics of several different experimtal arrangements, provide an overview of the many applitions for this technique in Sec. V, and discuss its use insensing of quantities other than temperature in Sec. VI. Stion VII compares the performance of phosphor thermomewith that of competing methods and ends with some sugg

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TABLE I. Early phosphor thermometry systems and the applications for which they were used, from about 1950 to 1980.

Author~s! Application Phosphor/method Design features Range/uncertainty~Year/Ref.!

Bradley Measurement of temperaturedistributions on a flatplate in supersonic flow

ZnCdS:Ag,Cu~NewJersey Zinc Co.!,intensity shift

Excitation at 365 nm, withfluorescence signal passedthrough a Wratten 2A filter

'8–25 °CDT560.2 °C

1953; 15

Grosset al.

Development of phosphor-based detectors ofthermal radiation

ZnS:Ag:Cl, CdS:Ag,intensity change andwavelength shift

Spectrometer used to monitorintensity variations and wavelengthshift of heated target

'10–40 °CNo error estimatepresented by author

1960; 38, 39

Czysz andDixon

Thermal mapping of modelsin wind tunnels for heattransfer measurements

Radel in phosphors~U.S. Radium Co.!,intensity shift

Surface thermocouple heat transfergauges used to eliminate theabsolute intensity measurements

Range unspecifiedDT'0.2 °C

1966;21, 22, 23

Brenner Measurement of collectorjunction temperaturesin power transistors

Radel in No. 2090~U.S. Radium Co.!,intensity shift

ASA 400 B&W film used to sensefluorescence intensities withreadout via microdensitometry

170–210 °CDT52.0 °C

1971; 40

Fry Integrated circuit Radel in No. 3251 Fiberoptic conveys fluorescencefrom the IC to the detector

60–100 °C 1971; 41

Kusamaet al.

Determination of thetemperature of colortelevision screens

Y2O2S:Eu,line shift of the5D0→7F2 transition

Sony TrinitronTM color CRT used,with beam currents varied from100mA to 1 mA per RGB channel

'25–50 °CDT51.0 °C

1976; 29

Pattison Measurement of surfacetemperatures on rotatingturbogenerators

Levy West W800 intransparent lacquer,intensity shift

Fiberoptic cables used to conveyexcitation and emission signalsthrough the generator housing

No specific rangeor error estimatepresented by author

1977; 42

Wickersheimand Alves

Measurements requiringisolation from or immunityto electromagnetic fields

Eu31-doped, rare-earthoxysulfides,line-intensity ratios

Prefiltration of UV excitationradiation by reflection fromseries of dichroic mirrors

9–250 °CNoise equivalenttemp.50.025 °C

1979; 13

Sholesand Small

Thermal dosimetry duringrf-hyperthermia treatmentsof malignant tumors

Al2O3:Cr31 ~0.05%!,

decay lifetime ofR lines

Mechanical shutter exposes theruby sample to the excitationsource for 10 ms intervals

35–46 °C~Partial range!DT50.3 °C

1980; 17

Samulski andShrivastava

Thermal dosimetry duringrf-hyperthermia treatmentsof malignant tumors

Calcium and zinc–cadmiumsulfides,integrated response

0.75 ms pulses from xenon lampused as excitation; data weretaken at 1 min intervals

20–50 °CProbe resolution50.3–0.4 °C

1980; 35, 36

McCormack Measurements requiringelectrically insulating andchemically inert probes

BaClF:Sm21

decay lifetime of the5D0→7F0 transition

Plastic-clad silica fiber withNA50.25 was used to maximizethe optical power collected

'25–200 °CDT565 °C

1981; 37

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II. BACKGROUND

A. The nature of phosphorescent materials

Prior to discussing those properties of phosphors imptant to thermometry, some relevant terminology shouldintroduced. Luminescence refers to the absorption of eneby a material, with the subsequent emission of light. Thisa phenomenon distinct from blackbody radiation, incandcence, or other such effects that cause materials to glohigh temperature. Fluorescence refers to the same proceluminescence, but with the qualification that the emissionusually in the visible band and has a duration of typica1029–1023 s. Phosphorescence is a type of luminescencgreater duration,'1023–103 s. Emission, luminescencephosphorescence, and fluorescence are closely relatedand sometimes are used interchangeably.2

A phosphor is a fine white, or slightly colored, powddesigned to fluoresce very efficiently. Color television acathode-ray tube~CRT! displays employ phosphor screethat emit visible light then seen by the viewer. The sourceenergy that excites this fluorescence is a beam of electrOther examples of phosphor uses are in fluorescent spaints, fluorescent lighting, photocopy lamps, scintillatoand x-ray conversion screens. For the last, a beam of x

2616 Rev. Sci. Instrum., Vol. 68, No. 7, July 1997


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provides the excitation. Other excitation sources are ntrons, gamma rays, ultraviolet~UV! light and, in some casesvisible light. Many different phosphors are manufacturedthese and other applications. The lighting and display indtries have provided the impetus for the study of thousandphosphor materials over the last several decades, and muknown about them.3–5

In terms of their composition, phosphors may be dividinto two classes: organic and inorganic. It is the latter thave primarily but not exclusively found use in thermometInorganic phosphors consist of two components: a host inganic compound and an activator~or doping agent! fromwhich the light is emitted. Such materials do not have toin the powdered form to be useful. Some phosphors arericated into macroscopic pieces of crystal or glass. In tform, certain of them are useful as solid-state laser meand can be thought of as phosphors having a singular parsize and number density.6–9 In this article, this more-generalized meaning of the term ‘‘phosphor,’’ includinsuch macroscopic solids, is intended and used.

B. History of the technique

Research on luminescing materials was largely themain of academia until the introduction of the fluorescelamp in 1938.4 After this, industrial laboratories werespurred into action by the growing need for commercial adomestic lighting, and also by the need for improved catho

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ray tubes. The degradation of luminescent output withcreasing temperature can be a detriment in lamp applicatiTherefore, thermal characterization was an important taskthose studying phosphors. Neubert suggested phosphofor thermometry as early as 1937.10 Urbach11,12 contributedto early phosphor thermometry approaches and applicatiTable I surveys some of the developments important to phphor thermometry from that time through the early 1980s

In 1950, magnesium fluorogermanate was synthesand used widely as a lamp phosphor.3 Eventually, in the1980s, it became the basis of commercial phosphor tmometry products.13,14 One of the first thermometric applications of a phosphor known to us was by Bradley15 who, in1952, measured the temperature distribution on a flat wein a supersonic flow field using a phosphor provided by Ubach. Fluorescence thermometry in the 1950s is surveyea paper by Byler and Hays16 of U.S. Radium Corporation~now USR Optonix, Inc.! which marketed the phosphoridentified by Urbach. In 1960, Maiman invented the rulaser, making use of the efficient fluorescence of ruby,which Cr31 is the activator in an Al2O3 host. Its temperaturedependent fluorescence properties have also been usethermometry.17–19 Trivalent rare-earth activators are nowidely used, particularly Tb31, Eu31, and Dy31, in a varietyof hosts. Interestingly, their use as laser media predatesuse in television picture tubes. For example, Chang20 oper-ated a Y2O3:Eu laser in 1963. Renowned for its relative isensitivity to temperature changes, we shall describe besituations wherein this material has nevertheless found uspractical high temperature (.600 °C) thermometry systemsAlso in the 1960s, Czyszet al.21–23 carried out phosphothermometry on models in wind tunnels at the Arnold Enneering Development Center~AEDC!, and this work hascontinued up to the present at this facility. They use phphors originally investigated by Urbach. Wickersheim acolleagues24–28 explored the possible use of rare-earth acvated materials in a variety of applications, including lasescintillators, neutron detectors, x-ray detectors, etc. Theirforts at determining the temperature sensitivity of phosphwere fruitful, and this work eventually led to the commecialization of thermometry systems based on fluorescence1977, Kusamaet al.29 exploited the thermal dependencecathode ray tube phosphors to study electron-beam heaeffects in color television applications.

It was not until the 1970s that the technological toobecame generally available for exploiting the thermal prerties of phosphor emission in a broader range of appltions. For instance, high quality optical fibers capable oftraviolet transmission appeared at that time. A group ledLeroux developed approaches based on utilizing the tperature dependence of fluorescence intensity and tempresponse30–33for surface thermometry purposes. By applyiphosphor to the tip of an optical fiber, Wickersheim aAlves13 produced the optical analog of a thermocouple.1979, Jameset al.34 mentioned several methods for extracing temperature from phosphor fluorescence measuremincluding phosphor time decay determination. Also pursuthis approach were Samulski and colleagues35,36 in 1980 andMcCormack37 in 1981.

Rev. Sci. Instrum., Vol. 68, No. 7, July 1997


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Also, from the beginning, lasers have been usefulstudying and characterizing phosphors. In many situationlaser is the most appropriate source of illuminationphosphor-based thermography, even over relatively lasurfaces. Moreover, the introduction of new types of lasmakes possible new classes of thermal phosphor apptions. For instance, significant advances in phosphor thmometry in the 1980s were made possible by the advenshort-pulse UV lasers.

Finally, over the past 15 years, the many advanceselectro-optics and solid state electronics have enabled msurements of fluorescence decay times at the nanoselevel. Prior to this, the capture and analysis of such fpulses lay mostly in the domain of experimental nuclear aparticle physics and required expensive, sophisticated insmentation.

Histories and reviews of phosphor-based thermomehave been written by Dowell43 and Baumann.33 Thoroughbibliographies of the literature covering the research on thmographic phosphors carried out in the 1980s have bassembled44,45and described46 elsewhere; the present revieseeks to provide additional citations to the more recent wo

C. Physical principles of fluorescence thermometry

It has long been generally known that the brightnessenergy-to-light conversion efficiency of phosphorescent mterials is temperature dependent. There are several waystemperature dependence is manifested. Since a numbefactors underlies the intrinsic nature of the fluorescence pcess, we begin with an overview of it.

1. The fluorescence process

The fundamental aspects of the fluorescence of solidterials are well understood. Prior to excitation, the materiaelectronic levels are populated in the ground state. A meto deposit energy in the material is required in order to exca higher electronic state. This may be accomplished byposure to electromagnetic radiation~visible or ultravioletlight, or x or gamma rays!, particle beams~electrons, neu-trons, or ions!, or, as is the case for semiconductors, eleccal current. The atomic configuration will not typically remain permanently excited but will either return to its groustate or assume an intermediate level. Conservation pciples dictate that the amount of energy absorbed mustbe released. This may be manifested by emission of a phwith energy equal to that of the energy-level difference,transfer of energy via quantized vibrational~phonon! ex-change in the material, or by other more complicated eneexchange mechanisms. Generally the lifetime of the excstate is short, on the order of only a few milliseconds. Nther the longer lived phosphorescent states nor the electraps that may be present in some phosphors to varyinggrees have typically been utilized for fluorescence thermoetry. ~The thesis of Peatman47 presents additional discussioof the processes involved in these phenomena.!

Many of the materials that fluoresce efficiently and thhave been found useful for a variety of laser, display, liging, fluorographic, and thermographic applications are thfor which the luminescence originates from a deliberat

2617Remote thermometry


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added impurity. As an example, a host material suchAl2O3 is transparent and nonfluorescent until Cr31 is added.It then becomes red and fluoresces with properties that mit useful in the design of lasers and thermometry systeThe dopant concentration is usually a few percent or lesthat nonradiative deexcitation exchange is minimizedtween the atoms~i.e., the luminescent centers are said toisolated!.

Fluorescence as referred to here is the emission ocring from electronic transitions and is usually in the visibregion of the spectrum, but may also be in the near infra~IR!, as from Nd, or in the UV, as is typical for Gd. Figurepresents the energy-level diagrams for various well-knostates of some rare earths that are commonly used asluminescent center in a thermometry application.48

It is generally true that the fluorescence spectral propties of any material will change with temperature. This isin part because the Boltzmann distribution governs the ptitioning of the populations in the various participating vibrtional levels of the ground, excited, and emitting stateschange in intensity distribution~including width and positionof spectral lines! results since individual oscillator strengthvary in accordance with the selection rules and the FranCondon principle. Kusamaet al.29 explored this forY2O2S:Eu. They measured the temperatures of a comm

FIG. 1. The energy-level diagrams for various well-known states of sorare earths that are commonly used as the luminescent center in a theretry application~after Ref. 48!.



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cially available CRT using cathode ray stimulation and susequent observation of the line shifts.

The temperature dependence of these processes castriking when there is competition with states which contefor nonradiative deexcitation pathways. The rate of chanof the population of an emitting state, 2, to a ground stateis the sum of a constant, purely radiative spontaneous esion, A1,2, and a nonradiative component,W1,2, which istemperature dependent.

k51/t5A1,21W1,2. ~1!

The measured lifetime,t, is the reciprocal of the decarate, k. One model for temperature dependence for sofluorescence materials used for thermometry is basedthermal promotion to a nonemitting electronic state followby nonradiative relaxation. For europium phosphors, thiscited as a charge transfer state~CTS!. This is illustrated inFig. 2 which shows an energy level diagram based onmodel that Fonger and Struck used to describe the therquenching of oxysulfides.49,50The excited electronic states othis atom are designated by5Dj where j50, 1, 2, and 3.Once the particular5Dj state is excited, by whatever meanits population may be increased or decreased via severaergy transfer pathways. A radiative transition to the groustate is possible with a probability rate ofaj . Note that thisis the reciprocal of the spontaneous, unquenched lifetiDepending on the temperature, the vibrational distributionthe excited state will be given by a Boltzmann distributioAt low temperature, states in resonance with the CTS seeFig. 2 are improbable. However, at higher temperature anificant fraction of the distribution will be at energies corrsponding to the CTS and the transition to the CTS becom

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FIG. 2. The configuration coordinate diagram for La2O2S:Eu~after Ref. 49!.

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likely. The rate is therefore proportional toe(2DE/kT) whereDE is the energy difference between the5Dj and the CTstates and the constant of proportionality isaCTS,j . Feedingfrom the CTS down to the5Dj is possible. Deactivation fromthe CTS is nonradiative with a rate ofaCTS. The rates to andfrom the CTS, according to Fonger and Struck49,50are about1011–1012 s21, i.e., on the order of a lattice vibration ratFor continuous illumination, they note that the intensitygiven by

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whereA is related toaCTS andaCTS,j , T is temperature,E isenergy, andk is the Boltzmann constant. Other models fsome phosphors have a similar dependence.

For some materials, a temperature-dependentexcitation pathway based on multiphonon emission51 exists.The multiphonon decay rate is

Wm5G@~12e~hn/kT!!2N•e~2g•DE!#, ~3!

whereN is the number of photons required to reach tenergy of the transitionDE, andG andg are characteristicsof the material. This explanation has been applied to YAhosts,52–56 among others. If the energy difference betwethe ground and emitting states is four to five times the hiest vibrational frequency of the surrounding lattice, thensufficiently high temperatures a multiphonon transition islowed and the transition probability is temperature depdent.

2. Factors influencing the fluorescence process

a. Dopant concentration.The parameters describing thluminescence characteristics of any given phosphor arependent on the concentration of the activating impurOverall intensity, relative spectral distribution, decay timrise time, and response to temperature are all affectesome degree. It is standard practice when characterizingthermal response of a given phosphor to measure its briness as a function of dopant concentration. A representacase57 is shown in Fig. 3. These data illustrate that a hidopant concentration of Eu in a host of Y2O3 results in most

FIG. 3. Fluorescence intensity vs dopant concentration for Eu in a hosY2O3 ~after Ref. 57!.



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of the luminescence being concentrated into the5D0 emis-sion line ~at 611 nm! that is of importance to color televisions. They further illustrate that the more activator centthere are in a phosphor, the more it will fluoresce, up topoint. However, when the concentration levels reach a ctain point, another nonradiative deexcitation pathwaycomes important. As the activator density is increased,probability that an excited activator will transfer energy noradiatively to a neighboring dopant ion increases. The arrat a cutoff in this process is usually referred to as concention quenching. For other dopants, even when in the sahost, the concentration that produces the maximum brigness will be different. For example, the optimuconcentration4,58 of Sm or Dy in Y2O3 is 0.5%.

Generally, one seeks to maximize the brightness ofphosphor. With respect to thermometry applications, hoever, there are other considerations that may warrant theof different strategies. For instance, at high concentratithe fluorescence decay profile may not be that of simsingle-exponential decay. This is important for decay-timbased approaches since multiexponential and nonexponedecay profiles are more difficult to model.~Multiexponentialdecay is not necessarily a problem if measurements basecontinuous line intensities are being made though.! Thesemore complex wave forms can make calibration and danalysis difficult but not intractable. An example of this phnomenon arises in the use of YAG:Tb, which is a good phphor for high temperature thermometry. At low concenttions, about 5/6 of the excitation energy drives the5D3 states~blue bands between 350 and 450 nm! while the remainderexcites the5D4 states~blue-green and green bands at 4and 544 nm!. At sufficiently high dopant levels, there is aenergy transfer from the higher to the lower states. Wherthe latter states exhibit exponential decay at up to 3% dopconcentration, the former appear to follow the Fo¨rstermodel,59,60 the decay function for which is

x~ t !/x~0!5exp@2t/t2N/N0~pt/t!1/2#, ~4!

whereN053/(4pR03) with R0 being the critical distance fo

energy transfer (>1 nm), N is the dopant concentration inumber of atoms per cm3, andt is the unquenched spontaneous lifetime. The implications and role of the Fo¨rstermodel with respect to fluorescence thermometry arecussed by Dowell.60,61The spectral emission distribution cabe a sensitive function of dopant concentration. An examof this62 is illustrated in Fig. 4 for Y2O2S:Eu. At low Euconcentrations, the shorter wavelength lines are strongerthey are at higher concentrations. Kusamaet al.29 noted thatthis is particularly important for intensity based thermometry. They indicate that the5D2 emission of Y2O2S:Eu isespecially sensitive to Eu concentrations. In comparing teperature dependent ratios of this line to a5D0 emission line,a difference of 0.1 mol % between samples will vary tratio by about 5% and lead to an estimated error of 15 °

Finally, rise times are affected by dopant levels. Thoccurs especially when one level feeds another as noteHellier63 and Rys-Williams and Fuller.64 Table II containsthe results of measurements of rise time versus Eu contration in Y2O3.

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b. Saturation effects.Saturation effects on YAG:Tb, another phosphor of interest in thermometry, are discussedde Leeuw and ’t Hooft.65 A survey of the study of saturatioeffects is given there.

High incident fluxes, whether from laser, particle beaor any other excitation source, can lead to luminescence sration effects. When this occurs, phosphor efficiency chanas a function of the incident flux. In typical applications, th

FIG. 4. Spectral emission distribution for Y2O2S:Eu at dopant concentrations of ~a! 0.1% and~b! 1.0%.

TABLE II. The dependence of the rise time of the5D0→7F2 transition onthe concentration of Eu31 in Y2O3:Eu

31 ~after Ref. 64!.

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would manifest itself as a spurious temperature change ifflux were to increase to the point of causing saturation.illustrative example arises in the description of saturationY2O3S:Eu by Imanagaet al.

66 where the relative dopant concentrations were 0.1% and 4%. A nitrogen laser~337 nm!focused to a spot size of about 1 mm2 was used for short-duration~4 ns! photon excitation. For cathode ray excitatioa 10 kV electron beam of 5ms duration was used andnumber of effects were noted. With the latter form of extation, the spectral distribution changed as a function of influence beginning at about 0.1mA/cm2. The effect was morepronounced at higher concentrations. Above a certain threold value, overall intensity decreases, but for the lower ccentration, this threshold value itself is lower and the ratedecrease is faster than at higher concentrations. Increathe beam voltage, independently of beam current, willgrade the saturation of this phosphor according to Yamamand Kano.67 Conversely, the situation may improve for somphosphors over a certain range, as observed in the casZnS and Zn2SiO4 by Dowling and Sewell.68 For short-pulseexcitation via nitrogen laser, the saturation effects are emore complex. Moreover, saturation is itself a temperatudependent phenomenon. Generally, for all concentratiothe saturation thresholds are lower at higher temperatuSaturation is enhanced at higher concentrations, but the rwill be different from one emission line to another.

There is evidence for a decrease in fluorescence detime with increasing laser fluence. Imanagaet al.66 suggestseveral possible mechanisms that may cause luminescesaturation behavior in this case. Beam-induced temperarise was ruled out as a possible cause since saturationless pronounced for the most temperature-dependent esion lines, as well as for other reasons,66 although this mightbe the dominant mechanism at work in other cases. Therdynamic considerations, coupled with experimental studcan help determine if beam-related effects will be a problin a given situation. The results found by Imanagaet al.66

may likely be due to the increased probability of having twexcited dopant atoms in close proximity to each other. Edently, when this occurs, an additional nonradiative dexcitation path is allowed. For instance, one of the exciatoms might transfer its energy to its neighbor, thus creaa doubly excited atom which nonetheless can emit onlyfluorescence photon rather than two, thus decreasingemission rate.

In most applications, one does not have the opportunto deliver a surfeit of laser excitation due to the necessitytransporting beams long distances, usually over optical fibwhich have limited fluence-handling capacity~see the dis-cussion on the design of fiberoptic-based systems belo!.Saturation becomes a relatively important problem durlaboratory calibrations, however, when a more nearly idoptical arrangement is employed, i.e., one having higavailable laser power and finer focusing.

Finally, we note that Raueet al.69 have studied saturation effects in Tb-doped phosphors excited by cathodebeams. They have developed a model that is consistentthe results of excited state absorption experiments carriedon a variety of these materials.

Remote thermometry


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c. Impurities.There are inevitably small amounts of udesired species in solid solution in the host material, athese change the atomic electronic environment experienby the activators so as to either augment or hinder phospperformance. For instance, at concentrations greater thppm, transition metal impurities will decrease phosphbrightness. This is because they absorb at wavelengths slar to those of the typical activators, thus effectively stealexcitation energy and decreasing the number of excited flrescence centers. Moreover, nonradiative energy tranfrom an excited activator to such impurities is efficient, thincreasing the decay rate and quenching the emission.

Even so, it is possible to mix powdered phosphors witgreat variety of substances, from epoxies to inorganic biers, without affecting emission characteristics. In fact,ramic binders can be flame sprayed, plasma sprayed, etclong as the result is a suspension of mixed materials. Perly chosen additives can also play a positive role. Forample, addition of small amounts of Dy and Tb or PrY2O3:Eu has the effect of decreasing the lifetime by as muas a factor of 3 with little or no change in quantuefficiency.70 Having a faster response can be important whusing phosphor thermometry to interrogate, e.g., a hspeed rotating shaft that has a ‘‘viewing time’’ thatlimited71,72 ~see Sec. IV C!.

d. Sensitizers.A sensitizer is a material that, when addto a phosphor, increases the fluorescence output. Webe9 ina survey of rare-earth laser research, formulated a tablrare-earth ions that have been used as active laser mediaincluded in it the corresponding rare earths used as sensers for each. The sensitizing dopant will absorb energy arather than emitting fluorescence, it transfers its energy tomain dopant from which an optical transition occurs. Fexample, he notes that erbium has been found to be a stizer for dysprosium, while gadolinium is a sensitizer fterbium. A prospective sensitizer must exhibit no absorptat the emission wavelength of interest. It must also habsorption bands that do not steal excitation energy fromactivator. Finally, it must have energy levels that are abthe fluorescing line which feed the activator but which do nquench it.73

e. Quantum efficiency.Quantum efficiency,QE , is a pa-rameter often used to help scale a phosphor’s ability to pduce bright light. Knowledge of the approximate quantuefficiency of a phosphor can aid in determining expecsignal levels. Numerically, the quantum efficiency is thetio of the number of photons absorbed by the phosphor tonumber that are emitted. IfQE>0.8, a material is considereto be an efficient phosphor.QE is not as dominant a criterionfor fluorescence thermometry as it is for lighting and dispapplications. Figure 5 presents the emission intensities fvariety of activators4 in YVO4. With Eu as an activator, theresulting phosphor is most efficient and useful in thermoetry only above the quenching temperature of'500 °C. Onthe other hand, a Dy activator in this host will produce dectimes that make the phosphor62 useful in the range from 300to 500 °C.~The ratio of two emitting lines in this case caalso be used to make measurements at somewhat lowerperatures.! It is interesting to note that YVO4:Nd is being



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used as a laser medium despite its low efficiency. FYVO4:Eu, QE568% at the optimum concentration of 0.0mol % ~253.7 nm excitation! whereas QE588% forYVO4:Dy at a concentration of 0.003 mol %. The latterclearly a more efficient phosphor, except for the fact thatconcentration dependence is more pronounced.

Efficiency is a wavelength-dependent phenomenTherefore, if, e.g., instrumentation availability prevents tuse of mid-UV excitation with a phosphor that happens toefficient in that band, then a substitute material better suito the wavelength of the excitation source should be uinstead.

FIG. 5. The emission intensities for various rare-earth activators in YV4

~reprinted from R. C. Ropp,Luminescence and the Solid State~ElsevierScience, Amsterdam, 1991!, p. 438, with the kind permission of ElsevieScience, Amsterdam.

FIG. 6. Temperature dependence of La2O2S:Eu spectra.



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3. Fluorescence intensity

A comparison of the two spectra of La2O2S:Eu presentedin Fig. 6 shows that the relative intensities of the lines n465 and 512 nm are temperature dependent. In fact,generally the case that certain of the lines in these phospget weaker, i.e., become less bright, as the temperature omaterial is increased. A plot of intensity versus temperatis shown in Fig. 7. Clearly, for this material, there is a regiwhere the selected emission line is no longer sensitivetemperature change. At a particular value, viz., the ‘‘quening temperature’’~as discussed above!, the strength of theemission line falls off drastically. The quenching tempetures and the slopes of the temperature dependencies usdiffer for each type of phosphor and for each of the emisslines within the spectrum of a given phosphor. In some cathere may be an increase in intensity with temperature ovgiven range of temperatures, although the opposite is mtypical.

4. Fluorescence lifetime

When a phosphor is excited by a pulsed source, thesistence of the resulting fluorescence can be observedviding that the length of the source pulse is much shothan the persistence time of the phosphor’s fluoresceThis is illustrated in Fig. 8 for the 514 nm line oLa2O2S:Eu monitored at two different temperatures. Theminescence intensity decays exponentially in this case,cording to the relation:

I5I 0e2t/t, ~5!

FIG. 7. Emission intensity vs temperature for La2O2S:Eu ~after Ref. 49!.



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where t ~the lifetime or decay time! is the standard 1/e('37%) folding time for the fluorescence. The decay timeoften a very sensitive function of temperature and, therefoa determination of its value constitutes a very useful methof thermometry. A portion of a calibration curve foLa2O2S:Eu is shown in Fig. 9 for three of its emission line

5. Fluorescence line shift

Each emission line is characterized by a wavelengthwhich the intensity is maximum. Its value may chanslightly with temperature, and this is termed a line shAlso, an emission line has a finite width, called the linwidth, which is often designated by the spectral width at hthe maximum line intensity. Linewidth and line shift changas a function of temperature are generally small, and areoften used in fluorescence thermometry. However, as prously mentioned, Kusamaet al.29 utilized this approach forcathode-ray-tube thermometry. As shown in Fig. 10, th

FIG. 8. Phosphor decay characteristics at 65 and 75 F~La2O2S:Eu at 514nm!.

FIG. 9. Fluorescence decay time vs temperature for La2O2S:Eu.

Remote thermometry


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observed a shift to the blue of about 0.2 nm in going fro215 to 72 °C. The data they presented indicate that theshift has an approximately quadratic dependence onquency.

6. Absorption band and excitation spectratemperature dependence

The absorption spectra of many phosphors consistrelatively broad band at the blue or ultraviolet end of tspectrum, along with sharper absorption features in theible and near-IR. The sharper features are often dueatomic transitions of the dopant atom and, as noted abexhibit some temperature sensitivity. The broad absorpband, however, since it results from direct interaction wthe host, may show a more marked temperature depende

FIG. 10. Linewidth and line position at215 and 72 °C for Y2O2S:Eu~afterRef. 29!.



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Bugos74 studied the temperature-dependent excitatspectra of a number of thermographic phosphors. The reswere very instructive, because an excitation spectrum pvides an alternate means of determining the positionstrength of absorption features in a material. Whereassorption spectra are obtained by measuring the amounlight transmitted through a specimen as a function of walength, an excitation spectrum is determined by monitorthe intensity of an emission line while the excitation wavlength is varied. An example is shown in Fig. 11, whicpresents the temperature-dependent excitation spectra75 ofY2O3:Eu. Within the resolution of the Perkin-Elmer mod650-10S spectrometer ('1 nm) used in that work, no discernible change was found in the atomic-transition bawidths. Even so, the charge-transfer band is seen to mtoward the red end of the spectrum by about 30 nm per 50~i.e., 0.6 nm/°C.! The peak of the absorption for this materiincreases up to about 400 °C.~At this point, the heat generated by the oven placed inside the spectrometer could bto affect the measurement.! We note that at room temperature this phosphor is not excited efficiently by standasources such as a nitrogen laser~337 nm!, a tripled YAGlaser ~355 nm!, or a long-wavelength mercury vapor lam~365 nm!. Even so, these particular sources are efficienabsorbed at higher temperatures and are quite useful inregime.

The temperature dependence of the absorption wasploited by Turleyet al.76 to infer the energy of a CO2 laserby its heating of a phosphor sample. For this, constant etation was applied to a YVO4:Dy phosphor by a flashlampthat was filtered to illuminate at 365 nm. This wavelengthweakly absorbed by the phosphor at room temperatWhen the 100 ns laser pulse heated the sample, the abtion and the resulting~detected! fluorescence increased iproportion to the incident laser energy. The associated t

on of
FIG. 11. The temperature-dependent excitation spectra of Y2O3:Eu, as measured with a Perkin-Elmer model 650-10S spectrometer with a resolutiapproximately 1 nm.


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perature rise did not exceed the quenching temperature ophosphor.

III. PRACTICAL DESIGN CONSIDERATIONS

A. Choice of phosphor

Figure 12 shows lifetime versus temperature for sevematerials that have been tested at Oak Ridge National Laratory ~ORNL! and collaborating institutions.77 The choiceof phosphor for a given application will clearly dependthe temperature range that must be spanned. However,are additional considerations: chemical compatibility btween phosphor, substrate and the surrounding environmfor instance, is one such concern. Fortunately, many ofmost efficiently fluorescing materials are very inert; in fasome are ceramics. As another example, the optical bground in a given situation may make the wavelength ofphosphor emission an important consideration. In respohowever, we note that one can select from among phospthat emit anywhere between the infrared and the ultravioFigure 13 shows blackbody emission curves for temperatof 800, 1200, and 1600 K and prominent emission lines fr

FIG. 12. Lifetime vs temperature of selected phosphors.

FIG. 13. The blackbody emission curves for temperatures of 800, 12001600 K along with prominent emission lines from phosphors containing~611 nm! and Dy ~488 nm!.



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Eu ~611 nm! and Dy ~488 nm!. The blackbody emission athe latter wavelength is more than an order of magnituless.

It is sometimes convenient or even necessary to usphosphor that emits several lines, each with differing thermcharacteristics. As an example, YVO4:Dy ~the commerciallamp phosphor discussed above! exhibits bright bands centered at 575 and 484 nm. These lines have nearly idenlifetimes and both quench at'300 °C. The intensities ofboth of these bands decrease slightly with temperatureanother weak band~at ambient! at 453 nm grows instead.62

We have found that where the temperatures w,300 °C, the intensity ratio of the 453 and 484 nm linprovides the means for making measurements down to rotemperature if not below. Above that, the lifetime approamust be used.

B. Bonding techniques

In some critical applications, the most important techcal problem encountered is that of bonding~i.e., securingadhesion of! the phosphor material to the surface undergothermography.78 In what follows, we discuss the consideations that typically underlie the choice of a bonding methdescribe the various phosphor bonding processes, and eine the techniques used to evaluate and test bondstrengths.

1. Selection criteria

There are several criteria to consider in choosing a boing method.~1! The temperature range of interest will narow the field of potential phosphors that might be used iparticular application, and it is a primary considerationselecting a bonding method. As discussed below, some ohigh-temperature bonding processes cannot be used withtain of the more chemically and thermally sensitive phophors, thus further restricting the choice.~2! The heat fluxthrough the measurement surface and the surface-to-airperature differential may be important. The phosphor coatmust be thin enough to approximate isothermal conditioacross the layer. On the other hand, through careful designonisothermal condition may be established in a twphosphor layer, thus creating a heat-flux gauge as a reThe realization of one such gauge will be discussed latethe article. A thin coating is also a necessity when fast thmal transients must be measured.~3! Any thermal cycling ofthe surface and the temperature range of such cycles mutaken into account. The larger the temperature excursionmore important it is to choose, test, and verify the appropate adhesion method since cracking and breakup of the cing may otherwise result from repeated cycling.~4! The sam-ple’s physical environment can be a cause for concern.example, if the measurement must be made in a hitemperature reducing or oxidizing atmosphere, the phospmust be synthesized from chemically compatible materiIf erosion is perceived as a potential problem due to gasparticles impinging at high Mach numbers, then a smoand physically durable coating is required.~5! The substratecharacteristics must be such that neither contamination ofphosphor nor chemical reactions with it will occur. In add

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Remote thermometry


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tion, any substantial mismatch between the thermal expsion coefficients of the phosphor and the substrate becoimportant, especially if rapid changes of temperature arepected.~6! The size of the surface to be thermographicacharacterized and/or its location will determine whethercoating must be applied in the laboratory with special equment or whether it can be done on-site with appropriate ptable equipment~a discussion of the various approaches flows below!.

For the least critical applications, the coating processbe relatively straightforward. Typically, a slurry of phosphis mixed with an available epoxy, paint, glue, or other bining agent and then brushed or daubed onto the surface. Iviscosity of the mixture is low enough, an air brush may abe used. The binder may have to be suitably transparenultraviolet light to achieve satisfactory excitation of thphosphor. Some surfaces have enough porosity or coarseto even allow sufficient bonding with just the rubbing of thphosphor onto them. In general, however, one seeks totain a very robust coating and approaches more speciathan these are needed.

2. Surface preparation

Especially when adhesion is critical, surface cleanlinand roughness are important concerns. In such casesabrasive-blast spray~e.g., sand blasting! can both clean andappropriately roughen the surface. A certain degree of rouness is required to provide a ‘‘tooth’’ to which the phosphbinder can adhere, since it is sometimes difficult to obtadhesion to a surface that is too smooth. Even so, blast cling should proceed only until, e.g., the mirrorlike sheenmetallic surfaces is lost; pitting or other damage to the sstrate should be avoided. Ultrasonic cleaning may be usean alternative, as well as immersion or wiping with convetional solvents such as acetone. Scrubbing with nonabrabrushes to remove grit may be beneficial, as is cleanblasting or spray. Last, the piece may be oven dried to mmize oxide formation and the absorption of water, andbake off other contaminants.

3. Chemical bonding

When phosphor thickness is not an issue, chemicals wadhesive properties can be mixed and applied to the surfEpoxies are typically useful to about 350 °C. Above thisvariety of binder materials, some based, e.g., on silicasilicones and silicates, water-mix refractories, cellulosic metc., can be used, possibly in excess of 1200 °C. Bore79

summarizes some of the possibilities and issues with regto their use. Sperex™M VHT-1 is representative of a variof such binders that have been tested and put in use. Wia cure time of 1 h each at 200, 400, and 600 °C in successis recommended. In situations where this is not possiblepractical, a room temperature cure may suffice if there isexposure to solvents.

4. Electron-beam vapor deposition

Electron-beam~e-beam! vapor deposition produces thiyet durable coatings. The phosphor must be either cold or



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pressed into a disk shape and mounted in the depositiontem’s vacuum chamber. Instrumentation typical of this aproach was assembled by Turleyet al.80 and consisted of a61 cm ~24 in.! vacuum chamber evacuated to 331028 Pa(431026 Torr), with the beam set to'40–50 mA at 6 kV.Deposition rates vary from about 0.2 to 2mm/h. For e-beamdeposition of phosphor films, significant increases in phphor efficiency can be achieved by postprocess anneaAmong the advantages of this approach to bonding arefine degree of film-thickness control that is obtainable, athe fact that there is no chemical binding agent present.latter can absorb and scatter excitation light and generdecrease efficiency. The high temperatures generated ae-beam point of impact irreversibly break down some phphors, such as the oxysulfides, and postannealing in thisoffers no benefit.

5. Radio frequency sputtering

The radio-frequency~rf! sputtering process is functionally similar to e-beam deposition of phosphors, and it hbeen investigated as a bonding method in a wide varietysituations.81,82In this technique, the target achieves the vapphase by momentum exchange, a mechanical procespostbonding anneal should also be carried out once theis deposited to the proper thickness. In one carefully plantest of instrumentation designed to make temperature msurements in turbine engines, samples of three phospwere prepared by the rf sputtering, e-beam, and chembinder approaches. After annealing and exposure to 8 h in aturbine-simulating burner rig, the fluorescence efficiencyall specimens was compared to the pure phosphor powThe results, presented in Table III, from Turleyet al.,80

showed a spread in the efficiencies that ranged from 30%60%. Since rf sputtering is a technique that is quite similare-beam deposition, there is not likely to be a general distition or advantage of one approach over the other in termbonding performance.

6. Laser ablation

Laser-ablation deposition is a proven method for proding high temperature superconducting thin films.83 Highpeak-power laser pulses are used to almost instantaneodeposit energy into a small-surface volume of a targWithin a few nanoseconds, the irradiated layer is broughtemperatures of>20 000 °C. The heated material is ejectat high speeds and may be collected on a suitable substIf the substrate is held above a certain temperature andsequently cooled at a proper rate, then a high quality crysline thin film results. Under different conditions, a thin amophous layer is produced. Optical materials and certphosphor-related materials such as ZnS, CdS, and Y2O3 havebeen deposited via laser ablation.83,84

C. Measurement strategy

The physical variables most typically measured whcharacterizing the temperature dependence of thermographosphors are the intensity, lifetime, and line shift of s



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lected spectral features. Any particular one of these mayfer the best measurement strategy in a given application

Intensity-based approaches generally require the splest instrumentation and, therefore, tend to be the leastpensive to implement. Their disadvantage lies in the difficties associated with maintaining calibration, since this clof experimental arrangement requires mechanical, electrand environmental stability. When possible, the signal is nmalized to a temperature-independent optical signal atother wavelength. Even when this is done, however,chromatic differences of optical elements will make chanin element positions, thermal expansion, etc., significsources of error. An example would be a temperature depdence of the chromatic dispersion of lenses used for fluocence collection.

Fluorescence lifetime approaches do not suffer thisadvantage. On the other hand, analysis of the decay rateis generally more complex. Economy-of-scale has madinexpensive to obtain simple analog-to-digital electrocomponents of the kind needed to measure fluorescenctensities. However, fast multichannel devices such astransient digitizer are needed for lifetime measurements,they are still relatively costly. In any case, the decay ratesmeasured in terms of time and frequency, and the mettherefore offers comparatively less uncertainty since thquantities can generally be determined with greater accuthan can, e.g., optical intensities.

Methods based on changes in spectral features, sucshifts in line position or variation of linewidths, do not suffe

TABLE III. Intensities and fractional survival of three different phospho~Note: Each phosphor was bonded by rf sputtering, chemical agent,e-beam deposition.!

Sample/binder~mm!Relativeintensitya % remaining

Y2O3:Eu

Sputtered~1–2! 0.029 33Sputtered~3–5! 0.16 56Binder (,35) 0.089 24Binder ('35) 0.58 0.6e-beam~9–13! 0.37 62e-beam~24–27! 0.44 61

YVO4:Eu

Sputtered~1–2! 0.80 42Sputtered~3–5! 1.01 52Binder (,35) 1.9 49Binder ('35) 0.72 43e-beam~9–13! 1.1 32e-beam~24–27! 0.72 52

YAG:Tb

Sputtered~1–2! 0.04 51Sputtered~3–5! 0.097 58Binder (,35) 0.56 5.4Binder ('35) 0.32 28e-beam~9–13! 0.29 86e-beam~24–27! 0.29 35

aThe intensities were measured relative to the luminescence-peak heiga hot-pressed sample of Y2O3:Eu that was used as a standard~after Ref.80!. Comparisons were made using a laboratory spectrophotometer.



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the calibration problems of amplitude-based approachHowever, their principal drawback is that the thermal sentivity of such effects is usually quite low relative to either thintensity or the lifetime approaches.

The decay time method is preferred when making htemperature measurements in situations where there is ea significant flame or combustion background, or whereblackbody emission is bright compared to the phosphor flrescence. For measurement of moving surfaces, decaymethods do not require recalibration except at very high sface speeds. Measurements on a high speed motor arecussed later in this article, and in full detail elsewhere72

Figure 14 shows expected temperature dependence oratio of fluorescence to blackbody emission for two of tdopants most commonly used in phosphor thermomebased on reasonable assumptions. This kind of~dc! back-ground can be discriminated against quite effectivelyspectral and temporal filtering. Clearly, in these situatiophosphors producing narrow-band emissions have an adtage over those that have a broader emission spectrum.

D. Particle size versus decay rate

There is a change in fluorescence lifetime with a chanin the size of the phosphor particles. An example of thissmall particles is cited by Christensenet al.,85 where theyobserved an increase from 436 to 598ms for a decrease inparticle size from 0.42 to 0.11mm. Because of this and otheeffects, the fluorescence decay time of virtually any phphor sample may actually be a composite of the individdecay lifetimes arising from the distribution of particle sizeFrom the purely practical standpoint, then, since some phphor preparation methods yield large agglomerated particsimply grinding the phosphor to a fine powder will affect thlifetime. This should not be overlooked during bonding stuies and calibration procedures.

An explanation for this observation can be found in tquantum theory of the decay rates of electric-dipole em

FIG. 14. The ratio of fluorescence to blackbody for Eu and Dy emisslines vs temperature.

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Remote thermometry


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sions. It predicts that the spontaneous lifetime of fluorcence is inversely proportional to the index of refraction ofluorescent material.85 Specifically, the refractive indexnc inthe vicinity of an ion isolated in a crystal will be differenfrom the indexns in the vicinity of the same ion when it islocated in a small~e.g.,,600 nm! particle. The relationshipbetween the indices and the lifetimes is

ts5nctc /ns5t0 /ns , ~6!

where thets , tc , andt0 are the lifetime values for the ioniemission in the small particle, crystal and vacuum casesspectively. For small particles immersed in a liquid, the litime scales as the inverse of the index of refraction ofliquid.

E. Magnetic field effects

Only very large magnetic fields have a significant effeon the emission characteristics of thermographic phosphMannik et al.86 obtained calibration curves of decay timversus temperature for the 514 nm emission lineY2O2S:Eu between 70 and 120 °C, first without an applfield and then in the presence of a 0.8 T field. They foundapparent change. At sufficiently high field strengths,course, the degenerate energy levels will be split and, hethe spectrum will change. This is the well-known Zeemeffect, a standard tool in spectroscopy for making enelevel assignments and understanding spectra. As an exaof the field strengths required to exhibit it, however, Alvet al.26,27,87 needed a 5 T field to produce splitting of lessthan 0.5 nm for europium and terbium in some oxysulfidLine broadening was an even smaller effect. Their expments were done at liquid nitrogen and liquid helium teperatures to insure that the emission linewidths themsewere narrow enough to permit resolution of the effects.

F. Estimation of signal strength

Quantum yield is the ratio of the number of fluorescenphotons produced in a particular emission band to the nber of photons incident on the sample. Once the quanyield is known or determined experimentally, signal levemay be estimated. The quantum yield may be measureusing an optical source to illuminate a phosphor samplenormal incidence. The resulting fluorescence is then motored at some angle with respect to normal using a deteof known diameter so that the solid angle of collectiondetermined. It is assumed that the phosphor acts as a Lbertian source where, following Noelet al.:88

~A0 /A1!•I5I 0 cosu, ~7!

HereA0 is the area of the hemisphere defined by the distaof the detector surface from the phosphor,A1 is the area ofthe detector surface,I 0 is the total amount of fluorescencemitted into the 2p-steradian hemisphere, andI is the fluo-rescence measured by the detector. This relation is usedetermineI 0 . The quantum yield,Qy , is then simply

Qy5I 0 /I i , ~8!



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whereI i is the intensity of the source which strikes the phophor. Noelet al.88 followed this procedure to determine thquantum yield of the different emission bandsGd2O2S:Tb, finding that 1%,Qy,6%.

Knowing the quantum yield, the number of fluorescenphotons striking a detector surface for any geometry cancalculated from the intensity of the incident source, the clection solid angle, and the losses introduced by intervenoptical elements~such as lenses and filters!, scatterers, andabsorbers. The signal strength is subsequently a functiothe detector sensitivity and the properties of the associaelectronics.

An example is illustrated in Fig. 15 where a 3 mJ im-pulse of collimated light illuminates a phosphor layer, wha photomultiplier tube with an intervening lens and filtdetect the fluorescence. Assuming thatQy55%, one cancalculate the number of photons,F, detected and the size othe resulting signal:

F5~E/hn!•Qy•h•V•cosu, ~9!

whereV5pa2/2pr 2 is the solid angle subtended by the leof radiusa at distancer , h is the combined transmissioefficiency of the lens and filter, andu is the angle at whichthe fluorescence is observed with respect to the normal.serting the reasonable values ofE5331023 J, Qy50.05,h50.3, a52.5 cm, r525 cm, andu530°, we find thatF>531011 photons.

The efficiency,e, with which a photocathode convertincoming photons to electrons is nominally 0.25. The libated electron is multiplied by sequential collisions with tphotomultiplier dynodes such that a gain,G, of between104 and 107 is obtained.~The exact value ofG will depend

FIG. 15. The experimental arrangement for determining the fluorescesignal strength.



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The total charge,Q, that is detected is therefore

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wheree51.602310219 C, the charge of the electron. FoG5106 and with 3 mJ of illumination, the total charge acumulated would thus beQ>2.831022 C. The temporalsignature of the resulting detector current will be the famildecaying exponential wave form of the laser-induced flrescence process. The value of the detector current averover a 1 speriod under conditions of continuous illuminatioin the present example would therefore be 2.831022 C/s,i.e., 28 mA. This is, of course, a huge number and virtuaany type of photodetector would be saturated or overdriwell before this. On the other hand, the example does iltrate that large numbers of photons~and, therefore, large signal levels! can be obtained rather straightforwardly.

IV. INSTRUMENTATION AND EXPERIMENTALTECHNIQUES

A. The generic thermometry system

A phosphor-based optical thermometry system will geerally consist of at least the following components:~1! asource of excitation energy,~2! a means to deliver the energto the target~typically beam-steering elements or a fiberopbundle if the energy is in the optical spectrum!, ~3! a fluo-rescing medium that is bonded to the target and illuminaby the incident flux,~4! an optical system to collect antransport the fluorescence that is subsequently generated~5!a detector or an array of detectors to monitor the fluorescesignal, and~6! a data acquisition and analysis system tultimately yields the target’s temperature. Figure 16 shoan elementary yet interesting illustrative example of this geral arrangement.~The particular system depicted was dsigned to demonstrate the feasibility of making remote, ncontact measurements of the temperatures of and strainhigh voltage devices such as transformers and other comnents in electrical substations in the Tennessee Valleythority’s power grid.!

In this arrangement, the energy source is a pulsed2laser of 3 ns pulse length, emitting in the ultraviolet at 3

FIG. 16. The fundamental elements of a remote fluorescence thermomsystem.



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nm. Many other sources, either continuous or pulsed,also be used to produce fluorescence. These include filamand arc lamps, various lasers and flashtubes and even flaas well as beams of electrons, neutrons, ions, and otherticles. In the present example, the laser light is guided tofluorescent target by two mirrors. The first is front surfacfor beam steering, and the second is dichroic and henceto reflect ultraviolet light while transmitting visible light. Thexcitation beam strikes a band of phosphor that is bononto a heated cylindrical brass target mounted on the shaa motor so that either static or dynamic measurements camade. The fluorescence emanates from each illuminapoint on the surface of the target into a solid angle ofpsteradians. A significant fraction of this light thus falls withthe acceptance angle of the collection optics, in this cshown as a single plastic Fresnel lens of 15 cm diameOften, a more complex optical arrangement of lenses anfiberoptics is required for pulse delivery and fluorescencollection. The details depend on the particular experimesituation that must be addressed. The La2O2S:Eu phosphorused here produces simultaneous, bright emissions at sewavelengths across the visible spectrum. Because theperature dependence of the lines varies, a narrow bandfilter is used to allow the desired wavelength to reachdetector, in this case a photomultiplier tube~PMT!. ThePMT, of course, transduces the fluorescence decay siinto an electrical wave form, which is then digitized, aquired, and analyzed. In this particular example, tcomputer-based analysis system averages the data, takelogarithms of the averaged values, and calculates the slopthe resulting~presumably straight! line to arrive at the fluo-rescence decay time. The temperature of the targeted suis subsequently revealed by access to a look-up table ofcay times versus temperatures that is stored in memory.

For phosphors quenching at extremely high tempetures, the fluorescence pulses are typically quite fast, sotimes only nanoseconds in length. Moreover, the puheights will be very small if the fluorescence signal is weaBecause of these factors, PMTs are often the detectorchoice in phosphor thermography systems. The avalanphotodetector~APD! has been considered as an alternativethe PMT,89 since it is equally fast in some applications.90,91Itis not unusual, however, to find silicon photodiodes~photo-voltaic mode!, charge-coupled devices~CCDs! and otherelectro-optic detectors used in applications where the flrescence signals are slower and stronger.

The source of illumination and the requisite detectiequipment can typically be located at line-of-sight distanof up to '10 m from the target surface. For surfaces naccessible by a line-of-sight path, optical fibers may be uto convey the illumination and fluorescence signals. Moover, the use of fibers opens the possibility of multiplexiof the excitation and decay pulses, thus increasing the nber of regions on the target that can be monitored at the stime.

The equipment can be configured in a portable arranment for field applications and, as mentioned above, teperatures of either static or moving surfaces can be demined. A general advantage of this and related rem

try

Remote thermometry


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optical techniques is the freedom from electrical interfereoffered by the elimination of electromagnetically sensititransducers at~and leads to and from! the target’s location.

B. Calibration methodology

To provide valid thermometric data, the response ophosphor-based temperature sensor must be calibragainst a known standard. At the very least, this requiresthe quantity of interest~line intensity, lifetime, etc.! be mea-sured as a function of temperature under known and repable conditions, with a stable and well-characterized thmometer serving as the reference. Ideally, the calibrameasurements are carried out with a transfer standard. Iand if the resulting data are taken under an appropriatetocol, traceability of the measurements to the InternatioTemperature Scale92 ~ITS-90!, as maintained by the variounational standards laboratories,93,94can be established. Sincthese general features of the calibration process are larthe same for all the different fundamental methods of phphor thermometry, we shall concentrate in what followsjust one of them, fluorescence lifetime versus temperatMoreover, as per the defined scope of this review, mosthe discussion is aimed at those experimental arrangemthat arenoncontactin nature. There remains, however, sustantial overlap with the calibration processes appliedthose systems that are not~e.g., a fiberoptic probe with phosphor bonded onto the tip, which is then touched to a surfor inserted into a volume!.

Table V in Sec. VII A includes estimates of the postcabration measurement errors claimed for several of the difent phosphor thermometry systems that have been descin the recent literature. The sources of error that contributthe overall error budget in the general case are discussethe remainder of the text here.

1. General approach and thermal considerations

As discussed above, many different workers have invtigated the temperature dependence of phosphor fluoresclifetimes. The establishment of that dependence withincontext of a quantitative calibration procedure calls for apropriate attention to details, many of which are not of pticular interest in other measurement arrangements.

The overall scheme of a relatively generic calibratisystem95 is shown representatively in Fig. 17. As might bexpected, it constitutes a special case of the arrangemshown in Fig. 16. The phosphor to be evaluated is eitbonded onto a substrate with a high-temperature adhesiva small amount of it is spread into a ceramic boat. Tsample is then centered inside an oven or furnace and siclad optical fibers are used to convey the luminous signinto and out of the high-temperature cavity. The fibersinserted into the cavity either through a viewport~if avail-able! or directly through the thermal insulation of the wallA dye laser driven by a pulsed nitrogen laser is used to tthe excitation signal to the wavelength of interest. Tdecaying-exponential fluorescence wave forms are gleafrom the optical background with a narrow band filter aconverted to an electrical pulse train by a PMT. A usevariation involves substitution of a monochromater for t



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optical filter, thus extending the flexibility of the arrangment. A digital sampling oscilloscope or some other waform acquisition device is used to collect the fluorescensignals, which are then analyzed by computer to yieldfluorescence lifetime. A signal derived from the laser pulseused to trigger the data acquisition sweep of the oscilloscoor otherwise enable a synchronous detection technique.probe of the reference thermometer~e.g., a platinum resis-tance thermometer or a well-characterized thermocouple! isplaced in direct contact with the phosphor sample insidehigh-temperature cavity and its readings are stored incomputer, along with the decay time measurements, ascavity is ramped up and down in temperature. In that waylook-up table of decay times versus reference temperatcan be generated for use in the field.

The overall accuracy and repeatability of the calibratidepend critically on the quality and robustness of the reence standard and the stability and uniformity of the higtemperature cavity’s thermal field. As a practical mattthen, the reference thermometer must typically be encasean armored probe or it must have a surface passivation lto minimize the effects of corrosion due to the oxidation thoccurs during cycling to high temperatures. Bare-wire thmocouple junctions are particularly prone to a varietywell-known ‘‘aging’’ effects that degrade their measuremeaccuracy. For example, one study of type K thermocoupcarried out in our laboratories96 showed that shifts averagin2% can arise in the temperature readings of thermocoucycled even once between room temperature and 950Because of their ubiquity, however, few types of instrumetation have been studied as carefully as thermocouplesplatinum resistance thermometers. As a result, there is aliterature that describes them and provides guidelinestheir ~repeated! use in high-temperature environments.

A more insidious problem is that created by thermal gdients in the high-temperature cavity. The lack of a totauniform distribution of temperatures within the oven or funace means that it is generally necessary to keep the pphor target and the reference thermometer fixed in placein physical contact with each other during a calibration ruMoreover, the location of the target and the reference th

FIG. 17. The characteristic experimental arrangement used to calibrattemperature dependence of fluorescence.



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mometer within the thermal cavity should be noted so tthey can be repositioned to those points during any suquent recalibrations. To investigate this problem, Lutz devoped a computer-aided data acquisition system that mtored an array of NIST-traceable thermocouples. It was uto map the thermal gradients of a Lindberg model 51442furnace driven by a model 59344 digital controller.97 ~Thisparticular furnace was subsequently used in investigationthe metrological aspects of phosphor thermometry.98! Theplanar array of nine thermocouples was moved fromfront to the back of the 19.05 cm313.34 cm335.56 cm fur-nace cavity with data then acquired at 15 mm intervals othe range from 50 to 1200 °C. The maximum measugradients99 at 100 and 1000 °C, respectively, were 0.11 a0.73 °C/mm, with a measurement resolution of 0.02 °C/mA 12.5 mm ('0.5 in.) uncertainty in the repositioning of thphosphor target relative to the reference thermometer cthus lead to an error of'1% at temperatures of 1000 °C, bthe error can obviously be reduced if care is taken in resitioning the target.

One remedy to the thermal gradient problem~at least atrelatively low temperatures! was devised by Crovini andFernicola100 at the Colonnetti Institute. They custom built aoven that was subsequently used in calibration studiesfluorescence decay thermometers. Their system is smallscale than the furnace described above, in that it hatemperature-regulated inner chamber'80 mm in length.The components inside this chamber consists of a phospholder block and a mating piece in which are mountedoptical fibers used for illumination and observation. Theblocks can be either fixed in place to insure positional repability or moved with precision relative to each other bybellows-coupled drive to simulate noncontact thermomeconditions. The temperature uniformity over the specimenphosphor under study is60.03 °C. This makes it possible tachieve resolution in temperature as sensed with theirglass samples~Schott, type LG-760! of 60.2 °C over therange from220 to 100 °C. Circulating thermostatted fluand an integral electrical heating coil are used to establishoven’s operating temperature over this range and evenyond ~up to 200 °C, with a stability of60.03 °C!. Furtherdetails of their work are available elsewhere.101

Other relevant studies include one in which a precurto the prototypical calibration system shown in Fig. 17 wdeveloped by Cateset al.102One of the more recent furnacebased calibration systems for Y2O3:Eu and YAG:Tb phos-phor thermometers was described by Alaruriet al.103 Theyused an alignment laser to adjust the positions of the besteering and collection optics relative to the phosphor tarFurther discussion of thermal influences on the calibratmethodology has been provided by Dowellet al.95 and Gil-lies et al.104

2. Optical considerations

Several optical components are typically neededhandle the excitation and fluorescence pulses in a phosthermometry system. Thermal and mechanical instabilitiethe optical fibers used in such systems include dopant mition, silica devitrification, and variation of the material’s in



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dex of refraction, all produced by exposure to high tempeture. At high enough temperatures, the small but finabsorption coefficients of some optical materials can emake them emitters, although typically in the infrared. Theeffects and related phenomena produce variations in the flguiding properties of the optical fibers, leading to losssignal strength and/or resultant degradation of the measment’s signal-to-noise ratio~SNR!.

Milcent and colleagues105 have previously presented andiscussed these problems and carried out a careful investion of the way in which the properties of optical fibers usin thermometry systems change with temperature. In partlar, they studied the response of both unclad and metal-silica fibers to thermal cycling and heating-rate variatioThe fibers were part of an optical pyrometer that had a msurement range of 100–1000 °C. They found that heatingmetal-clad fiber to'900 °C led to variations of up to 20 °Cin the measured temperatures, i.e., deviations of'2.2%.When the silica-clad fibers were heated to 700 °C, the resing variations in the measured temperatures were roughlysame, but the total power guided by the fiber decreasedabout 4%.

Another source of thermomechanical damage to optfibers that can affect the calibration~or even limit the usabil-ity! of remote thermometry systems is that resulting frominternal focusing of high energy pulses emitted by the extation laser. Allisonet al.106 reviewed the work of others anexplored this problem within the context of laser-inducfluorescence. They used a Nd:YAG laser to drive pulsesup to 8 mJ with 5 ns durations through fibers having codiameters ranging from 400 to 1000mm. They found that thedamage threshold for the fiber under study varied withfocal length of the coupling lens, the structure and compotion of the fiber, and the nature of the fiber’s surface preration. They noted in particular that careful alignment of tinput beam with the axis of the fiber is necessary to minimthe optical energy density at the first internal reflection poTheir results provide guidelines for the fluences that cansustained at the input faces of both plastic-clad and hard-silica fibers. These findings become particularly significawhen attempts are made to increase laser power in ordeoffset the rolloff in signal strength associated with eithsaturation phenomena or quenching of a particular phospat the high end of its useful temperature-sensing range.extension of their study that assesses other attenuamechanisms, such as those due to nonlinear optical effecfiber materials, environmental influences, and ionizing radtion, is available elsewhere.107 ~As an aside, we note that thwork on pulsed laser damage to optical fibers106 led to thedevelopment of a method for generating self-imaging effein such waveguides and the resulting conceptualizationnew potential means of sensing temperature.108!

Maintaining careful alignment between componentspoints downstream from the input coupling face of the fibis also an important consideration in maximizing the excition intensity delivered to the phosphor target. In particulreduction of the lateral offset between any two butt splicesthe optical fiber pathway is of particular importance. Dowet al.109,110measured the lateral offset power losses of a

Remote thermometry


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riety of step-index optical fibers scanning point sourcesorder to quantify this problem. Their results are presentedgraphical mappings of the attenuation in transmitted poversus the size of the lateral offset. Quantitative estimatethe residual misalignment that can be tolerated in a giexperimental arrangement can thus be made, and all ofallows inference of how the SNR is affected by misaligment.

In many situations a lens is used to focus the ligemerging from the optical fiber onto the phosphor target,also to collect the fluorescence signal and couple it intoreturn fiber. Should the lens be inside the high-temperacavity, its figure may change due to thermal expansionwill the dimensions of the standoffs or other structural ements that hold it. If these changes produce large enoshifts in the position of the excitation beam’s focal porelative to the phosphor target, then the level of excitatenergy density at the surface of the target will begin tocrease. At very high temperatures, the fluorescence sigfor most phosphors are typically quite weak due to quening, whereas the background glow due to blackbody emsion is relatively strong. Since a decrease in the availaexcitation energy results in a weaker fluorescence sigthermomechanically driven shifts in the position of the focpoint of the coupling lens may thus limit the range ovwhich the phosphor thermometry system can be fully cbrated.

To deal with the problem of obtaining precise positioing of a lens inside a high-temperature cavity, Hanftet al.111

developed the kinematic lens displacement device showFig. 18. It is in essence a high-temperature translation stconsisting of alumina and ceramic-foam components. Tlens is placed into a holder that can be maneuvered updown along a linear slide rod by turning a winch that plaout an alumina thread that is attached to the holder. The sof the winch extends through the insulated walls of the fnace and can thus be rotated from outside the htemperature cavity. The phosphor sample to be calibrateplaced in a removable tray located in the base of the devAs shown in Fig. 18, a set of vertical guides~seen mountedon the base! is provided in order to keep the lens holder frorotating about the axis of the linear slide rod. In this arran

FIG. 18. A kinematic lens displacement device used to adjust the positiooptical elements within high-temperature furnace cavities.



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ment, one rotation of the winch corresponds to'2.4 cm oftravel by the lens holder. The overall positional repeatabiof the device is better than 1 mm and, when at temperatthe system can be adjusted at a maximum rate of'12 rota-tions per min. This relatively simple mechanism providesvery satisfactory means of adjusting the position of the leplaced inside a phosphor thermometry calibration furnac

As discussed in Sec. IV A, the transducer of choicemost phosphor thermometry systems is a photomultiptube, although avalanche photodiodes and other kinds oftectors are also employed. The types of noise, drift, and ouncertainties that arise when making measurementssuch devices are well understood and will not be dwelt uphere. We note only that Dowell carefully investigated tperformance characteristics of the PMTs used inmetrological-grade phosphor thermometry systems, andtailed discussions~including error assessments! of his find-ings are available elsewhere.98,110

3. Signal processing considerations

When making fluorescence lifetime measurements, acaying exponential wave form with noise and dc offsettypically presented to the signal processing system. Compite signals consisting of multiexponential wave forms aencountered for some phosphors~see Sec, II C/2a!, and thiscan complicate the calibration process. The more rouproblem, though, is that of measuring the parameters osingle-exponential wave form.

Two broad central issues that arise in synthesizingappropriate signal processing system are~1! the choice of thedata acquisition and signal averaging devices and~2! thechoice of the algorithm used to analyze and reduce the dThese issues have been faced many times in the physbiological, and engineering sciences with the result thavast effort has gone into the creation of methodologies tpermit the precise estimation of the 1/e lifetimes of exponen-tially decaying signals. An overall assessment of the genproblem is far beyond the scope of this article, and westead comment briefly on only those parameter estimatechniques that have been developed and used withincontext of calibrating phosphor thermometry systemGuides to the extensive literature on the more general plem are available elsewhere.98,112

As an introductory historical note, we mention somethe early work done on laser-induced fluorescence at ORThat effort focused on the development of synchronoustection techniques for making precise measurements ofheights of exponential pulses in the presence of backgronoise. Lock-in amplifiers are the standard class of instrumused for this purpose. They have output measuremerrors113 that are inversely proportional to the SNR of thraw signal over the range 1023,SNR,1, i.e., over the‘‘worst case’’ range of highly corrupted signals, a consquence of their function as a bandwidth reduction filter.114

Several workers, both before and since that time, have inporated phase-sensitive detection and frequency domtechniques into phosphor thermometry systems, and arepresentative examples are mentioned elsewhere in thiticle.

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Calibration studies of some of the various lifetime mesurement techniques benefited from this early work at ORin that the methodology of summing noise onto knowsignals,113 while certainly not a novel idea, neverthelesslowed for the identification of many types of systematicrors in the instrumentation chains used to measure the flrescence signals. For instance, when a Tektronix 7854 disampling oscilloscope~DSO! was used to acquire the fluorescence signal, average it, and then calculate the exponedecay time constant, the resulting measurement errors wfound to decrease with continued averaging, but not withlimit.115 Exploration of this point led to the establishmentan optimal sampling strategy for this DSO as a functionthe SNR at the input of the instrument. Similar studies wcarried out with an EG&G/PARC Corp. model 162 boxcaverager110 and a novel phase-locked digital filter116 for thepurpose of evaluating the measurement errors intrinsicthese devices. The general conclusion of this line of wwas that, while it was possible to characterize the systemerrors in any of the devices used to recover a wave fofrom noisy data, the most accurate overall system calibtions required postrecovery assessment of the computaterrors in the time-constant estimation algorithm as well.

To that end, a series of studies examining the errorsvariety of algorithms running on the thermometry systemhost computer~see Fig. 17! was undertaken. This series bgan with a preliminary empirical investigation of the prolems encountered in making a simple least-squares fittwo-parameter exponential curve to the phosphor fluocence data.117 A more sophisticated subsequent studyvolved numerical Monte Carlo testing of both two-point anonlinear least-squares algorithms, with Gaussian-distribunoise added onto the simulated signals.118 Comparing thetwo algorithms at a simulation SNR of;40:1, the two-data-point estimation had an uncertainty of about 4%, while tof the least-squared error estimation was only 0.1%. Tcomparison of the theoretical predictions with actual msurements made on Y2O3:Eu was favorable.119 The use ofMarquardt least-squared error algorithms and KullbacLeibler stochastic distance analysis allowed for ever mprecise modeling and validation studies,98,120including thoseincorporating the dc offset of the wave form into the modbeing tested. The final results were similar in nature to thfound in the experimental evaluations of the various waform-acquisition instruments: one can establish an optimobservation time that should be used in measuring the enential decay in order to minimize the lifetime-estimatierror caused by the noise.119 More recently Zhanget al.121

implemented Prony’s method for improving computationspeed, achieving a 98% decrease in run time. Sunet al.122

extended this approach for the double exponential fluocence decay process.

4. Low temperature arrangements

The discussion so far has focused on the calibrationphosphor thermometers over the range from room tempture through 1000 °C and above. There have been sevattempts at making fluorescence-based thermometers



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work at cryogenic temperatures as well, and we describemeasurement and calibration features of some of thosetems in what follows.

The main instrumentational difference encounteredworking at low temperatures, of course, is the change frofurnace-based to a cryostatic experimental arrangement,the concomitant presence of liquid cryogens. Selection ofphosphor is also an important issue, as many of these mrials exhibit little or no temperature dependence in their litimes and intensities at low temperatures. As an examDowell and Gillies123 found that the fluorescence lifetime oY2O3:Eu ~6.8%! changed by less than 3% between rootemperature and 77.5 K.~The phosphor and optical fiberhad been mounted in a Teflon holder and submergedbath of liquid nitrogen. The temperature standard in this cwas a Cryogenic Consultants model DTG 200 resistathermometer.!

Simmonset al.,124 however, found in a calibration studof La2O2S:Eu that the5D3 transition line~434 nm! of thismaterial exhibited a strong temperature dependence dowliquid nitrogen temperatures. They extended their studyinclude observations of the 657 nm emission lineMg4~F!GeO6:Mn and built a cryostatic sample holder thwas used to make noncontact temperature measuremenboth materials over the range from 4 to'140 K, with thecooling provided by liquid helium.125 A photo of the experi-mental apparatus is shown in Fig. 19~a! and a cross-sectionaview of the sample holder immersed in the helium bathshown in Fig. 19~b!. The data taken during calibration runwith this system established that the relationships betwthe temperature,T, of the phosphors and their emission lifetimes,t, are given by

T~t!5194.00~2.362 log t!, La2O2S:Eu,

4 K,T,80 K, ~11a!

T~t!538.3~4.052 log t!, La2O2S:Eu,

80 K,T,125 K, ~11b!

T~t!52341.0~0.742 log t!, Mg4~F!GeO6:Mn,

4 K,T,140 K, ~11c!

whereT is in degrees kelvin andt is in microseconds.Krausset al.126 also studied La2O2S:Eu at low tempera-

tures~over the range from 193 to 293 K!. They measured theratio of the 5D3 and 5D1 lines of a sample bonded onliquid-nitrogen-cooled copper plate that was mounted inevacuated test cell, and achieved a resolution of 0.25 Ktheir calibration runs.

The performance of several other types of fluorescethermometry systems has also been characterized at lowperatures. One example is the attenuation-based, holmdoped optical fiber sensor of Yatagheneet al.,127 which wasdesigned for use at liquid nitrogen temperatures. Anothethe molecular luminescence cryothermometer of PetrinMaki128 that employed photoexcitation of thioketonesmeasure temperatures at 20 K and below. A ruby crythermometer described by Anghelet al.129 was calibratedagainst a type-K thermocouple to a precision of60.3 K over

Remote thermometry


FIG. 19. Low temperature calibration system.~a! Photograph of the cryostatic sample holder.~b! Thermography system diagram.

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the range from 200 to 300 K. The fluorescence of the crywas excited by a green light-emitting diode~LED!, and thedecay lifetime was the quantity of interest. Zhanget al.130

reviewed the various types of Cr31-doped thermometricschemes. In particular, they evaluated YAG:Cr31 as a fluo-rescence lifetime-based thermal sensor material overrange from 77 to 900 K using their own data and thatothers. They developed a very useful calibration formula ttakes into account the energetics of the atomic transitithat occur during the fluorescence process. The generalclusion of their effort was that sensors of this type shothus be satisfactory in meeting a variety of industrial thmometry needs. In addition, this same research grou131

demonstrated that the size of a ruby crystal affects the cbration when used in the cryogenic region. This is duefluorescence reabsorption.

C. Thermometry of moving surfaces

A phosphor target mounted on a surface moving at hspeeds may translate or rotate by nonnegligible amountsing the period required to make a decay time-based measment of its temperature. Virtually any kind of light deliverand collection scheme used in the measurement systemhave a limited field of view. As a result, the fluorescinregion may move significantly within the field of view, pehaps even completely into and then out of it, during tmeasurement period. This possibility is depicted in Fig.which illustrates how measurements were made on the ature of a high-speed motor, using only a single optical fibto collect the emission.72 The total amount of light reachingthe detector will change with time as the fluorescing ssweeps through the acceptance numerical aperture of thelection fiber. The efficiency of light collection will vary as



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function of rotation angle, being a maximum on the fiber aand dropping to zero at points sufficiently far off the axThe time dependence of the signal will be given by the pruct of the exponential time dependence and the~changing!optical collection efficiency factor. This time-dependent eficiency factor,F, may be determined in several ways. Fexample, a temperature independent fluorescence linebe observed. Allisonet al.72 noted that the 538 nm emissiolifetime of the phosphor used in that study was temperatindependent over the expected range of interest~room tem-perature to 100 °C!. The point-to-point intensity ratio of themoving signal,I v , to the stationary signal,I s , both at 538nm, yields the functional dependence of the efficiency facat that particular speed. Thus,

FIG. 20. The experimental arrangement used to measure the armatureperature of an operating, high-speed, permanent-magnet motor.



FIG. 21. Illustration of the effect of motion on the fluorescence signal.

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Two such idealized signals are depicted in Fig. 21. Whthis function is used to normalize the acquired temperatudependent signal,I v(t), which for Allison et al. was at awavelength of 514 nm, the intrinsic time dependenI c(t), is revealed. If the logarithm of the curve fit through tresulting data turns out to be a straight line, then the option has most likely been carried out properly, since tresult would match the static case. The relationships shobe

I c~ t !5I v~ t !/F, ~13!

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wheret is the characteristic decay time andA is a constant.A different approach to thermometry of moving surfac

was developed by Leroux and Taboue´132 within the contextof a broader study of the thermometric properties of the Zclass of phosphors. In their work, a cylindrical spindle wrotated about its axis at speeds up to 400 rps while beconvectively and radiatively heated. A circumferential baof phosphor and, separately, a spot of phosphor on the ccal tip of the rotor’s axis were illuminated with UV light, anthe shift in wavelength~i.e., the change in color! of the phos-phor patterns was observed while the shaft was spinnMeasurements of the rotor’s surface temperature overrange from 20 to 200 °C could be made with uncertaintieslarger than those obtained in similar measurements on ssurfaces, viz.,'2% at 200 °C.

D. Transient thermometry

Because of the very low ‘‘thermal mass’’ or heat capaity of a thin film of phosphor, and the fact that thtemperature-dependent processes occur over periodserned by the atomic vibrations in the host lattice, i.,1 ns, phosphor thermometry can measure rapidly chan



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temperatures. As discussed above, Turleyet al.76 used apulsed CO2 laser to produce temperature rises that wprobably in excess of 100 °C/ms.

A different approach was taken by Tobinet al.133 Intheir case, a nichrome wire coated with YVO4:Eu was con-tinuously illuminated with an ultraviolet lamp. A fast-actinmechanical switch connected the wire to a capacitorachieve a temperature rise requiring only a few milliseconto equilibrate. By following the ratio of two of the emissiolines having different temperature responses, heating rateabout 100 °C/ms were tracked and determined.

E. Surface and volumetric thermography

As conceived originally, phosphor thermography wastended foremost to be a means of depicting two-dimensiotemperature patterns on surfaces. In fact, over its first thdecades of existence, the predominant use of the technwas in imaging applications in aerodynamics. The methwas termed, ‘‘contact thermometry’’ since the phosphor win contact with the surface to be monitored.

An alternate approach suggested by Urbach called ‘‘pjection thermography’’ involved the optical transfer and prjection of a thermal image onto a phosphor screen for ssequent analysis.11,12 Figure 22 shows the calibration curveobtained by Byler and Hays16 of the four phosphors derivedfrom Urbach’s work as being useful for this purpose. Thwere applied to the surface of interest by either spraythem on or by painting them while in a phosphor/acetmixture. At least one manufacturer prepared and marketetape consisting of a phosphor backing on a thermoplaresin that provided adhesion.16 Generally, the data acquisition system was simply a photographic camera. Usinglaroid™ ASA 3000 film, Dixon and Czysz21–23at the AEDCnoted that, under their conditions of illumination, the phophors emit 1026–1025 W/cm2 before the onset of quenching. Quantitative analysis was performed by point-to-podensitometry measurements of the transmission of the ptographic film. They resolved temperature differences

Remote thermometry


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'0.2 °C. It is important to note that these applications wintensity- rather than decay-rate-based approaches. Thiseral technique has been and continues to be a standardused in aerodynamic heating studies in wind tunnels. Toverall approach, however, has largely been overshadoby the introduction of modern infrared thermal imaging tecniques, several of which have evolved into commercial pructs that are used in a wide range of industrial and scienapplications.

Buck developed a phosphor imaging system for transaerodynamic measurements made in a hypersonic windnel at NASA’s Langley Research Center.134–136His approachused a three-color video camera to view a fluorescing phphor mixture. The ratio of the intensity of the blue part of temission band to that of the green provided the signainterest, as it was a function of temperature for the Rad2090 and 3003 phosphors that were used. The techncalled for the collection of several dozen images during as transient heating test, a task made possible with compuaided data acquisition.

Bizzak and Chyu137,138noted that the more conventionthermometry methods are not satisfactory for temperaand heat transfer measurements that must be made inrapidly fluctuating conditions peculiar to the microscale evironment. They suggested that thermal equilibrium onatomic level might be achieved within 30 ns, and, therefoto be useful in performing microscale thermometry thestrumentation system must have a very rapid response tMoreover, its spatial resolution should approach the sizean individual phosphor particle, typically just a fraction ofmicron. In their effort to develop a phosphor imaging systbased on La2O2S:Eu, they used a frequency-tripled Nd:YAlaser. The image was split and the individual beams wdirected along equal-length paths to an intensified CCDtector, a technique similar to that of Gosset al.139 The gateduration was 40ms and the ratio of the5D2 to 5D0 lineintensities yielded the temperature. Significantly, they wable to determine how the measurement accuracy varied

FIG. 22. Some phosphors used for thermometry in the 1960s. RanZnCdS:Ag:Ni; range II: ZnCdS:Ag:Ni; range III: ZnS:Cu:Ni; and range IVZnS:Cu~from Ref. 16, reprinted with permission of the American Sociefor Nondestructive Testing!.



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measurement area. The maximum error found for a surfthe size of which was represented by a 131 pixel in theirvideo system was 1.37 °C, with an average error of o0.09 °C.

Taliaferro et al.140 used an image-intensified Cohmodel 012A03 CCD camera to monitor the fluorescencetensity profiles of La2O2S:Eu- and Y2O2S:Eu-coated surfaceover the range from room temperature to 200 °C. The extation source was an ultraviolet lamp, having peak emissat 254 nm and producing an irradiation intensity9 mW/cm2 at a distance of 2.5 cm. Narrow band pass optifilters blocked undesired wavelengths from the camera.images were stored and processed by computer, withtemperature-dependent surface brightnesses subsequtranslated into false-color thermograms for ease of visualterpretation. The precision of the imaging system was sthat thermal gradients of 0.02 °C/pixel~at nominal tempera-tures of 35 °C! could be resolved over the surface. Noet al.141 report the results of related experimental studiestwo-dimensional phosphor thermography.

It has often been suggested that combustion flames cbe diagnosed by seeding them volumetrically with phospparticles. In fact, the flames themselves can in somestances serve to excite the luminescence. Sweet and Wh142

describe phosphor excitation in a hydrogen diffusion flaand note that this method for producing optical emissioreceived considerable attention in the early part of the 2century. Their experiment involved coating a surface wthe phosphor to be tested, and then placing it in the flaThe particular materials under study were Y2O3, La2O3,Gd2O3, and Lu2O3 doped with either Tb or Eu. A conclusioof their work was that the excitation mechanism involvcollisions with excited neutral OH molecules.

By the 1980s, the performance of video cameras, smented detectors, CCD array detectors, and laboratory cputers provided greater data acquisition power and expmental design flexibility, leading to several nedevelopments. A particularly clever conception by Goet al.143–145 at System Development Laboratories involvthe visualization of the condensed-phase combustion of srocket propellant. They impregnated the fuel under test wYAG:Dy, and used the ratio of anF-level band at 496 nm toaG-level band at 467 nm as the signal of interest. Becausthermalization, the intensity of the 467 nm band increasedcomparison with the 496 nm band over the range from abient to the highest temperature they were able to att1673 K. At that temperature, the blackbody emission intduced a significant background component into the sigeven within the narrow pass band of the spectrometerwas employed. For this reason, they used aQ-switchedNd:YAG laser~frequency tripled to 355 nm!. The detectorsin this arrangement included an intensified~1024-element!diode array which was gated on for a period of 10ms. Theyalso used an intensified CCD detector,139perhaps the first useof such a device for phosphor thermometry. To simulcombustion in the laboratory, the phosphor was mixed witlow melting point~400 K! plastic and the resulting blend waheated with a focused CO2 laser which ignited a flame thaeroded the surface. A time history of the disintegrating pl

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F. Optical fiber fluorescence thermometry

It has been a general goal of this review to concentron noncontact methods of phosphor thermometry. Howethere have been instances above~and some to follow below!where it has been useful and appropriate to incorporate sdiscussion of the various phosphor-tipped fiberoptic teperature sensors and the physical principles underlying thIt is outside the scope of this review, though, to provideextensive survey of such devices, especially since othhave done so already.1 Therefore, the additional commentain this subsection will be limited to a brief overview of thgeneral class of instrumentation.

Grattan, Palmer, and colleagues in the U.K. haveported the results of studies on several different kindsdecay-time-based fiberoptic thermometers. Some of thepers in their long series of articles describe sensors in whthe tip of the fiberoptic probe consists of a fluorescing ement made of either ruby,19,146 alexandrite,147

YAG:Nd,148,149or YAG:Cr31,130,150among other materialsThese devices have been used with visible151 and infrared152

excitation, with fluorescence reference channels,153,154and innarrow low-temperature bands130 and higher wide-temperature bands.155 A review of early developments in thfield was published by Grattan156 in 1987, and the recenbook by Grattan and Zhang1 provides a very thorough anup-to-date discussion of the field as a whole.

The background research underpinning what was tocome another well-known effort within the field of fiberoptthermometry was begun by Wickersheim, Buchanan,colleagues in the 1960s. They studied the luminescencseveral rare-earth-doped oxides aoxysulfides,24–28,87,157–161many of which would prove to beuseful in phosphor thermometry. Subsequently, LuxtCorp.162 developed techniques for making robust fiberopprobes with thermal phosphor tips. Wickersheim, Sun, aothers13,14,163–166have described the general operating prciples of the thermometric instrumentation systems thatcorporate these transducers. Many applications have bfound for this kind of system, including various uses in clincal medicine~Sec. V A! and in the maintenance of electricmachinery~Sec. V B!. A variation of the prototypical Lux-tron design allows the phosphor target to be mountedmotely on the surface to be measured, rather than fixedrectly onto the tip of the optical fiber.14,165,166When used inthis manner, noncontact determinations of the temperatura moving or otherwise difficult-to-access surface canmade in largely the same fashion as would be done with‘‘generic’’ system described earlier~Sec. IV A!. A series oftechnical notes167–170outlines some probe configurations aexperimental arrangements useful in making nonconmeasurements.

There may be circumstances where fluorescence tmometers based on visible-light excitation may have soadvantages over competing methods in terms of both eas



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design and practicality of the source. Bosselmannet al.18 andFernicola and Crovini171–172 have investigated this usinYAG:Cr crystals on fibers illuminated with a laser diodemitting in the red. In both cases, the excitation wavelenwas 636 nm, and the emission was from the well-knownRlines which in this host are at 687 nm. There were sligvariations in their four samples, but the lifetimes we'2 ms at ambient temperature, and decreased by abouorder of magnitude at 300 °C. Their common interest in ttechnique stemmed from various automotive applications

One final example of fiber-based thermometry is twork of Mauriceet al.173 In their technique, the ratio of in-tensities of the4S3/2→4I 15/2 and

2H11/2→4I 15/2 transitions ismonitored in a segment of Er-doped fiber pumped byargon ion laser beam passed through a dichroic mirror. Tfound that the intensity ratio varied by about a factor ofover the range from 300 to 1000 K, thus making this a ptentially useful method around which to design thermome

V. APPLICATIONS

A. Biophysical and biomedical uses

Many workers over the past several decades have intigated the properties of organic materials via fluorimetryvarious kinds, and the general technique is now a standtool in science and engineering. A few of the specific effowithin this broad field that have focused on improving mesurements of fluorescence decay lifetimes include the usmode-locked lasers to study extremely fast fluorescencecesses~Wild et al.174!, the development of cross-correlatiophase and modulation fluorimetry~Alcala et al.175,176!, andthe use of digital electronics to synthesize parallel phafluorimetry systems~Feddersenet al.;177 see also the discussion of limitations of this technique given by Alcala178!. Allof this work has been ongoing while noncontact phospthermometry has been under development, with the croover region being the incorporation of phosphors into optifibers to make fluorescence-based sensors. Some backgron the general features of this class of sensor is proviabove, and a thorough discussion is available elsewhere1

The resulting instrumentation has been very usefulmonitoring physiological variables. For instance, Alcala, Yand Yeh179 describe a digital phosphorimeter witfrequency-domain data processing that is able to extractvalues of the partial pressure of oxygen in blood. The devhas a drift of less than 1% per 100 h of use. At aboutsame time, Alcala, Liao, and Zheng180 designed and built aphosphorescence lifetime thermometer that has alexancrystals bonded to the tip of an optical fiber. It operates othe range from 15 to 45 °C, has an accuracy of better t0.2 °C, and measures the lifetime of the alexandrite’s He–stimulated phosphorescence with a reproducibility of 0.3

The commercially available FLUOROPTIC™ thermometers of Luxtron Corp.~see Sec. IV F! have found many usein the biomedical arena since their introduction. One partilarly significant medical application of these instruments hbeen in clinical hyperthermia.181,182The small outer diameteof the optical fiber (,1 mm) allows for insertion of theprobe into standard catheters, and the established prec

Remote thermometry


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of measurement (,0.1 °C) is obtainable in the presencelarge electromagnetic fields.183 This makes the instrumentcompatible with rf induction and other hyperthermia sytems, with minimal perturbation of the electromagnefields.184 ~In fact, others185 have also taken advantage of thnonperturbative nature of thermoluminescent phosphorsmaking dosimeters for microwave fields.! A Luxtron 2000Bthermometer was used in the firstin vivo studies of dynamicboundary hyperthermia,186 in which the uncertainty in theboundary of thermally driven cell death in porcine brain wfound to be controllable to within,0.6 mm. Samulski haswritten a succinct, useful review187 that includes discussionof the role of photoluminescent thermometry in clinical abiomedical research.

Ikeda, Sun, and Phillips188,189designed a blood flow-ratesensor that incorporated a FLUOROPTIC™ thermomeThis device inferred the flow rate from direct measuremof temperature change due to heat transfer through thesor, in analogy to hot-wire anemometry. A variety of othfiber-optic-based blood flow sensors has been describethe literature~see Tjinet al.190 for a review!, but they aretypically based on Doppler anemometry rather than on htransfer.

Shrum and colleagues191 have devised a molecular fluorescence thermometry technique that derives its signal fthe shift in wavelength with temperature of the fluid in whian organic compound~BTBP! is dissolved. The sensitivity othe method is'0.05 nm/°C, and they used it to measutemperature changes in both associated and nonpolar flto within 62 °C over the range from 15 to 70 °C. This tecnique can be thought of as a liquid-based equivalent of sostate phosphor thermometry: low concentrations of BTBP~aslittle as 1027 M! can seed the host fluid and produce deteable temperature-sensitive fluorescence in somewhatsame way as small concentrations of the rare-earth dofunction in thermographic phosphors.

Finally, within the realm of biomedical applications otemperature-dependent phosphors, Shearer and Kimb192

have noted that the temperature dependence of the phosscreens used in medical x-ray fluoroscopy systems can athe quality of the images. They examined several typesscreens that were illuminated by typical levels of x-ray flto quantify the effect on overall emission efficiency.BaPbSO4 sample at 50 °C exhibited a 60% decrease in toefficiency relative to a sample held at 0 °C. A similar reswas found by Morgan193 for calcium tungstate and by Trouand Kelly,194 who studied zinc cadmium sulfide. Rare-eascreens made from LaBrO:Tb and La2O2S/Gd2O2S:Tb ex-hibited efficiency changes of,5% over this range. We nothere that a greater temperature dependence would bepected for some emission lines of the terbium-doped oxyfides, particularly at low terbium-dopant concentratioBuchanan and colleagues195 have also investigated rareearth-doped phosphors for use in x-ray transduction.

B. Electrical machinery

The immunity of optical signals to electromagnetic iterference makes phosphor thermometry very useful in d



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nostic studies of electrical machinery. For instance, Manand colleagues at Ontario Hydro have developed a phospbased technique for monitoring the temperature of the roin a large turbogenerator.86,196,197The motivation for doingthis is to seek out any ‘‘hot spots’’ that might develop in throtor of the generator as a result of several possible cauincluding overheated insulation. They used a pulsed nitrolaser as the optical source, and conveyed its light alongall-silica optical fiber having a core diameter of 600mm.~Silica fibers are generally preferred over plastic-clad fibbecause of their higher pulsed-power damage thresholdtheir superior transmission characteristics in the ultraviol!A plastic-clad fiber collected the fluorescence and conveit to a photomultiplier tube. They used a microprocessbased, custom-designed data analysis system196 to determinethe fluorescence decay times. A silicone binder, mixed 5by weight with phosphor, was painted in a 5-cm-wide striaround the circumference of the rotor, with a gap in the strthat functioned as a fiducial line. As noted above, their cbration and characterization studies established that extemagnetic fields did not affect the phosphor lifetimes. Tpresence of a thin film of oil arising from the lubricants usin this kind of machinery was found to marginally degrathe signal, but measurements were still possible. In fitests197,198involving a 540 MW generator, they used the 51nm lines of the phosphors Y2O2S:Eu ~1%!, Gd2O2S:Eu~0.5%!, and La2O2S:Eu ~1%!. While the first two materialsexhibited a strong signal and a marked temperature dedence from 60 to 150 °C, they were also subject to satution. The last material did not saturate, however, and msurements were made with it up to temperatures of betw70 and 80 °C. Above that level, the signals were weak athe associated lifetimes were very short, i.e.,,1ms. Thesurface velocity of the rotor was'207 m/s. The spatial resolution of the measurements was 4 mm, and their accurwas62 °C.

Hot spots also develop in electrical transformers, aFLUOROPTIC™ thermometers have been used to determthe temperatures of the windings,199,200the leads,201 and thetransformer tank.202 Other applications that call for the sensor to function in the presence of large electromagnetic fieinclude fiber optic thermometry-based measurements ofcrowave power183,203,204and rf susceptibility.205

C. Gas centrifuges

During the 1980s, workers from several institutions fibegan using phosphor thermometry for making nonconmeasurements on the gas centrifuges being developed foU.S. Department of Energy’s uranium enrichment prograThese devices are large right circular cylinders driven at vhigh speeds. Their function is to separate U235 from the natu-ral isotopic distribution in UF6 gas to create enriched fuel fonuclear power reactors. The flow pattern inside the machand, hence, the separative efficiency of the centrifugeaffected by thermal variations on the rotor surface. The gof maximizing separative work performance provided timpetus for developing a remote thermometry system copatible with a high speed mechanism of this type.



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FIG. 23. Schematic diagram of the phosphor thermography system developed for measuring the wall temperature at points around the periphery ofgas centrifuge.
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Making the measurement by taking the ratio of linetensities was the first approach considered, but the hspeed of the centrifuge surface complicated the calibratiothe phosphor. In addition, mechanical concerns dictatespacing of several centimeters between the tips of the opfibers used for delivery/collection of the light, and the mesurement surface. This latter point meant that beam spwould be non-negligible, and implied that a bright sourceillumination would be needed to counter the falloff of intesity with distance. A portable nitrogen laser was incorporainto the experimental arrangement and emission lifetimeasurements were made on La2O2S:Eu. This phosphor hadalready been studied by others, and it exhibited thermsensitive emission lines in the temperature range of interBenchtop tests on low speed rotors demonstrated feasibby establishing the technique’s precision (1/3 °C) and cfirming the utility of the fiberoptic, timing, and signasynchronization206,207components of the apparatus.

Figure 23 presents a schematic diagram of the expmental arrangement developed for use with a full-sizedcentrifuge. Circumferential strips of phosphor were painon the rotor at various locations along its axis. The pujitter in the trigger circuit for the laser was in the worst caa few hundred nanoseconds, enabling excellent spatial rlution of the temperature measurements made on the pphor bands.

D. Gas turbines

The useful results obtained by applying phosphor thmometry to high speed gas centrifuges led to efforts aimemeasuring the temperatures of surfaces and structures inturbine engines, particularly those of the rotating bladesstationary vanes. The history of that work is documentedseries of papers by Noelet al.208–215 and colleagues,216

which includes a review of the results obtained throu



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1993.217 Several other groups have also independentlyplied phosphor thermometry to turbomachinery, and in wfollows we provide a synopsis of the work that has bedone in this field.

One of the original intents of the ORNL phosphor themometry program was to provide a means of sensingtemperatures of objects within the combustion-flame enronment. The first experimental tests of the technique wcarried out inside a high-altitude simulation wind-tunnelthe AEDC.218 In these tests, the exhaust from the aft end oPratt & Whitney~PW! F100 engine impinged on a variablearea extractor~VAE!, a cone-shaped centerbody that aidspressure recovery and the pumping process. Measuremwere made of the VAE surface temperature, both with awithout an afterburner plume intervening between the sface and detection optics. The VAE was water coolthough, so its temperature was consequently rather low, tcally 150 °C. The laser-induced fluorescence signals frthe phosphor layer bonded on the VAE surface were succfully detected despite the optical background created byplume. This is shown in Figure 24 which is a print mafrom a 16 mm film of the afterburner and VAE. The whispot is the phosphor luminescence. The resulting data mit possible to track the temperature change of the VAE ding the course of afterburner ignition transients.

Subsequent engine experiments were performed aburner-rig installation, i.e., a test cell capable of exposturbine blades to an engine-simulating environment. Tusual arrangement involves mounting several blades omechanical carousel which is then rotated at relatively lspeeds through a jet flame. Phosphor thermometry testTobin et al.219 in one such installation demonstrated tworkability of the technique up to 1100 °C. Other studihave been carried out in spin pits, where the higher rotatiospeeds and the intermediate-range temperatures encoun

Remote thermometry


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in operating turbine engines can be simulated, albeit withthe exhaust flame present.

A number ofin situ experiments have also been carriout, including measurements made on first-stage stator vinside a PW 2037 engine,220 on the vanes of a turbine disrunning near full speed~14 600 rpm! in an advanced turbineengine gas generator~ATEGG! system,214 on the compo-nents of a PW joint technology developmental eng~JTDE!,217 and inside an experimental turbine engine at Vginia Polytechnic Institute and State University.220 Collec-tively, these tests have demonstrated that phosphors basmaterials such as yttrium oxide, yttrium vanadate, andtrium aluminum garnet~YAG! can survive for extended periods, even when bonded in place in the first stage of abine where the blades are adjacent to the burner. Usoptical signals were obtained at temperatures well in excof 1000 °C, and separate calibration studies221,222 showedthat the fluorescence decay parameters of some of the pphors were still readily measurable at 1200 °C. The preferdesign of the optical probe consisted of a two-fiber confiration with internal optical elements coated so as to minimstray reflections, laser-induced damage to the input fiber,the optical background due to blackbody radiation. On alated note, studies of the heat transfer through combusengine components have also been undertaken with thegraphic phosphors.223

Several others have also explored turbine engineother aerospace applications of phosphor thermometry.instance, Alaruri and colleagues103,224 exposed samples oY2O3:Eu to the combustion flows in a burner rig. The gvelocities in the rig ranged from 245 to 407 m/s at roughatmospheric pressure. Data were taken over the rang400–1000 °C, with the overall accuracy of the measuremeestimated to be63%. They noted that the phosphor’s flurescence was easily distinguishable from the luminescedue to the fuel pulses~JP-5 aviation fuel was used!.

Phillips and Tilstra225 describe a ruggedized, thermographic phosphor temperature sensor based on the detime approach and designed using a 200mm optical fiber toconvey the fluorescence signals. It was tested over the rafrom 275 to 350 °C, at altitude simulations of up to 21.3 k

FIG. 24. Afterburner flame impinging on the variable-area extractor. Twhite spot is phosphor luminescence viewed through the flame.



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Simonset al.226 describe calibrations of additional phophor materials of use for moderately high temperature msurements.

VI. MEASUREMENT OF OTHER QUANTITIES

A. Heat flux

The determination of the heat flux through a surfaceimportant in a variety of scientific and engineering applictions. Noelet al.88,227 and Turleyet al.228 pursued develop-ment of several embodiments of thermal phosphor-baheat flux gauges. A standard expression relates the heatq, to the insulator thickness,d, thermal conductivity,K, andtemperature difference,DT, across an insulating barrier:

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FIG. 25. Photograph of the fluorescing heat-flux gauge.

FIG. 26. Photograph of the heat-flux range gauge attached to a turblade.



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As originally conceived, a phosphor-based heat flux gawould consist of a sandwich of two different types of phophors separated by a UV-transparent insulator. The firstalization of such a device incorporated Gd2O2S:Tb, in whichthe ratio of theD3 state at 415 nm to theD4 state at 490 nmserved to provide the temperature of the top surface.bottom surface was made of La2O2S:Eu, and its temperaturwas derived from the ratio of the emission of theD2 band at514 nm to that of theD0 band at 614 nm. A demonstratioarrangement utilized a hot-air stream to apply heat tosuch gauge bonded onto a water-cooled aluminum contaLevels of heat flux up to 40 kW/m2 could be measured in thiway.

A second realization of this method consisted of an arof discrete circular and triangular spots of phosphor. Tmade it possible to obtain a two-dimensional determinatof heat flux through the surface of interest. Also, only ophosphor is required when, for instance, one-half of a secircle of it is deposited on the top of the insulator with tmatching half bonded onto the bottom. Figure 25 isclose-up photograph of a fluorescing heat-flux gauge wmatching triangular phosphor layers. Figure 26 depictsgauge attached to a turbine blade. Figure 27 shows theperimental arrangement for this type of heat-flux gauge.

Baumann33 has also made heat-flux measurements usthermographic phosphors. Wind tunnel models made of thdifferent materials~steel, Macor™ glass ceramic, and Nocoat™ 4000 silicone elastomer!, all of which had thin coat-ings of Y2O2S:Eu ~0.15%! applied, were monitored duringblow-down tests in a hypersonic flow facility. The respontime of the measurement system was typically less thanms, and the values of the heat flux measurements obtawith his technique compared well with the standard spgauge methods.

FIG. 27. Laboratory heat-flux gauge system.



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As a practical point, we note that the oxysulfide-basphosphors begin to degrade chemically in air at ab450 °C. However, other phosphors may be used for thisplication when higher temperatures will be involved. Aother limitation encountered in developing this conceptuse at high temperatures is the difficulty in finding a suitainsulator. To circumvent this, Noel has developed a theand a resulting design that eliminates the need for an invening insulator in the gauge.88 As a related matter, we notthat a detailed calculation of the heat flow through a thin fisubjected to laser heating has been carried out by Abraand Halley.229

B. Pressure and strain

Pressure will generally affect a material’s luminescenand spectral absorption properties. However, the naturethis effect is not as well studied or understood as areeffects of temperature on luminescence. The applicationpressure may be viewed as the imposition of compresstrain from all directions. Hence, knowledge of a materiapressure dependence also lends insight into its strain dedence. On the atomic level, strain results in a changechemical bond lengths and atomic orbital configuratioThis perturbs the electric and magnetic fields seen byactivator and, hence, the spectral energy level manifoThese environmental changes modify the fluorescence perties, but the precise mechanism will vary from one phphor to another. For the case of La2O2S:Eu, the applicationof pressure~compressive strain! has the effect of increasingthe energy of the CTS. The CTS parabola as seen in Fiwould be placed higher and, consequently, the value ofdifference between the excited electronic state and the lwhere the CTS intersects the ground state would becreased. In this case, assuming a constant ambient temture, the increased pressure has an effect similar to thacooling the phosphor, i.e., certain of the lines, e.g., the5D2

lines for La2O2S:Eu, get brighter and the decay times glonger. Tensile strain will have the reverse effect for tsame emission line of this phosphor.

Table IV presents a survey of the existing data onpressure dependence of various phosphor, glass, and cryline materials. The effect of pressure on fluorescence delifetime ~at constant temperature! is more pronounced in theoxysulfides than it is in other materials. Webster aDrickamer230,231 have a theory that appears to explain tpressure dependence of the luminescence of these oxfides; it involves the CTS discussed earlier. Most authothough, have been more concerned with the effects of psure on line shifts. For example, Haugen232 discusses the usof line shifts of ruby for making pressure measurementsfact these line shifts are often used for pressure calibratiodiamond anvil cells. Also suggested are the trivalent ioCe13, Pr13, Eu13, and Er13, possibly in fluoride hosts sincethey have transitions that can be transmitted by optical fibThe pressure dependence of the emission intensities,widths, and peak locations for ZnS:Eu12 and Y2O2S:Eu

13 isgiven by Zhizhonget al.233 For the last, the emission frequency changed by about 30 cm21 in going from 0 to 4 GPa,

Remote thermometry


TABLE IV. Survey of the effects of high pressure on the luminescence characteristics of several fluorescent materials.

Material ReferencePressurerange

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SensitivityTemperature

effects CommentsDl/DP DI /DP Dt/DP

Gd2O2S:Tb 243, 244 5 GPa 5D3 ••• 100% decrease t30.1 over2 GPa

••• MaximumDt/DP

5D4 2 cm21/GPa 25% decrease0–5 GPa

,5% change0–5 GPa

••• •••

La2O2S:Eu31 231, 243, 244 12 GPa 5D2 5 cm21/GPa 310 in

3.5 GPat310 in3.5 GPa

See the text Shift of CTSimportant

5D1 6 cm21/GPa Mild decrease No change5D0 4 cm21/GPa Mild decrease No change

La2O2S:Tb 244 5 GPa 5D4 3 cm21/GPa 100% increasein 5 GPa

No change ••• 5D3 lines weakin this host

YVO4:Eu 244 5 GPa 5D0 3 cm21/GPa 10% decreasein 5 GPa

t30.9in 5 GPa

••• •••

5D1 7 cm21/GPa 50% decreasein 5 GPa

t30.67in 5 GPa

••• •••

5D2 9 cm21/GPa No change No change ••• •••

YVO4:Dy 243, 244 5 GPa 4F9/2–6H13/2 7 cm21/GPa 6% decrease

0–5 GPat30.9in 5 GPa

See the text •••

4F9/2–6H15/2 10 cm21/GPa 45% decrease

0–5 GPaMild decrease •••

Y2O2S:Eu31 233 7 GPa 5D0 and

5D1 5 cm21/GPa 20.24/GPa ••• ••• •••

Gd2O2S:Tb 243, 244 5 GPa 5D3 ••• 100% decrease0–5 GPa

t30.1in 2 GPa

••• MaximumDt/DP

5D4 2 cm21/GPa 25% decrease0–5 GPa

,5% change0–5 GPa

••• •••

CaWO4:Nd31 237 7 GPa 4I 11/2 0.727 nm/GPa

at 290 K(7 cm21/GPa)

••• ••• dl/dT5

2431024

nm/K

4.5 timessofter thanAl2O3; smallcrystals used

YAG:Eu31 240 7 GPa 591 nm 0.197 nm/GPa(6 cm21/GPa)

••• ••• to 973 K,2531024

nm/K

Expressionderived forDl5 f (P,T)

LaOCl:Eu31 238 14 GPa Many 1–10 cm21/GPa ••• ••• ••• Ar ion laserexcitation

YAG:Tb31 228 8 GPa 5D4→7FJ 1–10 cm21/GPa ••• ••• ••• Ar ion laserexcitation

Al2O3:Cr 234 10 GPa R1 0.366 nm/GPa(8 cm21/GPa)

••• ••• ••• •••

Al2O3:Cr 235 1 GPa R1 ••• 30% increase0–0.8 GPa

3.4 ms at0.8 GPa

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while the intensity decreased tenfold over the same ranTheir results complement those obtained by Webster.230

More recent work on ruby has been done by GuptaShen,234 who include a review of the literature in their aticle. They found the pressure sensitivity of ruby toslightly less than 10 cm21/GPa. Liu et al.235 measured theeffects of tensile stress on ruby emission, finding a shift2 cm21/GPa, i.e., roughly a third of the shift due to pressuVos and Schouten236 have investigated the temperature dpendence of the pressure response of the ruby absorlinesR1 andR2 . Beales and Goodman,237 meanwhile, haveexplored the temperature and pressure dependence of linthe spectrum of CaWO4:Nd, finding dl/dT524



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31024 nm/K and a pressure sensitivity of 7 cm21/GPa foremission at 1.06 nm. They note that the results are similarother rare-earth-doped crystals, and that they chose thisterial because it is not as hard as sapphire and, thereshould exhibit a larger pressure dependence. Chicolleagues238 found line shifts of up to 10 cm21/GPa forLaOCl:Eu13 and similar results239 for YAG:Tb. Arashi andIshigane240 tested the pressure dependence of YAG:Eu31

finding that it exhibited a sensitivity of 6 cm21/GPa in its591 nm fluorescence, a value only slightly less thansensitivity of ruby. Doped alkali halides show a pronouncchange in absorption position and shape, especially



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31, YAlO3:Eu31, Gd2O3:Eu

31, YbAG:Eu31, andGdAG:Eu31.! Drotning and Drickamer241 considered Tl1

and In1 in particular. In contrast to the rare-earth phosphothe spectral properties of these materials are rather brSealset al.242 have also studied the pressure and temperasensitivities of certain inorganic phosphors.

Allison et al.,243 in conjunction with Gleason,244 used adiamond anvil cell to study the possibility of employing thpressure sensitivity of thermographic phosphors asmechanism that would underlie a new class of strain gauIn particular, the pressure dependence of the fluoresceproperties of YVO4:Dy, Y2O3:Dy, and Gd2O2S:Tb were in-vestigated. For YVO4:Dy, the fluorescence lifetime changelittle with either pressure or temperature below 300 °However the relative intensities of narrow emission lines tcomprise the emission band centered at 575 nm do chawith temperature. The ratio of emission at 575.3 nm to tha572.3 nm is constant up to a threshold value of presswhere there is an increase to a new level. The value ofthreshold varies from 4.5 GPa at 20 °C to 0.3 GPa at 150

Measurements of the pressure dependence of the lifeof Gd2O2S:Tb were made and compared with La2O2S:Eu.These two phosphors exhibit the strongest pressure dedence of all the materials surveyed. The former decreasedecay time by an order of magnitude with an applicationonly 2 GPa, while the europium phosphor increases byorder of magnitude with an application of 3.5 GPa. Alsotapered sapphire rod was used to apply force toLa2O2S:Eu sample. As expected, the set of lines betweenand 515 nm increased in intensity and the emission line537 and 538 nm decreased. The applied pressure wasmated to be between 0.5 and 1 GPa. The response omaterials clearly demonstrates that these oxysulfidesgood candidates for further study of strain-induced effewith potential for practical measurement of strain in indutrial situations.~A survey of some possible applicationsavailable elsewhere.245! There may be other molecular spcies that have even greater pressure sensitivities.

Work that is closely related to the pressure- and strasensing studies described above includes that of Prinscolleagues.246,247They employ thermographic phosphors aa digital imaging technique to make simultaneous measments of surface temperatures and strain fields. The tempture measurements are derived from the fluorescence insity of the phosphor. The strain, meanwhile, is measuredgeometric interference: a set of lines etched onto the phphor is imaged under different strain conditions, and theages are superimposed to reveal a strain-dependent m´pattern. Comparisons with strain gauges reveal an untainty of 60.05% over a 1.8% strain dynamic range.~Sim-pson and Welch248 also developed a noncontact, optoeletronic method for strain measurement based on a diffeprinciple of operation. In it, the shape changes of a mulobed geometric form on the surface of a rotating target wmonitored and analyzed, with the results yielding the paraeters of the strain field.! Gallery et al.249 have usedtemperature- and pressure-sensitive paints based



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rhodamine B and platinum octaethylporphyrin, respectiveto characterize the behavior of an airfoil model in a witunnel operated at Mach numbers of up to 0.56. Their iming system used a Pulnix model TM-745 CCD camerarecord the luminescence signals. This method of presssensing for aerodynamic applications via fluorescent matals has grown rapidly the past five years. Finally, we nthat the characteristics of optical fibers change as a funcof applied pressure. For example, tensile stresses in the rof 0.05–4 GPa will significantly change the transmission325 nm for a 125-mm-diam fiber of'20 m length. Further-more, for this excitation, the ratio of emission bands at 4and 620 nm changes.250 ~Pure silica optical fibers havemechanical strength of about 5 GPa at room temperaturepractice, as compared with a theoretically predicted value22 GPa.! Allison et al.251 have investigated various expermental arrangements aimed at exploiting this effect indesign of optomechanical sensors, while Egalon aRogowski252 have studied how the modal patterns in a fibvary with axial strain.

C. Flow tracing

Nakatani et al.253–255 developed techniques for usinphosphor particles as tracers to aid in visualizing the flowsliquids. Both continuous ultraviolet lamps and a nitrogenser served as sources of illumination for the different cofigurations of this method. The phosphors with which thworked included various combinations of ZnS, CdS, aCaS. The last material has a lifetime of'1.5 s, making itparticularly well suited for measuring the flow of slowlmoving liquids. Streak photographs were used to trackseeded fluid, yielding distance versus time relationshipsmade it possible to resolve the flow’s velocity and directioTheir experimental results and analyses revealed the wvariety of conditions over which the phosphor particles cobe used to make high fidelity mappings of the flow field.

Keyes and Sartory256 have also used the luminescenproperties of thermographic phosphors in fluid dynamicsperiments. Their goal was to evaluate the Ludwieg modeldeterminations of local fluid drag.

D. Particle beam characteristics

Phosphors and other scintillating materials can be uto study the structure of high energy ion and electron beaindicating the position, profile, and flux density of the beaas a function of phosphor brightness. Also, if the beam crent is large enough to heat the material, then an appropmethod of phosphor-based thermometry can be used to dnose the beam. Over time, as a significant dose accumulthe phosphor’s efficiency will deteriorate due to radiatidamage. This process, which manifests itself as a decreabrightness, was first noted by Birks and Black.257,258 Theyestablished the relationship between phosphor intensityI ,and total accumulated dose,N:

I /I 051/@11~N/N1/2!#. ~16!

Here, I 0 is the original zero-dose brightness andN1/2 is thecharacteristic dose at which the brightness decreases to

Remote thermometry


es

TABLE V. Performance characteristics of several relatively recent, representative thermographic phosphor temperature measurement systems.

Author~s! Application Phosphor/method Design features Range/uncertainty Referenc

Manniket al.

Noncontact thermometry of540 MW electrical generatorrotor surfaces

Gd2O2S:Eu, Y2O2S:Euand La2O2S:Eu,decay lifetime

Ambient magnetic field of thegenerator was'1 T; measurementhad spatial resolution of 4 mm

41–73 °CDT562.0 °C

1987; 86, 196,197, 198

Dowell Design and evaluation ofa thermal phosphor-basedtemperature standard

Y2O3:Eu~single crystals!,decay lifetime

Fluorescence lifetimes weremeasured relative to acesium-beam atomic clock

'30–800 °CDT56.4 °C~at '800 °C!

1989; 98

Noelet al.

Remote measurement of vaneand blade temperatures inan operating turbine engine

YVO4:Eu, Y2O3:Euand YAG:Tb,decay lifetime

Phosphor layers from 4 to 35mmthick applied by rf sputtering,e-beam deposition, chemical binder

450–1300 °Crelative error'62%

1991; 212–214

Draina andAnderson

Temperature control of ICwafers during plasma etchand chemical vapordeposition processes

Luxtron MIH probesand custom-designedSEMATECH probe

Measurements made in vacuum andat low pressures relative to atype-K thermocouple or a PRT

25–400 °CDT'610 °C

1992; 262

Alaruriet al.

Monitoring temperaturesof turbine engine partsin a burner rig

Y2O3:Eu ~4.52%!and YAG:Tb ~5%!,decay lifetime

Air-cooled fiberopticprobe with sapphire lenswas developed; itsdamage threshold was 3.5 J/cm2

27–1095 °Caccuracy quoted as63% for Y2O3:Eu

1993; 103, 244

Chyu andBizzak

2D surface temperaturemeasurements/general heattransfer studies

La2O2S:Eu13,

intensity ratio of512–620 nm lines

80 mJ excitation energy suppliedby laser beam of 6.4-mm-diamduring 8 ns pulses

18–60 °CDT560.5 °C

1994; 137, 263

Luxtron Corp. Commercially availableapparatus used in a widevariety of measurements

MFG2 phosphor-tippedfiberoptic probes,decay lifetime

Excitation and fluorescence returnpulses transmitted over the sameoptical fiber~model 790 systems!

2200–450 °CDT560.1 °C rms, atcalibration point

1993; 166

Anghelet al.

Development of probesuseful in low temperatureindustrial applications

Ruby crystal withfiberoptic couplings,decay lifetime

Green LED used to excite sensor,overall size of probe 4310 mm,calibrated with type-K thermocouple

'270–30 °CDT560.3 °C

1995; 129

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half of its original level. Hence,N1/2 is an indicator of aphosphor’s susceptibility to radiation damage. In recwork, Hollerman et al.259,260 studied the proton-beaminduced damage of several common phosphor materialsand 60 MeV, and found that the dose associated with ohalf decay ranged from (1–20)31015 protons/cm2. One ofthe interesting results was that raising the temperature fambient to 150 °C actually increasedN1/2 somewhat. Evi-dently, the higher temperatures promoted some healingatomic-level damage sites. Such information may be of snificance in assessing the use of phosphors in nuclear reas well as particle beam applications.

Hollerman and colleagues261 have already explored thpossible use of Gd2O2S:Tb for this class of application, withthe specific goal being to develop a beam-positioning sysfor high energy accelerators. They used a 45 MeV probeam to irradiate a surface painted with this material in orto determine the change in the total integrated intensitythe fluorescence as a function of temperature. Their measments showed that the intensity dropped by a factor of 2going from 113 to 133 °C, thus indicating the potential importance of thermal effects in selecting a phosphor for usbeam positioning systems.

VII. DISCUSSION

A. Performance characteristics

Phosphor thermometry systems can be divided rouginto two broad categories: those designed for laboratoryvestigations and those designed for field applications. Wthere are many performance characteristics that are mutuimportant to both general classes of apparatus, a numbe



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significant differences also exists between them. The labtory systems are typically used to explore the performalimits of the technique under highly controlled conditionfor instance, varying only one experimental or environmenparameter at a time during calibration studies. On the othand, field-grade systems must be very robust in nature,the phosphor target in particular being able to withstandtreme levels of thermal and mechanical shock, as occwhen measurements are taken, for example, on surfacespinged by high speed, combusting flows.

Because of the diverse range of situations over whphosphor thermometry has been employed, it is instructivintercompare several recent realizations of the techniquestablish a sense of the overall performance limits of modsystems. To that end, Table V presents a summary ofcharacteristics of several different but largely representaexperimental arrangements, including laboratory-gradefield-deployed setups. From Table V, and a perusal ofdiscussions in the preceding sections, it is evident thattemperature-dependent characteristics of a wide varietyphosphors have been studied to date. These include sincrystal and powdered materials, ceramic oxides and oxyfides, and many others as well. The thermometric properof these materials have been exploited over the range froto over 1800 K. The fluorescence lifetime approach to demining the sample’s temperature is preferred to alternamethods in many types of field applications, especially thwhere large and broadband optical background signals~e.g.,blackbody radiation! are present. Of course, ratios otemperature-dependent line intensities, or simply measments of the individual line intensities as a function of teperature, also provide useful thermometric data. This is p



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ticularly the case for those phosphors used in the rang100–300 °C which, when operated below their quenchtemperature, exhibit relatively bright optical lines. The phophor target can be interrogated on the surface to whichbonded by light channeled through optical fibers, the tipswhich may be placed arbitrarily close to the surface of intest. Alternatively, a laser beam can be steered onto the taby mirrors, over paths of up to 10 m. Phosphor thermogphy is generally capable of spatial resolutions on the orde1 mm, and the phosphors typically exhibit no sensitivitytheir fluorescence parameters to ambient magnetic fieldat least 1 T.~Extremely fine spatial resolutions, 0.7mm, havebeen reported by Kolodner and colleagues,264,265who usedrare-earth chelate films to make fluorescently imaged surtemperature profiles of metal–oxide–semiconductor fieldfect transistor MOSFET circuits. They have used simicompounds to make noncontact temperature measuremof a glass resin undergoing reactive-ion etching.266!

The measurement uncertainties found in this classthermometry system range from approximately 0.1% toof the temperature reading, with some authors reporsmaller uncertainties in particular experimental arranments. While the development of an error budget is omeaningful within the context of a given apparatus, factthat generally contribute to the overall measurement einclude, for example, the chemical purity of the phosph~contaminant dopants may make the decay time multiexnential in nature!, the presence of uncharacterized nonlineaties in the transducer and signal conditioning electron~e.g., rectification errors!, and mathematical or computational peculiarities in the algorithm and processor usedreduce the data~e.g., baseline offset error and other modeliinadequacies!. In addition to these types of systematic errthere are also several sources of random error~phototubeshot noise, etc.! that introduce statistical uncertainty into thresults. Section IV B contains detailed discussions of sevof the sources of experimental error and signal-to-noise rdegradation that have to be dealt with during calibration ophosphor thermography system.

Ultimately, of course, the quantity of interest is the teperature of the phosphor target itself, which is presumaalmost identical to that of the substrate to which it is bondIn many experimental arrangements, the level of breakdoin the exactness of this presumption constitutes the limitfactor on the final accuracy with which the temperaturethe substrate can be determined. This is because the pphor coating~no matter how thin! may have enough heacapacity and thermal conductivity to alter its thermal enronment. If so, then the temperature of the substrate madifferent from that of the coating and, of importance, frowhat it would be in the absence of the coating. This is ually not a problem at ambient temperatures where the hfluxes are small and both emissivity and blackbody radiatplay only a negligible role. However, at higher temperaturand especially in applications calling for measurementsside of turbine engines, high temperature wind tunnels, eit may be necessary to make a thermal model of the msurement zone to determine whether or not the heat tran



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process will impose a limit on the ultimate accuracy of tmeasurement.

For the idealized situation shown in Fig. 28, a simpmodel was developed for use in estimating the size ofuncertainties introduced by this effect into high temperataerospace applications.267 It relies on a perturbative solutionof the steady-state heat conduction problem. The emissivand thermal conductivities for a given coating and substrmay not always be known but, as seen in Table VI, rouestimates suffice to bracket the error. The baseline case aparametric sensitivity analysis of it are depicted in Table V

B. Comparison with other techniques

One of the primary virtues of any type of noncontathermometry is the elimination of electrical leads andother interconnections between the object under studythe sensor element~s! and associated read-out instrumention. In fact, in many cases the presence of leads introduinsurmountable mechanical roadblocks, e.g., making thercouples, thermistors, noise thermometers, etc., seldomfor measuring the surface temperatures of moving objeWhile sliprings are sometimes employed as an interfacetween the sensor leads and the read-out device in suchations, they are typically limited in maximum speed of oeration. Also, the relative motions of the slipring componecan generate electrical noise that can corrupt the signal. Fa broader perspective,any measurement environment thhas a high EMI/RFI background can lead to inductive copling of electrical noise into thermocouple and resistivtemperature-device leads, whether the sensor is statimoving. This latter point is a well-known problem andgreat deal of effort has gone into the design of signal contioning circuits that have high levels of common-mode rejetion and other electronic features that work to maximizesignal-to-noise ratio. Even so, electrical pickup can placevere constraints on a variety of measurement circumstan

Robinson and Beams268 attempted to address the mesurement of temperatures on high speed rotors by desiga miniature radio telemetry system that was able to transa temperature-dependent signal. With it, they could resotemperature differences of 0.01 °C on rotors spinningspeeds of up to 300 rotations per second. However, on-bcircuitry and battery power were required, and an arraythe devices would have been needed to obtain even anproximate mapping of the surface temperature of the rot

In contrast to all of these limitations, phosphor thermoraphy is a leadless, completely noncontact technique, andphoto-optical nature of the signal makes it unsusceptible

FIG. 28. Simplified thermal model geometry.

Remote thermometry


TABLE VI. Thermal model error analysis.

ParameterBasecase

Increasedspot

diameterThickercoating

Thinnersubstrate

Increasedcoating

emissivity

Increasedsubstrateemissivity

Decreasedcoatingthermal

conductivity

Decreasedsubstratethermal

conductivityIncreased gastemperature

Coating diameter 12 mm 25 mm ••• ••• ••• ••• ••• ••• •••Coating thickness 0.15 mm ••• 1.5 mm ••• ••• ••• ••• ••• •••Substrate thickness 6 mm ••• ••• 3 mm ••• ••• ••• ••• •••Coating emissivity 0.33 ••• ••• ••• 0.9 ••• ••• ••• •••Substrate emissivity 0.5 ••• ••• ••• ••• 0.9 ••• ••• •••Coating thermalconductivity

2.76W/mK

••• ••• ••• ••• ••• 0.276 ••• •••

Substrate thermalconductivity

45W/mK

••• ••• ••• ••• ••• ••• 18 •••

Hot gas temperature 920 K ••• ••• ••• ••• ••• ••• ••• 1250 KThermal radiation source 920 K ••• ••• ••• ••• ••• ••• ••• •••

Estimated error 11% 10.8% 110% 11% 2% 20.7% 10% 0.7% 0.8%

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EMI/RFI pickup. Only a thin film of phosphor is needed othe surface under study, and no on-board circuitry is requfor interrogation. Although most phosphor thermograpsystems do use optical fibers to launch and recover thedent and fluorescence pulses in proximity to the target,fractive and reflective components have been used to sbeams along free-space paths of up to 10 m withdifficulty.218

Competing noncontact techniques include radiomeIR thermography and optical pyrometry. Very sophisticaand highly developed versions of both classes of instrumtation are commercially available, and a substantial manuturer’s literature describes the performance characteristicthe pertinent devices. The thermometric resolution preseobtained via either technique is typically 0.1 °C.

Some IR thermography systems require liquid nitrogcooling of the detector array, while others use solid-stdevices to accomplish this. They can thermally image objeat distances both near and far from the camera, withspatial resolution of the resulting thermograms governedeither case by the granularity of the detector’s pixelationsome measurement situations, it may be useful and pereven necessary to interchange with the thermal target bimaged a blackbody source that is peaked at the properperature. This could help to normalize the response ofcamera to the thermal background against which it musterate. On the other hand, measurements of a phosphor’srescence lifetime can be made in the presence of substaamounts of blackbody background, and with good sparesolution even when the target is distantly located.

Optical pyrometry is a well-established technique thhas found many applications in science and industry. Wattempting to use a pyrometer that is not located in clproximity of its target, though, any intervening haze or othobscuring atmospheric effect can mask the signal, thus ming this technique relatively sensitive to the environmentwhich it must function. Moreover, for maximum accuracy,is important to know how the emissivity of the surface undobservation varies with temperature. Phosphor thermograis largely immune to both of these limitations, thus allowiit to be of use in certain of the situations where parame



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measurements of temperature are either difficult or impracal. Of course, advances are always being made in pyromtoo, and improved instrumentation that can overcome vous measurement hurdles is now becoming available.269,270

C. Suggestions for further research

1. Recommendations for improvement

As new applications for phosphor thermometry aristhere will be a concomitant need for target materials thave luminescence characteristics appropriate to the msurement being undertaken. For instance, although manthe phosphors that have been studied to date emit atlonger visible wavelengths, i.e., at the yellow-to-red endthe spectrum, some color-sensitive applications mightfor a phosphor that emits at shorter wavelengths, nearblue-to-violet end of the spectrum. Cunninghamet al.271

have made preliminary measurements of the temperaturependence of the excitation and emission spectra of eightferent phosphors that emit in the blue region when excitedultraviolet light. Their study covered the temperatures fro20 to 350 °C. It would be beneficial to explore the declifetime dependence on temperature of these materials,to extend the overall study to a higher range of temperatu

Similarly, Allison et al.272 have recently observed thaLuPO4 codoped with Dy~1%! and Eu~2%! has six differentspectral lines with temperature-dependent intensities. Moover, each of these lines has a different thermal sensitiv~One of them, at 453 nm, actually increases in intensitythe temperature of the material rises.! The simultaneousavailability of these multiple fluorescence signals suggethe possibility of using two or more of them to make diffeential measurements of temperature. Such an arrangemight introduce a certain amount of ‘‘optical common-morejection’’ into the measurement. This could potentially improve the signal-to-noise ratio in situations where intensbased remote thermometry is made difficult by the preseof a large blackbody background. More extensive studwill be needed to determine the optimum ratio of dopantsthis material, to fully characterize the temperature sensitiv



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of its fluorescence, and to evaluate its robustness. Therescence properties of other codoped phosphors shoulinvestigated as well.

At temperatures in the range of 1000 °C and above,fluorescence lifetimes of virtually all phosphors are veshort. Once the lifetime drops below about 100ms, thespeed, resolution, and accuracy requirements placed ondata acquisition system typically call for the use of sophicated transient digitizers and fast pulse analyzers. Equipmof this type is used routinely in nuclear, particle, and laphysics laboratories. If more were done to adapt it for usephosphor thermometry systems, then the range over wtemperature measurements could be made with this tnique might be extended.

Improvements in the methods of phosphor bondwould also be welcome. A particularly valuable advanwould arise from the development of a technique that woproduce thin, durable, spray-on coatings like those creaby flame and plasma deposition, but without the needhigh-temperature combusting gases and expensive appa

Finally, the potential of thermographic phosphorssense still other physical quantities should continue toexplored. Interesting steps along this line have been takeEgalon, Rogowski and colleagues.273–275They investigatedthe possible use in chemical sensing applications offilms of fluorescent material placed at the core/claddingterface of fiberoptic probes. Their publications review tliterature that is relevant to this effort and describe the resof a preliminary experiment done with large diameter~9mm! glass tubing coated with Y2O3:Eu

31 powder that wasmixed into a polymer binder.

2. Thermographic phosphor temperature standards

The kelvin stands apart from the other base units ofSysteme International d’Unite´s ~SI! in that it is not realizedin terms of universally reproducible atomic quantities.~Thekilogram, too, is still defined in terms of an artifactual stadard.! The intriguing possibility of establishing a temperture scale based on the atomic transitions of luminescphosphors has been suggested as a means of addressinsituation.110 In such a scheme, the relationship betweendecay lifetime of a particular excited state of a given phphor and its temperature would have to be predicted reliafrom an appropriate quantum mechanical model. Moreothe fluorescence lifetime of the phosphor must be measin terms of the ‘‘ticks’’ of an atomic clock and the systemaerrors of the measurement system must be well understIf these conditions could be met, then one could establisprinciple an absolute scale of temperature based on atotransitions over the range for which the phosphor in usetemperature-dependent fluorescence.

This would be an ambitious undertaking, of course, ait would ultimately require the resolution of many importametrological issues.~Even the existing realization of thkelvin via the ITS-90, which has been studied carefullymany years,93,94 remains the subject of close metrologicscrutiny.276! The intermediate step of investigating singlcrystal thermal phosphors as temperature transfer standhas already been taken277 though, with LaPO4:Eu having



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been studied for this purpose over the range from room tperature through 650 °C. LuPO4:Eu has also been studied.

278

Dowell98 developed a NIST-traceable thermometry systbased on single crystals of Y2O3:Eu, with the goal in mindof exploring the metrological issues of its use in absolthermometry. In fact, the time base for his data collectand analysis system was calibrated with a FrequencyTime Systems model FTS 4050 cesium-beam atomic clbuffered by a Spectracom 8140 distribution amplifier. Hwork demonstrated that precision of 0.6% relative to existstandards could be obtained with phosphor thermometrtemperatures of up to'1060 °C. It also revealed severainteresting sources of systematic uncertainty that can conute to the error budget of such systems. Meanwhile othnotably Zhang, Grattan, and colleagues~including workers atthe Colonnetti Institute!,130,150 have developed very usefumodels of the theoretical dependence of the lifetimeYAG:Cr31 on temperature, and confirmed them with expemental measurements taken through 900 K. Some otheroretical work that interrelates fluorescence lifetime and teperature was discussed in Sec. II C1.

The results of these efforts, along with those of relastudies by various others, suggest that phosphor thermommay make it possible to realize the kelvin over a continuorange of temperatures~perhaps through 1300 K and abov!without the need for interpolation between fixed points.~Aninteresting aside is that some work has already been donusing the melting points of refractory ceramics, i.e., undopphosphors, as secondary high-temperature standards,279 thusmaking these materials potential contributors to a future vsion of the existing variety of temperature scale as well.! Avery significant effort would be required to bring a phosphbased, atomic-transition realization of the kelvin into extence, but interest in the possibility has prompted researcthe subject at a growing number of standarlaboratories.280,281

ACKNOWLEDGMENTS

The authors acknowledge the efforts of the many graate students from the University of Virginia, the Universiof Tennessee, and various other colleges and universwho have done much to establish the scientific and engining foundations of phosphor thermography over the pastcade. They also thank the large number of colleagues atOak Ridge National Laboratory, the Los Alamos NationLaboratory, Wright-Patterson Air Force Base, the Universof Virginia, the University of Tennessee, EG&G EnergMeasurements, Inc., and the International Bureau of Weigand Measures for their interest in this work, for many stimlating and useful discussions, and/or for their ongoingsearch efforts in this area. The authors give special acknoedgement to W. A.~Bill ! Stange of Wright Laboratory forhis continued support of much of the effort described abofor well over a decade. His influence has made possibledevelopment of the method for important aerospace andbine engine applications. Finally, the authors thank the woers at the Science and Engineering Library of the Universof Virginia ~Physics Branch Librarian James Shea in partilar!, and those at the Oak Ridge National Laboratory Libra

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