IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-18, NO. 11, NOVEMBER 1971

A Pyroelectric Thermal Imaging Camera Tube MICHAEL F. TOMPSETT

Abstract-This paper proposes the use of a thin slice of a pyro- electric single crystal as the target of an electron beam scanned camera tube. This target would be sensitive to thermal images gen- erated by black body radiation. It is shown that lo4 resolvable pic- ture points per cmz of a TGS target at a frame rate not lower than 10 Hz and a thermal resolution of 1°C in the scene should be obtain- able. A pyroelectric thermal imaging camera tube based on these principles has been built and its operation will be described else- where [l].

INTRODUCTION

T HER3IAL imaging is the general term applied to techniques for producing images of objects and scenes using self-emitted black body radiation

instead of scattered illumination as in conventional imaging systems. This thermal radiation has a spectral distribution given by the Planck law [ 2 ] and n-hich is primarily in the infrared. For a body at a temperature of 20"C, the energy density peaks at a wavelength of 10 p and has a total energy density of up to 40 m\Y.cm-2. For increases in temperature, the total energy density a t all wavelengths increases and the peak moves to- wards shorter wavelengths. If images could be made using this radiation, then contrast would arise both from differences in temperature and from emissivity dif- ferences in the scene. A thermal imaging device n-ith sufficient resolution and discrimination would then pro- vide a method of detecting thermal objects and seeing in total darkness. In order to make a system that is in any n-ay comparable with a normal television system would require a device that gives at least lo4 resolvable picture points and a picture frame rate of 10 Hz or faster. To give useful picture contrast, a thermal dis- crimination of 1°C in objects having the same emissivity or differences in emissivity of a fe\v percent for objects at the same temperature is required.

Thermal imaging systems have been described [3]- [j] and are commercially available. In these systems the image is mechanically scanned point-by-point across a single detector that may be either a cooled photoconductor or a pyroelectric bolometer. In these systems only part of the imaged radiation is collected a t any given time so tha t if all the radiation incident on the imaged area could be utilized, for example, by using charge storage, then an improvement in sensitivity is expected. The use of cooled photoconductive targets sensitive in the infrared and scanned b57 an electron

beam has been attempted [ 6 ] , [ 7 ] using impurity-doped semiconductors. However, the use of a compound semi- conductor such as mercury cadmium telluride [SI is required for detecting 8-13-pm radiation. Cooling of the target may be avoided by the use of a holometric type of target. In this approach the incident radiation from the scene heats the target and a temperature sensitive physical property is used to detect the tem- perature rise. The system to be described uses a slice of a pyroelectric single crystal i n n-hich heating causes a change in the spontaneous electrical polarization along the polar axis of the crystal. This change in polariza- tion may be detected using an electron beam scanned across the target as in a television camera tube. There are several ways of operating the tube and obtaining a signal, which will be described in this paper.

Crystals having pryoelectric properties make up 10 of the 2 1 crystal classes n-ithout a center of symmetry [ 9 ] and many of these have appreciable pyroelectric coefficients [ lo] , [ll]. However, one readily available material with favorable properties is triglycine sulphate (TGS). I t may be regarded as only one of man?' pos- sible materials, but i t does possess several advantages. I t has a high pyroelectric coefficient, a lon- dielectric constant, and is easily groum from water solution into large crystals that are readily thinned into large plates normal to the polar axis [4], [12]. I t mal- be poled as a single domain in either direction by applying a small electric field. A pyroelectric thermal detector \\-as originally described and evaluated by Cooper [ lo] . The first suggestion that TGS might be used in a camera tube was made by Hadni [I31 and the proposal \\-as adopted by LeCarvennec [14]. However, the principle here was to use the material as a temperature-sensitive dielectric in the region of the Curie temperature of 49.2"C where the sensitivity is very great. The dielectric constant ET varies from 16 200 at the Curie temperature to approximately 1500, 1°C on either side of it. Keeping the target temperature near the Curie temperature, LeCarvennec established a potential Vo across the tar- get using a metal electrode on one side and a scanned electron beam on the other. \i-hen the temperature changed by A T , a voltage A V is produced across the target because of the change in dielectric constant and is given by

(1) ;\lanuscript received AIarch 3, 1971; revised June 2, 1971. The author was with the English Electric \.aIve Company

Limited, Chelmsford, England. He is now xvith Bell Telephone Laboratories, Inc., %Iurray Hill, X. J. 07974.

Hon-ever, to make a camera tube using this mode of operation is not regarded as very satisfactor?., pri-

TOkIPSETT: PYROELECTRIC THERhf AL IMAGING CAMERA TUBE 1071

marily because of the problems of making large crystal slices having a uniform polarization curve and of main- taining a uniform elevated temperature over its area during tube operation. *A fundamental problem of oper- ating at the Curie temperature would be random switching of electric domains. Pel'ta et al. [ l j ] have proposed using TGS mounted externally to the camera tube on a metal plug faceplate, but the interplug capacitance would render the required spatial resolu- tion unobtainable.

THE PYROELECTRIC IOD DE

The essential feature of the p)-roelectric effect is that the electrical polarization P, of a crystal shon-ing the effect varies with temperature. If the crystal slice has a thickness d and dielectric constant ET at temperature T, then for a temperature change of A T , a voltage AV across the crystal is generated \\-here

The p>-roelectric figure of merit for a given material may be denoted

1 dP, 4T = -(F) ET T

so that

d A V = - - T A T . ( 4

€ 0

For TGS a t 20°C, +T=3X10-9 C cm-?OC-l [ll]. The camera tube that is being proposed I\-ould oper-

a te in n-hat may be called the pyroelectric mode. That is, the thin pyroelectric crystal or target is exposed to radiation from the scene. This causes local changes in temperature and corresponding changes in electrical polarization. If one surface of the crystal is rendered electrically conducting and is held a t a fixed potential, then a potential distribution corresponding to the changes in polarization is formed on the nonconducting surface. \\-hen an electron beam is scanned across this surface and sufficient charge is deposited to restore a uniform surface potential, a video signal will be gen- erated i n the electrical connection to the conducting side. So\\- the nature of the polarization charges in- duced h>- the temperature changes means that once these charges have been neutralized in the electron beam readout process, the target temperature must be restored to its former value before they may reappear. This may be done l )>- using a shutter interposed between the target and the scene. During this restoration period, opposite polarit>- charges and potentials will be gen- erated and the charge already landed by the electron heam has to be removed, unlike conventional camera tube operation. It is assumed in all this work that the

target is a perfect insulator and that charge does not leak through the target.

THERMAL DYSAMICS

Since the video signal obtained from a target is de- pendent on the temperature changes that take place between electron beam scans, the thermal dynamics of the target have to be considered. .Assume that the tar- get, when vie\\-ing the shutter, has an equilibrium tem- perature of T, and that n-hen i t is exposed to a uni- formly radiating scene, there is an extra radiant density of A W absorbed in the target. If the shutter is opened a t time t=0 , the instantaneous temperature Ti of the target is given by

Ti - T , = AT,(1 - exp - ti.) (5)

\\-ith A I;t' S

ATm = - and T = - G G

\\.here S is the thermal capacity per unit area of target and G is the total thermal radiance and conductance from the target at temperature T,.

Considering a part of the target freshll, exposed to a thermal highlight, the target will initially warm up rapidly when the shutter is open and then cool slo\vly n-hen the shutter closes. -At times large compared to the thermal time constant 7, a dynamic equilibrium becomes established \\-hen the amounts of heating and cooling are equal. The temperatures of the target resulting from the sudden removal of a thermal highlight will just be an exponential decay with the thermal time con- s tant if the radiation incident on the target from the scene and the shutter are considered equal. So\\- the signal read from the target is proportional to the tem- perature change between scans and, depending on the polarity of the pyroelectric material and the mechanics of the electron beam, either the positive or the negative signals might be read. In certain systems it might be possible to read both positive and negative signals and invert one before display. I t is obvious that the exact response of a pyroelectric camera tube to a thermal transient in the scene being imaged is calculable but dependent on both the material and the operation of the tube. Therefore there is lag inherent in any pyroelectric target as a function of the thermal time constant of the target. To obtain maximum sensitivity consistent with minimum lag, the thermal time constant of the target should be approximately the shutter period. For long time constants the voltage swing on the target is de- pendent on the shutter period only.

The question of the spatial resolution of a pyroelectric target used with a shutter as already described is an interesting one. -A stationary delta function hot object may be considered imaged onto the target. In the ab- sence of the shutter, the heat from the hot image would spread siden-ays and set up a n equilibrium hot area

with an appreciable half \\-idth. If a simple thermal- sensitive property of the target \\-ere being used, the spatial resolution would be poor. Hou-ever, using the shuttered pyroelectric mode, onll- temperature changes between scans are recorded, so that a nonuniform tem- perature distribution gives a zero output as long as i t is constant. In the region of the hot image there will be temperature oscillations at the shutter frequency, but these will decrease with distance exponentially from the image n-ith a half n-idth on the order of K(~ jps ) - l , ob- tained by solving the time-dependent heat equation, where K is the conductivity, p is the density, and s is the specific heat of the target material. IVith a shutter frequency f= 10 Hz and the appropriate values [12] of K , p , and s for TGS, the calculated half I\-idth of the thermal oscillations due to an image point is cm. This suggests that a resolution of lo4 image points per cm2 of target should be obtainable on TGS for a station- ary image. In the nonequilibrium situation, resolution will be less as the temperature rises around the hot image point, i.e., resolution is subject to lag.

SENSITIVITY OF A PYROELECTRIC TARGET The sensitivity of a pyroelectric target used for ther-

mal imaging in a television camera tube will be calcu- lated. For these purposes, assume that the temperature swing AT of the target during a first shutter open period viewing a fresh thermal object is given by

AT = AT,/? (6)

where AT, is the difference of equilibrium temperature above the surrounding temperature that an unshuttered target would attain if freely suspended in vacuum and y is a constant.

For such a target, assuming perfectly transmitting optics with numerical aperture (N-A), vie\\-ing a scene a t a temperature AT, above ambient

combined with (6) we have

Equations (4) and (8) then define the voltage su-ing A V that ]vi11 be produced on the target as

E O

Examination of (9) Ivould suggest that A V is propor- tional to slice thickness. Hoxvever, this is not really the case because increasing the thickness merely increases the thermal capacity and increases 7 and y. Hence, for a thin target the voltage sensitivity is independent of thickness as long as all the thermal radiation is absorbed in it. Thus the relevant figure of merit for the voltage

sensitivity of the pJ.roelectric material in this applica- tion should Ile

where s is the specific heat. Equation (9) may be enumerated to give the rnag-

nitude of the voltage differences that might be gen- erated. lye may consider a target of TGS \\-ith d = 2 0 pm and for n-hich +T = 3 X 10p9C cm-?OC-l. \Yith a 10-Hz shutter and thermal equilibrium time constant 7 = 4 s, (4) and ( 5 ) give y = 80. Consider a perfect F(l) lens and AT,= 1°C; then (9) gives

A V = 112 m\-/OC.

The corresponding charge difference Aq generated on the charged surface is given by the change in polarization so that

Substituting from (8) gives

(NX)2 ap, Aq = -(z) ATo.

2Y T

This may be evaluated in the case of TGS for n-hich [I11

and with y = 80 and an F(l) lens

I t is of interest to see n-hat magnitude of video current might be generated from the TGS target used as an example so far. If the charge \\-ere read off in a normal TI7 field period of 0.16 ms and with a thermal highlight of 10°C in the scene, the peak signal current would be 30 nA for every cm2 of target area. This is not an unrea- sonable value and, using the most recent preamplifiers [16], a 1°C scene temperature difference would be ob- servable. The signal-to-noise ratio in the video signal is dependent only on the charge readout per scan and not on the speed of scan as long as the amplifier band- width is kept at the minimum to give the required resolution [ 171.

I t should be pointed out that the above signal cur- rent and indeed all the voltages and charge differences, which it has been estimated could appear on the target, have been calculated for an ideal system. An imperfect optical imaging system, reflection and imperfect ab- sorption of the target, and a restricted optical band- width, say, 8-14 pm for enhancing image contrast, 11-ill all cause reductions in sensitivity in a practical system.

TOOhlPSETT: PYROELECTRIC THERlMAL IMAGING CAMERA TUBE

ELECTROX BEA1\.i READOUT

has been sho\\-n above, the target variations of potential and charge on which READ mechanisms might be based are very small. In addition, it is assumed that the target is nonconducting and therefore any charge tha t is read off has to be returned via the electron beam during the second part of the shutter cycle. Several ap- proaches are possible, although the suitability of each depends very much on the pyroelectric material and its properties.

The simplest method of electron beam readout is that of the cathode potential stabilized (CPS) vidicon [18] \\-here the video signal is taken from the target electrode, n-hich is capacitively coupled to the scanned surface of the target. Only electrons may be deposited in this low- voltage scanning mode and other methods must be sought to restore the target potential. One possibility [l ] is t ha t positive ions could be produced from the residual gas in the tube by an electron beam scanning across the target but biased so that i t does not land on the target. These positive ions would be attracted to the target and thereby neutralize the previouslq- landed electrons by landing positive charge. The major prob- lems \\-auld be of obtaining control and uniformity.

;Inother mode of possible operation is tha t of anode potential stabilization (XPS) [19] when both positive and negative signals are produced corresponding to the phase of the shutter. This mode is much less uniform [I91 than that of CPS, so tha t a more complex mode using a combination of the two may be necessary. The shutter target is first scanned in APS so as to render the surface slightly more positive than the cathode po- tential used in CPS mode. I t is then discharged uni- formly in CPS. -After having been exposed to the scene, the output signal is read off again in CPS mode. The process is then repeated in synchronism with the shutter. This dual mode type of operation also opens up the possibility of using the return beam orthicon or isocon [ 2 0 ] , [ 2 1 ] mode of operation. These modes stabilize the target potential as in CPS mode, but either the nonlanded or scattered electrons are returned to an electron multiplier. This overcomes the noise limitation of the preamplifier used in the normal vidicon mode. In a return beam tube, signal-to-noise ratio is essentially that of the shot noise in the return beam. However, a monochromatic beam may be required to prevent the electrical lag that 11-ould otherwise occur in the high- capacitance pyroelectric targets. The lower the value of E T for a given value of (dP,/dT),, the greater is the advantage in using return beam. The major uncertainty in applying -IPS is that the target is bombarded by an electron beam of relatively high energy. The energy has to be greater than the secondary emission-crossover potential for the material and this is normally on the order of 100 eV. Xt these energies the target material could suffer electron bombardment damage, particularly an organic material like TGS.

1073

Another method in \I-hich the signal current is not limited by the displacement charge of the target makes use of a principle described by Verster [ L 2 ] . X mesh anode is placed just behind or evaporated onto the pyroelectric target. The operation of the tube is then such that the potentials generated across the pyroelec- tric target by the usual imaging and shuttering arrange- ments modulate the electrostatic fields in the vicinity of the anode. By a suitable choice of static potentials, it can be arranged that the landing of a scanned electron beam onto the anode is modulated and a video signal obtained from the anode. Again a low value of ET is advantageous in this mode so as to maximize the poten- tial differences across the target.

A pyroelectric target also suggests itself for true mirror electron beam readout. Here a nonlanding elec- tron beam is scanned across the target such that it is locally deflected by the microfields generated by polari- zation changes in the target. Several systems can be envisioned to give both direct imaging and electrical readout. In direct imaging cases the electron beam is reaccelerated onto a phosphor screen where an image of the potential gradients on the target is produced. Such systems have been described by Shn-artze [ 2 3 ] and by Barnett and Sixon [24]. L411 these systems have a limited field of vie\\-.

A return beam sl-stem operating in nonlanding mir- ror mode could be used to provide an electrical readout. An electron beam scanned across the target u-ould bias it negative such that electrons no longer landed. The beam would then be deflected according to the micro- fields generated by the charge pattern at the surface of the target. So that the deflection is dependent on the amplitude of the microfields and not their periodicity [ 2 5 ] , i t would be necessary to impose a single spatial frequency on them. This could be done by placing a mesh in a conjugate plane of the infrared imaging sys- tem. The deflection of the returned beam could then be detected by a pair of neighboring dynodes. The differen- tial of the required signal along the line scan direction n-ould then be obtained. This could be integrated to give the required image on a TV monitor. "sing the results of Barnett et al. [ 2 5 ] - [ 2 7 ] and the target sen- sitivity already calculated (9), a deflection sensitivity approaching 0.04 rad/"C temperature difference in the scene may be calculated for TGS under ideal condi- tions.

SCMMARY

This paper shonx the basic feasibilitJ- of making a thermal imaging camera tube using a slice of pyroelec- tric crystal. The charges and voltages developed on the best available targets by the small temperature changes in the scene being imaged are sufficiently large that electron beam scanning methods may be used to pro- duce TV type displays. There are several possible com- promises but lo4 resolvable picture points per cm2 in the case of T G S a t a frame rate of 10 Hz giving thermal

1074 IEEE TRANSACTIONS ON ELECTRON DEVICES, NOVEMBER 1971

resolution better than 1°C on the scene would seem to be theoretically achievable. Greater sensitivity would be possible if more sophisticated return beam methods n-ere used, particularly for materials with a high pyro- electric coefficient and a 101v dielectric constant. A h y viable design of thermal imaging camera tube is pred- icated by the perfornlance requirements, the avail- ability of large slices of the pyroelectric crystal, and practical considerations of their use in a vacuum tube.

-4 pyroelectric thermal imaging camera tube based on these principles has been designed and its operation will be described elsewhere [ 11.

REFERENCES [l] IT. SI. \Treathall, to be published. 121 See, for examde. P. \X7. Kruse. L. D. ;\IcGlauchlin, and R. B.

SIcQuistan, Eds,’, Elements of Infrayed Technology, Generation, Transnzission and Detection. New York: \YileY. 1962.

[3] E. 31. \Vormser, “Sensing the invisible world,” d p p l . Opt . , vol. 7, _ ,

PP. 1667-1671,1968. [4] R . IV. -4stheimer and F. Schwarz, “Thermal imaging using

pyroelectric detectors,” .4ppl. Op t . , vol. 7 , pp. 1687-1695, 1968. [5] S. S. Borg, “Thermal imaging xvith real time picture presenta-

tion,“ - 4 p p l . Opt . , vol. 7, pp. 1697-1703, 1968. [6] R. \T. Redington and P. J . van Heerden, “Doped silicon and

germanium photoconductors as targets for infrared television camera tubes,” J . Opt . Soc. d n z e r . , vol. 19, pp. 997-1001, 1959.

171 11. Berth and J . J . Brissot, “Targets for infrared television camera tubes,“ PhiliPs Tech. Rw., vol. 30, pp. 210-277, 1969.

[ 8 ] P. \V. Kruse, “Photon effects 111 Hg,-,Cd,T,,” A p p l . Op t . , vol.

191 F. P. Tona and G. Shirane. Ferroelectric C~\ ,s tuIs . S e w York: 1, pp. 687-692, 1965.

. . l Iacm~llan, 1962.

[lo] J. Cooper, “SIinimum detectable power of a pyroelectric thermal receiver,” ReLl. Sci. Instr., vol. 33, pp. 92-95, 1962.

[26]

~ 7 1

H. P. Beerman, “Pyroelectric infrared radiation detectur,” .-liner. Cemnz. SOC. Biiii., vol. 16, pp. 737-740, 1967. E . H. Putley, “Solid state devices for infrared detection,“ J . .Sc.i. Instr., vol. 43, pp. 857-868, 1966. A . Hadni, Y . Henniger, R. Thomas, P. Vergnat, and B. \Yynche,

appllcation to the detection of infrared radiation,” J . Phys . ”On , the properties of some pyroelectric materials and their

LeCarvcnnec, “Research on a new target for thermal television,” (Paris), vol. 26, pp. 345-360, 1965.

in .4d~lunces in Electron. und Electron. Phys., Proc. 4th S y m p . Photo-Elertron. InlaRc: I)e;sices, vol. 2 8 , pp. 256-272, 1969. S. D. Pel’ta, L. SI. Dun, 1;. K. Souik, and 1. S. Rez, “The use of

Phys. ,vol . 13, pp. 157-158, 1968. pyroelectrics in television camera tubes,“ Radio Eng. Electl.orc.

H . Breirner, \J’. Holm, and S . L. Tan, “.I color television camera with Plumbicon camera tubes,” Philips Tech. Rev. , vol. 28, pp,

coupled devices: Design considerations,” this issue, pp. 986-992. G. F. ;\meIio, IY. J. Bertram, and SI. F. Tompsett, “Charge

photoconductive camera tube,” Electronics, vol. 23, pp. 70-73, P. K. Ii’eitner, S. V. Forgue, and R. R. Goodrich, “The vidicon

3iay 1950, 1. Dresner, “The hinh-beam-velocit~ vidicon,” RC.4 R e x , vol.

336-351, 1967.

2 2 , pp. 305-321, 196i. X. D. CoDe and H. Borkan. “lsocon scan--ri low-noise, wide dynamic range camera tube scanning technique,” .-lppl. Opt., vol. 2 , pp. 253-260, 1963.

light levels,” J . Roy. Telee. Sur . , vol. 11, pp. 261-266, 1967. U. P. JIouser, P. C. Kuggles, and S . .A. Siark, “Tube for low

J . L. I-erster, “On the use of gauzes in electron optics,” Philips Res . Rep . , vol. 18, pp. 592-606, 1963. \I7. Sch\vartze, “Resolution limiting geometrical optical effects in the mirror electron microscope,” Opi ik , vol. 23, pp. 614-626,1.966.

electron tnicroscopv.” Opl ik , vol. 26, pp. 310-32.5, 1967/1968. 31. E. Barnett and \Y. C. Nixon, “Electrical contrast 111 mlrror

31. E. Barnett and L. England, “Distortions in shadow projec- tion electron mirror images of periodic structures,” Opt ik , vol.

Phys . L e f t . , vol. 12, pp. 229-230, 1968. Si. E. Barnett, “I-oltage resolution 01 theelectron mirror,” A p p l .

31. E. Barnett and 0. I . Szentesi, “Electron mirror observation of an interdigital electrode structure,” J . Sri. Inst. ( J . PhTs. E . ) ,

2 7 , pp. 311-352, 1968.

VOI. 2 , pp. 855-858, 1969.