June 1980 / Vol. 5, No. 6 / OPTICS LETTERS 225

Vacuum-ultraviolet spectral-irradiance calibrations: methodand applications

W. R. Ott, J. M. Bridges, and J. Z. Klose

National Bureau of Standards, Washington, D.C. 20234

Received February 4, 1980

A method to determine the spectral irradiance of a radiation source in the vacuum ultraviolet through the use ofrecently developed spectral-radiance standards is described. The method has been applied between 138 and 310nm, and the spectral irradiances of several different light sources have been measured on an absolute scale with es-timated uncertainties less than 10%.

The quantitative measurement of the intensity of anunknown radiation source is often done by comparingthe output of the source with that of a standard radia-tion source. To a large extent, such calibrations in thevacuum ultraviolet (VUV) are limited by the scarcityof convenient radiation standards. In recent years,various sources have been developed as standards ofspectral radiance in the VUV.1-4 However, there arestill no standard sources of spectral irradiance, exceptfor certain synchrotron radiation facilities, for wave-lengths shorter than 200 nm. As a first step towardestablishing standard sources of spectral irradiance inthe VUV, this Letter describes a method of calibratingthe spectral irradiance of an unknown radiation sourcebased on an existing standard source of spectral radi-ance. The method is applied to several sources that arebeing applied as irradiance sources in various experi-mental situations, e.g., on board the Space Shuttle.

Spectral radiance (W cm-2 nm- t sr-') is the powerradiated from a specific emitting surface (cm2 ) in acertain wavelength band (nm) within a given solid angle(sr). The spectral irradiance (W cm-2 nm- 1) definesthe radiant power incident upon a specific target area(cm2 ) in a certain wavelength band (nm). The root ofthe problem of establishing a spectral-irradiance scalebased on a radiance scale is that the radiance is normallycalibrated only for a small portion of the light source,usually that portion that can be considered homoge-neous. Thus the radiation source must be used in sucha way that only radiation from the calibrated areareaches the measurement system. This can be done bythe use of optical imaging or by the use of collimatingapertures.5 ' 6 For applications in the VUV, the methodusing collimating apertures has proven more practicaland is the basis for the measurements described here.

The collimating aperture method is as follows. Aradiation source that is homogeneous over a certainemitting area and whose spectral radiance has beenpreviously determined is situated a given distance froma monochromator. A pair of apertures, one at the en-trance slit of the monochromator (the field aperture)and the other as close to the source as possible (thesource aperture), is chosen so that only the radiationfrom the homogeneous portion of the source is mea-

sured. The spectral irradiance at the field aperture isgiven by the product of the known spectral radiance ofthe source and a geometric factor dependent on theaperture dimensions and their locations relative to thelamp. This geometric factor contains the informationon the effective area of the emitting source, the solidangle of the radiation beam incident upon the fieldaperture, and the irradiated area.

In principle, these quantities can be determined bymeasurement. However, a direct determination of thegeometric factor is unnecessary, as is shown by the fol-lowing discussion. The response of a spectroradiometeras a function of wavelength to a suitably collimatedradiance standard is essentially a measure of the systemdetection efficiency on a relative scale. If the samespectroradiometer is irradiated with an unknown sourceand the response is measured again, the spectral irra-diance of the unknown source can be determined, alsoon a relative scale. Then, provided that the wavelengthrange of calibration extends to the visible or near-UVregion in which standard sources of irradiance areavailable, the absolute spectral irradiance of the un-known source can be determined at one wavelengthwithin the calibrated wavelength range. This absolutevalue can then be used to normalize the relative scaleof irradiance to an absolute scale.

The calibration procedure is illustrated in Fig. 1. IfEX (X) is the spectral irradiance at wavelength X of thesource whose irradiance is to be determined, the re-sponse S, (X) of the spectroradiometer to this radiationsource [see Fig. 1(a)] is

S. (X) = E. (X) aX), (1)

where a (X) is the spectroradiometer system efficiency.The response SR (X) to the apertured radiance standard[see Fig. 1(b)] is

SR () = LR W(Xa) Y, (2)

where y is the geometric factor discussed above and LRis the spectral radiance of the radiance standard. Therelative spectral irradiance can then be determined:

(3)E. k) = -y SX) LR(X) -SR(X)

0146-9592/80/060225-03$0.50/0 © 1980, Optical Society of America

226 OPTICS LETTERS / Vol. 5, No. 6 / June 1980

sx

(a)Diffuser

Unknown

Stop

SLX(b) Monochromator

Radiance |

Standard 55R Source

Aperture

Fig. 1. Schematic of calibration procedure. (a) Spectrora-diometer is irradiated by the lamp whose spectral irradianceis to be evaluated, (b) efficiency is determined on a relativescale by irradiation with a stopped-down radiance stan-dards.

The absolute spectral irradiance is determined bycomparing the unknown source with an already existingirradiance standard at some convenient wavelength. Ifit is more suitable, this can be done on a separate spec-troradiometer setup, for example, one that is dedicatedto spectral-irradiance measurements in the visible. Theresponses of this system (primed) both to the source tobe calibrated, S', and to the already existing irradiancestandard, Si, are measured at one wavelength, X0, sothat the spectral irradiance of the unknown source,E. (Xo), is given by

E.(Xo) = Sr(Xo) EI(o),

In the case of an extended homogeneous radiationsource whose dimensions are large compared with thesource aperture, it can be shown that Fraunhofer dif-fraction effects introduce no wavelength dependenceand that the geometric factor is wavelength indepen-dent. One can appreciate this qualitatively, perhaps,by realizing that some percentage of the radiation fromeach radiating point does not pass through the fieldaperture because of diffraction. However, one can al-ways find a complementary point in the domain of theextended homogeneous source that exactly makes upfor this loss. As discussed later, by substituting a dif-ferent-sized aperture we showed that the inhomogeneityof the real sources used was insufficient to cause anymeasurable effects that were due to diffraction.

The vacuum spectroradiometer, as already indicatedschematically in Fig. 1, consisted of a solar-blind pho-tomultiplier and a spectrometer with a 2-mm-diameterfield aperture mounted on a MgF2 diffusing windowlocated 50 mm in front of the entrance slit. The dif-fuser was located some distance in front of the entranceslit in order not to overfill the grating and thereby in-crease the quantity of scattered light (less than 2% of theweakest signal.)

The light source used as a standard of spectral radi-ance in the near and vacuum ultraviolet was an argonmini-arc2 previously calibrated with respect to a pri-mary standard, a wall-stabilized hydrogen arc.' Both

100

(4)

where E1( 0o) is the spectral irradiance of the standardsource.

If Eq. (3), with X = X0, is combined with Eq. (4), -y canbe eliminated to give

E. \) = S' X (X0) SR (Xo) E, (X0 ) S. (X)S' (Qo) SX (XO) LR (XO) SR(X)

From Eq. (5) the absolute irradiance of the unknownsource can be determined over the complete wavelengthrange of the radiance standard.

The considerations given above assume first that thesystem efficiency of the spectroradiometer employeddoes not depend on the angle at which radiation entersand second that diffraction effects' are not significant.Since several sources with different dimensions are usedand since a monochromator grating is used whose re-flection efficiency is not expected to be uniform, a dif-fuser located directly behind the field aperture is nec-essary to ensure that the first assumption is correct. AMgF2 window, ground on one side, was used as the dif-fusing element. Although it cannot be expected to beas good a diffuser as an integrating sphere, it was shownto be suitable at least over a relatively small range ofangles. For the sources described in this Letter, theuncertainty in the spectral irradiance that is due to thenonideal diffusing properties of the diffusing windowwas measured to be less than 1%.

We must also consider possible effects of diffraction.

a

E 1

2 o-

lo-

(30 W)

X. LAMP(15W) T

10 I | I e

0 50 100 150 200 250 300 350

A (nm)

Fig. 2. Absolute spectral irradiance measured as a functionof wavelength at a distance of 50 cm from the field aperturefor five different continuum sources with the indicated powerrequirements. The spectrum of the D2 lamp with MgF2window below 170 nm, measured for a 1-nm bandpass, is apseudo-continuum made up of blended lines. Shown forcomparison purposes are spectra of the 250-MeV NationalBureau of Standards synchrotron radiation facility, for a beamcurrent of 10 mA and a field aperture distance of 19 m, and thetungsten-quartz-halogen lamp, for the standard 50-cm dis-tance.

June 1980 / Vol. 5, No. 6 / OPTICS LETTERS 227

aC

-5'250 270 290 310

A (nm)

Fig. 3. Percentage difference in the near UV between spec-tral-irradiance calibrations using conventional techniques(essentially using a tungsten-quartz-halogen irradiancestandard and integrating sphere) and the method here in-troduced for VUV calibrations (essentially using a mini-arcradiance standard and diffusing window). The shaded arearepresents the uncertainty in the tungsten-lamp calibration.The error bars represent the imprecision in the measurements.The measurements were normalized at 280 nm.

sources have been used before for radiometric purposesand have been subjected to radiometric-scale inter-comparisons, both internally and international. Theuncertainty in the absolute spectral radiance is esti-mated to be within 6% between 138 and 330 nm. Themini-arc was located 50 cm from the field stop. A0.3-mm-diameter aperture placed 5 cm from the mini-arc center was used to restrict the size of the radiatingarea to about 0.5 mm in diameter.

The light sources calibrated were the following: a30-W deuterium lamp with quartz window, a 30-Wdeuterium lamp with MgF2 window, an argon mini-arcwithout collimating aperture, a higher-powered argonarc (the maxi-arc), and a noble-gas dimer lamp. Theresults, in addition to several reference spectra, areshown in Fig. 2. The total systematic uncertainty in thecalibrations, including the uncertainties in the primarysource calibrations and the transfer procedure, is esti-mated to be less than 10%. Although the sources gen-erally emit radiation at shorter wavelengths than shown,the short-wavelength limit is currently set at about 138nm because of the low signal obtained from thestopped-down mini-arc radiance standard at shorterwavelengths. The broad structure apparent in the D2lamp spectrum below 165 nm is due to blending ofLyman-band molecular lines and is to some extent de-pendent on spectral resolution. The bandpass for thismeasurement was about 1 nm. Not shown in the figureare several lines in the argon-arc spectra that are due toresidual-gas impurities.2 The irradiances were put onan absolute scale by measuring the absolute spectralirradiance of the lamps at 280 nm, using a tungsten-quartz-halogen lamp as an irradiance standard. Aseparate spectroradiometer utilizing a BaSO4-coatedintegrating sphere, a predispersing monochromator, andan analyzing spectrometer was used for these mea-surements. As shown previously, one measurement of

this type is sufficient, in combination with the relativemeasurements performed on the vacuum-irradiancefacility, to determine the absolute irradiance at allwavelengths.

As a check on the method, it is possible to carry outtungsten-lamp calibrations at other wavelengths be-tween 250 and 350 nm and thus overdetermine thesystem of measurements. Figure 3 illustrates the per-centage difference between the spectral irradiance ofthe deuterium lamp based on the tungsten-lamp mea-surements (on the integrating-sphere-double-disper-sion spectroradiometer) and the spectral irradiancebased on the argon mini-arc measurements (on thediffusing-window-vacuum-monochromator spectro-radiometer). The measurements are in agreementwithin 3%. However, perhaps because the point at 250nm is a little high, one can perceive a slight trend towardan increasing difference at the shorter wavelengths, i.e.,the radiance-based values seem to go lower than thetungsten-lamp-based values. Such a trend would beexpected if diffraction played an important role. Inorder to check on whether the argon-arc plasma wasbeing sufficiently resolved by the collimating systemand whether diffraction effects were indeed being ac-counted for by virtue of the extended nature of thesource, additional measurements were taken with a1-mm aperture used as the field aperture. Thesemeasurements should be more sensitive to possiblediffraction effects since a smaller percentage of thediffracted beam from each radiating point passesthrough the field aperture. The spectral distributionof the signal was exactly the same as it was with the 2-mm-diameter aperture, indicating that the data are notsignificantly affected by diffraction. The measure-ments also indicate that the spatial resolution was suf-ficient to define a relatively homogeneous region aboutthe arc axis and that the radiance values applied to thedata were appropriate. Thus, if there is a trend in thedata of Fig. 3, diffraction effects do not seem to be thecause of it. Future scale intercomparisons, most likelywith synchrotron-based calibration groups, are plannedin order to extend our range of comparison and to ex-amine our measurement methods further.

This work was partially supported by the NationalAeronautics and Space Administration.

References

1. W. R. Ott, K. Behringer, and G. Gieres, Appl. Opt. 14,2121(1975).

2. J. M. Bridges and W. R. Ott, Appl. Opt. 16, 367 (1977).3. P. J. Key and R. C. Preston, Appl. Opt. 16, 2477 (1977).4. H. Kaase and K. H. Stephan, Appl. Opt. 18, 2275 (1979).5. W. R. Ott, J. M. Bridges, and P. A. Voigt, in Proceedings

of the 5th International Conference on Vacuum Ultravi-olet Radiation Physics, M. C. Castex, M. Povey, and N.Povey, eds. (Centre National de la Recherche Scientifique,Meudon, France, 1977), Vol. 3, p. 123.

6. R. D. Saunders, W. R. Ott, and J. M. Bridges, Appl. Opt.17, 593 (1978).

7. J. M. Bridges, W. R. Ott, E. Pitz, A. Schulz, D. Einfeld, andD. Stuck, Appl. Opt. 16, 1788 (1977).

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