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This section was established to reduce the lead time for the publication of Letters containing new, significant material in rapidly advancing areas of optics judged compelling in their time­liness. The author of such a Letter should have his manuscript reviewed by an OSA Fellow who has similar technical interests and is not a member of the author's institution. The Letter should then be submitted to the Editor, ac­companied by a letter of endorsement from, the

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Spectral irradiance measurements: effect of uv-produced fluorescence in integrating spheres

Robert D. Saunders and William R. Ott

U.S. National Bureau of Standards, Washington, D.C. 20234. Received 31 January 1976. Sponsored by W. R. Hunter, U.S. Naval Research Labo­ratory.

The spectral irradiance of a 30-W deuterium arc lamp was determined by comparing it with a tungsten quartz-halogen lamp calibrated for spectral irradiance in the 250-350-nm region. Because the deuterium lamp and the tung­sten lamp subtend different solid angles, four independent measurements were carried out by using the following en­trance optics to the spectrometer: (1) a BaSO4 coated in­tegrating sphere; (2) a Halon1 coated integrating sphere; (3) no sphere, but a ground-Suprasil diffuser in front of the entrance slit; and (4) no sphere and no diffusing element. The last two measurement setups produced essentially the same results. However, the spectral irradiances deter­mined for the deuterium lamp, with the use of integrating spheres with different coatings, showed the systematic dif­ferences from measurement 3 given in Table I. This table shows, for example, that the spectral irradiance of the deu­terium lamp at 340 nm is about 17% high when using the Halon coated sphere. The results using the BaSO4 coated sphere show a somewhat smaller difference. It has been suggested2 that these differences could have been caused by the accumulation of tobacco smoke on the surface of the spheres. However, while such an effect could explain the BaSO4 results in Table I, it is unlikely that it can account for the exceptionally high difference when using the Halon sphere. If it is at all significant, the effect of smoke con­tamination would be most serious for the BaSO4 sphere since it was coated several years ago, and the Halon sphere, the one with the large difference, was coated only 6 months prior to the measurements. Both spheres have been in the same environment since the Halon sphere was coated. In­deed, the explanation for the large difference is that the in­tegrating sphere coated with Halon is fluorescing due to in­cident short wavelength radiation. The fraction of the sig­nal due to fluorescence is extremely small in the case of the tungsten lamp since the spectral irradiance increases rapid­ly toward the longer wavelengths. The opposite is true in

Table I. The Percentage Differences: (1) Between the D2 Lamp Spectral Irradiance as Determined by use of a BaSO4 Integrating Sphere and as Determined by use of a

Ground-Suprasil Diffuser; (2) Between the D2 Lamp Spectral Irradiance as Determined by use of a Halon Integrating Sphere and as Determined by use of

the Ground-Suprasil Diffuser.

Table II. Transmission of 0-53 Filter as Determined wi th two Different Integrating Spheres Using both a Tungsten

Lamp and a Deuterium Lamp

April 1976 / Vol. 15, No. 4 / APPLIED OPTICS 827

the case of the deuterium lamp. The spectral irradiance increases toward the shorter wavelengths with the result that the uv-produced fluorescence at longer wavelengths is a significant fraction of the measured radiation output from the Halon integrating sphere. This explanation was verified by measuring the transmission as a function of wavelength of a Corning 0-53 uv (260-nm cutoff) filter, placed in front of the entrance port of the integrating spheres, using both the tungsten lamp and the deuterium lamp as irradiation sources. If any fraction of the mea­sured radiation is due to the uv-produced fluorescence, the transmission of the cutoff filter at the longer wavelengths should appear to be low. The results are shown in Table II.

It was concluded that one must take account of possible fluorescence effects when making spectral irradiance mea­surements using integrating spheres, especially on radia­tion sources that have an uv output which is significant compared to the output at longer wavelengths.

Certain commercial materials are identified only to spec­ify adequately the experimental procedure. In no case does such identification imply recommendation or endorse­ment by the National Bureau of Standards.

References 1. Trademark of Allied Chemical. 2. W. R. Venable, U.S. National Bureau of Standards; private

communication.

828 APPLIED OPTICS / Vol. 15, No. 4 / April 1976