Abstract

Thermopiles used for absolute radiometry in the far ultraviolet spectral region are generally calibrated with visible and near infrared radiation. Three possibilities for the failure of the sensitivity of thermopiles to be independent of wavelength have been investigated. It was found that gold–black typical of that used on thermopiles for far ultraviolet applications reflectively scattered 2–4% of the incident far ultraviolet radiation. Since the same gold–black scattered a similar fraction of incident visible and near infrared radiation, the correction for this effect was small. An analysis of surface sensitivity maps indicated that a thermopile of 0.04-sec time constant showed no wavelength variation of sensitivity when either dc detection or ac synchronous detection with 13 cps chopping was used. The most significant wavelength-dependent phenomenon was found to be the photoelectric effect. Ejected electrons carry a significant amount of energy away from gold-blacked thermopiles in the spectral region below 1600 Å. The maximum correction determined for a particular thermopile was 5.2% at λ 735 Å.

© 1965 Optical Society of America

Comparison of an Ionization Chamber and a Thermopile as Absolute Detectors in the Extreme Ultraviolet

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