The desire for high-accuracy infrared sources suitable for low-background seeker/tracker calibrations pushes the limits of absolute cryogenic radiometry and blackbody design. It remains difficult to calibrate a blackbody at irradiance levels below 1 nW cm~(-2) using electrical substitution radiometry. Ideally, the blackbody temperature should be chosen so that most of the emitted power lies in the spectral range of interest. This constraint frequently necessitates the use of small apertures (less than 1 mm diameter) to achieve the required reduction in power. However, the dimensions of small apertures are difficult to determine accurately. Also, the effects of diffraction and systematic problems such as aperture heating and light leaks become amplified. To diagnose these problems and to calculate diffraction effects, aperture dimensions must be known accurately. We describe a technique of radiometrically deducing the diameter of small apertures in the blackbody in situ utilizing the cryogenic blackbody calibration.
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