Characteristics have been measured for a series of transmission optical devices that are considered for use in transforming solar disc radiances into a top of the atmosphere solar radiance source which can be used to calibrate terrestrial radiance measurements derived from orbiting remote sensing instruments. The types of transmission optical elements that were evaluated are a diffraction hole-plate, plates of fused silica and glass with ground surfaces, several types of microlens arrays and "engineered" diffusers. Collimated light from a diffuse source with approximately the angular solar diameter was imaged onto a CCD camera by a long focal length achromat at multiple narrow band wavelengths. The optical transform device was inserted into the beam of collimated radiation on the source side of the achromatic lens.

Several "engineered" diffusers (structured microlens arrays) were characterized in collimated light from a diffuse disc source which either overfilled the CCD or had approximately the angular diameter of the Sun. The engineered diffusers were inserted into the collimated source radiation in front of a long focal length achromat which imaged the diffuse source onto a 1024x1024 array of 25 micrometer square pixels of a research grade CCD camera. The randomization of structured microlens characteristics produced a uniform and well defined angular radiance source at multiple wavelengths from 400 nm to 1000 nm. The materials which can be used to fabricate the "engineered" diffusers make their use practical for using the Sun as an on-orbit radiometric calibration source from the ultraviolet through the thermal infrared. Their optical characteristics make their use possible in a simple on-orbit self calibration mode which is useful in the radiometric calibration of high resolution hyperspectral remote sensing instruments operating at geosynchronous altitudes. The characteristics of the engineered diffuser are compared with a diffraction effect hole-plate, a ground 2-sided fused silica plate, and square and hexagonally filled microlens arrays. Both the engineered diffusers and the diffraction effects from the multiple micrometer diameter holes in a thin metal plate (hole-plate) produced diffuse radiance source images with negligible direct transmittance components.

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