Private life of an integrating sphere: the radiant homogeneity of the Descent Imager-Spectral Radiometer calibration sphere.

Perturbations to an integrating sphere's radiant uniformity by a protruding optical system are mapped. A sphere provides the Descent Imager-Spectral Radiometer sensor head (DISR SH), a practical platform for many of its subsystems' calibrations, but the resulting nonuniformity threatens the goal of accurate radiometry. An apparatus with a nearly identical shape is built to measure the radiance variations. The radiance field is found to be very homogeneous (0-2% variations) over two-thirds of the sphere's surface area, with larger excursions near the SH, away from active fields of view. This provides an accurate platform to transfer absolute calibrations.