Characteristics of an internally illuminated calibration sphere

It is noted that in recent years, a new class of instruments with fields of view of many degrees has been developed for measuring the earth radiation budget (ERB). To determine the applicability of internally illuminated spheres to such instruments with their large fields of view, a series of measurements is carried out to examine the uniformity of the exitant radiance at the aperture of the sphere and the uniformity with angle as an instrument views other than normal in the sphere. The sphere has a diameter of 76.2 cm and a circular aperture of 29.8 cm. The spatial uniformity across the aperture is found to be 1.9 percent or better for full twelve-lamp illumination and 2.6 percent for one-lamp illumination. The angular uniformity from normal to + or - 40 deg is found to be 3 percent or better.