An Integrating Sphere As A Precision Radiometric Calibration Source

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Abstract
A 20-in.-diameter integrating sphere has been designed to provide a precision radiometric calibration source over a spectral range of 0.02 to 50 μm for the Earth Radiation Budget Experiment (ERBE) instruments and the reference active cavity radiometers (ACR$). The calibration source described herein has unique specifications to provide simulated earth albedo plus longwave background emittance to these instruments, in as near uniform an irradiance as possible, with fields of view ranging from 3 to 168 deg conical. The widest field of view simulates the limb-to-limb earth input with a "space-ring" of liquid nitrogen around the entrance aperture. This paper describes: (1) how the measurement of a distributed shortwave source of radiation has been used to calibrate the ERBE instruments in radiation regimes representing earth-reflected solar radiation, (2) the design of an integration sphere comparing the theoretical throughput irradiance levels to the measured levels using reference ACR radiation measurement devices, and (3) methods of controlling the shortwave intensity and wall temperature, as well as monitoring the changes in these levels due to changes in loading as measurement devices are moved into and out of the exit aperture.

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