Radiance stabilization of a calibration sphere source using two spectral bands

In order to increase the repeatability of a radiance calibration sphere source we have developed a technique to digitally control the light source so as to maintain a constant radiance output. In this experiment a data acquisition system was used to read the photocurrents from two detectors that monitor the radiance at different wavelengths, the required change in the lamp current needed to maintain constant radiance was then calculated. Three different radiance control algorithms were tested: single band control, two-band minimum difference control and two-band spectral shape control. An electronic circuit using two moderate precision, digital-to-analog converters and a voltage-controlled power supply was developed to stabilize a small spherical integrating source. The test data show that two (or, by inference, more) spectral bands can be used in the radiance control of a sphere source with different algorithms for calculating the radiance stabilization correction function.