Radiance uniformity measurement of a miniaturized integrating sphere light source for microscope radiance responsivity calibration

An LED-based integrating sphere light source (LED-ISLS) was designed and fabricated for radiance responsivity calibration of microscopes. The LED-ISLS was composed of a miniaturized integrating sphere, an LED chip, a thin circular aperture, and was successfully applied to calibrate the radiance responsivity of a home-built microscope system. Several issues remain to be discussed. Firstly, the area of the circular aperture, which served as an exit port, was measured by comparing the light flux through an area-certified standard aperture and the circular aperture. The measurement uncertainty was smaller than 0.2%. Secondly, the radiance uniformity on the exit port was discussed. Simulations and experiments were conducted to evaluate the effects of circular aperture edge thickness and integrating sphere internal material reflection parameters on the radiance uniformity. From the simulation it was found out that as the edge thickness increases, the radiance uniformity drops drastically; the Bidirectional Reflectance Distribution Function (BRDF) also has impact on the radiance uniformity of the LED-ISLS. By reducing the BRDF from 0.9 to 0.6, the radiance uniformity at the exit port was reduced by about 63%. The radiance uniformity was also evaluated experimentally by using a 1 mm small aperture to scan the different small areas of the exit port. The radiance non-uniformity across the exit port of the LED-ISLS was about 5% from the scanning results. This work further investigated the radiance non-uniformity of the LED-ISLS and offered ideas of technical optimization of the LED-ISLS for better radiance responsivity calibration performance.