Methods to reduce the size-of-source effect in radiometers

Abstract

In radiometry, photometry and radiation thermometry, accurate measurements of the radiance, luminance or the radiance temperatures of sources requires a knowledge of the contribution from the surroundings to the measured signal from the target area. The dependence of the radiometer or the radiation thermometer on the area surrounding the target area is described as the size-of-source effect (SSE), and minimizing the radiometer's sensitivity to SSE is critical in the lowest-uncertainty optical measurements. We describe the dominant effects that influence the SSE, and show that the SSE can be reduced to <5 × 10−5 as measured using a 50 mm diameter radiance source with a 2 mm diameter, central obscuration. The SSE is found to be dependent on the internal scatter and the optical design of the radiometer. For testing the contributions to SSE, a radiometer is constructed with a 50.8 mm diameter lens in f/12 geometry with a 1 mm diameter target size. If the internal radiometer scatter is reduced, then the SSE is found to be primarily dependent on the scatter from the objective lens such as surface finish, internal lens scatter and the particulate contamination of the lens. Various combinations of objective lenses are measured for SSE, and the relative merits of increasing optical performance at the expense of additional optical elements are also discussed.