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

TLDR

Radiometry requires knowledge of surrounding contributions to the measured signal, and the size‑of‑source effect (SSE) describes how the radiometer’s sensitivity to surrounding area limits lowest‑uncertainty optical measurements. The study aims to identify the dominant factors influencing SSE and demonstrates that it can be reduced to below 5 × 10⁻⁵ using a 50 mm radiance source with a 2 mm central obscuration. To evaluate SSE contributions, the authors built a radiometer with a 50.8 mm f/12 lens and 1 mm target, then measured SSE across various objective lens combinations to assess trade‑offs between optical performance and added elements. SSE depends on internal scatter and optical design; when internal scatter is minimized, the primary remaining source of SSE is lens surface finish, internal scatter, and particulate contamination.

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.