Size-of-Source Effect Difference Between Direct and Indirect Methods of Radiation Thermometers

摘要

The size-of-source effect (SSE) of radiation thermometers is generally determined using direct and indirect methods. The present paper describes the difference in the SSE obtained using the two methods. Three thermometers at different SSE levels, namely, 1.5 × 10~(-4), 1.05 × 10~(-3), and 2.44 × 10~(-3), are selected. SSEs with 5 mm to 96 mm aperture diameters are higher in the indirect than in the direct method. Three factors contribute to this difference: the light reflection from the inside face and edge of the aperture to the black spot, the relay reflections from the thermometer and background, and the decreased radiance caused by obstruction of light by a black spot. Thus, the difference in the SSEs results from the difference in the principle of the two methods and not from the SSE levels of the thermometers themselves. The difference is negligible to be considered in a number of radiation thermometers, but they must be taken into consideration in top-level intercomparisons.