Improvement of size of source effect measurement of standard radiation thermometers

The size of source effect (SSE) influences the calibration uncertainty of the radiation thermometer. For each standard thermometer, the SSE must be measured. If the size of the radiance source is different, the output of the radiation thermometer must be corrected using its SSE. The SSE was measured in indirect technique since 1995 at National Metrology Institute of Japan. The SSE was measured by using an integrating sphere of 400 mm in inner diameter and 140 mm in exit opening diameter. The radiation thermometer was focused onto a black spot and a uniform bright area. The definition of the SSE is the ratio between the signal at the black spot and that of the bright area. The qualities of the black spots are the important factors that influence the uncertainty of the SSE. The uniformity and stability of the radiance source are other factors. To make the size of the black spot more precise, we renewed the black spot in 2005. Although the transmittances for the wavelengths of 650 nm and 900 nm were smaller than 0.001%, we found that the transmittances were 0.01% to 0.06%, 0.06% to 0.4%, and 0.1% to 0.5% for the wavelengths of 1600 nm, 2000 nm and 2200 nm, respectively. The SSE results were corrected using their transmittances of the black spots. The effect of the reflectance of the black spot was also a problem.