An Alternative Calculation of the Size-of-Source Effect Uncertainty for Non-uniform Sources

For a correct determination of the size-of-source effect (SSE), it is necessary to use a large (e.g., 50-mm diameter) radiation source of uniform spectral radiance. The non-uniformity of the source can be an important component of the uncertainty because it is difficult to obtain sources with good uniformity over the whole aperture. In the indirect method to measure the SSE, the component due to the source non-uniformity comes from the measurement of S(L,∞), usually measured outside the spot that blocks radiation from the radiation thermometer target. By expressing the SSE as an experimental sum of successive contributions of radiating rings at the source aperture, the uncertainty due to the non-uniformity can be reduced. In this article, the calculus of the SSE and its uncertainty for the usual equation is compared with the calculus based on an experimental equation that considers the SSE as a sum of the contributions from radiating rings.