Determination and comparisons of aperture areas using geometric and radiometric techniques

One of the major uncertainties associated with radiance and irradiance meas urements stems from the aperture system and a knowledge of the aperture are a. High-quality apertures, with a roundness of 0.1 mu m, can now be manufac tured by diamond-turning techniques. The quality of these high-precision ap ertures is such that their geometrical diameter can then be measured with a n uncertainty of +/-0.05 mu m using the internal-diameter measuring machine developed at the National Physical Laboratory (NPL), UK. Optical technique s have also been developed at the NPL and the World Radiation Center (WRC), Switzerland, so that the ratio of the radiometric areas of two apertures c an be determined and this ratio compared with the geometrical ratio as calc ulated from their diameters. Results are presented of these comparisons usi ng apertures from the NPL and the WRC.