Determination and comparisons of aperture areas using geometric and radiometric techniques

One of the major uncertainties associated with radiance and irradiance measurements stems from the aperture system and a knowledge of the aperture area. High-quality apertures, with a roundness of 0.1 mu m, can now be manufactured by diamond-turning techniques. The quality of these high-precision apertures is such that their geometrical diameter can then be measured with an uncertainty of +/-0.05 mu m using the internal-diameter measuring machine developed at the National Physical Laboratory (NPL), UK. Optical techniques 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 can be determined and this ratio compared with the geometrical ratio as calculated from their diameters. Results are presented of these comparisons using apertures from the NPL and the WRC.