Diffraction loss in radiometry.

Diffraction loss in radiometry has gained in importance recently because of an increased interest in longer wavelengths and the continuous improvement in experimental accuracy. The deviation from geometrical optics now contributes significantly to the errors of experiments. Previous research has concentrated on geometries classified as F1 and F2, leaving an intermediate case yet to be investigated. This intermediate case has some interesting behavior, as it is in this envelope of geometries that it is possible to have zero diffraction loss. We designate this intermediate geometric regime as F3. We introduce a numerical regime to calculate diffraction loss for intermediate geometries, which is also highly efficient for the F1 and F2 regimes.