Diffraction corrections in radiometry: comparison of two different methods of calculation.

We have studied the effect of diffraction on the flux received by a detector located in the fully illuminated region cast by a circular aperture which is placed between a point source and the detector. This case was studied previously by Steel, De, and Bell, who have derived a diffraction correction formula based on an intensity distribution approximation due to Focke. We have derived a diffraction correction formula based on a different type of approximation. The corrections predicted by this formula are much greater than those predicted by Steel et al.'s formula. The validity of our formula and reasons for the discrepancy between the two methods are discussed; some supporting experimental evidence is presented. The case of complex radiation is discussed; it is shown that, under certain conditions, the calculation of diffraction cor rections is very simple.