Radiometric errors caused by diffraction from circular apertures: edge effects.

The diffraction corrections associated with a circular aperture are calculated for the case of a point source and a detector aperture having a diameter approximately equal to that of the illuminated region. This investigation is made for monochromatic and complex (tungsten) radiation; two types of detectors are considered: a typical silicon diode and a neutral detector. The intensity distribution near the edge is calculated for the same cases. Some experimental results are also presented to corroborate the calculations and to suggest the behavior with extended sources. We show that strong diffraction effects are present close to the shadow edge even with a source-detector combination having a very extended wavelength band. In radiometric measurements, a suitable compromise between diffraction effects, stray light, and vignetting effects can be achieved by having the detector aperture diameter approximately half that of the illuminated region. An extended source will reduce diffraction effects. Finally, some suggestions are made with respect to making diffraction corrections for a series of apertures.