Chopping factors for circular and square apertures.

For the cases of circular and square apertures, calculations are presented for the beam size dependence of the chopping factor that is the factor relating beam intensity to that component of the intensity passing a filter tuned to the chopping frequency. For small beams, a value of 0.45 applies, but as the beam size approaches the chopping blade size, its value decreases and becomes geometry dependent. Complete numerical calculations are given for the case of a circular chopping wheel, but it is shown that analytic results of an approximation are usually sufficient. Harmonics of the chopping factor are also presented.