ABSTRACT
This chapter examines the role of the exit pupil in diffraction calculations. There we show that, for an axial pencil of rays, no primary diffracted rays other than those from the edge of the aperture stop can reach the image point, and that therefore the exit pupil is a natural place to switch from ray tracing to diffraction theory. From the examination of the difference between the Debye and Kirchhoff theories, we infer that a single transition plane is adequate when this difference is small. For if the difference between the two theories is small, we can move the aperture from its actual location to infinity without changing the field near focus appreciably. There are several ways in which the angular spectrum can be computed in the Kirchhoff approximation. The Debye approximation to the angular spectrum is an asymptotic approximation in which one retains only the contribution of interior stationary points.
