Strehl ratio with low sensitivity to spherical aberration

Abstract
The influence that Seidel spherical aberration has on the irradiance distribution along the optical axis, or the Strehl ratio versus defocus, is expressed in terms of a differential operator. This permits us to describe separately the following two effects: a focal shift and a shape variation. We analyze two annular apodizers that reduce the shape variation of the Strehl ratio versus defocus.