| Image quality criteria are used in optical design programs as merit functions during the optimization process. The most common are the r.m.s. spot size, the wavefront variance and the Hopkins criterion. The particular choice of an image quality criterion is especially impotant during the final aberration-balancing and weighting stages of the design process.; The spot size and the wavefront variance criteria are generally regarded as applicable in the corresponding extremes of large and small wavefront degradations. However, many optical systems are designed and operate in an intermediate domain where there is no analytical justification for using any of these criteria. In practice though, the variance criterion has been widely used outside the domain of small aberrations.; This work explores the relationship between the OTF and different physically meaningful image quality criteria in the domain of moderately large aberrations (roughly r.m.s. wave aberrations between 0.08 and 2 wavelengths). This is mainly done by examining the OFT curves for different aberration combinations along with the corresponding wavefront contour maps and isometric plots, aberration difference-function contour maps, point spread functions, distribution functions and other associated properties.; The insight thus gained, shows the limitations of the variance criterion in the domain of moderately large aberrations due to its increased degeneracy with respect to the OFT. This can be partly dealt with by using the equally-distributed variance condition. More importantly, this work demonstrates the merits of the proper use of the Strehl Ratio as an image quality criterion in this domain. Maximization of the Strehl Ratio and the use of an additional relatively simple shape control criterion result in "better" MTF curves. Different methods of implementing both the Strehl Ratio and the shape control criteria are reviewed. The investigation concentrates on radially symmetrical systems with circular, unvignetted pupils. However, some of the conclusions seem to be valid for more general pupil functions.; Finally, some promising empirical relations between the Strehl Ratio and the aberration coefficients have been shown that may provide directly the balancing ratios for Strehl Ratio maximization. |
