Lens design merit functions: rms image spot size and rms optical path difference.
The chief lens design problem is to get all the optical paths from object point to image point equal within a small fraction of a wavelength to ensure that spherical wavefronts will be converging on all image points in the field of view. The LASL lens design program minimizes the lateral deviations of the rays from their ideal image points. Results given here show that this procedure also minimizes the optical path difference and that there is a linear relationship between the rms image spot size and the rms optical path difference.
Brixner
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