Diffraction Limited

The term diffraction limited implies that the performance of an optical system is limited by the physical effects of diffraction rather than imperfections in either the design or fabrication. A common means of determining if a system is diffraction limited is to compute or measure the optical path difference. If the peak to valley OPD is less than one quarter wave, then the system is said to be diffraction limited.

There are many other ways of determining if a system is diffraction limited, such as Strehl ratio, RMS OPD, standard deviation, maximum slope error, and others. It is possible for a system to be considered diffraction limited by one method and not diffraction limited by another method.

On some OpticStudio plots, such as the MTF or Diffraction Encircled Energy, the diffraction limited response is optionally shown. This data is usually computed by tracing rays from a reference point in the field of view. Pupil apodization, vignetting, F/#'s, surface apertures, and transmission may be accounted for, but the optical path difference is set to zero regardless of the actual (aberrated) optical path.

For systems which include a field point at 0.0 in both x and y field specifications (such as 0.0 x angle and 0.0 y angle), the reference field position is this axial field point. If no (0, 0) field point is defined, then the field coordinates of field position 1 are used as the reference coordinates instead.