Optical Path Difference (OPD) and The Rayleigh Criteria

OPD is an extremely useful measure of the performance of an imaging optical system. If the wavefronts proceeding to a given point image are spherical, concentric, and centered at the point image for a given field of view, then the imagery will be geometrically perfect, or diffraction limited. As shown earlier, the image will then be a perfect Airy disk. This is, in effect, the reverse of our earlier example where we threw a rock into a pool of water to illustrate the wave nature of light and diffraction. If we think of the water waves traveling in reverse to where the rock entered the water, we will emulate light imaging to a point image. By definition, the wavefronts will be perfectly spherical, concentric, and centered where the rock entered the water. Recall also that rays are perpendicular to the wavefronts. It is thus clear that if the wavefronts are spherical, concentric, and centered at a point in the image, then the rays will all come to that same point as defined by the center of curvature of the wavefronts. As we learned earlier, diffraction at the limiting edge of the pupil will create an Airy disk, which is the reason why we do not have a perfect point image.

Consider Fig. 4.1 where we show a hypothetical lens with a perfectly spherical reference wavefront and a real wavefront. The real wavefront departs from sphericity due to aberrations induced by the...