Reference OPD

The Optical Path Difference, or OPD, is of value in optical design computations because the OPD represents the phase error of the wavefront forming an image. Any deviations from zero OPD contribute to a degradation of the diffraction image formed by the optical system.

Because the exit pupil is the image of the stop in image space, the exit pupil represents the only location in image space where the beam has a clearly defined edge. The illumination at the exit pupil is generally smoothly varying in amplitude and phase, and there is a clearly defined boundary between regions of zero amplitude and nonzero amplitude. A reasonable assumption is that there are no diffraction effects apparent in the wavefront when viewed in the exit pupil. This is asymptotically true if all apertures in the optical system are large compared to the stop limited beam size incident upon each aperture. Even if the exit pupil is virtual, which is often the case, the exit pupil still defines the only location in image space where the beam is diffraction free.

As the wavefront propagates from the exit pupil towards the image surface, the beam profile becomes complex in amplitude and phase, and the wavefront extends over all space due to the effects of diffraction. For this reason, the phase error as measured in the exit pupil is uniquely and critically important to the description of the wavefront and image quality.

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