The Retardation for light results display the absolute difference (as a length) between the phases of horizontally-polarized and vertically-polarized light as it passes through the part from the direction named in the result.
These results are created at the end of a 3D Warp analysis.
The retardation indicates how out-of-step the phases of horizontally-polarized and vertically-polarized light became by passing through the part. This becomes important if the light was initially polarized, or subsequently passes through another optical element which further polarizes the light. If so, the transmitted light may vary in brightness or exhibit colored bands.
The meaning of "horizontal" and "vertical" depends on the orientation of the light source. They are mutually orthogonal planes of polarization, the intersection of which contains the incident light beam, for each location on the part. The value displayed is for the planes which maximize the phase difference, i.e., the worst case.
This result is meaningful only on the sides of the part not facing the light source. The result is absent for sides of the part facing the light source because there has been no phase shift at this point.
Values larger than 0.25 of the incident light wavelength significantly affect the polarization of the transmitted light. A tolerance of 0.1 wavelength or even less is advisable in optically important sections of the part.