Phase Correction
The multiple internal reflections cause a polarization-dependent phase-lag of the transmitted light, in a manner similar to a Fresnel rhomb. This must be suppressed by multilayer phase-correction coatings applied to one of the roof surfaces to avoid unwanted interference effects and a loss of contrast in the image.
In a roof prism without a phase correcting coating s-polarized and p-polarized light each acquire a different geometric phase as they pass through the upper prism. When the two polarized components are recombined interference between the s-polarized and p-polarized light results in a different intensity distribution perpendicular to the roof edge as compared to that along the roof edge. This effect reduces contrast and resolution in the image perpendicular to the roof edge. Giving an inferior image compared to that from a porro prism erecting system. This roof edge diffraction effect may also be seen as a diffraction spike perpendicular to the roof edge generated by bright points in the image.
This effect can be seen in the elongation of the Airy disk in the direction perpendicular crest of the roof as this is a diffraction from the discontinuity at the roof crest.
The unwanted interference effects are suppressed by vapour depositing a special dielectric coating known as a phase-compensating coating on the roof surfaces of the roof prism. This coating corrects for the difference in geometric phase between s- and p-polarized light so both have effectively the same phase shift and no interference degrades the image.
Read more about this topic: Roof Prism
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