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Message: <blockquote data-quote="Kevin Purcell" data-source="post: 1279555" data-attributes="member: 68323">The interferograms show the phase consistency of the light bundle coming through the optic.Neither of these two articles are that good but they give you a basic idea: take a coherent beam, split it in two, pass one through the item under test then combine the two beams to see the interference fringes.If you don't have any "device under test" in the path you should see a series of straight (vertical) fringes.<a href="http://en.wikipedia.org/wiki/Interference" target="_blank">http://en.wikipedia.org/wiki/Interference</a><a href="http://en.wikipedia.org/wiki/Interferometer" target="_blank">http://en.wikipedia.org/wiki/Interferometer</a><a href="http://en.wikipedia.org/wiki/Michelson_interferometer" target="_blank">http://en.wikipedia.org/wiki/Michelson_interferometer</a>My interpretation:Fig 6: nice vertical regularly space fringes through the glass sample. The sample is homogeneous at the measurement wavelength.Fig 7: interferogram is twisted and contorted through the glass so all the rays in the bundle do not have the same phase delay.Not all optical glasses are created equally. Note that I suspect that if you looked through these glass blocks with the naked eye you wouldn't see a difference. I suspect you might not even see a problem in porro prism design either (as you on't need phase coherence for that to work properly) but it makes a difference in a roof prism where the input beam is split in two and recombined. That's why you need phase correction. But phase correction won't do any good if the beams coming though the prism alrady have their phase "mangled" by the glass.Figure 13 and 14 are views with two different Schmidt-Pechan prisms in the light path. You can see the outline shape of the prism (take a look at the WP article for a diagram) with the roof to the right. Figure 13 shows a phase-corrected prism where the phase-correction is working. The output beam shows consistent slight fringes across the exit phase of the prism. OK, the fringes you can see are very slightly bent. I think in this case they've tuned the delay in the reference path so they show minimum fringes (if this was perfect you'd cancel out the fringes on the image through the prism would be black). By doing this you can see the deviations from perfect phase. So you can see the phase is uniform across the prism.Figure 14 shows an uncorrected roof prism. You can see bright fringes from one side of the roof and dim fringes from the other side of the roof so the light passing through the prism does not have the same phase delay for each side of the roof (for the path twisting to the right and the path twisting to the left). If you change the delay in the reference path (i.e. change the phase of the reference beam) you'd see the bright pattern dim and the dim pattern brighten until eventually the upper part would be bright fringes and the lower part are dim. The phases are different.Note the MTF measuresments too: the contrast increases at higher spatial frequencies with phase correction. The phase correction is (most) needed to improve resolution when the "lines" in the "resolution chart" are perpendicular to the roof edge direction.<a href="http://en.wikipedia.org/wiki/Optical_transfer_function" target="_blank">http://en.wikipedia.org/wiki/Optical_transfer_function</a>One interesting point is I've looked a quite a few roof prisms bins to see if I can see the roof edge. Sometimes you can; sometimes you can't. I would hope you couldn't see that edge. The sharper the knife-edge of the roof is the more difficult it should be to see (but at the lower end of bins I find the correlation is rather weak). And the sharper the edge the less it should scatter (rather than reflect) messign up overall contrast (another reason why cheap roof look worse). I noted the orientation of the roof isn't a standard direction. I've not seen one where the roof is either vertical or horizontal (a feature perhaps to avoid messing up vertical or horizontal lines which we have in the built environment). And of the ones I've seen the roofs are at 90 degrees to each other in each barrel. I suspect this is a deliberate effort to compensate for the reduction in resolution: when one eye is "good" then the other is "bad". Of course it doesn't quite work when you have a dominant eye (which I think almost everyone has).  But every little helps.BTW, I suspect the images were a lot better in the original publication ... I think this might be a scan of a photocopy.The more I understand about roof prisms the more I appreciate porro prisms bins <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite2" alt=";)" title="Wink    ;)" loading="lazy" data-shortname=";)" /></blockquote>

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