Ed -

Question. Since we are discussing diffuse light, is it the distance of the light source or the person viewing the test that is important. Consider the old standby flat tester pictured in all the telescope making texts (attachment) where a diffuse monochromatic light is housed above the piece being tested. The diffuse light shines down through a pane of glass at 45 degrees angle to the person viewing the test. This pane of glass acts as a beam splitter, so the light, after producing fringes, reflects out to the person viewing the test. Would stepping back several feet from the test set up accomplish the same thing?? I often do that when I want a quick sanity check of something I'm working on. For critical evaluation I use my Fizeau interferometer that provides collimated light.

Thanks
Dick Parker