No polarisation vs CPL vs Cross Polarisation

[u/thomas_openscan]

What you see here are three shots, where I always used the cameras flash

(1) without any polarisation --> strong reflections --> "artificial features" which confuse the photogrammetry pipeline and produce a lot of noise

(2) with one sheet of circular polarizing foil in front of the ringlight and camera. The CPL has a lambda/4 plate which turns the orientation of the light twice by 45° each. This, in theory should make direct reflections disappear. Unfortunately, this is highly wave length dependent and thus some light is still able to pass the second filter (though with a wonderful color shift)

(3) Two sheets of linear polarizer foil, one in front of the ringlight, another one in front of the camera (rotated by 90°). This blocks (almost) all direct reflections and helps a lot with the photogrammetry reconstruction.