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Beam splitting with polarizing cube - better than with variable density wheel (better stability and less interferences)
At center: half-wave plate, made from LCD display ripped from 1982 "Donkey Kong" Nintendo game. According to my measurements, it has just a few percent loss (from reflections) and at least 100:1 extinction ratio, which is more than enough for suppressing interferences with the film at Brewster angle. I checked it to work equally well at both 514nm and 633nm. It is mounted at 45 degree angle and so rotates the (reference) beam into horizontal polarization.
At right: another half-wave plate, which can be rotated in order to smoothly change the reference/object beam ratio.