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i found a reasonably succinct explanation of the maltese cross i saw when looking through a Pockels cell with crossed polarizers. for some reason, the introductory optics texts i've seen don't cover it.

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Glassblowers use crossed polarizers to see if there's residual strain in a piece of glassware, since it'll change the polarization of the light.

yeah, looking at large windows through a polarizing filter (or even polarizing sunglasses) can be a somewhat unnerving way to visualize stress birefringence. (atmospheric scattering or Brewster's angle reflection provides the (partially) polarized source.) a Pockels cell uses a uniaxial crystal with faces cut perpendicular to its optic axis, so the maltese cross pattern is (in retrospect) expected because i was using a non-collimated white light source. hardened eyeglass lenses are supposed to show the pattern as well, due to deliberately induced frozen stresses, but i'd have to dig around a bit to find a pair to test.

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