I've performed some experiments for projecting conoscopic interference patterns. In some set-ups are used two circular polarizers one before and one after the birefringent crystal. I can imagine that the second polarizer will align the orientation of the wave so that they can interfere but I don't understand why the first polarizer is needed. Theoretically, I could be able to see interference even without it because the interference is due to the interaction between the extraordinary and ordinary ray. So why they usually put a first polarizer and thy call the second analyzer? Do linear polarizer and circular polarizer gives different results?
Typically the first polariser in any crossed polarisation measurement is used to obtain a clean input polarisation state. Typically the quality of the polarisation directly out of a laser will be worse than that after it has passed through the first "cleaning" polariser.