On the gamut of colors seen through birefringent airplane windows.
Colors in birefringent airplane windows seen through polarizing filters are not especially pure nor are all colors equally probable. This is a consequence of the shape of transmission vs wavelength for a retarder-polarizer combination illuminated by partially polarized skylight. Although transmission can have a single peak in the blue or green, a peak in the red is accompanied by one in the blue. This and the general blueness of skylight are why the purest colors seen in airplane windows are greens, yellowish-greens, and purples (mixtures of red and blue). Purer colors and a wider gamut are obtained for birefringent sheets interposed between dichroic polarizing filters and illuminated by tungsten light.
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