Color with birefringent retarding layers

An optical retarding layer has different refractive indices for various directions of polarization of the light. When placed between crossed polarizers it causes color brightening. These interference colors occurring with birefringence are determined by the position of the main axes of the layer relative to the direction of polarization of the light and the retardation And, where An is the difference in main refractive indices and d is the thickness of the layer. The brightest colors are obtained for... 

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