The contrast ratio and color shift are very important optical properties in Liquid Crystal Displays (LCDs).  These quantities can be improved using optimized compensation films. However, we found there is a limit to those optical properties even though the films are designed perfectly. The limit, discussed in this paper, is found to be due to internal reflections and resulting interference in the LCD.

              LCDs have many isotropic and anisotropic layers, and the incident light is transmitted and multi-reflected at their interfaces. This results in two types of the interference. The first is the interference of the e-ray and o-ray by themselves, and the second one is the interference between e-ray and o-ray. The first type has higher frequency in the wavelength space, and is related to the optical path lengths of the e-ray and o-ray independently. The second type has a lower frequency, and depends on the residual birefringence of the black state. So, as the residual birefringence increases, there is a greater contribution from the second type of interference.

              Considering the optical properties of the LCD, the first type interference could affect the black level almost equivalently for all visible wavelengths, so it does not affect the color of the black state. On the other hand, the second type could cause a color shift because of the lower frequency pattern of the interference in the wavelength space.

              As an example, consider the case of a 5.53micron cell with a residual retardation that is a function of wavelength but has a value of 10 to 300nm at 550nm wavelength, that is compensated exactly at all wavelengths. If we consider an index mismatch of 0.5, 0.66 for o-ray and e-ray respectively, the calculated transmittance curves are shown below. From these results, for the case of a residual retardation of 100nm, the estimated maximum contrast ratio is around 500:1, and there is a blue shift in the coloration.