1. Technical Field
This invention relates to beam splitters and, more particularly, to parallel plate polarizing and color beam splitters for liquid crystal displays.
2. Discussion
The angle at which a thin-film MacNeille polarizer is effective is determined by the indices of refraction of three materials. The first two materials are the alternating materials that make up the thin-film reflecting stack. The third material is the surrounding medium whose index of refraction may be different than either of the two materials used for the thin-film stack.
The thin-film stack is typically made of alternating layers of a first material with a high index of refraction (n.sub.H) and a second material with a low index of refraction (n.sub.L) . At a special angle, Brewster's angle, the reflectance for P-polarized light entirely vanishes. In the low index of refraction material, this angle is computed by ##EQU1## In the high index material, the corresponding angle is the complement of .theta..sub.B. The object of the thin-film polarizer design is to use Brewster's angle to minimize the reflectance of P-polarized light while maximizing the reflectance of S-polarized light.
If the index of refraction of the external medium (n.sub.E) is not equal to n.sub.L, additional refraction takes place. The polarizing angle .theta..sub.E in the external medium can be found using Snell's Law: ##EQU2## Because the interface between the stack and the external medium does not operate at Brewster's angle when n.sub.E .noteq.n.sub.L, a small amount of reflectance of P-polarized light is generated by the interface.
High contrast also occurs when the color and polarization beam splitters are parallel. The most convenient angle for a polarizing and/or color beam splitters is usually 45.degree.. A 45.degree. prism takes less space or volume than a prism operating at a higher angle.
A beam splitter which operates at 45.degree. is impossible with a two-component system (n.sub.E =n.sub.L) for the following reason. The only solutions to the equation ##EQU3## requires that n.sub.H =n.sub.L. However, when n.sub.H =n.sub.L, there is no reflection for P-polarized or S-polarized light. Therefore, the third material for the external medium must be introduced to keep the external angle at 45.degree. while permitting the internal Brewster angle to be larger than 45.degree.. This, however, means that the index of refraction of the external medium must be larger than the index of refraction of the low index material in the stack. The only suitable external medium having a sufficiently high index of refraction and sufficiently low birefringence to allow high-contrast operation at 45.degree. is oil.
Liquid oil prisms include a thin-film polarizing stack formed on a thin plate which is placed in a tank containing oil. The oil, however, is source of additional problems. A large external medium index of refraction will normally facilitate superior polarizing, but as the index of refraction goes up, the quality of the oil usually goes down and gives rise to additional problems. The liquid oil prism provides a less than perfect match to the corresponding glass substrate. Undesirable absorption of blue light is also common. The intense beams of light traversing the liquid oil prism also cause chemical decomposition. Absorption of infrared and visible energy also increases thermally generated convection currents and pressure on the tank which requires special methods and apparatus for relief of the currents and pressure.
Therefore, it would be desirable to provide polarizing and color beam splitters which operate at an incidence angle of 45.degree. and operate in an external medium which does not have the disadvantages of index of refraction oil and liquid prisms. Preferably the polarizing and color beam splitters have high contrast.