Liquid crystal cell using the electrically controlled birefringence effect and a uniaxial medium of negative optical anisotropy usable therein

A liquid crystal cell assembly using the electrically controlled birefringence effect and having spaced apart sides, one of which is exposed to an incident light, said assembly comprising:

• a nematic liquid crystal molecular layer of positive optical anistropy;
• at least two electrodes, one on either side of said liquid crystal layer, with the electrode located on said one side exposed to incident light being transparent;
• the molecules of said liquid crystal layer being substantially oriented in a homeotropic direction in the absence of a voltage between said electrode;
• two complementary circular polarizer means for polarizing said incident light located on either side of said electrodes;
• and
• at least one layer of medium for compensating the birefringence of said nematic liquid crystal layer in its homeotropic structure for improving oblique observation of said cell located between at least one of said polarizer means and said electrode adjacent thereto, said compensating layer being a uniaxial medium of negative optical anistropy having an axis of symmetry parallel to the homeotropic direction and an extraordinary axis parallel to said axis of symmetry, said medium having three principal optical indices each corresponding to an axis, one of said indices being weaker than the other two indices, and that axis which corresponds to said weak index being parallel to said homeotropic direction.