Cell with a double liquid crystal layer using the electrically controlled birefringence effect and process for producing a uniaxial medium with negative optical anisotropy usable in said cell

A liquid crystal cell using the electrically controlled birefringence effect, wherein it comprises an assembly, one of whose sides is to be exposed to an incident light and which comprises a first nematic liquid crystal layer having a negative dielectric anisotropy, a first group of electrodes having at least one first and one second transparent electrode placed on either side of the first layer, the first electrode being on said one side, a second nematic liquid crystal layer having a negative dielectric anisotropy parallel to the first layer and separated therefrom, a second group of electrodes having at least one first and one second electrode, said second layer being placed on the side of the second electrode of the first group with at least one first and one second electrode of said second group placed on either side of the second layer, the first electrode of the second group being on the side of the second electrode of the first group, at least the first electrode of the second group being transparent, wherein the cell also comprises, at least on said side of the assembly which is to be exposed to the incident light, a means for polarizing said incident light, the molecules of said first and second layers being substantially oriented in a homeotropic direction in the absence of a voltage between the first and second electrodes in each group, the axes of the molecules of the first layer being substantially parallel to a first axis when an excitation voltage is applied between the first and second electrodes of the first group and the axes of the molecules of the second layer being substantially parallel to a second axis when said excitation voltage is applied between the second and first electrodes of the second group, and wherein said cell also has means for compensating the birefringence of the assembly of the first and second liquid crystal layers in its homeotropic structure so that during the application of the excitation voltage, the first and second axes form the same angle with the homeotropic direction and are respectively projected parallel to the latter and on a plane perpendicular thereto, in accordance with two axes which have the same direction and opposed orientations and which are parallel to the main observation plane of the cell.