Abstract: Active matrix screen for the color display of television images or pictures, control system and process for the production of the screen.The screen utilizes the electrically controlled birefringence effect and comprises an assembly having a nematic liquid crystal layer (2) with a positive optical anisotropy between an active matrix having transparent control electrodes (8) and a transparent counterelectrode (10) equipped with colored filters (13) and two polarizing means (12,14), which are complimentary of one another and are located on either side of the assembly. The optical path in the layer is at the most equal to approximately 0.6 micrometer. The system comprises means for amplifying the blue, green and red video signals, whereof the respective gains are regulated so as to be able to display a neutral white shade.

Abstract: Liquid crystal cell using the electrically controlled birefringence effect and processes for the production of the cell and a uniaxial medium or material of negative optical anistropy usable therein.The cell can comprise two crossed rectilinear polarizers, two glass plates provided with transparent electrodes and placed between said polarizers, a liquid crystal layer between the plates and, between a plate and a polarizer, a layer of a negative uniaxial thermoplastic polymer having its extraordinary axis perpendicular to the plates. This layer can be obtained by heating the polymer up to the isotropic state, under a uniform presure on the polarizers, cooling and eliminating the pressure. Application to the production of data display devices.

Abstract: A device for determining the contrast of a display screen as a function of the observation direction. This device comprises a first convergent objective (12) for forming the image (19) of the Fourier transform of an elementary surface (9) of a screen (6) in the image focal plane (Fi) of the first objective, a second convergent objective (14) for projecting the image of the transform on to an array (16) of detectors (18) arranged in matrix-like manner, each detector producing an electric signal proportional to the light intensity from the elementary surface in accordance with a given observation direction and a diaphragm (20) in the vicinity of the second objective and whose aperture defines a surface, which must be equal to that of the elementary surface, as well as a computer for processing the electric signal produced by each detector in order to determine said contrast.

Abstract: Cell with a double liquid crystal layer using the electrically controlled birefringence effect and process for producing a negative optical anisotropy uniaxial material usable in said cell.The cell can comprise two crossed rectilinear polarizers, three glass plates provided with transparent electrodes and placed between said polarizers, two liquid crystal layers between the plates and, between a plate and a polarizer, a sheet of the medium, whose extraordinary axis is perpendicular to the plates. This sheet can be obtained by heating a thermoplastic polymer up to the isotropic state, under a uniform pressure on the polarizers, cooling and eliminating the pressure. The cell is designed in such a way that the molecules of the respective layers tilt in two directions forming the same angle with the homeotropy direction, when an exciting voltage is applied between the electrodes, said directions, viewed in projection on a plate, also being opposite in the main observation plane.

Abstract: Liquid crystal cell having a homeotropic structure, with compensated birefringence for said structure.The cell comprises a liquid crystal layer which can have a hometropic structure and electrodes located on either side of the layer and whereof at least one is transparent. It also comprises means for compensating the birefringence of the liquid crystal layer in its homeotropic structure, so that the cell has a high contrast for said structure, in the case of an oblique observation made in a given observation plane.Application to the construction of data display means.

Abstract: Illuminating device for a large screen making it possible to uniformly illuminate said screen by means of a light arriving at a low angle of incidence .theta., wherein it comprises n.p light sources and n.p optical focusing elements in matrix form, as well as means for ensuring that the light beams from each source do not encounter the optical elements associated with the other sources.

Abstract: The invention relates to a method of controlling an optical characteristic of a material and it is concerned also with devices and image-forming means for carrying out the method.The method of control is particularly advantageous in devices comprising a plurality of points distributed in the form of a matrix. The optical characteristic K of the material is controlled by an energization having a magnitude G for a duration t, the values G and t being so selected as a function of the time response K (G, t) of the characteristic K at an energization stage of magnitude G that K (G.sub.O, t.sub.O) is equal to K.sub.O.