Abstract: The invention relates to focussing sensors for measuring the deviation between the point of convergence of a beam of radiant energy and the surface of an object receiving this beam. According to the invention, a degree of asymmetry is first introduced into this beam, after which the image of this convergence point if formed on photoelectric sensors enabling the distortions of this image to be measured. These sensors supply signals which are dependent upon said deviation and enable it to be measured.This invention relates to optical focussing sensors by which it is possible to measure the focussing deviation of a beam of radiant energy of which the point of convergence has to be kept in coincidence with the surface of an object which it is desired to illuminate by a spot.

Abstract: The invention relates to the optical reading of a support comprising a track along which is arranged a series of information items which optically translate a carrier wave angularly modulated by the information. The read-out system uses a single lens for projecting a quasi-punctiform spot onto the support; a photodetector comprising four cells arranged in a square and asymmetrical illumination of the lens to ensure detection of the information and simultaneous supply of defocussing and tracking deviation signals.

Abstract: A flexible record disc engraved in the form of impressions along a track located in a ring-shaped zone is protected with a second flexible disc adhering at its central portion to the non-engraved part of the record disc. In the case where read-out is carried out on a stabilizing air cushion, the air remaining between the two discs is displaced by centrifugal force, so that good conditions of optical read-out are achieved.

Abstract: This invention relates to a projection device of which the optical device having a focal length is variable under the effect of an electrical signal and comprises a hollow transparent lens into which a liquid nematic crystal is introduced. Under the effect of the electrical signal applied between two transparent electrodes respectively arranged on the two inner surfaces of the lens, the index of the liquid crystal for the direction of incidence of the projected beam is capable of varying as a result of the oscillation of the molecules of the liquid crystal, the position of the focussing point of the projected beam varying in consequence.

Abstract: The present invention relates to the protecting against scratching and fouling of a data carrier which carries a relief impression readable optically by transmission. The data carrier in accordance with the invention is protected by a transparent protective layer with substantially smooth faces, which adheres to the peaks in the relief of the impression in such a way as to prevent the troughs in said relief from being filled up.

Abstract: The present invention relates to the protection of relief impressions formed in order to store information on a data carrier which can be optically read out by transmission. The invention relates to a data carrier the relief impression in which is protected by at least one layer transparent vis-a-vis the read-out radiation. The protective layer has an index of refraction that differs from that of the data carrier, per se. Thus, a refractive boundary at the relief impression is maintained and the information encoded in the diffractive relief pattern is preserved.

Abstract: A device for detecting the error in focussing a read-out beam on the impression. Two photodetector cells located respectively in front of and behind the optical axis of the system, in relation to the direction of transfer, produce two electrical signals the difference between which is the read-out signal, a control signal being obtained by a comparison of the phases of the two signals, one of which is phase-shifted so that the optimum focussing point corresponds to zero phase-shift.

Abstract: A device for focussing a read-out light beam on a moving data carrier. It comprises an objective lens which causes said read-out beam to converge on the data carrier; a cylindrical lens arranged in the path of a light beam reflected by the data carrier; and photo-electric means for detecting the reflected beam and arranged in such a fashion that the light spot obtained there, normally substantially circular in shape, is distorted by elongation if focussing of the read-out beam on the data carrier is incorrect.

Abstract: The present invention relates to the optical read-out of a diffractive track carried by a data carrier. The object of the invention is an optical read-out device in which concentrated illumination of the track is effected by at least two collateral read-out spots straddling same, in order to produce a track eccentricity signal by subtraction of the voltages furnished by two groups of photo-electric transducers sensitive to the spread in the beams which form said spots.