Optical focussing device
AssigneeThomson-Brandt
InventorsClaude Bricot, Jean Claude Lehureau
Life of this patent
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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.
Claims7
A device for focussing a read-out light beam onto the reflective surface of a moving data carrier, said device comprising an objective lens which causes the read-out beam to converge upon said reflective surface, optical guidance means for guiding, towards an observation plane, a reflected beam consisting of at least one fraction of said read-out beam reflected by said reflective surface;
- optical means arranged in the path of one of said beams for introducing astigmatism on the optical axis of said objective lens;
- said reflective surface and said observation plane being located respectively in first and second imaging planes;
- said beams respectively projecting first and second light spots in said first and second imaging planes;
- photodetector means being positioned in said observation plane for detecting the shape of the second of said light spots;
- said photodetector means producing a control signal, controlling the focussing of said read-out beam.
Dependencies are inferred from the claim text.