Optical focussing device

Claim 1

A device as claimed in claim 1, wherein said photo detector means comprise four adjacent photoelectric cells 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; four adjacent photoelectric cells being positioned in said observation plane for detecting the shape of the second of said light spots; electrical means processing the electrical signals furnished by said photoelectric cells for producing a control signal, controlling the focussing of said read-out beam; said electrical means comprising an adder having four inputs respectively supplied from the respective outputs of said photo-electric cells; the output signal of said adder being representative of the information recorded on to said data carrier.