Optical focussing-error detection system

Claim 1

An optical system for detecting deviations between an image plane of an objective system and a reflective surface on which an image is to be formed by the objective system, said apparatus comprising means for producing a beam of radiation which is directed along a first path through the objective system and onto the reflective surface, at least a portion of said beam incident on said reflective surface being reflected thereby back through the objective system along a portion of said first path, means disposed in said portion of said first path for separating the reflected radiation from said beam passing along said first path towards the reflective surface, said separating means including an optical element which both diverts said radiation reflected by said reflective surface along a second path different from said first path and introduces astigmatism solely in said reflected radiation diverted along said second path, and radiation responsive means disposed in said second path for producing a signal indicative of the deviation between said image plane and said reflective surface.

Claim 9

An apparatus for reading a record carrier having a radiation reflecting information structure containing information in a form readable by a beam of radiation, said apparatus comprising means for producing a read beam of radiation, an objective system for focussing said read beam to a spot on said information structure so that the radiation is modulated by said information structure in accordance with information stored thereby, at least a portion of the modulated radiation being reflected by the information structure back through said objective system, means for separating the reflected radiation from said read beam traveling towards said record carrier, said separating means being disposed between said objective system and said beam producing means in the path traversed by said read beam and said reflected radiation and including an optical element which both diverts said reflected radiation along a second path which is different from the path traversed by said read beam traveling towards said record carrier and introduces astigmatism solely in said reflected radiation diverted along said second path, and radiation responsive means disposed in said second path, said radiation responsive means producing a signal indicative of deviation of said information structure from a predetermined position in response to variations in the shape of said second spot due to the astigmatism introduced by said optical element.

Claim 10

An apparatus for reading a record carrier having a radiation reflecting information structure containing information in a form readable by a beam of radiation, said apparatus comprising means for producing a read beam of radiation, an objective system for focussing said read beam to a spot on said information structure so that the radiation is modulated by said information structure in accordance with information stored thereby, at least a portion of the modulated radiation being reflected by the information structure back through said objective system, means for separating the reflected radiation from said read beam traveling towards said record carrier, said separating means being disposed between said objective system and said beam producing means in the path traversed by said read beam and said reflected radiation and including an optical element which both diverts said reflected radiation along a second path which is different from the path traversed by said read beam traveling towards said record carrier and introduces astigmatism solely in said reflected radiation diverted along said second path, and radiation responsive means disposed in said second path, said radiation responsive means producing a signal indicative of deviation of said information structure from a predetermined position in response to variations in the shape of said second spot due to the astigmatism introduced by said optical element.

Claim 12

An apparatus as claimed in claims 8 or 9 wherein said radiation responsive means includes a radiation sensitive detector comprised of four sub-detectors.