Parkhurst & Oliff
Method and apparatus for reproducing optically an information recorded on a record medium along tracks
 In a method and apparatus for detecting an information signal and a focussing error signal of an objective lens with respect to a video disc on which main and sub beams emitted from a laser light source are to be focussed as light spots by the objective lens, the main and sub beams reflected by the video disc are made incident as parallel light fluxes upon a detection prism surface which is set substantially at a critical angle with respect to the main beam, but at an angle smaller or larger than the critical angle with respect to the sub beam, and two light fluxes of the reflected main beam which situate on respective sides of a boundary plane including the central light ray and perpendicular to a plane of incidence, and are reflected by the reflection surface are separately received by two light receiving regions which are divided along a boundary plane including the main beam reflected by the reflection surface and perpendicular to the plane of incidence. The focussing error signal is derived as a difference between output signals from the two light receiving regions and the information signal is derived as a sum of the output signals from the two light receiving regions. The sub beam transmitted through and refracted by the reflection surface is received by a light detector and a tracking error signal is derived by detecting a variation in a light amount of the sub beam impinging upon the light detector. 
Olympus Optical Co. Ltd.
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Tohru Kiichi Kenichi
Musha Kato Ito