Parkhurst & Oliff
Method and apparatus for detecting a focussing condition of an objective lens
 A method and apparatus for detecting a focussing error signal of an objective lens with respect to a video disc on which a light beam emitted from a laser light source is to be focussed as a light spot by the objective lens. A light flux reflected by the video disc is made incident upon a detection prism surface which is set substantially at a critical angle with respect to a central light ray in the reflected light flux, and two light fluxes 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 central light ray 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. A relay lens is arranged between the reflection surface of the detection prism and the light receiving regions and the relay lens forms an image of a pupil of the objective lens on the light receiving regions. 
Olympus Optical Co. Ltd.
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G11B G11B G11B
inventional additional inventional
Kenichi Kiichi Tohru
Ito Kato Musha