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 peripheral light fluxes reflected by the video disc, and the peripheral light fluxes reflected by the reflection surface are separately received by two light receiving regions which are spaced apart from each other. The focussing error signal is derived as a difference between output signals from the two light receiving regions. 
Olympus Optical Co. Ltd.
G G G
G11B G11B G11B
inventional inventional additional
Kenichi Kiichi Tohru
Ito Kato Musha