Parkhurst & Oliff
Method and apparatus for detecting focussing error signal of objective lens
 In order to derive a focussing error signal of an objective lens with respect to a video disc on which a light beam is focussed by the objective lens, the P-polarized light beam is made incident upon a prism and then upon a thin film which is applied to the prism and has a refractive index higher than that of the prism, and the light beam reflected by a boundary surface between the thin film and the air is detected by a photo-detector having two light receiving regions. The thin film is made of material having a higher refractive index than that of the prism. Between the prism and the thin film there may be interposed at least one additional thin film in such a manner that the lower and higher refractive index thin films are alternately arranged. 
Olympus Optical Co., Ltd.
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inventional additional inventional inventional
Kiichi Kenichi Tohru
Kato Ito Musha