| 摘要 |
Apparatus for reading optically detectable indicia on a record carrier, such as video information which is recorded in the form of pits on a record medium, for example, a video disc. A source of coherent light transmits a beam to the record carrier. A hologram is interposed between the coherent light source and the carrier, the hologram serving to focus the beam on the record carrier. The intensity of the focussed beam is modulated by the recorded indicia, and at least one photodetector is disposed in an optical path traversed by the modulated beam so as to detect the intensity thereof. In a preferred embodiment, the hologram focusses the beam onto the surface of the record medium whereat it is modulated and reflected therefrom to the photodetector. Plural photodetectors may be provided in separate optical paths traversed by the reflected light so as to produce outputs which, upon comparison with each other, represent deviations, or tracking errors, in the location of incidence of the light beam on the record medium. In yet another embodiment, the hologram directs the coherent light to the record medium along two different optical paths so that the reflection of one beam can be used to read out the recorded information and the reflection of the other beam can be used to detect focussing fluctuations. In a still further embodiment, another hologram is used to receive and direct reflected light to the photodetector.
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