Focus detector which serves to split off a portion of a detected light beam only when the detected light beam is not refocused at an expected refocus point

摘要

The present invention is directed to a focus detector, one embodiment of which includes a light source, a collimator lens, a beam splitter and an objective lens. The light emitted by the light source is collimated by the collimator lens and subsequently passed through the beam splitter. The objective lens focuses the light passing through the beam splitter on a rotating disk. The light reflected by the disk passes through the beam splitter and is directed to a sensor lens. The sensor lens directs the light beam to an optical element arranged in a light path adjacent a refocus point of the light beam. The optical element is so arranged that the light beam is split only when it is not refocused at the expected refocus point.