Optical scanning unit for tracked data carrier - has two concentric detectors sensing focussing faults in beam ring shaped by conical body or holographic lens

摘要

The unit has a radiation-sensitive detection system consisting of two detectors, one around the other. The detectors are separated by a ring. A difference in the outputs from the detectors indicates a fault in focussing. The innermost detector is ring-shaped. The beam is given a ring shape by a conical transparent body in its path between the detection system and the unit's beam splitter. Alternatively, the conical body is a halographic lens.