Optical distance measuring for optical storage automatic focussing - using additional source and lenses for accuracy and sensitivity

摘要

An optical distance measuring system for automatic focussing in optical storage systems controls an objective which focusses a light beam onto a storage plate over variable distance. It is designed to be simply constructed, very accurate, and highly sensitive and to avoid the signal-to-noise, measurement sensitivity and incidence direction stabilisation problems associated with semi-transparent reflector main beam systems. The system uses an additional light source, two lenses and a position sensitive photodiode arranged to produce a light spot in the objective focal plane or in the storage plane. One lens forms the spot image on the photodiode so that it moves with distance variation between plate and objective and the photodiode produces a position signal.