Apparatus for reading a flat reflecting record carrier with autofocusing means

Claim 1

Apparatus for reading a flat reflecting record carrier on which information is recorded in the form of an optically readable track structure, comprising a radiation source emitting a read beam of radiation, beam splitting means for dividing said read beam into a read sub-beam and an auxiliary sub-beam spaced from said read sub-beam and propagating in substantially the same direction therewith, an objective system means for focussing said read sub-beam on the track structure of said record carrier, radiation sensitive signal detection means for converting the read sub-beam as modulated by the information in the track structure into an electrical signal, an opaque member in the path of the auxiliary sub-beam between said beam splitting means and said objective system means, said opaque member being provided with an aperture arranged outside the optical axis of said objective system means, the aperture limited portion of the auxiliary sub-beam passing through said aperture thereby passing excentrically through said objective system means to said record carrier and reflecting off the surface of said record carrier to pass again through said objective system and to be converged thereby at a point in space, said point being at a predetermined location in response to a desired distance between the record carrier and the objective system, and a pair of radiationsensitive elements symmetrically arranged about said predetermined location and secured to said opaque member.