Self-compensating focus system for a reflective video disc

Claim 1

A self-compensating focus system for an optical video playback device for reading a program record which is reflective to a beam of energy, said record having one surface that is a plain reflector and an opposite surface through which a reading beam may gain access to information stored in said record, said system comprising: means, including a first objective lens disposed across a first optical path, for focusing a reading beam of energy on said one surface of said record; means, including a plurality of at least partially reflecting surfaces, for directing energy reflected from said one surface of said record and returned through said first lens along a second optical path to an intermediate focus and then to the opposite surface of said record; means, including a second objective lens disposed across said second optical path, for imaging said intermediate focus onto said opposite surface of said record, the magnification factor of said first lens being related to the magnification factor of said second lens so that changes in the spacing of said one surface relative to said first lens produce compensating changes in the spacing of said intermediate focus relative to said second lens so that said reading beam remains focused on said opposite surface of said record; and means responsive to energy from said reading beam reflected from said opposite surface of said record for developing an output signal representing information stored in said record.