Self-compensating focus system for optical video playback device

Claim 1

In a self-compensating focus system for an optical video playback device which includes a source of beam energy for reading a record having information stored in a track disposed thereon, said record having one surface that is a reflector and an oppositely disposed surface through which said reading beam may gain access to said track, the improvement which comprises: means, including a plurality of mirrors, at least one of which is a semi-transparent mirror, for establishing an optical path between said reflector surface and said opposite surface for directing energy reflected from said surfaces; means, included in said optical path, comprising a first lens having a predetermined focal length for focusing said reading beam of energy on said reflector surface of said record; a second lens having a predetermined focal length, also included in said optical path, for focusing beam energy reflected from said reflector surface onto said track, said first and second lenses being spaced apart a distance equal to the sum of their individual focal lengths to form an afocal optical relay for focusing said beam energy reflected from said reflector surface onto said track, the ratio of the numerical aperture of said second lens to the numerical aperture of said first lens being substantially equal to √2, and photoreceptor means disposed adjacent said semi-transparent mirror and responsive to beam energy reflected from said track for deriving an output signal representative of the information stored in said track.