Optical system for reflective mode video playback apparatus

Claim 1

In an optical video playback device for scanning the storage track of a video record ot the type that is reflective to a reading beam derived from a source of light energy, an improved optical system for deriving an output signal which comprises: a light detector; means for directing a reading beam along a predetermined optical path extending from said energy source to a reading plane and thence to said light detector; its back focal plane located inside said objective, positioned in said path for focusing said beam on the storage track of a record located in said reading plane and for returning energy reflected from said record track along said path to said light detector; a mirror, displaceable about a reference axis, disposed across said path with said reference axis effectively positioned in said back focal plane of said reading lens; an optical relay interposed between said mirror and said achromat objective for effectively positioning said mirror axis in said back focal plane of said achromat objective, said optical relay comprising an intermediate lens for forming a light spot to be projected by said achromat objective onto said record track and for imaging said mirror axis into said back focal plane of said achromat objective, said intermediate lens being spaced from said mirror axis by a distance equal to f 1 [(1+f 1 x)], where f 1 is the focal length of said intermediate lens and x is the Newtonian back conjugate distance specified for said achromat objective, and spaced from said achromat objective a distance equal to f 1 +x+f', where f' is the focal length of said achromat objective; and a beam-splitter also disposed across said path and spaced from said mirror in the direction of said source for deflecting reflected energy out of said path toward said light detector.