Achromatic depth-of-field correction for off-axis optical system

Claim 1

An optical system for projecting an image of a lightreflective subject having an effective principal plane onto a viewing screen, comprising: means for directing a beam of unmodulated, collimated white light onto said subject, said directing means having an optical axis nonorthogonally disposed relative to the effective principal plane of said subject: light-projecting means, including a lens having its principal plane substantially parallel to the plane of said screen and angularly disposed relative to the effective principal plane of said subject, for receiving subject-modulated light reflected from said subject and directing it toward said screen; and means for substantially achromatically orthogonalizing said subject-modulated light relative to the principal plane of said light-projecting lens, said orthogonalizing means comprising a prism located adjacent said subject in the paths of both said beam of unmodulated, collimated white light and said subjectmodulated light, said prism having a rear planar surface closely adjacent to said subject and oriented to be angularly disposed relative to the principal plane of said light-projecting lens, and a front planar surface angularly disposed relative to the principal plane of said light-projecting lens as well as angularly disposed relative to the plane of said rear prism surface, the planes of said front and rear prism surfaces converging at a location on the opposite side of the optical axis of said light-projecting lens from the location of the optical axis of said unmodulated light directing means.