Importance

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Abstract

An in-focus image of an information-bearing region within an optical memory medium is discriminated from an out-of-focus image so as to reduce errors in reading information represented by the information-bearing region within the optical memory medium by producing a probe beam and a reference beam from a wideband point source, producing antisymmetric spatial properties in the reference beam, converting the probe beam to a beam focused to a line in the information-bearing region, producing an in-focus return probe beam, and producing antisymmetric spatial properties in the in-focus return probe beam. Then the in-focus return probe beam is spatially filtered and passed through a dispersal element to focus it to a line in a detector plane. The reference beam is spatially filtered and passed through a dispersal element to focus it to the line in the detector plane. A beam from an out-of focus image point is spatially filtered and passed through a dispersal element. The spatially filtered reference beam is interfered with the spatially filtered beam from the out-of-focus image point and the spatially filtered in-focus return probe beam. The spatially filtered in-focus return probe beam is detected as an interference term between the spatially filtered reference beam and the spatially filtered in-focus return probe beam by means of the detector. An amplitude of an interference term between an amplitude of the spatially filtered out-of-focus image beam and an amplitude of the spatially filtered reference beam is thereby substantially reduced, and reduces errors in data produced by the detector to represent the information being read.

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First Claim