Missing order defect detection apparatus

Claim 1

A flaw detection apparatus for detecting defects in a record surface having uniformly spaced track convolutions said apparatus comprising: first means for providing a beam of light directed toward a region of the surface of said record along an incident beam path; second means for focusing said beam of light to illuminate a region of said surface, said illuminated region being sufficiently large to span a plurality of tracks; the structure of the track convolutions in the surface region illuminated by said light beam serving as a diffraction grating for diffracting the light from said illuminated region to form a zero diffraction order beam of light and higher diffraction order beams of light, and wherein short wavelength variations in track depth exist along the length of said track convolutions, the structure of said track convolutions and said short wavelength variations serving as a two-dimensional diffraction grating for diffracting the light from the illuminated region into said zero diffraction order beam of light, said higher diffraction order beams of light, and into additional higher diffraction order beams of light which are substantially affected by said short wavelength variations; third means for establishing relative motion between said surface and said beam path in such a manner that a succession of regions of said surface are scanned by said light beam; fourth means for directing one of said higher diffraction order beams, to the exclusion of said additional higher diffraction order beams, to a first location; light detection means having a photosensitive surface positioned to intercept said one higher diffraction order beam at said first location and to provide a detection signal in response to said one higher order diffraction beam; and fifth means, responsive to said detection signal developed by said light detection means, for indicating illumination of a defect in said surface.