Defect detection system

Abstract

Defect detection apparatus, for optically inspecting a spiral groove of a video disc record, directs a coherent light beam at the grooved surface of the disc. The incident beam, focused at a point beyond the disc surface, illuminates the grooved surface with a light spot that spans a plurality of convolutions of the groove. Relative motion is established between the disc surface and the incident beam in a manner causing the illuminating spot to rapidly scan the groove surfaces in a coarse spiral pattern. The structure of the illuminated groove convolutions, absent any defects, serves as a diffraction grating for diffracting the light into an undeviated zero diffraction order cone of light that converges at a first location in a plane spaced from the disc surface and into deviated higher diffraction order cones of light that converge at additional locations in said plane separated from the first location. A photodetector is positioned to be in registry with the first location and spaced from said additional locations. A blocking means is placed over a central region of the photodetector to normally intercept the zero diffraction order light, thereby preventing the conversion of light energy to electric energy by the photodetector when defect-free groove regions are illuminated. When a defect exists in the illuminated region of the groove, unblocked regions of the photodetector receive light and the photodetector produces electrical signals indicative of the presence of a defect. A servo means, responsive to departures of the zero order cone axis from a desired orientation, is employed for controlling the orientation of the axis of the incident beam to oppose such departures, in order to preclude false defect indications due to warpage or unevenness of the disc record.