Importance

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Abstract

An image of a scanning position on a faceplate on a light receiving region defined by an arrangement of n light receiving elements such that an amount of light received by the light receiving region becomes a peak at a center of the light receiving region in an arranging direction of the light receiving elements and is gradually reduced substantially symmetrically toward both ends thereof in the same direction. Therefore, if there is no defect in the surface of the faceplate, levels of light receiving signals of the light receiving elements arranged substantially symmetrically in position on both sides of the light receiving region with respect to the center thereof as a reference are substantially equal and there is no substantial difference therebetween. When there is a recessed or protruded defect in a scan position of the faceplate surface, the image on the light receiving region is shifted in either direction from the center by light reflected by a side slope portion of the recessed or protruded defect and the difference of the levels of the light receiving signals in symmetrical positions becomes large. The defect is detected when the difference is larger than a predetermined value.

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First Claim