GB1433126A - Detection of blemishes in surfaces - Google Patents

Abstract

1433126 Counting imperfections in surfaces FERRANTI Ltd 26 July 1973 [29 July 1972] 35611/72 Heading G4D The invention relates to counting blemishes in the surface of objects, which objects have odd shapes but whose surface, or the projection of the surface on to a plane, has an axis of symmetry. Apparatus for counting such blemishes has means arranged to scan the surface, or plane, line by line and derive an electric signal representing charges in reflectivity of the surface; means, responsive to the electric signal, producing an edge pulse each time the scan crosses the edge of the surface on to the surface; means generating a datum pulse at a predetermined point in each scan where the scan crosses the axis of symmetry; and means deriving gating pulses of substantially twice the period between an edge pulse and a succeeding datum pulse to gate the electric signal into a counter which thus counts only blemishes lying between the edges of the surface. The scanning apparatus (Fig. 1, not shown) employs a light source such as a laser, a rotating multifacetted mirror drum and a photodetector receiving light scattered from the surface. In Fig. 3 the photodetector output is applied to a threshold circuit 14 which passes only large pulses corresponding to the edges of the surface. Each leading edge pulse sets a bi-stable 15 which is reset by the next datum pulse at B. The output of bi-stable 15 is fed to an integrating amplifier 16 whose output is a rising voltage during period between edge pulse and datum pulse and a falling voltage for a following equal period. A second threshold circuit 17 detects when the output D is above a small value and opens the gate 25 to counter 26. A coincidence circuit 32 compares the trailing edge pulse with the gating pulse and operates an alarm or stops the count if the axis of symmetry does not coincide with the datum pulses. In an alternative arrangement (Fig. 5, not shown) the amplifier 16 is replaced by a counter which counts up during the period between edge pulse and datum pulse and then down at the same rate, the gate 25 being open while the count is above zero.