Proceedings PaperThree Laser Scanning Instruments For Automatic Surface Inspection
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Three laser scanning instruments will be described for the inspection of surfaces; particular emphasis will be put on the components to provide special optical features. The first instrument is a pinhole detector for strip materials which can also be used for surface inspection. Two independent alternate scan lines are produced to highlight holes of various angles within the strips. A large, efficient, light guide collection system collects the transmitted light, and an acousto-optic modulator reduces the laser beam power when the scan is beyond the edges of the strip. An ancillary channel of the instrument collects part of the specular light reflected from the strip surface of one of the two scans and, by means of a double reflection at the inspected surface, such effects as gross tilt and heave of the strip do not affect the inspection process. Another strip inspection instrument using two scan lines is employed to detect all classes of defect in clear strip, such as glass or plastic. The two scan lines in this case are created simultaneously and are arranged to scan, on the far side of the strip, two linear gratings which are arranged to be exactly out of phase with each other. The scan of the light transmitted by the gratings should thus normally be constant and any deviation from this indicates the presence of defects in the strip. The third instrument has been designed to inspect the internal surfaces of cylinder bores for surface defects, the position and size of ports and surface finish. Two versions will be described, one using a probe containing a single clad optical rod and a second with a more complex fibre optic assembly.