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For some years we are dealing with the development of inspection's systems able to cope in a simple accurate and reliable way with the specific requirements herein involved. Our systems are based on a discreet triangulation procedure where the topographic information is obtained from the horizontal shift incurred by the bright spot created, by an oblique collimated light beam, on a surface when it is displaced vertically. A laser beam is focused onto a small, diffraction limited, spot on the surface and is made to scan it over the desired region. The sample is placed on XY high accuracy step motion table to perform the scanning of the sample' areas of interest. The bright spot is perpendicularly imaged onto a photo-sensitive detection system and its position obtained. The corresponding horizontal spot's shift on the reference plane is then computed. At each sampled point the distance between the surface and a reference plane is then easily calculated allowing the calculation of statistical surface characterization parameters and the tridimensional reproduction of surface's relief. The decision about the right photo-sensitive detection system is our major concern in this communication. Special attention is paid to the use of CCD's linescan cameras and of the so called PSD's identifying the specific inspection situations where each one represents the best choice. |
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