Surface inspection by optical triangulation.pdf

Surface inspectiofl b optical triangulation Manuel F. M. Costa Universidade do Minho, Departamento de FIsica Tel. +351 53 604327(/20); Fax. +351 53 604339(/92) P-4719 BRAGA CODEX PORTUGAL ABSTRACT We will descnbe an optical non contact microtopographer based on an active, discrete triangulation procedure, showing that this kind of triangulation based surlace inspection system can be applied, not only to large distance range sensing, but also to smaller samples or smother surfaces with resolutions that can be driven down to the sub micron range. In our system 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 horizontal position of the bright spot is perpendicularly imaged onto a linescan camera and the information about the individual detectors that are activated, above a certain controllable intensity threshold level, is used to compute the corresponding horizontal spots shift on the reference plane. Thus we can compute the distance between the surface and a reference plane at each sampled point. A map of the surface topography can then be built and statistical surtace characterization parameters may be calculated. Our laboratory setups configuration is quite versatile and it has been used for different inspection tasks like: thickness measures and relief mapping of polyethylene films and thin sputtered cooper, tin dioxide and silver films, several kinds of fabrics; and roughness measure and topographic inspection of polyethylene molds and graphite samples. Presented will also be the portable version of our system and also the setup for large distance dimensional assessment. 1. INTRODUCTION In modem manufacturing quality control requirements are stringent and the subjective, although valuable, human visu