Surface inspection by an optical triangulation method

Abstract

A noncontact optical microtopographer based on an active, discrete triangulation procedure has been under development for some time at the physics department of the University of Minho. Our experi- ence has shown that this kind of triangulation-based surface inspection system can be applied, not only to long-distance range sensing, but also to smaller samples or other surfaces with height resolutions that can go down to the submicron range. In our system, the topographic information is obtained from the horizontal shift of 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 cam- era and the information on the individual detectors that are activated, above a certain controllable intensity threshold level, is used to compute the corresponding horizontal spot's 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 surface characterization parameters may be calculated. Our versatile laboratory setups have been used for different inspection tasks such as thickness measurements and relief mapping of different kinds of films (polyethylene, thin sputtered copper, tin dioxide and silver films) and several kinds of fabrics, and roughness measure and topo- graphic inspection of polyethylene molds and graphite samples. In this communication we briefly describe our method. The current configura- tion of our laboratory setup is described. A hand-held version of our system and also the setup for long-distance dimensional assessments are briefly presented. © 1996 Society of Photo-Optical Instrumentation Engineers. Subject terms: rough surfaces; microtopography; electro-optical sensors; rough- ness.