Simple laboratory set-up for the fringe-projection method

Abstract

In recent years, optical fringe-projection and other optical interferometric techniques for surface profiling have received much attention because they are whole-field and non-contacting; very high data processing speeds can be achieved using computer image-processing techniques. These advantages over many other mechanical probe-based techniques are particularly useful for the measurement of large surfaces as well as for micro-systems at the sub-micron level. In the fringe-projection technique, a reference optical grating is first generated and then projected onto the surface of interest. For a given optical set-up, the distribution of the reference grating is perturbed in accordance with the profile of the test surface, thereby enabling direct derivation of surface profiles from measurements of the perturbed fringe distribution. The reference gratings are readily generated with a Michelson interferometer, which uses a beam-splitting cube and mirrors - these optical elements are readily available in all laboratories. A major drawback of this technique is the need for good vibration isolation, as otherwise unstable fringes will be generated. Alternatively, beam-splitting cubes with coated reflective surfaces can be used, but this would not allow adjustment of the frequency of the generated fringes. This paper describes a very simple method of generating and projecting optical grating for surface profiling. The working principle is based on the reflection-refraction of a commercial beam-splitting cube. By carefully adjusting the orientation of the laser beam, the frequency of the grating can be varied. A distinct advantage of this method over the Michelson interferometer lies in its ability to generate stable carrier fringes under lax vibration isolation conditions.