Optical measurement of waviness on specular surfaces by Fringe Reflection Technique, FRT

The need for accurate measurements of long range low amplitude topographic structure of specular (glossy) surfaces has been increased in automotive and aircraft industries. Optical measurement techniques are the most successful techniques to measure different surface structures with high resolution and high accuracy and at high speed. However, for glossy, specular surfaces many commercial techniques based on surface scattering fail. Fringe Reflection Technique (FRT), mimicking the human observation process of specular surfaces, is then a robust and suitable solution for measuring these surfaces. In our research we are interested in measuring waviness in the micron range over cm long spatial wavelengths. The artefact investigated is a flat casted and painted composite surface. By using a simple and non-expensive FRT-setup, combined with image analysis algorithms high resolution data were obtained. The results match well with reference data obtained by a Coherix interferometer with height measures of some ten micrometers over waviness having a lateral structure of several cm.