Three-dimensional measurement of specular free-form surfaces with a structured-lighting reflection technique

A structured-lighting reflection technique was developed in order to detect and measure small wavinesses and curvature defects on specular free-form surfaces. It can reconstruct the 3D relief of specular free-form surfaces and display the curvature at each point. A calibrated camera observes the reflection of a retro-illuminated LCD panel through the free- form surface. The use of a coded lighting technique and the knowledge of the setup geometry allow to locate each observed point on the LCD panel. Using the principle of inverse ray tracing, a surface modelled with Bezier polynomials is fitted to the observed data. Unlike structured-lighting projection techniques which are directly sensitive to the topography of the surface to be inspected, the structured-lighting reflection techniques are essentially sensitive to the gradient and thus enable the detection and measurement of curvature defects which are imperceptible using the projection techniques.