Color and shape measurement with a three-color photometric stereo system

The purpose of our research has been to develop a system for measurement of microtopography based on the photometric stereo principle. In a previous system, in which we used two illumination directions facing each other, we had difficulties in detecting topography variations perpendicular to the illumination direction. With the new measurement system we avoid this problem by use of three colored and two white illumination modules. The integration problem that occurred when the gradient was known in more than one direction was solved by use of weight functions in the spatial-frequency domain. The results show that true three-dimensional surface height functions can be obtained with the new method. This is a significant improvement from the two-source system, in which we could make only estimates of profiles perpendicular to the illumination direction. To quantitatively evaluate the new measurement system, the results were compared with measurements of a mechanical stylus instrument. Comparisons between mechanical and optical measurements show a coefficient of determination r2 between 0.71 and 0.81 for the new system and r2 between 0.57 and 0.88 for the two-source system.