High speed fringe projection for fast 3D inspection

Abstract

Fringe projection techniques have been widely used for inspection of free form surfaces for quality inspection or reverse engineering purposes. For inline 3D-inspection systems maximum measuring speed is of vital interest. Typically, image acquisition and processing rates of up to 10'000 frames/s are state of the art. In order to exceed this value, we propose a fringe projection concept which uses a high speed CMOS camera with in pixel phase calculation. The camera can record up to 1 million frames/s. An analogue calculation is realized in every pixel to extract the phase of the temporarily modulated light. In order to determine a phase, the illumination light must be modulated with a quarter of the frame rate of the image acquisition device, in our case with up to 250 kHz. In fringe projection techniques, the projected fringes must be shifted with respect to the inspected surface. Mechanical phase shifting of the fringes becomes the crucial problem in ultra high speed fringe projection. We have investigated a new way to generate 250 kHz phase shifted fringes. In this paper, we present the new fringe projection technique and discuss the results of our high speed 3D measuring device.