Mid-wave infrared 3D sensor based on sequential thermal fringe projection for fast and accurate shape measurement of transparent objects

Structured light sensors based on diffuse reflection of projected patterns are widely used to measure the 3D shape of objects, e.g., in industry, medicine, or cultural heritage. Unfortunately, there exist many objects made of uncooperative materials, i.e., materials with optical properties such as being glossy, transparent, absorbent, or translucent, which cannot be measured reliably by this measurement technique. In the last years, we presented a two-step method based on thermal pattern projection which allows the determination of the object surface of these uncooperative objects. In the first step, a multi-fringe thermal pattern is projected onto the measurement object. In the second step, a mid-wave infrared (MWIR) stereo camera records the thermal radiation that is absorbed and re-emitted by the object surface. This system allows us to measure transparent objects. However, the measurement time is with tens of seconds up to minutes quite long and the measurement accuracy should be improved. In this contribution, we present a projection principle of a fast sequentially scanning fringe leading to a significant reduction in measurement periods down to the second range or even below while increasing the measurement accuracy. The work includes a characterization of our MWIR 3D setup for both projection principles, the multi-fringe and sequential fringe one, regarding the measurement accuracy and speed. We show measurement results obtainted with both projection techniques for an object made of different material classes.