Thermal single-shot 3D shape measurement of transparent objects: optimization of the projected statistical LWIR pattern

Fast and non-contacting 3D shape measurements of objects for quality assurance, human machine interaction, or robot handling, e.g., in the industrial sector, have become well established. Recently, we have successfully combined thermography and triangulation to tackle the challenge of measuring the 3D shape of uncooperative materials, i.e., materials with optical properties such as being glossy, transparent, absorbent, or translucent. Therefore, we have developed the principle of thermal 3D measurements, a two-step process consisting of (1) the projection and absorption of projection patterns in the thermal infrared and (2) the stereo recording of heat patterns re-emitted by the object surface. We match image points by evaluating the temporal normalized cross correlation between pixels in both camera image stacks. In order to measure dynamic scenes, the previously achieved measurement times of a few seconds must be reduced by at least one order of magnitude to the range < 0.1 s. For this purpose, we apply established single-shot methods from the visible spectral range to our thermal 3D approach. Instead of temporal sequences of multi-fringe patterns or scanning single fringes, we now project statistical point patterns and record only one thermal stereo image pair. In this paper, we theoretically investigate our approach by using a simulation model for thermal point pattern generation on static measurement objects. We analyze the temporal and spatial behavior of the heat patterns taking the material parameters into account. Finally, we show a first thermal single-shot 3D measurement.