Thermal light section sensor for real-time 3D measurement of transparent objects

Fast and non-contacting 3D imaging of objects has become well established, e.g., for quality assurance, human machine interaction, or robot handling in the industrial sector. In recent years, we have successfully combined active thermography and triangulation to tackle the challenge of measuring the 3D shape of objects with optical properties such as being glossy, transparent, absorbent, or translucent. We have shown that such optically uncooperative objects can be directly measured by a shift in the wavelength from the visible (VIS) and near-infrared spectral range into the thermal infrared. Until now, this so-called thermal 3D imaging works well with stereo camera setups and allows for measuring even dynamic scenes when applying high-end components. However, the use of two cutting-edge infrared cameras makes thermal high-speed 3D sensors rather expensive. Therefore, we have developed a real-time thermal 3D sensor, which utilizes a single camera and allows for the measurement of transparent objects on continuously moving conveyor belts. For this purpose, we adapted the classical line-structured light method from the VIS to the thermal 3D measurement principle. Due to the change in spectral range and elimination of one of the two cameras, we were forced to redesign underlying principles for the calibration and the measurement itself. In this contribution, we present our experimental setup of this real-time thermal light section sensor and illustrate the challenges that arise in when applying the well-known method from the VIS in the thermal infrared. In addition, we characterize the measurement accuracy depending on the speed of the conveyor belt and the evaluation method.