3D shape measurement of glass and transparent plastics with a thermal 3D system in the mid-wave infrared

Structured light projection techniques are well established and an integral part of optical, accurate, and fast threedimensional (3D) measurements. Most problems occur when the projected patterns are not diffusely reflected at the objects' surfaces. Therefore, we present a new optical 3D mid-wave infrared (MWIR) system based on a ""shape from heating"" approach with thermal pattern projection. Thus, the three-dimensional shape of materials that are transparent, translucent, or reflective in the visible wavelength range (e.g., glass, plastics, or carbon-fiber-reinforced material) can now be measured optically, contactless, and without a prior process of painting. The system operates with a stereo-vision setup of two cooled MWIR cameras (3-5 mm) and a CO2 laser projection unit to heat up the objects' surfaces with aperiodic patterns. A stack of thermal images can be used to find corresponding points in both MWIR cameras and to calculate the 3D information of the surface geometry. In this paper, we introduce the setup and measurement principle of the 3D MWIR system and show some relations between various system parameters (masks, temperature contrast, and material parameters) and the measurement accuracy. We demonstrate the capabilities of our sensor by presenting an impressive 3D result of a real object.