Landmann, Martin: Optically measuring the front and rear surface shape of transparent objects

Abstract in Englisch:

Recently, we have made significant progress in evaluating the three-dimensional surface shape of optically challenging objects, especially transparent objects. Our method involves projecting long-wave infrared (LWIR) laser lines, inducing thermal patterns on the object’s surface. Using two thermal cameras in stereo arrangement, we were able to quickly measure the outer geometry of objects in less than 0.1 s. Additionally, we have set a new standard by capturing and reproducing real-time dynamic deformations of transparent objects at a 3D frame rate of 20 Hz. Many industrial processes call for determining not only the front but also the rear surface shape, i.e., the material thickness. This is crucial for identifying weak points or potential material savings, for instance, in ampoules. Existing methods for the simultaneous measurement of surface shape and material thickness (e.g., computer tomography) are complex, expensive, slow, and cannot be integrated into production lines. As a result, e.g., container glass manufacturers are actively seeking an alternative solution. We aim to provide such a solution by enhancing our current process. Instead of a CO2 laser line at λ = 10.6 μm wavelength, which is absorbed at the object’s surface and does not penetrate the material, we use a wavelength in the short-wave infrared (SWIR). At this shorter wavelength, the laser radiation travels through commercially available glasses. At the rear surface, the radiation is partly reflected and reaches the front surface again. Along its path, the radiation is absorbed and leaves a heat trace behind. Whereas common glasses are translucent in the SWIR, they are generally opaque in the LWIR range. Consequently, while some SWIR radiation penetrates the object, LWIR cameras detect heat only at its front surface: (1) at the entering laser line and (2) at the position of the exiting line. Our goal is to use these two thermal signal positions to determine both the front and rear 3D surface shape, and thus the material thickness. In this paper, we investigate our approach theoretically using a simulation model. The model is used to generate thermal points on static measurement objects and determine appropriate parameters such as laser power, angle of incidence, and irradiation time. Furthermore, we analyze the temporal and spatial behavior of the thermal points, considering the material parameters. With the obtained simulated results, we subsequently demonstrate an initial experimental setup. In this setup, the two thermal signals are evaluated on a glass plate for different angles of incidence to determine the material thickness.