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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.
We previously presented a technique calibrating a multiaperture array projector (MAAP) for performing high-speed three-dimensional (3-D) surface topology measurements with aperiodic sinusoidal fringe structured illumination. Although a stereo-camera setup used to be required for triangulation, the presented MAAP calibration technique re-enables photogrammetric 3-D measurements to be made using a single CCD or CMOS camera. For such a monocular-view photogrammetric system, intrinsic camera calibration, intrinsic projector calibration, and extrinsic system calibration are vital to obtain high-measurement performance. However, there is currently no such technique that can comprehensively perform the required extrinsic system calibration when using an MAAP and a single-camera setup. A comprehensive optimization-based method is proposed and the resulting measurement performance is evaluated. With the new calibration technique, we are able to achieve a surface standard deviation of ∼50 μm.
In this contribution, we present new 3D sensor technologies based on three different methods of near-infrared projection technologies in combination with a stereo vision setup of two cameras. We explain the optical principles of an NIR GOBO projector, an array projector and a modified multi-aperture projection method and compare their performance parameters to each other. Further, we show some experimental measurement results of applications where we realized fast, accurate, and irritation-free measurements of human faces.
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