Optical 3D Measurement of Glass and Transparent Plastics

Fig. 1: Laboratory setup of an optical 3D-sensor for measuring transparent plastics and glasses.
© Fraunhofer IOF

Fig. 1: Laboratory setup of an optical 3D-sensor for measuring transparent plastics and glasses.

When it comes to non-contact, fast, and highly precise 3D measurements of objects, optical stereo-vision systems developed by the Fraunhofer IOF have proven their efficacy for certain applications in industry and research. Based on active pattern projection, new 3D systems with wavelengths in the near infrared offer accurate, irritation-free 3D measurements of humans, e.g. for applications in the field of human-machine interaction.

The basic principle of these 3D systems is the active projection of optical patterns onto the objects’ surfaces, which works successfully in the visible or near-infrared wavelength range for a multitude of materials. Yet 3D image errors always arise if the optical properties of certain materials such as glass or transparent plastics interfere with the process of diffuse reflection.

Fig. 2: Thermal image of plastic glasses with a thermal pattern on the surface (top). Reconstructed 3D point cloud with color-coded depth information (bottom).
© Fraunhofer IOF

Fig. 2: Thermal image of plastic glasses with a thermal pattern on the surface (top). Reconstructed 3D point cloud with color-coded depth information (bottom).

Fraunhofer IOF has developed a new method which makes the 3D measurement of these reflective or transparent materials possible. For this purpose, thermal patterns (in the infrared about 10 μm) are absorbed from the object’s surface and their re-emission is analyzed with thermographic cameras. The patterns are generated based on a CO2 laser and an adapted GOBO principle which is described in: S. Heist et al.: High-speed three-dimensional shape measurement using GOBO projection, Opt. Laser Eng. Vol. 87 (2016). Glass and transparent plastics become absorbent by changing the spectral range to the thermal infrared.

The top of figure 2 shows a thermal image of glasses made from plastics with a typical thermal pattern on the object surface. A thermal contrast of about 1 to 2 Kelvin between pattern and surface is sufficient for the further image processing. The bottom of figure 2 shows a complete 3D point cloud of the glasses which was reconstructed by correlating the image stacks of two thermal imaging cameras after various pattern projections.