3D shape measurement by thermal fringe projection: optimization of infrared (IR) projection parameters

Structured light projection techniques in the visible spectrum of light (VIS) are widely used for fast, contactless, and nondestructive optical three-dimensional (3D) shape measurements. For instance, 3D reconstruction can be performed with a stereo camera system combined with corresponding pixel search and triangulation. Recently, we increased the measurement speed significantly by GOBO projection of aperiodic sinusoidal fringes. Due to their optical properties, such as being glossy, transparent, absorbent, or translucent, some materials cannot be measured in VIS. Changing the spectral range from VIS to infrared (IR) allows measuring the 3D shape of some of these materials. Instead of diffuse reflection of structured light in VIS, the absorption of structured light in IR (e.g., CO2 laser at 10.6 mu m) combined with energy conversion and re-emission of light according to Planck's law is used. Detection can be carried out at 3-5 mu m. Depending on optical and thermal material properties (e.g., complex spectral refractive index, thermal conductivity, specific heat capacity, emissivity), the parameters of the projection unit have to be adjusted, e.g., intensity or illumination time. In this paper, we investigate the influence of material, geometry, and irradiation parameters on the temperature contrast. We present a simulation tool based on the Beer-Lambert law and heat diffusion equation for irradiation-induced thermal pattern on the object's surface. It allows to determine optimized irradiation settings for well-known material and geometry parameters. We compare the simulation outputs with experimental results.