Within the Collaborative Research Centre 1153 Tailored Forming a process chain is being developed to manufacturehybrid high performance components made from different materials. The optical geometry characterizationof red-hot workpieces directly after the forming process yields diverse advantages, e.g., the documentation ofworkpiece distortion effects during cooling or the rejection of deficient components in an early manufacturingstate.Challenges arise due to the high components temperature directly after forming (approximately 1000 °C): Theapplied structured light method is based on the triangulation principle, which requires homogeneous measurementconditions and a rectilinear expansion of light. This essential precondition is violated when measuringhot objects, as the heat input into the surrounding air leads to an inhomogeneous refractive index field. Theauthors identified low pressure environments as a promising approach to reduce the magnitude and expansionof the heat induced optical inhomogeneity. To this end, a vacuum chamber has been developed at the Instituteof Measurement and Automatic Control.One drawback of a measurement chamber is, that the geometry characterization has to be conducted througha chamber window. The sensors light path is therefore again affected - in this case by the window's discreteincrease of refractive index, and also due to the different air density states at sensor location (density at ambientpressure conditions) and measurement object location (density at low pressure conditions). Unlike the heatinduced deection effect, the light path manipulation by the window and the manipulated air density state inthe chamber are non-dynamic and constant over time.The reconstruction of 3-D geometry points based on a structured light sensor measurement directly dependson the mathematical model of detection and illumination unit. The calibration routine yields the necessarysensor model parameters. The window light refraction complicates this calibration procedure, as the standardpinhole camera model used for entocentric lenses does not comprise enough degrees of freedom to adequatelyparametrize the pixel-dependent light ray shift induced by thick vacuum windows. Telecentric lenses only mapparallel light onto a sensor, therefore the window induced ray shift is constant for all sensor pixels and can bedirectly reproduced by the so-called affine camera model.In this paper, we present an experimental calibration method, and corresponding calibration data and measurementresults for a structured light sensor with and without measurement window. The sensor comprises atelecentric stereo camera pair and an entocentric projector. The calibration of the telecentric cameras is conductedaccording to the well-known affine camera model. The projector is used as feature generator to solvethe correspondence problem between the two cameras. The calibration data illustrates that the window refractioneffect is fully reproduced by the affine camera model, allowing a precise geometry characterization ofobjects recorded through windows. The presented approach is meant to be used with the aforementioned vacuumchamber to enable a geometry characterization of hot objects at low pressure levels.
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