Structured light sensor with telecentric stereo camera pair for measurements through vacuum windows

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

Calibration routine for a telecentric stereo vision system considering affine mirror ambiguity

A robust calibration approach for a telecentric stereo camera system for three-dimensional (3-D) surface measurements is presented, considering the effect of affine mirror ambiguity. By optimizing the parameters of a rigid body transformation between two marker planes and transforming the two-dimensional (2-D) data into one coordinate frame, a 3-D calibration object is obtained, avoiding high manufacturing costs. Based on the recent contributions in the literature, the calibration routine consists of an initial parameter estimation by affine reconstruction to provide good start values for a subsequent nonlinear stereo refinement based on a Levenberg–Marquardt optimization. To this end, the coordinates of the calibration target are reconstructed in 3-D using the Tomasi–Kanade factorization algorithm for affine cameras with Euclidean upgrade. The reconstructed result is not properly scaled and not unique due to affine ambiguity. In order to correct the erroneous scaling, the similarity transformation between one of the 2-D calibration plane points and the corresponding 3-D points is estimated. The resulting scaling factor is used to rescale the 3-D point data, which then allows in combination with the 2-D calibration plane data for a determination of the start values for the subsequent nonlinear stereo refinement. As the rigid body transformation between the 2-D calibration planes is also obtained, a possible affine mirror ambiguity in the affine reconstruction result can be robustly corrected. The calibration routine is validated by an experimental calibration and various plausibility tests. Due to the usage of a calibration object with metric information, the determined camera projection matrices allow for a triangulation of correctly scaled metric 3-D points without the need for an individual camera magnification determination

2D refractive index field measurements in air in different pressure scenarios

The optical geometry characterization of wrought hot components can help to quantify material distortion effects during air-cooling. The component's shrinkage behavior is affected by inhomogeneous heat dissipation due to the object's complex geometry and - in case of hybrid materials - differing thermal expansion coefficients. As optical triangulation techniques rely on the rectilinear expansion of light, the hot component's heat input into the surrounding medium air influences the reachable accuracy of optical geometry measurements due to an inhomogeneous refractive index field around the hot component. In previous work, the authors identified low pressure measurements in air as a possible approach to reduce the magnitude and expansion of the inhomogeneous refractive index field above cylindrical high-temperature objects and thereby allow precise geometry acquisition. We now present experimental data of the 2D refractive index field above a hot cylinder in different pressure scenarios using the well-known background oriented schlieren (BOS) method in order to illustrate the decrease in refractive index variations dependent on the pressure state. For this purpose, a ceramic rod is placed in a vacuum chamber and heated up to temperatures of about 1000°C. Using a monochromatic camera, a wavelet background and an optical ow algorithm, the developing 2D refractive index field for a low pressure scenario is compared to ambient pressure conditions. The experimental data illustrates a reduction in the convective heat flow above the hot heating rod at lower pressure values and therefore a homogenization of the density-coupled refractive index in air, validating former simulation results. © 2018 SPIE

Development of a reconstruction quality metric for optical three-dimensional measurement systems in use for hot-state measurement object

Optical three-dimensional (3-D) geometry measurements are state of the art when it comes to contactless quality control and maintenance of the shape of technical components that exclude tactile measurements due to filigree or internal structures. Optical inspection methods are also characterized by a fast and high-resolution 3-D inspection of complex geometries. And due to their noncontact principle, they can carry out measurements in places that would otherwise not be accessible due to harsh environmental conditions or specimens such as hot forged parts. However, there are currently no methods to estimate the reconstruction quality for the optical 3-D geometry measurements of hot objects. The mainly used geometric measurement standards cannot be used for the characterization of hot measurements since the calibrated geometrical values are not transferable to high temperatures. For the development of such a metric, we present the fundamentals of the concepts and algorithms for an estimation of the reconstruction quality are presented and evaluated using a two-dimensional simulation model. The generated findings were applied to the 3-D geometry measurement of a hot object in a laboratory environment. The results are compared with general state-of-the-art reconstruction quality metrics

Electronic excitations and structure of Li2IrO3 thin films grown on ZrO2:Y (001) substrates

Thin films are a prerequisite for application of the emergent exotic ground states in iridates that result from the interplay of strong spin-orbit coupling and electronic correlations. We report on pulsed laser deposition of Li2IrO3 films on ZrO2:Y (001) single crystalline substrates. X-ray diffraction confirms preferential (001) and (10-1) out-of-plane crystalline orientations with well defined in-plane orientation. Resistivity between 35 and 300K is dominated by a three-dimensional variable range hopping mechanism. The dielectric function is determined by means of spectroscopic ellipsometry and, complemented by Fourier transform infrared transmission spectroscopy, reveals a small optical gap of ≈300 meV, a splitting of the 5d-t2g manifold, and several in-gap excitations attributed to phonons and possibly magnons

Design, setup, and evaluation of a compensation system for the light deflection effect occurring when measuring wrought-hot objects using optical triangulation methods

In this paper, we present a system to compensate for the light deflection effect during the optical geometry measurement of a wrought-hot object. The acquired 3D data can be used to analyze the formed geometry of a component directly after a hot forging process without waiting for the needed cooling time to room temperature. This may be used to parameterize the process and to detect defect components early in the production process, among others. The light deflection as the deviation from the linear path of the light is caused by an inhomogeneous refractive index field surrounding the hot object. We present the design and setup for a nozzle-based forced air flow actuator, which suppresses the light deflection effect. The design process includes a simulation of the developing field, as well as of the interaction of the field with an external forced air flow. The cooling effect of the air flow is evaluated, and conclusions are drawn from the conflicting interests of good measurement conditions against the forced cooling of the hot object. The findings are then implemented in the physical setup of the suppression system. The system is evaluated using a previously established method based on optical triangulation and fringe projection. Other occurring effects and their influence on the evaluation are considered and discussed. © 2020 by the authors. Licensee MDPI, Basel, Switzerland