Wansong Li

Evaluation methods for gradient measurement techniques

Many optical metrology methods deliver 2D fields of gradients, such as shearography, Shack-Hartmann sensors and the fringe reflection technique that produce gradients for deformation, wave-front shape and object shape, respectively. The evaluation for gradient data usually includes data processing, feature extraction and data visualization. The matters of this talk are optimized and robust processing methods to handle and prepare the measured gradients. Special attention was directed to the fact that optical measurements typically produce data far from ideal behavior and that parts of the measured area are usually absent or invalid. A robust evaluation must be capable to deliver reliable results with non perfect data and the evaluation speed should be sufficient high for industrial applications. Possible data analysis methods for gradients are differentiation and further integration as well as vector processing when orthogonal gradients are measured. Evaluation techniques were investigated and optimized (e.g. for effective bump and dent analysis). Key point of the talk will be the optimized data integration that delivers the potential of measured gradients. I.e. for the above mentioned examples: the deformation, wave-front and object shape are delivered by successful data integration. Local and global existing integration methods have been compared and the optimum techniques were combined and improved for an accelerated and robust integration technique that is able to deal with complicated data validity masks and noisy data with remaining vector rotation which normally defeats a successful integration. The evaluation techniques are compared, optimized and results are shown for data from shearography and the fringe reflection technique (, which is demonstrated in talk “High Resolution 3D Shape Measurement on Specular Surfaces by Fringe Reflection”).

Object adapted pattern projection: Part I: generation of inverse patterns

Abstract Fast and robust 3D inspection is of big importance for industrial quality control. Therefore, reliable optical techniques are needed that use as few images as possible for measurement. One promising technique for this aim is the inverse fringe projection which has the following advantages: The technique includes the information of a preceding measurement into the projected inverse pattern. Therefore, it is possible to do differential measurement using only one camera frame for each state. Because the camera takes images with user-defined patterns (e.g. fringes with constant frequency), one always has optimized patterns for sampling independently of the object shape. The hardware needs are as low as just a programmable projector and a standard camera. Till now the technique had drawbacks concerning the robust pattern generation and the quantitative (instead of only qualitative) evaluation of differential phases. In this paper, we concentrate our investigations on the robust pattern inversion. We show how the process can be simplified by separating the different inversion problems. Different methods and examples for generating the inverse pattern will be shown and compared to each other.

Vision ray calibration for the quantitative geometric description of general imaging and projection optics in metrology

Exact geometric calibration of optical devices like projectors or cameras is the basis for utilizing them in quantitative metrological applications. The common state-of-the-art photogrammetric pinhole-imaging-based models with supplemental polynomial corrections fail in the presence of nonsymmetric or high-spatial-frequency distortions and in describing caustics efficiently. These problems are solved by our vision ray calibration (VRC), which is proposed in this paper. The VRC takes an optical mapping system modeled as a black box and directly delivers corresponding vision rays for each mapped pixel. The underlying model, the calibration process, and examples are visualized and reviewed, demonstrating the potential of the VRC.

Absolute optical surface measurement with deflectometry

Deflectometry utilises the deformation and displacement of a sample pattern after reflection from a test surface to infer the surface slopes. Differentiation of the measurement data leads to a curvature map, which is very useful for surface quality checks with sensitivity down to the nanometre range. Integration of the data allows reconstruction of the absolute surface shape, but the procedure is very error-prone because systematic errors may add up to large shape deviations. In addition, there are infinitely many combinations for slope and object distance that satisfy a given observation. One solution for this ambiguity is to include information on the object’s distance. It must be known very accurately. Two laser pointers can be used for positioning the object, and we also show how a confocal chromatic distance sensor can be used to define a reference point on a smooth surface from which the integration can be started. The used integration algorithm works without symmetry constraints and is therefore suitable for free-form surfaces as well. Unlike null testing, deflectometry also determines radius of curvature (ROC) or focal lengths as a direct result of the 3D surface reconstruction. This is shown by the example of a 200 mm diameter telescope mirror, whose ROC measurements by coordinate measurement machine and deflectometry coincide to within 0.27 mm (or a sag error of 1.3μm). By the example of a diamond-turned off-axis parabolic mirror, we demonstrate that the figure measurement uncertainty comes close to a well-calibrated Fizeau interferometer.

Measuring deformations with deflectometry

Phase-measuring deflectometry is a powerful method to measure reflective surfaces. It is relatively easy to extract slope and curvature information from the measured phase maps; however, retrieving shape information depends very sensitively on the calibration of the camera and the geometry of the measurement system. Whereas we have previously demonstrated shape uncertainties below 1 μm, the range below 100 nm is currently inaccessible to deflectometric shape measurement. On the other hand, the astounding sensitivity of deflectometry can be put to good use for deformation measurements. The evaluation of corresponding shape differences rather than absolute shapes is much less susceptible to system calibration errors and its resolution is given mostly by the measurement system’s sensitivity. We give an overview of recent progress in difference deflectometry. Firstly we show results from solar mirror substrates under load to detect flaws with high sensitivity. Secondly we present a preliminary simulation study of achievable deformation-measurement uncertainties to assess the feasibility of deflectometric characterisation of actuator performance and gravity sag for the mirror segments of the European Extremely Large Telescope (E-ELT). Results for the relevant Zernike terms show reliable detection of Zernike coefficients at the 25 nm level. Random artefacts related to noise in the phase measurements are seen to translate into bogus Zernike terms, and we discuss possible mitigation techniques to enhance the sensitivity and accuracy further.

Object-adapted inverse pattern projection: generation, evaluation, and applications

Fast and robust 3D quality control as well as fast deformation measurement is of particular importance for industrial inspection. Additionally a direct response about measured properties is desired. Therefore, robust optical techniques are needed which use as few images as possible for measurement and visualize results in an efficient way. One promising technique for this aim is the inverse pattern projection which has the following advantages: The technique codes the information of a preceding measurement into the projected inverse pattern. Thus, it is possible to do differential measurements using only one camera frame for each state. Additionally, the results are optimized straight fringes for sampling which are independent of the object curvature. The ability to use any image for inverse projection enables the use for augmented reality, i.e. any properties can be visualized directly on the objects surface which makes inspections easier than with use of a separated indicating device. The hardware needs are low as just a programmable projector and a standard camera are necessary. The basic idea of inverse pattern projection, necessary algorithms ane found optimizations are demonstrated, roughly. Evaluation techniques were found to preserve a high quality phase measurement under imperfect conditions. The different application fields can be sorted out by the type of pattern used for inverse projection. We select two main topics for presentation. One is the incremental (one image per state) deformation measurement which is a promising technique for high speed deformation measurements. A video series of a wavering flag with projected inverse pattern was evaluated to show the complete deformation series. The other application is the optical feature marking (augmented reality) that allows to map any measured result directly onto the object under investigation. The general ability to straighten any kind of information on 3D surfaces is shown while preserving an exact mapping of camera image and object parts. In many cases this supersedes an additional monitor to view results and allows an operator to investigate results on the object, directly.

Erzeugung und Auswertung von objektangepassten inversen Projektionsmustern (Generation and Evaluation of Object Adapted Inverse Patterns for Projection)

Abstract Die inverse Streifen- und Muster-Projektionstechnik wird vorgestellt und mit Anwendungsbeispielen verknüpft. Die Technik erlaubt eine schnelle und robuste 3D-Qualitätskontrolle sowie eine schnelle Formprüfung, da nur ein Kamerabild zur Auswertung jeweils eines Zustandes benötigt wird. Beliebige Muster können unabhängig von der Objekthöhenstruktur an das Objekt angepasst werden. Beliebige (gemessene) Bilddaten können für den Experimentator direkt auf dem Objekt an den korrespondierenden Positionen markiert werden (Augmented Reality).

Applications of inverse pattern projection

Fast and robust 3D quality control as well as fast deformation measurement is of particular importance for industrial inspection. Additionally a direct response about measured properties is desired. Therefore, robust optical techniques are needed which use as few images as possible for measurement and visualize results in an efficient way. One promising technique for this aim is the inverse pattern projection which has the following advantages: The technique codes the information of a preceding measurement into the projected inverse pattern. Thus, it is possible to do differential measurements using only one camera frame for each state. Additionally, the results are optimized straight fringes for sampling which are independent of the object curvature. The hardware needs are low as just a programmable projector and a standard camera are necessary. The basic idea of inverse pattern projection, necessary algorithms and found optimizations are demonstrated, roughly. Evaluation techniques were found to preserve a high quality phase measurement under imperfect conditions. The different application fields can be sorted out by the type of pattern used for inverse projection. We select two main topics for presentation. One is the incremental (one image per state) deformation measurement which is a promising technique for high speed deformation measurements. A video series of a wavering flag with projected inverse pattern was evaluated to show the complete deformation series. The other application is the optical feature marking (augmented reality) that allows to map any measured result directly onto the object under investigation. Any properties can be visualized directly on the object’s surface which makes inspections easier than with use of a separated indicating device. The general ability to straighten any kind of information on 3D surfaces is shown while preserving an exact mapping of camera image and object parts. In many cases this supersedes an additional monitor to view results and allows an operator to investigate results on the object, directly.