Marcus Petz

Reflection grating photogrammetry : a technique for absolute shape measurement of specular free-form surfaces

While for non-reflecting surfaces a variety of optical techniques is available that allow the flexible geometric measurement of free-form surfaces, established approaches for the testing of specular surfaces are limited to basic geometries or slight deviations from an assumed reference geometry. As not only the intensified use of aspheric optics but also the increasing quality standards for technical surfaces call for an enhanced measurement range, the authors have developed two related techniques for the direct three-dimensional measurement of specular reflecting surfaces. These techniques are based on the observation of the mirror image of a grid-like reference structure and apply principles that are well-known from measurement systems for non-reflecting surfaces, such as photogrammetry and structured illumination, to the evaluation of specular surfaces. While the first of these approaches works with an active triangulation process based on one camera and a pseudo three-dimensional reference structure, the second one utilises a stereo-photogrammetric camera system in conjunction with a merely two-dimensional reference structure. Both systems allow the unambiguous measurement of reflecting free-form surfaces and may, by the use of multiple wavelength and photogrammetric stitching techniques, be extended to the measurement of rather complex geometries. Besides the fundamental mode of operation of this so-called reflection grating photogrammetry, the properties of a suitable reference structure will be presented in this contribution. Furthermore the photogrammetric calibration procedure and the used calibration models will be discussed. Finally the measurement uncertainty is evaluated based on both, experimental and theoretical considerations.

Optical three-dimensional metrology with structured illumination

A variety of techniques have been developed to measure the three-dimensional shape of an object using structured illumination. The measurement of objects with diffusely reflecting surfaces by means of projected patterns can be considered a standard technique. Also more recently, methods for the measurement of specularly reflecting or transparent surfaces by evaluation of the images of regular patterns have been published. In this paper, we suggest a systematic treatment of all of these methods as generalized active or passive triangulation techniques.

Reflection grating method for 3D measurement of reflecting surfaces

Within the recent years, 3D measurement of shape has gained importance due to increasing tolerance requirements in quality assurance. Optical methods are well suited for this task since they provide 3D-shape of surfaces with high density of information while working contactless and fast. In many cases, the surfaces of the inspected workpieces are reflecting and may not be modified for measurement purposes. If so, common methods such as fringe-projection-systems cannot be used since they require a non-reflecting surface. In this paper, an optical measurement technique is presented that unlike known optical methods allows the direct and unambiguous determination of 3D-coordinates of reflecting surfaces based on the observation of a separate reference grid structure and its image reflected by the inspected surface. For the calculation of 3D-coordinates from the recorded images of the reference grid, photogrammetric methods and phase shifting techniques are applied. Finally, application examples are given, depicting the potential of the presented measuring method.

Measurement of optically effective surfaces by imaging of gratings

Due to the increasing use of aspherical optics, there is a demand for innovative measuring methods that allow the flexible testing of a wide variety of optical components. With the measuring method presented in this paper the imaging properties of the optical component under test may be determined by making it part of an imaging system consisting of an electronic camera with lens, a special grid pattern and the object itself. The grid pattern is taken as a reference object whose resulting image recorded by the camera is evaluated in order to obtain the properties of the tested optical component. In favor of a high measurement accuracy phase shifting techniques and photogrammetric methods are used in the course of the evaluation of the reference structure and the characterization of the tested optical surface. Unlike other techniques the presented measuring method does not require particular previous knowledge of the tested component and may be applied to both reflective and refractive optical surfaces. Measuring examples for reflective and refractive components are given, demonstrating the potential of the method.

Force transducer based on a geometric-optical displacement sensor

The vast majority of current force measurement methods determine the deformation of an elastic body to measure the effective force. The leading sensor technology in this market segment is the resistance strain gauge. Because of the measurement principle the stain gauge has some drawbacks. For instance, the measurement suffers from the so-called creeping and a local weakening of the elastic element is necessary to create sufficient local strains. This is also a reason for this technique being limited with respect to the measurement of dynamic loads. In order to overcome these limitations, in this paper a new contactless working geometric-optical displacement transducer is proposed. An optical lever principle is used to reach the required high position resolution to measure the force-induced deformation of an elastic body.

Vorhersage des Phasenrauschens in optischen Messsystemen mit strukturierter Beleuchtung

Zusammenfassung In dieser Arbeit wird eine Methode vorgestellt, mit der sich ein Schätzwert für das Phasenrauschen in optischen Messsystemen mit strukturierter Beleuchtung direkt aus den aufgezeichneten Grauwerten berechnen lässt. Das verwendete Rauschmodell basiert auf einer Richtlinie zur Charakterisierung von Bildsensoren (EMVA 1288), so dass die benötigten Parameter direkt einem konformen Datenblatt entnommen werden können. Alternativ wird in dieser Arbeit auch eine Methode beschrieben, mit der die Parameter ohne zusätzlichen Geräteaufwand experimentell ermittelt werden können. Abstract

Unsicherheit bei der Datenfusion dimensioneller Messungen

Zusammenfassung Bei der Auswertung von Messdaten wird eine detaillierte Analyse der Messunsicherheit gefordert. Werden dimensionelle Daten mehrerer Sensoren fusioniert, so umfasst dieser Prozess üblicherweise Transformationen und Ausgleichsrechnungen. Die daraus resultierende Messunsicherheit ist häufig nicht direkt bestimmbar, kann aber mittels Monte-Carlo-Methoden abgeschätzt werden. Das grundlegende Normungsdokument dafür ist GUM Supplement 2. Anhand mehrerer Simulationsrechnungen zur Datenfusion für die Formmessung zylinderischer Werkstücke werden Effekte aufgezeigt, welche die resultierende Unsicherheit entweder erhöhen oder verringern können. Abstract The processing of measurement data requires a detailed analysis of the measurement uncertainty. Data fusion in dimensional metrology includes transformation and estimation stages based on solving an optimization problem. The structure of the measurement uncertainty has to be considered for the analysis of these stages. The basic normative document to be applied is GUM Supplement 2. The uncertainty propagation is simulated by the Monte Carlo method. Examples with data fusion for collecting and improving data are given for the form measurement of cylindrical objects.

Model-Based Deflectometric Measurement of Transparent Objects

The measurement of the entire 3-dimensional geometry of a transparent object like a lens is a challenging task for established measurement techniques. In addition to some process-specific drawbacks, like the long measurement time for scanning techniques or the small robustness for interferometric approaches, many optical measurement techniques suffer from the fact that they have to measure the front and rear surface of such a specimen in reflection, which implies some major drawbacks:

Evaluation of LCD monitors for deflectometric measurement systems

With deflectometric measurement methods there are powerful systems available today that are capable of measuring the geometry of specular surfaces or the power distribution of refractive optics in a fast and flexible manner without influencing the measurement object by tactile probing. They are based on the general principle to image a known spatially coded reference structure via the unknown measurement object onto an optically calibrated camera. As a representation of the reference structure LC-displays are very suitable as they provide a high flexibility in the generation of spatial coding patterns like sinusoidal fringes. As the characteristics of the reference structure have a huge impact on the resolution, the accuracy and the measurement range of the whole system, in this work two displays with different LCD technologies are analysed, compared and evaluated especially for deflectometric applications. The main focus is on the quality of gray-value rendering and the dependency between the characteristic curve and the observation angle. The experimental results corroborate the theoretical finding that IPS-technology is superior to TN- and MVA-displays in terms of an observation-angle independent shape of the grayscale-characteristic curve. So IPS should be the technologyof- choice when selecting a LC-display for a deflectometric measurement system.

Rasterreflexions-Photogrammetrie zur Messung spiegelnder Oberflächen (Reflection Grating Photogrammetry for the Measurement of Specular Surfaces)

Abstract Die berührungslose dreidimensionale Messung spiegelnd reflektierender Freiformflächen bereitet trotz neuerer Fortschritte auf dem Gebiet der Asphärenprüfung noch immer erhebliche Schwierigkeiten. Die in diesem Beitrag vorgestellten geometrisch-optischen Messverfahren nach dem Rasterreflexionsprinzip ermöglichen durch Kombination mit photogrammetrischen Auswertetechniken die punktweise und eindeutige Erfassung nahezu beliebig geformter spiegelnder Oberflächen.