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.
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.
For fast, accurate and robust shape measurement of specular surfaces there are several powerful measurement techniques based on deflectometry. For boundary surfaces of transparent objects like refractive optics, on the contrary, deflectometry is so far limited to the measurement of only one surface in reflection. This is unsatisfying from a metrological point of view as the geometrical relation between both surfaces, which substantially defines the optical function, is lost. In this work a new deflectometric approach is presented that works in transmission and allows the simultaneous measurement of both surfaces of refractive optics. The basic idea of the approach is calculating the unknown surface geometry of a transparent object by iteratively adapting a surface model to the observed light ray deflections. The main problem in this case is the ambiguity of the refraction at the two boundary surfaces of a lens, as there are multiple possible solutions that produce the same measurement results. This is solved by combining four different views on the object under test, which allows to find an unambiguous solution. An experimental measurement setup is presented and results of different simulations and tests are discussed in this paper.
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.
At the Technical University Braunschweig a robot for the assembly of micro structures with an assembly positioning uncertainty below 1 μm is under development. In order to reach the demanded accuracy and robustness, an optical sensor is used. First a 2D-Sensor was tested. To reach the required cycle time a substantial reduction of the response time of this 2D-sensor was necessary. By the application of optimised 2D-algorithms the image processing takes place in video real time now. The extensive software update required an examination of the repeatability results of the 2D-sensor (standard deviation of 0.1 μm in both axes). An overview of the reproducibility and the dynamic cooperation with the parallel robot are presented. A prototype of a 3D-sensor for the robot was built. The reached reproducibility is presented. The 3D-sensor integrated into the robot allows the execution of assembly tasks with a positioning uncertainty better than 1 μm. For the assembly tasks the accurate position of the structures must be determined, which is marked by a defined pattern of circular marks. By photogrammetric measurement of the marks and a pattern recognition the spatial position of the structure is determined. The measurement volume is 7,5 x 11 x 6 mm<sup>3</sup>. To show the efficiency of the 3D-sensor a first test of its cooperation with the robot is described.
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.
In photogrammetry several images of an object taken from different positions in space, are combined to calculate 3D geometrical data. This concept can be scaled down to dimensions in the mm or sub-mm regime. In this paper the application of microphotogrammetry for the measurement of strain fields in material testing is presented. The working principle and an experimental measurement setup are described and results of two application examples are given. Microphotogrammetry is compared with speckle-interferometry as an alternative approach for strain field measurement. As an additional implementation of the microphotogrammetric approach the current state of development of a precision assembly system with a positioning control using CCD-cameras is discussed.
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.