11 February 2011 3D metrology system using an active triangulation with high dynamic range
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We build up a new type of 3D metrology system for measuring the 3D shape of micro-structures and for reverse engineering techniques. The measurement principle is an active triangulation with a high dynamic range. The optical imaging is realized with a stereomicroscope. This features a field of view from square-centimeters down to square-millimeters at a constant triangulation angle. One port of the stereomicroscope is used to project measurement labels onto a measurement object and the second port to observe them by a camera. A calibration enables the assignment of 3D coordinates to the position of a measurement label in the camera image. To generate measurement labels we use a projection device that consists of a collimated, white light power LED illuminating a digital mirror device (DMD). The use of a DMD features the quick generation of user definable measurement labels with high brightness and contrast. Due to working with different magnifications and examining surfaces with different properties, the size and the spacing of projected measurement labels has to be adaptable. During a measurement as many measurement labels as possible should be visible in the camera image. Therefore the acquisition of measurement data from bright and dark surface areas requires a high dynamic range. The measurement labels in the camera image are distinguished with a temporal coding. This identification enables the adaption of the brightness of each measurement label in the projection pattern. As a result the dynamic of the measurement system is expanded by the dynamic of the projection device.
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D. Härter, C. Müller, H. Reinecke, "3D metrology system using an active triangulation with high dynamic range", Proc. SPIE 7932, Emerging Digital Micromirror Device Based Systems and Applications III, 79320E (11 February 2011); doi: 10.1117/12.873998; https://doi.org/10.1117/12.873998

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