When three dimensional measurements are conducted with fringe projection, the quality of the grating used for the generation of the fringes is important. It has a direct influence on the achievable depth resolution in a given measurement setup. In the past, Ronchi grating or gratings written in nematic liquid-crystal displays or in digital micromirror devices have been used. We report on the application of a reflective ferroelectric liquid-crystal-on-silicon display as the fringe-generating element in a setup based on a stereo microscope. With this device the depth resolution of measurements by use of phase-shifting algorithms can be significantly improved compared with the application of a Ronchi grating or a nematic liquid-crystal display.
Three-dimensional optical topometry of technical surfaces becomes increasingly important for the control of industrial processes. However, the local reflectance of the surface of the investigated sample often varies within a wide range, making accurate measurements by fringe projection difficult. We demonstrate the use of a liquid-crystal spatial light modulator as the fringe-generating element in a standard stereo microscope. With this device the brightness of the projected patterns can be adapted pixelwise. This technique leads to a significant improvement of the results of our measurements with a phase-shifting algorithm.
When three-dimensional optical topometry of technical surfaces is performed, one major problem is that often the local reflectance of the object's surface varies within a wide range. This leads to overexposed and underexposed areas on the detector, where no measurements can be made. To overcome this problem, we have developed a method that extends the dynamic range of an imaging system. As an example we implemented this method to a measurement system that is based on fringe projection with a data projector and a color CCD camera. By projection of quasi-monochromatic fringes a different dynamic range in each of the three color channels of the camera is achieved. Hence the overall dynamic range of the system is increased by a factor larger than 5.
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