We present a method based on color encoding for measurement of transient 3D deformation in diffuse objects. The object is illuminated by structured light that consists of a fringe pattern with cyan fringes embedded in a white background. Color images are registered and information on each color channel is then separated. Surface features appear on the blue channel while fringes on the red channel. The in-plane components of displacement are calculated via digital correlation of the texture images. Likewise, the resulting fringes serve for the measuring of the out-of-plane component. As crossing of information between signals is avoided, the accuracy of the method is high. This is confirmed by a series of displacement measurements of an aluminum plate.
The fringe projection (FP) method is an outstanding tool for reconstructing painted surfaces. This technique, which has been used for conservation and digitization, does not damage the artwork and can reach sub-millimeter accuracy. To carry out this type of analysis, it is necessary to achieve the most accurate measurements possible. Measuring the precision that a projector-camera-object arrangement can achieve is a complex task. In this paper, we show an experimental method used to measure the accuracy of this technique with instrumentation within the reach of most conservation laboratories. The method consists of capturing, as a reference model, a stepped cylindrical Nylamid® pyramid, as a construction whose shape, size, and manufacturing accuracy are known with high precision. The pyramid has eight well-defined steps, which are fashioned with an accuracy more exact than that of the fringe projection method. The height of each step was measured, obtaining the mean and variance of the height measurements fitted to a Gaussian distribution. In this work, we show the measured heights of the steps, obtained by varying the period of the fringes. The smallest detectable step height was less than 44.1 µm; however, this was obtained with a variance in the order of the step height. The smallest detectable step height with a small variance was 0.1008 mm. In addition to this accuracy measurement, a qualitative evaluation of a painting was carried out, finding the presence of possible superimposed thin layers, fabric, and microcracks, which commonly occur in the drying and aging processes. Further research would provide an experimental measurement of the method’s accuracy and its variance as essential for obtaining a confidence criterion that could then be applied to the model of the painting’s surface.
Three-dimensional displacement in multi-colored objects is measured by combining fringe projection (FP) and digital image correlation (DIC). Simultaneity of measurements through both techniques is warranted by encoding their signals on the RGB channels of color images. By separating the illuminating sources for each technique and by using ultraviolet light for DIC, contrast and amplitude of the registered signals are enhanced, enabling displacement measurement of multi-colored objects in dynamic events. The objects may even present a relatively large dynamic range of intensity levels. Proper selection of the light sources is supported by spectral analyses of the components of the system and by evaluation of the contrast of the registered images. Experimental results reveal that displacement measurements with relatively high accuracy can be obtained.
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