24 February 2005 Deflectometric inspection of diffuse surfaces in the far-infrared spectrum
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Deflectometry has proven to be a very precise and reliable technique for the detection and measurement of bumps, dents, waviness and scratches on specular surfaces. Phase shifted fringe patterns are successively reflected at the surface and the spatial distortion of these reflected patterns is observed with a camera to extract information about the shape of the surface. Up to now, deflectometry could not be used for diffuse reflecting surfaces, because specular reflection does not occur. With the system developed at our institute it is now possible to inspect even diffuse reflecting surfaces like unpolished metal or plastics using the deflectometric measuring principle. Hereby the fact is exploited that a surface becomes specular when the reflected light has sufficiently large wavelength compared to the surface roughness. For the diffuse surfaces mentioned above the adequate range of the electromagnetic spectrum is far-infrared. In our approach a reflected infrared pattern is observed with a thermal camera. By analyzing four images of the phase shifted pattern an image is calculated, which contains information about local surface curvature. The presented method has been successfully tested for the inspection of the diffuse surfaces of unpainted car body parts.
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Jan W. Horbach, Jan W. Horbach, Soeren Kammel, Soeren Kammel, } "Deflectometric inspection of diffuse surfaces in the far-infrared spectrum", Proc. SPIE 5679, Machine Vision Applications in Industrial Inspection XIII, (24 February 2005); doi: 10.1117/12.586612; https://doi.org/10.1117/12.586612

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