Shape and surface characterization using diffused light

Detta är en avhandling från Luleå tekniska universitet

Sammanfattning: Diffuse reflection from various surfaces have been studied using several different methods in order to draw conclusions about the surfaces. First of all the aim was to perform fast and accurate shape measurements of molded steel objects on-line in a production system. Two measurement methods were used, stereoscopic imaging with projected fringes and dual wavelength digital holography. Information from the CAD model of the object was used to speed up the shape determination, mainly by enhancing the performance of the phase unwrapping. The uncertainty of the shape result was between 3.5 µm and 40 µm depending on the measurement method. The request on conformity between the object and the CAD model varied between 0.1 mm and 1.6 mm. To overcome the influence of speckles in the case of using interference fringes in the stereoscopic imaging an investigation of speckle reduction using a moving aperture was performed. The technique resulted in a decrease in the speckle contrast of about 60%. Secondly, diffuse reflections that come not only from the surface, but also from beneath it were studied. Measurements were done both on ice surfaces with the aim to identify changes in the surface structure and on glass spheres and sand grains with the aim to detect different moist content in granular material. The ice measurements showed an increase in the back reflection and a decrease in the specular reflection when the surface went from being smooth to being rough, a fact that can be useful when considering the roadgrip of icy roads. Depending on the water content in the granular materials the moist measurements when illuminating the surfaces with white light showed differences in the shape of the reflection distribution in the plane of incidence. Given these differences a simple model describing the water content was proposed. All experiments, except the ones on ice, have been performed in the laboratory. The results have been analyzed and compared to simulations, model estimations, and in the case of the shape measurements with the corresponding CAD models, all with relatively good outcome. In this thesis the work is presented in six papers. One about the speckle reduction method, three about the shape measurements and two about the ice surface structure and the moist in granular material, respectively.

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