DYNAMIC RANGE ENHANCMENT IN DIGITAL FRINGE PROJECTION AND LASER INTERFEROMETRY
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Abstract
GELAREH BABAIE. Dynamic range enhancement in digital fringe projection and laser interferometry. (Under the direction of DR. FARAMARZ FARAHI)Optical metrology is the science and technology concerning measurements with light. Surface texture and 3D profile measurement are very important in many applications such as production automation, robot vision, virtual reality, medical image diagnosis, and other fields. Fringe projection profilometry and laser interferometry systems are two powerful optical methods for surface shape and texture measurements. These techniques offer the advantages of non-contact operation, full-field acquisition, high resolution, and fast data processing. As a result, these techniques are widely used in different fields where fast and accurate measurement is required. However, despite the widespread use of these systems, there are several limitations in acquiring and analyzing interferometric data. One serious limitation, which significantly decreases the SNR in both fringe projection and laser interferometry systems, is the limited dynamic range of imaging systems. For example, a typical 8-bit camera can represent the intensity level between 0 to 255 grayscale, while the optical reflectivity of objects under the measurement towards the camera can be much wider. As a result, some areas in the image taken by the camera are saturated while other areas are underexposed. Consequently the true intensity values and phase information of the corresponding saturated and underexposed areas cannot be correctly retrieved for metrology purposes. Therefore, achieving a system with high dynamic range is very desirable in the field of optical metrology. This research effort, in part, is focused on addressing the dynamic range problem in both digital fringe projection and laser interferometry techniques. Spatially Varying Pixel Intensity (SVPI) technique is introduced as a tool to enhance dynamic range in optical metrology tools specifically in digital fringe projection and laser interferometer systems. The results obtained demonstrate that this technique can increase the signal to noise ratio in the fringe projection and laser interferometer system, hence improving the measurement precision.In the second part of the research, profilometry based on optical wavelength conversion is introduced as a new technique for 3D profilometry of objects composed of unique raw materials, such as dark, non-reflective objects. A solid can absorb light as the result of intraband transitions. When the energy of the incident light exceeds the energy bandgap of the solid, the electrons absorb the energy of the light and electron jumps from the ground energy level to the higher excited level. The reverse process can happen when the electrons in the excited states drop to the lower level by either emitting a photon (radiative process) or by releasing heat (non-radiative process). The amount of light absorption, percentage of absorbed light that is converted to another wavelength, depends on the optical properties of the material that the object is made from. New fringe projection systems based on wavelength conversion are presented in this thesis that offer increased signal to noise ratio as well as increasing the effective numerical aperture of the measurement systems.