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In industry, there are needs to accurately measure the 3-D profile of edges parts in order to evaluate edge
condition. Optical methods are increasingly used for this purpose due to its advantages such as being non-contact,
fast, accurate, and easy to integrate with software for data acquisition and feature analysis. We
utilized structured line projection technology to measure edge's profile. In this method a structured light
distribution, created by the transmission of a sinusoidal grating, was projected onto the inspected parts at a
certain incidence angle. The projected light lines were deformed due to the depth change on the edge
surface. A CCD camera sitting at a different angle was used to record the deformed fringes. From the
deformed fringes the 3D surface profile was extracted based on triangulation principal. Because the
projected grating pattern can be interpreted as an interference pattern, we used the spatial carrier phase
shifting and phase unwrapping method as in classic interferometry to extract the phase information from
the intensity distribution of fringes. Due to the limited depth-of-range of the fringe image and depth-of-focus
of the imaging lens on the CCD camera, the observed deformed fringes have different widths and
frequencies at different depth. According to the definition of depth-of-focus, the fringe width out of focus
can be on the order of the square root of 2 wider than that which is in focus. The width change can also be
due to a tilt across the object. This width change affects the accuracy of the spatial carrier phase shifting
and subsequently the accuracy of extracted profile. In this paper, we proposed to monitor the change of the
fringe width with imaging depth. According to the width of the fringes, we defined a parameter called
compressing rate, to use in computing the edge profile. For different edge types, the compressing rate was
optimized in order to get the profile that can match the results from traditional the methods. By using this
method, the system repeatability can be improved significantly.
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