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In optical measurement of pipe threads at the manufacturing plant level, achieving uncertainty of within 1 μm was a problem because the object position and shape are not all the same. The authors propose a measurement method by designing an optical specification to suppress the diffraction effect and adopting optical shading/aberration correction and subpixel edge-detection processing. In this method (Quadruplet-Camera system), 4 CCD line cameras are positioned opposite to 4 parallel light sources in order to measure four points of the pipe circumference, and thread images are acquired by moving the Quadruplet-Camera system in the pipe axial direction. In the usual optical approach, the numerical aperture (NA) of the lens is increased to improve optical resolution. However, as NA increases, diffracted rays which form a diffraction pattern are observed in the edge area of a cylindrical object, and this causes measurement uncertainty. Increasing NA also results in a narrower depth of field (DOF), which causes instability in the measurement results at a manufacturing plant. The authors designed a compatible optical specification, and concluded that stability of measurement, which means eliminating the diffraction effect and securing a wide DOF, should take precedence over high optical resolution for application to a manufacturing plant, and adopted optical shading/aberration correction and subpixel edge-detection processing in order to compensate for lower optical resolution. In this manuscript, first, we explain the proposed method and confirmation experiments in the laboratory. We also explain a new optical measurement system based on the concept described above in a manufacturing plant to confirm the effectiveness of the method. We concluded that the measurement system has sufficient performance, i.e., uncertainty within 1 μm, for use as a practical system.
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