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A vision sensor utilizing circular structured light is proposed to deal with the in-pipe profile detection. Circular structured light is firstly thrown by the ring laser projector and then reflected by a cone-shaped mirror to intersect with the inner surface of the pipe. The intersection is taken as the representative of the contour of the section by the charge coupled device (CCD) camera and gets processed by a subsequent algorithm to extract relevant information. The imaging principle is analysed to relate the pixel value to the real dimension, and lead to the promotion of an effective and feasible calibration method which adopts a series of pipes with known diameters as calibration objects. This method figures out the mathematic relation between the pixel value and real dimension by interpolation instead of modelling the imaging system directly. With the calibrated system, pipes with known diameters are employed in the experiment. The feasibility and accuracy have been improved compared with the existing techniques.