How calibrate a laser camera system fystem for 3D measuring

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I have a camera and a laser line. I would like to make 3D mesasurements, but I have some difficulties to calibrate the system. There are lens distorsions, and how can I get measures in mm. I have seen that it is possible to get intrinsic parameters to get distorsion information, but how can I convert pixels in mm.

Thank you for your help

Jeff