Analysis of measurement accuracy of quadrilateral hole using optical inner-diameter-measuring machine

In this paper, the measurement error in a gap larger than 0.3 mm of a quadrilateral hole sample, using the optical non-contact measuring machine, is discussed theoretically and experimentally. By projecting spot image onto the sample´s inside wall and detects the reflected image by CCD camera, the position of inside wall of sample can be measured. The inner diameter of the sample can be measured from distance between the positions of each inside walls. Experimental results showed that the measurement error of the gap varies with the gap size of sample. Comparison with theoretical simulation results, it was verified that the spherical aberrations of the lens in the measuring machine is the cause of this error. With the maximum spherical aberration of the lens of 50μm, the maximum measurement error of the gap is +1.0μm for a 0.3 × 0.3 mm square hole, and is -0.30 - -0.35μm for the quadrilateral hole with the gap larger than 0.6 mm. These theoretical results are in good agreement with the experimental results. The measurement error is always negative when the spherical aberration is assumed to be 0.