Development of an Optical Hole-Diameter Measuring Instrument: Optical Analysis, Fundamental Experiment, Trial Manufacture and Performance Test

An optical hole-diameter measuring instrument has been developed. The instrument, which consists of two laser diodes, one photo sensitive detector, two condenser lenses and some mirrors, is intended to measure, based on trigonometry, the distances to the hole wall on two sides alternately. The optical, fundamental analysis and experiment showed that it was possible to measure the distances to the object having not only a mirror-like plane surface but also a rough, cylindrical surface such as a drilled one. Using the instrument, which was manufactured on trial, it was found that the width between block gages and the hole diameter of a ring gage could be measured within the accuracy of 10μm and that it was influenced by the position of the measuring probe and the inclination of the measured object. The deviation from the desired values may be attributed to the impropriety of calibration, the misalignment of irradiation of two laser beams and the nonlinearity of optical and electrical elements.