Krass & Young
Optical, interferometric hole gauge
 An optical gauge for measuring the dimension of a hole in a workpiece employs an interferometric technique using a partially coherent radiation source. A beam of the radiation is directed to a beam splitter positioned a known distance from one of the known surfaces to produce a first sub-beam extending to a mirror and a second sub-beam along the axis of measurement. The mirror is adjustably positioned along the first sub-beam axis by a micrometer head. A radiation detector positioned on the opposite side of the beam splitter from the mirror receives light reflected by the second workpiece surface and light reflected by the mirror. Interference between the numerous wavelengths in the two beams results in a mixture of constructive and destructive interference at all positions of the mirror except that position where the path lengths are exactly equal. When the mirror is adjusted to this position, all wavelengths which make up the beams interfere constructively and a high amplitude signal is generated by the detector. The light at the detector has a speckle pattern and each detector element is approximately the size of a speckle. Three such sensors may be combined in a single gauge suspended in a bore for measuring the distances at 120 degree angles from the gauge to the bore wall and the true diameter is computed from the three distances. 
Environmental Research Institute of Michigan
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Lauren M.
Peterson