Optical, interferometric hole gauge

A gauge for measuring the radius of a bore hole comprising:

• a package of three optical sensors for suspension within a bore hole for measuring the distance in three directions from the center of the package to the surface of the hole;
• a partially coherent radiation source for supplying the three sensors;
• each sensor having an optical axis, a beam splitter in the housing supported on the optical axis to receive said beam of light so as to divide said beam into first and second sub-beams projecting along mutually perpendicular paths, the second sub-beam being directed to the hole surface to illuminate a local portion of the surface at one side of the hole, a mirror disposed normally to the path of the first sub-beam to receive the first sub-beam from the beam splitter and reflect the first sub-beam back to the beam splitter, means for moving said mirror along the path of the first sub-beam so as to adjust its distance from said beam splitter, and an optical detector positioned on the opposite side of the beam splitter from the reflective surface for receiving reflections from the reflective surface and from the hole surface and where constructive interference occurs;
• and
• control means coupled to the detector outputs for sensing constructive interference and coupled to the mirrors for moving each of the mirrors to a position where the constructive interference occurs, the control means including means for calculating the bore hole radius from the distances measured for each sensor.

The method of measuring the distance along a line between first and second separated surfaces comprising:

• illuminating a beam splitter with a partially coherent beam of light so as to create a first sub-beam extending along a first path and a second sub-beam extending along a second path at least partially coincident with said line for illumination of the first of said surfaces;
• positioning the beam splitter a determinable distance from the second of said surfaces;
• positioning a mirror for movement along the first path;
• positioning an optical detector on the opposite side of the beam splitter from said mirror for receiving light reflected from the mirror and from the first of the surfaces;
• analyzing the output of said detector;
• moving the mirror along said first path until the output of the detector peaks, and measuring the optical distance of the mirror from the beam splitter at the peak, whereby the measured distance will equal the optical distance of said beam splitter from the first surface along said second path;
• calculating the determinable distance as a function of the measured distance;
• and
• summing the measured distance and the calculated distance thereby determining the distance between said first and second surfaces.