A glass plate thickness measuring instrument wherein a beam of radiation is impinged on the glass to provide a pair of beams reflected from the front and back surfaces of the glass plate respectively. The two beams are made to fall on a radiation responsive device in the form of a charge coupled device, which provides pulsed image outputs of the beams in the form of staircase waveforms of a truncated Gaussian distribution. Electronic circuity, including a microprocessor, is employed to calculate the center of gravity of an area under both waveforms above a threshold value, and the distance between the respective centers of gravity calculated to provide a measure of the thickness of the material.