The probe described is capable of accurately measuring the displacement of a plane reflecting surface along its normal without making mechanical contact with it. The image of an illuminated grating of special construction is formed on the surface to be observed, and the light reflected from this surface then forms an image of equal size on a second, exactly similar, grating. The disposition of the second image relative to the second grating depends upon the position of the probe relative to the plane surface and changes in this disposition cause variations in the amount of light transmitted which are detected photoelectrically. It is therefore possible to set the probe at a reproducible distance from ther eflecting surface by moving it until a predetermined level of light transmission is reached. Successive settings relative to a fixed surface indicate that the standard deviation of a single setting of the particular probe described is about 0.4 μ (1.6 × 10^-5 in.). When the probe is used with a calibrated line standard, displacements may be measured with a standard deviation of 0.7 μ (2.8 × 10^-5 in.).