Abstract: An optical gauge having a laser or other such light source, the beam from which is cyclically directed upon a photo-detector by a rotating plane mirror, the time at which or during which in each revolution of the plane mirror the beam impinges upon the photo-detector being commensurate with a linear parameter of a workpiece upon which the beam is also directed, either constantly, as in the first disclosed embodiment, or cyclically as the beam is moved across the workpiece and occluded thereby during a portion of each rotation of the mirror, as in the second disclosed embodiment. The point on the rotating mirror at which the beam is reflected lies at the focal point of a fixed, parabolic mirror.