Abstract: A non-contact gage for continuously measuring the diameter of a moving member such as a wire or rod employs a source of radiant energy, a scanner and a radiant energy detector aligned across the path of travel of the member whose diameter is to be measured. Operation of the scanner causes a composite signal to be generated in the detector and a component of that signal is proportional to the diameter of the member. The diameter proportional component is separated electronically and fed to a read out device. Inherent non-linearities or inaccuracies such as may be caused mechanically by the scanner mechanism are electronically minimized or eliminated in the detection and display circuitry.