PURPOSE:To permit a measured value to be directly obtained in the form of an electric signal without passing through any mechanical system, by detecting the difference in phase between light-receiving element outputs corresponding to the light beams from two measuring points transmitted or reflected by a scanning chart which is displaced at a predetermined speed. CONSTITUTION:Light beams La, Lb parallel to each other emitted from measuring points a, b, respectively, pass through a scanning chart 1 and a condenser lens 2 to enter light-receiving elements 3a, 3b of a light-receiving device 3, respectively. The scanning chart 1 has a pattern changing in transmissivity in the direction of the distance between the measuring points a, b at a predetermined period and is displaced by a driver at a predetermined speed in the direction of the distance between the measuring points a, b. The photoelectric outputs of the light-receiving elements 3a, 3b corresponding to the transmitted light beams La, Lb, respectively, have rectangular waves different in phase from each other. The difference in phase between both signals is proportional to the distance D between the two points and is represented by K1D (wherein K1: proportionality constant).
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