JPH05341043

PURPOSE:To provide an optical length measuring device which measures length by sensing the moving amount with high precision and on the real time basis over the whole region from the standstill state due to out-of-touching till the running speed both in the regular and reverse directions. CONSTITUTION:Two light fluxes branched are shifted into different frequencies by acousto-optical elements AOHS 1, 2 'and separated into the reference light and measuring light by half mirrors HM 1, 2. The, measuring light is cast onto the surface of a moving long stretching object 12 in such a way as intersecting, and the diffused light is sensed. The reference light and diffused light are photoelectrically transduced to form a reference signal and measuring signal, and judgement is made which of the fed signals has higher frequency in accordance with the frequency levels each time one period of frequency difference between the reference signal and measuring signal is generated, and pulses corresponding to the moving amount specified by the optical constant are generated to serve for sensing the moving amount of the long stretching object.