MEASUREMENT OF OPTICAL LENGTH

PURPOSE:To enable a highly accurate measurement of length by determining it with the harmonic detection of the output of a photo detector employing a modulated irradiation light when a shield length is detected for an irradiation light to an object to be measured. CONSTITUTION:A light from a linear light source 20 which is flashed synchronizing an oscillation frequency f0 of an oscillator 22 irradiates an object 20 to be measured while the light not shielded is collected to an Si photo diode linear array with a lens 14. A photo detector 12 is driven at a frequency of fs to allow the scanning of a multiplexer 36. The frequency f0 is now 5 to 10 times larger than fs. The output of the multiplexer 36 undergoes a harmonic detection at the frequency f0 with a detector 40. A pulse with a peak value above the set value is selected with a comparator 42 and sent to a counter 46 which counts pulses at each scanning cycle 1/fs receiving from a clock pulse generator 44. The signal is given to an arithmetic device 48 to determine the width of the object 10 at a high accuracy.