| 摘要 |
PURPOSE:To measure the speed of even a body which has projections and recesses by using a photodetector consisting of plural photodetecting elements, providing an actuator which moves a lens for convergence, and controlling the actuator based on variation in the ratio of light intensity among the respective photodetecting elements. CONSTITUTION:Laser luminous flux 9 emitted by a laser light source 1 is split 2 into two parallel pieces of laser luminous flux and they are converged 3 on the surface of a body 8 to be measured. Light 10 scattered by the surface of the body 8 is converged 3 again on the photodetector 6 through the lens 5 for convergence. Light beams having two kinds of frequency deviations interfere with each other on the detector 6 and the frequency of variation in its intensity corresponds to the moving speed of the body 8, so a frequency component is detected 7 from the output signal of the detector 6, thereby obtaining the moving speed of the body 8. Further, the ratio of the outputs of the respective photodetecting elements of the detector 6 is computed 11 and then the detector 6 decides whether or not the surface of the body 8 is too close to the condenser lens 3. The actuator 4 is moved according to the decision result to control the position of the lens 3, so that the moving speed of even the body 8 having projections and recesses can be measured.
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