A photoelectric tachometer
Kouyu Miyazaki · Journal of Physics E Scientific Instruments · 1968
A beam of light is reflected on to a phototransistor from the surface of a rotating specimen. No additional reflecting or absorbing surfaces are attached to the sample but use is made of the periodic nature of the reflected light intensity due to small surface variations. The rate of rotation of the sample can be determined by detecting coincidence between the modulated phototransistor signal and a similar signal which has been subjected to a known delay by a shift register.