This method employs a light source and phototransistor or other photoelectric converter to convert rotational speed into countable pulses. The light detecting methods include the incident light method and the reflected light method.
Incident light method:
A slotted disk attached to the rotating part rotates between the light source and the receiver photocell. This produces a pulsed signal that is proportional to the number of slots in the disk. This system has the merit that if the number of slots is increased, higher resolution measurement can be made.
Reflected light method:
The surface of the rotor is patterned with marks of different degrees of light reflectivity, such as a black and white pattern or highly directional reflective tape. on-and-offr photocell outputs a pulsed signal generated by the light reflected back to it from the marks in an on-and- off fashion.
The photoelectric sensors described above use either a light-emitting diode or an ordinary electric bulb as a light source, and a phototransistor or other such device can be used for the receiver photocell.
The photoelectric sensing method allows detection of rotational speed without touching the rotor, so it is used for detecting high-speed rotation. The reflected light method places no load on the rotor, so it is suitable for measuring low - torque rotations.