This datasheet tells the reader information around speed sensor, mounting, and photoelectric. In the datasheet the reader can find explanation like sensor, and the speed sensor.
There are lots of discussion such as light source, photoelectric speed sensor, and photoelectric speed are presented in the datasheet. Below are grabbed from this datasheet:
The Model 21000E photoelectric speed sensor is used with IRD balancing instruments to detect the rotating speed of a part. This speed sensor operates on a retro-reflective principle and requires reflective tape to be attached to the rotor. The optical system is coaxial and minimizes problems associated with ambient light. For best performance, the sensing axis of the photocell should be placed at a slight angle from perpendicular to the reflective tape. An indicator light on the side of the sensor tells when the sensor is being triggered by the tape. Two threaded mounting nuts are supplied. This sensor is furnished as a standard accessory with most IRD Balancing instruments.
In addition, this datasheet contains information such as range, and speed.