Angle encoders
Tachometric measurements

Strictly speaking, they are not speed measuring devices. In fact, angle encoders are more commonly used to control motor speeds.

Most encoders used for speed measurement or control are optical angle encoders. Such a device is based on a disk divided into N sectors. A light source illuminates this rotating disk, and the optical signals transmitted by the sectors are picked up on the other side of the disk by a photoreceiver, then processed by a signal conditioner to recover an electrical signal whose frequency is directly proportional to the speed of rotation. For a disc with N sectors, the resolution is 360°/N.