Encoder
A digital optical encoder is a device that converts motion
into a sequence of digital pulses. By counting a single bit
or by decoding a set of bits, the pulses can be converted
to relative or absolute position measurements.
Most rotary encoders are composed of a glass or plastic
code disk with a photographically deposited radial
pattern organised in tracks. As radial lines in each track
interrupt the beam between a photoemitter-detector
pair, digital pulses are produced.