The purpose of an encoder is to act as a position feedback device as part of an integrated control system. The encoder is an electro-optomechanical device that attaches to a rotating shaft providing angular position information to a counter or controller. An optical encoder uses a code disk with a pattern of opaque and clear lines normally engraved or deposited on the surface of the disk. A light source shines on the code disk, and the transmitted light is passed through the code disk to a photodetector such as a linear array. Rotary encoders usually have two or more tracks making it possible to determine the direction of rotation, specific position during shaft rotation, and resolution of position. Incremental encoders produce a series of pulses to keep track of motion, position, and velocity and are used in applications when knowledge of absolute position upon power loss is not required. Absolute encoders keep track of their position at all times and should be used when position data must be retained after loss of power.