A key component of optical rotary encoders is a code disk mounted on the encoder shaft . This disk is made of unbreakable plastic that has a concentric pattern of transparent and opaque areas. Infrared light from an LED  shines through the code disk, onto an array of photoreceptors . As the shaft turns, a unique combination of photoreceptors are illuminated or blocked from light by the pattern on the disk. For multiturn models, there is an additional set of code discs arranged in a gear train . As the main encoder shaft rotates, these discs are geared together to turn like the wheels of an odometer. The rotational position of each disc is monitored optically and the output is a count of the net number of rotations of the encoder shaft.