Rotary encoder disks are round or disk-shaped and have evenly-spaced holes or markings around their perimeter. They are usually made of metal, plastic, glass or paper; however, products that are made from other materials are also available. Typically, rotary encoder disks are made by cutting slots in the material, or by printing alternating dark and light areas. Rotary encoder disks that use optical techniques are paired with a light emitting diode (LED) and optical detector to "read" slot patterns as they pass in front of the optics. Devices that are paired with optical elements become rotary encoders. These are used to detect both the speed and direction that an encoder disc is turning.