In a sense, the optical rotary encoder output is binary; it is either light or dark.
tracks etched into the disk. Periodically, these tracks have an opening, called a window. The windows of every track are offset, so that each revolution of the optical rotary encoder has a different light/dark pattern.
On the other side of the optical rotary encoder is the mask, which is usually connected directly to a sensor, to increase the resolution. The mask, like the disk, is also grooved with corresponding tracks and windows. As the light source shines through the moving disk onto the mask of the optical rotary encoder, the different, unique light patterns illuminate the sensor.
Each optical rotary encoder track contains one or two sensors. Each sensor outputs its individual signal, and the sensors outputs are slightly offset from each other, creating slightly offset outputs. The two offset signals interact differently with the light source, creating a unique optical rotary encoder pattern. The pattern is thequadrature.
Since each arc revolution has a completely different pattern, an optical rotary encoder can be exceptionally accurate.