The disc of optical rotary encoder ！

The disc has opaque and transparent segments and passes between the LED and detector to intermittently interrupt a light beam. The detector tracks the series of light exposures it receives and sends that information to the processor that extracts motion information.

Two rotary optical encoder subtypes exist: Incremental and absolute.

Incremental encoders are named for their output, consisting of the two square waves, each corresponding to an increment of rotation. Typically, the LED directs rays through a convex lens that focuses the light into a parallel beam; the beam passes through a grid diaphragm, which splits it to produce a second beam of light 90° out of phase. Light passes from A and B channels through a disc onto the photovoltaic or photodiode array. The disc rotation creates a light-dark pattern through the clear and opaque disc segments.

This pattern is read and processed by a photodiode array and decoding circuitry; beams A and B are each received by a separate diode and converted into two square-wave signals ... 90° out of phase, commonly known as quadrature output. It’s then fed into a processor that can process the signal to determine the number of pulses, direction, speed, and other information. Incremental encoders can also have a third channel with a single segment slot or reference that is used to zero or home the device. Alternatively, an incremental sine-wave encoder produces quadrature sine waves (sine and cosine) instead of square quadrature. Arctangent functioning yields arbitrary levels of resolution.

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