Encoders are sensors that measure the positional changes of an object using scales (gratings) that are attached to the object. Canon developed the SR sensor, an ultra-compact detection head with a built-in point light source and a high-sensitivity, low-noise photo IC, and the MRA (Micro Roofmirror Array) scale with enhanced light utilization efficiency. Measuring only 4 x 5 mm, this detection head is capable of high-resolution (0.1µm) position detection with significantly reduced power consumption.
Incorporated into Canon's ultra-compact encoder for controlling focus and zoom lens positioning, the SR sensor and MRA scale are used in the company's digital video camcorders, EF lenses for EOS-series SLR cameras, and broadcast television lenses.
Other types of encoders include ultrahigh-resolution laser rotary encoders that rely on optical diffraction and interference.

SR Sensors and MRA Scales