The most widespread encoders use the optical (or photoelectric) technology to detect signals; this differentiates in two main scanning methods: the transmissive and the reflective one.

Transmissive optical scanning

In the transmissive optical encoder, the scan system is based on the rotation of a graduated disc - or code wheel -  patterned with alternating opaque and clear (transparent) segments; the code wheel is illuminated by an infrared light source positioned perpendicularly to the sensor. The disc beams its image on the surface of several receivers, opportunely masked by another grid (called “collimator”) with the same pitch as the other. The receivers have the task to sense the variations of light that take place during the rotation of the disc, converting them into corresponding electric pulses. In linear transducers the operating principle is similar, with the difference that the motion is detected on a linear reading system.

 

Incremental optical encoder	Absolute optical encoder

Reflective optical scanning

The reflective optical system is also based on the photoelectric scanning of a code wheel, but in this type of technology the light source and the receiver are in the same surface mount package: the light source illuminates the code wheel formed by darker segments alternated with reflective ones, where the light is reflected and detected by the sensor (receiver), which will transform the variations in corresponding pulses, as per the transmissive system. This type of reading allows reducing the size of the device while maintaining its performances; it’s an ideal solution for those applications that require miniaturized encoders with a high resolution.