Mayhew Labs writes:
The Rotary Encoder LED Ring is a position indicator for rotary encoders. Traditional knobs with indicator lines do not give accurate representation of an encoder’s value since rotary encoders have no start or end point.
Rotary encoders are used as input devices in applications where a single encoder has many functions or where a control responds to the rotation rate of an encoder. In these applications, the Rotary Encoder LED Ring offers a visual representation of how an encoder is reacting to the input as well as the current position or control value.
Rotary Encoder LED Ring - [Link]