Optomechanical mouse

Alternatively called the ball mouse, the optical-mechanical or optomechanical mouse consists of a ball that rolls one of two wheels inside the mouse. Each wheel contains a circle of holes or notches, allowing an LED (light-emitting diode) to shine through and be detected by a sensor. As the wheel spins these lights represent an x-axis or y-axis for the mouse pointer on your screen. This mouse is more accurate than a mechanical mouse that uses only wheels and rollers, however, is not as good as an optical mouse.

Mechanical mouse, Mouse terms, Optical mouse