Here are the important characteristics of mice and trackballs:
Mice are available that use the following mechanisms:
Mechanical mice use a rubber-coated ball that contacts the mouse pad. Moving the mouse causes the ball to move, which in turn causes one or both of the internal cylindrical rollers with which the ball is in contact to move. These two internal rollers are oriented at 90° to each other, which allows one to respond to horizontal mouse movement and the other to vertical mouse movement. Connected to the end of each roller is a wheel with many small notches around its circumference. As the wheel rotates, these notches alternately pass or block light from an LED aimed to impinge on a sensor. The rate at and duration for which the sensors see the light flickering correspond to how fast, how far, and in what direction the ball is moving. Most mice and all trackballs are mechanical. Modern mechanical mice are inexpensive and reliable, but require frequent cleaning.
Early mechanical mice provided limited resolution, were relatively unreliable, and required very frequent cleaning. Manufacturers addressed these problems by introducing optical mice, which substituted an optical sensor for the mouse ball. Reducing the number of moving parts greatly increased reliability. Because the optical mouse was a sealed unit, cleaning was needed much less often. The sole drawback was that the sensor of an optical mouse required a special mouse ...