With traditional optical mice, an LED illuminates the surface. The light reflected back is captured via a system of lenses and then sent to the sensor. This creates a photograph of the surface. If the mouse moves, each photo will be different. Then all the sensor has to do is to analyze the differences to determine the coordinates. When we say "photo," we obviously don't mean a realistic portrait of your desk, but rather snapshots that bring out the irregularities and the structure of its surface. This makes it easier to compare the differences and determine movement.