How They Work

• A digital mouse illuminates the surface under the mouse using an LED or laser. An optical sensor, which is similar to a tiny digital camera, is present in the base of the mouse. The optical sensor captures images of the surface under the sensor, at a rate of approximately 1,512 frames per second. A digital signal processor calculates the differences between each frame, to work out how far the mouse has moved in the horizontal and vertical directions. The processor then sends the horizontal and vertical coordinates to the computer to update the mouse pointer position on the computer's screen.
  
  

Digital Image Correlation

• The digital signal processor, or DSP, uses digital image correlation to calculate how far the mouse has moved in the x and y directions. You can demonstrate this by drawing two dots 1 cm apart on a clear plastic sheet. On an identical sheet, draw another two dots with the same spacing but in a different location. Place one sheet on top of the other and move it until the dots on the top overlay the dots on the bottom sheet. If the overlap at the top and side of the sheets is measured, it gives the distance the dots have moved horizontally and vertically. Optical mice use an image of the surface underneath the optical sensor to build reference points, and then calculate how far those reference points have moved in the next image. Repeated over 1,500 times a second, this method allows tiny distances to be measured, making optical mice far more precise than the older mechanical mice.
  
  

Digital Mice Versus Mechanical Mice

• A mechanical mouse uses a ball in its base to register horizontal and vertical movement. As the ball rotates, it turns two rollers, each fitted with a disc containing a number of evenly spaced slits. An LED is located on one side of the disc and a light sensor on the other. As the ball moves the rollers, light shining through the slits creates pulses on the output of the light sensor. A processor counts the pulses to measure the horizontal and vertical movement. Mechanical mice have many disadvantages compared to optical mice. Only a limited number of slits fit on the detector disc, limiting the resolution, and the ball often picks up dirt and dust, making it stick and hindering movement. The ball only works on a rough surface, such as a fabric mouse mat, or it skates across the surface and does not register movement correctly.
  
  

Types of Digital Mice

• Two types of digital optical mice are in use at the time of publication and vary in the method they use to illuminate the surface under them. Some optical mice use a red LED to light the surface, as these are cheap to manufacture and the silicon optical sensor works well with red light. More expensive optical mice use an infrared laser for illumination, and the focused light produced allows better tracking and response times. Infrared lasers also work on a wider range of surfaces than LEDs, with some laser mice able to work on glossy and reflective surfaces, such as glass or mirrors.