Optical Devices

Purely optical mice completely eliminate the need for moving parts in the mouse (aside from those in the buttons). They do this by using a light beam that bounces off a reflective surface underneath the mouse, as depicted in Figure 15.12. This surface must contain some form of texturing for the light sensor to detect, much as an opto-mechanical mouse detects changes in the light when the spoked wheel attached to a roller moves.

Light Source

Light Sensor

Figure 15.12

Optical mice use a light beam to detect movement against a surface.

Light Source

Light Sensor

Figure 15.12

Optical mice use a light beam to detect movement against a surface.

Chapter 15

Optical trackballs work in much the same way, but because the user turns a ball, optical trackballs don't eliminate the ball. Instead, the ball is covered in a pattern of dots, and instead of rollers, the trackball uses a sensor like the one used in an optical mouse.

Optical mice and trackballs offer advantages in reliability over mechanical and opto-mechani-cal designs. Because they don't rely on moving parts in their sensors, these devices' sensors aren't as likely to become clogged with dirt. These devices also aren't as likely to suffer from broken parts. Optical mice and trackballs do require occasional cleaning, however. An optical mouse typically uses felt pads as a buffer between its bottom and the desk or pad on which it rests, and these felt pads can accumulate dust over time, which increases drag. Similarly, the contact points on which an optical trackball's ball rests can accumulate dust and dirt. In extreme cases, an optical device's sensors might become clogged with dirt, which must be blown out. Such occurrences are quite rare, however. Cleaning optical devices is typically easier than is cleaning mechanical and opto-mechanical devices.

Most optical mice suffer from a problem in that they require special mouse pads. These pads are reflective but are imprinted with a pattern of dots that the mouse uses to track movement. If you lose or damage the pad, the mouse becomes useless. A few optical mice, such as Microsoft's IntelliMouse Optical, can track on a wide variety of textured surfaces, such as blue jeans or wood-grain desktops. Because optical trackballs texture their balls, they don't require special pads.

In the grand scheme of pointing devices, optical mice and trackballs are fairly rare. Microsoft (http://www.microsoft.com) and Mouse Systems (http://www.mousesystems.com) manufacture most of the optical mice available today, whereas Logitech

(http://www.logitech.com) produces most of the optical trackballs available today. All three companies also produce conventional opto-mechanical devices.

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