A dead pixel is a defect or malfunction that occurs in the LCD screen of a computer monitor, appearing as a small dot or square that is always dark or always light rather than displaying a color to form part of the picture on the monitor. A dead pixel also might be always one color rather than being either dark or light. Each type of dead pixel is caused by a slightly different malfunction in the monitor. Dead pixels also can occur in the image sensors used in digital cameras. In these cases, the pixel does not sense the correct light levels, creating defects in the resulting photographs.
When a bad pixel appears as a dark spot on the screen, the cause can be traced to a transistor flaw. The transistor that controls the pixel is stuck in an "on" position, so that current constantly flows, making it impossible for light to reach the layer below, where the red, green or blue color would be generated for the pixel to work correctly. In a light or white-colored dead pixel, the opposite occurs. One or more of the transistors is not functioning, so that the light that reaches the RGB layer is not differentiated into the correct color.
Another form of dead pixel occurs when the RGB layer is defective. This usually appears as a pixel that stays a single color but typically is not black or white. A pixel also can malfunction because of broken bonds between the electrode layer of the screen and the board for the video driver. Called a Tape Automated Bonding, or TAB error, this type of malfunction occurs most often because the monitor has experienced extreme heat or cold or because of physical damage. A TAB error does not affect a single pixel but typically involves an entire row, so it appears as a black horizontal or vertical stripe across the monitor.
If a monitor appears to have dead pixels, it can be checked using a dead pixel test, usually a set of software tests that determine where the dead pixel is and how it is malfunctioning. A dead pixel test can be found on numerous online sites, along with instructions about how to fix the problem. In some cases, there is no available correction, and the monitor must be replaced. Dead pixels are sometimes referred to as stuck pixels, and if a pixel is actually stuck rather than dead, it is more likely that it can be corrected easily.