Dealing with dead pixels
Turned on or "lit" pixels are a fairly common occurrence in LCD manufacturing. Like their CRT
counterparts, LCD manufacturers have set limits as to how many defective pixels are acceptable for
a given LCD panel, based on user feedback and manufacturing cost data. The goal in setting these
limits is to maintain reasonable product pricing while minimizing distraction from defective pixels for
maximum user comfort. Considering the number of pixels contained in an LCD panel, this defective
rate is quite minute. For example, a panel with a native resolution of 1024 x 768 pixels contains a
total of 2,359,296 red, green and blue pixels per panel (1024 x 768 x 3 = 2,359,296). Therefore, a
panel with 20 lit pixels would have a sub-pixel defect rate of: (20/2,359,296)*100 = 0.0008%.
Figure 1: Cross-Section View of an LCD Panel
Fortunately, the most widely-used operating system, Windows (3.xx/95/98/NT/2000), and Windows-
based applications tend to mask, or hide, the phenomenon of turned-on pixels due to their reverse
video of black characters on a white background. Older DOS/text-based applications will suffer by
comparison when the graphics mode calls for white characters on a black background. Using these
DOS-based applications will "highlight" the occasional red, green or blue turned-on pixel, causing
possible concentration problems for some users.
Setting a Standard
In the interest of setting a standard for allowable pixel faults in a display for manufacturers, an ISO
specification (ISO 13406-2) was established. The specification determined four fault classes of LCD
displays, with each having a certain number of pixel faults allowed (see Table 1). Class 1 LCD
displays are considered “perfect,” allowing zero pixel or sub-pixel faults. However, this standard has
been viewed as virtually impossible or much too costly for LCD display manufacturers to consider
for mass production. Class 2, which allows for five pixel or sub-pixel faults, has been an extremely
difficult standard for manufacturers to achieve in mass production but a select few have managed
to be certified. This groundbreaking achievement is a major breakthrough in the visual display
industry and has set the bar for others to follow. Consumers should be aware, though, that Class 2
