The LCD’s controller for a modern active-matrix LCD addresses a specific
row and then a specific column; the pixel at the point where a positive
charge and a ground meet receives power and goes dark. Active-matrix
designsinclude tiny transistors at each meeting point of a column and
row, yielding a sharper and faster-to-refresh image.Everything else is a matter of little details. The amount of voltage can be
adjusted to make the dot fully dark or just barely darkened; a typical LCD
has the equivalent of a 256-level grayscale. The other part of the equation
involves color, which is created through the use of three subpixels with red,
green, or blue filters.
Doing the math ...................................................................................
Each of the LCD points on a typical laptop’s screen can be set at an intensity
from 0 to 255, which is the same as saying there are 256 possible settings.
With three pixels at each location for a color monitor, that means an LCD’s
palette is made up of about 16.8 million colors (256 red ×256 green ×256 blue =
16,777,216).
For the record, that range of colors is much wider than most humans can dis-
cern. However, each color is a specific digitalvalue; a standard television set
uses a continuous and essentially infinite range of analogcolors and bright-
ness levels to display our daily drivel and some extraordinarily gifted (or
extremely picky) viewers claim to prefer an analog image to a digital one.
Dead pixels ..........................................................................................
A modern laptop with a screen resolution of 1,280×800 has 1,024,000 address-
able points and three times that when you consider subpixels with colored
filters. That’s a lot of little points of darkness, and the reality is that some
pixels may be dead on arrival while others may fade away with time. If you
use a white background on your LCD, a dead pixel may appear black or gray;
if you use a black background, a dead pixel may only be noticeable if it lights
up white or colored.
And so it is not unusual to have a few dead spots, or even more than a few. In
fact, many laptop makers will not make substitutions for an LCD under war-
ranty until the percentage of bad pixels is more than a few percent, which is
quite a few.
Dead pixels, alas, are not repairable. The only option is to install a new screen
at a cost of several hundred dollars... and even then you have no guarantee
that there won’t be a few gaps here and there. The good news: The human eye
and brain have an incredible ability to fill in the gaps.
Chapter 12: Seeing the Light: LCDs and Video 189