Hardware Reference
In-Depth Information
FIGURE 3.2
LCDmonitor
An LCD screen has two polarized fi lters, between which are liquid crystals. In order for light
to appear on the display screen, it must pass through both fi lters and the crystals. The second
fi lter, however, is at an angle to the fi rst, so by default nothing passes through. By applying
current to the crystal, you can cause it to twist, which also twists the light passing through it.
If the light twists so that it matches the angle of the second fi lter, it can pass through the fi lter
and light up an area of the display. On a color LCD, an additional fi lter splits the light into
separate cells for red, green, and blue.
CRTs
You may occasionally also see an older type of monitor, the
cathode ray tube (CRT)
. This
is a boxy monitor, like an old-style television.
A CRT is essentially a large vacuum tube. At the back of the CRT is a long, narrow
neck containing a cathode, and at the front is a broad, rectangular surface with colored
phosphors on it. When the cathode is heated, it emits negatively charged electrons. Those
electrons are attracted to the positively charged front of the CRT where they strike the
phosphors and cause them to light up. In a color CRT, there are three electron guns, one
for each color: red, green, and blue. The actual colors are created by the phosphors on the
screen. For each
pixel
(colored dot) on the screen, there are three phosphors—red, green,
and blue—arranged in a triangle formation called a
triad
. Each electron gun works only on
dots of a certain color. So, for example, if a certain pixel is supposed to be purple, the red
and blue guns will fi re at it, but the green gun won't.
