INPUT/OUTPUT 95
(a)
(b)
y
z
Rear glass plate
Liquid crystal
Rear
electrode
Rear
polaroid
Front glass plate
Front electrode
Front polaroid
Bright
Dark
Light
source
Notebook computer
􀀀
@ @€ €À À􀀀 Figure 2-5. (a) The construction of an LCD screen. (b) The grooves on the rear
and front plates are perpendicular to one another.
display, for example, the rear electrode might have 640 vertical wires and the
front one might have 480 horizontal ones. By putting a voltage on one of the vertical
wires and then pulsing one of the horizontal ones, the voltage at one selected
pixel position can be changed, making it go dark briefly. By repeating this pulse
with the next pixel and then the next one, a dark scan line can be painted, analogous
to how a CRT works. Normally, the entire screen is painted 60 times a
second to fool the eye into thinking there is a constant image there, again, the
same way as a CRT.
The other scheme in widespread use is the active matrix display. It is considerably
more expensive but it gives a better image so it is winning ground.
Instead of just having two sets of perpendicular wires, it has a tiny switching element
at each pixel position on one of the electrodes. By turning these on and off,
an arbitrary voltage pattern can be created across the screen, allowing for an arbitrary
bit pattern.
So far we have described how a monochrome display works. Suffice it to say
that color displays uses the same general principles as monochrome displays, but
that the details are a great deal more complicated. Optical filters are used to
separate the white light into red, green, and blue components at each pixel position
so these can be displayed independently. Every color can be built up from a
linear superposition of these three primary colors.