Hardware Reference
In-Depth Information
crystal, the intensity of the light exiting from it can be controlled electrically. This
property can be exploited to construct flat panel displays.
An LCD display screen consists of two parallel glass plates between which is a
sealed volume containing a liquid crystal. Transparent electrodes are attached to
both plates. A light behind the rear plate (either natural or artificial) illuminates
the screen from behind. The transparent electrodes attached to each plate are used
to create electric fields in the liquid crystal. Different parts of the screen get dif-
ferent voltages, to control the image displayed. Glued to the front and rear of the
screen are polarizing filters because the display technology requires the use of
polarized light. The general setup is shown in Fig. 2-33(a).
Liquid crystal
Rear glass plate
Rear
electrode
Rear
polaroid
Front glass plate
Front electrode
Front polaroid
y
Dark
z
Bright
Light
source
Notebook computer
(b)
(a)
Figure 2-33.
(a) The construction of an LCD screen. (b) The grooves on the rear
and front plates are perpendicular to one another.
Although many kinds of LCD displays are in use, we will now consider one
particular kind, the
TN
(
Twisted Nematic
) display, as an example. In this display,
the rear plate contains tiny horizontal grooves and the front plate contains tiny ver-
tical grooves, as illustrated in Fig. 2-33(b). In the absence of an electric field, the
LCD molecules tend to align with the grooves. Since the front and rear alignments
differ by 90 degrees, the molecules (and thus the crystal structure) twist from rear
to front.
At the rear of the display is a horizontal polarizing filter. It allows in only hor-
izontally polarized light. At the front of the display is a vertical polarizing filter. It
allows only vertically polarized light to pass through. If there were no liquid pres-