Flat Panel Displays
CRTs are far too bulky and heavy to be used in notebook computers, so a
completely different technology is needed for their screens. The most common
one is LCD (Liquid Crystal Display) technology. It is highly complex, has
many variations, and is changing rapidly, so this description will, of necessity, be
brief and greatly simplified.
Liquid crystals are viscous organic molecules that flow like a liquid but also
have spatial structure, like a crystal. They were discovered by an Austrian botanist
(Rheinitzer) in 1888, and first applied to displays (e.g., calculators, watches) in
the 1960s. When all the molecules are lined up in the same direction, the optical
properties of the crystal depend on the direction and polarization of the incoming
light. Using an applied electric field, the molecular alignment, hence the optical
properties, can be changed. In particular, by shining a light through a liquid 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
used to create electric fields in the liquid crystal. Different parts of the screen get
different voltages, to control the image displayed. Glued to the front and rear of
the screen are polaroids because the display technology requires the use of polarized
light. The general setup is shown in Fig. 2-5(a).
Although many kinds of LCD displays are in use, we will now consider one
particular kind of display, the TN (Twisted Nematic) display as an example. In
this display, the rear plate contains tiny horizontal grooves and the front plate contains
tiny vertical grooves, as illustrated in Fig. 2-5(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 polaroid. It only allows in horizontally
polarized light. At the front of the display is a vertical polaroid. It only
allows vertically polarized light to pass through. If there were no liquid present
between the plates, horizontally polarized light let in by the rear polaroid would
be blocked by the front polaroid, making the screen uniformly black.
However the twisted crystal structure of the LCD molecules guides the light
as it passes and rotates its polarization, making it come out horizontally. Thus in
the absence of an electric field, the LCD screen is uniformly bright. By applying
a voltage to selected parts of the plate, the twisted structure can be destroyed,
blocking the light in those parts.
Two schemes are commonly used for applying the voltage. In a (low-cost)
passive matrix display, both electrodes contain parallel wires. In a 640 ´ 480