Hardware Reference
In-Depth Information
ent between the plates, horizontally polarized light let in by the rear polarizing fil-
ter would be blocked by the front polarizing filter, making the screen black.
However the twisted crystal structure of the LCD molecules guides the light as
it passes and rotates its polarization, making it come out vertically. 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, block-
ing the light in those parts.
Two schemes can be used for applying the voltage. In a (low-cost)
passive
matrix display
, both electrodes contain parallel wires. In a 1920
1080 display
(the size for full high-definition video), for example, the rear electrode might have
1920 vertical wires and the front one 1080 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. A
pixel
(originally a picture element, is a colored dot from which all digital images are
built). By repeating this pulse with the next pixel and then the next one, a dark
scan line can be painted. Normally, the entire screen is painted 60 times a second
to fool the eye into thinking there is a constant image there.
The other scheme in widespread use is the
active matrix display
. It is more
expensive but it gives a better image. Instead of just having two sets of perpendic-
ular wires, it has a tiny switching element at each pixel position on one of the elec-
trodes. By turning these on and off, an arbitrary voltage pattern can be created a-
cross the screen, allowing for an arbitrary bit pattern. The switching elements are
called
thin film transistors
and the flat panel displays using them are often called
TFT displays
. Most notebook computers and stand-alone flat panel displays for
desktop computers use TFT technology now.
So far we have described how a monochrome display works. Suffice it to say
that color displays use the same general principles as monochrome displays but 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 superposi-
tion of these three primary colors.
Still new screen technologies are on the horizon. One of the more promising is
the
Organic Light Emitting Diode
(
OLED
) display. It consists of layers of elec-
trically charged organic molecules sandwiched between two electrodes. Voltage
changes cause the molecules to get excited and move to higher energy states.
When they drop back to their normal state, they emit light. More detail is beyond
the scope of this topic (and its authors).
×
Video RAM
Most monitors are refreshed 60-100 times per second from a special memory,
called a
video RAM
, on the display's controller card. This memory has one or
more bit maps that represent the screen image. On a screen with, say, 1920
×
1080
