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B
Appendix
LCD Panel Display
Liquid Crystal Displays (LCDs) are categorized as non- emissive display devices, in that
respect, they do not produce any form of light like a Cathode Ray Tube (CRT). LCDs either
pass or block light that is reflected from an external light source or provided by a back/side
lighting system. There are two modes of operation for LCDs during the absence of an
electric field (applied Power); a mode describes the transmittance state of the liquid crystal
elements. Normal White mode: the display is white or clear and allows light to pass through
and Normal Black Mode: the display is dark and all light is diffused. Virtually all displays in
production for PC/Workstation use are normal white mode to optimize contrast and speed.
A dot matrix LCD display works is as follows: A twisted nematic (TN) LC display consists of
two polarizers, two pieces of glass, some form of switching element or electrode to define
pixels, and driver Integrated Circuits (ICs) to address the rows and columns of pixels. To
define a pixel (or subpixel element for a color display), a rectangle is constructed out of
Indium Tin Oxide -- a semi-transparent metal oxide (ITO) and charge is applied to this area
in order to change the orientation of the LC material ( change from a white pixel to a dark
pixel).
Polarizers
Polarizers are an integral part of a LCD display, possessing the unique
property of only passing light if it is oriented in a specific (oriented) direction. To utilize this
phenomena in TN LC displays, the bottom polarizer orients incoming light in one direction.
The oriented light passes through the LC material and is either unaltered or "bent" 90
degrees. Depending on the orientation of the top polarizer, this light will either pass through
or be diffused. If the light is diffused, it will appear as a dark area. Figure 3 is a simple
illustration of the sequence of events that occur when light passes through a simple twisted
nematic LC display.
Passive and Active Matrix Displays
Addressing and color technology describes the
method employed to transfer charge (data or the display image) from the outside world to
the display. Unlike a CRT, which is just a surface of phosphors scanned with a beam of
electrons in a vacuum, a liquid crystal display is an array of conductors with metal (or metal
like) lines running in both horizontal and vertical directions. For the case of a CRT, the
electrons travel through a resistance free medium (vacuum) and deliver a clear consistent
signal. The charge traveling through the metal lines of a LCD matrix is affected by the
properties of the metal. As a result, the magnitude and waveform of the applied charge can
vary from one end of the display to the other. This variation imposes limitations on display
quality and capabilities. Active matrix displays can cost twice as much as an equivalent
passive matrix display and add more than $1000 to the cost of a notebook form-factor
computer. However, active matrix displays produce a stunning and bright image without
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