The
Liquid Crystal Display is the most used type of
display
in
modern portable electronic devices. It replaced
the
plasma display and the
Vacuum fluorecent display
in portable
PCs,
calculators and
portable electronic games,
since it has much
lower power consumption.
The basic display is built up like this:
_________________ polarization filter
----------------- glass
================= Transparent electrode
liquid crystals
================= transparent electrode
----------------- glass
================= polarization filter
----------------- mirror or backlight
The
polarization filter lets only light
polarized the right
way
through. The glass is there to hold everything together.
Since the LCD itself doesn't
emit light an
external lightsource
is needed. Either we
reflect light back by the use of a
mirror
or a
backlight is used. Some LCDs have a
combination of the
two. By using a
semi transparent mirror and a
backlight,
these LCDs are called
hybrid LCDs. The backlight can be
LEDs,
electroluminescent backlight panels
or
fluorecent tubes. The electrodes are made from
indium tin oxide.
The liquid
consists of
organic crystaline molecules. These
crystals
change the polarization of the light.
In polarized
light the
lightwaves only move in one
direction. This means that if light
moves
through a
horisontal filter the light will be
horizontaly polarized(HP)
A
vertical filter only lets verticaly polarized(VP) through.
This also means HP. light is blocked by a vertical filter.
The crystals can change the polarization or let light it
pass unchanged
depending on orientation. This orientation changes if
voltage is
applied
and thus we can change the light so that it is either blocked or not.
Usualy
AC is used
since DC causes an
electrolytic effect and over time
breakes down the liquid.
By
adjusting how much the crystals turn we can get
several
shades of grey. By
adding colour filters we can make colour
displays. The
AMLCD(also called TFT) is a
further development of this
technology.