LIQUID CRYSTAL DISPLAY
• History of LCD’s?
– First discovered in 1888, by Austrian botanist
– He melted a curious cholesterol-like
substance (cholesteryl benzoate), it first
became a cloudy liquid and then cleared up
as its temperature rose. Upon cooling, the
liquid turned blue before finally crystallizing.
– Eighty years passed before RCA made the
first experimental LCD in 1968.
• What are Liquid Crystals
– Liquid crystals are substances that do not
melt directly to the liquid phase but first pass
through a paracrystalline stage in which the
molecules are partially ordered. In this stage a
liquid crystal is a cloudy or translucent fluid
but has some of the optical properties of a
– These are the main elements in a Liquid
• Liquid Crystal Types
– Smectic liquid crystals have molecules
parallel to one another, forming a layer, but
within the layer no periodic pattern exists.
– Nematic types have the rod-like molecules
oriented parallel to one another but have no
– Cholesteric types have parallel molecules,
and the layers are arranged in a helical, or
• Kinds of LCD
Twisted Nematics (TN)
– A particular liquid crystal used by LCD’s is the
liquid crystal, called twisted nematics (TN).
– Applying an electric current to these liquid
crystals will untwist them to varying degrees,
depending on the current's voltage.
– LCD’s use these liquid crystals because they
react predictably to electric current in such a
way as to control light passage.
Supertwist (STN) LCD’s
– Although twisted nematic LCD’s may be
driven in a time multiplexed fashion to
increase the amount of information displayed,
they are restricted in terms of reduced
contrast and limited viewing angle.
– To achieve more highly multiplexed displays,
supertwist technology is employed.
• How LCD’s work
– LCD’s work using these 4 concepts:
• Light can be polarized.
Liquid crystals can transmit and change polarized
The structure of liquid crystals can be changed by
There are transparent substances that can conduct
An LCD is a device that uses these four
facts in a surprising way!
– If we apply an electric charge to liquid crystal
molecules, they untwist! When they straighten
out, they change the angle of the light passing
through them so that it no longer matches the
angle of the top polarizing filter.
– Consequently, no light can pass through that
area of the LCD, which makes that area
darker than the surrounding areas.
Illustration of a simple LCD
A simple LCD display from a
• Colored LCD’s
– An LCD that can show colors must have three
subpixels with red, green and blue color
filters to create each color pixel.
– Through the careful control and variation of
the voltage applied, the intensity of each
subpixel can range over 256 shades.
– Combining the subpixels produces a possible
palette of 16.8 million colors (256 shades of
red x 256 shades of green x 256 shades of
Diagram of a colored
Uses of LCD:
1. Computers 4. Laptops
2. Thermometers 5. Cellphones
3. Mood Rings