Projection
television displays utilizing LCD technology have been around on mass
production scale. During most of this time period the devices were front-type
projection units. The display unit was mounted on a table or hung from a
ceiling to be projected to a wall or screen. This sufficed for most commercial
applications and in some home use. Recent years have seen an explosion in the
number of rear-type LCD projection televisions.
Their
all-in-one design eliminates the need for unsightly equipment and wires
normally found in front projection setups. Advances in screen design have
allowed the new rear-projection televisions to generate bright, crisp video
with improved viewing angles that rivals front projection devices. Geometric distortion and convergence issues
are virtually non-existent.
All
of the items discussed can be applied to Front Projection, Rear Projection, and
Direct-View LCD display units. The video process circuits and light box
assemblies function the same way. The only difference between the two is how
the generated image is projected. Since most homes will have the rear
projection unit, the descriptions to follow will focus on them.
LIQUID CRYSTAL TECHNOLOGY
Three
items are required for and LCD display to function: A back-light source,
polarizing of the light source, and liquid crystals to manipulate the polarization of this light.
LIGHT POLARIZATION
Normal
light can be transmitted anywhere along a 360-degree plane. This is especially
true for scattered light being reflected off of random surfaces. This is why
polarizing sunglasses are so effective.
a polarizing filter that only allows light on a vertical plane is used
to filter the incoming back-light. If another filter is placed in front, allowing only horizontal phased light to
pass, the light is effectively blocked. LCD devices use this basic principle to
control the amount of light passing through.
POLARIZING FILTERS
LIQUID CRYSTALS
Although
liquid crystals come in many different forms, the key difference between the
types is the arrangement of the crystals. Some have randomly arranged crystals
while others are arranged in a specific pattern. Other differences include how
they react to temperature, pressure, magnetic fields, and electrical current.
The crystals used in LCD display devices are know as “chiralnematic”. As the
crystals are arranged in layers, the crystals naturally twist slightly with
each subsequent layer. Layers can be added until the crystals complete a
90-degree “twist”. This twist in the crystalline structure can be used to take
a certain polarized light and shift
its
phase accordingly. The other characteristic of a nematic-type crystal is it
ability to react to an electric potential. If an electrical potential is
applied to the crystal layers, the twisted crystals will begin to “un-twist” in
an amount proportionate to electrical potential until, when enough potential is
reached, they line up perfectly. This is how liquid crystals are used to
control light and generate images on a display device.
Figure
illustrates how the naturally occurring twist in the crystalline layer rotates
the incoming polarized light to match the polarized plane of the second filter.
In this normal state, the crystals rotate the polarized light 90-degrees to
match the plane of the outgoing polarizing filter allowing the back-light to
pass through.
LIQUID CRYSTAL EFFECT ON POLARIZED LIGHT
Figure below illustrates, an electrical
potential is applied to fully “un-twist” the crystals. The polarized back-light is now perpendicular to the outgoing filter and no light will pass. By varying
this electrical potential, the amount of effect on the twisted crystals can be
altered to a point where linear control of light output is achieved.
VOLTAGE EFFECT ON LIQUID CRYSTALS
CREATING COLOR
All that is required for LCD pixels to create
color is to place a color filter in front of each pixel. By using red, green,
and blue color filters, the required primary colors are generated to produce
the millions of color variations needed for graphics and video display. Modern
LCD technology uses what is known as Thin-Film Transistor (TFT) technology.
Each pixel has its own transistor and capacitor, which increase the contrast
rating of the LCD due to the increased retention of charge.
This helps to dramatically increase the response time for
each pixel as they are scanned. Control of each pixel is simply a matter of
addressing a particular column and individually activating each pixel in that
row with a properly timed address pulse on the horizontal plane. The higher the
pulse level, the more the crystals align, producing a lower light output.
TFT LCD TECHNOLOGY
DIRECT VIEW LCD
This type of display device uses the methods
described previously to generate video by placing vertical columns of red,
green, and blue filters over a liquid crystal layer. Thin-Film Transistors
control the amount of light passing through each pixel. The light source is
generated behind the LCD array. Fluorescent lamps are the most common to use. A
diffuser plate distributes the light from the lamps to provide uniform
brightness to all areas of the screen. A polarizing sheet is installed next to
allow only one plane of light to pass. This light enters the LCD structure and
is twisted 90-degrees. Another polarizing sheet is placed in front of the
pixels at exactly 90 degrees. With no voltage present to “twist” the crystals
into alignment, full passage of the back-lighting is allowed. Control of the
light output from each pixel is now possible by scanning the matrix of pixels
using carefully timed pulses at the horizontal and vertical planes of the
columns and rows.
DIRECT VIEW LCD PANEL [TYPICAL]
