LCD is winning the race to be the dominant display technology for HDTV.
According to DisplaySearch, in 2007, the market for LCD (liquid crystal
diode) TVs was estimated at $27.4 billion. LCD TV shipments rose a
stronger than expected 52% year to year in the second quarter of 2008 to
nearly 7.5 million units (79.8% of total HDTV sales).
The LCD
screen on a LCD HDTV is a selective light filter. It does not in itself
emit light, but selectively filters light passing through the display
from the back. Most HDTVs include either a single sheet of luminescent
plastic or sidelighting with fluorescent lamps to provide the backlight.
Both techniques are difficult to control.
Many consumers focus
on screen resolution when selecting a High Def (HDTV) system. Is this
HDTV 720p, 1020i, or 1020p? However, when an expert group, the SMPTE
(Society of Motion Pictures and Television Engineers), recently ranked
the importance of screen resolution, it ranked fourth. The SMPTE ranked
contrast ratio, color saturation, color temperature and grayscale ahead
of screen resolution in importance. The Imaging Science Foundation
reached a similar conclusion. Backlighting is the key component to
improvement in contrast ratio and grayscale.
It is important that
an LCD system allows for adjustment of the level of backlighting, and
preferably, performs this function automatically. This will improve the
contrast of the picture. Also, a reduced backlight level will also
reduce power consumption and extend lamp life, as the power used and
lamp lifetime is directly related to the backlight level. A lamp with a
50,000 hour lifetime at 6 milliamps may only have a 30,000 hour lifetime
if the current is 7 milliamps, just 1 milliamp higher (Example: Sharp
Electronics LCD Display).
Another important factor in the choice
of backlighting is the consideration of "blur". Blur occurs when there
is rapid motion in the programming, such as sports. When the picture
changes quickly, the pixels can lag in response. Effective backlight
engineering can reduce this significantly by strobing the backlighting,
so that the individual pixels do not remain lit longer than needed.
Examples of this technique include the Philips "Aptura" and the Samsung
"LED Motion Plus".
Sidelighting
With sidelighting, the
backlight is located in the back of the LCD panel, and to each side to
create panel illumination. To make the light more uniform, a light
diffuser is located between the lamps and the rear of the optical filter
module. Most LCD systems that use sidelighting use cold cathode
fluorescents (CCF) lamps. This technology provides a very bright white
light. The lifetime of the lamps is usually between 10,000 to 20,000
hours (6-12 years at a rate of 5 hours of use per day).
The most
important disadvantage is that the intensity of light cannot be varied
locally, and sidelighting can look non-uniform. Some CCF lamps contain
mercury. This is an ecological disadvantage.
The major drawbacks:
The intensity of light cannot be varied locally. This results in a lower contrast picture.
The light can be non-uniform.
Some CCF lamps contain mercury.
Conventional Backlighting
Most
backlighting is performed with a single sheet of electroluminescent
plastic. These panels can age (though improvements in this area have
reduced this problem). Electroluminescent Panel backlighting uses
colored phosphors to generate light. Displays using this technique can
be thin and lightweight, and provide even light distribution.
The major drawbacks:
Limited life of 3,000 to 5,000 hours to half brightness (2-3 years at a rate of 5 hours of use per day).
The intensity of light cannot be varied locally.
LED Backlighting
LED
backlighting uses a matrix of LEDs that can be controlled individually
to offer very good control. In this configuration, a large number of
LEDs are mounted uniformly behind the display. Brightness improvements
in LEDs have made them more practical for backlighting. NEC pioneered
this technique with their monitor, the 2180WG. Other companies, such as
Samsung, have recently introduced versions of this technology. This
should push prices down (the Samsung model cost less than 1/3 the price
of a comparable NEC monitor).
This system offers local dimming
technology and can achieve 1,000,000:1 dynamic contrast ratio. True
actual black levels (the "Holy Grail" of LCD) are attainable by shutting
off a pixel's light source. This can eliminate the grayish black
picture typical of many LCD HDTVs. Groups of LEDs can be locally
controlled to produce more light, increasing the brightness when
necessary.
The problem with conventional white LEDs is that their
spectrum is not ideal for photographic reproduction because they are
basically blue LEDs with a yellow phosphor on top. Their color spectrum
has two peaks, one at blue and another at yellow. White LEDs that are
based on a group of red, green and blue LEDs are ideal for the RGB color
filters of an LCD and can result in the most saturated colors. Sony's
"Triluminos" LED backlight system is an example of this technique.
Sometimes,
these HDTVs have a row of LEDs across the top and bottom of the
display. This improves the brightness and color reproduction, and
enables almost complete uniformity across the display.
LED
backlights are much more efficient that alternative techniques. And
since the LED backlight waste less energy, less heat is generated, that
must be managed with air conditioning. LED backlights offer a long
operating lifetime of more than 50,000 hours (more than 25 years of use
at a rate of 5 hours a day).
LED backlit systems may not show
well in a typical retail environment. Typically, the screens are
somewhat reflective in order to achieve the high contrast ratio, and
subsequently, a brightly lit showroom may not flatter this technology.
In a more typical less-lit environment, this problem should not be as
noticeable.
Engineering Concerns
The electrical
engineering is simpler with LED backlights. Cold cathode fluorescent
lamps (CCF) require high AC voltages (1,500 VAC) at startup, and
operating voltages of 700 VAC or more. When the lamp is new it may take
much less voltage to start, but with age the voltage requirements
usually increase, complicating a designer's problem. An
electroluminescent panel requires a voltage of about 100 VAC @ 400
Hertz.
A power inverter (to generate VAC) is needed for both CCF
and electroluminescent panels to operate with a DC power supply. This is
a significant expense, in cost, size, and weight of the system. The
best inverter for this application permits the output voltage to
automatically increase as the lamps age, extending the useful life of
the lamps.
LCD systems with LED backlighting utilize low voltage
DC, usually 12 or 24 VDC. This results in a smaller, simpler, and more
reliable system. LED backlit systems must be well cooled or their
performance can suffer. The LEDs generate less heat than CCF lamps, but
the heat is very concentrated. Operating at high temperatures reduces
the LED lifetime and shifts their light spectrum, while reducing output
luminance.
OLED
A new technology that you will be hearing
about in the future is Organic LED (OLED). This technology has a
completely different backlight technology, it doesn't need one. Because
of this, they draw much less power. And because there is no backlight,
an OLED system has a larger viewing angle than an LED system. An OLED
display is much thinner than an LCD display. The response time for OLED
is faster than normal LCD screens. An average of 8 to 12 milliseconds in
response time is normal for a LCD compared to 0.01 milliseconds in
response time for an OLED.
Sony introduced an 11" HDTV at the
Consumer Electronics Show in Vegas. The new 11" Sony TV is called the
"XEL-1". It's very thin (1/10") and currently sells for $2500. Samsung
demonstrated a 31" OLED HDTV with 1080P resolution at a tradeshow in
Asia in November. LG, Toshiba, Matsushita (Panasonic), and others are
investing hundreds of millions of dollars, developing this extremely
promising technology. Toshiba has plans to sell a 30" OLED display in
2009. Samsung has announced they will sell large screen OLED HDTVs in
2010.
OLED displays have already been used for some time in
digital cameras, cell phones and other devices with relatively small
panels, because they are very energy efficient, which is very important
in portable devices.
In theory, OLED displays can be more
inexpensively manufactured than LCD or plasma displays, meaning that
they should not be as expensive. Remember that the first large LCD and
Plasma displays were much more expensive when they were first
introduced.
Summary
LCD HDTVs and monitors with LED array
backlighting are now the approach of choice because they exhibit better
image quality while saving power. The ability to locally control light
output is key to achieving high contrast ratios.
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