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HOME REVIEWS CONTACT ABOUT March 14,
2009
LCD Backlighting for HDTV
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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).
An LCD
as used in HDTV is basically 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.
Most
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
primary 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
Drawbacks:
o
The intensity of
light cannot be varied locally. This
results in a lower contrast picture.
o
The light can be
non-uniform.
o
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
Drawbacks:
o
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).
o
The intensity of light
cannot be varied locally.
LED Backlighting
LED
backlighting uses a matrix of white 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 white 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.
At one
time, a significant disadvantage of LED backlighting was the consumption of
energy, especially with large displays. But
advances in recent years have improved the efficiency to the point that energy
efficiency is now an advantage. 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.
LED
systems 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. 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, Toshiba, Matsushita (Panasonic), and
others are investing hundreds of millions of dollars, also trying to develop
this extremely promising technology.
Toshiba has plans to sell a 30" OLED display in 2009.
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 is now the
approach of choice because it exhibits better image quality while saving
power. The ability to locally control
light output is key to achieving high contrast ratios. Long term, OLED has the potential to rival
LED systems in price and performance.