A backlight is a form of illumination used in liquid crystal displays
(LCDs). As LCDs do not produce light by themselves (unlike, for
example cathode ray tube (CRT) displays), they need illumination
(ambient light or a special light source) to produce a visible image.
Backlights illuminate the LCD from the side or back of the display
panel, unlike frontlights, which are placed in front of the LCD.
Backlights are used in small displays to increase readability in low
light conditions such as in wristwatches, and are used in smart
phones, computer displays and LCD televisions to produce light in a
manner similar to a CRT display. A review of some early backlighting
schemes for LCDs is given in a report Engineering and Technology
History by Peter J. Wild.
Simple types of LCDs such as in pocket calculators are built without
an internal light source, requiring external light sources to convey
the display image to the user. Most LCD screens, however, are built
with an internal light source. Such screens consist of several layers.
The backlight is usually the first layer from the back. Light valves
then vary the amount of light reaching the eye, by blocking its
passage in some way. Most use a fixed polarizing filter and a
switching one, to block the undesired light.
1 Light source types
2.2 LED backlights
4 Reflective polarizers
5 Power consumption
7 External links
Light source types
The light source can be made up of:
Light-emitting diodes (LEDs)
An electroluminescent panel (ELP)
Cold cathode fluorescent lamps (CCFLs)
Hot cathode fluorescent lamps (HCFLs)
External electrode fluorescent lamps (EEFLs)
Formerly, incandescent lightbulbs
An ELP gives off uniform light over its entire surface, but other
backlights frequently employ a diffuser to provide even lighting from
an uneven source.
Backlights come in many colors.
Monochrome LCDs typically have yellow,
green, blue, or white backlights, while color displays use white
backlights that cover most of the color spectrum.
Colored LED backlighting is most commonly used in small, inexpensive
White LED backlighting is becoming dominant. ELP
backlighting is often used for larger displays or when even
backlighting is important; it can also be either colored or white. An
ELP must be driven by relatively high[specify] voltage AC power, which
is provided by an inverter circuit.
CCFL backlights are used on larger
displays such as computer monitors, and are typically white in color;
these also require the use of an inverter and diffuser. Incandescent
backlighting were used by early LCD panels to achieve high brightness,
but the limited life and excess heat produced by incandescent bulbs
were severe limitations. The heat generated by incandescent bulbs
typically requires the bulbs to be mounted away from the display to
18 parallel CCFLs as backlight for an LCD TV
For several years (until about 2010), the preferred backlight for
matrix-addressed large LCD panels such as in monitors and TVs was
based on CCFLs, either by using two CCFLs at opposite edges of the LCD
or by an array of CCFLs behind the LCD (see picture of an array with
18 CCFLs for a 40-inch LCD TV). Due to the disadvantages in comparison
with LED illumination (higher voltage and power needed, thicker panel
design, no high-speed switching, faster aging), LED backlighting is
becoming more popular.
See also: LED-backlit LCD
LCD with LED matrix backlight
LED backlighting in color screens comes in two varieties: white LED
backlights and RGB LED backlights.
White LEDs are used most often
in notebooks and desktop screens, and make up virtually all mobile LCD
screens. A white LED is typically a blue LED with broad spectrum
yellow phosphor to result in the emission of white light. However,
because the spectral curve peaks at yellow, it is a poor match to the
transmission peaks of the red and green color filters of the LCD. This
causes the red and green primaries to shift toward yellow, reducing
the color gamut of the display. RGB LEDs consist of a red, a blue,
and a green LED and can be controlled to produce different color
temperatures of white. RGB LEDs for backlighting are found in high end
color proofing displays such as the HP DreamColor LP2480zx monitor or
HP EliteBook notebooks, as well as more recent consumer-grade
displays such as Dell's Studio series laptops which have an optional
RGB LED display.
RGB LEDs can deliver an enormous color gamut to screens. When using
three separate LEDs (additive color) the backlight can produce a color
spectrum that closely matches the color filters in the LCD pixels
themselves. In this way, the filter passband can be narrowed so that
each color component lets only a very narrow band of spectrum through
the LCD. This improves the efficiency of the display since less light
is blocked when white is displayed. Also, the actual red, green, and
blue points can be moved farther out so that the display is capable of
reproducing more vivid colors.
A new method to further improve the color gamut of LED-backlit LCD
panels is based on blue LEDs (such as GaN) illuminating a layer of
nanocrystal phosphors, so-called Quantum Dots (QD), which convert
the blue wavelengths to the desired longer wavelengths as
narrow-bandwidth green and red colors for optimal illumination of the
LCD from behind. The manufacturer, Nanosys, claims that the color
output of the dots can be tuned precisely by controlling the size of
the nanocystals. Other companies pursuing this method are
PLC (UK), QD Vision and 3M, a licensee of Nanosys.
Sony has adapted
Quantum Dot technology from the US company QD Vision to introduce
LCD TVs with an improved edge-lit LED backlight marketed under the
term Triluminos in 2013. With a blue LED and optimized nanocrystals
for green and red colors in front of it, the resulting combined white
light allows for an equivalent or better color gamut than that emitted
by a more expensive set of three RGB LEDs. At the Consumer Electronics
Show 2015, Samsung Electronics, LG Electronics, the Chinese TCL
Sony showed QD-enhanced LED-backlighting of LCD
CCFL backlighting has also improved in this respect. Many LCD models,
from cheap TN-displays to color proofing S-IPS or S-PVA panels, have
wide gamut CCFLs representing more than 95% of the
There are several challenges with LED backlights. Uniformity is hard
to achieve, especially as the LEDs age, with each LED aging at a
different rate. Also, the use of three separate light sources for red,
green, and blue means that the white point of the display can move as
the LEDs age at different rates; white LEDs are also affected by this
phenomenon, with changes of several hundred kelvins being recorded.
White LEDs also suffer from blue shifts at higher temperatures varying
from 3141K to 3222K for 10 °C to 80 °C respectively.
Power efficiency may also be a challenge; first generation
implementations could potentially use more power than their CCFL
counterparts, though it is possible for an LED display to be more
power efficient. In 2010, current generation LED
displays can have significant power consumption advantages. For
example, the non-LED version of the 24"
Benq G2420HDB consumer display
has a 49W consumption compared to the 24W of the LED version of the
same display (G2420HDBL).
To overcome the aforementioned challenges with RGB and white LED
backlights an 'advanced remote phosphor'  LED technology has been
developed by NDF
Special Light Products, specifically for high-end and
long-life LCD applications such as cockpit displays, Air Traffic
Control displays and medical displays. This technology uses blue pump
LEDs in combination with a sheet on which phosphorous luminescent
materials are printed for colour conversion. The principle is similar
to Quantum Dots, but the phosphors applied are much more robust than
the quantum dot nano-particles for applications that require long
lifetime in more demanding operational conditions. Because the
phosphor sheet is placed at a distance (remote) of the LED it
experiences much less temperature stress than phosphors in white LEDs.
As a result, the whitepoint is less dependent on individual LEDs, and
degrading of individual LEDs over lifetime, leading to a more
homogenous backlight with improved colour consistency and lower lumen
The use of LED backlights in notebook computers has been growing. Sony
has used LED backlights in some of its higher-end slim
since 2005, and
Fujitsu introduced notebooks with LED backlights in
2006. In 2007, Asus, Dell, and Apple introduced LED backlights into
some of their notebook models. As of 2008[update],
Lenovo has also
announced LED-backlit notebooks. In October 2008, Apple announced that
it would be using LED backlights for all of its notebooks and new
24-inch Apple Cinema Display, and one year later it introduced a new
LED iMac, meaning all of Apple's new computer screens are now LED.
Almost every laptop with a 16:9 display introduced since September
2009 uses LED-backlit panels. This is also the case for most LCD
television sets, which are marketed in some countries under the
misleading name LED TV, although the image is still generated by an
Most LED backlights for LCDs are edge-lit, i.e. several LEDs are
placed at the edges of a lightguide, which distributes the light
behind the LC panel. Advantages of this technique are the very thin
flat-panel construction and low cost. A more expensive version is
called full-array or direct LED and consists of many LEDs placed
behind the LC panel (an array of LEDs), such that large panels can be
evenly illuminated. This arrangement allows for local dimming to
obtain darker black pixels depending on the image displayed.
LED backlight are often dynamically controlled using the video
information (dynamic backlight control or dynamic “local
dimming” LED backlight, also marketed as HDR, high dynamic range
television, invented by Philips researchers Douglas Stanton, Martinus
Stroomer and Adrianus de Vaan).
Using PWM (pulse-width modulation, a technology where the intensity of
the LEDs are kept constant, but the brightness adjustment is achieved
by varying a time interval of flashing these constant light intensity
light sources), the backlight is dimmed to the brightest color
that appears on the screen while simultaneously boosting the LCD
contrast to the maximum achievable levels
If the frequency of the pulse-width modulation is too low or the user
is very sensitive to flicker, this may cause discomfort and
eye-strain, similar to the flicker of CRT displays. This can
be tested by a user simply by waving a hand or object in front of the
screen. If the object appears to have sharply defined edges as it
moves, the backlight is strobing on and off at a fairly low frequency.
If the object appears blurry, the display either has a continuously
illuminated backlight or is operating the backlight at a frequency
higher than the brain can perceive. The flicker can be reduced or
eliminated by setting the display to full brightness, though this may
have a negative impact on image quality and battery life due to
increased power consumption.
For a non-ELP backlight to produce even lighting, which is critical
for displays, the light is first passed through a lightguide - a
specially designed layer of plastic that diffuses the light through a
series of unevenly spaced bumps. The density of bumps increases
further away from the light source according to a diffusion equation.
The diffused light then travels to either side of the diffuser; the
front faces the actual LCD panel, the back has a reflector to guide
otherwise wasted light back toward the LCD panel. The reflector is
sometimes made of aluminum foil or a simple white-pigmented surface.
The LCD backlight systems are made highly efficient by applying
optical films such as prismatic structure to gain the light into the
desired viewer directions and reflective polarizing films that recycle
the polarized light that was formerly absorbed by the first polarizer
of the LCD (invented by Philips researchers Adrianus de Vaan and
Paulus Schaareman), generally achieved using so called DBEF films
manufactured and supplied by 3M. These polarizers consist of a
large stack of uniaxial oriented birefringent films that reflect the
former absorbed polarization mode of the light. Such reflective
polarizers using uniaxial oriented polymerized liquid crystals
(birefringent polymers or birefringent glue) are invented in 1989 by
Philips researchers Dirk Broer, Adrianus de Vaan and Joerg
Brambring. The combination of such reflective polarizers, and LED
dynamic backlight control make today's LCD televisions far more
efficient than the CRT-based sets, leading to a worldwide energy
saving of 600 TWh (2017), equal to 10% of the electricity consumption
of all households worldwide or equal to 2 times the energy production
of all solar cells in the world.
The evolution of energy standards and the increasing public
expectations regarding power consumption have made it necessary for
backlight systems to manage their power. As for other consumer
electronics products (e.g., fridges or light bulbs), energy
consumption categories are enforced for television sets. Standards
for power ratings for TV sets have been introduced, e.g., in the USA,
EU, and Australia as well as in China. Moreover, a 2008
study showed that among European countries, power consumption is
one of the most important criteria for consumers when they choose a
television, as important as the screen size.
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