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Encyclopedia
Liquid crystal display
A liquid crystal display is a flat panel display, electronic visual display, or video display that uses the light modulating properties of liquid crystals . LCs do not emit light directly....
s (LCDs). As LCDs do not produce light themselves (unlike for example Cathode ray tube
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
(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 frontlight
Frontlight
A frontlight is a means of illuminating a display device, usually a liquid crystal display , which would otherwise be viewed in ambient light. This improves its performance in poor lighting conditions. Frontlights are inferior to backlights and are usually found as an auxiliary feature.An LCD...
s, which are placed in front of the LCD. Backlights are used in small displays to increase readability in low light conditions, and in computer display
Computer display
A monitor or display is an electronic visual display for computers. The monitor comprises the display device, circuitry, and an enclosure...
s and LCD televisions to produce light in a manner similar to a CRT display.
Simple types of LCD displays are built without an internal light source, requiring external light sources to convey the display image to the user. Modern LCD screens, however, are built with an internal light source. Such LCD screens consist of several layers. The backlight is usually the first layer from the back. But in order to create screen images, a mechanism is needed to regulate the light intensity of the screen's pixels. For this, light valve
Light valve
A light valve is a device for varying the quantity of light, from a source, which reaches a target. Examples of targets are computer screen surfaces, or a wall screen in the case of a light projector....
s are used that vary the amount of light reaching the target by blocking its passage in some way. The most common element is a polarizing filter to polarize the light from the source in one of two transverse directions and then passing it through a switching polarizing filter, to block the path of undesirable light.
Light source types
The light source can be made up of:- Incandescent light bulbIncandescent light bulbThe incandescent light bulb, incandescent lamp or incandescent light globe makes light by heating a metal filament wire to a high temperature until it glows. The hot filament is protected from air by a glass bulb that is filled with inert gas or evacuated. In a halogen lamp, a chemical process...
s - light-emitting diodeLight-emitting diodeA light-emitting diode is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting...
s (LEDs) - An electroluminescentElectroluminescenceElectroluminescence is an optical phenomenon and electrical phenomenon in which a material emits light in response to the passage of an electric current or to a strong electric field...
panel (ELP) - One or more Cold Cathode Fluorescent LampCold cathodeA cold cathode is a cathode used within nixie tubes, gas discharge lamps, discharge tubes, and some types of vacuum tube which is not electrically heated by the circuit to which it is connected...
s (CCFL) - One or more Hot CathodeHot cathodeIn vacuum tubes, a hot cathode is a cathode electrode which emits electrons due to thermionic emission. In the accelerator community, these are referred to as thermionic cathodes. The heating element is usually an electrical filament...
Fluorescent LampFluorescent lampA fluorescent lamp or fluorescent tube is a gas-discharge lamp that uses electricity to excite mercury vapor. The excited mercury atoms produce short-wave ultraviolet light that then causes a phosphor to fluoresce, producing visible light. A fluorescent lamp converts electrical power into useful...
s (HCFL)
An ELP gives off uniform light over its entire surface, but other backlights usually employ a diffuser to provide even lighting from an uneven source.
Backlights can be any color. Monochrome
Monochrome
Monochrome describes paintings, drawings, design, or photographs in one color or shades of one color. A monochromatic object or image has colors in shades of limited colors or hues. Images using only shades of grey are called grayscale or black-and-white...
LCDs usually have yellow
Yellow
Yellow is the color evoked by light that stimulates both the L and M cone cells of the retina about equally, with no significant stimulation of the S cone cells. Light with a wavelength of 570–590 nm is yellow, as is light with a suitable mixture of red and green...
, green
Green
Green is a color, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 520–570 nanometres. In the subtractive color system, it is not a primary color, but is created out of a mixture of yellow and blue, or yellow and cyan; it is considered...
, blue
Blue
Blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal...
or white
White
White is a color, the perception of which is evoked by light that stimulates all three types of color sensitive cone cells in the human eye in nearly equal amounts and with high brightness compared to the surroundings. A white visual stimulation will be void of hue and grayness.White light can be...
backlights, while color displays use white backlights that cover most of the color spectrum.
Usage
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
power, which is provided by an inverter
CCFL inverter
A CCFL inverter is a device for providing drive power to a Cold Cathode Fluorescent Lamp . CCFLs are often used as inexpensive light units in electrical devices.-Applications:#widely used in backlights for LCDs...
circuit. CCFL backlights are used on large displays like computer monitors, and are usually white in color. These also require the use of an inverter and diffuser. Incandescent backlighting can be used when very high brightness is desired, but a drawback is the limited life of incandescent bulbs, and the amount of heat generated, which often means that the bulb needs to be mounted away from the display.
LED backlights
HP EliteBook
HP EliteBook is a brand of premium business-class notebooks and mobile workstations made by Hewlett-Packard. The EliteBook series, which slots above the lower-end ProBook series, was introduced in August 2008.-Features:...
notebooks, as well as newer 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
Gamut
In color reproduction, including computer graphics and photography, the gamut, or color gamut , is a certain complete subset of colors. The most common usage refers to the subset of colors which can be accurately represented in a given circumstance, such as within a given color space or by a...
to screens. When using three separate LEDs (additive color
Additive color
An additive color model involves light emitted directly from a source or illuminant of some sort. The additive reproduction process usually uses red, green and blue light to produce the other colors. Combining one of these additive primary colors with another in equal amounts produces the...
) the backlight can produce a color spectrum that closely matches the color filters in the LCD pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
s themselves. In this way, the filter passband
Passband
A passband is the range of frequencies or wavelengths that can pass through a filter without being attenuated.A bandpass filtered signal , is known as a bandpass signal, as opposed to a baseband signal....
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
RGB color model
The RGB color model is an additive color model in which red, green, and blue light is added together in various ways to reproduce a broad array of colors...
points can be moved farther out so that the display is capable of reproducing more vivid colors. CCFL backlighting has also improved in this respect. Many current 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 NTSC
NTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
color specification.
There are several challenges with LED backlights. Good uniformity is harder to achieve, especially as the LEDs age, with each LED possibly aging at a different rate. Also, the use of three separate light sources for red, green, and blue means that the white point
White point
A white point is a set of tristimulus values or chromaticity coordinates that serve to define the color "white" in image capture, encoding, or reproduction. Depending on the application, different definitions of white are needed to give acceptable results...
of the display can move as the LEDs age at different rates. Aging also occurs with White LEDs, with changes of several 100K 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 also can be a challenge. First generation implementations could use the same or more power as 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
BenQ
BenQ Corporation is a Taiwanese multi-national company that sells and markets consumer electronics, computing and communications devices under the "BenQ" brand name, which stands for the company slogan Bringing Enjoyment and Quality to life .- Company :BenQ sells and markets technology products,...
G2420HDB consumer display has a 49W consumption compared to the 24W of the LED version of the same display (G2420HDBL).
The use of LED backlights in notebook computers has been growing. Sony
Sony
, commonly referred to as Sony, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan and the world's fifth largest media conglomerate measured by revenues....
has used LED backlights in some of its higher-end slim VAIO
VAIO
VAIO is a sub-brand used for many of Sony's computer products. Originally an acronym of Video Audio Integrated Operation, this was amended to Visual Audio Intelligent Organizer in 2008 to celebrate the brand's 10th anniversary...
notebooks since 2005. Fujitsu
Fujitsu
is a Japanese multinational information technology equipment and services company headquartered in Tokyo, Japan. It is the world's third-largest IT services provider measured by revenues....
introduced notebooks with LED backlights in 2006. In 2007, Asus
ASUS
ASUSTeK Computer Incorporated is a multinational computer technology and consumer electronics product manufacturer headquartered in Taipei, Taiwan. Its product range includes motherboards, desktops, laptops, monitors, tablet PCs, servers and mobile phones...
, Dell
Dell
Dell, Inc. is an American multinational information technology corporation based in 1 Dell Way, Round Rock, Texas, United States, that develops, sells and supports computers and related products and services. Bearing the name of its founder, Michael Dell, the company is one of the largest...
, and Apple introduced LED backlights into some of their notebook models. , Lenovo has also announced LED-backlit notebooks, and other companies like HP
Hewlett-Packard
Hewlett-Packard Company or HP is an American multinational information technology corporation headquartered in Palo Alto, California, USA that provides products, technologies, softwares, solutions and services to consumers, small- and medium-sized businesses and large enterprises, including...
will also be marketing LED-backlit notebooks in the near future. In October 2008, Apple announced it will be using LED backlights for all its notebook and its new 24-inch Apple Cinema Display
Apple Cinema Display
The Apple Cinema Display was a line of flat panel computer monitors introduced in September 1999 by Apple Inc. It was initially sold alongside the older line of Studio Displays, but eventually replaced them. In July 2011, Apple replaced it with the Apple Thunderbolt Display...
, and one year later it introduced a new LED iMac
IMac
The iMac is a range of all-in-one Macintosh desktop computers built by Apple. It has been the primary part of Apple's consumer desktop offerings since its introduction in 1998, and has evolved through five distinct forms....
, meaning all of Apple's computer screens are now LED. Almost every laptop with 16:9 display introduced since September 2009 uses LED-backlit panels.
Flicker due to backlight dimming
LED backlights are often dimmed by applying pulse-width modulationPulse-width modulation
Pulse-width modulation , or pulse-duration modulation , is a commonly used technique for controlling power to inertial electrical devices, made practical by modern electronic power switches....
to the supply current, switching the backlight off and on again like a fast strobe light
Strobe light
A strobe light or stroboscopic lamp, commonly called a strobe, is a device used to produce regular flashes of light. It is one of a number of devices that can be used as a stroboscope...
. If the frequency of the pulse-width modulation is too low and 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 it is operating at a frequency that is too high for the brain to perceive. The flicker can be reduced or eliminated by setting the display to full brightness, though this has a negative impact on image quality and battery life due to increased power consumption.
Diffusers
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 plasticPlastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...
that diffuses
Photon diffusion
Photon diffusion is a situation where photons travel through a material without being absorbed, but rather undergoing repeated scattering events which change the direction of their path. The path of any given photon is then effectively a random walk...
the light through a series of evenly-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
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...
to guide otherwise wasted light back toward the LCD panel. The reflector is sometimes made of aluminum foil, sometimes merely a white-pigmented surface or, as in the 3M Vikuiti ESR, consists of hundreds of polymer layers of alternating low and high refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
. Between the lightguide and the LCD panel is usually placed a reflective polarizer
Polarizer
A polarizer is an optical filter that passes light of a specific polarization and blocks waves of other polarizations. It can convert a beam of light of undefined or mixed polarization into a beam with well-defined polarization. The common types of polarizers are linear polarizers and circular...
film which vastly increases efficiency by repeatedly reflecting any unpolarized light back, which would otherwise be absorbed by the LCD's rear polarizer.