An LED-backlight LCD television is an LCD TV that uses LED backlighting instead of the CCFLs used in traditional LCD televisions. It is not a true LED display but is called "LED TV" by some manufacturers.[1] The use of LED backlighting has a dramatic impact, resulting in a thinner panel, less power consumption and heat dissipation, and a brighter display with better contrast levels.

The LEDs can come in three forms:

Dynamic RGB LEDs which are positioned behind the panel White Edge-LEDs positioned around the rim of the screen using a special

diffusion panel to spread the light evenly behind the screen (the most common) A full-array of LEDS which are arranged behind the screen but are incapable of

dimming or brightening individually

LED backlighting techniques

A LED-backlit TV.RGB dynamic LEDs

This method of backlighting allows dimming to occur in locally specific areas of darkness on the screen. This can show truer blacks, whites and PRs[clarification needed] at much higher dynamic contrast ratios, at the cost of less detail in small bright objects on a dark background, such as star fields.[2]

Edge-LEDs

This method of backlighting allows for LED-backlit TVs to become extremely thin. The light is diffused across the screen by a special panel which produces a uniform color range across the screen.

Full Array LEDs

Many brands use LED backlighting technology and may offer a range of benefits over CCFL LCD TVs such as reduced energy consumption, better contrast and brightness, greater colour range, more rapid response to changes in scene and a capacity to provide the means to render an image more accurately.[3]

Differences between LED-backlit and CCFL-backlit LCD displaysLED-backlit LCD TVs differ from conventional CCFL-backlit LCD TVs in the following:[4]

Produce images with greater dynamic contrast. With Edge-LED lighting they can be extremely slim. Models on the market can

be approximately one inch thick. Offer a wider color gamut, especially when RGB-LED backlighting is used.[5]

Less environmental pollution on disposal. Higher price. Generally 20-30% lower power consumption.[citation needed]

TechnologyTV manufacturers can use an LED backlight instead of the Cold Cathode Fluorescent Lamps (LCD-CCFL) used in most LCD televisions. LCD-based televisions described as 'LED TVs' are different from self-illuminating Organic light-emitting diode (OLED), OEL or AMOLED display technologies. In the UK, the ASA (Advertising Standards Authority) has made it clear in prior correspondence that it does not object to the use of the term 'LED TV', but does require it to be clarified in any advertising. There are several methods of backlighting an LCD panel using LEDs including the use of either White or RGB (Red, Green and Blue) LED arrays positioned behind the panel; and Edge-LED lighting, which uses white LEDs arranged around the inside frame of the TV along with a light diffusion panel to spread the light evenly behind the LCD panel.

Compared to regular CCFL backlighting, an LED backlight provides higher brightness and improved color gamut. However advancements in CCFL technology mean wide color gamuts and lower power consumption are also possible. Cost is the principal barrier to wide use of LED backlighting on LCD televisions.

The variations of LED backlighting do offer different benefits. The first commercial LED backlit LCD TV was the Sony Qualia 005 (introduced in 2004) and featured RGB LED arrays to produce a color gamut around twice that of a conventional CCFL LCD television. This was possible because the combined light output from red, green and blue LEDs produces a more pure white light than is possible with a single white light LED. RGB LED technology continues to be used on selected Sony BRAVIA LCD models, with the addition of 'local dimming' which enables excellent on-screen contrast through selectively turning off the LEDs behind dark parts of a picture frame.

Edge LED lighting was first introduced by Sony (September 2008) on the 40 inch BRAVIA KLV-40ZX1M (referred to as the ZX1 in Europe). The principal benefit of Edge-LED lighting for LCD televisions is the ability to build thinner housings (the

BRAVIA KLV-40ZX1M is as thin as 9.9mm). Others have also introduced Edge-LED lit LCD televisions with extremely thin housings.

LED-backlit LCD TVs are considered a more sustainable choice, with a longer life and better energy efficiency than plasmas and conventional LCD TVs.[6] Unlike CCFL backlights, LEDs also use no mercury in their manufacture. However, other elements such as gallium and arsenic are used in the manufacture of the LED emitters themselves, meaning there is some debate over whether they are a significantly better long term solution to the problem of TV disposal.

Because LEDs are able to be switched on and off more quickly than CCFL displays and can offer a higher light output, it is theoretically possible to offer very high contrast ratios. They can produce deep blacks (LEDs off) and a high brightness (LEDs on). However, measurements made from pure black and pure white outputs are complicated by Edge-LED lighting not allowing these outputs to be reproduced simultaneously on-screen.

In September 2009 Nanoco Group announced that it has signed a joint development agreement with a major Japanese electronics company under which it will design and develop quantum dots for LED Backlights in LCD televisions Quantum dots are valued for displays, because they emit light in very specific gaussian distributions. This can result in a display that more accurately renders the colors than the human eye can perceive.

LCD televisionFrom Wikipedia, the free encyclopediaJump to: navigation , search This article is about Liquid crystal display (LCD) based televisions. For LCD technology, see Liquid crystal display.

A generic LCD TV, with speakers on either side of the screen.

Liquid-crystal display televisions (LCD TV) are television sets that use LCD technology to produce images. LCD televisions are thinner and lighter than CRTs of similar display size, and are available in much larger sizes. When manufacturing costs fell, this combination of features made LCDs practical for television receivers.

In 2007, LCD televisions surpassed sales of CRT-based televisions worldwide for the first time,[citation needed] and their sales figures relative to other technologies are accelerating. LCD TVs are quickly displacing the only major competitors in the large-screen market, the plasma display panel and rear-projection television. LCDs are, by far, the most widely produced and sold television display type.

LCDs also have a variety of disadvantages. Other technologies address these weaknesses, including organic light-emitting diodes (OLED), FED and SED, but as of 2011 none of these have entered widespread production.

Contents[hide]

1 Description o 1.1 Basic LCD concepts o 1.2 Addressing sub-pixels o 1.3 Building a display

2 Comparison o 2.1 Packaging o 2.2 Efficiency o 2.3 Image quality

2.3.1 Response time 2.3.2 Contrast ratio 2.3.3 Color gamut

3 History o 3.1 Early efforts o 3.2 High-definition o 3.3 Market takeover o 3.4 Recent research o 3.5 Competing systems

4 Environmental effects 5 See also 6 References

7 External links

[edit] Description

This section does not cite any references or sources.Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (June 2009)

[edit] Basic LCD concepts

LCD television at home together with PlayStation 3 and some other equipment

LCD televisions produced a black and colored image by selectively filtering a white light. The light is typically provided by a series of cold cathode fluorescent lamps (CCFLs) at the back of the screen, although some displays use white or colored LEDs instead. Millions of individual LCD shutters, arranged in a grid, open and close to allow a metered amount of the white light through. Each shutter is paired with a colored filter to remove all but the red, green or blue (RGB) portion of the light from the original white source. Each shutter–filter pair forms a single sub-pixel. The sub-pixels are so small that when the display is viewed from even a short distance, the individual colors blend together to produce a single spot of color, a pixel. The shade of color is controlled by changing the relative intensity of the light passing through the sub-pixels.

Liquid crystals encompass a wide range of (typically) rod-shaped polymers that naturally form into thin layers, as opposed to the more random alignment of a normal liquid. Some of these, the nematic liquid crystals, also show an alignment effect between the layers. The particular direction of the alignment of a nematic liquid crystal can be set by placing it in contact with an alignment layer or director, which is essentially a material with microscopic grooves in it. When placed on a director, the layer in contact will align itself with the grooves, and the layers above will subsequently align themselves with the layers below, the bulk material taking on the director's alignment. In the case of an LCD, this effect is utilized by using two directors arranged at right angles and placed close together with the liquid crystal between them. This forces the layers to align themselves in two directions, creating a twisted structure with each layer aligned at a slightly different angle to the ones on either side.

LCD shutters consist of a stack of three primary elements. On the bottom and top of the shutter are polarizer plates set at right angles. Normally light cannot travel through a pair of polarizers arranged in this fashion, and the display would be black. The polarizers also carry the directors to create the twisted structure aligned with the polarizers on either side. As the light flows out of the rear polarizer, it will naturally follow the liquid crystal's twist, exiting the front of the liquid crystal having been rotated through the correct angle, that allows it to pass through the front polarizer. LCDs are normally transparent.

To turn a shutter off, a voltage is applied across it from front to back. the rod-shaped molecules align themselves with the electric field instead of the directors, destroying the twisted structure. The light no longer changes polarization as it flows through the liquid crystal, and can no longer pass through the front polarizer. By controlling the voltage applied across the crystal, the amount of remaining twist can be selected. This allows the transparency of the shutter to be controlled. To improve switching time, the cells are placed under pressure, which increases the force to re-align themselves with the directors when the field is turned off.

Several other variations and modifications have been used in order to improve performance in certain applications. In-Plane Switching displays (IPS and S-IPS) offer wider viewing angles and better color reproduction, but are more difficult to construct and have slightly slower response times. IPS displays are used primarily for computer monitors. Vertical Alignment (VA, S-PVA and MVA) offer higher contrast ratios and good response times, but suffer from color shifting when viewed from the side. In general, all of these displays work in a similar fashion by controlling the polarization of the light source.

[edit] Addressing sub-pixels

A close-up (300×) view of a typical LCD display, clearly showing the sub-pixel structure. The "notch" at the lower left of each sub-pixel is the thin-film transistor. The associated capacitors and addressing lines are located around the shutter, in the dark areas.

In order to address a single shutter on the display, a series of electrodes is deposited on the plates on either side of the liquid crystal. One side has horizontal stripes that form rows, the other has vertical stripes that form columns. By supplying voltage to one row and one column, a field will be generated at the point where they cross. Since a metal electrode would be opaque, LCDs use electrodes made of a transparent conductor, typically indium tin oxide.

Since addressing a single shutter requires power to be supplied to an entire row and column, some of the field always leaks out into the surrounding shutters. Liquid crystals are quite sensitive, and even small amounts of leaked field will cause some level of switching to occur. This partial switching of the surrounding shutters blurs the resulting image. Another problem in early LCD systems was the voltages needed to set the shutters

to a particular twist was very low, but that voltage was too low to make the crystals re-align with reasonable performance. This resulted in slow response times and led to easily visible "ghosting" on these displays on fast-moving images, like a mouse cursor on a computer screen. Even scrolling text often rendered as an unreadable blur, and the switching speed was far too slow to use as a useful television display.

In order to attack these problems, modern LCDs use an active matrix design. Instead of powering both electrodes, one set, typically the front, is attached to a common ground. On the rear, each shutter is paired with a thin-film transistor that switches on in response to widely separated voltage levels, say 0 and +5 volts. A new addressing line, the gate line, is added as a separate switch for the transistors. The rows and columns are addressed as before, but the transistors ensure that only the single shutter at the crossing point is addressed; any leaked field is too small to switch the surrounding transistors. When switched on, a constant and relatively high amount of charge flows from the source line through the transistor and into an associated capacitor. The capacitor is charged up until it holds the correct control voltage, slowly leaking this through the crystal to the common ground. The current is very fast and not suitable for fine control of the resulting store charge, so pulse code modulation is used to accurately control the overall flow. Not only does this allow for very accurate control over the shutters, since the capacitor can be filled or drained quickly, but the response time of the shutter is dramatically improved as well.

[edit] Building a display

A typical shutter assembly consists of a sandwich of several layers deposited on two thin glass sheets forming the front and back of the display. For smaller display sizes (under 30 inches), the glass sheets can be replaced with plastic.

The rear sheet starts with a polarizing film, the glass sheet, the active matrix components and addressing electrodes, and then the director. The front sheet is similar, but lacks the active matrix components, replacing those with the patterned color filters. Using a multi-step construction process, both sheets can be produced on the same assembly line. The liquid crystal is placed between the two sheets in a patterned plastic sheet that divides the liquid into individual shutters and keeps the sheets at a precise distance from each other.

The critical step in the manufacturing process is the deposition of the active matrix components. These have a relatively high failure rate, which renders those pixels on the screen "always on". If there are enough broken pixels, the screen has to be discarded. The number of discarded panels has a strong effect on the price of the resulting television sets, and the major downward fall in pricing between 2006 and 2008 was due mostly to improved processes.

To produce a complete television, the shutter assembly is combined with control electronics and backlight. The backlight for small sets can be provided by a single lamp using a diffuser or frosted mirror to spread out the light, but for larger displays a single lamp is not bright enough and the rear surface is instead covered with a number of

separate lamps. Achieving even lighting over the front of an entire display remains a challenge, and bright and dark spots are not uncommon.

[edit] Comparison

A 19" Sony LCD TV

[edit] Packaging

In a CRT the electron beam is produced by heating a metal filament, which "boils" electrons off its surface. The electrons are then accelerated and focused in an electron gun, and aimed at the proper location on the screen using electromagnets. The majority of the power budget of a CRT goes into heating the filament, which is why the back of a CRT-based television is hot. Since the electrons are easily deflected by gas molecules, the entire tube has to be held in vacuum. The atmospheric force on the front face of the tube grows with the area, which requires ever-thicker glass. This limits practical CRTs to sizes around 30 inches; displays up to 40 inches were produced but weighed several hundred pounds, and televisions larger than this had to turn to other technologies like rear-projection.

The lack of vacuum in an LCD television is one of its advantages; there is a small amount of vacuum in sets using CCFL backlights, but this is arranged in cylinders which are naturally stronger than large flat plates. Removing the need for heavy glass faces allows LCDs to be much lighter than other technologies. For instance, the Sharp LC-42D65, a fairly typical 42-inch LCD television, weighs 55 lbs including a stand,[1] while the late-model Sony KV-40XBR800, a 40" 4:3 CRT weighs a massive 304 lbs without a stand, almost six times the weight.[2]

LCD panels, like other flat panel displays, are also much thinner than CRTs. Since the CRT can only bend the electron beam through a critical angle while still maintaining focus, the electron gun has to be located some distance from the front face of the television. In early sets from the 1950s the angle was often as small as 35 degrees off-axis, but improvements, especially computer assisted convergence, allowed that to be dramatically improved and, late in their evolution, folded. Nevertheless, even the best CRTs are much deeper than an LCD; the KV-40XBR800 is 26 inches deep,[2] while the

LC-42D65U is less than 4 inches thick[1] – its stand is much deeper than the screen in order to provide stability.

LCDs can, in theory, be built at any size, with production yields being the primary constraint. As yields increased, common LCD screen sizes grew, from 14 to 30", to 42", then 52", and 65" sets are now widely available. This allowed LCDs to compete directly with most in-home projection television sets, and in comparison to those technologies direct-view LCDs have a better image quality. Experimental and limited run sets are available with sizes over 100 inches.

[edit] Efficiency

LCDs are relatively inefficient in terms of power use per display size, because the vast majority of light that is being produced at the back of the screen is blocked before it reaches the viewer. To start with, the rear polarizer filters out over half of the original un-polarized light. Examining the image above, you can see that a good portion of the screen area is covered by the cell structure around the shutters, which removes another portion. After that, each sub-pixel's color filter removes the majority of what is left to leave only the desired color. Finally, to control the color and luminance of a pixel as a whole, the light has to be further absorbed in the shutters. 3M suggests that, on average, only 8 to 10% of the light being generated at the back of the set reaches the viewer.[3]

For these reasons the backlighting system has to be extremely powerful. In spite of using highly efficient CCFLs, most sets use several hundred watts of power, more than would be required to light an entire house with the same technology. As a result, LCD televisions end up with overall power usage similar to a CRT of the same size. Using the same examples, the KV-40XBR800 dissipates 245 W,[2] while the LC-42D65 dissipates 235 W.[1] Plasma displays are worse; the best are on par with LCDs, but typical sets draw much more.[4]

Modern LCD sets have attempted to address the power use through a process known as "dynamic lighting" (originally introduced for other reasons, see below). This system examines the image to find areas that are darker, and reduces the backlighting in those areas. CCFLs are long cylinders that run the length of the screen, so this change can only be used to control the brightness of the screen as a whole, or at least wide horizontal bands of it. This makes the technique suitable only for particular types of images, like the credits at the end of a movie. In 2009 some manufacturers[5] made some TVs using HCFL (more power efficient than CCFL). Sets using LEDs are more distributed, with each LED lighting only a small number of pixels, typically a 16 by 16 patch. This allows them to dynamically adjust brightness of much smaller areas, which is suitable for a much wider set of images.

Another ongoing area of research is to use materials that optically route light in order to re-use as much of the signal as possible. One potential improvement is to use microprisms or dichromic mirrors to split the light into R, G and B, instead of absorbing the unwanted colors in a filter. A successful system would improve efficiency by three

times. Another would be to direct the light that would normally fall on opaque elements back into the transparent portion of the shutters. A number of companies are actively researching a variety of approaches, and 3M currently sells several products that route leaked light back toward the front of the screen.[3]

Several newer technologies, OLED, FED and SED, have lower power use as one of their primary advantages. All of these technologies directly produce light on a sub-pixel basis, and use only as much power as that light level requires. Sony has demonstrated 36" FED units displaying very bright images drawing only 14 W, less than 1/10 as much as a similarly sized LCD. OLEDs and SEDs are similar to FEDs in power terms. The dramatically lower power requirements make these technologies particularly interesting in low-power uses like laptop computers and mobile phones. These sorts of devices were the market that originally bootstrapped LCD technology, due to its light weight and thinness.

[edit] Image quality

A traveler pocket-size LCD TV

Early LCD sets were widely derided for their poor overall image quality, most notably the ghosting on fast-moving images, poor contrast ratio, and muddy colors. In spite of many predictions that other technologies would always beat LCDs, massive investment in LCD production, manufacturing, and electronic image processing has addressed many of these concerns.[6]

[edit] Response timeThis section does not cite any references or sources.Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (June 2009)

For 60 frames per second video, common in North America, each pixel is lit for 17 ms before it has to be re-drawn (20 ms in Europe). Early LCD displays had response times on the order of hundreds of milliseconds, which made them useless for television. A combination of improvements in materials technology since the 1970s greatly improved this, as did the active matrix techniques. By 2000, LCD panels with response times around 20 ms were relatively common in computer roles. This was still not fast enough for television use.

A major improvement, pioneered by NEC, led to the first practical LCD televisions. NEC noticed that liquid crystals take some time to start moving into their new orientation, but stop rapidly. If the initial movement could be accelerated, the overall performance would be increased. NEC's solution was to boost the voltage during the "spin up period" when the capacitor is initially being charged, and then dropping back to normal levels to fill it to the required voltage. A common method is to double the voltage, but halve the pulse width, delivering the same total amount of power. Named "Overdrive" by NEC, the technique is now widely used on almost all LCDs.

Another major improvement in response time was achieved by adding memory to hold the contents of the display – something that a television needs to do anyway, but was not originally required in the computer monitor role that bootstrapped the LCD industry. In older displays the active matrix capacitors were first drained, and then recharged to the new value with every refresh. But in most cases, the vast majority of the screen's image does not change from frame to frame. By holding the before and after values in computer memory, comparing them, and only resetting those sub-pixels that actually changed, the amount of time spent charging and discharging the capacitors was reduced. Moreover the capacitors are not drained completely; instead, their existing charge level is either increased or decreased to match the new value, which typically requires fewer charging pulses. This change, which was isolated to the driver electronics and inexpensive to implement, improved response times by about two times.

Together, along with continued improvements in the liquid crystals themselves, and by increasing refresh rates from 60 Hz to 120 and 240 Hz, response times fell from 20 ms in 2000 to about 2 ms in the best modern displays. But even this is not really fast enough because the pixel will still be switching while the frame is being displayed. Conventional CRTs are well under 1 ms, and plasma and OLED displays boast times on the order of 0.001 ms.

One way to further improve the effective refresh rate is to use "super-sampling", and it is becoming increasingly common on high-end sets. Since the blurring of the motion occurs during the transition from one state to another, this can be reduced by doubling the refresh rate of the LCD panel, and building intermediate frames using various motion compensation techniques. This smooths out the transitions, and means the backlighting is turned on only when the transitions are settled. A number of high-end sets offer 120 Hz (in North America) or 100 Hz (in Europe) refresh rates using this technique. Another solution is to only turn the backlighting on once the shutter has fully switched. In order to ensure that the display does not flicker, these systems fire the backlighting several times per refresh, in a fashion similar to movie projection where the shutter opens and closes several times per frame.

[edit] Contrast ratio

Even in a fully switched-off state, liquid crystals allow some light to leak through the shutters. This limits their contrast ratios to about 1600:1 on the best modern sets, when measured using the ANSI measurement (ANSI IT7.215-1992). Manufacturers often

quote the "Full On/Off" contrast ratio instead, which is about 25% greater for any given set.[7]

This lack of contrast is most noticeable in darker scenes; in order to display a color close to black, the LCD shutters have to be turned to almost full opacity, limiting the number of discrete colors they can display. This leads to "posterizing" effects and bands of discrete colors that become visible in shadows. which is why many reviews of LCD TV's mention the "shadow detail".[8] For contrast, the highest-end LCD TVs offer regular contrast ratios of 2,000,000:1.

Since the total amount of light reaching the viewer is a combination of the backlighting and shuttering, modern sets can use "dynamic backlighting" to improve the contrast ratio and shadow detail. If a particular area of the screen is dark, a conventional set will have to set its shutters close to opaque to cut down the light. However, if the backlighting is reduced by half in that area, the shuttering can be reduced by half, and the number of available shuttering levels in the sub-pixels doubles. This is the main reason high-end sets offer dynamic lighting (as opposed to power savings, mentioned earlier), allowing the contrast ratio across the screen to be dramatically improved. While the LCD shutters are capable of producing about 1000:1 contrast ratio, by adding 30 levels of dynamic backlighting this is improved to 30,000:1.

However, the area of the screen that can be dynamically adjusted is a function of the backlighting source. CCFLs are thin tubes that light up many rows (or columns) across the entire screen at once, and that light is spread out with diffusers. The CCFL must be driven with enough power to light the brightest area of the portion of the image in front of it, so if the image is light on one side and dark on the other, this technique cannot be used successfully. Displays backlit by full arrays of LEDs have an advantage, because each LED lights only a small patch of the screen. This allows the dynamic backlighting to be used on a much wider variety of images. Edge-lit displays do not enjoy this advantage. These displays have LEDs only along the edges and use a light guide plate covered with thousands of convex bumps that reflect light from the side-firing LEDs out through the LCD matrix and filters. LEDs on edge-lit displays can be dimmed only globally, not individually.

The massive on-paper boost this method provides is the reason many sets now place the "dynamic contrast ratio" in their specifications sheets. There is widespread debate in the audio-visual world as to whether or not dynamic contrast ratios are real, or simply marketing speak.[9][10] Reviewers commonly note that even the best LCD displays cannot match the contrast ratios or deep blacks of plasma displays, in spite of being rated, on paper, as having much higher ratios.

[edit] Color gamutThis section does not cite any references or sources.Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (June 2009)

Color on an LCD television is produced by filtering down a white source and then selectively shuttering the three primary colors relative to each other. The accuracy and quality of the resulting colors are thus dependent on the backlighting source and its ability to evenly produce white light. The CCFLs used in early LCD televisions were not particularly white, and tended to be strongest in greens. Modern backlighting has improved this, and sets commonly quote a color space covering about 75% of the NTSC 1953 color gamut. Using white LEDs as the backlight improves this further.

In September 2009 Nanoco Group announced that it had signed a joint development agreement with a major Japanese electronics company under which it will design and develop quantum dots for use in LED backlights in LCD televisions.[11] Quantum dots are valued for displays, because they emit light in very specific Gaussian distributions. This can result in a display that more accurately renders the colors that the human eye can perceive. Quantum dots also require very little power since they are not color filtered.

History

An LCD TV hanging on a wall in the Taipei World Trade Center during the Computex Taipei show in 2008.

Early effortsThis section does not cite any references or sources.Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (June 2009)

Passive matrix LCDs first became common in the 1980s for various portable computer roles. At the time they competed with plasma displays in the same market space. The LCDs had very slow refresh rates that blurred the screen even with scrolling text, but their light weight and low cost were major benefits. Screens using reflective LCDs required no internal light source, making them particularly well suited to laptop computers.

Refresh rates of early devices were too slow to be useful for television. Portable televisions were a target application for LCDs. LCDs consumed far less battery power then even the miniature tubes used in portable televisions of the era. The earliest commercially made LCD TV was the Casio TV-10 made in 1983.[12] Resolutions were limited to standard definition, although a number of technologies were pushing displays towards the limits of that standard; Super VHS offered improved color saturation, and

DVDs added higher resolutions as well. Even with these advances, screen sizes over 30" were rare as these formats would start to appear blocky at normal seating distances when viewed on larger screens. Projection systems were generally limited to situations where the image had to be viewed by a larger audience.

Nevertheless, some experimentation with LCD televisions took place during this period. In 1988, Sharp Corporation introduced the first commercial LCD television, a 14" model. These were offered primarily as boutique items for discerning customers, and were not aimed at the general market. At the same time, plasma displays could easily offer the performance needed to make a high quality display, but suffered from low brightness and very high power consumption. However, a series of advances led to plasma displays outpacing LCDs in performance improvements, starting with Fujitsu's improved construction techniques in 1979, Hitachi's improved phosphors in 1984, and AT&T's elimination of the black areas between the sub-pixels in the mid-1980s. By the late 1980s, plasma displays were far in advance of LCDs.

[edit] High-definitionThis section does not cite any references or sources.Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (June 2009)

It was the slow standardization of high definition television that first produced a market for new television technologies. In particular, the wider 16:9 aspect ratio of the new material was difficult to build using CRTs; ideally a CRT should be perfectly circular in order to best contain its internal vacuum, and as the aspect ratio becomes more rectangular it becomes more difficult to make the tubes. At the same time, the much higher resolutions these new formats offered were lost at smaller screen sizes, so CRTs faced the twin problems of becoming larger and more rectangular at the same time. LCDs of the era were still not able to cope with fast-moving images, especially at higher resolutions, and from the mid-1990s the plasma display was the only real offering in the high resolution space.

Through the halting introduction of HDTV in the mid-1990s into the early 2000s, plasma displays were the primary high-definition display technology. However, their high cost, both manufacturing and on the street, meant that older technologies like CRTs maintained a footprint in spite of their disadvantages. LCD, however, was widely considered to be unable to scale into the same space, and it was widely believed that the move to high-definition would push it from the market entirely.

This situation changed rapidly. Contrary to early optimism, plasma displays never saw the massive economies of scale that were expected, and remained expensive. Meanwhile, LCD technologies like Overdrive started to address their ability to work at television speeds. Initially produced at smaller sizes, fitting into the low-end space that plasmas could not fill, LCDs started to experience the economies of scale that plasmas failed to achieve. By 2004, 32" models were widely available, 42" sets were becoming common, and much larger prototypes were being demonstrated.

[edit] Market takeover

LCD TV for public viewing in a Hong Kong bus

Although plasmas continued to hold an arguable picture quality edge over LCDs, and even a price advantage for sets at the critical 42" size and larger, LCD prices started falling rapidly in 2006 while their screen sizes were increasing at a similarly rapid rate. By late 2006, several vendors were offering 42" LCDs, albeit at a price premium, encroaching on plasma's only stronghold. More critically, LCDs offer higher resolutions and true 1080p support, while plasmas were stuck at 720p, which made up for the price difference.[13]

Predictions that prices for LCDs would drop rapidly through 2007 led to a "wait and see" attitude in the market, and sales of all large-screen televisions stagnated while customers watched to see if this would happen.[13] Plasmas and LCDs reached price parity in 2007, at which point the LCD's higher resolution was a winning point for many sales.[13] By late 2007, it was clear that LCDs were going to outsell plasmas during the critical Christmas sales season.[14][15] This was in spite of the fact that plasmas continued to hold an image quality advantage, but as the president of Chunghwa Picture Tubes noted after shutting down their plasma production line, "Globally, so many companies, so many investments, so many people have been working in this area, on this product. So they can improve so quickly."[13]

When the sales figures for the 2007 Christmas season were finally tallied, pundits were surprised to find that LCDs had not only outsold plasma, but also outsold CRTs during the same period.[16] This evolution drove competing large-screen systems from the market almost overnight. Plasma had overtaken rear-projection systems in 2005.[17] The same was true for CRTs, which lasted only a few months longer; Sony ended sales of their famous Trinitron in most markets in 2007, and shut down the final plant in March 2008.[18] The February 2009 announcement that Pioneer Electronics was ending production of the plasma screens was widely considered the tipping point in that technology's history as well.[19]

LCD's dominance in the television market accelerated rapidly.[13] It was the only technology that could scale both up and down in size, covering both the high-end market for large screens in the 40 to 50" class, as well as customers looking to replace their

existing smaller CRT sets in the 14 to 30" range. Building across these wide scales quickly pushed the prices down across the board.[16]

Current sixth-generation panels by major manufacturers such as Sony, Sharp Corporation, LG Display, Panasonic and the Samsung have announced larger sized models:

In October 2004, Sharp announced the successful manufacture of a 65" panel. In March 2005, Samsung announced an 82" LCD panel.[20]

In August 2006, LG Display Consumer Electronics announced a 100" LCD television[21]

In January 2007, Sharp displayed a 108" LCD panel under the AQUOS brand name at CES in Las Vegas.[22]

[edit] Recent researchThis section does not cite any references or sources.Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (June 2009)

Some manufacturers are also experimenting with extending color reproduction of LCD televisions. Although current LCD panels are able to deliver all sRGB colors using an appropriate combination of backlight's spectrum and optical filters, manufacturers want to display even more colors. One of the approaches is to use a fourth, or even fifth and sixth color in the optical color filter array. Another approach is to use two sets of suitably narrowband backlights (e.g. LEDs), with slightly differing colors, in combination with broadband optical filters in the panel, and alternating backlights each consecutive frame. Fully using the extended color gamut will naturally require an appropriately captured material and some modifications to the distribution channel. Otherwise, the only use of the extra colors would be to let the looker boost the color saturation of the TV picture beyond what was intended by the producer, but avoiding the otherwise unavoidable loss of detail ("burnout") in saturated areas.

[edit] Competing systems

In spite of LCD's current dominance of the television field, there are several other technologies being developed that address its shortcomings. Whereas LCDs produce an image by selectively blocking a backlight OLED, FED and SED all produce light directly on the front face of the display. In comparison to LCDs, all of these technologies offer better viewing angles, much higher brightness and contrast ratio (as much as 5,000,000:1), and better color saturation and accuracy, and use less than 1/10 as much power. In theory, they are less complex and less expensive to build.

Actually manufacturing these screens has proved more difficult than originally imagined. Sony abandoned their FED project in March 2009,[23] but continue work on their OLED sets. Canon continues development of their SED technology, but announced that they will not attempt to introduce sets to market for the foreseeable future.[24]

Samsung has been displaying OLED sets at 14.1, 31 and 40 inch sizes for some time, and at the SID 2009 trade show in San Antonio they announced that the 14.1 and 31 inch sets are "production ready".[25]

[edit] Environmental effectsSee also: Electronic waste

The production of LCD screens uses nitrogen trifluoride (NF3) as an etching fluid during the production of the thin-film components. NF3 is a potent greenhouse gas, and its extensive half-life may make it a potentially harmful contributor to global warming. A report in Geophysical Research Letters suggested that its effects were theoretically much greater than better-known sources of greenhouse gasses like carbon dioxide. As NF3 was not in widespread use at the time, it was not made part of the Kyoto Protocols and has been deemed "the missing greenhouse gas".[26]

Critics of the report point out that it assumes that all of the NF3 produced would be released to the atmosphere. In reality, the vast majority of NF3 is broken down during the cleaning processes; two earlier studies found that only 2% to 3% of the gas escapes destruction after its use.[27] Furthermore, the report failed to compare NF3's effects with what it replaced, perfluorocarbon, another powerful greenhouse gas, of which anywhere from 30% to 70% escapes to the atmosphere in typical use.

Organic light-emitting diodeFrom Wikipedia, the free encyclopediaJump to: navigation, search

Demonstration of a flexible OLED device

A green emitting OLED device

An organic light emitting diode (OLED) is a light-emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compounds which emit light in response to an electric current. This layer of organic semiconductor material is situated between two electrodes. Generally, at least one of these electrodes is transparent.

OLEDs are used in television screens, computer monitors, small, portable system screens such as mobile phones and PDAs, watches, advertising, information and indication. OLEDs are also used in light sources for space illumination and in large-area light-emitting elements. Due to their early stage of development, they typically emit less light per unit area than inorganic solid-state based LED point-light sources.

An OLED display functions without a backlight. Thus, it can display deep black levels and can be thinner and lighter than liquid crystal displays. In low ambient light conditions such as dark rooms, an OLED screen can achieve a higher contrast ratio than an LCD using either cold cathode fluorescent lamps or the more recently developed LED backlight.

There are two main families of OLEDs: those based upon small molecules and those employing polymers. Adding mobile ions to an OLED creates a Light-emitting Electrochemical Cell or LEC, which has a slightly different mode of operation.

OLED displays can use either passive-matrix (PMOLED) or active-matrix addressing schemes. Active-matrix OLEDs (AMOLED) require a thin-film transistor backplane to switch each individual pixel on or off, and can make higher resolution and larger size displays possible.

Contents[hide]

1 History 2 Working principle 3 Material technologies

o 3.1 Small molecules o 3.2 Polymer light-emitting diodes o 3.3 Phosphorescent materials

4 Device Architectures o 4.1 Structure o 4.2 Patterning technologies o 4.3 Backplane technologies

5 Advantages 6 Disadvantages 7 Manufacturers and Commercial Uses

o 7.1 Samsung applications o 7.2 Sony applications o 7.3 LG applications

8 See also 9 References 10 Further reading

11 External links

[edit] HistoryThe first observations of electroluminescence in organic materials were in the early 1950s by A. Bernanose and co-workers at the Nancy-Université, France. They applied high-voltage alternating current (AC) fields in air to materials such as acridine orange, either deposited on or dissolved in cellulose or cellophane thin films. The proposed mechanism was either direct excitation of the dye molecules or excitation of electrons.[1][2][3][4]

In 1960, Martin Pope and co-workers at New York University developed ohmic dark-injecting electrode contacts to organic crystals.[5][6][7] They further described the necessary energetic requirements (work functions) for hole and electron injecting electrode contacts. These contacts are the basis of charge injection in all modern OLED devices. Pope's group also first observed direct current (DC) electroluminescence under vacuum on a pure single crystal of anthracene and on anthracene crystals doped with tetracene in 1963[8] using a small area silver electrode at 400V. The proposed mechanism was field-accelerated electron excitation of molecular fluorescence.

Pope's group reported in 1965[9] that in the absence of an external electric field, the electroluminescence in anthracene crystals is caused by the recombination of a thermalized electron and hole, and that the conducting level of anthracene is higher in energy than the exciton energy level. Also in 1965, W. Helfrich and W. G. Schneider of the National Research Council in Canada produced double injection recombination electroluminescence for the first time in an anthracene single crystal using hole and electron injecting electrodes,[10] the forerunner of modern double injection devices. In the same year, Dow Chemical researchers patented a method of preparing electroluminescent cells using high voltage (500–1500 V) AC-driven (100–3000 Hz) electrically-insulated one millimetre thin layers of a melted phosphor consisting of ground anthracene powder,

tetracene, and graphite powder.[11] Their proposed mechanism involved electronic excitation at the contacts between the graphite particles and the anthracene molecules.

Device performance was limited by the poor electrical conductivity of contemporary organic materials. This was overcome by the discovery and development of highly conductive polymers.[12] For more on the history of such materials, see conductive polymers.

Electroluminescence from polymer films was first observed by Roger Partridge at the National Physical Laboratory in the United Kingdom. The device consisted of a film of poly(n-vinylcarbazole) up to 2.2 micrometres thick located between two charge injecting electrodes. The results of the project were patented in 1975[13] and published in 1983.[14][15]

[16][17]

The first diode device was reported at Eastman Kodak by Ching W. Tang and Steven Van Slyke in 1987.[18] This device used a novel two-layer structure with separate hole transporting and electron transporting layers such that recombination and light emission occurred in the middle of the organic layer. This resulted in a reduction in operating voltage and improvements in efficiency and led to the current era of OLED research and device production.

Research into polymer electroluminescence culminated in 1990 with J. H. Burroughes et al. at the Cavendish Laboratory in Cambridge reporting a high efficiency green light-emitting polymer based device using 100 nm thick films of poly(p-phenylene vinylene).[19]

[edit] Working principle

Schematic of a bilayer OLED: 1. Cathode (−), 2. Emissive Layer, 3. Emission of radiation, 4. Conductive Layer, 5. Anode (+)

A typical OLED is composed of a layer of organic materials situated between two electrodes, the anode and cathode, all deposited on a substrate. The organic molecules are electrically conductive as a result of delocalization of pi electrons caused by conjugation over all or part of the molecule. These materials have conductivity levels ranging from insulators to conductors, and therefore are considered organic semiconductors. The

highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) of organic semiconductors are analogous to the valence and conduction bands of inorganic semiconductors.

Originally, the most basic polymer OLEDs consisted of a single organic layer. One example was the first light-emitting device synthesised by J. H. Burroughes et al., which involved a single layer of poly(p-phenylene vinylene). However multilayer OLEDs can be fabricated with two or more layers in order to improve device efficiency. As well as conductive properties, different materials may be chosen to aid charge injection at electrodes by providing a more gradual electronic profile,[20] or block a charge from reaching the opposite electrode and being wasted.[21] Many modern OLEDs incorporate a simple bilayer structure, consisting of a conductive layer and an emissive layer.

During operation, a voltage is applied across the OLED such that the anode is positive with respect to the cathode. A current of electrons flows through the device from cathode to anode, as electrons are injected into the LUMO of the organic layer at the cathode and withdrawn from the HOMO at the anode. This latter process may also be described as the injection of electron holes into the HOMO. Electrostatic forces bring the electrons and the holes towards each other and they recombine forming an exciton, a bound state of the electron and hole. This happens closer to the emissive layer, because in organic semiconductors holes are generally more mobile than electrons. The decay of this excited state results in a relaxation of the energy levels of the electron, accompanied by emission of radiation whose frequency is in the visible region. The frequency of this radiation depends on the band gap of the material, in this case the difference in energy between the HOMO and LUMO.

As electrons and holes are fermions with half integer spin, an exciton may either be in a singlet state or a triplet state depending on how the spins of the electron and hole have been combined. Statistically three triplet excitons will be formed for each singlet exciton. Decay from triplet states (phosphorescence) is spin forbidden, increasing the timescale of the transition and limiting the internal efficiency of fluorescent devices. Phosphorescent organic light-emitting diodes make use of spin–orbit interactions to facilitate intersystem crossing between singlet and triplet states, thus obtaining emission from both singlet and triplet states and improving the internal efficiency.

Indium tin oxide (ITO) is commonly used as the anode material. It is transparent to visible light and has a high work function which promotes injection of holes into the HOMO level of the organic layer. A typical conductive layer may consist of PEDOT:PSS [22] as the HOMO level of this material generally lies between the workfunction of ITO and the HOMO of other commonly used polymers, reducing the energy barriers for hole injection. Metals such as barium and calcium are often used for the cathode as they have low work functions which promote injection of electrons into the LUMO of the organic layer.[23] Such metals are reactive, so require a capping layer of aluminium to avoid degradation.

Single carrier devices are typically used to study the kinetics and charge transport mechanisms of an organic material and can be useful when trying to study energy transfer processes. As current through the device is composed of only one type of charge carrier, either electrons or holes, recombination does not occur and no light is emitted. For example, electron only devices can be obtained by replacing ITO with a lower work function metal which increases the energy barrier of hole injection. Similarly, hole only devices can be made by using a cathode comprised solely of aluminium, resulting in an energy barrier too large for efficient electron injection.[24][25][26]

[edit] Material technologies

[edit] Small molecules

Alq3,[18] commonly used in small molecule OLEDs.

Efficient OLEDs using small molecules were first developed by Dr. Ching W. Tang et al.[18] at Eastman Kodak. The term OLED traditionally refers specifically to this type of device, though the term SM-OLED is also in use.

Molecules commonly used in OLEDs include organometallic chelates (for example Alq3, used in the organic light-emitting device reported by Tang et al.), fluorescent and phosphorescent dyes and conjugated dendrimers. A number of materials are used for their charge transport properties, for example triphenylamine and derivatives are commonly used as materials for hole transport layers.[27] Fluorescent dyes can be chosen to obtain light emission at different wavelengths, and compounds such as perylene, rubrene and quinacridone derivatives are often used.[28] Alq3 has been used as a green emitter, electron transport material and as a host for yellow and red emitting dyes.

The production of small molecule devices and displays usually involves thermal evaporation in a vacuum. This makes the production process more expensive and of limited use for large-area devices than other processing techniques. However, contrary to polymer-based devices, the vacuum deposition process enables the formation of well controlled, homogeneous films, and the construction of very complex multi-layer structures. This high flexibility in layer design, enabling distinct charge transport and charge blocking layers to be formed, is the main reason for the high efficiencies of the small molecule OLEDs.

Coherent emission from a laser dye-doped tandem SM-OLED device, excited in the pulsed regime, has been demonstrated.[29] The emission is nearly diffraction limited with a spectral width similar to that of broadband dye lasers.[30]

[edit] Polymer light-emitting diodes

poly( p -phenylene vinylene) , used in the first PLED.[19]

Polymer light-emitting diodes (PLED), also light-emitting polymers (LEP), involve an electroluminescent conductive polymer that emits light when connected to an external voltage. They are used as a thin film for full-spectrum colour displays. Polymer OLEDs are quite efficient and require a relatively small amount of power for the amount of light produced.

Vacuum deposition is not a suitable method for forming thin films of polymers. However, polymers can be processed in solution, and spin coating is a common method of depositing thin polymer films. This method is more suited to forming large-area films than thermal evaporation. No vacuum is required, and the emissive materials can also be applied on the substrate by a technique derived from commercial inkjet printing.[31][32] However, as the application of subsequent layers tends to dissolve those already present, formation of multilayer structures is difficult with these methods. The metal cathode may still need to be deposited by thermal evaporation in vacuum.

Typical polymers used in PLED displays include derivatives of poly( p -phenylene vinylene) and polyfluorene. Substitution of side chains onto the polymer backbone may determine the colour of emitted light[33] or the stability and solubility of the polymer for performance and ease of processing.[34]

While unsubstituted poly(p-phenylene vinylene) (PPV) is typically insoluble, a number of PPVs and related poly(naphthalene vinylene)s (PNVs) that are soluble in organic solvents or water have been prepared via ring opening metathesis polymerization.[35][36][37]

[edit] Phosphorescent materials

Ir(mppy)3, a phosphorescent dopant which emits green light.[38]

Main article: Phosphorescent organic light-emitting diode

Phosphorescent organic light emitting diodes use the principle of electrophosphorescence to convert electrical energy in an OLED into light in a highly efficient manner,[39][40] with the internal quantum efficiencies of such devices approaching 100%.[41]

Typically, a polymer such as poly(n-vinylcarbazole) is used as a host material to which an organometallic complex is added as a dopant. Iridium complexes [40] such as Ir(mppy)3

[38] are currently the focus of research, although complexes based on other heavy metals such as platinum[39] have also been used.

The heavy metal atom at the centre of these complexes exhibits strong spin-orbit coupling, facilitating intersystem crossing between singlet and triplet states. By using these phosphorescent materials, both singlet and triplet excitons will be able to decay radiatively, hence improving the internal quantum efficiency of the device compared to a standard PLED where only the singlet states will contribute to emission of light.

Applications of OLEDs in solid state lighting require the achievement of high brightness with good CIE coordinates (for white emission). The use of macromolecular species like polyhedral oligomeric silsesquioxanes (POSS) in conjunction with the use of phosphorescent species such as Ir for printed OLEDs have exhibited brightnesses as high as 10,000 cd/m2.[42]

[edit] Device Architectures

[edit] Structure

Bottom or top emission: Bottom emission devices use a transparent or semi-transparent bottom electrode to get the light through a transparent substrate. Top emission devices[43][44] use a transparent or semi-transparent top electrode emitting light directly. Top-emitting OLEDs are better suited for active-matrix applications as they can be more easily integrated with a non-transparent transistor backplane.

Transparent OLEDs use transparent or semi-transparent contacts on both sides of the device to create displays that can be made to be both top and bottom emitting (transparent). TOLEDs can greatly improve contrast, making it much easier to view displays in bright sunlight.[45] This technology can be used in Head-up displays, smart windows or augmented reality applications. Novaled's[46] OLED panel presented in Finetech Japan 2010, boasts a transparency of 60–70%.

Stacked OLEDs use a pixel architecture that stacks the red, green, and blue subpixels on top of one another instead of next to one another, leading to substantial increase in gamut and color depth, and greatly reducing pixel gap. Currently, other display technologies have the RGB (and RGBW) pixels mapped next to each other decreasing potential resolution.

Inverted OLED: In contrast to a conventional OLED, in which the anode is placed on the substrate, an Inverted OLED uses a bottom cathode that can be connected to the drain end of an n-channel TFT especially for the low cost amorphous silicon TFT backplane useful in the manufacturing of AMOLED displays.[47]

[edit] Patterning technologies

Patternable organic light-emitting devices use a light or heat activated electroactive layer. A latent material (PEDOT-TMA) is included in this layer that, upon activation, becomes highly efficient as a hole injection layer. Using this process, light-emitting devices with arbitrary patterns can be prepared.[48]

Colour patterning can be accomplished by means of laser, such as radiation-induced sublimation transfer (RIST).[49]

Organic vapour jet printing (OVJP) uses an inert carrier gas, such as argon or nitrogen, to transport evaporated organic molecules (as in Organic Vapor Phase Deposition). The gas is expelled through a micron sized nozzle or nozzle array close to the substrate as it is being translated. This allows printing arbitrary multilayer patterns without the use of solvents.

Conventional OLED displays are formed by vapor thermal evaporation (VTE) and are patterned by shadow-mask. A mechanical mask has openings allowing the vapor to pass only on the desired location.

[edit] Backplane technologies

For a high resolution display like a TV, a TFT backplane is necessary to drive the pixels correctly. Currently, Low Temperature Polycrystalline silicon LTPS-TFT is used for commercial AMOLED displays. LTPS-TFT has variation of the performance in a display, so various compensation circuits have been reported.[43] Due to the size limitation of the excimer laser used for LTPS, the AMOLED size was limited. To cope with the

hurdle related to the panel size, amorphous-silicon/microcrystalline-silicon backplanes have been reported with large display prototype demonstrations.[50]

[edit] Advantages

Demonstration of a 4.1" prototype flexible display from Sony

The different manufacturing process of OLEDs lends itself to several advantages over flat-panel displays made with LCD technology.

Lower cost in the future: OLEDs can be printed onto any suitable substrate by an inkjet printer or even by screen printing,[51] theoretically making them cheaper to produce than LCD or plasma displays. However, fabrication of the OLED substrate is more costly than that of a TFT LCD, until mass production methods lower cost through scalability. Roll-roll vapour-deposition methods for organic devices do allow mass production of thousands of devices per minute for minimal cost, although this technique also induces problems in that multi-layer devices can be challenging to make.

Light weight & flexible plastic substrates: OLED displays can be fabricated on flexible plastic substrates leading to the possibility of flexible organic light-emitting diodes being fabricated or other new applications such as roll-up displays embedded in fabrics or clothing. As the substrate used can be flexible such as PET.,[52] the displays may be produced inexpensively.

Wider viewing angles & improved brightness: OLEDs can enable a greater artificial contrast ratio (both dynamic range and static, measured in purely dark conditions) and viewing angle compared to LCDs because OLED pixels directly emit light. OLED pixel colours appear correct and unshifted, even as the viewing angle approaches 90° from normal.

Better power efficiency: LCDs filter the light emitted from a backlight, allowing a small fraction of light through so they cannot show true black, while an inactive OLED element does not produce light or consume power.[53]

Response time: OLEDs can also have a faster response time than standard LCD screens. Whereas LCD displays are capable of between 2 and 8 ms response time offering a frame rate of +/-200 Hz,[citation needed] an OLED can theoretically have less than 0.01 ms response time enabling 100,000 Hz refresh rates.

[edit] Disadvantages

LEP display showing partial failure

An old OLED display showing wear Current costs: OLED manufacture currently requires process steps that make it

extremely expensive. Specifically, it requires the use of Low-Temperature Polysilicon backplanes; LTPS backplanes in turn require laser annealing from an amorphous silicon start, so this part of the manufacturing process for AMOLEDs starts with the process costs of standard LCD, and then adds an expensive, time-consuming process that cannot currently be used on large-area glass substrates.

Lifespan: The biggest technical problem for OLEDs was the limited lifetime of the organic materials.[54] In particular, blue OLEDs historically have had a lifetime of around 14,000 hours to half original brightness (five years at 8 hours a day) when used for flat-panel displays. This is lower than the typical lifetime of LCD, LED or PDP technology—each currently rated for about 25,000 – 40,000 hours to half brightness, depending on manufacturer and model.[55][56] However, some manufacturers' displays aim to increase the lifespan of OLED displays, pushing their expected life past that of LCD displays by improving light outcoupling, thus achieving the same brightness at a lower drive current.[57][58] In 2007, experimental OLEDs were created which can sustain 400 cd/m2 of luminance for over 198,000 hours for green OLEDs and 62,000 hours for blue OLEDs.[59]

Color balance issues: Additionally, as the OLED material used to produce blue light degrades significantly more rapidly than the materials that produce other colors, blue light output will decrease relative to the other colors of light. This

differential color output change will change the color balance of the display and is much more noticeable than a decrease in overall luminance.[60] This can be partially avoided by adjusting colour balance but this may require advanced control circuits and interaction with the user, which is unacceptable for some users. In order to delay the problem, manufacturers bias the colour balance towards blue so that the display initially has an artificially blue tint, leading to complaints of artificial-looking, over-saturated colors. More commonly, though, manufacturers optimize the size of the R, G and B subpixels to reduce the current density through the subpixel in order to equalize lifetime at full luminance. For example, a blue subpixel may be 100% larger than the green subpixel. The red subpixel may be 10% smaller than the green.

Efficiency of blue OLEDs: Improvements to the efficiency and lifetime of blue OLEDs is vital to the success of OLEDs as replacements for LCD technology. Considerable research has been invested in developing blue OLEDs with high external quantum efficiency as well as a deeper blue color.[61][62] External quantum efficiency values of 20% and 19% have been reported for red (625 nm) and green (530 nm) diodes, respectively.[63][64] However, blue diodes (430 nm) have only been able to achieve maximum external quantum efficiencies in the range between 4% to 6%.[65]

Water damage: Water can damage the organic materials of the displays. Therefore, improved sealing processes are important for practical manufacturing. Water damage may especially limit the longevity of more flexible displays.[66]

Outdoor performance: As an emissive display technology, OLEDs rely completely upon converting electricity to light, unlike most LCDs which are to some extent reflective; e-ink leads the way in efficiency with ~ 33% ambient light reflectivity, enabling the display to be used without any internal light source. The metallic cathode in an OLED acts as a mirror, with reflectance approaching 80%, leading to poor readability in bright ambient light such as outdoors. However, with the proper application of a circular polarizer and anti-reflective coatings, the diffuse reflectance can be reduced to less than 0.1%. With 10,000 fc incident illumination (typical test condition for simulating outdoor illumination), that yields an approximate photopic contrast of 5:1.

Power consumption: While an OLED will consume around 40% of the power of an LCD displaying an image which is primarily black, for the majority of images it will consume 60–80% of the power of an LCD – however it can use over three times as much power to display an image with a white background[67] such as a document or website. This can lead to reduced real-world battery life in mobile devices.

Screen burn-in: Unlike displays with a common light source, the brightness of each OLED pixel fades depending on the content displayed. The varied lifespan

of the organic dyes can cause a discrepancy between red, green, and blue intensity. This leads to image persistence, also known as burn-in.[68]

UV sensitivity: OLED displays can be damaged by prolonged exposure to UV light. The most pronounced example of this can be seen with a near UV laser (such as a Bluray pointer) and can damage the display almost instantly with more than 20 mW leading to dim or dead spots where the beam is focused. This is usually avoided by installing a UV blocking filter over the panel and this can easily be seen as a clear plastic layer on the glass. Removal of this filter can lead to severe damage and an unusable display after only a few months of room light exposure.

[edit] Manufacturers and Commercial Uses

Magnified image of the AMOLED screen on the Google Nexus One smartphone using the RGBG system of the PenTile Matrix Family.

A 3.8 cm (1.5 in) OLED display from a Creative ZEN V media player

OLED technology is used in commercial applications such as displays for mobile phones and portable digital media players, car radios and digital cameras among others. Such portable applications favor the high light output of OLEDs for readability in sunlight and their low power drain. Portable displays are also used intermittently, so the lower lifespan of organic displays is less of an issue. Prototypes have been made of flexible and rollable displays which use OLEDs' unique characteristics. Applications in flexible signs and lighting are also being developed.[69] Philips Lighting have made OLED lighting samples under the brand name 'Lumiblade' available online.[70]

OLEDs have been used in most Motorola and Samsung colour cell phones, as well as some HTC, LG and Sony Ericsson models.[71] Nokia has also recently introduced some OLED products including the N85 and the N86 8MP, both of which feature an AMOLED display. OLED technology can also be found in digital media players such as the Creative ZEN V, the iriver clix, the Zune HD and the Sony Walkman X Series.

The Google and HTC Nexus One smartphone includes an AMOLED screen, as does HTC's own Desire and Legend phones. However due to supply shortages of the Samsung-produced displays, certain HTC models will use Sony's SLCD displays in the future,[72] while the Google and Samsung Nexus S smartphone will use "Super Clear LCD" instead in some countries.[73]

Other manufacturers of OLED panels include Anwell Technologies Limited,[74] Chi Mei Corporation,[75] LG,[76] and others.[77]

DuPont stated in a press release in May 2010 that they can produce a 50-inch OLED TV in two minutes with a new printing technology. If this can be scaled up in terms of manufacturing, then the total cost of OLED TVs would be greatly reduced. Dupont also states that OLED TVs made with this less expensive technology can last up to 15 years if left on for a normal eight hour day.[78][79]

Handheld computer manufacturer OQO introduced the smallest Windows netbook computer, including an OLED display, in 2009.[80]

The use of OLEDs may be subject to patents held by Eastman Kodak, DuPont, General Electric, Royal Philips Electronics, numerous universities and others.[81] There are by now literally thousands of patents associated with OLEDs, both from larger corporations and smaller technology companies [1].

[edit] Samsung applications

By 2004 Samsung, South Korea's largest conglomerate, was the world's largest OLED manufacturer, producing 40% of the OLED displays made in the world,[82] and as of 2010 has a 98% share of the global AMOLED market.[83] The company is leading the world OLED industry, generating $100.2 million out of the total $475 million revenues in the global OLED market in 2006.[84] As of 2006, it held more than 600 American patents and more than 2800 international patents, making it the largest owner of AMOLED technology patents.[84]

Samsung SDI announced in 2005 the world's largest OLED TV at the time, at 21 inches (53 cm).[85] This OLED featured the highest resolution at the time, of 6.22 million pixels. In addition, the company adopted active matrix based technology for its low power consumption and high-resolution qualities. This was exceeded in January 2008, when Samsung showcased the world's largest and thinnest OLED TV at the time, at 31 inches and 4.3 mm.[86]

In May 2008, Samsung unveiled an ultra-thin 12.1 inch laptop OLED display concept, with a 1,280×768 resolution with infinite contrast ratio.[87] According to Woo Jong Lee, Vice President of the Mobile Display Marketing Team at Samsung SDI, the company expected OLED displays to be used in notebook PCs as soon as 2010.[88]

In October 2008, Samsung showcased the world's thinnest OLED display, also the first to be 'flappable' and bendable.[89] It measures just 0.05 mm (thinner than paper), yet a Samsung staff member said that it is "technically possible to make the panel thinner".[89] To achieve this thickness, Samsung etched an OLED panel that uses a normal glass substrate. The drive circuit was formed by low-temperature polysilicon TFTs. Also, low-molecular organic EL materials were employed. The pixel count of the display is 480 × 272. The contrast ratio is 100,000:1, and the luminance is 200 cd/m². The colour reproduction range is 100% of the NTSC standard.

In the same month, Samsung unveiled what was then the world's largest OLED Television at 40-inch with a Full HD resolution of 1920×1080 pixel.[90] In the FPD International, Samsung stated that its 40-inch OLED Panel is the largest size currently possible. The panel has a contrast ratio of 1,000,000:1, a colour gamut of 107% NTSC, and a luminance of 200 cd/m² (peak luminance of 600 cd/m²).

At the Consumer Electronics Show (CES) in January 2010, Samsung demonstrated a laptop computer with a large, transparent OLED display featuring up to 40% transparency[91] and an animated OLED display in a photo ID card.[92]

Samsung's latest AMOLED smartphones use their Super AMOLED trademark, with the Samsung Wave S8500 and Samsung i9000 Galaxy S being launched in June 2010. In January 2011 Samsung announced their Super AMOLED Plus displays[93] - which offer several advances over the older Super AMOLED displays - real stripe matrix (50% more sub pixels), thinner form factor, brighter image and a 18% reduction in energy consumption.

[edit] Sony applications

Sony XEL-1, the world's first OLED TV.[94] (front)

Sony XEL-1 (side)

The Sony CLIÉ PEG-VZ90 was released in 2004, being the first PDA to feature an OLED screen.[95] Other Sony products to feature OLED screens include the MZ-RH1 portable minidisc recorder, released in 2006[96] and the Walkman X Series.[97]

At the Las Vegas CES 2007, Sony showcased 11-inch (28 cm, resolution 960×540) and 27-inch (68.5 cm, full HD resolution at 1920×1080) OLED TV models.[98] Both claimed 1,000,000:1 contrast ratios and total thicknesses (including bezels) of 5 mm. In April 2007, Sony announced it would manufacture 1000 11-inch OLED TVs per month for market testing purposes.[99] On October 1, 2007, Sony announced that the 11-inch model, now called the XEL-1, would be released commercially;[94] the XEL-1 was first released in Japan in December 2007.[100]

In May 2007, Sony publicly unveiled a video of a 2.5-inch flexible OLED screen which is only 0.3 millimeters thick.[101] At the Display 2008 exhibition, Sony demonstrated a 0.2 mm thick 3.5 inch display with a resolution of 320×200 pixels and a 0.3 mm thick 11 inch display with 960×540 pixels resolution, one-tenth the thickness of the XEL-1.[102]

[103]

In July 2008, a Japanese government body said it would fund a joint project of leading firms, which is to develop a key technology to produce large, energy-saving organic displays. The project involves one laboratory and 10 companies including Sony Corp. NEDO said the project was aimed at developing a core technology to mass-produce 40 inch or larger OLED displays in the late 2010s.[104]

In October 2008, Sony published results of research it carried out with the Max Planck Institute over the possibility of mass-market bending displays, which could replace rigid LCDs and plasma screens. Eventually, bendable, transparent OLED screens could be stacked to produce 3D images with much greater contrast ratios and viewing angles than existing products.[105]

Sony exhibited a 24.5" prototype OLED 3D television during the Consumer Electronics Show in January 2010.[106]

In January 2011, Sony announced the NGP handheld game console (the successor to the PSP) will feature a 5-inch OLED screen.[107]

On 17 February 2011, Sony announced its 25" OLED Professional Reference Monitor aimed at the Cinema and high end Drama Post Production market.[108]

[edit] LG applications

As of 2010 LG produces one model of OLED television, the 15 inch 15EL9500[109] and has announced a 31" OLED 3D television for March 2011

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LED backlight design for large displaysMike CaddyThursday 24 February 2011 04:15

LEDs use a DC supply, making them simpler to drive, there is no inverter, which improves efficiency, and their power consumption varies nearly linearly with brightness, simplifying power management. As a result the majority of small to medium displays are now fitted with them, writes Mike Caddy.

Despite their advantages and growing popularity, there are also drawbacks associated with LED backlights. A white LED isn’t truly white. It is actually a blue LED fitted with a yellow phosphor to give the impression of white light, and its spectral curve has gaps in the green and red parts of the spectrum. To achieve the very best colour balance, premium quality LCD displays are fitted with RGB backlights.

Most displays of any size will require more than one LED for an acceptable level of brightness, and good uniformity is harder to achieve, especially as the LEDs age.

Power efficiency can also be a challenge. Though LED displays are normally more power efficient than CCFL, this is not a given and some implementations use the same or more power than their CCFL counterparts.

LEDs are however continually improving in terms of light output, efficiency and lifetime. LED brightness has increased so that fewer devices are potentially required per display. Manufacturers have used MEMS and other light guide technologies to spread illumination evenly over a large area to give the maximum brightness and uniformity.

Other sophistications include full frame LED lighting, where the LCD panel is divided into up to 240 segments, and the brightness of the LED backlight can be varied locally, to produce a ‘blacker black’ in dark areas of the screen and simultaneously to reduce power. Ultra thin screens can be created by using edge lighting.

Making the most of your LED backlight

LCD displays vary greatly in their performance, and some displays can achieve a required level of ‘readability’ with less help from the backlight than others. The key parameters to look at are brightness, contrast ratio and viewing angle.

Brightness is a relatively well-standardised parameter, and is quoted in candelas per square metre in a darkened room with all pixels white at maximum backlight drive. Contrast ratio values are less easy to compare as there are a number of ways of interpreting this measurement, but it is fundamentally the ratio of the luminance of the brightest color (white) to that of the darkest color (black) that the system is capable of producing.

Viewing angle is more subjective.

Display brightness is affected by the transmission ratio of a TFT display. A small proportion of each pixel is obscured by the thin film transistor controlling it. Technologies such as low temperature polysilicon (LTPS) reduce the size of this transistor.

Whilst the performance figures can provide a guide to drawing up a shortlist of potential display options, the best advice for applications where good visibility is required under demanding conditions, and power consumption is also an issue, is to mock up the application on a number of display alternatives and measure the backlight power in each case. Factors to be considered are the ambient light level in the environment in which the display is to be used, and the likely viewing angle. Sometimes varying the colour in which key information is presented can also have an impact on display performance.

OLED – the no backlight alternative

The driving of an LCD itself consumes very little power, and the power consumption of the display system is almost all down to the backlight – which always illuminates the whole display area unless it can be switched off.

OLED by contrast is an emissive technology. Each pixel emits its own light – so when it is off, it produces no light and consumes almost no power. Unlike backlit LCD displays therefore, OLEDs produce a true black, and their contrast ratio is much higher, typically 10,000:1 compared with 400:1 for a conventional TFT display. They are also brighter, partly because they don’t require the pair of polarizers which filters out half of the light emitted by the backlight in an LCD display.

Aesthetically OLED technology wins hands down over LCD, with much improved brightness and contrast. The response time of an OLED display is typically 50μs versus 25ms of LCD, meaning full motion-video is faster and grayscale rendition is far superior.

Despite its higher cost and shorter lifetime, OLED is being used in a growing number of cool consumer products, including the Sony X-series Walkman, the Nokia N85 and the Microsoft Zune HD. The technology is now also becoming available on the industrial market, and OLED display options in a range of sizes (0.79in to 7.0in) and resolutions (64x48 to 480x272) are offered, supported by development and evaluation kits.

OLED power management

The power consumption of an OLED display is not a fixed value but varies depending on the image being displayed. In typical video and image display applications, it can be as low as 25% of the theoretical ‘maximum’ power that would be consumed if all pixels were fully illuminated.

For applications where power is a major concern, image design can contribute to reducing consumption. For example, displaying an image in negative mode (white text on a dark background) can be much more efficient than in positive mode (dark text on a white background), since you need to switch on only around one-tenth of the pixels.

Pixel brightness also impacts power consumption. The relationship isn’t quite linear, but this is a good first order approximation. Power can be saved not only by reducing the brightness of all pixels when ambient light levels permit, but also by context-sensitive brightness management – for example by dimming menu areas that aren’t available. 

Reducing brightness can also help extend the life of the display. Where power really is a challenge, changing the colour of frequently displayed menu items could also be considered, as the red and green pixels are more efficient than the blue. A research team from British Columbia has been able to design “energy aware” colour sets that gave energy savings of around 40% compared to a standard colour palette.

Refresh rate also has an impact on power consumption due to the capacitative characteristics of OLED pixels. A very high frame frequency increases power consumption by increasing the number of charging cycles. It can also cause the display to dim, as the pixels don’t have time to charge fully during each refresh cycle. Although the contrast setting can be increased to compensate, this further increases power consumption. The refresh rate should be set as low as possible without causing visible image ‘flickering’. A suitable nominal value is 75Hz though it is sometimes possible to get away with as little as 60Hz.

Since OLEDs are an emissive technology, very slow degradation of pixels occurs with continuous use. In addition to using stand-by and time-out modes to reduce ageing, it is worth considering a screen saver. The number of pixels used in the screen saver and their brightness should be managed as above. 

OverviewMany visual equipment including TVs, cameras and printers are evolving to provide aricher palette of colors. In the past, hardware limitations made it impossible toreproduce all colors perceived by the human eye. However, advances in display andprinting technology are making it possible to reproduce a wider range of colors.Display devices which boasted the widest color gamut have also made significantadvances to reproduce colors that are richer and closer to natural hues. Examplesinclude LCD which employs WCG-CCFL and CRT which uses phosphors for widegamut color reproduction. Along with those advances, the demand for even morerealistic colors increased among users. This led to the growing popularity of LED as abacklight source and by using LED’s superior ability to reproduce colors, display makershave been able to bring greater satisfaction to users.LED Monitor’s Color GamutLED monitor refers to a monitor that uses LED as a backlight instead of fluorescentlamps. Just as CRT and LCD monitors use RGB as primary colors to realize diversecolors, RGB LED is used in LED monitors to create white and acts as a backlight unit.01 LED BackLightSAMSUNG ELECTRONICS CO., LTD< LED BackLightLED backlighting provides a wider color gamut compared to previous monitors as LED’scolor wavelengths have a higher purity level than fluorescent lamps. As illustrated below,an LED monitor claims a broader range of reds and greens for a more extensive colorgamut and natural color expressions, compared to a WCG-CCFL monitor. Additionally,as a semiconductor device, LED monitors have the added advantage of a longer lifespan, typically lasting over 100,000 hours.As seen above, LED monitors are capable of delivering a color gamut of 114% to NTSCstandards. Created by the US National Television Systems Committee, NTSC is atelevision system standard for display devices, various broadcast equipment and videosignal levels. An LCD monitor with a 82% NTSC color gamut means it is able to cover82% of the NTSC RGB color gamut. Therefore, 114% NTSC signifies a display capableof producing the entire NTSC color gamut, solving the problem of the low 82% NTSC(limited color depth and diversity) which had frequently been cited as a major weaknessof CCFL LCD displays. In sum, by expanding the color gamut, newer displays havebeen able to reproduce previously impossible colors.SAMSUNG ELECTRONICS CO., LTD400030002000100000.80.60.40.20300 400 500 600 700 800Wavelength (nanometers) 0 0.2 0.4 0.6 0.8Intensity(counts)CIE ChromacityDiagramLED BLACKLIGHT DISPLAYCCFL BLACKLIGHT DISPLAY520540560580

600620650770nm114% NTSC82% NTSC02 LED BackLightWhile the majority of current CCFL LCD displays are unable to express colors outsidethe sRGB space, LED monitors are able to do so, even covering Adobe RGB. As AdobeRGB is the standard color profile for cameras and printers, this means that LEDmonitors eliminate the differences in color that occur due to a varying color gamutbetween visual devices.Samsung LED monitors provide a calibration tool called “Natural Color” to effectivelyutilize the characteristic of LED monitors and to more accurately represent colors. Thetool is not limited to simple calibration but enables emulation to the color conditiondesired by the user, resulting in more precise color expression.SAMSUNG ELECTRONICS CO., LTDLED BackLight LED BackLightWIDE GAMUT RGBADOBE RGB 1998S RGB03 LED BackLightNatural ColorThe same image shown on different display devices can appear to be in different colors.This is because display devices have a different color gamut and a different method ofshowing colors due to hardware characteristics. This explains why R:255 on twodisplays can still appear to be different.CMS (Color Management System) is used to rectify this discrepancy. CMS maps thecolor characteristics of each device with independent color spaces to produce an imagewith color that is uniform and independent of a device. This method was suggested bythe International Color Consortium (ICC) and requires a profile which enables themapping for each device with independent color spaces. The more accurate the profile,the more accurate the CMS. Although color-related device makers provide their ownprofiles for their devices, there will inevitably be some differences even among the samemodel produced by the same company. The calibration process provides a moreaccurate profile. By measuring the output characteristics of a display for input signalsfrom 0 to 255, a profile is made by reflecting the characteristics of each display. This, inturn, allows the user to even correct color changes resulting from the aging or differentsettings of a device.SAMSUNG ELECTRONICS CO., LTDR:255 R:255Original Image RED 255 Digital DATA Different RED on thesame model monitors.What is the keyto solve this problem?04 LED BackLightFurthermore, the ‘Emulation’ function in Natural Color allows users to change colorspaces to sRGB or Adobe RGB even without CMS tools such as Photoshop or viewerprogram. If the Emulation function is executed after selecting a color space such as

sRGB or Adobe RGB, a corresponding ICC file is created. This file can also be saved onan OS and LED monitor, like a calibration file. When the saved emulation values matchthe saved settings of an image, the same image color can be reproduced even withouta separate CMS tool. The saved emulation values can be accessed through a hot-keyfunction on the monitor.Natural Color also checks the uniformity of a device and provides revision, so that

accurate color reproduction is possible even as the device ages. BacklightFrom Wikipedia, the free encyclopediaJump to: navigation, search This article is about backlights in liquid crystal displays. For the rear window of an automobile, see Car glass. For the lighting design practice, see Backlighting (lighting design).

Animation of an LCD, both unlit and with electroluminescent backlight switched on.

A backlight is a form of illumination used in liquid crystal displays (LCDs). As LCDs do not produce light 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, and in computer displays 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 valves 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.

LCD Back Light

Contents[hide]

1 Light source types 2 Usage

o 2.1 LED backlights 2.1.1 Flicker due to backlight dimming

3 Backlight diffusers 4 References

5 External links

[edit] Light source typesThe light source can be made up of:

Incandescent light bulbs light-emitting diodes (LEDs) An electroluminescent panel (ELP) One or more Cold Cathode Fluorescent Lamps (CCFL) One or more Hot Cathode Fluorescent Lamps (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 LCDs usually have yellow, green, blue or white backlights, while color displays use white backlights that cover most of the color spectrum.

[edit] Usage

LCD with CCFL backlight

Colored LED backlighting is most commonly used in small, inexpensive LCD panels. White LED backlighting is becoming more common. 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 voltage AC power, which is provided by an inverter 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.

[edit] LED backlights

See also: LED-backlit LCD television

LCD with LED matrix backlight

LED backlighting in color screens comes in two flavors: white LED backlights and RGB LED backlights[1]. White LEDs are used most often in notebooks and desktop screens, and in virtually all mobile LCD screens. A white LED is actually a blue LED with yellow

phosphor to give the impression of white light. The spectral curve has big gaps in the green and red parts. RGB LEDs consist of a red, a blue, and a green LED and can be controlled to produce different temperatures of white. RGB LEDs for backlighting are found in high end color proofing displays such as HP DreamColor LP2480zx monitor or selected HP EliteBook 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 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 little 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. 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 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 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.[2] 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.[citation needed] In 2010, current generation LED displays can have significant power consumption advantages. For example, the non-LED version of the 24" Benq G2420HDB[3] consumer display has a 49W consumption compared to the 24W of the LED version of the same display (G2420HDBL[4]).

The use of LED backlights in notebook computers has been growing. Sony has used LED backlights in some of its higher-end slim VAIO notebooks since 2005. 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, Lenovo has also announced LED-backlit notebooks, and other companies like HP 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, and one year later it introduced a new LED iMac, meaning all of Apple's computer screens, except the 30-inch Cinema HD Display, are now LED. Almost every laptop with 16:9 display introduced since September 2009 uses LED-backlit panels.

[edit] Flicker due to backlight dimming

LED backlights are often dimmed by applying pulse-width modulation to the supply current, switching the backlight off and on again like a fast strobe light. 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 [5] . 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.

[edit] Backlight diffusersIn order for a non-ELP backlight to produce even lighting (which is critical for LCDs), the light is first passed through a lightguide - a specially-designed layer of plastic that diffuses 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 to guide otherwise wasted light back towards the LCD panel. The reflector is sometimes made of aluminum foil, sometimes merely a white-pigmented surface or, as in the 3M Vikuiti ESR, consisting of hundreds of polymer layers of alternating low and high index. Between the lightguide and the LCD panel is usually placed a reflective polarizer film which vastly increases efficiency by repeatedly reflecting any unpolarized light back, which would otherwise be absorbed by the LCD's rear polarizer.