The Evolution of LED Backlights

Author: Adam Simmons
Last updated: 2nd August 2013

 

The rise of LED

LED (Light Emitting Diode) backlights are ‘interesting’ for the consumer as they help make a thinner, lighter and more efficient display. It is also a winner from a marketing perspective, with manufacturers keen to draw an artificial distinction between their ‘LED’ (backlit) monitors and their ‘LCD’ monitors. This blindly lead people to believe that the technology was completely distinct from ‘LCD’ and not just a change in backlight type from CCFL (Cold Cathode Fluorescent Lamp) to LED. The rapid brightness adjustments also allow manufacturers to make better use of the ‘dynamic contrast’ feature that we often criticise in our reviews. Whilst the practicalities of adjusting the entire backlight to suit the overall scene darkness are questionable it certainly allows the insanely big and misleading numbers game to be played with contrast ratios.

To many consumers the situation seemed win-win with the end product being thinner, lighter, free from mercury and arsenic and more energy efficient – consuming less power and generating less heat. Looking deeper into the monitors using the technology, as they became more prevalent, it soon became clear that there was still room for CCFL backlighting. The drive to make things thinner may please some users aesthetically, but it also has its downsides. Although considerable variability can be seen on models with both CCFL and LED backlights, many LED models tend to be particularly thin and susceptible to being flexed both during and after manufacture. This can exacerbate luminance uniformity problems and in particular cause issues with backlight bleed and clouding.

Thin, attractive and relatively delicate

Thin, attractive and relatively delicate

But the main drawback of most current LED backlight technologies is the narrower spectral range of the light they emit when compared to WCG (Wide Colour Gamut) CCFL backlights. This was a key reason for some manufacturers being slow to drop CCFL backlighting on some of their ‘professional’ models – almost exclusively to create wider colour gamuts necessary for image processing and viewing extended colours gamuts such as Adobe RGB. Despite these potential drawbacks the technology was adopted as ‘standard’ by many manufacturers, primarily for ecological reasons and to suit the majority of the market.

RGB LED – a rare breed

A fairly narrow selection of LED-backlit monitors actually overcame the colour gamut limitation (and then some) by employing ‘triads’ of LEDs (red, green and blue) to create broad-spectrum white light. This rare alternative to WLED (White Light Emitting Diode, the common implementation explored below) was known as RGB-LED backlighting. Some notable models include the XL20, XL24 and XL30 from Samsung, a manufacturer who were amongst the first to broadly introduce LED backlight technologies to both monitors and TVs. Although RGB-LED designs flaunted colour gamuts that even WCG-CCFL backlights couldn’t generally reach the technology never really took off. There were simply too many drawbacks; cost, size, weight, differential degradation of the LEDs (leading to colour imbalances across the screen over time) and relatively poor energy efficiency.

RGB-LED's considerable colour gamut

RGB-LED's considerable colour gamut



WLED – a modern approach

Unlike these RGB triad designs, most modern LED backlight solutions involve placing a border (or in some case clusters) of ‘white’ LEDs behind or at the side of the LCD matrix, often near the edges and using a diffuser to spread the light across the screen. Despite being called ‘white’ LEDs they actually emit a blue light which passes through a yellow phosphor to give a more neutral white and provide the red and green components of the image. Early iterations of the technology (those circa 2009-10) tended to suffer from an obvious and uncorrectable blue bias. As manufacturers became more familiar with the technology and were able to tweak the backlights, phosphor coatings and the LCD panels this tint became more workable. Despite these advances many WLED backlights used in modern monitors still suffer from certain imbalances when it comes to the spectrum of light they produce. The graphic below represents the relative intensity of light at various wavelengths for a ‘typical’ modern WLED backlight.

Typical WLED spectrum

Typical WLED spectrum

You can see a distinct peak of spectral energy in the ‘blue’ region, specifically at 450nm (light considered ‘pure blue’). This comes from the blue diode of the backlight which is typically composed of InGaN (indium gallium nitride). A much weaker spectral response of less than a third the intensity can be observed between 500nm and 700nm, corresponding to the ‘yellow’ light of the typical scintillator phosphor coating; YAG (yttrium aluminium garnet). In combination the InGaN and YAG components of the backlight produce ‘white’ light with a native colour temperature (white point) determined by the ratio of InGaN to YAG.

This light is filtered through the red, green and blue subpixels of the monitor to produce a wide range of colours and allow further refinement of the white point. After filtering a considerable amount of the initial spectral energy of the backlight is lost; the ‘filter’ is far from perfect and the initial spectral imbalance of the backlight is still an underlying issue. Provided the filters are working as intended (i.e. the monitor is properly calibrated) your typical WLED-backlit monitor will be able to make good use of the strong ‘pure blue’ spectral component to produce strong ‘pure blue’ colours. The red and green components (originating from the yellow light of the YAG phosphor coating) are relatively weak. These gaps in spectral energy and relative lack of intensity for wavelengths other than 450nm restrict the colour gamut of a typical LED-backlit monitor to roughly the sRGB colour space.

Typical WLED colour gamut

Typical WLED colour gamut

Looking at the colour reproduction in greater detail you will also find that the ‘pure blue’ component can become overpowering. When you mix this with the relatively diminutive yellow component (greens and reds) there will be some weaknesses evident. This is particularly true for shades that are mostly blue but contain a slight mixture of the other colours; it may seem counter-intuitive but most WLED-backlit monitors are not very good at displaying certain shades of blue!

It is a similar story for many standard-gamut CCFL-backlit monitors when producing green shades. There is typically a spectral peak at green and secondary peaks at blue and red. In this example the peaks at red and blue are 40% the intensity of the green maximum. A crucial point to note, though, is that the relative intensity of these peaks and the distribution of energy for surrounding wavelengths varies considerably depending on the phosphors used.

Typical CCFL spectrum

Typical CCFL spectrum



Improving the phosphors

For CCFL backlights a wide variety of phosphors can be used including ones that produce a wide colour gamut (WCG-CCFL). Although the spectrum above is fairly typical for a standard-gamut CCFL backlight there has classically been more variation here than for WLED backlights. But things are moving forward; there are a growing number of exceptions when it comes to the light emitted by WLED backlights and it seems that some recent developments in LED backlighting technology have started to redefine our expectations of the technology.

Samsung, one of the key modern panel manufacturers, has really embraced WLED backlighting and was the first panel manufacturer to adopt it universally for all new models. They launched a number of ‘Premium’ models in their Series 7, Series 8 and Series 9 screens which broke some of the traditional technological boundaries. These models include the (T/S)(23/27)(A750/950), (S24/27)A850 and the S27B970D. They make use of improved phosphors with enhanced spectral qualities to increase energy in the ‘yellow’ region. This is particularly true for the high-end PLS models which show significant improvement in this area. These enhanced phosphors improve the coverage in the red and green sections of the gamut but also expand the range of blue shades that can be produced. Many models from other manufacturers, including the Dell U2713HM and Apple LED Cinema Display also use enhanced phosphors to enhance the gamut. Many more recent models include these enhanced phosphors, too.

S27B970D colour gamut - reaching beyond sRGB

S27B970D colour gamut - reaching beyond sRGB



Back to the drawing board

Although it can be nice from some perspectives to reach a bit beyond sRGB, as it allows slightly greater vibrancy, you would really want to reach the next ‘standard’ of gamut for colour-critical work and to really unlock vibrancy potential. This is exactly what panel manufacturers are now aiming to do, with LG Display starting to integrate a modified type of WLED backlight called GB-LED (also known as GB-R LED or GB-r LED). Rather than using a blue diode coated in yellow phosphor, the backlights combine blue and green diodes with a red phosphor. As illustrated below, this creates strong and distinct spectral peaks for blue, green and red rather than giving a blue peak and broad ‘yellow’ region. This technology is currently being implemented in AH-IPS (‘Advanced High-performance In-Plane Switching’) panels such as the 24” LM240WU9, 27” LM270WQ3 and 30” LM270WQ6. These are designed to provide 99% Adobe RGB coverage and 104% NTSC coverage which actually exceeds the 98% Adobe RGB and 102% NTSC typical of WCG-CCFLs.

GB-LED spectrum

GB-LED spectrum

There are already some monitors on the horizon that use GB-LED backlights, including the LG 27EA83, Dell U2413 and U2713H. These monitors aren’t cheap, though, partly because the backlight configuration is relative expensive to implement compared to your standard WLED design. There is an ongoing development from a US-based company called Nanosys which could offer a lower cost alternative. The technology is called ‘Quantum Dot Enhacement Film’ (QDEF) and it changes the LED backlight as we know it. Blue diodes are still used, but the phosphor coating and diffuser are replaced with a special film of nanoscopic phosphors called ‘Quantum Dots’ as illustrated below.


QDEF - Quantum Dot Enhancement Film

QDEF - Quantum Dot Enhancement Film

The Quantum Dots are found in their trillions on the film. They can be tuned physically (by alternating their size) to control the wavelengths of light emitted once they are excited by a light source. The blue component is provided in abundance in the light emitted from the diode itself whilst the red and green components can be provided by the specially tuned Quantum Dots. This provides the three distinct spectral peaks at ‘blue’, ‘green’ and ‘red’ that are required for coverage of extended colour spaces. The sort of spectrum produced by this system is quite comparable to the Adobe RGB WLED design. The spectrum yielded is illustrated by the following graph, taken from Jeff Yurek’s (Product Marketing Manager of Nanosys) blog ‘dot color’.

QDEF spectrum

QDEF spectrum

To find out more about where the technology stands from a PC monitor perspective we spoke to Jeff Yurek directly. He told us that the initial goal was to integrate QDEF films into portable displays such as tablet PCs but that he hopes to see good interest gather from manufacturers of larger displays, too.

An important attraction of QDEF is its easy integration into existing LCD designs – the film is thinner than a typical sheet of paper and simply replaces existing components. It is also cost-neutral, in contrast to the expensive multi-diode and enhanced phosphor designs currently being employed by LG Display. The ‘naked’ blue diode does not need a separate phosphor treatment and instead passes light through the film which is of comparable cost to the phosphor and diffuser arrangement. Furthermore the film itself has demonstrated suitable lifetime for use in TVs and monitors with an equivalent operating lifetime of well over 30,000 hours (which is comparable to some of the better LED backlights out there today).

The primary purpose of QDEF technology is to bring extended colour spaces to the user without compromising the form, cost or function of existing LCD designs. The film is currently designed to provide complete Adobe RGB coverage with the potential to extend beyond the NTSC colour space. As with GB-LED, this gives displays the potential to more closely mimic the sorts of colours we can see in the real world and create scenes which are more vivid and realistic.


Making use of those extra colours

In order to accurately output this vivid and colourful content, though, the content itself must be specifically written with extended colour spaces such as Adobe RGB in mind. Currently the only sorts of users who can properly take advantage of this are colour professionals, photographers and designers who can create and process broad gamut content. As extended colour gamut capabilities become more common the sRGB boundary becomes something that is emulated rather than a native technological restriction. It is only natural that once devices become more universally capable of properly supporting extended colour gamuts we would see a shift away from the confines of the sRGB colour space. Designers, film makers and others in the ‘industry’ that we have spoken to are keen to see this as it allows them to better express their creative efforts and bring the consumer the kind of engrossing entertainment experience they crave. Jeff Yurek reiterated this and pointed out that Pixar Animation Studios, for example, use a massive colour palette for their creations but a lot of the shade detail is missed once it’s scaled down and outputted in sRGB.

Adopting a broader colour space isn’t something that will happen overnight and there is certainly the need for the hardware to properly support the sRGB colour space as well. This can be done with some degree of success through emulation modes which are common on wide colour gamut monitors. But there may be some confusion if developers start pumping out content designed to be viewed using broad gamut monitors whilst others are still using a standard gamut – we can’t rely on this being clearly labelled as such. Jeff pointed out that there is software out there that can intelligently map sRGB content onto extended colour spaces. This is designed to improve the vividness of the image without massive and unrealistic oversaturation whilst maintaining appropriate shade variety. Similar technology could also be implemented on the hardware level – which could intelligently detect whether the source material is designed for sRGB viewing and in need of ‘correction’ or already designed for extended spaces and hence left untouched.

Conclusion

When LED backlighting first took off manufacturers were all too keen to promote what were essentially misleading or even fabricated performance benefits. As the technology became adopted quite broadly it became all too clear that the situation wasn’t a ‘win-win’ in favour of the sender ‘white LED’ (WLED) backlight. In some areas, particularly colour gamut coverage, CCFLs could offer significant and clearly visible advantages. LCD panel manufactures have now begun raising the bar in this respect by experimenting with improved phosphors and alternative diode arrangements to enhance colour gamut.

Some interesting developments are also running in parallel with this. Big panel manufacturers including LG Display, Samsung and AU Optronics (AUO) are keenly developing alternative technologies to LCDs such as OLED and QLED. These promise enhanced colour gamuts, amazing contrast and excellent responsiveness. But for use in desktop monitors there are still some significant technical and economic challenges to overcome. Such monitors are some way off being commercially viable in the consumer sector.

Quantum Dot LEDs (QLED)

Quantum Dot LEDs (QLED)

Another interesting technology which is much closer to becoming a reality involves the use of light-emitting Quantum Dots in existing LCD designs; Quantum Dot Enhacement Film (QDEF). This provides superior colour performance to current basic LED backlights. Unlike advanced diode and phosphor arrangements the QDEF film works in place of the phosphor coatings on simple blue diodes and can be implemented by manufacturers without additional material cost. As with LG’s enhanced diode and phosphor arrangements (GB-LED) and alternative technologies such as OLED and QLED, this is designed to push the colour gamut far beyond the restrictive sRGB standard.

We will see more and more monitors comfortably edge past the cramped sRGB colour space and properly display alternative standards such as Adobe RGB without resorting to overly bulky or power-hungry technologies. This will give content creators the opportunity to really give scenes the look they intend with truly vivid, spectacular and true-to-life colours. That’s a very exciting prospect for game developers, film producers, artists and designers – and of course the consumer at the other end.



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