FEATURE
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LEDs to cool off?

William Payne finds that LEDs are leading the way in some applications, but are already being replaced in others

Just at the moment, the coolest, sexiest laptop on the market is the Apple MacBook Air. Its key selling points include just how thin and light it is, the clarity of its display, and its long battery operation. All are the result of Apple’s choice of LEDs for backlighting the display instead of more conventional, bulkier and power-hungry CCFL technologies.

The Air epitomises the increasing status of LEDs as a technology for cutting edge, energyefficient and eco-friendly illumination, whether in displays, machine vision, advertising or industrial applications.

According to industry analyst Strategies Unlimited, the LED market was worth around $4.5bn last year. Growth is increasing, hitting 9.5 per cent, up from 6 per cent for the two previous years.

By 2012, the company reckons the market will be worth $11.4bn. Fastest growth will come in the lighting and illumination sectors, although mobile and appliance displays will constitute the largest single segment. Other major growth areas include automotive, fixed monitors and TVs, and advertising billboards.

For manufacturers, the attractions of LEDs include greater energy efficiency, better environmental characteristics – including the absence of heavy metals – and lower weight and size, allowing manufacturers to create ever smaller devices – like Apple’s Air. Continued market LED growth is being driven by higher power throughputs – which are key factors in greater uptake in automotive, illumination and machine vision applications – improved cost, superior contrast, and wider colour spreads.

Dispersing heat more effectively is a key factor in improving performance. Brighter LEDs require more power. Unlike conventional lighting, LEDs do not radiate outwards; as they are diodes, they accumulate heat at the base. This increases the potential for circuit failure. LED packages are now focused on more efficient heat dispersion, including slug designs, thermal vias, heat sinks, pipes and cooling fans.

7 x 7cm2 two colour fluorescent white PIN OLED produced at Novaled on IPMS ITO Al grid substrate.

Vendors are improving LED luminosity not just by ramping up their power output, but reducing light loss within the LED. Reflective layers coupled with opaque substrates are being designed into new LEDs to replace crystalline substrates, cutting light loss through diffusion.

One factor that is likely to push further growth in the LED market is falling prices. Overcapacity and stiff competition among suppliers is leading to falling prices. The mobile phone industry has been especially active in pushing prices down, especially for low-end blue, red and green LEDs. Blue LEDs, used as backlights in mobile phones, have fallen by around 70 per cent in four years. White LEDs, by contrast, have maintained price levels.

But it’s not all growth. Some of the current major growth areas for LEDs – such as signs and displays – could be vulnerable to other, cheaper, technologies, despite the rapid growth being marked up by LEDs at present. LEDs owe at least some of their growth to their potential as a disruptive technology – they are able to perform a similar function to existing applications, but at a lower cost or greater efficiency. In some instances, they are displacing lasers, in others, conventional lighting.

However, in areas such as signs and displays, other technologies, such as digital ink, could prove a cheaper, more effective option than LEDs. Video displays using LEDs have made large inroads in advertising. These are state of the art, and offer major advantages, such as the ability to provide high quality moving images and sharp, bright colours, providing 24-hour displays.

Despite these touted advantages of LEDs, two of Europe’s biggest operators of outdoor advertising have opted for digital ink technology rather than LEDs for high profile electronic billboard programmes. Clear Channel Outdoor has used digital ink for a series of 10 large outdoor displays throughout London. JCDecaux has also selected digital ink technology for a series of high profile billboards throughout Europe.

Other European operators are opting for LED displays, and some, such as Clear Channel, are using both digital ink and LED technologies. The reason for this apparent confusion is the impact of regulation and environmental requirements. European countries such as the UK tend to limit the size of billboards that can be displayed. They also ban the use of video on roadside sites, as these could prove a dangerous distraction.

Lighting tile with 35cm2 active area using a white copolymer.

All this gives digital ink an advantage over LEDs. Digital ink works effectively in smaller displays, and is both more energy efficient and cost effective. It also works better with outdoor displays during daylight hours, since direct sunlight enhances rather than blurs the screen. Its inability to provide video is not an issue, as regulations ban this.

In sites where larger displays are allowed, and where video is permitted, LEDs have established an advantage over digital ink. The US typically allows much larger roadside billboard displays than European countries. Likewise, in European rail terminals, much larger billboards are allowed, and video is permitted. In both these settings, LEDs are the dominant display technology.

One application where LEDs have distinct advantages – and are enjoying increasing growth – is machine vision. These advantages include monochromatic wavelength, long life, low power, fast strobing and instantaneous switching. Ultraviolet LEDs, in particular, are used in machine vision applications for high contrast inspection of fluorescing properties, such as sealed compounds or greased fittings. UV LEDs are also used in machine vision applications in cosmetics and pharmaceutical manufacturing.

Inspecting pharmaceuticals as they are manufactured and packaged is a highly demanding process, requiring a large number of checks and separate inspections. In some cases, mistakes can lead to potentially disastrous consequences for patients. Unsatisfactory inspection regimes or insufficient batch record systems can also result in drugs licensing regulators such as Europe’s EMEA and the US FDA suspending manufacturers’ plant licenses, effectively closing down a multi-billion dollar factory. The stakes for accurate inspection of pharmaceuticals couldn’t be higher.

There is a downside to tough inspection regimes, however. Since the process of pharmaceutical inspection is so complex and rigorous, inspecting as many as 500 separate drugs packets a second is highly demanding. Lot and date codes have to be checked. Overfilling or empty slots have to be detected. Cross contamination of different drugs, and any broken capsules also have to be inspected for. Finally, the print quality of labelling has to be inspected.

Faced with these daunting requirements, many automated inspection systems working on high speed production lines often produce a high number of false rejects. Perfectly good, expensively manufactured drug packages are often discarded as a result, increasing overall costs.

German pharmaceutical inspection specialist Scanware Electric has designed a high speed inspection system that uses a complex array of LEDs to perform the necessary inspection steps with greater functionality.

As a result, accuracy of the inspection process has been increased, and the number of false positive rejections cut.

Speed – and clarity of image capture – are the keys to Scanware’s drugs inspection system. The company has developed its own bespoke LED panels comprising up to 3,000 LEDs. The ability of the LEDs to switch instantaneously between different wavelengths is crucial, providing a sequence of clear images at different light levels in rapid succession. Embedded systems are able to process the LED-produced high contrast images instantly, allowing accurate complex, multi-step inspection on high-throughput production lines.

A machine vision application that is likely to win less public acclaim than drugs manufacture is traffic monitoring. Infrared LED arrays have begun to replace halogen lamps in some roadside speed monitors. Nestor Traffic Systems is one company that has adopted Siemens’ Nerlite LED arrays for traffic monitoring.

 

A warm white coloured OLED research sample measuring 35 x 35mm2.

LEDs have a number of advantages over conventional halogen in traffic control systems. They feature lower energy consumption, while having a lifetime average of 50,000 hours, compared to a typical 300 hours for a halogen bulb. Moreover, LEDs have fast switching and provide better resolution and recognition of car number plates. LED flash is also less blinding for car drivers than halogen units, allowing for safer forward image capture of vehicles.

A development that could boost luminosity substantially, not only without requiring greater power but actually increasing LED energy efficiency, has been developed by a student at Rensselaer Polytechnic in New York. Martin Schubert’s innovation could also lead to even thinner and lighter LED displays, and cut the amount of toxic metals present in mobile devices.

Schubert observed that normal LEDs produce polarised light – from the side. Previous readings had been done from the top, and had not observed this characteristic. Schubert discovered that ordinary LEDs produce polarised light at a ratio of 7:1 from the side. He then created a filter that made that polarised light usable.

The development could have a big impact on displays, making them thinner, more energy efficient and environmentally friendly. At present, LEDs used in displays produce non-polarised light. This light is then passed through a sandwich of filters to polarise it. This increases power consumption, as the LED light needs to be sufficiently bright to penetrate the polarising filters.

Schubert’s innovation means that the polarising filter layers can be dispensed with. This will not only mean even thinner and lighter displays than at present, but a sizeable cut of around 50 per cent in energy consumption. Polarised LEDs could provide a cheaper, more efficient alternative to conventional fluorescent backlighting, replacing a major source of heavy metal environmental pollution. Organic LEDs promise even greater luminosity and power efficiency, combined with a raft of new applications. OLEDs couple LED technology with plastic or flexible substrates, opening up new opportunities in labelling, large panel displays, and lighting.

A European project of 24 universities and manufacturers, such as Philips and Osram, is aiming to improve the commercialisation of OLED technologies. The OLLA consortium has already achieved a white OLED of 25 lm/W with a 5,000 hour lifetime. Green OLEDs have been demonstrated that can achieve 120 lm/W efficiency, far outperforming what can be achieved with fluorescent tubes.

OLLA has set itself the objective of demonstrating an industrially viable white light OLED light tile of 15 x 15cm or larger by this year. The target application is general lighting, and the aim is to produce the first commercial flat light source with high energy efficiency.

A rather different ‘organic’ approach to improving LED performance and efficiency is being taken by Cincinnati University researcher Andrew Steckl. He is aiming to intensify the luminescence and efficiency of LEDs by adding organic materials – specifically salmon sperm. Professor Steckl is working with the US Air Force in developing these new devices, which he has labelled BioLEDs.

Steckl is adding salmon sperm to LEDs because it produces much brighter devices with broader colour ranges. DNA from the sperm creates an electron trap that generates an increased number of photons. As DNA is far larger, and more complex, than most molecules, it serves as an efficient electron barrier, slowing down electrons, and increasing photon output by as much as 100 per cent. The salmon DNA is turned into thin film layers – providing an organic alternative to silicon layers.

Any source of DNA would serve the same purpose. Steckl opted for salmon sperm because his Air Force collaborators have access to large amounts of it. Salmon sperm is a waste product of the fishing industry, where it is thrown away. But plant or other animal DNA would be equally applicable. BioLED materials, such as salmon sperm, are cheaper and more environmentally friendly than their conventional counterparts. They also provide higher performance and greater efficiency. Steckl believes that exploring the properties of organic materials will reveal manyfar more efficient substances that can be utilised in industrial processes – effectively exploiting the four billion year development process that is evolution.

Steckl’s ultimate aim is to develop BioLEDs that are based in their entirety on organically derived materials. They will be renewable, biodegradable, far cheaper, and – he believes – possess greater performance and efficiency.

From trendy laptops to fish sperm, today’s LEDs cover a wider range of applications and technologies than ever. But their potential for improving energy efficiency, device mobility, and biodegradability mark them out as a key technology of tomorrow.