Europa Science Network: Europa Science | Scientific Computing World | Electro Optics | Research Information

LED ILLUMINATION

LED there be light

Nick Morris predicts a bright future for LEDs

Talk to anyone in the industry at the moment about the latest advances in illumination and there is one topic that comes up again and again; the high brightness LED. The recurring message at the recent EuroLED conference, hosted by the UK Photonics Cluster, was that high-brightness LEDs offer many potential advantages over traditional incandescent bulbs and fluorescent tubes: LEDs are highly reliable, with a long service life; they consume relatively little energy; lower operating voltage means they are safer to handle, a particular concern in outside applications where moisture has to be taken into account; and they project less heat into the illuminated field, another important consideration.

A field where LEDs show great promise is in architectural applications. The highly directional nature of LED illumination, alongside the saturated colour output they offer, provides architects with lighting options, both internal and external, that simply aren't possible using conventional lighting. This also makes LEDs particularly suitable for so-called task lighting - highlighting particular architectural or decorative features, or providing local spot lighting.

The Yunxi Tower in Yixing, China.

One particular example of LEDs being used to great effect is in the Yunxi Tower in Yixing in the Zhejiang Province of China, 150km south east of Shanghai. The tower forms the centrepiece of a garden, designed in the style of an ancient Chinese garden. The tower itself is designed in the Tang architectural style, and is set on an island in a lake. The garden forms part of a large project known as 'New Century, New Yixing' and has become a key tourism site, attracting visitors from China and beyond.

When designing the tower, the architects incorporated modern building techniques to strengthen the traditional Chinese structure. As part of this they wanted to use the latest lighting technology to maximise the impact of the tower, particularly by highlighting the intricate wooden structure of the building - especially at night, both when seen directly and via its reflection in the surrounding lake.

The architects, the Suzhou Park and Garden Architecture Institution, approached LED specialist Primolite, based in Shanghai, with the task of developing a lighting system for the tower. Primolite worked with the Quanhui Lighting Company to develop the system, which is based on Luxeon LEDs from Philips Lumileds.

Different luminaires were used throughout the tower to highlight various aspects of the building, such as the pagoda on top of the tower, the columns between the floors, and other features, such as the layered profile of the roof. The small size of the LEDs allowed Primolite to create luminaires that were compact enough to be almost unseen by visitors to the tower, but which were bright enough to highlight the decorative details of the tower. In total the LED lighting system consumes 8kW, significantly less than an alternative halogen or incandescent system which, it is estimated, would have consumed 21kW. At night the tower forms a lantern that can be seen from up to 3km away.

LEDs are fast becoming a viable alternative to halogen and high intensity discharge lamps for automotive designers. According to Dr Karsten Eichhorn of automotive lighting firm Hella, speaking at EuroLED: 'LEDs are now an established technology for rear lights, brake lights and indicators, but are not yet ready to be used for the main headlights of cars.' However, some new models, such as Audi's A8 luxury saloon, are using LED arrays for daytime running lights, which can improve safety. Eichhorn added: 'Main beam headlights using LEDs are currently at the prototype stage of development; we're still waiting for the available flux to increase before LEDs finally replace high intensity discharge lamps. However, we expect that by 2008 you can expect to see cars using LEDs for all lighting.'

The Scion FUSE 2006 concept car uses LEDs for all its major lighting systems.

Scion - a North American division of Toyota - unveiled a concept car featuring LED technology at the New York International Auto Show 2006. The Scion FUSE 2006 was designed to highlight possible future design directions and new technologies for the company. Visopia, a lighting design company based in Los Angeles, designed the lighting system for the sports coupe using PerkinElmer's ACULEDs (All Colour Ultrabright LEDs). LEDs were used for all the major lighting systems on the concept car, including high and low beam headlights, and fog lights. A distinctive feature of the design is that the red-green-blue balance of the LEDs can be varied, meaning the driver can personalise the car by changing the actual emitted colour of all the lights - where local laws allow.

With the ever-increasing cost of fuel and electricity, designers must take account of energy consumption in any and all lighting applications. One of the most common forms of illumination that can be seen anywhere and everywhere in the developed world is street lighting - the omnipresent orange glow of sodium-discharge lamps can be found on any city street or major road at night. Any saving that can be made in energy use and cost per unit, however small, could have a major impact on overall energy use if implemented on a large scale.

Carl Clarke of Advanced LEDs believes it is only a matter of time before we see LED arrays replacing high and low-pressure sodium lamps on our streets: 'Currently, lamps need to be changed every three or four years. Each visit to replace a lamp entails costs both in terms of the engineer's time and possible disruption of traffic. The long life and robust nature of LED arrays means that they will represent a real cost saving when installed.'

Advanced LEDs is also developing solar powered lighting arrays, which offer even greater environmental benefits. So far, such systems from Advanced LEDs are being used for security and amenity lighting, and will very soon be added to its LED street lamps. Clarke pointed out: 'Every square metre of photovoltaic panels used has the capacity, over its lifetime, to reduce carbon dioxide emissions by two tonnes.'

The LED Lantern from LEDtronics stores solar energy to give up to 25 hours of useful light.

LEDtronics, based in California, also develops solar powered lighting systems using LEDs. For example, it has just released a new Bright White LED Compact Solar-Powered Lantern, which uses stored energy gathered using photovoltaic cells to power an array of 24 LEDs. In bright sunlight the batteries in the lantern can be recharged in 12 hours, giving a maximum of 25 hours of usage time. The lantern is particularly useful for applications where a gas or petrol-burning lantern would usually be used, such as for camping or for emergency home lighting. However, the LED lantern is much safer - it does not give off as much heat as a fuel burning light, and all the safety risks associated with storing the flammable fuel are removed.

The air transport industry is regularly criticised for the part it plays in global contributions to energy use and carbon dioxide emissions. As the number of flights continues to rise, alongside the building of bigger airports to cope with these flights, any savings that can be made in overall energy consumption must be welcomed. One area where LEDs could be used to make such savings is in airfield lighting. Airfield ground lighting is designed to help pilots take off, land and taxi safely round the airfield in all weather conditions. A typical airport might have 800 lights marking out the runway, and 1,000 lights for the various taxiways. A large hub with multiple runways, such as London's Heathrow, might have many times this number. Such a large number of lamps consume a huge amount of power, so any increase that can be made in power efficiency will have real benefits in terms of overall running costs of the airport.

After a series of tests to compare the operation of LED lamps against tungsten filament lamps in various weather conditions, such as fog, the Civil Aviation Authority has licensed the use of LED lamps for some applications in aerodromes in the UK.

LEDs can be used to make very large display screens, with thousands of pixels.

Many airport terminals now contain large video display screens, which show passenger information, video advertising and entertainments. LEDs are useful for such applications, as they have specific and narrow spectral emission characteristics, and can be turned on and off very quickly. LEDs offer much more saturated colours than other display methods, and very high contrast. In such displays each pixel usually consists of three different monochromatic LEDs, emitting red, green and blue light. The light from each must be combined in order to give a full palette of colours. The LED ColorDichroics system from Unaxis Optics uses a series of filters to reflect and transmit the light from different LED sources, combining the different colours into one single light beam. Hence the light from the LEDs can be combined to give a display screen with the full spectrum of colours available.

However, large screens, which can contain many thousands of LEDs, do have problems - the intensity of each individual LED in each pixel can vary considerably, meaning the display doesn't look uniform when seen from a distance. Buying highly specified LEDs from manufacturers can be prohibitively expensive. However, Opsira offers a tool that allows the intensity of each LED in a display to be tested and calibrated individually, meaning a uniform brightness can be sought without having to purchase expensive components. The Luca system uses a camera to scan up to 720 pixels at once. The data gathered is sent via FireWire to the controlling system, and from there to the computer controlling the display, which can then adjust each pixel until all the units are giving off the same light intensity.

Because the output of an LED is typically highly directional - the intensity can change significantly with viewing angle - it is very important for a display designer to know the angular characteristics of the LEDs being used. Because the angular characteristics of the LED are set during the design and production of the unit, the testing must be carried out at this stage. The IS-LI system from Radiant Imaging uses an imaging sphere, developed with Philips, to measure far field luminous intensity, radiant intensity, CIE chromaticity coordinates and correlated colour temperature, all as a function of angle, for LEDs. With this information to hand the designer of an LED display can choose the LEDs most suitable for the display.

High-brightness LEDs look set to saturate the marketplace over the coming years - time will tell what market share they capture. If, as predicted, they reach and surpass the efficacy of traditional incandescent bulbs and lamps they will offer significant cost savings.