Making light work of illumination

LEDs are stealing a march on conventional lighting. Their benefits, in terms of size, cost and energy savings, make them attractive for all manner of mainstream uses. Jon Excell reports.

Thomas Edison’s incandescent light bulb, for decades the universally accepted symbol for innovation, is under threat from a technology that is more efficient, lasts longer and will soon be cheaper to produce: the LED.

Unlike incandescent and fluorescent lamps, solid-state lighting (light-emitting diodes or LEDs) and OLEDS (organic light-emitting diodes) creates light without producing heat. A semiconducting material converts electricity directly into light, which makes the light very energy efficient.

Long used as simple indicator lamps on digital watches, toys and clocks, LED technology has improved recently to such an extent that it is beginning to make a big impression in the wider lighting industry. The tiny lights have, for example, been appearing in car headlights and traffic lights. The reason is that LEDs have become both brighter and more efficient, and as the cost of semiconductor materials has plummeted they have also become cheaper.

The technology’s rapid development is following Haitz’s Law. Rather like the LED equivalent of Moore’s Law, this is a rule of thumb based on observations by Roland Haitz (a former head of R&D at Agilent Technologies) that continuous improvement in LED efficiency has resulted in light output per package doubling every 18 to 24 months. So whereas in the early 1970s small-instrument LEDs consumed about 100MW and generated a light output of around 0.2 lumens, today’s high-brightness white-LED chips consume 5W and generate 150 lumens.

These figures compare favourably with a 20W halogen lamp (which generates 400 lumens) and a 100W tungsten lamp (which generates 1,000 lumens). If Haitz’s Law continues to hold, and there are few signs that it won’t, experts believe that a 5W LED should be able to replace a 20W halogen lamp within three years and a 100W tungsten lamp in less than five years.

Given these energy savings, solid-state lighting offers potentially enormous environmental benefits over conventional lighting. In the US studies carried out as part of the state-sponsored Next-Generation Lighting Initiative have shown that by replacing all traditional lighting with solid-state lighting the entire country could enjoy a 50 per cent reduction in electricity used for lighting, a 10 per cent reduction in total electricity consumption, and a reduction in carbon emissions of nearly 30 million tons per year.

While the first infrared LED was introduced in 1961, the technology was never considered a serious contender as a source of illumination until the invention of the blue LED in the mid-1990s. By combining red, green and blue sources, it became possible to generate white light for the first time.

This discovery prompted a number of companies to become seriously involved in the development of solid-state lighting. Perhaps most notably, in 1998 Philips Lighting joined forces with LED manufacturer Agilent Technologies to form Lumileds, which, with its Luxeon light sources, has since become a world leader in the development of white and bright LEDs.

The most direct way of generating white light from LED sources is to combine the light from separate red, green and blue LEDs to produce white light. Although this method is efficient and yields a tuneable colour palette (ideal, for example, for mood lighting), balancing the light output of the three LEDs requires complex drive and control circuitry. Variable ageing effects of the three LED colours can also lead to a drift in colour point over time.

An alternative approach, and the one adopted by Lumileds, is to integrate a yellow phosphor into the coating on top of a blue LED chip. White light is then produced by additive mixing of yellow light emitted by the phosphor with blue light from the LED. ‘Thanks to extensive research towards a conformal phosphor coating process, a small source size and excellent colour uniformity over the viewing angle is obtained compared to other deposition methods,’ said Dr. Dietrich Bertram, LED expert at Philips Research.

However, as the light produced is lacking in red wavelengths (which means that illuminated red objects lose some of their colour), current research at Philips and Lumileds is aimed at developing new red and green phosphors suitable for use with the blue LED chips. This is expected to allow the production of white LEDs using true red, green and blue additive mixing to give a wider range of colour temperatures and excellent colour rendering.

Solid-state lighting is already penetrating the market for neon signs and the flexibility it offers is being taken up in outdoor advertising and entertainment. About 30 per cent of the traffic lights in the US and a high percentage in countries like the Netherlands are LEDs. These are considered to be far more reliable and less prone to failure or damage than tungsten lamps. In the automotive sector interior and braking lights using LEDs are also being developed. This is partly because of their design flexibility but also for safety reasons – they light up faster, an important consideration for braking lights.

A number of European countries and most US cities are now replacing their incandescent traffic lights with LED units. The current generation of LED traffic signal modules use around 10 per cent of the energy used by incandescent lamps. An incandescent lamp uses 155W of power while an LED module uses an average of 10W to light the same unit. Additionally, LED modules have an expected life of five years or more, and are more reliable.

California was one of the first places to wake up to the benefits of solid-state lighting and the California Department of Transportation (Caltrans) has converted over 90 per cent of the 4,500 state-owned traffic signals.

According to Steve Prey from the Caltrans Energy Conservation programme, the estimated annual cost of energy spent over one year on incandescent traffic signals throughout the state is about $11.2m (£6.7m) – and the introduction of LED-based traffic signals has shaved almost $10m (£6m) off that annual bill. He added that the energy savings with LED signals are 9.3MW in grid load demand and 81.6 million kWh of energy.

It is not just in replacing existing lighting that LEDs are expected to make an impact, however. The ease with which they can interface with computers means that lighting arrays can be programmed to do things that could never be done before. It’s now possible, for example, to continuously vary the colour and intensity of inorganic LEDs to create different ambiences reflecting the mood of a piece of music or movie. On the range of applications open to LEDs Bertram said, ‘Solid-state lighting is set to create a revolution in the lighting industry by providing novel solutions for traditional lighting applications and exciting new applications for lighting that we can barely imagine at present.’

However, before we get too carried away, Edison’s light bulb hasn’t quite lost the battle. White LEDs, although coming down in cost, are still many times more expensive than incandescent light bulbs. Despite the manifold advantages of solid-state lights, they’re unlikely to attain the ubiquitous status of the tungsten bulb until they become cheaper.