A solid future for lighting
Over the next decade, incandescent bulbs and fluorescent tubes are likely to give way to illumination based on solid-state semiconductors
IT SOUNDS like the start of a bad joke to ask how many companies it takes to change a light bulb. But the truth is that plenty are trying. The lighting industry thinks that the light bulb is dead and that the days of the fluorescent tube are numbered. It predicts that the future will be illuminated by light-emitting diodes (LEDs).
Although they are expensive to manufacture, LEDs are more efficient than bulbs at converting electricity into light. If existing trends continue they will soon be more efficient than tubes. And manufacturing costs are coming down at the same time as efficiency rises (see chart).
The diodes are better than existing lighting in other ways, too. They are durable, glowing brightly for tens of thousands of hours; and they are robust, resisting shock and not flickering. They have neither glass parts nor delicate filaments. Coloured ones are already used in traffic lights across America and, in a few years, white LEDs should be brightening homes and offices. They also offer novel possibilities. Coupled to fibre-optic cable, they could generate light centrally and route it through buildings. Lamps might work not with a dimmer switch, but with a rainbow dial that turned them from white to red, green, blue or any shade in between. Wallpaper that glows and twinkling table-tops also stray from the realm of the outlandish to become feasible notions.
An LED consists, in the simplest case, of two layers of semiconductor sandwiched together. One layer is “n-type”, meaning that it has an excess of negatively charged electrons. The other is “p-type” and is populated by “holes” left behind after electrons have been removed from the crystal structure. These holes have, in effect, a positive charge. When a current is applied across the sandwich, electrons and holes are pushed towards the junction between the n- and p-type materials. There, they meet and annihilate one another, releasing energy in the form of light. The colour of this light depends on the material that has been used to build the LED.
Although it has been known for decades that many semiconductors emit red or green light, it has proved hard to coax one into producing higher-energy blue light. Success came in 1995 when Shuji Nakamura, working for Nichia, a Japanese company, managed to build a blue LED from gallium nitride (work for which he recently won an innovation award from The Economist). This mattered because it takes all three primary colours to make white light—and if you want to replace bulbs and tubes, white is what you need.
In the end, it didn't quite happen that way. Attempts to build white-light emitters out of three coloured LEDs failed because the brightness of each component changed with temperature and age, so the colour of the composite light drifted off white. However, blue LEDs turned out to be the key to white light after all. The way to extract white light from a blue LED is to coat the top of the device with a layer of phosphor powder. The phosphor—a compound called yttrium-aluminium garnet—absorbs part of the blue light and is thereby “pumped” into an excited state. When it relaxes, the energy it has absorbed is re-emitted as yellow light. The combination of blue and yellow produces a rough approximation of white light.
Such quick and dirty white light is now available in the market. Lumileds, set up by Agilent Technologies and Philips Lighting, and based in San Jose, California, is one of the leading manufacturers of white LEDs. A 5W version (the most powerful so far) of the firm's Luxeon Star will be available this month. It produces the same amount of light as a 10W incandescent bulb. By putting these devices together in groups of 12, Lumileds has created a product that can stand in for a household bulb. Nichia, GELcore (a collaboration between General Electric and Emcore, a semiconductor company based in Somerset, New Jersey) and Osram also sell “white” LEDs that work this way.
All of these companies are now busy working on the next generation of white-light emitters—ones whose output will be truly white. One way to achieve this is to employ an LED that emits ultraviolet light (UV, which is even more energetic than blue). The advantage of this approach is that many phosphors absorb UV efficiently. The recipes have been improved by years of development for fluorescent tubes, in which the UV is supplied by an excited gas rather than a semiconductor. By altering the balance of phosphors that emit at red, green and blue frequencies, the colour of the light can be controlled to account for vagaries of taste. People are, it seems, picky about the colour that they read by, preferring a red-tinted “warm” white to the blue-shifted “cool” spectrum of sunlight.
The best solution would be the one originally envisaged—to combine red, green and blue light generated directly by LEDs. That would waste less energy than “downgrading” UV into visible colours with phosphors. It would, however, mean building a feedback system that can cope with the irregularities of the LEDs in response to heat and age.
Color Kinetics, a Boston-based company, has used the multi-LED approach to build digitally controlled lamps that produce a light show with hundreds of colours. B/E Aerospace, a firm that designs interiors for airliners, plans to install such lamps in its products. According to Color Kinetics' website, the lamps will provide “a sunset ambience to encourage relaxation and sleep; a sunrise programme to awaken and re-energise passengers after an extended flight; and custom effects featuring an airline's signature colours to enhance its brand.” You have been warned.
The really big money will come when LEDs can compete on price with everyday incandescent and fluorescent lighting. That is a market worth $12 billion around the world, according to Robert Steele, an analyst with Strategies Unlimited in Mountain View, California.
A light is judged by the number of lumens it throws (a lumen is a measure of brightness that accounts for the sensitivity of the human eye). The number of lumens per watt (lm/W) measures a light's efficiency, while the number of dollars per lumen measures its cost. On both counts, solid-state lighting has some way to go.
The best white LEDs on the market emit 25 lm/W, which is almost twice as efficient as an equivalent tungsten-filament light bulb, but barely a third as good as a fluorescent tube. To become competitive, the devices need to reach 80 lm/W. To rule the world, 150 lm/W is probably required. If progress continues at the rate of the past 30 years, this will be reached by 2010. That is a big if, but the efficiency of blue and UVLEDs should improve dramatically as better ways are found to build them.
Just in case progress should be slower, governments are sticking their noses in. Japan has created a “Light for the 21st Century” project that will run until 2004. America is also considering applying a little industrial policy: $480m will be made available over the coming decade if the “next generation lighting initiative” survives in the energy bill now being debated in Congress. With luck, though, the technology will rise above such interference. Light bulbs could then be consigned to the dustbin for ever.
This article appeared in the Science & technology section of the print edition under the headline "A solid future for lighting"
From the October 5th 2002 edition
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