Scientists have produced materials which have a photonic band gap, namely a range of wavelengths of light which are unable to pass through the material. This is achieved by structuring the materials in precisely designed patterns at the nanoscopic scale. The result has been called 'semiconductors of light'.
Light emitted from traditional forms of illumination is effectively a by-product of heat. The incandescent light bulb operates at 2000°C so clearly much of the energy it uses is wasted.
There is an alternative which leans heavily on quantum theory, which states that an atom's electrons emit energy whenever they jump from a high to a low energy level. Provided that the difference between the two levels is in the right range, the surplus energy is manifest as a flash of light - a photon. The wavelength and thus the colour of the light is determined by the size of the energy gap and this, in turn, depends on the atoms involved.
The principle behind the semiconductor light-emitting diode is that quantum transitions can take place within a solid. In an appropriate material some electrons are free to move whilst others are bound to the atoms. The difference between the energy state of these two types of electron is termed the 'band gap'. The application of a small electric current to a diode made from a semiconductor raises electrons to a higher energy state. As the electrons pass through the band gap they generate photons at a wavelength determined by the size of the gap. An appropriate semiconductor produces photons in the visible spectrum to create the LED.
Up to the present it has not been possible to make an LED that emits pure white light. To get close to this goal a combination of LEDs emitting red, green and blue light mixes these hues to produce a warm white light. The blue light has been a stumbling block but recently researchers in Japan made a chip using the semiconductor gallium nitride which emits blue light. This completed the triad of hues.
One of the criteria for judging the quality of artificial light is the degree to which it approximates to sunlight. This is measured by a colour rendering index (CRI) in which 100 represents absolute affinity. Incandescent bulbs have a score of 95 against the best LEDs, which achieve 85. However, it is at the level of efficiency that LEDs leave the rest of the field behind. Incandescent bulbs produce 10-20 lumens per watt whilst LEDs emit 100 lumens per watt. Furthermore, they have a life expectancy of about 50,000 hours. Their durability is measured in years, perhaps even decades.
However, there is a problem with the whitish light produced by mixing blue, red and green LEDs in that the human eye is not as sensitive to light at the blue and red ends of the visible spectrum as it is to the green. Consequently much of the power used to generate blue and red light is wasted. This problem has been addressed by Frederick Schubert of the Centre for Photonics Research at the University of Boston. In January 2000 it was reported that he had produced an LED which took into account the sensitivities of the human eye. The result was an LED emitting white light at the highest possible efficiency.
His answer was a device called a 'photon recycling semiconductor' (PRS-LED), which uses electrical power to generate photons at a single wavelength. Some of the photons are recycled to produce light at a different wavelength. The two wavelengths are calculated to produce the effect of white light. Schubert's device uses blue and orange to achieve this effect. The blue light is produced by an LED made of gallium, nitrogen and indium. The second semiconductor layer consists of gallium, indium and phosphorus. The band gap of this second layer material has been adjusted to produce orange light. The claimed efficiency of this device is 330 lumens per watt (see Fig. 14.1).
This LED is still in the laboratory and the research effort is focused on improving its CRI and improving its efficiency so that a PRS-LED measuring less than a square centimetre will emit as much light as a 60 watt bulb whilst using only three watts of power. This highlights another virtue of this technology - its compactness. The absence of a glass bulb and bulky connections makes this a truly versatile form of illumination capable of being integrated into walls, ceilings and even floors. There are also moves to create LEDs which respond to changes in daylight level by adjusting their colour and brightness.
Secondary source absorbs some blue light and emits -orange light
Primary source emits blue light
Orange light sä
Aluminium, gallium, indium & phosphorus
Sapphire substrate Gallium, nitrogen, indium Contact
Figure 14.1 The Schubert Photon Recycling Semiconductor (PRS-LED) (derived from New Scientist article 'The end of light as we know it', 8 January 2000)
Even more radical is the suggestion by Ton Begemann of Eindhoven University that an LED lighting system might also be a carrier of information. He claims it would be possible to modulate the power supply to the LED in such a way as to enable it to carry digital information but at speeds too fast for the eye to detect. A pager, sensor or computer would be able to decode the message, making the light on the desk a node in a communication system via the electricity supply network.
The greatest benefit of all would be in the saving of energy. In most commercial buildings lighting is the main consumer of electricity. Colin Humphreys, a materials scientist at Cambridge University, estimates that LEDs would cut lighting bills by 80%. The reduction in carbon dioxide emissions due to lighting would also approach that percentage. Transposed to the scale of a nation, if all the light sources in the USA were converted to LEDs, this would cancel out the need for new power stations for 20 years, assuming the present rate of increase in electricity consumption of 2.7% per year.
As yet LEDs are not cost-effective set against conventional light sources. A Schubert 60 watt equivalent LED would cost $100. However, this is a technology which is bound to succeed, especially if the avoided cost of pollution and carbon dioxide emissions is factored into the cost-benefit analysis. Already 8% of US traffic signals use red LEDs. Developments in the technology coupled with economies of scale should enable LEDs to swamp the market within a decade. This will qualify them as one of the leading technologies in the fight to combat global warming.
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