快猫短视频

DON鈥橳 you just hate it when you flick on the light switch only to hear that
annoying 鈥減link鈥�? Time to hunt down a new bulb, clamber up on a rickety
stepladder and risk burnt fingers, a painful fall or even
electrocution鈥攋ust to get some light. Accidents caused by light bulbs put
more than 3500 people in hospital in Britain in 1997, according to the Royal
Society for the Prevention of Accidents.

The good news is that a foolproof replacement is on the way. Slowly but
surely, small semiconductor chips called light-emitting diodes (LEDs) are taking
over from the light bulb. These devices can give out red, green or blue
wavelengths, which together give the impression of the warm white light thrown
out by light bulbs. LEDs use less energy than light bulbs, and the latest
developments promise even more efficient white light LEDs. Best of all, these
devices will run quite happily for a decade or more without flickering or dying.
Equip your house with LEDs and you may never change a light again.

Traditional forms of lighting, from the electric light bulb all the way back
to Paleolithic lamps that burned animal fat, glow because they are hot. Every
hot body radiates heat and light according to the law of radiation derived by
the German physicist Max Planck in 1900. The problem is that only a fraction of
the energy emitted by a hot body is visible light. In a modern incandescent
bulb, with a tungsten filament operating at more than 2000 掳C, much of the
electrical power is wasted in producing radiation we cannot see but which we
feel as heat. This is a serious loss at a time when the threat of global warming
is making energy conservation a hot topic, and when demand for electricity in
the US over the past decade has increased at 2.7 per cent annually.

Luckily, there is a better way to get light. Quantum theory tells us that an
atom鈥檚 electrons emit energy whenever they jump from a high energy level to a
lower one. If the difference between the two levels is in the right range, the
excess energy is emitted as a flash of light鈥攁 single photon. The
wavelength and therefore the colour of this light is determined by the precise
size of the energy gap, which in turn depends on which atoms are involved. Thus
a tube of neon gas glows red when energised by an applied voltage. Other gases
give different colours: argon glows blue, for example.

Similar quantum transitions can take place in semiconducting solids. In these
materials, some electrons are free to move while others remain bound to the
atoms. The difference between the energy states of these two types of electrons
is known as the band gap. Applying a small voltage to a diode made from a
semiconducting material raises electrons to higher energies. As they then
cascade down through the band gap, they generate photons at a wavelength
corresponding to the size of the gap. With a suitable semiconductor, this leads
to a photon of visible light, and what you鈥檝e got is a light-emitting diode.

For years, semiconducting LEDs came in any colour as long as it was red. But
even with this limitation, LEDs have some huge advantages over red light bulbs.
They are rugged and much more efficient than incandescent bulbs, producing
bright light from tiny amounts of power. This has made them ideal for
applications such as indicator lights on instrument panels and rear lights for
bicycles and motorcycles. Red LEDs are even appearing on many new cars, arranged
in glowing lines along the rear window as a bright additional brake light. But
engineers at Visteon Automotive Systems based in Dearborn, Michigan, have a more
radical idea.

They are planning to equip cars with diode lasers. Each diode laser is built
using a light-emitting semiconductor mounted inside a tiny cavity made from a
pair of mirrors. These will completely change the way car indicators and brake
lights are designed.

In conventional car rear lights, white light bulbs are enclosed in parabolic
reflectors and lenses to concentrate and direct the light, which is then
filtered to produce a bright beam of red light. Diode lasers would do away with
all this paraphernalia. They generate tight beams of intense red light, so there
is no need for large plastic lamp housings at the back of the car.

Light beams

At the moment, researchers at Visteon are working on rear signal lights for
cars made with a single red diode laser providing 200 to 500 milliwatts of light
energy, using 80 per cent less power than a conventional bulb. And these diode
lasers should last for years鈥攆or the entire life of the vehicle, says
engineer Jeff Nold of Visteon. According to Nold, these lights should be in
production within five years.

As the market for coloured LEDs expands, their cost has plummeted, opening up
applications traditionally dominated by light bulbs. Already, about 8 per cent
of traffic lights in the US have been fitted with red LEDs鈥攁 trend that
Europe and Asia are following.

Researchers are also experimenting with a range of semiconductors made from
atoms such as gallium, aluminium, indium and arsenic. The result is LEDs in a
range of colours, from red to yellow to green鈥攖he small shining dots that
adorn virtually every piece of electronics.

However, these LEDs are not much use for lighting your living room or office.
We want our artificial light to be more like sunlight, with the same mix of red,
green and blue that we call 鈥渨hite light鈥�. But until recently LEDs that did this
were out of the question because one vital component was missing鈥攁 blue
light-emitting diode. For years, the blue LED eluded researchers until finally,
in 1993, a small Japanese company called Nichia Chemical Industries announced
that it had succeeded in creating tiny chips of the semiconductor gallium
nitride that emitted bright blue light
(快猫短视频, 29 March 1997, p 28).
This triggered a revolution. Using this blue LED it was at last possible to
make LEDs that emitted white light.

Miniature lamp

The LEDs themselves do not emit white light. They use the blue LED to provide
photons that excite atoms in a layer of phosphor that sits on top of the LED
chip, operating like a tiny fluorescent lamp. As there is a mix of atoms in the
phosphor, the result is broadband white light.

How closely an artificial light source matches sunlight is represented by a
value of between 0 and 100 on the Colour Rendering Index. A value of 100
represents a perfect match between the colour spectrum of a given source and
daylight. Incandescent light bulbs have a CRI of 95, whereas most fluorescent
lamps have CRIs of only 55 to 75. Some of the best white LEDs sit somewhere in
between, with a respectable 85.

But if light bulbs have a better CRI, they lose out badly when it comes to
the efficiency with which they turn electrical energy into light. Fluorescent
lamps have efficiencies ranging from 50 to 90 lumens per watt of electrical
power, compared to between 10 and 20 for light bulbs. LEDs can do even better,
with efficiencies of more than 100 lumens per watt.

Not surprisingly, lighting manufacturers have been quick to spot the
potential of these new light sources. In 1997, two major
corporations鈥擯hilips Electronics of the Netherlands and Hewlett-Packard of
Palo Alto, California鈥攂egan collaborating in a venture called LumiLeds to
develop coloured LED lighting, including general-purpose illumination for homes
and offices. General Electric and Osram are following suit, and setting up their
own separate LED lighting ventures. The major light bulb manufacturers are
clearly beginning to take note.

But until now, white LEDs have remained relatively expensive and less
efficient than researchers had hoped. Part of the problem, says Fred Schubert of
the Center for Photonics Research at Boston University, is that phosphors emit
over a broad band that doesn鈥檛 match the eye鈥檚 sensitivity. The eye is not as
sensitive to the blue and red ends of the visible spectrum as it is to green, so
much of the electrical power used to generate these wavelengths in a white LED
is wasted.

But things are looking better for the white LED. Last month, at a meeting at
the Institute of Electrical and Electronics Engineers in Washington DC, Schubert
announced a new way of generating white light. His idea, patent pending, takes
into consideration the colour sensitivity of the human eye, and he claims the
design will lead to a white LED with the highest possible efficiency.

Schubert proposes a source he calls a photon recycling semiconductor LED
(PRS-LED). This device uses electrical power to generate photons at a single
wavelength, some of which are then 鈥渞ecycled鈥� to make light at a second
wavelength. The two wavelengths are selected to create the sensation of white.
Schubert鈥檚 proof-of-principle prototype device uses the colours blue and orange
(see Diagram).
The blue light is radiated by an LED made of gallium, nitrogen
and indium operating at 3.5 volts. Some of this light enters a second
semiconductor layer made of aluminium, gallium, indium and phosphorus, where the
energy of the light raises the electrons to a higher level. The band gap of this
material has been chosen so that the electrons emit orange light. The result, as
Schubert has demonstrated, is a mixture that we perceive as white.

Schubert calculates that his device would have a maximum efficiency of 330
lumens per watt. This is a potential efficiency over three times as great as the
best fluorescent lamps, and between 15 and 30 times that of incandescent lamps,
which is a powerful incentive for further research.

Even with such a high efficiency, there are other factors to consider before
PRS-LEDs can light up your home. Although the light from Schubert鈥檚 prototype
looks white, it does not have a high CRI. As Schubert says, it would not
faithfully display the colours of a Renoir painting. But he predicts that the
CRI of his device can be improved, and that it will be possible to make PRS-LEDs
with differently tinged whites, from cool to warm.

Schubert鈥檚 prototype PRS-LED is tiny, but he is optimistic about scaling it
up. Work is under way to deposit blue LEDs on silicon wafers, the semiconductor
used in computer chips, which are up to 30 centimetres across鈥攖he 鈥渟ize of
a small pizza鈥�, as he puts it. Such a wafer could be cut up into many light
sources, and Schubert estimates that a high-efficiency PRS-LED measuring less
than a square centimetre would emit as much light as a 60-watt bulb, while using
only 3 watts of electrical power.

As well as being energy efficient, LEDs have other advantages too: they are
almost unbreakable, they generate little heat and, best of all, they are much
longer lived. The average incandescent light bulb lasts just 1500 hours鈥攊n
the region of 6 months of normal use. LEDs can last 50 000 hours, says Schubert.
If you move into a house fitted out with white LEDs you may never need to change
a light again.

Cheap and easy

There would be huge savings in the cost of electricity too. If every existing
light source in the US were converted to a high-efficiency alternative, no new
power stations would be needed for 20 years. And according to Colin Humphreys, a
materials scientist at the University of Cambridge, LED lighting could slash
your lighting bill by 80 per cent.

Perhaps most attractive of all is the compactness of semiconductor LEDs.
Since they don鈥檛 require glass bulbs or large connectors, their small profile
would offer new freedom in lighting design. LED lighting panels could be fully
integrated into walls, ceilings and floors, for instance. 鈥淗ow about lights that
adapt their colour, beam width and brightness to the changing daylight,鈥� says
Ton Begemann of the Eindhoven University of Technology, who is also senior
vice-president of Philips Lighting. The result could be smart street lights that
adapt to weather conditions or to street usage. Or, suggests Begemann, how about
an LED lighting system that transmits information? LEDs only need low voltages
to drive them, and it would be simple to modulate the power supply with a
rapidly varying signal to convey a digital message. The light would be modulated
at speeds too fast for the eye to detect, but a pager or a sensor in your
computer would be able to decode it. The lights above your desk could be used as
a communications system, sending messages from desk to desk, or from building to
building through the electricity supply.

Despite the impressive advantages of LED lighting, no one expects it to kill
off the light bulb overnight. Conventional lighting is cheap to install. With
present semiconductor technology, Schubert鈥檚 60-watt LED might cost about
$100. Until LED prices fall, the uptake will be slow.

But Begemann expects that in the next twenty years, LEDs will replace about
half of the bulbs in cars and traffic signals. Blue-green LEDs are already
undergoing trials at locations in Britain and America, and Singapore is about to
refit 60 000 traffic lights with LED signals.

There are other considerations too. Clever though their ideas are, the
inventors of advanced light sources mustn鈥檛 forget the human element. Through
its colour and brightness, light sets a mood and heightens emotion, much like
the background music in a film. So it鈥檚 no wonder that people become attached to
some types of lighting. For instance, red-tinged 鈥渨hite鈥� light makes complexions
look better. Which is why 19th-century theatres used to charge more for
candlelit seats than for those illuminated by gaslight. And why one London
theatre delayed converting to electric light because its leading actress found
it unflattering.

No matter how good the efficiency of LEDs or how perfect their colour
rendering, intimate candlelit dinners are unlikely to go out of fashion. And who
knows, the incandescent light bulb that you now treat with such disdain, may one
day become a treasured possession, brought out for that special romantic
occasion.