How Light Bulbs Work
by Tom Harris
Before the invention of the light bulb, illuminating the world after thesun went down was a messy, arduous, hazardous task. It took a bunch of candles or torches to fully light up a good-sized room, and oil lamps, while fairly effective, tended to leave a residue of soot on anything in their general vicinity.
When the science of electricity really got going in the mid 1800s, inventors everywhere were clamoring to devise a practical, affordable electrical home lighting device. Englishman Sir Joseph Swan and American Thomas Edison both got it right around the same time (in 1878 and 1879, respectively), and within 25 years, millions of people around the world had installed electrical lighting in their homes. The easy-to-use technology was such an improvement over the old ways that the world never looked back.
The amazing thing about this historical turn of events is that the light bulb itself could hardly be simpler. The modern light bulb, which hasn't changed drastically since Edison's model, is made up of only a handful of parts. In this article, we'll see how these parts come together to produce bright light for hours on end.
Light Basics
Light is a form of energy that can be released by an atom. It is made up of many small particle-like packets that have energy and momentum but no mass. These particles, called light photons, are the most basic units of light. (For more information, see How Light Works.)
Atoms release light photons when their electrons become excited. If you've read How Atoms Work, then you know that electrons are the negatively charged particles that move around an atom's nucleus (which has a net positive charge). An atom's electrons have different levels of energy, depending on several factors, including their speed and distance from the nucleus. Electrons of different energy levels occupy different orbitals. Generally speaking, electrons with greater energy move in orbitals farther away from the nucleus. When an atom gains or loses energy, the change is expressed by the movement of electrons. When something passes energy on to an atom, an electron may be temporarily boosted to a higher orbital (farther away from the nucleus). The electron only holds this position for a tiny fraction of a second; almost immediately, it is drawn back toward the nucleus, to its original orbital. As it returns to its original orbital, the electron releases the extra energy in the form of a photon, in some cases a light photon.
The wavelength of the emitted light (which determines its color) depends on how much energy is released, which depends on the particular position of the electron. Consequently, different sorts of atoms will release different sorts of light photons. In other words, the color of the light is determined by what kind of atom is excited.
This is the basic mechanism at work in nearly all light sources. The main difference between these sources is the process of exciting the atoms.
In the next section we'll look at the different parts of a light bulb.
LED Light Bulb Basics
An LED is what's called a "solid-state lighting" technology, or SSL. Basically, instead of emitting light from a vacuum (as in an incandescent bulb) or a gas (as in a CFL), an SSL emits light from a piece of solid matter. In the case of a traditional LED, that piece of matter is a semiconductor.
Stated very simply, an LED produces light when electrons move around within its semiconductor structure.
A semiconductor is made of a positively charged and a negatively charged component. The positive layer has "holes" -- openings for electrons; the negative layer has free electrons floating around in it. When an electric charge strikes the semiconductor, it activates the flow of electrons from the negative to the positive layer. Those excited electrons emit light as they flow into the positively charged holes. (For a complete explanation, see How Light Emitting Diodes Work.)
The problem with LEDs as primary home lighting is that while they emit a lot of light, the structure of an LED causes some of that light to get trapped inside. So an LED bulb has traditionally been dimmer than an incandescent bulb, and most people want their lamps and ceiling fixtures to be pretty bright.
Recently, though, LEDs bulbs have brightened up. You can now find LED replacement bulbs that emit light equivalent to a 60-watt incandescent light bulb, which makes them a viable technology for basic lighting needs at home.
And in some ways, they're much more than viable: An LED replacement light bulb called GeoBulb emits 60-watt equivalent light using 7.5 watts of power [source: Sundance].
And in other ways, less than viable: A 60-watt-equivalent LED bulb costs about $100.
Which brings us to the pros and cons of LED light bulbs.
Some LED bulbs can last up to 50,000 hours.
Image courtesy C. Crane Company Inc.
Advantages of LED Light Bulbs
While you won't find LEDs in too many household lighting fixtures these days, there are a couple of good reasons to want them there in greater numbers.
First, there's the reduced energy use. The LED method of producing light loses far less energy to heat than do other lighting technologies. It's dramatically more efficient than the vacuum/filament method used in incandescent bulbs -- sometimes around 85 percent more efficient; and it's even about 5 percent more efficient than the CFL's plasma-tube approach [source: Taub].
A single light fixture stocked with a 60-watt incandescent bulb consumes about 525 kWh of electricity in a year; put a GeoBulb LED bulb in that light fixture, and the annual energy use is more like 65 kWh [source: Sundance]. The annual CO2 reduction is in the hundreds of pounds for a single lamp.
But energy-efficiency is just part of the story. The other part is time-efficiency: You could go 20 years without having to change an LED light bulb. Solid-state lights like LEDs are more stable light sources than incandescent or fluorescent bulbs, and the difference is startling: A typical incandescent bulb lasts about 750 hours; a Geobulb lasts 30,000 hours [source: Sundance].
Some LED bulbs last up to 50,000 hours [source: Linden].
Because of that time benefit, things get a bit more muddled when you get into the cost issue. A 60-watt LED replacement bulb runs in the area of $100, and even the lower-output versions, used for things like spot lighting, will cost between $40 and $80. That's compared to a $1 incandescent and a $2 fluorescent bulb.
The reality is, even at $100 for a single bulb, LEDs will end up saving money in the long run, because you only need one every decade or two and you spend less money on home lighting, which can account for about 7 percent of your electric bill [source: Greener Choices]. But the upfront cost is still pretty prohibitive. Lots of people simply can't spend a thousand dollars for 10 light bulbs.
The other primary LED issue -- degradation in the color of the light to something bluish -- has been solved in newer models. LEDs can produce the same soft, white light as a regular bulb. (Although Energy Star does recommend looking for the Energy Star label when shopping for LED bulbs, since the organization tests for color stability as part of its certification criteria.)
So price is really the only problem with LED light bulbs right now. But that could change pretty soon.
How LED Light Bulbs Work
by Julia Layton
LEDs are poised to take over household lighting.
Image courtesy C. Crane Company Inc.
Concerns about LED Light Bulbs
LEDs are poised to take over household lighting. Or so believe lots of research scientists and some big lighting companies. Philips, for one, has stopped putting money into research and development of fluorescent technologies and is now putting all of its energy into LEDs [source:Taub]. General Electric devotes half its R&D budget to solid-state lighting [source: GE].
If you look at what the scientists are saying about LEDs, the picture does look pretty rosy. Breakthroughs are popping up at a breakneck pace. Two recent developments are predicting a major price reduction in the high-efficiency, long-lasting bulbs.
One has to do with the light-loss issue in bulb design. One way to release more of an LED's light is to put microscopic holes in the casing. The problem is that making all of those holes is time-consuming and expensive. A team of researchers at the University of Glasgow in Scotland has found a new way to do it, though. They've found that using a nano-imprint lithography technique can reduce the time and expense of putting billions of holes in tiny LEDs.
Another team of scientists, this one at the University of Cambridge in England, approached the issue from the supply side. They've found a new, less-expensive way to create gallium-nitride semiconductor material, a common basis of LED lighting. At present, gallium-nitride semiconductors are grown on expensive sapphire wafers. The new way uses silicon wafers and affects a dramatic reduction in manufacturing cost for the LEDs.
And what's more, these new gallium-nitride-based bulbs are rated at 100,000 hours, are three times more efficient than CFLs, and should cost less than $3 a piece [source: Evans]. Gallium-nitride LED light bulbs could be on the market by 2012.
The lighting industry in general expects LED costs to come down quickly. Lighting Science Group, a company that develops and manufactures LED lighting, estimates a 50 percent price reduction within two years [source: Linden]. Businesses, which have much more extreme lighting expenses and so can recoup upfront bulb expenditures very quickly, will likely flock to LEDs with that kind of price drop. However, homeowners may wait until the bulbs hit the $3 mark the Cambridge researchers are aiming for.
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