Incandescent Lightbulbs Home Page
10 Most Recent Questions and Answers (out of 42)
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MLA Citation: Bloomfield, Louis A. "Incandescent Lightbulbs Home Page" How Everything Works 10 Dec 2017. 10 Dec 2017 <http://www.howeverythingworks.org/prints.php?topic=incandescent_light_bulbs&page=0>.
1531. What does it mean if a light bulb uses 60 watts? B, Los Angeles
The watt is a unit of power, equivalent to the joule-per-second. One joule is about the amount of energy it takes to raise a 12 ounce can of soda 1 foot. A 60 watt lightbulb uses 60 joules-per-second, so the power it consumes could raise a 24-can case of soda 2.5 feet each second. Most tables are about 2.5 feet above the floor. Next time you leave a 60-watt lightbulb burning while you're not in the room, imagine how tired you'd get lifting one case of soda onto a table every second for an hour or two. That's the mechanical effort required at the generating plant to provide the 60-watts of power you're wasting. If don't need the light, turn off lightbulb!

1424. Is the total energy savings still significant for long tube fluorescent lights, as compared to incandescent lights, when you consider the energy involved in manufacturing all the components of the lights? — AB, San Antonio, TX
Yes, fluorescents are more energy efficient overall. To begin with, fluorescent lights have a much longer life than incandescent lights—the fluorescent tube lasts many thousands of hours and its fixture lasts tens of thousands of hours. So the small amount of energy spent building an incandescent bulb is deceptive—you have to build a lot of those bulbs to equal the value of one fluorescent system.

Second, although there is considerable energy consumed in manufacturing the complicated components of a fluorescent lamp, it's unlikely to more than a few kilowatt-hours—the equivalent of the extra energy a 100 watt incandescent light uses up in a week or so of typical operation. So it may take a week or two to recover the energy cost of building the fluorescent light, but after that the energy savings continue to accrue for years and years.


1316. Is there any equipment that can track people in a large, dense forest? — BRAR, India
To track someone in a forest, he must be emitting or reflecting something toward you and doing it in a way that is different from his surroundings. For example, if he is talking in a quiet forest, you can track him by his sound emissions. Or if he is exposed to sunlight in green surroundings, you can track him by his reflections of light.

But while both of these techniques work fine at short distances, they aren't so good at large distances in a dense forest. A better scheme is to look for his thermal radiation. All objects emit thermal radiation to some extent and the spectral character of this thermal radiation depends principally on the temperatures of the objects. If the person is hotter than his surroundings, as is almost always the case, he will emit a different spectrum of thermal radiation than his surrounds. Light sensors that operate in the deep infrared can detect a person's thermal radiation and distinguish it from that of his cooler surroundings. Still, viewing that thermal radiation requires a direct line-of-sight from the person to the infrared sensor, so if the forest is too dense, the person is untrackable.


1301. How does a halogen bulb work and is it really better than a regular bulb?
A halogen bulb uses a chemical trick to prolong the life of its filament. In a regular bulb, the filament slowly thins as tungsten atoms evaporate from the white-hot surface. These lost atoms are carried upward by the inert gases inside the bulb and gradually darken the bulb's upper surface. In a halogen bulb, the gases surrounding the filament are chemically active and don't just deposit the lost atoms at the top of the bulb. Instead, they react with those tungsten atoms to form volatile compounds. These compounds float around inside the bulb until they collide with the filament again. The extreme heat of the filament then breaks the compounds apart and the tungsten atoms stick to the filament.

This tungsten recycling process dramatically slows the filament's decay. Although the filament gradually develops thin spots that eventually cause it to fail, the filament can operate at a higher temperature and still last two or three times as long as the filament of a regular bulb. The hotter filament of a halogen bulb emits relatively more blue light and relatively less infrared light than a regular bulb, giving it a whiter appearance and making it more energy efficient.


1253. How does an acetylene miner's lamp work? How does a propane gas lamp work? Why do gas lamps need a mantle and what is the mantle made of? — DK, Washington, DC
An acetylene miner's lamp produces acetylene gas through the reaction of solid calcium carbide with water. An ingenious system allows the production of gas to self-regulate—the gas pressure normally keeps the water away from the calcium carbide so that gas is only generated when the lamp runs short on gas. In contrast, a propane lamp obtains its gas from pressurized liquid propane. Whenever the propane lamp runs short on gas, the falling gas pressure allows more liquid propane to evaporate.

Only the propane lamp needs a mantle to produce bright light. That's because the hot gas molecules that are produced by propane combustion aren't very good at radiating their thermal energy as visible light. The mantle extracts thermal energy from the passing gas molecules and becomes incandescent—it converts much of its thermal energy into thermal radiation, including visible light. Mantles are actually delicate ceramic structures consisting of metal oxides, including thorium oxide. Thorium is a naturally occurring radioactive element, similar to uranium, and lamp mantles are one of the few unregulated uses of thorium.

The light emitted by these oxide mantles is shorter in average wavelength than can be explained simply by the temperature of the burning gases, so it isn't just thermal radiation at the ambient temperature. The mantle's unexpected light emission is called candoluminescence and is thought to involve non-thermal light emitted as the result of chemical reactions and radiative transitions involving the burning gases and the mantle oxides.

In contrast, the acetylene miner's lamp works pretty well without a mantle. I think that's because the flame contains lots of tiny carbon particles that act as the mantle and emit an adequate spectrum of yellow thermal radiation. Many of these particles then go on to become soot. A candle flame emits yellow light in the same manner.

One last feature of a properly constructed miner's lamp, a safety lamp, is that it can't ignite gases around it even if those gases are present in explosive concentrations. That's because the lamp's flame is surrounded by a fine metal mesh. This mesh draws heat out of any gas within its holes and thus prevents the flame inside the mesh from igniting any gas outside the mesh.


1176. How are incandescent light bulbs made? — SU
The glass enclosures are made from a ribbon of hot glass that's first thickened and then blown into molds to form the bulb shapes. These enclosures are then cooled, cut from the ribbon, and their insides are coated with the diffusing material that gives the finished bulb its soft white appearance.

The filament is formed by drawing tungsten metal into a very fine wire. This wire, typically only 42 microns (0.0017 inches) in diameter is first wound into a coil and then this coil is itself wound into a coil. The mandrels used in these two coiling processes are trapped in the coils and must be dissolved away with acids after the filament has been annealed.

The finished filament is clamped or welded to the power leads, which have already been embedded in a glass supporting structure. This glass support is inserted into a bulb and the two glass parts are fused together. A tube in the glass support allows the manufacturer to pump the air out of the bulb and then reintroduce various inert gases. When virtually all of the oxygen has been eliminated from the bulb, the tube is cut off and the opening is sealed. Once the base of the bulb has been attached, the bulb is ready for use.


1165. I understand that an ear thermometer measures a person's temperature by studying the thermal radiation emitted by their ear. What is the farthest range that a person can emit thermal radiation that can still be received? Does this range depend on how hot the inner person is? — M
The thermal radiation that a person emits is mostly infrared light and, like all light, it can travel forever if nothing gets in its way. In principle, if you can observe something through a telescope, you can also measure its temperature. For example, astronomers can measure the temperature of a distant star by studying the star's spectrum of thermal radiation.

However, there are several complications when using this technique to measure a person's temperature. First, anything that lies between the person and you, and that absorbs or emit thermal radiation, will affect your measurement. That's because some of the thermal radiation that appears to be coming from the person may be coming from those in between things. Fortunately, air is moderately transparent to thermal radiation but many other things aren't. In fact, to get an accurate reading of person's temperature, you'd have to cool the telescope and the light detector so that they don't add their own thermal radiation to what you observe. You'd also have to use a mirror telescope because glass optics absorb infrared light.

Second, the temperature that you observe will be that of the person's skin and not their inner core temperature. That's because the person's skin absorbs any infrared light from inside the person and it emits its own infrared light to the world around the person. You can't observe infrared light from inside the person because the person's skin blocks your view. All you see is their skin temperature.


1164. How does an ear thermometer work so quickly? — SN, West Covina, California
An ear thermometer examines the spectrum of thermal radiation emitted by the inner surfaces of a person's ear. All objects emit thermal electromagnetic radiation and that radiation is characteristic of their temperatures—the hotter an object is, the brighter its thermal radiation and the more that radiation shifts toward shorter wavelengths. The thermal radiation from a person's ear is in the invisible infrared portion of the light spectrum, which is why you can't see people glowing. But the ear thermometer can see this infrared light and it uses the light to determine the ear's temperature. The thermometer's thermal radiation sensor is very fast, which accounts for the speed of the measurement.

1152. I was told by an electrician to use 130-volt bulbs, which he said were outlawed by the electric bulb makers because they last so long. He said that electricians can buy them and not the public. I found them and have used them for 5 years and he is right! They last forever. Why is that? How do they compare to more energy efficient lights? — J
When you use a bulb designed for 130 volts in a fixture that operates at 120 volts, the bulb's filament runs at less than its rated temperature. This temperature change has two consequences—one good and one bad. The good news is that operating the filament at less than its normal temperature slows the evaporation of tungsten atoms and prolongs the filament's life. That's why your bulbs are lasting so long. The bad news is that incandescent bulbs become much less energy efficient as you lower their filament temperatures. The light emitted by the filament is thermal radiation and its color spectrum and brightness depend almost exclusively on its temperature. These 130-volt bulbs emit redder and dimmer light than a normal bulb and they are significantly less energy efficient as a result. Incandescent bulbs already emit far more invisible infrared light than visible light and operating them at reduced temperatures only makes this problem worse. I recently read the statement "this bulb burns cooler than a normal bulb" on a package of super-long-life bulbs—as though burning cooler was a good thing rather than a serious shortcoming.

As energy becomes more and more precious, making the most of it becomes more and more important. I would suggest saving these 130-volt bulbs for fixtures that are so difficult to reach that you want to avoid changing bulbs at all costs. In more easily accessible fixtures, replacing bulbs is only a minor inconvenience associated with improved energy efficiency. Better still, switch to fluorescent lamps—which are much more energy efficient than even the best incandescent lamps.


1090. How does the temperature of a fire correspond to its color. How hot is blue fire? How hot is yellow fire? — SF, Lake Almanor, CA
The hotter the fire, the more green and blue light it emits. The dimmest glow that you can see in a darkened room appears when a surface is about 400° C. The dull red of a heat lamp is about 500° C. A candle's yellow glow is about 1700° C. A normal incandescent lamp is about 2500° C. And the sun is about 5800° C. Blue fire would be hotter still, except it's usually colored artificially by the presence of excited atoms. Atomic emissions are colored because atoms can't emit all colors in order to produce a normal spectrum of thermal radiation. Instead, they preferentially emit only specific colors. That's why when you burn copper, you see blue-green light, even when the copper isn't very hot. The copper atoms just can't emit red or yellow light, even though those would be the more appropriate colors at the temperature of the burning copper.

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