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Name: Lisa
Status: Other
Grade: Other
Location: VA
Country: United States
Date: May 2006


Question:
In a recent discussion with friends I noted that halogen light bulbs burn hotter than incandescent light bulbs, which burn hotter than fluorescent light bulbs. I was told that I was wrong, because two light bulbs with the same wattage would by definition produce the same amount of heat. I believe the person who made that statement is confusing wattage--the energy used--with how efficiently that energy is used, i.e., the less efficient, the more energy is lost as heat.

Also, I believe that he failed to take into account the different technology used by the various types of light bulbs to turn the wattage into light. So, my questions are--would an incandescent light bulb, a halogen light bulb, and a fluorescent light bulb of the same wattage produce the same amount of heat when in use; and, if not, why not?



Replies:
Yes, they would.

You are right that halogen bulbs generally burn hotter than incandescents, and that incandescents are hotter than fluorescents. Your friend is right that all the energy used by any light bulb will eventually end up as heat, and this includes the energy initially emitted as visible light. It would be great if light emitted by a bulb would keep being light as it bounced around the room. If it did, we would only have to turn a light bulb on for a moment, and the room would stay illuminated after we turned the bulb off.

The question of efficiency is about how much energy is required to produce a given amount of useful light. A more efficient bulb will produce more light from a given amount of energy, and will therefore produce less heat for a given amount of illumination, than a less efficient bulb.

Tim Mooney


Hi, Lisa. I believe you touched on the major difference. The wattage is the amount of energy used, and does not tell you how much goes into making heat and how much goes into making light. That is why manufacturers put actual light output ratings (in lumens) on compact fluorescent light bulbs, as well as a comparison (for example "produces as much light as a 60W incandescent light bulb").

Bulbs of different types but the same wattage would produce different amounts of heat. The more efficient the bulb, the less heat and the more light for a given wattage.

David Brandt


I will not answer your question, but will give you some ways to frame the questions better.

Energy goes into the bulb in the form of power = voltage * current. Energy comes out of the bulb as heat or light. The light from a bulb goes out into the room, where it lights things up, but eventually dissipates into heat. This is thermodynamics. So if two bulbs pull the same amount of current (voltage is usually fixed, at 120 VAC etc), then the two bulbs would heat up the room the same. So far your friend may be more correct.

So **if** this is the definition of "wattage" on a bulb -- the product of the input current the bulb draws times the (standard input) 120 volts ac, then equal wattage produces equal heat. So far this has to be correct -- its "just physics", conservation of energy etc. But now I also get confused, by all the definitions light-makes use. Still another definition. The amount of power coming out as "light" is measured in lumens. But the brightness of light has to include the human -- after all if all the "light" were infrared, or ultraviolet, we would still think the room is dark. So people say 1 watt of light is 680 lumens of light, but strictly speaking only if the light is a perfect green color. This is a detail. Lumens are perceived light output power from a bulb, it is what you want from a "light". So if you are an interior designer, you would typically compare lights that produce equal lumens, not lights that take in equal watts from the wall socket. (Too bad really, because it would be nice to be more up front on the electricity bill aspect.)

Some bulbs tilt the ratio of light out to heat out. This could increase lumens (out), without necessarily increasing watts (in). Since what we usually want is light, such a more "efficient" light would be designed to draw less current (hence watts in), to produce the same light (lumens out) as the standard filament bulb.

To a home-owner, the "standard" is the Edison filament light bulb. It uses a lot of its watts input producing heat output. To get the same amount of lumens light output, I think a fluorescent bulb needs only about 1/3 (check my figures) the current input. So it heats the room less.

But I think people can create confusion. An interior designer might **define** "wattage" essentially as the lumens (out). Now all bets are off, because we do not know the input watts, only that fraction of output lumens or watts that goes into light. We do not know how much goes into heat. We do not know how much power came from the wall socket. So in this case, you may be more correct.

Still another term confuses me too: The "color" of the light. This is entirely human perception I think. A filament bulb is yellow-ish compared to say sunlight. A fluorescent bulb might sometimes be more blue-ish. People consider blue a cold color, yellow a warm color, so I think hardware stores refer to such fluorescent as a "colder" light. But fluorescent bulb makes might change the phorphors in their bulb and make them seem more "warm". These terms are somewhat useless to your electric bill. It has nothing to do with the hot-to-touch temperature, it seems.

I have been told at lighting stores that people now days do not compare the color quality of lights to natural sunlight, but to an Edison filament lamp. People do not want to mimic the sun, but what they have gotten used to with these filament lamps. If true, then new bulb makes, such as LED's, might follow this path, creating still more definitions involving human perception.

Finally back to hot-to-touch that you mention. You might guess that hotter-to-touch bulbs are putting out more of their output power in the form of heat, and perhaps therefore less in the form of light. But there is more to it. You put out the same amount of body heat all the time, but you are warmer if you wear a coat. So your temperature, "hot-to-touch" temperature, is not the whole story of how much heat you are putting out. I cannot answer your exact question, I do not know. But temperature is not heat lost, it is just a part of the story.

So this is not a complete answer, but it helps you know things **if** you know how other people define things.

Steve Ross


You win. Wattage and light output (measured in lumens or candelas) are not the same. See:

http://en.wikipedia.org/wiki/Lumen_%28unit%29

for detailed explanation. Two classic examples of high power but low light output are a microwave oven and a conventional oven. Both generate radiation but not much light. An incandescent light is basically a "black body" whose radiation output depends solely (to a good approximation) on the absolute temperature. Fluorescent lights, the flash on your camera, LED's, and a laser pointer each emit bright light in a increasingly narrow wavelengths. Very bright lights but powered by lower and lower power sources. Of course energy is conserved, so the total energy output rate defines its wattage, but each process for producing visible light has its own efficiency.

Vince Calder


The bulbs would produce different amounts of heats, as you suspect. The power rating of a bulb (wattage) is the total amount of power consumed. In an inefficient light bulb, most of the power will be turned into heat, and a small amount will be turned into light. In an efficient light bulb, some of the power will be turned into heat, and a lot will be turned into light. In a perfect bulb, all of the power will be turned into light, and none into heat.

Bob Erck



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