Light Efficiency, Heat, and Wattage
Country: United States
Date: May 2006
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
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.
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.
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.
You win. Wattage and light output (measured in lumens or candelas)
are not the same. See:
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.
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
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Update: June 2012