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Light Bulb Color and Temperature
Name: Shawanna
Status: Student
Grade: 6-8
Location: MD
Country: United States
Date: December 2006
Question:
Do different color light bulbs produce different
amounts of heat?
Replies:
Hi Shawanna,
There is no significant difference in the amount of heat
produced, between different colors of light bulbs of the same
wattage.
To give you a little more detail, all standard filament-type
lights waste around 95% of their power as heat, and only
about 5% of the power they use is used to make white light.
So, a 100 watt light bulb may be using 100 Watts of
electrical power, but it is producing only about 5 Watts of
actual white light, and a whopping 95 Watts of heat!
All a colored light is, is a white light that has its glass
bulb painted with a colored paint to filter out all but the
desired color. For example, a red light has translucent red
paint covering the glass bulb that blocks all the other
colors except red. The energy contained in all the rest of
the colors that are blocked, is turned into heat as well. The
same is true for green lights, blue lights, yellow lights,
and so on. It really makes little difference which color the
light is, they all have similar, low efficiency, and
therefore they all generate about the same amount of heat.
Note that this is not true for LED lights (such as the new
LED Christmas tree lights). Not only are LED lights a little
more efficient than normal lights (more light, less heat),
they also only produce only one color in the first place. So
there is no need to waste energy to filter out unwanted
colors. A red LED, for example, produces only red light and
no other color; there are no unwanted colors to filter out.
Therefore LED lights produce more light, and less heat than
standard colored filament lights that must be filtered to get
a desired color.
Regards,
Bob Wilson
Shawana-
I think they produce different amounts of heat, but only rather
small differences, around 1% of the total power the bulb takes from
the power line.
This is because they are really inefficient in the first place. If
a filament-bulb takes 100 Watts of power, it only makes about 2
Watts of pure white light. The rest is heat, in two forms. Then
the colored film coating on the glass bulb decides which colors in
that white light to pass, and which colors to block and turn into heat.
The bulb does make some invisible Infra-Red (IR) light, maybe 10-20
Watts out of 100W. We cannot see it, but everywhere this IR light
shines it gets felt as heat. If the paint is transparent to this IR
light, the heat is felt wherever the visible light shines strongly,
such as on your skin 1-2" away from the bulb. But if the paint
blocks the IR light, it gets turned into heat in the glass bulb,
which feels hot if you touch the glass, but 1" away sideways or
below the bulb you will not feel it. The hot glass heats the air
touching it, and the hot air rises, so you might feel warmth 1" away
right above the bulb.
It can be a tricky thing to do an experiment on. You need to figure
out where all parts of the heat are going, and catch and measure them all.
Filament-bulbs are very biased towards the red end of the
rainbow. They do not make much blue light, and they make more
yellow, and lots more red and IR light. With a bulb of clear glass
they are best at heating objects near them.
But a yellow bulb only blocks blue light, which is the color emitted
least by this lamp, so yellow glass will not reduce the "thrown
heat" very much. Dark Blue, on the other hand, blocks
red-yellow-green, and maybe IR too, and it only lets the weak blue
light through, so it probably has the weakest "thrown heat" on nearby objects.
You may have seen "heat-lamps" over food in a cafeteria, shining
rather orangish and keeping the food a little bit warm.
You can bet that orangish color film doesn't block any IR
light. The metal reflector steers all the thrown heat into one
general direction, so it really makes the warming stronger.
Jim Swenson
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Update: June 2012
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