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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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