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Name: Nakita
Status: Student
Grade: 9-12
Location: PA
Country: United States
Date: March 2008

Do light bulbs that produce much heat also produce much light?

Hi Nakita,

A typical ordinary tungsten filament light bulb "throws away" about 95% of its electrical energy as heat. The remaining 5% is converted into light. This is true for a small 40 Watt lamp or a large 150 watt one. Halogen lights are a little better than this, being up to about 10% efficient. As you can see, any type of tungsten filament lamp has pretty poor efficiency. You can think of them as "heating elements that also happen to produce a little light".

Higher wattage bulbs do produce more heat, but that is because they use more power. They also produce more light in the process. But the ratio of light output to heat output (that is, "efficiency") remains much the same whether the light is a small one or a large one.

There are some special tungsten lamps that are designed to produce more heat and less light. These are generically referred to as "heat lamps" and are often installed in bathroom ceilings to keep you warm after you get out of the shower. They are the same as ordinary lights, except that the filaments are designed to operate at a lower temperature compared to normal lamps. The lower temperature results in less light, and more infrared heat.


Bob Wilson

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