# Calculate efficiency for an incandescent light bulb?

1. Dec 11, 2008

### germblaster

How does one calculate efficiency for an incandescent light bulb? Assume the bulb is 40 W, has a tungsten filament at temp. ~ 2800 K. The bulb would be used for heating in a device.
If we assume blackbody:
From Wien's displacement law, the maximum wavelength is 9.99E-7 m. Fromt the blackbody curve for this temp., it appears that only ~10% of the radiation is emitted in the visible spectrum. Does this mean that the rest goes into heating?

Is if safe to assume an efficiency of greater than 90%?

2. Dec 11, 2008

### mgb_phys

Re: Bulbs

If you are using the bulb as a heater, then in a closed room it's heating efficiency is 100%.

If you are using it in something like a fish tank you would subtract the visible part that escapes - it's a bit less than that because quite a bit of infrared will go through the glass as well.

3. Dec 11, 2008

### germblaster

Re: Bulbs

4. Dec 11, 2008

### mgb_phys

Re: Bulbs

You're pretty much going to get 40W out of it - the power changes a little as the filament heats up, and I don't know good the engineering tolerances are on light bulbs, so it might be 40W +- 10%

5. Dec 13, 2008

### Redbelly98

Staff Emeritus
Re: Bulbs

40W is not enough to cook food.

But assuming you were using enough power to cook, any radiation (visible or otherwise) that is absorbed by the food will heat it. The answer will come down to the particular geometry of your setup, plus the emissivity of the food which will change as it darkens while cooking.

For a simple toaster, I'd put the efficiency at less than 50%, since at least half the radiation is directed away from the toast. Factor in that the toast, while it is still white, will be reflecting a significant portion of the radiation.