Number of Photons Per Second from a One-Watt Bulb

I am going to give up Pilsners.

— The Lenten sacrifice made by an engineer. His wife wanted him to give up beer entirely, but that was too much to ask.


Figure 1: Incandescent Lightbulb.

Figure 1: Incandescent Light bulb (Source).

An article on Stumbleupon called "8 shocking things we learned from Stephen Hawking's book" considered the following statement from The Grand Design shocking.

A 1-watt night-light emits a billion billion photons each second.

We can easily compute this number after making a few assumptions:

  • We are talking about an incandescent bulb (Figure 1).
  • The incandescent bulb is ~4% efficient – this means that 4% of the input power goes into making visible light.
  • We will only be considering photons in the range of human vision. There are actually many more photons emitted with wavelengths outside the range of human vision.

Figure 2 shows my calculations.

Figure 2: Calculation of the Number of Photos from a 1W Incandescent Bulb.

Figure 2: Calculation of the Number of Photons from a 1W Incandescent Bulb.

Easy enough calculation. However, I never have seen a 1 W nightlight bulb. I think the smallest I have seen is 7 W.

 
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