Example Problem

How close do you have to be to a 100 W light bulb to observe light of the same intensity as that from a 5 mW laser with a 3 mm diameter spot size?

For the laser beam the intensity is:

S = Power/area = 0.005 W / πr2 = 0.005 W / (π 0.00152 )

This is about 700 W/m2

Assume the light bulb emits energy uniformly in all directions, and that the bulb is about 5% efficient. Most of the energy is given off as heat, so only about 5 W is emitted as visible light.

The energy spreads out uniformly in all directions, so imagine a sphere of radius r centered on the light bulb. The intensity is:

S = Power/area = 5 W / 4πr2 = 700 W/m2

This gives r = 2.4 cm, about as large as the glass enclosing the filament.