On Wed, 14 Aug 2013 17:59:31 +0100, Chris Hogg wrote:

If a bottle delivers say 50 watts of light, then each bottle will have
to have a cross-sectional area of 50/0.045 = 1110 cm^2, or a diameter
of about 37.6 cm. So they'd have to be even bigger than Jeff
suggested, which makes me think the estimate of 40-60 watts is
somewhat exaggerated.

I think you are missing an efficiency factor. I suspect the light you
get from the pop bottle is similar in level to that which you get
from a 40-60 watt tungsten incandescent bulb. Incandescent light
bulbs are horribly inefficient, less than 5%.

Reversing the calculation, a 2 litre pop bottle has a diameter of
about 9.5 cm, so a cross sectional area of about 71 cm^2, and would
give a visible light output of 71x0.045 = 3.2 watts.

3.2 Watts of real light or 64 Watts of incandescent assuming 5%
efficiency.
Most incandescent bulbs won't be that efficient...

--
Cheers
Dave.