For the non-physicists among us, in plain English...

http://www.brillianz.co.uk/data/documents/Lumen.pdf

And whilst a direct comparison cannot be made because the two terms (lumens and watts) refer to totally different things, if you're looking for a replacement bulb in Tesco then the following might help...

http://www.clarkhoward.com/watt-to-lumen-lightbulb-conversion/

It does to some extent for CFLs. Small ones with thin tubes and lots of folds are much less efficient than bigger ones with straight tubes. GU10 CFLs are spectacularly inefficient.

LEDs are the reverse. They become slightly less efficient (and less reliable) as they get hotter.

A gents loo??????????

Watt are you on about gusty?!

luwimmin surely?

Rgds

Damon

And ladettes...

Rgds

Damon

<blockquote><cite>Posted By: fostertom</cite>Striking how lumen/W efficiency increases considerably, for incandescents, as W goes up.</blockquote>

Presumably because as the temperature changes the spectrum of a black body moves so more is in the visible range ???

http://en.wikipedia.org/wiki/File:Planck%27s_law_vs_frequency.png

If that's the cause I suppose it might not be true for animals (cameras?) that have a different response curve ???

Two distinct effects here. It depends whether you are comparing bulbs with different rated wattages, or comparing the same bulb run at two different power levels.

(1) a 150W bulb is more efficient than a 40W if both are run at their rated voltage. This is the phenomenon fostertom noted 4 days ago. I think it is something to do with heat lost by conduction through the filament support wires. Could be complicated, though.

(2) any incandescent bulb, run above its rated voltage, will consume more watts and also emit more lumens per watt, i.e. become more efficient, for the reason CWatters gives. But as fostertom points out, the bulb life is reduced. Conversely, bulbs become hugely inefficient when dimmed. If you want lower lighting in a room, it's more efficient to turn off some of the lights altogether, than to turn them all down with a dimmer.

You might like to think about the claim commonly made by manufacturers, that dimmers 'save energy'. Justified, or not?

Oh and by the way - thanks, marktime and Joiner, for those links.

But please, everyone, when reading http://www.brillianz.co.uk/data/documents/Lumen.pdf do remember that LED technology has advanced greatly since 2006 when the document was written.