http://en.wikipedia.org/wiki/Luminous_efficacy Here is an excellent source on the topic, courtesy beenthere. As I said on another thread I had two experiences with trying to use 277 volt florescents at 240 volts where the light was very poor. This is only a 13% drop in power and the unwise would probably meant this was only a 13% drop in luminosity. But no, the drop in luminosity was greater than 50 percent as far as I could tell. This puzzled me and still does even after reading the wiki article above. That article explained luminous flux and the eye's ability to see only certain wavelengths of life to be the culprit. Or did I miss something? I recently bought a package of florescent bulbs purporting to give out the same amount of light as 100 watt incandescents, but using only 23 watts of power. Does anyone know just what it is that makes this happen? Perhaps I missed something in that article.