Originally Posted by smasha

http://en.wikipedia.org/wiki/Luminous_efficacy

anything that claims to produce more than 683 lm/W is science fiction. as a point of reference, an XML T6 is about 40% efficient, producing about 270 lm/W.

the battery type and number of batteries (chemistry, voltage, Ah, etc) will dictate how much power is available to the light, and how long that power will last.

then there are losses on the optics, the electronics, the batteries, the LED, the contacts...