Conservative measurement of output out the front. If Surefire lists a light as having 65 lumens, you know that you'll get, bare minimum, 65 lumens of light out the front. Usually you get a bit more since Surefire tests a large sampling of the same model, and then rates the average output across that wide sampling. This leads to rather conservative output numbers. Some companies also list lumens in terms of real lumens, as out the front. But they use a different standard than Surefire. Many companies still list over-inflated emitter lumens. (Output measured at the emitter itself, before the head is attached. Once the head is in place, output drops down by 1/3 or even as much as 1/2.) Since folks don't use flashlights without the head attached, measuring output at the emitter is retarded. But it's a nice way of pretending that a company's product is better and brighter than it actually is.
