randochap said:As far as light output, etc. let's just note that not all lumens are created equal so this measurement can be misleading Lumen quotes do not tell us much about the "kind" of light that gets put down the road. All I can say is that the light output from an Edelux light for instance is truly stunning. I can't imagine that that I could benefit from any more light. I can comfortably descend at 50+kph on unlit, narrow winding roads. Motorists will dim their headlights a kilometre away.
Actually, there is nothing at all misleading about "lumens." A lumen is a watt weighted to the photopic response of the human eye. Therefore, a lumen is a measure of power output. Power out is power out. It's cut and dry. There are other factors, though. A big one is the sensitivity of the human eye to the spectral range of the light being used. The human eye's spectral range is centered at a green/yellow wavelength. That's no surprise. Our sun's visible spectrum is basically centered there. What this means is the eye is most sensitive to light that is yellow/green, like the Sun's output. What this means for LED users is that blue white light--that some LED's use--isn't ideal for human's to see by at night. Not only will blue-white at a given frequency seem less intense than yellow-green, it will also yield poorer contrast. Good contrast is essential for night time riding.
As important as power output is, illuminance is even more important. Illuminance is power per unit area. A simple to think about this is to think about a laser. Laser beams are narrow--on the order of 1-2 mm in diameter--so they pack a lot of power into a very small spot on an illuminated surface. A bare lightbulb, however, essentially spreads it's power everywhere, so less power is concentrated, everything else being equal, in a given area.
So, you have to consider power output (lumens), illuminance (lumens/sq. meter, a result of beam pattern), and light center wavelength (color). FWIW, at the center wavelength of the eye's response (555 nm), there are 683 lumens per watt. Be careful using that figure, though, because that conversion factor varies with wavelength in the spectrum.
If a manufacturer gives a power in watts, I tend to shy away, as either they don't firmly grasp how the eye responds to light or they're trying to do a little misdirection by using units that others don't use.