Originally Posted by ajbaudio
On lights: As an electrical engineer and amateur photographer, I must point out that the
Watt is unit of measure for
physical power. Watts can tell you how much power a light may consume, but its cannot tell you exactly how "bright" the light will be. For example, a 100 W incandescent light bulb may appear brighter or darker
to the human eye than a 100 W halogen light bulb, depending on bulb type, fixture/enclosure, etc.
A far more useful unit of measure for light power or "luminous flux" is the
lumen. The lumen will tell you how bright something appears
to the human eye. Most packaging for household light bulbs have the number of lumens printed on the box. Cycling light manufacturers really should do the same.
Steven Scharf's
website elaborates more on this from a cycling perspective.
Lumens may not be all that useful either because it doesn't tell you where the light is going, it's just a measure of how much total light is being produced by the bulb. It doesn't tell you much about how well the lamp is designed. You could have two lights with the exact same # of lumens and one could be much more effective if it focuses the light where you need it most.
Watts are not bad to go by if you're comparing LED to LED, Halogen to Halogen, etc. Watts are not a good thing to go by if you're comparing different technologies. The best measure is to compare them side by side and see what works best for you.