This question has doubtless been answered elsewhere on these forums; what you are asking is not specific to headlamps, but a more general question of how light intensity is measured. Might be worth a quick search to see what you turn up. There was a discussion of this subject recently in one of the headlamp forums, but it got a little...intense. I found this reference useful: http://en.wikipedia.org/wiki/Lux to explain some of the differences.
