kickstart

Its not exactly accurate, but..........

Lumen is how bright a light source appears when you look at it.

Lux is how brightly a specific area is illuminated by a light source.

this isn't the correct definition of the terms, but represents a practical application of how it relates to bicycle lights.

noglider 

@kickstart, you're right. It's not precise, but it seems useful.

When I explain basic electrical theory, I say voltage is the amount of force and amperage is the quantity of current. If you put your thumb on the end of a garden hose, you decrease the quantity and increase the force. I guess candela is analogous to force whereas lumens are the analogy to amperage (current or quantity). I don't know where candela is useful for us, though.

Garfield Cat

So the bare LED puts out energy and somebody puts up some kind of "object" in its way in order to make a measurement. To me that bare LED is scattered all over the place in space. LED is itself an object, maybe a flat piece of material. Turn it one way and the energy will scatter that way or whichever way.

But if that bare LED is directed (focused) with a lense, maybe the measurement of that one object might change. I think of a very focused laser beam which can do a lot of energy at a fixed point. Or even a magnifying glass and the sun on a tiny ant.

I'm trying to find an analogy. To me it seems the lumens doesn't count for that much. What's important is the measurement at the object, the ant.

How about that old Dean Martin song: When a moon hits your eye like a big pizza pie
That's amore

Read more: Dean Martin - That's Amore Lyrics | MetroLyrics