Lumens represents the total luminous flux of the lamp (basically light ouput).

Candle power represents the concentration of light (lumens per steradian). A steridian is a measure of solid angle (imagine a cone, where the lamp is at the tip).

Imagine a laser beam - many you can buy use an LED, but the lumen output is very small. But as the beam is incredibly narrow, the candlepower rating is high! This is a good example of how the lumen and candle power measurments are not direct.

Now imagine a naked flame burning. It is emitting light in all directions and not concentrated or focused, not a narrow beam, so its candela rating is going to be lower. If it is emmiting 1 candela in all directions (over basically a sphere), its total light output in lumens is 4pi x the candela rating, which is 12.57 as there are 4pi steradians in a sphere. If you block half the light ie so it shines on just one half of the inside of the sphere, its candela rating is still one (the focus or concentration has not changed per unit solid angle) but the light output has, so it will be 6.3 lumens.

How does this relate to bike lights? Lumens is the only way to compare. As can be seen from the above, a candela rating is very dependant on the focus of the beam and can be artifically made to look higher. Plus it shows no indication of how wide that beam is.

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