Before we can start an in-depth discussion about beam patterns and biking or specifically mountain biking, we first have to better understand a few terms and how they relate. This is where a lot of discussions about lighting get muddled. Where people come to the conclusion that more lumens are better, which is true, to a point. Let’s define in layman terms what the various terms are.
Lumens: Essentially the “potential energy” that a light source has. We must think of light as a source of energy, just like the energy in a battery. The more lumens a source has, the more potential energy it has to put light where we want it, and either to see further, or wider, or just waste that energy elsewhere.
Candela: This is the intensity of light in a certain direction. This is very hard to measure from a real-world standpoint without very expensive goniophotometers, however, it is what lighting and optical designers use when designing a beam pattern so you will often hear it being used or see it in lighting standards such as FMVSS 108 for automobiles, or StVZO for bike lighting in Europe. It is also critical in optical design equations and calculations.
Lux: This is the measure of how many lumens are spread over a certain area at a certain distance. This is easily measured with inexpensive handheld lux meters, this is the amount of light hitting a calibrated sphere and this number is VERY dependent on distance. Why? Because the equation for finding lux is:
Lux = candela/ (distance (m)^2)
So one can see that for every meter we move away from the source, lux is cut in half. It’s a logarithmic equation. Lux meters are cheap ways for us to figure out the lighting power of a lamp, as well as backtrack the numbers to determine the beam pattern effectiveness, beam width, and where energy is spread around in a beam pattern. You will often find legitimate companies rating their lux at 10 meters, this is a fair industry accepted the standard for both automobile and bikes even this is also considered for the mountain bike lightings.