Back to the science of measuring brightness! (and comparing headlamps!) As we discussed in the previous post, lumens are a measure of the total brightness emitted by a light source. It can be a useful number when comparing headlamps, as when stacking up the new and old Petzl headlamp series highlighted in my September 13 post (a reader request—thank you for the question).
Old Petzl Tikka: 26 lumens
New Petzl Tikka 2: 40 lumens
Old Tikka Plus: 35 lumens
New Tikka Plus 2: 50 lumens
Old Tikka XP: 40 lumens (boost mode)
New Tikka XP 2: 60 lumens (boost)
So it's pretty clear that the new Petzl line is 30-50 percent brighter than its predecessors. Most manufacturers—Petzl, Princeton Tec, Black Diamond, SureFire, etc.—spec out their lights in lumens, thus providing a (rough) standard for comparison. Keep in mind that manufacturers measure lumens differently; there's no industry standard. Also remember that lumens are measuring the total light output—a narrow or spot beam will be brighter than diffuse, broad area lighting. Thus a second useful measure to look at is the maximum distance a headlamp or flashlight can illuminate, which will vary depending on lumens and beam width.
A few headlamp makers—most notably Mammut—list brightness in terms of lux. Lux is a measure of brightness over a specific area, which varies with distance from the light source. The number of lumens, remember, is a fixed amount for any given light source. But obviously, if you move an object farther and farther away from the light source, it will become more and more dim. Lux provides some key information for how bright something will be illuminated at a certain distance. For headlamps, you will often see lux measured at 2 meters, the typical distance you need for lighting up the trail at your feet for night-time hiking.
Lux and lumens are related: One lux is equal to one lumen per square meter. Consider a simple example from our last post: a single burning candle radiating light in all directions. As previously noted, the candle emits around 12.5 lumens. If you enclose the candle in a perfect sphere one meter in diameter, the light will strike an area of ~12.5 square meters (the surface of the sphere: 4 • π • r²), giving us a lux of 1. If you move out another meter, your two-meter sphere will quadruple in surface area to ~50 m², thus reducing lux to 0.25. Now imagine that you were able to capture and reflect all that light into a single, narrow beam that illuminated exactly one square meter at a meter distance. Your lux now would be 12.5.
When evaluating headlamps by lux, consider a few standards of comparison. A full moon on a clear night produces 0.25 lux. Indoor home lighting is usually between 50-100 lux. Sunrise/sunset is around 400 lux. Full daylight out of direct sun is 10,000-25,000 lux. Headlamps vary anywhere between 10-700 lux at 2 meters.
There is no easy way to convert lumens to lux, since lux varies depending on how narrow and effectively a headlamp focuses the total lumens into a beam. So what's the upshot of all these lumenating and luxing? Next time you shop for a headlamp, you can decipher the jargon.
“Equipped” is an AMC Outdoors blog, written by Matt Heid.