For most people, one of the most important features of a headlamp or flashlight is its brightness. This can be a surprisingly difficult thing to compare, since headlamp manufacturers and retailers often list brightness according to several different measures: candlepower, lumens, lux, and watts. Today's entry explores the first two.
CandlepowerThese days, candlepower is a seldom-used unit yet is the simplest to comprehend—and it relates directly to understanding lumens. Simply put, one candlepower is the amount of light emitted by a single normal burning candle. Over the years, the "candle" used to specifically define this standard unit has varied from a pure spermaceti candle (19th century) to a carbon filament in a specific incandescent lamp (early 20th century). In 1948, the international unit candela replaced the term candlepower, though it's considered to be closely equivalent to the old measure. So, again, one candela is approximately equivalent to the light emitted by a single burning candle.
Lumens are a measure of how bright the human eye perceives a light source to be (as opposed to the total power of light emitted). Understanding how lumens are calculated is much harder to wrap your mind around than candelas. First off, there's a linear relationship between candelas and lumens: If a light source doubles in brightness from 5 to 10 candelas, its lumens also double. Beyond that, it gets complicated. Lumens are equal to candelas times steradians, which are a measure defined by the ratio between the radius of a sphere, a solid angle at the center of the sphere, and the area on the surface of the sphere created by that solid angle. (Visit the wikipedia page if you really want to know.)
The upshot for headlamps and flashlights is this. If a one-candela light source is unobstructed in all directions, it emits 12.56 lumens. If light is completely obstructed or absorbed without reflection in half the directions, it emits half the number of lumens. Flashlights and headlamps, though they emit only a narrow beam, capture much of the otherwise obstructed light with reflective coatings or lenses that gather light over a wider area but focus it on a narrower area. So a one-candela flashlight that beams out in only 10 percent of directions wouldn't emit just 1.25 lumens. It would more likely emit something like 5-10 lumens, depending on how effectively it gathered all the light to focus into one beam.
Thus, as a very rough way to conceptualize brightness, divide the number of lumens by somewhere between 5-10 (to account for the varying amount of total light captured in the beam). So the total brightness of a 60-lumen headlamp would be roughly equivalent to 6-12 burning candles. If the headlamp focuses those lumens into a narrow beam, that beam will be the brighter—and go farther—than a wider beam. To put it into other terms that are easy to relate to: A 100-watt light bulb emits approximately 1,750 lumens.
We'll talk more about watts and lux—a measure of brightness over a specific area—in the next exciting post!
“Equipped” is an AMC Outdoors blog, written by Matt Heid.