Beam distance

Also called effective beam distance, throw, beam throw, usable beam distance

Beam distance is how far from a lamp its beam has faded to whatever threshold of light the maker counts as useful, and the flashlight industry's convention puts that threshold at 0.25 lux, which works out as twice the square root of the lamp's peak candela.

What is beam distance?

A figure in metres answering the question everybody asks first, which is how far the thing throws. It is a threshold, not a limit. Light does not stop at any particular distance, it only gets thinner, so a beam distance means nothing until somebody decides how thin still counts.

The convention comes from flashlights. ANSI/NEMA FL 1, approved as an American National Standard in August 2009 and now maintained by the portable lighting trade body PLATO, is the document that put beam distance on the box, and PLATO's own account of it is the plainest in print: the distance, in metres, at which the light projects a useful amount of light, measured at 0.25 lux, which PLATO glosses as roughly a full moon on a clear night in an open field.

The arithmetic follows from the inverse square law and takes about ten seconds. Illuminance falls with the square of distance, so a lamp of I candela delivers I divided by d squared lux at d metres. Set that equal to 0.25 and the distance is twice the square root of the candela. A lamp measuring 100,000 candela at its brightest point has a 632 metre beam distance. One measuring 250,000 has a kilometre. Nobody paces this out in a field: it is calculated from a peak intensity read in a laboratory, and FL 1's own beam distance procedure ends in a step titled final calculations.

Why two brands' beam distances do not compare

Because the threshold is a choice and almost nobody states which one they made. Move it from 0.25 lux to 1 lux and every distance in the catalogue halves without a thing changing about the lamp. Nothing in the number itself tells you which happened.

KC HiLiTES publishes candela and beam distance in the same table, which makes its method checkable, and the answer is that there is more than one method. Divide the published candela by the square of the published distance and you get the threshold that row implies. Its 30 inch FLEX Array combo bar, 270,000 candela and 1,039 metres, implies 0.25 lux exactly, though that bar is discontinued and the figures come from a 2017 capture. Its FLEX ERA 1 pod implies exactly 1 lux on all three of its lenses: 4,510 candela and 67.2 metres on flood, 16,200 and 127.3 on spread, 31,000 and 176.1 on spot. Its FLEX ERA 3 implies about 1.3 lux on both of its modes. Its FLEX ERA 4 implies about 1.53 lux on the combo lens and about 0.52 lux on the spot lens, so that product's two rows do not agree with each other. Every one of its current products in that set implies a lux or more, so the flashlight convention describes the discontinued bar and nothing else on the shelf.

One brand, four products, four different answers to the question the number is supposed to settle.

None of that makes the lamps bad or the figures invented. The FLEX ERA 1's three rows agree with each other to a tenth of a metre, which is the signature of a real calculation from a real measurement. What it means is that the column headed beam distance is not one quantity. It is several quantities sharing a name, and comparing them across brands, or even across one brand's shelf, compares nothing.

What to compare instead

Candela. Peak intensity is the physical thing a distance figure is derived from, it is the unit every lighting standard is written in, and two candela figures can be set side by side whoever published them. Where a maker gives you both, divide as above and you have learned their method. Where a maker gives you a distance and no candela, you have a number with nothing to attach it to.