Head torch lumens and run time: how to read the box before you buy

What the FL1 standard really measures

A figure is only as good as the method behind it. For portable lights, the reference is an American standard, ANSI/PLATO FL1, which sets out the method for six measurements, from light output to water resistance [1]. It is voluntary. But a manufacturer that prints the FL1 logo on its packaging must follow those methods for every figure on the box, not just the most flattering one [1].

Run time, the figure that misleads most

On paper, a 40-hour run time promises 40 hours of light. The standard says otherwise: the clock starts 30 seconds after switching on and stops when the output falls to 10% of its initial value [1, 2]. The threshold is not arbitrary: according to the standard, it is the level at which people usually change their batteries [1].

Everything then depends on the shape of the curve. An unregulated torch feeds its LED directly: it is very bright for the first few minutes, then dimmer and dimmer as the hours pass, and the higher its starting output, the faster that output drops [3]. A regulated torch, by contrast, holds a constant output for a set period, which is the run time you can actually use, and then often switches to reserve lighting [3]. Two torches can therefore show the same run time without offering anything like the same night.

Enlarge diagram

Petzl puts things in perspective with an order of magnitude: 100 lumens for 50 hours would take the equivalent of 75 AAA batteries, or 1.1 kg on your head [3]. When a spec sheet pairs a very high output with a very long run time, the two figures often come from different modes: the output from the maximum mode, the run time from the minimum mode [3]. You cannot have both at once.

Lumens, candela, beam distance: three figures for three questions

The lumen figure counts all the light that leaves the torch. The candela figure measures the intensity at the heart of the beam, where it is strongest. Beam distance, finally, is the distance at which the light falls to 0.25 lux, roughly the light of a full moon on a clear night in open country [1, 2]. Two torches with the same output can therefore reach very different distances: beam distance depends on output, but above all on the shape of the beam [2]. A wide beam lights up the camp nicely; a narrow one reaches a long way down the path.

Enlarge diagram

Let’s do the maths: 1,000 candela gives a beam distance of about 63 m, and 2,500 candela a beam distance of 100 m. To double the distance, you need four times the intensity: the inverse square law, on which the standard rests, drives a hard bargain [1].

Water and impact resistance: what IPX4, IPX7 and IPX8 mean

• IPX4: splashing water from any direction must do it no harm. A downpour, yes; a dunk in a mountain stream, no.
• IPX7: temporary immersion, under standardised conditions (the FL1 test is done under 1 m of water).
• IPX8: prolonged immersion, under conditions tougher than for IPX7, which the manufacturer must state [1].

For impact, the standard drops the torch six times onto concrete, from at least 1 m, and it must come out without cracks or failures [1]. A greater height on the packaging means the torch has passed the same tests from that height.

Batteries, rechargeables and the cold

The energy on board governs everything else: for the same energy reserve, more light means less run time, and you never get a high output and a record run time at the same time [3]. In serious cold, the type of battery matters too. According to Petzl, disposable lithium batteries perform better than alkaline and Ni-MH batteries at extreme temperatures, and work from -20 to +50 °C [4]. Before putting them in a torch, check its instructions to make sure it accepts them.

Comparing two head torches in five minutes

1. Look for the FL1 logo: without it, nothing tells you how the figures were measured.
2. Compare run times in the mode you will actually use, rather than in the weakest mode.
3. Check the light-output curve where there is one: a sustained output beats a peak at switch-on.
4. Choose the beam distance for the job: camp calls for a wide beam, walking at night for one that reaches far.
5. Check the water resistance and, if you are heading into the cold, which batteries the torch accepts.

A figure without a unit is a promise without a contract; a figure without a method is not much better. The FL1 standard supplies the method: reading the small print is up to you.

Julien de Jurquet

Campervans, motorhomes, cycling and calculators

Julien de Jurquet writes for Outfarer about campervans, motorhomes, cycling and calculators. He reads instruction manuals to the very end and never lets a figure through without its unit, so that your choices rest on sums rather than promises.

All their articles