How many lumens and how much intensity? The difference on our own numbers

If more lumens meant more distance, choosing a flashlight would be easy. It does not. Our own range shows it like a textbook: the FL3 has 3,700 lumens and reaches 150 metres. The FL5 has 1,800 lumens – half as many – and reaches 325 metres. More than double. The explanation lies in the units.

Lumens are volume. Think of them as the amount of water a light pours out. Candelas are pressure – how narrow the nozzle is. The same volume can be sprayed wide or sent far in a tight cone. And because throw is calculated from candelas, not lumens, a weaker light with sharper optics can reach further.

The ANSI FL1 standard defines throw as the distance where illuminance falls to 0.25 lux – roughly what a full moon provides. From that comes a simple relation:

You can check it on our data. The FL1 has an intensity of 90,535 cd. The square root is 300.9, times two gives 602 metres – and the spec sheet quotes 600. The HL1 has 5,626 cd, root 75, times two equals exactly the 150 metres in the table.

The formula works in reverse too. If a maker quotes only throw, halve it, square it and you have candelas. That gives you a comparable number across brands – and above all it shows when a catalogue “throw” figure was invented.

The FL3 has seven LEDs and a conical beam. Its 3,700 lumens spread wide, so the whole scene in front of you is lit – but no part of the beam is concentrated enough to reach beyond 150 metres. For working in a yard, a garage or searching nearby, that is exactly what you want.

The FL5 goes the other way. A narrow spot, 1,800 lumens, 325 metres of throw. It lights the far side of a field, but what is at your feet you will see mostly from spill. Neither is better – they are two different tools.

And then there is a third way: the FL1 has spot and flood at once and switches between them. 5,000 lumens, 90,535 candelas, 600 metres. You pay for it with 776 grams.

High numbers sell well, but you will rarely use them. Turbo is the shortest mode on any light: the FL7 runs three hours at 2,050 lumens, the FL1 barely two at 5,000, the FL3 around three. In reality you will run lower levels – and there the numbers look completely different: the HL1 gives 200 lumens in Eco and lasts up to 230 hours.

In the end something other than the maximum matters, namely how the light behaves when you leave it running. Most step down from turbo after a while so they do not overheat or drain – and that is correct, not a fault. So the number to watch is the runtime at the level you will use, not the lumens on the box.

Let us reverse the formula on the FL3 and FL5 and work out the candelas implied by their quoted throw. For the FL3 at 150 metres: one hundred and fifty halved and squared – 5,625 cd. For the FL5 at 325 metres it comes to 26,406 cd.

And there is the whole point of the article in one figure: the FL5 has half the luminous flux but nearly five times the on-axis intensity. The difference was made by optics, not power. Run this calculation on two lights you are weighing up and you will learn more in thirty seconds than from the entire description.

The ANSI FL1 standard prescribes how lumens and throw are measured – thirty seconds after switch-on, on a fully charged battery, in an integrating sphere. The problem is that following it is voluntary. A less scrupulous maker quotes the figure from the first instant, when the LED is coolest and brightest, and such a number can be a third higher than what the light sustains.

So it is worth noting that the FL1's runtimes are quoted to ANSI FL1: Turbo 1 hour 55 minutes, High 4 hours 7 minutes, Medium 12 hours 43 minutes. When the specs give minutes rather than rounded hours, it usually means something was actually measured.

The second trap is step-down. A powerful light drops itself from turbo after a while so it does not overheat. That is not a fault, it protects the LED and the cell – but it means five thousand lumens is a peak figure, not an operating one.

A beam is described with three words. The hotspot is the bright centre, the spill is the scattered light around it and the corona is the transition between them. A light with a concentrated hotspot is a spot; one where the hotspot is barely there is a flood.

• Spot – searching at distance, scanning terrain, observing. The FL5 in our range.
• Flood – close work, lighting a place, walking. The FL3 in our range, and headlamps in flood mode.
• Hybrid – both in one light, switchable. The FL1 or the HL1.

A third, practical note. For walking in terrain a pure spot is a worse choice than a flood, even with better numbers. A sharp circle of light twenty metres ahead does not help you see where you are stepping, and your eye adapts to the bright centre, so you see the surroundings even less than without a light.

When comparing two lights, look at the trio of lumens – candelas (or throw) – runtime in the middle mode. Those three numbers give you a realistic picture. And then at the IP rating, to which we devoted a separate article. And if you also want to know what colour the light should be, carry on with the article on warm and cool white.