Lumens, lux and beam patterns explained

The number on a light's box is usually lumens. It's the total light the lamp gives off, in every direction. It doesn't tell you how much lands on the road in front of you, and that's the part you care about.

Three units, three jobs

Lumens measure total light output. One source puts it plainly: the luminous flux is of less value for bicycle lighting due to the importance of directionality¹. A bare bulb and a focused headlight can share a lumen figure and light very different things.

Candela measures how strong the light is in one direction. The candela (cd) is the SI unit of luminous intensity¹. A tighter beam gives more candela from the same lumens.

Lux measures how much light lands on a surface. It is equal to one lumen per square metre². Lux is the closest of the three to "what do I see on the ground?"

Why the same lumens can look different

Wikipedia's lumen page gives a worked example. A flux of 1,000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1,000 lux³. The same 1,000 lumens, spread out over ten square metres, produces a dimmer illuminance of only 100 lux³.

So spread matters as much as output. A wide, soft beam and a narrow, hard one can use the same lumens and still look nothing alike on a path.

What boxes do and don't tell you

Many boxes give only one figure. A European cyclists' group notes that, on some lights, the packaging just states a number of lumens or candela⁴. Some makers do more. One source says some manufacturers indicate the illuminance their front lights provide to the road at a point located a standard distance right in front of the bicycle¹.

When a box gives lux at a stated distance, that's a more useful clue than lumens alone. But numbers from different makers may be measured differently, so compare like with like only when the test is the same.

Beam patterns

The pattern is the shape of the light on the ground. A light can make a round spot, or it can be shaped into a wider, flatter pool. Your manual or spec sheet should say which one you're getting.

Standards can ask for this. A cyclists' group describing German rules lists that light must emit a specific beam with a dipped flat pattern⁴. The standards page explains what that rulebook covers.

What to look for on a box or spec sheet

• Is the figure lumens, candela or lux? They aren't interchangeable.
• If it's lux, at what distance?
• Does the maker name a beam shape, such as a cut-off?
• Does it name a standard or approval mark, and can you check that mark against the official rule?
• Does the manual say how to aim the light?

If the page doesn't say, ask the maker. A vague claim like "super bright" tells you nothing you can measure.