Lumen

Also called lm, luminous flux, raw lumens, effective lumens

A lumen is the SI unit of luminous flux, meaning the total amount of visible light a source puts out in all directions added together, which tells you how much light exists and nothing about where it goes.

What is a lumen?

It is the SI derived unit for luminous flux, which the SI Brochure lists as candela times steradian and the International Lighting Vocabulary defines as the change in luminous energy with time. In plain terms: total light output, weighted for how sensitive the human eye is to each wavelength, summed over every direction the source emits in.

It is worth knowing that the lumen is the more fundamental of the two in practice, even though the candela is the base unit. NIST puts it directly: although the candela is defined as an SI base unit, luminous flux is perhaps the most fundamental photometric quantity, because the other four are defined in terms of the lumen with geometric factors attached.

It is the number on the box, and it is the number every off-road lighting company leads with, because it is large and it goes up whenever you add another emitter.

Why lumens oversell a lamp

Because a lumen count is indifferent to optics, and optics are most of what a good lamp is. Take an LED emitting a fixed number of lumens and put it behind a bare lens or behind a precision reflector and you have the same lumen figure and wildly different lights. One lights the ground around your bumper. The other reaches four hundred metres.

Lumens count the light. Candela count the light that went where you pointed it.

Which is why regulators do not use them. SAE J583 and every headlamp standard in the world are written in candela at specified angles, because the question a standard is asking is where the light went, not how much of it was made.

Raw versus effective

Two figures appear on spec sheets. Raw lumens is the sum of what the LED chips are rated to emit, from the chip maker's data sheet. Effective lumens is what actually leaves the finished lamp after the lens, the reflector and the heat have taken their share. The gap between them is commonly thirty percent or more, and a manufacturer quoting raw lumens is quoting a number that never left the housing.