Lumen (lm)

The SI measure of luminous flux: the total visible light a source puts out.

The lumen measures output. It counts the visible light a source emits in every direction, weighted for how the human eye responds, which is why an infrared lamp can be pouring out energy and producing almost no lumens at all.

It matters more than it used to because of how bulbs are now sold. For a century the number on the box was watts, which measures electricity consumed rather than light produced — a serviceable guide only while every bulb worked the same way. Once filament, halogen, fluorescent and LED were all on the same shelf, watts told you nothing comparable, and the lumen took over. A traditional 60-watt bulb puts out roughly 800 lumens; an LED does the same on about nine watts.

Lumens, foot-candles and lux are frequently confused, and the distinction is simply where you are standing. The lumen is what leaves the bulb. The foot-candle and the lux are what reaches the page.