Lux
Also called lx, illuminance, one lux distance
A lux is one lumen falling on one square metre, so it measures light arriving on a surface rather than light leaving a source, and it is the unit behind the distance figures off-road lamps are advertised by, though makers differ over how much of it still counts.
What is lux?
The SI unit of illuminance, one lumen per square metre. The International Lighting Vocabulary defines illuminance as the density of incident luminous flux with respect to area at a point on a real or imaginary surface, which is a precise way of saying it describes the place you are standing rather than the lamp. It is what a light meter reads when you hold it up.
Some anchors. Direct summer sunlight is around 100,000 lux. An overcast day is a few thousand. A well-lit office is 300 to 500. Twilight is around 10. A clear full moon is about one.
The one lux distance
Off-road lamps are commonly specified by how far away their beam still delivers one lux, and that figure sounds impressive until you notice it is the full-moon threshold. It is the distance at which there is measurably light rather than the distance at which you can see.
It is still a useful specification, for two reasons. It is comparable between lamps in a way that lumens are not, and it converts straight into candela: because illuminance falls off with the square of distance, the one lux distance in metres is the square root of the lamp's peak intensity in candela. A 500 metre claim means 250,000 candela. A 1,000 metre claim means a million, which is worth being sceptical about. One lux is not the only threshold in use, though. The flashlight standard the industry borrows from counts down to a quarter of that, which doubles the distance for the same lamp, and our entry on beam distance works through what happens when a maker does not say which one it used.
See also
Seen in
Sources
- The International System of Units (SI), 9th edition, 2019, Bureau International des Poids et Mesures, read 2026-08-27
- CIE S 017, International Lighting Vocabulary, online browsable edition, Commission Internationale de l'Eclairage, read 2026-08-27The e-ILV term pages are free to read. The published standard is not.
- NIST Special Publication 250-37, NIST Measurement Services: Photometric Calibrations by Yoshihiro Ohno, National Institute of Standards and Technology, 1997, read 2026-08-27Superseded as a calibration document by SP 250-95, and it quotes the pre-2019 candela definition. Its side-by-side definitions of the three photometric quantities are unaffected and are the clearest in print.