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 one lux distance that off-road lamps are advertised by.

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.