Lux (lx)

The SI measure of illuminance, and the metric counterpart of the foot-candle.

Lux does exactly what the foot-candle does, against a square metre rather than a square foot. Because a square metre is 10.76 square feet, the same light spread over it produces a number 10.76 times larger, which is the entire relationship between the two units.

The figures worth carrying around: full daylight runs to perhaps 10,000 lux and direct sun far beyond it, an overcast day gives around 1,000, a well-lit office 500, and a domestic living room in the evening perhaps 50. Moonlight comes in under one.

It is the measure behind a great many specifications nobody reads — workplace lighting regulations, camera low-light ratings, the sensors that decide when streetlights come on. The unit is modern and metric, but the thing it measures is the same thing the Victorians were arguing about when they defined candlepower against a whale-oil candle.