Light & Optics

The imperial measure of illuminance, defined as the light from one standard candle falling on a surface one foot away.

1 lumen per square foot = 10.76 lux

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

1 lumen per square metre = 0.0929 foot-candles

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

Light of one candela over one steradian

The SI base unit of luminous intensity, successor to the candlepower it is named after.

1 candela = 1.019 candlepower

Defined by the Metropolitan Gas Act 1860 against a pure spermaceti candle of specified weight and burn rate.

1 candlepower = 0.981 candela