Candela (cd)

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

The candela is one of the seven SI base units, sitting alongside the metre, the kilogram and the second — which makes light the only everyday human perception with a base unit of its own. There is no SI base unit for loudness or for warmth as the skin feels it.

Its name and its size both come from the candle. One candlepower, defined by the Metropolitan Gas Act of 1860 against a spermaceti candle burning at a prescribed rate, equals 0.981 candela, so the modern unit is very nearly the old one carried forward rather than a fresh start.

What changed is how it is pinned down. The Victorian definition depended on a physical object of specified material and burn rate, which meant the standard could vary with the quality of the wax. The modern definition is written in terms of radiation at a fixed frequency, so it can be reproduced in any laboratory without anyone having to source whale oil.