The SI unit of brightness is still, at heart, one ordinary candle
Light a plain wax candle and it shines with roughly one candela, the official SI unit of luminous intensity. Getting from that flickering flame to today's definition took whale-oil candles, molten platinum and a precise shade of green light pitched where the human eye is most sensitive.
Candela is simply Latin for candle, and the older word survives in terms like foot-candle and candlepower. Before 1948, countries kept their own standards built around a carefully specified flame or lamp filament. The English candlepower was the light of a pure spermaceti candle weighing a sixth of a pound and burning 120 grains an hour; Germany, Austria and Scandinavia used the Hefnerkerze, tied to the Hefner lamp.
In 1884 Jules Violle suggested something more stable: the glow from one square centimetre of platinum at its melting point. Platinum suited the job because it melts hot, resists oxidation and can be purified, and its emission depends strictly on temperature. His unit came to about 60 English candlepower. In practice, surface impurities spoiled the results for half a century until researchers suspended a hollow thorium dioxide shell with a tiny hole in molten platinum, creating a black body whose glow depends only on temperature. The resulting new candle was proposed in 1937, announced in 1946 and renamed candela by the 9th General Conference on Weights and Measures in 1948.
Hot platinum proved awkward to reproduce, so in 1979 the definition switched to radiometry: a source of monochromatic light at 540 terahertz radiating one six-hundred-and-eighty-third of a watt per steradian in a given direction. That frequency, near a wavelength of about 555 nanometres, is green, the colour a daylight-adapted eye picks up best. Other colours need more raw power to look equally bright. The 2018 revision, in force from 20 May 2019, kept the same idea by fixing the number 683 as a constant.
Because the candela measures power per solid angle, focusing changes everything. A 25-watt compact fluorescent bulb emitting around 1,700 lumens evenly in every direction works out near 135 candelas. Squeeze the same output into a 20-degree beam and the intensity inside it soars to roughly 18,000.
Source: Candela
