Candela
SI base unit · quantity: luminous intensity
01 · Definition
The other six base units describe the world as it is. The candela describes the world as the human eye sees it — and that is written into the definition itself.
One candela is the luminous intensity of a source that, in a given direction, emits monochromatic radiation of frequency 540·10¹² hertz and has a radiant intensity in that direction of 1/683 watt per steradian. Every other wavelength is weighted by the luminosity function V(λ) — the averaged sensitivity of the human eye.
The candela says how much light goes in one direction, the lumen how much goes in every direction together. An isotropic source of 1 cd gives 4π ≈ 12.57 lumens; a narrow spotlight of the same 1 cd gives a fraction of a lumen.
02 · Conversion
Enter a luminous intensity — the passport will convert it
The historical candles — from the Hefner to the Violle — differ from the candela by whole factors, not by decimal places: every country kept its own flame, and every flame burned in its own way.
Screen brightness is measured in candelas per square metre: 300 cd/m² on an office monitor, 1000 cd/m² on an HDR television. That is luminance, not luminous intensity — intensity divided by the area of the emitting surface.
| Unit | Name | Value | Where it appears |
|---|---|---|---|
| mcd | millicandela | 130 000 | indicator LEDs, backlit buttons |
| cd | candela | 130 | the base notation of luminous intensity in SI |
| kcd | kilocandela | 0.13 | headlights, traffic lights, floodlights |
| Mcd | megacandela | 1.3·10⁻⁴ | lighthouses, searchlights |
| Gcd | gigacandela | 1.3·10⁻⁷ | sky beams, pulsed sources |
Φ = I · Ω: luminous intensity times solid angle. The prefixes are decimal; cd/m² is already luminance, not intensity.
03 · Orders of magnitude
from an indicator to a lighthouse beamA candle: one candela
04 · Measuring instruments
What candelas are measured with
An Ulbricht sphere collects the whole luminous flux of a lamp: it measures lumens, from which the mean luminous intensity is worked out.
It traces the source through every angle and plots the intensity distribution. This is how headlights, traffic lights and floodlights are certified.
A photodiode with a filter matched to the luminosity function. It measures illuminance in lux — a workplace, a street, a greenhouse.
Three silicon photodiodes in a trap absorb almost all the light: the primary standard that ties the candela to the watt.
05 · Writing rules
Both letters lowercase: cd, because it is the Latin for candle
The candela is named not after a person but after the Latin candela — a candle — so the symbol is lowercase. A capital «Cd» is the chemical element cadmium. The unit name takes no capital either, and the plural is «candelas» while the symbol stays cd.
06 · Neighbouring units
The candela is the starting point of lighting engineering: multiply by a steradian and you get the lumen; divide the lumen by an area and you get the lux; divide the candela by the area of the emitting surface and you get luminance in cd/m².
The inverse-square law: a 100 cd source gives 100 lx at one metre and only 4 lx at five.
07 · Historical section
When the unit of light was a real candle
A lamp burning amyl acetate with a wick of prescribed size. A standard you had to light and then wait for the flame to settle.
An agreed average of the national flame standards. The direct predecessor of the candela: in 1948 it was simply renamed and made more precise.
Carcel's oil lamp with a clockwork pump: it burned 42 grams of rapeseed oil an hour and shone almost as brightly as ten candles.
The first standard independent of any fuel: one square centimetre of molten platinum at its freezing temperature.
The only SI unit with the human eye written into its definition
The metre, the kilogram and the second do not depend on an observer. The candela does: the factor 683 lm/W and the luminosity function V(λ) come from averaged measurements on people in the 1920s. Photometry therefore has two parallel worlds — the radiometric one, in watts, which is the same for everyone, and the photometric one, in lumens, which is calibrated to our retina. Change the observer and the numbers change too: for night vision the peak shifts to 507 nm, and a separate scotopic curve is used.
Catalogue · The International System of Units
A passport for every quantity
Seven base units, twenty-two derived units with names of their own, and an archive of systems withdrawn from use. Each is a sheet of its own: definition, dimension, conversion, instruments, writing rules. In 36 languages.
Base units
open the candela's passportDerived units with names of their own
in terracotta — those whose definition contains the candelaHistorical systems
archive of notationsPassport language
36 languages. The symbol cd and the formulas are not translated — they are identical in every locale; what is translated is the description, the examples and the writing rules for each script.