A candle burns, its flame flickers, its halo breathes. Wax runs down the candle. A cone of light goes off to the right and cuts a solid angle of one steradian out of a sphere. A green 540 THz wave runs along the top. Below, a light-meter mark slides along a rail: 0.25 lx at 2 m, 1 lx at 1 m, 4 lx at 0.5 m. A recording light blinks.
Quantity data sheet
SI base unitrev. 2026-10Candela cd
SI base unit · quantity: luminous intensity — the light a source sends in one direction
The unit that was once a candle
In 1860 the gas companies of London argued over whose gas burned brighter. To compare the lamps, Parliament approved a standard: a spermaceti candle weighing a sixth of a pound that burns at 120 grains an hour. The brightness of a gas burner came to be stated in «candles».
A candle is a poor standard: the flame flickers, the wick chars, the wax varies. In time the candle was replaced by platinum heated to its melting point, and later by a number. Today the candela is the luminous intensity of a source that emits 1/683 watt per steradian at a frequency of 540 THz. An ordinary candle still shines at about one candela.
The 540 THz frequency was not chosen at random. It is green light with a wavelength of about 555 nm, the colour the eye is most sensitive to. That is why the candela is the only SI base unit with a human built in: it measures not the energy of light, but how bright it looks.
The candela symbol is two lowercase letters cd, with no full stop. Capital Cd is cadmium. The quantity is written as a capital I with a subscript v — luminous intensity. The unit name is lowercase: candela.
Luminous intensity and illuminance. The candela says how brightly a source shines in one direction. The lux says how much light has fallen on a surface. The same candle gives 1 lx at a distance of one metre and 0.01 lx at ten.
01 · Definition
steradian · lumen · eyeA source sends light in all directions, but not always equally. A torch shines forward and hardly at all backward. So luminous intensity is counted for each direction separately: how much light goes into a small cone around that direction.
The size of the cone is measured as a solid angle in steradians. One steradian cuts a patch of 1 m² out of a sphere of radius 1 m. Luminous flux in lumens divided by the solid angle is luminous intensity. One candela is one lumen per steradian.
The lumen counts light with a correction for the eye. Radiant power is multiplied by the sensitivity curve V(λ): it equals one at 555 nm and falls towards the violet and red ends. A red laser and a green one of the same power differ for the eye several times over, and so in candelas too.
How much light reaches the sheet
At the top is a dark room with a bench. On the left is a source, on the right a white screen with a light meter. Move the screen and the illuminance changes with the inverse square of the distance. Choose a source: a candle, an incandescent lamp, an LED torch with a narrow beam, a lighthouse. On the torch you can narrow the beam: the flux stays the same, the luminous intensity rises. The wavelength slider makes the source coloured at the same radiant power. Amber shows light, green the eye and the V(λ) curve, violet the angles and instruments. Below are three views. «Distance» is illuminance against r. «Eye» is the V(λ) curve and the current wavelength. «Diagram» is luminous intensity by direction.
Light gathered into a narrow cone
A 300 lm torch is weaker than a 1700 lm lamp in flux. But the lamp shines in all directions, while the torch gathers its light into a cone 10° wide. The solid angle of such a cone is 0.024 sr, and the luminous intensity at the beam centre comes out above 10 000 cd. That is why candelas are printed on the torch and lumens on the lamp box: different things, and both honest.
02 · Conversion
units · beam · historic candlesFrom candelas to lumens, lux and old candles
Enter a luminous intensity or a flux. The first tab shows the illuminance at different distances and the flux of a uniform source. The second converts a torch’s flux into candelas for different beam widths. The third gives historic «candles» in candelas.
| {k} | {v} | {note} |
In technology, luminous intensity varies by a factor of billions: from a thousandth of a candela on a panel indicator to tens of millions in a lighthouse. The eye copes with such a range because it sees not candelas but their logarithm: a source ten times stronger looks only two or three times brighter.
04 · Measuring instruments
Bunsen photometer · goniophotometer · radiometer · light meterLight is compared with light
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05 · Writing rules
cd lowercase · no full stopcd — two lowercase letters
The candela symbol is written in lowercase, with a space after the number. Multiples take prefixes: kcd, mcd. Luminous intensity is not confused with flux or illuminance: each has its own unit. For torches and headlamps, the intensity at the beam centre is given.
Cd is cadmium, and no full stop follows the symbol. A headlamp’s lumens are its flux, not the intensity in the beam. A candela is a lumen per steradian, not just a lumen. Illuminance is measured in lux. The unit name is lowercase, and the «candle» is an obsolete unit not used in the SI.
06 · Neighbouring units
photometry1 lm = 1 cd·sr — the whole flux.
lx1 lx = 1 lm/m² — light on a surface.
cd/m²Luminous intensity per square metre of a glowing surface.
07 · Historical section
archive · from the spermaceti candle to 683 lm/WHow a candle became a number
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The candela rests on an average eye
The V(λ) curve is not the eye of a particular person but an average from experiments in the 1920s. Observers were asked to compare the brightness of two coloured fields, and from their answers a table was derived, which the International Commission on Illumination adopted in 1924. Each person has their own curve, and in people with colour-vision deficiency it is noticeably shifted.
So an instrument that measures candelas first measures watts and then converts them using the table. A photometer with a filter that matches V(λ) works well for white light. For blue LEDs and narrow lines the filter errs more, and such sources are checked with a spectroradiometer.
On the right are two curves. Green is daylight vision with a peak at 555 nm, blue is night, twilight vision with a peak at 507 nm. In the dark, blue looks brighter than red, although candelas do not take this into account.
Catalogue of quantities
Light and photometry · 12 unit sheets
Index · all quantities on one pageThe lab at the entrance to the indexA drop of acid changes the colour of the juice, salt colours the flame, water freezes at a tap. Below them are all Simetrium sheets, by discipline and alphabetically.Open the index →
