The candela (/kænˈdɛlə/ or /kænˈdiːlə/; symbol: cd) is the
base unit of luminous intensity in the International System of Units
(SI); that is, luminous power per unit solid angle emitted by a point
light source in a particular direction.
1 Definition 2 Explanation
3 Origin 4 SI photometric light units
4.1 Relationships between luminous intensity, luminous flux, and illuminance
5 See also 6 References
Definition Like most other SI base units, the candela has an operational definition—it is defined by a description of a physical process that will produce one candela of luminous intensity. Since the 16th General Conference on Weights and Measures (CGPM) in 1979, the candela has been defined as:
The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 7014540000000000000♠540×1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
The definition describes how to produce a light source that (by definition) emits one candela, but does not specify the luminosity function for weighting radiation at other frequencies. Such a source could then be used to calibrate instruments designed to measure luminous intensity with reference to a specified luminosity function. An appendix to the SI Brochure makes it clear that the luminosity function is not uniquely specified, but must be selected to fully define the candela. The candela is sometimes still called by the old name candle, such as in foot-candle and the modern definition of candlepower. Explanation The frequency chosen is in the visible spectrum near green, corresponding to a wavelength of about 555 nanometres. The human eye, when adapted for bright conditions, is most sensitive near this frequency. At other frequencies, more radiant intensity is required to achieve the same luminous intensity, according to the frequency response of the human eye. The luminous intensity for light of a particular wavelength λ is given by
( λ ) = 683.002
( λ ) ⋅
( λ ) ,
displaystyle I_ mathrm v (lambda )=683.002 mathrm lm/W cdot overline y (lambda )cdot I_ mathrm e (lambda ),
where Iv(λ) is the luminous intensity, Ie(λ) is the radiant intensity and
( λ )
displaystyle textstyle overline y (lambda )
is the photopic luminosity function. If more than one wavelength is present (as is usually the case), one must integrate over the spectrum of wavelengths to get the total luminous intensity. Examples
A common candle emits light with roughly 1 cd luminous intensity. A 25 W compact fluorescent light bulb puts out around 1700 lumens; if that light is radiated equally in all directions (i.e. over 4π steradians), it will have an intensity of
displaystyle I_ text V = frac 1700 text lm 4pi text sr approx 135 text lm/sr =135 text cd
. Focused into a 20° beam, the same light bulb would have an intensity of around 18,000 cd within the beam. The luminous intensity of light-emitting diodes is measured in millicandelas (mcd), or thousandths of a candela. Indicator LEDs are typically in the 50 mcd range; "ultra-bright" LEDs can reach 15,000 mcd (15 cd), or higher.
Prior to 1948, various standards for luminous intensity were in use in
a number of countries. These were typically based on the brightness of
the flame from a "standard candle" of defined composition, or the
brightness of an incandescent filament of specific design. One of the
best-known of these was the English standard of candlepower. One
candlepower was the light produced by a pure spermaceti candle
weighing one sixth of a pound and burning at a rate of 120 grains
per hour. Germany, Austria and Scandinavia used the Hefnerkerze, a
unit based on the output of a Hefner lamp.
It became clear that a better-defined unit was needed. Jules Violle
had proposed a standard based on the light emitted by 1 cm2 of
platinum at its melting point (or freezing point), calling this the
Violle. The light intensity was due to the
The value of the new candle is such that the brightness of the full radiator at the temperature of solidification of platinum is 60 new candles per square centimetre.
It was then ratified in 1948 by the 9th CGPM which adopted a new name for this unit, the candela. In 1967 the 13th CGPM removed the term "new candle" and gave an amended version of the candela definition, specifying the atmospheric pressure applied to the freezing platinum:
The candela is the luminous intensity, in the perpendicular direction, of a surface of 1 / 600 000 square metre of a black body at the temperature of freezing platinum under a pressure of 101 325 newtons per square metre.
In 1979, because of the difficulties in realizing a
SI photometry quantities
v t e
Quantity Unit Dimension Notes
Name Symbol[nb 1] Name Symbol Symbol[nb 2]
Luminous energy Qv [nb 3] lumen second lm⋅s T⋅J The lumen second is sometimes called the talbot.
candela (= lm/sr)
candela per square metre
lux (= lm/m2)
Luminous exposure Hv lux second lx⋅s L−2⋅T⋅J Time-integrated illuminance
Luminous energy density ωv lumen second per cubic metre lm⋅s⋅m−3 L−3⋅T⋅J
Luminous efficacy η [nb 3] lumen per watt lm/W M−1⋅L−2⋅T3⋅J Ratio of luminous flux to radiant flux or power consumption, depending on context
See also: SI · Photometry · Radiometry
^ Standards organizations recommend that photometric quantities be denoted with a suffix "v" (for "visual") to avoid confusion with radiometric or photon quantities. For example: USA Standard Letter Symbols for Illuminating Engineering USAS Z7.1-1967, Y10.18-1967 ^ The symbols in this column denote dimensions; "L", "T" and "J" are for length, time and luminous intensity respectively, not the symbols for the units litre, tesla and joule. ^ a b c Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ or K for luminous efficacy.
Relationships between luminous intensity, luminous flux, and illuminance If a source emits a known luminous intensity Iv (in candelas) in a well-defined cone, the total luminous flux Φv in lumens is given by
Φv = Iv 2π [1 − cos(A/2)],
where A is the radiation angle of the lamp—the full vertex angle of
the emission cone. For example, a lamp that emits 590 cd with a
radiation angle of 40° emits about 224 lumens. See
illuminance at point
displaystyle text illuminance at point mathbf r text on dA text , E_ v (mathbf r )=sum _ i frac mathbf hat a cdot (mathbf r -mathbf r _ i ) mathbf r -mathbf r _ i ^ 3 I_ i .
In the case of a single point light source of intensity Iv, at a distance r and normally incident, this reduces to
( r ) =
displaystyle E_ v (r)= frac I_ v r^ 2 .
Look up candela in Wiktionary, the free dictionary.
Proposed redefinition of SI base units
^ CIE Scotopic luminosity curve (1951)
^ CIE (1931) 2-deg color matching functions
^ Judd–Vos modified CIE 2-deg photopic luminosity curve (1978)
^ a b Sharpe, Stockman, Jagla & Jägle (2005) 2-deg V*(l) luminous
efficiency function Archived 27 September 2007 at the Wayback Machine.
^ Wyzecki, G.; Stiles, W.S. (1982). Color Science: Concepts and
Methods, Quantitative Data and Formulae (2nd ed.). Wiley-Interscience.
^ "Base unit definitions: Candela". The
v t e
International System of Units
ampere candela kelvin kilogram metre mole second
Derived units with special names
becquerel coulomb degree Celsius farad gray henry hertz joule katal lumen lux newton ohm pascal radian siemens sievert steradian tesla volt watt weber
Other accepted units
astronomical unit bar dalton day decibel degree of arc electronvolt hectare hour litre minute minute of arc neper second of arc tonne atomic units natural units
Conversion of units Metric prefixes Proposed redefinitions Systems of measurement