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The candela ( or ; symbol: cd) is the
base unit
base unit
of
luminous intensity In photometryPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical o ...
in the
International System of Units The International System of Units, known by the international abbreviation SI in all languages and sometimes Pleonasm#Acronyms_and_initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wi ...
(SI); that is, luminous power per unit
solid angle In geometry, a solid angle (symbol: ) is a measure of the amount of the field of view from some particular point that a given object covers. That is, it is a measure of how large the object appears to an observer looking from that point. The poi ...

solid angle
emitted by a point light source in a particular direction. Luminous intensity is analogous to
radiant intensity In radiometry Radiometry is a set of techniques for measurement, measuring electromagnetic radiation, including visible light. Radiometric techniques in optics characterize the distribution of the radiation's power (physics), power in space, as op ...
, but instead of simply adding up the contributions of every
wavelength In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...

wavelength
of light in the source's spectrum, the contribution of each wavelength is
weighted A weight function is a mathematical device used when performing a sum, integral, or average to give some elements more "weight" or influence on the result than other elements in the same set. The result of this application of a weight function is a ...

weighted
by the standard
luminosity function (black) and scotopic (green) luminous efficiency functions. The photopic includes the CIE 1931 standard (solid), the Judd–Vos 1978 modified data (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data (dotted). The horizontal axis is wavel ...
(a model of the sensitivity of the human eye to different wavelengths). A common wax
candle A candle is an ignitable candle wick, wick embedded in wax, or another flammable solid substance such as tallow, that provides light, and in some cases, a Aroma compound, fragrance. A candle can also provide heat or a method of keeping time. ...

candle
emits light with a luminous intensity of roughly one candela. If emission in some directions is blocked by an opaque barrier, the emission would still be approximately one candela in the directions that are not obscured. The word ''candela'' is
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it became ...
for ''candle''. The old name "candle" is still sometimes used, as in ''
foot-candle A foot-candle (sometimes foot candle; abbreviated fc, lm/ft2, or sometimes ft-c) is a non-SI unit of illuminance In photometry, illuminance is the total luminous flux incident on a surface, per unit area Area is the quantity that expresse ...
'' and the modern definition of ''
candlepower Candlepower (abbreviated as cp or CP) is an obsolete unit of measurement for luminous intensity In photometry, luminous intensity is a measure of the wavelength In physics Physics (from grc, φυσική (ἐπιστήμη), physik ...
''.


Definition

The 26th
General Conference on Weights and Measures The General Conference on Weights and Measures (GCWM; french: Conférence Générale des Poids et Mesures, CGPM) is the supreme authority of the International Bureau of Weights and Measures The International Bureau of Weights and Measures (fr ...
(CGPM) redefined the candela in 2018. The new definition, which took effect on 20 May 2019, is:
The candela ..is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz, ''K''cd, to be 683 when expressed in the unit lm W–1, which is equal to , or , where the kilogram, metre and second are defined in terms of '' h'', '' c'' and Δ''ν''Cs.


Explanation

The frequency chosen is in the
visible spectrum The visible spectrum is the portion of the that is to the . in this range of s is called ' or simply . A typical will respond to wavelengths from about 380 to about 750 . In terms of frequency, this corresponds to a band in the vicinity of ...
near
green Green is the between and on the . It is evoked by light which has a of roughly 495570 . In systems, used in painting and color printing, it is created by a combination of yellow and ; in the , used on television and computer screens, ...

green
, corresponding to a wavelength of about 555 nanometres. The
human eye The human eye is a sense organ A sense is a biological system A biological system is a complex biological network, network which connects several biologically relevant entities. Biological organization spans several scales and are determined ...

human eye
, when
adapted In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process that fits organisms to their environment, enhancing their Fitness (biology), evolutionary fitness. Secondly, it is a state reached by the popula ...
for bright conditions, is most sensitive near this frequency. Under these conditions,
photopic vision Photopic vision is the vision of the human eye, eye under well-lit conditions (luminance level 10 to 108 Candela per square metre, cd/m2). In humans and many other animals, photopic vision allows color vision, color perception, mediated by cone cel ...
dominates the visual perception of our eyes over the
scotopic vision Scotopic vision is the vision of the eye under low-light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that can be visual perception, perceived by the human eye. Visible light is usual ...
. 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 :I_\mathrm(\lambda)= 683.002\ \mathrm \cdot \overline(\lambda) \cdot I_\mathrm(\lambda) , where ''I''v(''λ'') is the
luminous intensity In photometryPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical o ...
, ''I''e(''λ'') is the
radiant intensity In radiometry Radiometry is a set of techniques for measurement, measuring electromagnetic radiation, including visible light. Radiometric techniques in optics characterize the distribution of the radiation's power (physics), power in space, as op ...
and \textstyle \overline(\lambda) is the
luminosity function (black) and scotopic (green) luminous efficiency functions. The photopic includes the CIE 1931 standard (solid), the Judd–Vos 1978 modified data (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data (dotted). The horizontal axis is wavel ...
. If more than one wavelength is present (as is usually the case), one must integrate over the
spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum Continuum may refer to: * Continuum (measurement) Continuum theories or models expla ...

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 4''π''
steradian The steradian (symbol: sr) or square radian is the SI unit of solid angle. It is used in three-dimension thumb , 236px , The first four spatial dimensions, represented in a two-dimensional picture. In physics Physics (from gr ...

steradian
s), it will have an intensity of I_\text = \frac \approx 135\ \text = 135\ \text. *Focused into a 20° beam, the same light bulb would have an intensity of around 18,000 cd within the beam.


History

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 Spermaceti (from Greek ''sperma'' meaning "seed", and '' ceti'', the genitive form of "whale") is a wax , a typical wax ester. Image:Beeswax foundation.jpg, Commercial honeycomb foundation, made by pressing beeswax between patterned metal roller ...

spermaceti
candle weighing one sixth of a pound and burning at a rate of 120 
grains A grain is a small, hard, dry – with or without an attached or layer – harvested for human or animal consumption. A grain crop is a grain-producing plant. The two main types of commercial grain crops are s and . After being harvested, dry ...
per hour. Germany, Austria and Scandinavia used the Hefnerkerze, a unit based on the output of a
Hefner lamp 200px, Hefner lamp or in German Hefnerkerze The Hefner lamp, or in German ''Hefnerkerze'', is a flame lamp used in photometry that burns amyl acetate Amyl acetate (pentyl acetate) is an organic compound and an ester with the chemical formula CH3 ...
. It became clear that a better-defined unit was needed.
Jules Violle Jules Louis Gabriel Violle (16 November 1841 – 12 September 1923) was a France, French physicist and inventor. He is notable for having determined the solar constant at Mont Blanc in 1875, and, in 1881, for proposing a standard for luminous ...
had proposed a standard based on the light emitted by 1 cm2 of
platinum Platinum is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical el ...

platinum
at its melting point (or freezing point), calling this the Violle. The light intensity was due to the Planck radiator (a
black body A black body or blackbody is an idealized physical object, physical body that absorption (electromagnetic radiation), absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence (optics), angle of incidence. The ...

black body
) effect, and was thus independent of the construction of the device. This made it easy for anyone to measure the standard, as high-purity platinum was widely available and easily prepared. The '' Commission Internationale de l'Éclairage'' (International Commission on Illumination) and the CIPM proposed a "new candle" based on this basic concept. However, the value of the new unit was chosen to make it similar to the earlier unit candlepower by dividing the Violle by 60. The decision was promulgated by the CIPM in 1946:
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 The square metre ( international spelling as used by the International Bureau of Weights and Measures The International Bureau of Weights and Measures (french: Bureau international des poids et mesures, BIPM) is an intergovernmental organis ...
.
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 square metre of a black body at the temperature of freezing platinum under a pressure of  newtons per square metre.
In 1979, because of the difficulties in realizing a Planck radiator at high temperatures and the new possibilities offered by
radiometry Radiometry is a set of techniques for measuring Measurement is the quantification of attributes of an object or event, which can be used to compare with other objects or events. The scope and application of measurement are dependent on the ...
, the 16th CGPM adopted a new definition of the candela:
The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency and that has a
radiant intensity In radiometry Radiometry is a set of techniques for measurement, measuring electromagnetic radiation, including visible light. Radiometric techniques in optics characterize the distribution of the radiation's power (physics), power in space, as op ...
in that direction of  
watt The watt (symbol: W) is a unit of power Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equa ...

watt
per
steradian The steradian (symbol: sr) or square radian is the SI unit of solid angle. It is used in three-dimension thumb , 236px , The first four spatial dimensions, represented in a two-dimensional picture. In physics Physics (from gr ...

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 arbitrary (1/683) term was chosen so that the new definition would precisely match the old definition. Although the candela is now defined in terms of the
second The second (symbol: s, also abbreviated: sec) is the base unit of time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past, th ...
(an SI base unit) and the watt (a derived SI unit), the candela remains a base unit of the SI system, by definition. The 26th CGPM approved the modern definition of the candela in 2018 as part of the
2019 redefinition of SI base units Effective 20 May 2019, the 144th anniversary of the Metre Convention, the SI base units were redefined in agreement with the International System of Quantities. In the redefinition, four of the seven SI base units – the kilogram, ampere, ...
, which redefined the SI base units in terms of fundamental physical constants.


SI photometric light units


Relationships between luminous intensity, luminous flux, and illuminance

If a source emits a known luminous intensity ''I''v (in candelas) in a well-defined cone, the total
luminous flux In photometryPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical ...

luminous flux
''Φ''v in lumens is given by :''Φ''v = ''I''v − 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 MR16 for emission angles of some common lamps.Online converter
/ref> If the source emits light uniformly in all directions, the flux can be found by multiplying the intensity by 4π: a uniform 1 candela source emits 12.6 lumens. For the purpose of measuring illumination, the candela is not a practical unit, as it only applies to idealized point light sources, each approximated by a source small compared to the distance from which its luminous radiation is measured, also assuming that it is done so in the absence of other light sources. What gets directly measured by a
light meter A light meter is a device used to measure the amount of light. In , a light meter (more correctly an exposure meter) is used to determine the proper for a photograph. The meter will include either a or analog calculator which displays the corr ...
is incident light on a sensor of finite area, i.e.
illuminance In photometryPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical ob ...

illuminance
in lm/m2 (lux). However, if designing illumination from many point light sources, like light bulbs, of known approximate omnidirectionally-uniform intensities, the contributions to illuminance from incoherent light being additive, it is mathematically estimated as follows. If r''i'' is the position of the ''i''-th source of uniform intensity ''Ii'', and â is the unit vector normal to the illuminated elemental opaque area ''dA'' being measured, and provided that all light sources lie in the same half-space divided by the plane of this area, : \text \mathbf\text dA\text E_v(\mathbf) = \sum _. In the case of a single point light source of intensity ''Iv'', at a distance ''r'' and normally incident, this reduces to : E_v(r) = \frac.


See also


References

{{Authority control SI base units UCUM base units Units of luminous intensity