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The candela ( or ; symbol: cd) is the unit of luminous intensity in the
International System of Units The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and the world's most widely used system of measurement. ...
(SI). It measures luminous power per unit solid angle emitted by a light source in a particular direction. Luminous intensity is analogous to radiant intensity, but instead of simply adding up the contributions of every
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...
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 ...
by the standard luminosity function (a model of the sensitivity of the human eye to different wavelengths). A common wax
candle A candle is an ignitable wick embedded in wax, or another flammable solid substance such as tallow, that provides light, and in some cases, a fragrance. A candle can also provide heat or a method of keeping time. A person who makes 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 a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
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 or light intensity. The foot-candle is defined as one lumen per square foot. This unit is commonly used in lighting layouts in par ...
'' and the modern definition of ''
candlepower Candlepower (abbreviated as cp or CP) is a unit of measurement for luminous intensity. It expresses levels of light intensity relative to the light emitted by a candle of specific size and constituents. The historical candlepower is equal to 0.981 ...
''.


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 (BIPM), the intergovernmental organization established i ...
(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 electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called '' visible light'' or simply light. A typical human eye will respond to ...
near
green Green is the color between cyan and yellow on the visible spectrum. It is evoked by light which has a dominant wavelength of roughly 495570 nm. In subtractive color systems, used in painting and color printing, it is created by a combin ...
, corresponding to a wavelength of about 555 nanometres. The
human eye The human eye is a sensory organ, part of the sensory nervous system, that reacts to visible light and allows humans to use visual information for various purposes including seeing things, keeping balance, and maintaining circadian rhythm. ...
, when adapted for bright conditions, is most sensitive near this frequency. Under these conditions, photopic vision dominates the visual perception of our eyes over the scotopic vision. 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, ''I''e(''λ'') is the radiant intensity and \textstyle \overline(\lambda) is the photopic luminosity function. 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. The word was first used scientifically in optics to describe the rainbow of colors ...
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 A compact fluorescent lamp (CFL), also called compact fluorescent light, energy-saving light and compact fluorescent tube, is a fluorescent lamp designed to replace an incandescent light bulb; some types fit into light fixtures designed for in ...
puts out around 1700  lumens; if that light is radiated equally in all directions (i.e. over 4 steradians), it will have an intensity of I_\text = \frac \approx 135\ \text/\text = 135\ \text. *Focused into a 20° beam (0.095 steradians), 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 is a waxy substance found in the head cavities of the sperm whale (and, in smaller quantities, in the oils of other whales). Spermaceti is created in the spermaceti organ inside the whale's head. This organ may contain as much as of ...
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 The Hefner lamp, or in German ''Hefnerkerze'', is a flame lamp used in photometry that burns amyl acetate. The lamp was invented by Friedrich von Hefner-Alteneck in 1884 and he proposed its use as a standard flame for photometric purposes with a ...
. 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 Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver". Pla ...
at its melting point (or freezing point), calling this the Violle. The light intensity was due to the Planck radiator (a 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 The International Commission on Illumination (usually abbreviated CIE for its French name, Commission internationale de l'éclairage) is the international authority on light, illumination, colour, and colour spaces. It was established in 1913 ...
'' (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) or square meter (American spelling) is the unit of area in the International System of Units (SI) with symbol m2. It is the area of a square ...
.
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 electromagnetic radiation, including visible light. Radiometric techniques in optics characterize the distribution of the radiation's power in space, as opposed to photometric techniques, which ch ...
, 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 that direction of  
watt The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James ...
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 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) is the unit of time in the International System of Units (SI), historically defined as of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds ea ...
(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, 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 photometry, luminous flux or luminous power is the measure of the perceived power of light. It differs from radiant flux, the measure of the total power of electromagnetic radiation (including infrared, ultraviolet, and visible light), in tha ...
''Φ''v in lumens is given by :''Φ''''v'' = ''I''''v'' 2  − 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 A multifaceted reflector (often abbreviated MR) light bulb is a reflector housing format for halogen as well as some LED and fluorescent lamps. MR lamps were originally designed for use in slide projectors, but see use in residential lighting a ...
for emission angles of some common lamps. If the source emits light uniformly in all directions, the flux can be found by multiplying the intensity by 4: 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 photography, a light meter (more correctly an exposure meter) is used to determine the proper exposure (photography), exposure for a photograph. The meter will include either a Di ...
is incident light on a sensor of finite area, i.e. 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_\mathrm 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_ \mathrm v(r) = \frac.


SI multiples

Like other SI units, the candela can also be modified by adding a
metric prefix A metric prefix is a unit prefix that precedes a basic unit of measure to indicate a multiple or submultiple of the unit. All metric prefixes used today are decadic. Each prefix has a unique symbol that is prepended to any unit symbol. The pr ...
that multiplies it by a
power of 10 A power of 10 is any of the integer powers of the number ten; in other words, ten multiplied by itself a certain number of times (when the power is a positive integer). By definition, the number one is a power (the zeroth power) of ten. The fi ...
, for example millicandela (mcd) for 10−3 candela.


References

{{Authority control SI base units Units of luminous intensity