# Candela

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. Luminous intensity is analogous to radiant intensity, but instead of simply adding up the contributions of every wavelength of light in the source's spectrum, the contribution of each wavelength is weighted by the standard luminosity function (a model of the sensitivity of the human eye to different wavelengths).[4][5] A common wax 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 means candle in Latin.

Candela
Photopic (black) and scotopic[1] (green) luminosity functions. The photopic includes the CIE 1931 standard[2] (solid), the Judd–Vos 1978 modified data[3] (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data[4] (dotted). The horizontal axis is wavelength in nm.
General information
Unit systemSI base unit
Unit ofLuminous intensity
Symbolcd
Conversions
1 cd in ...... is equal to ...
international candles   ≈ 1.02 cp
Hefner Kerze   ≈ 1.11 HK

## 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:[6]

The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 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[7] makes it clear that the luminosity function is not uniquely specified, but must be selected to fully define the candela.

In 2018, the 26th General Conference on Weights and Measures approved a redefinition of the candela, which will take effect on May 20, 2019.[8] The new definition of the candela is:

The candela, symbol cd, is the SI unit of luminous intensity in a given direction. It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz, Kcd, to be 683 when expressed in the unit lm W–1, which is equal to cd sr W–1, or cd sr kg–1 m–2 s3, where the kilogram, metre and second are defined in terms of h, c and ΔνCs.

The candela is sometimes still called by the old name candle,[9] 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

${\displaystyle I_{\mathrm {v} }(\lambda )=683.002\ \mathrm {lm/W} \cdot {\overline {y}}(\lambda )\cdot I_{\mathrm {e} }(\lambda ),}$

where Iv(λ) is the luminous intensity, Ie(λ) is the radiant intensity and ${\displaystyle \textstyle {\overline {y}}(\lambda )}$ 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 ${\displaystyle I_{\text{V}}={\frac {1700\ {\text{lm}}}{4\pi \ {\text{sr}}}}\approx 135\ {\text{lm/sr}}=135\ {\text{cd}}}$.
• Focused into a 20° beam, the same light bulb would have an intensity of around 18,000 cd within the beam.

## Origin

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.[10]

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 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 (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.[11]

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.[12]

In 1979, because of the difficulties in realizing a Planck radiator at high temperatures and the new possibilities offered by radiometry, the 16th CGPM adopted the modern definition of the candela.[13] 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 (an SI base unit) and the watt (a derived SI unit), the candela remains a base unit of the SI system, by definition.[14]

## SI photometric light units

SI photometry quantities
Quantity Unit Dimension Notes
Name Symbol[nb 1] Name Symbol Symbol[nb 2]
Luminous energy Qv [nb 3] lumen second lm⋅s TJ The lumen second is sometimes called the talbot.
Luminous flux, luminous power Φv [nb 3] lumen (= candela steradians) lm (= cd⋅sr) J Luminous energy per unit time
Luminous intensity Iv candela (= lumen per steradian) cd (= lm/sr) J Luminous flux per unit solid angle
Luminance Lv candela per square metre cd/m2 L−2J Luminous flux per unit solid angle per unit projected source area. The candela per square metre is sometimes called the nit.
Illuminance Ev lux (= lumen per square metre) lx (= lm/m2) L−2J Luminous flux incident on a surface
Luminous exitance, luminous emittance Mv lux lx L−2J Luminous flux emitted from a surface
Luminous exposure Hv lux second lx⋅s L−2TJ Time-integrated illuminance
Luminous energy density ωv lumen second per cubic metre lm⋅s⋅m−3 L−3TJ
Luminous efficacy η [nb 3] lumen per watt lm/W M−1L−2T3J Ratio of luminous flux to radiant flux or power consumption, depending on context
Luminous efficiency, luminous coefficient V 1 Luminous efficacy normalized by the maximum possible efficacy
1. ^ 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
2. ^ 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.
3. ^ 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 MR16 for emission angles of some common lamps.[15][16]

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 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 ri 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,

${\displaystyle {\text{illuminance at point }}\mathbf {r} {\text{ on }}dA{\text{, }}E_{v}(\mathbf {r} )=\sum _{i}{{\frac {|\mathbf {\hat {a}} \cdot (\mathbf {r} -\mathbf {r} _{i})|}{|\mathbf {r} -\mathbf {r} _{i}|^{3}}}I_{i}}.}$

In the case of a single point light source of intensity Iv, at a distance r and normally incident, this reduces to

${\displaystyle E_{v}(r)={\frac {I_{v}}{r^{2}}}.}$

## References

1. ^ CIE Scotopic luminosity curve (1951)
2. ^ CIE (1931) 2-deg color matching functions
3. ^ Judd–Vos modified CIE 2-deg photopic luminosity curve (1978)
4. ^ a b
5. ^ Wyzecki, G.; Stiles, W.S. (1982). Color Science: Concepts and Methods, Quantitative Data and Formulae (2nd ed.). Wiley-Interscience. ISBN 0-471-02106-7.
6. ^ "Base unit definitions: Candela". The NIST Reference on Constants, Units, and Uncertainty. Retrieved 27 September 2010.
7. ^ "Mise en pratique for the definition of the candela and associated derived units for photometric and radiometric quantities in the International System of Units (SI)" (pdf). SI Brochure Appendix 2. Bureau International des Poids et Mesures. July 2015. Retrieved 2017-12-07.
8. ^ "Convocation of the General Conference on Weights and Measures (26th meeting)" (pdf). Versailles: Bureau International des Poids et Mesures. 13 November 2018.
9. ^ Merriam-Webster
10. ^ "Hefner unit, or Hefner candle". Sizes.com. 30 May 2007. Retrieved 25 February 2009.
11. ^ Barry N. Taylor (1992). The Metric System: The International System of Units (SI). U. S. Department of Commerce. p. 18. ISBN 0-941375-74-9. (NIST Special Publication 330, 1991 ed.)
12. ^ 13th CGPM Resolution 5, CR, 104 (1967), and Metrologia, 4, 43–44 (1968).
13. ^ 16th CGPM Resolution 3, CR, 100 (1979), and Metrologia, 16, 56 (1980).
14. ^ "The photometric base unit – the candela" (pdf). SI Brochure. Bureau International des Poids et Mesures. 7 September 2007.
15. ^ Theory
16. ^ Online converter
2014 Hobart International – Singles

Elena Vesnina was the defending champion, but she retired in the second round against Estrella Cabeza Candela.Garbiñe Muguruza won her first WTA singles title, defeating Klára Zakopalová in the final, 6–4, 6–0.

Candela, Apulia

Candela (Pugliese: Cannéla) is a town and comune in the province of Foggia in the Apulia region of southeast Italy.

Candela, Coahuila

Candela is a city in the central region (Región Centro) of the northern Mexican state of Coahuila. It is the municipal seat of the surrounding municipality of Candela. There were 1,669 inhabitants in 2000. In 2011, according to Wolfram Alpha, it had a population of 1,672.

It was founded in 1690 by General Alonso de León as a mission called San Bernardino de la Candela.

Candela Andújar

Candela Andújar Jiménez (Spanish pronunciation: [kanˈdela anˈduxaɾ]; born 26 March 2000) is a Spanish football forward currently playing for Barcelona B in the Spanish Second Division.

Candela Municipality

Candela is one of the 38 municipalities of Coahuila, in north-eastern Mexico. The municipal seat lies at Candela. The municipality covers an area of 2,305.5 km2.

As of 2005, the municipality had a total population of 1,672.

Candela Peña

María del Pilar Peña Sánchez (born 14 July 1973), better known as Candela Peña, is a Spanish actress.She is the only child of a couple who had a bar in Barcelona. When she was four years old she started to learn dance in the city and after finishing high-school she went to Seville to begin theatre classes there and eventually in Madrid. Encouraged by Pedro Almodóvar, she published the novel Pérez Príncipe, María Dolores in 2001.

She won the Goya Award in 2003 (for the movie Te doy mis ojos) after being nominated several times previously.

Candela per square metre

The candela per square metre (cd/m2) is the derived SI unit of luminance. The unit is based on the candela, the SI unit of luminous intensity, and the square metre, the SI unit of area.

Nit (nt) is a non-SI name also used for this unit (1 nt = 1 cd/m2). The term nit is believed to come from the Latin word nitere, to shine.As a measure of light emitted per unit area, this unit is frequently used to specify the brightness of a display device. The sRGB spec for monitors targets 80 cd/m2. Typically, calibrated monitors should have a brightness of 120 cd/m2. Most consumer desktop liquid crystal displays have luminances of 200 to 300 cd/m2. High-definition televisions range from 450 to about 1500 cd/m2.

Culcha Candela

Culcha Candela is a Dancehall, Hip hop, House and Reggae group from Berlin, Germany. Sources say they formed at different times but in the range of 2001-2003. Their lyrics range from political issues, such as "Una Cosa" or "Schöne, neue Welt" to party songs, such as "Partybus". The name Culcha Candela can be translated into English roughly as "hot" or "bright" culture.

Estrella Cabeza Candela

Estrella Cabeza Candela (Spanish pronunciation: [esˈtɾeʎa kaˈβeθa kanˈdela]; born 20 February 1987) is a Spanish professional tennis player.

Cabeza Candela has won 11 singles and 13 doubles titles on the ITF circuit in her career. On 13 May 2013, she reached her best singles ranking of world number 95. On 2 April 2012, she peaked at world number 176 in the doubles rankings. Her best result in a WTA tournament is semifinals, reached in Hobart 2014.

Félix Candela

Félix Candela Outeriño (Spanish pronunciation: [ˈfeliks kanˈdela outeˈɾiɲo]; January 27, 1910 – December 7, 1997) was a Spanish and Mexican architect who was born in Madrid and at the age of 26, emigrated to Mexico, acquiring double nationality.

He is known for his significant role in the development of Mexican architecture and structural engineering. Candela’s major contribution to architecture was the development of thin shells made out of reinforced concrete, popularly known as cascarones.

He was a teacher of Santiago Calatrava, which has had a great influence on Calatrava's works.

Grinding (dance)

Grinding, (also known as juking, freak dancing or freaking wining) is a type of close partner dance where two or more dancers rub or bump their bodies against each other, most often with a female dancer rubbing her buttocks against a male dancer's crotch area. The male dancer will typically place his hands on the female dancer's waist, hips, or buttocks.Grinding gained widespread popularity as a hip hop dance in night clubs, and eventually moved on to high school and middle school dances in the US and Canada where there have been cases of administrators attempting to ban it due to its explicit nature.A predecessor to grinding as a sexually charged high-contact social dance was "The Bump", popular in the 1970s, in which the contact between partners generally involved the hips or buttocks of one dancer "bumping" those of the other dancer in temporary contact. Other predecessor elements of grinding may be attributed to the 1987 film Dirty Dancing, and the lambada, a brief dance craze of the 1980s that featured grinding actions, as seen in the films The Forbidden Dance and Lambada. A more explicit form of the dance is known as daggering.

Lumen (unit)

The lumen (symbol: lm) is the SI derived unit of luminous flux, a measure of the total quantity of visible light emitted by a source. Luminous flux differs from power (radiant flux) in that radiant flux includes all electromagnetic waves emitted, while luminous flux is weighted according to a model (a "luminosity function") of the human eye's sensitivity to various wavelengths. Lumens are related to lux in that one lux is one lumen per square meter.

The lumen is defined in relation to the candela as

1 lm = 1 cd ⋅ sr.A full sphere has a solid angle of 4π steradians, so a light source that uniformly radiates one candela in all directions has a total luminous flux of 1 cd × 4π sr = 4π cd⋅sr ≈ 12.57 lumens.

Luminance

Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted or reflected from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square metre (cd/m2). A non-SI term for the same unit is the nit. The CGS unit of luminance is the stilb, which is equal to one candela per square centimetre or 10 kcd/m2.

Luminous intensity

In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the human eye. The SI unit of luminous intensity is the candela (cd), an SI base unit.

Photometry deals with the measurement of visible light as perceived by human eyes. The human eye can only see light in the visible spectrum and has different sensitivities to light of different wavelengths within the spectrum. When adapted for bright conditions (photopic vision), the eye is most sensitive to greenish-yellow light at 555 nm. Light with the same radiant intensity at other wavelengths has a lower luminous intensity. The curve which measures the response of the human eye to light is a defined standard, known as the luminosity function. This curve, denoted V(λ) or ${\displaystyle \textstyle {\overline {y}}(\lambda )}$, is based on an average of widely differing experimental data from scientists using different measurement techniques. For instance, the measured responses of the eye to violet light varied by a factor of ten[citation needed] .

Mice Parade is an American indie/shoegaze band from New York City, led by percussionist Adam Pierce. Mice Parade's albums were all initially released on Pierce's record label, Bubble Core Records and FatCat Records. "Mice Parade" is an anagram of Pierce's name.

Current band members are Adam Pierce, Doug Scharin (HiM), Dylan Cristy (The Dylan Group), Rob Laakso (Kurt Vile & the Violators), Caroline Lufkin, Dan Lippel (ICE, New Focus Recordings), Josh McKay (Deerhunter, Macha), and Gunnar Örn Tynes (múm).Their most recent album, Candela, was released on January 29, 2013. Allmusic noted that it "presented Pierce's eclectic, often globe-trotting influences in some of his most accessible songs yet".In December 2017, the band updated on their Facebook page that they are working on an album, have finished several new songs and are looking to return in full form in 2018.

SI base unit

The International System of Units (SI = Systeme Internationale) defines seven units of measure as a basic set from which all other SI units can be derived. The SI base units and their physical quantities are the meter for measurement of length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for temperature, the candela for luminous intensity, and the mole for amount of substance.

The SI base units form a set of mutually independent dimensions as required by dimensional analysis commonly employed in science and technology.

The names and symbols of SI base units are written in lowercase, except the symbols of those named after a person, which are written with an initial capital letter. For example, the metre (US English: meter) has the symbol m, but the kelvin has symbol K, because it is named after Lord Kelvin and the ampere with symbol A is named after André-Marie Ampère.

Several other units, such as the litre (US English: liter), are formally not part of the SI, but are accepted for use with SI.

Vincent Candela

Vincent Philippe Antoine Candela (French pronunciation: ​[vɛ̃sɑ̃ kɑ̃dəˈla]; born 24 October 1973) is a former French footballer. With the French national team, Candela won the 1998 FIFA World Cup, as well as UEFA Euro 2000.

Candela was a quick, offensive-minded, and technically skilled left wingback, who was effective at joining the attack as well as being capable in defence, due to his intelligence and tenacity. Although he preferred playing on the left flank, he was naturally right-footed, and was also capable of playing on the right, both as a full-back, and as a winger, and was an accurate crosser and set-piece taker.

West Edge Tower

2nd & Pike, also known as the West Edge Tower, is a 440-foot-tall (130 m) residential skyscraper in Seattle, Washington. The 39-story tower, developed by Urban Visions and designed by Tom Kundig of Olson Kundig Architects, has 339 luxury apartments and several ground-level retail spaces. The 8th floor includes a bar overlooking the Pike Place Market, located a block west.The project was also known as the Candela Hotel & Residences from 2007 to 2009 and was planned to include a hotel in addition to luxury condominiums. After Candela was removed from the project, the revised skyscraper proposal was approved in 2011 but would later undergo further changes in 2013. Construction did not begin until July 2015 due to financing issues that were later resolved. The building topped out in August 2017 and was completed in March 2018.

Women on the Verge of a Nervous Breakdown

Women on the Verge of a Nervous Breakdown (Spanish: Mujeres al borde de un ataque de nervios) is a 1988 Spanish black comedy-drama film written and directed by Pedro Almodóvar, starring Carmen Maura and Antonio Banderas. The film brought Almodóvar to widespread international attention: it was nominated for the 1988 Academy Award for Best Foreign Language Film, and won five Goya Awards including Best Film and Best Actress in a Leading Role for Maura.

The actual Spanish title refers to an ataque de nervios, which is not actually well translated as "nervous breakdown" (crisis nerviosa). Ataques de nervios are culture-bound psychological phenomena during which the individual, most often female, displays dramatic outpouring of negative emotions, bodily gestures, occasional falling to the ground, and fainting, often in response to receiving disturbing news or witnessing or participating in an upsetting event. Historically, this condition has been associated with hysteria and more recently in the scientific literature with post-traumatic stress and panic attacks.

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