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candela

(kăn-dĕl'ə) pronunciation

n. (Abbr. cd)

A unit of luminous intensity equal to ¹/₆₀ of the luminous intensity per square centimeter of a blackbody radiating at the temperature of solidification of platinum (2,046°K). Also called candle.

[Latin candēla, candle. See candle.]

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Computer Desktop Encyclopedia: candela

A unit of measurement of the intensity of light. Part of the SI system of measurement, one candela (cd) is the monochromatic radiation of 540THz with a radiant intensity of 1/683 watt per steradian in the same direction. Another way of putting it is that an ordinary wax candle generates approximately one candela. See SI, lumen and nit.

Measures and Units: candela

[Etymology: candle] luminous intensity. Symbol cd. (Metric) The base photic unit.

SI, Metric-m.k.s.A. 1979 The intensity, in a given direction, of a source that emits monochromatic radiation of a frequency 540 THz and that has a radiant intensity in that direction of ¹/₆₈₃ W·Sr^-1 (1.11~ hefner, 0.981~ international candle). Hence:
cd·sr = lumen for luminous flux.

Metric-m.k.s. Defined, as discussed below, on a standard candle.

Metric-c.g.s. Defined, as discussed below, on a standard candle. The following are among the coherent derived units:
cd·cm^-2 = stilb for luminance;
cd·sr·cm^-2 = phot for illuminance.

History

The candela is a rationalization of a variety of units originally based on standard candles, and called candle. The pioneer was the British candle of 1860, defined by the Metropolitan Gas Act for gaslights in London;
[Proc. Roy. Soc. London Ser. A Vol. 186, 204-7 (1946)] it was recognized internationally in 1881, in an electrotechnical context, specified as a spermacetti candle of ⅙ lb burning at 2 grains per minute, but without mention of wick or atmosphere. Replaced by a pentane lamp in 1898, it was agreed internationally in 1909, with the term international candle. Equivalent approximate values of other prior candles, all of which were rather roughly measured, include:

France	bougie-decimale	0.97 international candles
	carcel	9.7 international candles
USA	candle	1.06 international candles
Germany	hefner candle	0.903 international candles

the last continuing in use as a national standard for a considerable time.

The differing sensitivity of the eye to different wavelengths caused trouble in progressing from those lamps running at a temperature just below 2 000 K to electric tungsten filaments above 2 800 K. By 1924 appropriate conversion tables were agreed, and used until the adoption, in 1946, of a specification based on a black body set at 2 042 K, the freezing point of platinum (see below). Because of the round figure adopted for the area in the definition, this new candle was 1.9~ % less than its predecessor. The name candela followed in 1948, and a restatement of the black-body definition in 1967. However, results with the specification were proving to be too inconsistent, while related radiometric techniques offered superior precision and ease of realization. These circumstances led to the re-definition of 1979.

1946	CIPM: ‘4. The photometric units may be defined as follows:
	New candle (unit of luminous intensity) - 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’.
1948	9th CGPM ‘candela’ adopted in place of (new) candle.
1967-68	13th CGPM: ‘decides to express the definition of the candela as follows:
	The candela is the luminous intensity, in the perpendicular direction, of a surface of 1/600000 square metre of a black body at the temperature of freezing platinum under a pressure of 101 325 newtons per square metre’.
1979	16th CGPM: ‘decides
	1. The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of a frequency 540 × 10¹² hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. 2. The definition of the candela (at the time called new candle)…is abrogated.’see note below

[Le Système International d'Unités (Sèvres, France: Bureau International de Poids et Mesures, 1985)]

Architecture: candela

The International Standard unit of luminous intensity; closely approximates the formerly accepted unit known as the “international candle.”

Columbia Encyclopedia: candle,

in weights and measures, unit of luminous intensity; it is defined as ¹/₆₀ of the intensity of a black body, or ideal radiator, at the temperature at which platinum solidifies (2,046K). The candle is one of the fundamental units of the International System of Units; its official name is the candela. See photometry.

Veterinary Dictionary: candela

The SI unit of luminous intensity. Abbreviated cd.

Wikipedia: 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.

This article is about the unit of luminous intensity. For other uses, see Candela (disambiguation).

The candela (symbol: cd) is the SI base unit of luminous intensity (that is, power emitted by a light source in a particular direction, with wavelengths weighted by the luminosity function, a standardized model of the sensitivity of the human eye).

Definition

Since the 16th General Conference on Weights and Measures in 1979, the candela has been defined as follows:

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.

Note: The candela is sometimes still called by the old name candle [5], 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 nanometers. The human eye is most sensitive to this frequency, when adapted for bright conditions. 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

$I_v(\lambda)= 683.002\,\overline{y}(\lambda) I(\lambda)$

where $I v (λ)$ is the luminous intensity in candelas, $I (λ)$ is the radiant intensity in W/sr and $\overline{y}(\lambda)$ is the standard luminosity function. If more than one wavelength is present (as is usually the case), one must sum or integrate over the spectrum of wavelengths present to get the total luminous intensity.

A common candle emits roughly 1 cd. A 100 W incandescent lightbulb emits about 120 cd.

Origin

Prior to 1948, there existed a variety of standards for luminous intensity in use in various 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 standards was the candlepower.

It became clear that a better-defined unit was needed. The Commission internationale de l'éclairage (International Commission on Illumination) and the CIPM proposed a “new candle” based on the luminance of a Planck radiator (a black body) at the temperature of freezing platinum. The value of the new unit was chosen to make it similar to the earlier unit candlepower. 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.

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 (Resolution 5, CR, 104 and Metrologia, 1968, 4, 43-44) 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 Planck radiator at high temperatures, and the new possibilities offered by radiometry, 16th CGPM (1979, Resolution 3; CR, 100 and Metrologia, 1980, 16, 56) adopted the modern definition of the candela. The arbitrary (1/683) term was chosen so that the new definition would exactly match the old definition. Although the candela is now defined partly in terms of the watt, which is a derived SI unit of power, the candela remains a base unit of the SI system, by definition.[6]

SI photometric light units

SI photometry units
Quantity	Symbol	SI unit	Abbr.	Notes
Luminous energy	Q_v	lumen second	lm·s	units are sometimes called talbots
Luminous flux	F	lumen (= cd·sr)	lm	also called luminous power
Luminous intensity	I_v	candela (= lm/sr)	cd	an SI base unit
Luminance	L_v	candela per square metre	cd·m^–2	units are sometimes called nits
Illuminance	E_v	lux (= lm·m^–2)	lx	Used for light incident on a surface
Luminous emittance	M_v	lux (= lm·m^–2)	lx	Used for light emitted from a surface
Luminous efficacy	- - - -	lumen per watt	lm/W	ratio of luminous flux to radiant flux; maximum possible is 683.002
Units-interrelation diagram (full page)

References

The Unit of Luminous Intensity: Candela (cd). Retrieved on 2006-02-08.
Base unit definitions: Candela. The NIST Reference on Constants, Units, and Uncertainty. Retrieved on 2006-02-08.

External links

Candela: The SI Unit of Luminous Intensity

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

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Translations: Translations for: Candela

Dansk (Danish)
n. - enhed for lysstyrke

Français (French)
n. - (Phys, Mes) candela

Deutsch (German)
n. - (Optik) Kandela (int. Einheit zur Messung der Lichtintensität

Ελληνική (Greek)
n. - (φυσ.) (νέο) κηρίον

Español (Spanish)
n. - candela (unidad luminosa)

Svenska (Swedish)
n. - candela

中文（简体） (Chinese (Simplified))
烛光

中文（繁體） (Chinese (Traditional))
n. - 燭光

한국어 (Korean)
n. - 칸델라 (광도의 단위)

العربيه (Arabic)
‏(الاسم) شمعه, وحدة اضاءة‏

עברית (Hebrew)
n. - ‮יחידת-המידה התקנית הבינלאומית לעוצמת אור‬

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		Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2007. Published by Houghton Mifflin Company. All rights reserved. Read more
		Computer Desktop Encyclopedia. THIS COPYRIGHTED DEFINITION IS FOR PERSONAL USE ONLY. All other reproduction is strictly prohibited without permission from the publisher. © 1981-2008 Computer Language Company Inc. All rights reserved. Read more
		Measures and Units. A Dictionary of Weights, Measures, and Units. Copyright © Donald Fenna 2002, 2004. All rights reserved. Read more
		Architecture. McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc. All rights reserved. Read more
		Columbia Encyclopedia. The Columbia Electronic Encyclopedia, Sixth Edition Copyright © 2003, Columbia University Press. Licensed from Columbia University Press. All rights reserved. www.cc.columbia.edu/cu/cup/ Read more
		Veterinary Dictionary. Saunders Comprehensive Veterinary Dictionary 3rd Edition. Copyright © 2007 by D.C. Blood, V.P. Studdert and C.C. Gay, Elsevier. All rights reserved. Read more
		Wikipedia. This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Candela". Read more
		Translations. Copyright © 2007, WizCom Technologies Ltd. All rights reserved. Read more

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