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.

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). Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... Colorful spectrum simulation Source of image- Digitally created by Deborah S Krolls, December 13, 2004 Copyright status- all permissions granted by originator Fair use rationale- not applicable Description of the image-Spectral Simulation Other versions- none Relevant links (internal and external)- Spectral Simulation copyright Permission is granted by artist, Deborah... Colorful spectrum simulation Source of image- Digitally created by Deborah S Krolls, December 13, 2004 Copyright status- all permissions granted by originator Fair use rationale- not applicable Description of the image-Spectral Simulation Other versions- none Relevant links (internal and external)- Spectral Simulation copyright Permission is granted by artist, Deborah... Candela may refer to one of the following. ... The SI system of units defines seven SI base units: physical units defined by an operational definition. ... Luminous intensity is a measure of the energy emitted by a light source in a particular direction. ... The luminosity function is a standard function established by the Commission Internationale de lÉclairage to account for the variable sensitivity of the human eye to radiation at different wavelengths. ...

Definition

Since the 16th General Conference on Weights and Measures in 1979, the candela has been defined as follows: The General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures (CGPM, never GCWM). ...

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 hertz (symbol: Hz) is the SI unit of frequency. ... In physics, intensity is a measure of the time-averaged energy flux. ... The watt (symbol: W) is the SI derived unit of power, equal to one joule per second. ... The steradian (ste from Greek stereos, solid) is the SI derived unit of solid angle, and the 3-dimensional equivalent of the radian. ...

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 The optical spectrum (light or visible spectrum) is the portion of the electromagnetic spectrum that is visible to the human eye. ... Mossy, green fountain in Wattens, Austria. ... The wavelength is the distance between repeating units of a wave pattern. ... This article refers to the sight organ. ... In ocular physiology, adaptation is the ability of the eye to adjust to various levels of darkness and light. ...

where I_v(λ) is the luminous intensity in candelas, I(λ) is the radiant intensity in W/sr and 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. The luminosity function is a standard function established by the Commission Internationale de lÉclairage to account for the variable sensitivity of the human eye to radiation at different wavelengths. ... In most modern usages of the word spectrum, there is a unifying theme of between extremes at either end. ...

A common candle emits roughly 1 cd. A 100 W incandescent lightbulb emits about 120 cd. A close-up image of a candle showing the wick and the various regions of the flame. ... The incandescent light bulb uses a glowing wire filament heated to white-hot by electrical resistance, to generate light (a process known as thermal radiation). ...

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. 1948 (MCMXLVIII) was a leap year starting on Thursday (the link is to a full 1948 calendar). ...

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 International Commission on Illumination (usually known as the CIE for its French-language name Commission internationale de léclairage) is the international authority on light, illumination, colour, and colour spaces. ... As the temperature decreases, the peak of the black body radiation curve moves to lower intensities and longer wavelengths. ... General Name, Symbol, Number platinum, Pt, 78 Chemical series transition metals Group, Period, Block 10, 6, d Appearance grayish white Standard atomic weight 195. ... 1946 (MCMXLVI) was a common year starting on Tuesday. ...

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. cm redirects here, alternate uses: cm (disambiguation) A centimetre (symbol cm; American spelling: centimeter) is an SI unit of length. ...

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: 1967 (MCMLXVII) was a common year starting on Sunday of the Gregorian calendar (the link is to a full 1967 calendar). ...

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.[5] For the song by the Smashing Pumpkins, see 1979 (song). ... In telecommunication and physics, radiometry is the science of radiation measurement. ... The SI system of units defines seven SI base units: physical units defined by an operational definition. ...

SI photometric light units

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Look up si, Si, SI in Wiktionary, the free dictionary. ... luminous energy : visible radiant energy, visible light ; units are lumen*seconds or Talbots (T = lm*s) ; lumen = candela*steradian ... The lumen (symbol: lm) is the SI unit of luminous flux. ... Look up second in Wiktionary, the free dictionary. ... The lumen (symbol: lm) is the SI unit of luminous flux. ... Look up second in Wiktionary, the free dictionary. ... In photometry the Talbot (T) is a nonstandard unit of luminous energy. ... Luminous flux is a measure of the energy emitted by a light source in all directions. ... The lumen (symbol: lm) is the SI unit of luminous flux. ... The steradian (ste from Greek stereos, solid) is the SI derived unit of solid angle, and the 3-dimensional equivalent of the radian. ... The lumen (symbol: lm) is the SI unit of luminous flux. ... Luminous intensity is a measure of the energy emitted by a light source in a particular direction. ... The lumen (symbol: lm) is the SI unit of luminous flux. ... The steradian (ste from Greek stereos, solid) is the SI derived unit of solid angle, and the 3-dimensional equivalent of the radian. ... The SI system of units defines seven SI base units: physical units defined by an operational definition. ... Luminance (also called luminosity) is a photometric measure of the density of luminous intensity in a given direction. ... A square metre (US spelling: square meter) is by definition the area enclosed by a square with sides each 1 metre long. ... A square metre (US spelling: square meter) is by definition the area enclosed by a square with sides each 1 metre long. ... This article is about the unit of measure. ... Illuminance is the total luminous flux incident per unit area. ... The lux (symbol: lx) is the SI derived unit of illuminance or illumination. ... The lumen (symbol: lm) is the SI unit of luminous flux. ... A square metre (US spelling: square meter) is by definition the area enclosed by a square with sides each 1 metre long. ... The lux (symbol: lx) is the SI derived unit of illuminance or illumination. ... In photometry, illuminance is the total luminous flux incident on a surface, per unit area. ... The lux (symbol: lx) is the SI derived unit of illuminance or illumination. ... The lumen (symbol: lm) is the SI unit of luminous flux. ... A square metre (US spelling: square meter) is by definition the area enclosed by a square with sides each 1 metre long. ... The lux (symbol: lx) is the SI derived unit of illuminance or illumination. ... Luminous efficacy is a property of light sources, which indicates what portion of the emitted electromagnetic radiation is usable for human vision. ... The lumen (symbol: lm) is the SI unit of luminous flux. ... The watt (symbol: W) is the SI derived unit of power, equal to one joule per second. ... The lumen (symbol: lm) is the SI unit of luminous flux. ... The watt (symbol: W) is the SI derived unit of power, equal to one joule per second. ... Luminous flux is a measure of the energy emitted by a light source in all directions. ... Luminous flux or luminous power is the measure of the perceived power of light. ...

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.

For the Manfred Mann album, see 2006 (album). ... February 8 is the 39th day of the year in the Gregorian calendar. ... For the Manfred Mann album, see 2006 (album). ... February 8 is the 39th day of the year in the Gregorian calendar. ...

External links

• Candela: The SI Unit of Luminous Intensity