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| {{See also|Light intensity (disambiguation)}}
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| In [[Photometry (optics)|photometry]], '''luminous intensity''' is a measure of the [[wavelength]]-weighted [[Power (physics)|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]].
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| 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 <math>\textstyle \overline{y}(\lambda)</math>, 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.
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| ==Relationship to other measures==
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| Luminous intensity should not be confused with another photometric unit, [[luminous flux]], which is the total perceived power emitted in all directions. Luminous intensity is the perceived power ''per unit solid angle''. If a lamp has a 1 lumen bulb and the optics of the lamp are set up to focus the light evenly into a 1 [[steradian]] beam, then the beam would have a luminous intensity of 1 candela. If the optics were changed to concentrate the beam into 1/2 steradian then the source would have a luminous intensity of 2 candela. The resulting beam is narrower and brighter, though its luminous flux remains unchanged.
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| Luminous intensity is also not the same as the [[radiant intensity]], the corresponding objective [[physical quantity]] used in the measurement science of [[radiometry]].
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| ==Units==
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| Like other SI base units, the candela has an [[operational definition]]—it is defined by the description of a physical process that will produce one candela of luminous intensity. By definition, if one constructs a light source that emits monochromatic green light with a frequency of 540 THz, and that has a radiant intensity of 1/683 [[watt]]s per [[steradian]] in a given direction, that light source will emit one candela in the specified direction.<ref>{{Cite web
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| | title = Base unit definitions: Candela
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| | work = The NIST Reference on Constants, Units, and Uncertainty
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| | url = http://physics.nist.gov/cuu/Units/candela.html
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| | accessdate = 8 February 2008
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| }}</ref>
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| The frequency of light used in the definition corresponds to a wavelength of 555 nm, which is near the peak of the eye's response to light. If the source emitted uniformly in all directions, the total [[radiant flux]] would be about 18.40 mW, since there are 4π steradians in a sphere. A typical candle produces very roughly one candela of luminous intensity.
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| Prior to the definition of the candela, variety of units for luminous intensity were used 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 [[England|English]] standard: [[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 [[Grain (mass)|grains]] per hour. Germany, Austria, and Scandinavia used the [[Hefnerkerze]], a unit based on the output of a [[Hefner lamp]].<ref>{{Cite web|url=http://www.sizes.com/units/hefner.htm |title=Hefner unit, or Hefner candle |work=Sizes.com |date=30 May 2007 |accessdate=25 February 2009}}</ref> In 1881, [[Jules Violle]] proposed the ''[[Violle]]'' as a unit of luminous intensity, and it was notable as the first unit of light intensity that did not depend on the properties of a particular lamp. All of these units were superseded by the definition of the candela.
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| ==Usage== | |
| The luminous intensity for monochromatic light of a particular wavelength λ is given by
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| :<math>I_\mathrm{v} = 683 \cdot \overline{y}(\lambda) \cdot I_\mathrm{e},</math>
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| where
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| :''I''<sub>v</sub> is the luminous intensity in candelas (cd), | |
| :''I''<sub>e</sub> is the radiant intensity in watts per steradian (W/sr),
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| :<math>\textstyle \overline{y}(\lambda)</math> is the [[Luminosity function|standard luminosity function]].
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| 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 luminous intensity:
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| <!-- TBD: Is integral from 0 to infinity really correct, or should it better be 380nm to 780nm here? -->
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| :<math>I_\mathrm{v} = 683 \int^\infin_0 \overline{y}(\lambda) \cdot \frac{dI_\mathrm{e}(\lambda)}{d\lambda} \, d\lambda.</math>
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| ==See also==
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| *[[Radiance]]
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| *[[Brightness]]
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| ==References==
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| <references/>
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| {{SI_light_units}}
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| {{Use dmy dates|date=September 2010}}
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| <!--Category-->
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| {{DEFAULTSORT:Luminous Intensity}}
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| [[Category:Physical quantities]]
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| [[Category:Photometry]]
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Hello, I'm Joanne, a 23 year old from Richey, United States.
My hobbies include (but are not limited to) Poker, Vehicle restoration and watching The Simpsons.
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