Luminous intensity
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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]].
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 <math>V(\lambda)</math> or <math>\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.
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''.
Luminous intensity is also not the same as the [[radiant intensity]], the corresponding objective [[physical quantity]] used in the measurement science of [[radiometry]].
==Units==
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 540 THz light, 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
| title = Base unit definitions: Candela
| work = The NIST Reference on Constants, Units, and Uncertainty
| url = http://physics.nist.gov/cuu/Units/candela.html
| accessdate = 2008-02-08
}}</ref>
The 540 THz frequency used in the definition corresponds to a wavelength of about 555 nm, which is green light near the peak of the eye's response. Since there are about 12.6 steradians in a sphere, the total [[radiant flux]] would be about 18.40 mW, if the source emitted uniformly in all directions. A typical candle produces very roughly one candela of luminous intensity.
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. It was superseded by the candela in 1946.
==Usage==
The luminous intensity for monochromatic light of a particular wavelength <math>\lambda</math> is given by
:<math>I_v= 683I\,\overline{y}(\lambda),</math>
where
:<math>I_v</math> is the luminous intensity in candelas,
:<math>I</math> is the radiant intensity in W/sr,
:<math>\overline{y}(\lambda)</math> is the [[Luminosity function|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 luminous intensity:
:<math>I_v= 683 \int^\infin_0 I(\lambda)\,\overline{y}(\lambda) d\lambda.</math>
==See also==
*[[Intensity]]
==References==
<references/>
{{SI_light_units}}
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[[Category:Physical quantity]]
[[Category:Photometry]]
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