View source for Physics:Luminous energy

{{Short description|Perceived light energy}}
In [[Physics:Photometry (optics)|photometry]], '''luminous energy''' is the perceived [[Physics:Energy|energy]] of [[Physics:Light|light]]. This is sometimes called the '''quantity of light'''.<ref name="IEV">[http://www.electropedia.org/iev/iev.nsf/display?openform&ievref=845-21-037 electropedia], [[Organization:International Electrotechnical Vocabulary|International Electrotechnical Vocabulary]] (IEV) of the [[Organization:International Electrotechnical Commission|International Electrotechnical Commission]]: entry # 845-21-037</ref> Luminous energy is not the same as [[Physics:Radiant energy|radiant energy]], the corresponding objective [[Physics:Physical quantity|physical quantity]]. This is because the [[Biology:Human eye|human eye]] can only see light in the [[Physics:Visible spectrum|visible spectrum]] and has different sensitivities to [[Physics:Light|light]] of different [[Wavelength|wavelength]]s within the spectrum. When adapted for bright conditions ([[Biology:Photopic vision|photopic vision]]), the eye is most sensitive to light at a wavelength of 555&nbsp;[[Metre|nm]]. Light with a given amount of radiant energy will have more luminous energy if the wavelength is 555&nbsp;nm than if the wavelength is longer or shorter. Light whose wavelength is well outside the [[Physics:Visible spectrum|visible spectrum]] has a luminous energy of zero, regardless of the amount of radiant energy present.

The SI unit of luminous energy is the [[Lumen second|lumen second]], which is unofficially known as the [[Talbot (unit)|talbot]] in honor of William Henry Fox Talbot. In other systems of units, luminous energy may be expressed in basic units of energy.

==Explanation==
Luminous energy <math>Q_\mathrm v</math> is related to radiant energy <math>Q_\mathrm e</math> by the expression
<math display="block">Q_\mathrm v = 683.002\ \mathrm{lm/W} \cdot \int^\infin_0 Q_\mathrm e(\lambda) \overline{y}(\lambda) \, \mathrm d\lambda.</math>
Here <math>\lambda</math> is the wavelength of light, and <math>\overline{y}(\lambda) </math> is the [[Physics:Luminosity function|luminosity function]], which represents the eye's sensitivity to different wavelengths of light.

Luminous energy is the integrated [[Physics:Luminous flux|luminous flux]] in a given period of time:
<math display="block">Q_\mathrm{v} = \int_{0}^{T} \mathit{\Phi_\mathrm{v}}(t) \, \mathrm{d}t</math>

==See also==
*[[Physics:Coefficient of utilization|Coefficient of utilization]]
*[[Physics:Radiant energy|Radiant energy]]

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== References ==
{{Reflist}}

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{{DEFAULTSORT:Luminous Energy}}
[[Category:Physical quantities]]
[[Category:Photometry]]



{{Sourceattribution|Luminous energy}}