View source for Luminous energy

{{Short description|Perceived light energy}}
{{More citations needed|date=January 2021}}
In [[photometry (optics)|photometry]], '''luminous energy''' is the perceived [[energy]] of [[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], [[International Electrotechnical Vocabulary]] (IEV) of the [[International Electrotechnical Commission]]: entry # 845-21-037</ref> Luminous energy is not the same as [[radiant energy]], the corresponding objective [[physical quantity]]. This is because the [[human eye]] can only see light in the [[visible spectrum]] and has different sensitivities to [[light]] of different [[wavelength]]s within the spectrum. When adapted for bright conditions ([[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 [[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]], 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 [[luminosity function]], which represents the eye's sensitivity to different wavelengths of light.

Luminous energy is the integrated [[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==
*[[Coefficient of utilization]]
*[[Radiant energy]]

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== References ==
{{Reflist}}

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[[Category:Physical quantities]]
[[Category:Photometry]]


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