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Luminous energy
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In photometry, luminous energy is the perceived of . This is sometimes called the quantity of light. electropedia, International Electrotechnical Vocabulary (IEV) of the International Electrotechnical Commission: entry # 845-21-037 Luminous energy is not the same as , the corresponding objective physical quantity. This is because the can only see light in the and has different sensitivities to of different within the spectrum. When adapted for bright conditions (), the eye is most sensitive to light at a wavelength of 555 . Light with a given amount of radiant energy will have more luminous energy if the wavelength is 555 nm than if the wavelength is longer or shorter. Light whose wavelength is well outside the has a luminous energy of zero, regardless of the amount of radiant energy present.

The SI unit of luminous energy is the , which is unofficially known as the "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 Q_\mathrm v is related to radiant energy Q_\mathrm e by the expression Q_\mathrm v = 683.002\ \mathrm{lm/W} \cdot \int^\infin_0 Q_\mathrm e(\lambda) \overline{y}(\lambda) \, \mathrm d\lambda. Here \lambda is the wavelength of light, and \overline{y}(\lambda) is the luminous efficiency function, which represents the eye's sensitivity to different wavelengths of light.

Luminous energy is the integrated in a given period of time: Q_\mathrm{v} = \int_{0}^{T} \mathit{\Phi_\mathrm{v}}(t) \, \mathrm{d}t


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