In
photometry, luminous energy is the perceived
energy
Energy () is the physical quantity, quantitative physical property, property that is transferred to a physical body, body or to a physical system, recognizable in the performance of Work (thermodynamics), work and in the form of heat and l ...
of
light
Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
. This is sometimes called the quantity of light.
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International Electrotechnical Vocabulary
The International Electrotechnical Vocabulary (IEV) is a managed list of terms and definitions organized by the International Electrotechnical Commission (IEC), which is grouped in classes. It serves to promote the global unification of terminolo ...
(IEV) of the International Electrotechnical Commission
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: entry # 845-21-037 Luminous energy is not the same as
radiant energy
In physics, and in particular as measured by radiometry, radiant energy is the energy of electromagnetic radiation, electromagnetic and gravitational radiation. As energy, its SI unit is the joule (J). The quantity of radiant energy may be calcul ...
, the corresponding objective
physical quantity
A physical quantity (or simply quantity) is a property of a material or system that can be Quantification (science), quantified by measurement. A physical quantity can be expressed as a ''value'', which is the algebraic multiplication of a ''nu ...
. This is because the
human eye
The human eye is a sensory organ in the visual system that reacts to light, visible light allowing eyesight. Other functions include maintaining the circadian rhythm, and Balance (ability), keeping balance.
The eye can be considered as a living ...
can only see light in the
visible spectrum
The visible spectrum is the spectral band, band of the electromagnetic spectrum that is visual perception, visible to the human eye. Electromagnetic radiation in this range of wavelengths is called ''visible light'' (or simply light).
The optica ...
and has different sensitivities to
light
Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
of different
wavelength
In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats.
In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
s within the spectrum. When adapted for bright conditions (
photopic vision
Photopic vision is the vision of the eye under well-lit conditions (luminance levels from 10 to 108 cd/m2). In humans and many other animals, photopic vision allows color perception, mediated by cone cells, and a significantly higher vis ...
), the eye is most sensitive to light at a wavelength of 555
nm. 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
visible spectrum
The visible spectrum is the spectral band, band of the electromagnetic spectrum that is visual perception, visible to the human eye. Electromagnetic radiation in this range of wavelengths is called ''visible light'' (or simply light).
The optica ...
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" 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
is related to radiant energy
by the expression
Here
is the wavelength of light, and
is the
luminous efficiency 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:
See also
*
Coefficient of utilization
*
Radiant energy
In physics, and in particular as measured by radiometry, radiant energy is the energy of electromagnetic radiation, electromagnetic and gravitational radiation. As energy, its SI unit is the joule (J). The quantity of radiant energy may be calcul ...
References
Physical quantities
Photometry
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