Luminous energy
Encyclopedia
In photometry
, luminous energy is the perceived energy
of light
. This is sometimes also called the quantity of light.
Luminous energy is not the same as the 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 nm
. Light with the same power at longer or shorter wavelengths has a lower luminous energy.
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
.
Photometry (optics)
Photometry is the science of the measurement of light, in terms of its perceived brightness to the human eye. It is distinct from radiometry, which is the science of measurement of radiant energy in terms of absolute power; rather, in photometry, the radiant power at each wavelength is weighted by...
, luminous energy is the perceived energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
of light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
. This is sometimes also called the quantity of light.
Luminous energy is not the same as the radiant energy
Radiant energy
Radiant energy is the energy of electromagnetic waves. The quantity of radiant energy may be calculated by integrating radiant flux with respect to time and, like all forms of energy, its SI unit is the joule. The term is used particularly when radiation is emitted by a source into the...
, the corresponding objective physical quantity
Physical quantity
A physical quantity is a physical property of a phenomenon, body, or substance, that can be quantified by measurement.-Definition of a physical quantity:Formally, the International Vocabulary of Metrology, 3rd edition defines quantity as:...
. This is because the human eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
can only see light in the visible spectrum
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
and has different sensitivities to light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
of different wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s within the spectrum. When adapted for bright conditions (photopic vision
Photopic vision
Photopic vision is the vision of the eye under well-lit conditions. In humans and many other animals, photopic vision allows color perception, mediated by cone cells, and a significantly higher visual acuity and temporal resolution than available with scotopic vision.The human eye uses three types...
), the eye is most sensitive to light at a wavelength of 555 nm
Metre
The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...
. Light with the same power at longer or shorter wavelengths has a lower luminous energy.
Units
The SISi
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...
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
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
.
See also
- Coefficient of utilizationCoefficient of utilizationA coefficient of utilization is a measure of the efficiency of a luminaire in transferring luminous energy to the working plane in a particular area. The CU is the ratio of luminous flux from a luminaire incident upon a work plane to that emitted by the lamps within the luminaire...
- Radiant energyRadiant energyRadiant energy is the energy of electromagnetic waves. The quantity of radiant energy may be calculated by integrating radiant flux with respect to time and, like all forms of energy, its SI unit is the joule. The term is used particularly when radiation is emitted by a source into the...