All Topics  
Luminous efficacy

 

 
   Email Print
   Bookmark   Link
 

 

Luminous efficacy
 
 
  Topics Luminous efficacy Topic Home

Luminous efficacy is a figure of merit
Figure of merit

A figure of merit is a quantity used to characterize the performance of a device, system or method, relative to its alternatives. In engineering, figures of merit are often defined for particular materials or devices in order to determine their relative utility for an application....
 for light sources. It is the ratio of luminous flux
Luminous flux

In photometry , luminous flux or luminous power is the measure of the perceived power of light. It differs from radiant flux, the measure of the total power of light emitted, in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light....
 (in lumen
Lumen (unit)

The lumen is the SI unit of luminous flux, a measure of the perceived power of light. Luminous flux differs from radiant flux, the measure of the total power of light emitted, in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light....
s) to power
Power (physics)

In physics, power is the rate at which mechanical work is performed or energy is transmitted, or the amount of energy required or expended for a given unit of time....
 (usually measured in watt
WATT

WATT is a radio station broadcasting a News radio-Talk radio-Sports radio format. Licensed to Cadillac, Michigan, it first began broadcasting in 1945....
s). As most commonly used, it is the ratio of luminous flux emitted from a light source to the electric power consumed by the source, and thus describes how well the source does at providing visible light from a given amount of electricity. This is also referred to as luminous efficacy of a source.

The term luminous efficacy can also refer to luminous efficacy of radiation (LER), which is the ratio of emitted luminous flux to radiant flux
Radiant flux

In radiometry, radiant flux or radiant power is the measure of the total Power of electromagnetic radiation . The power may be the total emitted from a source, or the total landing on a particular surface....
.






Discussion
Ask a question about 'Luminous efficacy'
Start a new discussion about 'Luminous efficacy'
Answer questions from other users
Full Discussion Forum



Encyclopedia


Luminous efficacy is a figure of merit
Figure of merit

A figure of merit is a quantity used to characterize the performance of a device, system or method, relative to its alternatives. In engineering, figures of merit are often defined for particular materials or devices in order to determine their relative utility for an application....
 for light sources. It is the ratio of luminous flux
Luminous flux

In photometry , luminous flux or luminous power is the measure of the perceived power of light. It differs from radiant flux, the measure of the total power of light emitted, in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light....
 (in lumen
Lumen (unit)

The lumen is the SI unit of luminous flux, a measure of the perceived power of light. Luminous flux differs from radiant flux, the measure of the total power of light emitted, in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light....
s) to power
Power (physics)

In physics, power is the rate at which mechanical work is performed or energy is transmitted, or the amount of energy required or expended for a given unit of time....
 (usually measured in watt
WATT

WATT is a radio station broadcasting a News radio-Talk radio-Sports radio format. Licensed to Cadillac, Michigan, it first began broadcasting in 1945....
s). As most commonly used, it is the ratio of luminous flux emitted from a light source to the electric power consumed by the source, and thus describes how well the source does at providing visible light from a given amount of electricity. This is also referred to as luminous efficacy of a source.

The term luminous efficacy can also refer to luminous efficacy of radiation (LER), which is the ratio of emitted luminous flux to radiant flux
Radiant flux

In radiometry, radiant flux or radiant power is the measure of the total Power of electromagnetic radiation . The power may be the total emitted from a source, or the total landing on a particular surface....
. Luminous efficacy of radiation is a characteristic of a given spectrum
Electromagnetic spectrum

The electromagnetic spectrum is the range of all possible electromagnetic radiation frequencies. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation from that particular object....
 that describes how sensitive the human eye
Human eye

The human eye is a significant human sense organ. It allows humans conscious light perception, vision, which includes color differentiation and the perception of depth....
 is to the mix of wavelength
Wavelength

In physics, wavelength is the distance between repeating units of a propagating wave of a given frequency. It is commonly designated by the Greek language letter lambda ....
s involved. Which sense of the term is intended must usually be inferred from the context, and is sometimes unclear. The luminous efficacy of a source is the LER of its emission spectrum
Emission spectrum

The emission spectrum of an Chemical element or Chemical compound is the relative intensity of electromagnetic radiation of each frequency Emission by atoms or molecules of that element or compound when they are excited....
 times the conversion efficiency from electrical energy to electromagnetic radiation
Electromagnetic radiation

Electromagnetic radiation takes the form of wave propagation waves in a vacuum or in matter. EM radiation has an electric field and magnetic field component which oscillate in phase perpendicular to each other and to the direction of energy Wave propagation....
.

Efficacy and efficiency


In some other systems of units
Units of measurement

The definition, agreement and practical use of units of measurement have played a crucial role in human endeavour from early ages up to this day....
, luminous flux has the same units as radiant flux. The luminous efficacy of radiation is then dimensionless. In this case, it is often instead called the luminous efficiency or luminous coefficient and may be expressed as a percentage. A common choice is to choose units such that the maximum possible efficacy, 683 lm/W, corresponds to an efficiency of 100%. The distinction between efficacy and efficiency is not always carefully maintained in published sources, so it is not uncommon to see "efficiencies" expressed in lumens per watt, or "efficacies" expressed as a percentage.

Luminous efficacy of radiation


Explanation

Cie 1931 Luminosity
Wavelength
Wavelength

In physics, wavelength is the distance between repeating units of a propagating wave of a given frequency. It is commonly designated by the Greek language letter lambda ....
s of light outside of the visible spectrum
Visible spectrum

The visible spectrum is the portion of the electromagnetic spectrum that is visual perception to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light....
 are not useful for illumination because they cannot be seen by the human eye
Human eye

The human eye is a significant human sense organ. It allows humans conscious light perception, vision, which includes color differentiation and the perception of depth....
. Furthermore, the eye responds more to some wavelengths of light than others, even within the visible spectrum. This response of the eye is represented by the luminosity function
Luminosity function

The luminosity function or luminous efficiency function describes the average sensitivity of the human eye to light of different wavelengths....
. This is a standardized function which represents the response of a "typical" eye under bright conditions (Photopic vision
Photopic vision

Photopic vision is the visual perception of the eye under well-lit conditions. In humans and many animals, photopic vision allows color vision, mediated by cone cells....
). One can also define a similar curve for dim conditions (Scotopic vision
Scotopic vision

Scotopic vision is the monochromatic vision of the eye in low light. Since cone cells are nonfunctional in low light, scotopic vision is produced exclusively through rod cells so therefore there is no color perception....
). When neither is specified, photopic conditions are generally assumed.

Luminous efficacy of radiation measures the fraction of electromagnetic power which is useful for lighting. It is obtained by dividing the luminous flux
Luminous flux

In photometry , luminous flux or luminous power is the measure of the perceived power of light. It differs from radiant flux, the measure of the total power of light emitted, in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light....
 by the radiant flux
Radiant flux

In radiometry, radiant flux or radiant power is the measure of the total Power of electromagnetic radiation . The power may be the total emitted from a source, or the total landing on a particular surface....
. Light with wavelengths outside the visible spectrum
Visible spectrum

The visible spectrum is the portion of the electromagnetic spectrum that is visual perception to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light....
 reduces LER, because it contributes to the radiant flux while the luminous flux of such light is zero. Wavelengths near the peak of the eye's response contribute more strongly than those near the edges.

In SI
Si

Si, si, or SI may refer to :...
, luminous efficacy has units of lumen
Lumen (unit)

The lumen is the SI unit of luminous flux, a measure of the perceived power of light. Luminous flux differs from radiant flux, the measure of the total power of light emitted, in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light....
s per watt
WATT

WATT is a radio station broadcasting a News radio-Talk radio-Sports radio format. Licensed to Cadillac, Michigan, it first began broadcasting in 1945....
 (lm/W). Photopic luminous efficacy of radiation has a maximum possible value of 683 lm/W, for the case of monochromatic light at a wavelength of 555 nm (green). Scotopic luminous efficacy of radiation reaches a maximum of 1700 lm/W for narrowband light of wavelength 507 nm.

Mathematical definition

The dimensionless luminous efficiency measures the integrated
Integral

Integration is an important concept in mathematics, specifically in the field of calculus and, more broadly, mathematical analysis. Given a function ƒ of a Real number variable x and an interval [a, b] of the real line, the integral...
 fraction of the radiant power that contributes to its luminous properties as evaluated by means of the standard luminosity function
Luminosity function

The luminosity function or luminous efficiency function describes the average sensitivity of the human eye to light of different wavelengths....
. The luminous coefficient is

where
yλ is the standard luminosity function
Luminosity function

The luminosity function or luminous efficiency function describes the average sensitivity of the human eye to light of different wavelengths....
,
Jλ is the spectral power distribution
Spectral power distribution

In color science and radiometry, a spectral power distribution describes the power per unit area per unit wavelength of an illumination , or more generally, the per-wavelength contribution to any radiometric quantity ....
 of the radiant intensity.


The luminous coefficient is unity for a narrow band of wavelengths at 555 nanometre
Nanometre

A nanometre is a Units of measurement of length in the metric system, equal to one billionth of a metre .It is one of the more often used units for very small lengths, and equals ten ?ngstr?m, an internationally recognized non-International System of Units of length....
s.

Note that is an inner product between and and that is the one-norm
Norm (mathematics)

In linear algebra, functional analysis and related areas of mathematics, a norm is a function that assigns a strictly positive length or size to all vectors in a vector space, other than the zero vector....
 of .

Examples

Type
 		Luminous efficacy of radiation
(lm/W)		Luminous efficiency
 
Class M star (Antares
Antares

Antares is a red supergiant star in the Milky Way galaxy and list of brightest stars in the nighttime sky . Along with Aldebaran, Spica, and Regulus it is one of the four brightest stars near the ecliptic....
, Betelgeuse
Betelgeuse

Betelgeuse is a semiregular variable star located approximately 600 light-years away from Earth. It is the second brightest star in the constellation Orion and the ninth list of brightest stars in the night sky....
), 3000 K		304%
ideal black-body radiator at 4000 K47.5 7.0%
Class G star (Sun
Sun

The Sun , a G V star, is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 98.6% of the Solar System's mass....
, Capella
Capella (star)

Capella is the brightest star in the constellation Auriga , the list of brightest stars in the night sky and the third brightest star in the northern Celestial sphere, after Arcturus and Vega....
), 5800 K		8012%
natural sunlight
Sunlight

Sunlight, in the broad sense, is the total spectroscopy of the electromagnetic radiation given off by the Sun. On Earth, sunlight is Filter ed through the Earth's atmosphere, and the solar radiation is obvious as daylight when the Sun is above the horizon....
9314%
ideal black-body radiator at 7000 K95 14%
ideal monochromatic 555 nm source683 100%


Lighting efficiency

Artificial light sources are usually evaluated in terms luminous efficacy of a source, also sometimes called overall luminous efficacy. This is the ratio between the total luminous flux emitted by a device and the total amount of input power (electrical, etc.) it consumes. It is also sometimes referred to as the wall-plug luminous efficacy or simply wall-plug efficacy. The overall luminous efficacy is a measure of the efficiency of the device with the output adjusted to account for the spectral response curve (the “luminosity function”). When expressed in dimensionless form (for example, as a fraction of the maximum possible luminous efficacy), this value may be called overall luminous efficiency, wall-plug luminous efficiency, or simply the lighting efficiency.

The main difference between the luminous efficacy of radiation and the luminous efficacy of a source is that the latter accounts for input energy that is lost as heat
Heat

In physics and thermodynamics, heat is any transfer of energy from one body or thermodynamic system to another due to a difference in temperature....
 or otherwise exits the source as something other than electromagnetic radiation. Luminous efficacy of radiation is a property of the radiation emitted by a source. Luminous efficacy of a source is a property of the source as a whole.

Examples


The following table lists luminous efficacy of a source and efficiency for various light sources:
Category
 		Type
 		Overall
luminous efficacy (lm/W)		Overall
luminous efficiency
Combustioncandle
Candle

A candle is a source of light, and sometimes a source of heat, consisting of a solid block of fuel and an embedded candle wick.Today, most candles are made from paraffin....
0.3 0.04%
 gas mantle
Gas mantle

An incandescent gas mantle, gas mantle, or Welsbach mantle is a device for generating bright white light when heated by a flame. The name refers to its original heat source, existing gas lights which filled the street lighting of Europe and North America in the late 19th century, mantle referring to the way it was hung above the f...
 2 0.3%
Incandescent
Incandescent light bulb

The incandescent light bulb, incandescent lamp or incandescent light globe is a source of electric light that works by incandescence, ....
100 W tungsten incandescent (220 V)13.8 2.0%
 200 W tungsten incandescent (220 V)15.2 2.2%
 100 W tungsten glass halogen (220 V)16.7 2.4%
 200 W tungsten glass halogen (220 V)17.6 2.6%
 500 W tungsten glass halogen (220 V)19.8 2.9%
 5 W tungsten incandescent (120 V)50.7%
 40 W tungsten incandescent (120 V)12.6 1.9%
 100 W tungsten incandescent (120 V)17.5 2.6%
 2.6 W tungsten glass halogen (5.2 V)19.2 2.8%
 quartz halogen (12–24 V)243.5%
 photographic and projection lamps35 5.1%
Light-emitting diode
Light-emitting diode

A light-emitting diode , is an electronic light source. The LED was discovered in the early 20th century, and introduced as a practical electronic component in 1962....
white LED10–100 1.5–15%
Arc lamp
Arc lamp

An arc lamp or arc light is the general term for a class of lamps that produce light by an electric arc . The lamp consists of two electrodes typically made of tungsten which are separated by a gas....
xenon arc lamp
Xenon arc lamp

A xenon arc lamp is an Lighting source. Powered by electricity, it uses ionized xenon gas to produce a bright white light that closely mimics natural daylight....
30–50 4.4–7.3%
 mercury
Mercury (element)

Mercury , also called quicksilver or hydrargyrum , is a chemical element with the symbol Hg and atomic number 80. A heavy, silvery d-block metal, mercury is one of six elements that are liquid at or near room temperature and pressure....
-xenon
Xenon

Xenon is a chemical element represented by the chemical symbol Xe. Its atomic number is 54. A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts....
 arc lamp		50–55 7.3–8.0%
Fluorescent
Fluorescent lamp

A fluorescent lamp or fluorescent tube is a gas-discharge lamp that uses electricity to Excited state mercury vapor. The excited mercury atoms produce short-wave ultraviolet light that then causes a phosphor to fluorescence, producing Light....
9–26 W compact fluorescent57–72 8–11%
 T12 tube with magnetic ballast 60 9%
 T5 tube 70–100 10–15%
 T8 tube with electronic ballast 80–100 12–15%
Gas discharge
Gas-discharge lamp

Gas-discharge lamps are a family of artificial light sources that generate light by sending an electrical discharge through an ionization gas, i.e....
1400 W sulfur lamp
Sulfur lamp

The sulfur lamp is a highly lighting efficiency full-spectrum electrodeless lighting system whose light is generated by sulfur Plasma that has been Electron excitation by microwave radiation....
10015%
 metal halide lamp
Metal halide lamp

Metal halide lamps, a member of the high-intensity discharge family of lamps, produce high light output for their size, making them a compact, powerful, and efficient light source....
65–115 9.5–17%
 high pressure sodium lamp
Sodium vapor lamp

A sodium vapor lamp is a gas discharge lamp which uses sodium in an excited state to produce light. There are two varieties of such lamps: low pressure and high pressure....
85–150 12–22%
 low pressure sodium lamp
Sodium vapor lamp

A sodium vapor lamp is a gas discharge lamp which uses sodium in an excited state to produce light. There are two varieties of such lamps: low pressure and high pressure....
100–200 15–29%
Theoretical maximumGreen light at 555 nm683.002100%


Sources that depend on thermal emission from a solid filament, such as incandescent light bulb
Incandescent light bulb

The incandescent light bulb, incandescent lamp or incandescent light globe is a source of electric light that works by incandescence, ....
s, tend to have low overall efficacy compared to an ideal blackbody source because, as explained by Donald L. Klipstein, “An ideal thermal radiator produces visible light most efficiently at temperatures around 6300 °C (6600 K or 11,500 °F). Even at this high temperature, a lot of the radiation is either infrared or ultraviolet, and the theoretical luminous [efficacy] is 95 lumens per watt. Of course, nothing known to any humans is solid and usable as a light bulb filament at temperatures anywhere close to this. The surface of the sun is not quite that hot.” At temperatures where the tungsten
Tungsten

Tungsten , also known as wolfram , is a chemical element that has the symbol W and atomic number 74.A steel-gray metal, tungsten is found in several ores, including wolframite and scheelite....
 filament of an ordinary light bulb remains solid (below 3683 kelvins), most of its emission is in the infrared
Infrared

Infrared radiation is electromagnetic radiation whose wavelength is longer than that of visible light , but shorter than that of terahertz radiation and microwaves ....
.

SI photometry units


See also


  • Luminous coefficient
    Luminous coefficient

    The luminous coefficient is a coefficient that measures the integral fraction of the radiant power that contributes to its luminous properties as evaluated by means of the standard luminosity function....
  • Photometry
    Photometry

    Photometry can refer to:*Photometry , the science of measurement of visible light in terms of its perceived brightness to human vision*Photometry , the measurement of the flux or intensity of an astronomical object's electromagnetic radiation...
  • Light pollution
    Light pollution

    Light pollution, also known as photopollution or luminous pollution, is excessive or obtrusive artificial light. The International Dark-Sky Association , "The Light Pollution Authority," defines light pollution as: It obscures the stars in the night sky for city dwellers, interferes with astronomy observatory, and, like an...


External links

  • Hyperphysics
    HyperPhysics

    HyperPhysics is an educational resource about physics topics. The information architecture of the website is based on tree structure that organize topics from general to specific....
     has these that do not quite comply with the standard definition