Luminous efficacy is a measure of how well a light source produces visible light. It is the ratio of luminous flux to power. Depending on context, the power can be either the radiant flux of the source's output, or it can be the total power (electric power, chemical energy, or others) consumed by the source. Which sense of the term is intended must usually be inferred from the context, and is sometimes unclear. The former sense is sometimes called luminous efficacy of radiation, and the latter luminous efficacy of a source.
The luminous efficacy of a source is a measure of the efficiency with which the source provides visible light from electricity. The luminous efficacy of radiation describes how well a given quantity of electromagnetic radiation from a source produces visible light: the ratio of luminous flux to radiant flux. Not all wavelengths of light are equally visible, or equally effective at stimulating human vision, due to the spectral sensitivity of the human eye; radiation in the infrared and ultraviolet parts of the spectrum is useless for illumination. The overall luminous efficacy of a source is the product of how well it converts energy to electromagnetic radiation, and how well the emitted radiation is detected by the human eye.
Efficacy and efficiency 
In some systems of units, 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, 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.
The luminous coefficient is luminous efficiency expressed as a value between zero and one, with one corresponding to an efficacy of 683 lm/W.
Luminous efficacy of radiation 
Wavelengths of light outside of the visible spectrum are not useful for illumination because they cannot be seen by the human eye. 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. This is a standardized function which represents the response of a "typical" eye under bright conditions (photopic vision). One can also define a similar curve for dim conditions (scotopic vision). 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 by the radiant flux. Light with wavelengths outside the visible spectrum reduces luminous efficacy, 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, luminous efficacy has units of lumens per watt (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 fraction of the radiant power that contributes to its luminous properties as evaluated by means of the standard luminosity function. The luminous coefficient is
- yλ is the standard luminosity function,
- Jλ is the spectral power distribution of the radiant intensity.
The luminous coefficient is unity for a narrow band of wavelengths at 555 nanometres.
||Luminous efficacy of radiation
|Luminous efficiency[note 1]
|Typical tungsten light bulb at 2800 K||15||2%|
|Class M star (Antares, Betelgeuse), 3000 K||30||4%|
|ideal black-body radiator at 4000 K||54.7||8.0%|
|Class G star (Sun, Capella), 5800 K||93||13.6%|
|ideal black-body radiator at 7000 K||95||14%|
|ideal 5800 K black-body, truncated to 400–700 nm (ideal "white" source)||251[note 2]||37%|
|ideal monochromatic 555 nm source||683||100%|
Lighting efficiency 
Artificial light sources are usually evaluated in terms of 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 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.
The following table lists luminous efficacy of a source and efficiency for various light sources. Note that all lamps requiring electrical/electronic ballast are unless noted (see also voltage) listed without losses for that, reducing total efficiency.
luminous efficacy (lm/W)
luminous efficiency[note 1]
|Incandescent||100–200 W tungsten incandescent (230 V)||13.8–15.2||2.0–2.2%|
|100–200–500 W tungsten glass halogen (230 V)||16.7–17.6–19.8||2.4–2.6–2.9%|
|5–40–100 W tungsten incandescent (120 V)||5–12.6–17.5||0.7–1.8–2.6%|
|2.6 W tungsten glass halogen (5.2 V)||19.2||2.8%|
|tungsten quartz halogen (12–24 V)||24||3.5%|
|photographic and projection lamps||35||5.1%|
|Light-emitting diode||white LED (raw, without power supply)||4.5–150 ||0.66–22.0%|
|4.1 W LED screw base lamp (120 V)||58.5–82.9||8.6–12.1%|
|5.4 W LED screw base lamp (100 V 50/60 Hz)||101.9||14.9%|
|6.9 W LED screw base lamp (120 V)||55.1–81.9||8.1–12.0%|
|7 W LED PAR20 (120 V)||28.6||4.2%|
|7 W LED PAR30 (110-230 V)||60.0||8.8%|
|8.7 W LED screw base lamp (120 V)||69.0–93.1||10.1–13.6%|
|Theoretical limit (white LED with phosphorescence color mixing)||260.0–300.0||38.1–43.9%|
|Arc lamp||xenon arc lamp||30–50||4.4–7.3%|
|mercury-xenon arc lamp||50–55||7.3–8.0%|
|UHP – mercury-vapor arc lamp: initial, free mounted||58–78||8.5-11.4%|
|UHP – mercury-vapor arc lamp: rated, with reflector for projectors||30–50||4.4–7.3%|
|Fluorescent||T12 tube with magnetic ballast||60||9%|
|9–32 W compact fluorescent (with ballast)||46–75||8–11.45%|
|T8 tube with electronic ballast||80–100||12–15%|
|PL-S 11 W U-tube, excluding ballast loss||82||12%|
|Gas discharge||1400 W sulfur lamp||100||15%|
|metal halide lamp||65–115||9.5–17%|
|high pressure sodium lamp||85–150||12–22%|
|low pressure sodium lamp||100–200||15–29%|
|Plasma display panel||2-10||0.3–1.5%|
|Cathodoluminescence||electron stimulated luminescence||30||5%|
|Ideal sources||Truncated 5800 K blackbody[note 2]||251||37%|
|Green light at 555 nm (maximum possible luminous efficacy)||683.002||100%|
Sources that depend on thermal emission from a solid filament, such as incandescent light bulbs, 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. No substance 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 filament of an ordinary light bulb remains solid (below 3683 kelvins), most of its emission is in the infrared.
SI photometry units 
|Luminous energy||Qv [nb 2]||lumen second||lm⋅s||T⋅J [nb 3]||units are sometimes called talbots|
|Luminous flux||Φv [nb 2]||lumen (= cd⋅sr)||lm||J [nb 3]||also called luminous power|
|Luminous intensity||Iv||candela (= lm/sr)||cd||J [nb 3]||an SI base unit, luminous flux per unit solid angle|
|Luminance||Lv||candela per square metre||cd/m2||L−2⋅J||units are sometimes called nits|
|Illuminance||Ev||lux (= lm/m2)||lx||L−2⋅J||used for light incident on a surface|
|Luminous emittance||Mv||lux (= lm/m2)||lx||L−2⋅J||used for light emitted from a surface|
|Luminous exposure||Hv||lux second||lx⋅s||L−2⋅T⋅J|
|Luminous energy density||ωv||lumen second per metre3||lm⋅s⋅m−3||L−3⋅T⋅J|
|Luminous efficacy||η [nb 2]||lumen per watt||lm/W||M−1⋅L−2⋅T3⋅J||ratio of luminous flux to radiant flux|
|Luminous efficiency||V||1||also called luminous coefficient|
|See also: SI · Photometry · Radiometry|
- Standards organizations recommend that photometric quantities be denoted with a suffix "v" (for "visual") to avoid confusion with radiometric or photon quantities.
- Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ or K for luminous efficacy.
- "J" here is the symbol for the dimension of luminous intensity, not the symbol for the unit joules.
See also 
- Light pollution
- Wall-plug efficiency – a related principle, but slightly different
- Coefficient of utilization
- Defined such that the maximum value possible is 100%.
- Integral of truncated Planck function times photopic luminosity function times 683 W/sr, according to the definition of the candela. 
- 1 candela*4π steradians/40 W
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