Luminous efficacy

Luminous efficacy is a measure of how well a light source produces visible light. It is the ratio of luminous flux to power, measured in lumens per watt in SI. 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.[1][2][3] 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 or overall luminous efficacy.[4][5]

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 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

Luminous efficacy can be normalized by the maximum possible luminous efficacy to a dimensionless quantity called luminous efficiency. 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


The response of a typical human eye to light, as standardized by the CIE in 1924. The horizontal axis is wavelength in nm

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.

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 monochromatic light at a wavelength of 507 nm.

Mathematical definition

Luminous efficacy, denoted K, is defined as[6]



Photopic vision

Luminous efficacy of radiation
Luminous efficiency[note 1]
Typical tungsten light bulb at 2800 K 15[7] 2%
Class M star (Antares, Betelgeuse), 3000 K 30 4%
Ideal black-body radiator at 4000 K 54.7[8] 8%
Class G star (Sun, Capella), 5800 K 93[7] 13.6%
Ideal black-body radiator at 7000 K 95[8] 14%
Ideal 5800 K black-body, truncated to 400–700 nm (ideal "white" source) [note 2] 251[7][note 3][9] 37%
5800 K black-body truncated to ≥2% photopic sensitivity range[note 4] 292[9][10] 43%
2800 K black-body truncated to ≥2% photopic sensitivity range[note 4] 299[9][10] 44%
2800 K black-body truncated to ≥5% photopic sensitivity range[note 5] 343[9][10] 50%
5800 K black-body truncated to ≥5% photopic sensitivity range[note 5] 348[9][10] 51%
Maximum for CRI=95 at 5800K (5800 K black-body truncated asymmetrically) 310[9][10] 45%
Maximum for CRI=95 at 2800 K (2800 K black-body truncated asymmetrically) 370[9][10] 54%
Ideal monochromatic 555 nm source 683[11] 100%

Scotopic vision

Luminous efficacy of radiation
Luminous efficiency[note 1]
Ideal monochromatic 507 nm source 1699 lm/W[12] or 1700 lm/W[13] 100%
Spectral radiance of a black body. Energy outside the visible wavelength range (~380–750 nm, shown by grey dotted lines) reduces the luminous efficiency.

Lighting efficiency

Main article: Wall-plug efficiency

Artificial light sources are usually evaluated in terms of luminous efficacy of the source, also sometimes called wall-plug 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. The luminous efficacy of the source 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 luminous efficiency of a source, overall luminous efficiency or 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]
Combustion candle 0.3[note 6] 0.04%
gas mantle 1–2[14] 0.15–0.3%
Incandescent 100–200 W tungsten incandescent (230 V) 13.8[15]–15.2[16] 2–2.2%
100–200–500 W tungsten glass halogen (230 V) 16.7[17]–17.6[16]–19.8[16] 2.4–2.6–2.9%
5–40–100 W tungsten incandescent (120 V) 5–12.6[18]–17.5[18] 0.7–1.8–2.6%
2.6 W tungsten glass halogen (5.2 V) 19.2[19] 2.8%
tungsten quartz halogen (12–24 V) 24 3.5%
photographic and projection lamps 35[20] 5.1%
Light-emitting diode white LED (raw, without power supply) 4.5–150 [21][22][23][24] 0.6622%
4.1 W LED screw base lamp (120 V) 58.582.9[25] 8.612%
5.4 W LED screw base lamp (100 V 50/60 Hz) 101.9[26] 14.9%
6.9 W LED screw base lamp (120 V) 55.181.9[25] 8.112%
7 W LED PAR20 (120 V) 28.6[27] 4.2%
7 W LED PAR30 (110-230 V) 60[28] 8.8%
8.7 W LED screw base lamp (120 V) 6993.1[25][29] 10.113.6%
Theoretical limit for a white LED with phosphorescence color mixing 260300[30] 38.143.9%
Arc lamp carbon arc lamp 2-7[31] 0.29-1.0%
xenon arc lamp 30–50[32][33] 4.4–7.3%
mercury-xenon arc lamp 50–55[32] 7.3–8%
UHP – ultra-high-pressure mercury-vapor arc lamp: initial, free mounted 58–78[34] 8.5–11.4%
UHP – ultra-high-pressure mercury-vapor arc lamp: rated, with reflector for projectors 30–50[35] 4.4–7.3%
Fluorescent very low pressure mercury-vapor gas-discharge lamp with fluorescence as T12 tube with magnetic ballast 60[36] 9%
9–32 W compact fluorescent (with ballast) 46–75[16][37][38] 8–11.45%[39]
T8 tube with electronic ballast 80–100[36] 12–15%
PL-S 11 W U-tube, excluding ballast loss 82[40] 12%
T5 tube 70–104.2[41][42] 10–15.63%
70-150W Inductively Coupled Electrodeless Lighting System 71-84[43] 10-12%
Gas discharge 1400 W sulfur lamp 100[44] 15%
metal halide lamp 65–115[45] 9.5–17%
high pressure sodium lamp 85–150[16] 12–22%
low pressure sodium lamp 100–200[16][46][47] 15–29%
Plasma display panel 2-10[48] 0.3–1.5%
Cathodoluminescence electron stimulated luminescence 30 5%
Ideal sources Truncated 5800 K blackbody[note 3] 251[7] 37%
Green light at 555 nm (maximum possible luminous efficacy) 683.002[11] 100%

Sources that depend on thermal emission from a solid filament, such as incandescent light bulbs, tend to have low overall efficacy 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.”[20] At temperatures where the tungsten filament of an ordinary light bulb remains solid (below 3683 kelvins), most of its emission is in the infrared.[20]

Philips Classictone Standard Incandescent Lamps

The Philips Classictone Standard incandescent lamps have detailed specification[49] of output in lumen, and the luminous efficacy is listed below based on the specification at different nominal power ratings.

Power rated nom. (W)Output Overall (lm)Overall luminous efficacy (lm/W)
15[50] 120 8.00
25[51] 220 8.80
40[52] 415 10.4
60[53] 710 11.8
75[54] 940 12.5
100[55] 1340 13.4

Philips Classic Filament LED Lamps

The Philips Classic Filament LED Lamps have detailed specification[56] of output in lumen, and the luminous efficacy is listed below based on the specification at different nominal power rating.

Power rated nom. (W)Output Overall (lm)Overall luminous efficacy (lm/W)
4.3[57] 470 109
7.5[58] 806 108

SI photometry units

SI photometry quantities
Quantity Unit Dimension Notes
Name Symbol[nb 1] Name Symbol Symbol
Luminous energy Qv [nb 2] lumen second lm⋅s TJ [nb 3] Units are sometimes called talbots.
Luminous flux / luminous power Φv [nb 2] lumen (= cd⋅sr) lm J [nb 3] Luminous energy per unit time.
Luminous intensity Iv candela (= lm/sr) cd J [nb 3] Luminous power per unit solid angle.
Luminance Lv candela per square metre cd/m2 L−2J Luminous power per unit solid angle per unit projected source area. Units are sometimes called nits.
Illuminance Ev lux (= lm/m2) lx L−2J Luminous power incident on a surface.
Luminous exitance / luminous emittance Mv lux lx L−2J Luminous power emitted from a surface.
Luminous exposure Hv lux second lx⋅s L−2TJ
Luminous energy density ωv lumen second per cubic metre lm⋅s⋅m−3 L−3TJ
Luminous efficacy η [nb 2] lumen per watt lm/W M−1L−2T3J Ratio of luminous flux to radiant flux or power consumption, depending on context.
Luminous efficiency / luminous coefficient V 1
See also: SI · Photometry · Radiometry
  1. Standards organizations recommend that photometric quantities be denoted with a suffix "v" (for "visual") to avoid confusion with radiometric or photon quantities. For example: USA Standard Letter Symbols for Illuminating Engineering USAS Z7.1-1967, Y10.18-1967
  2. 1 2 3 Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ or K for luminous efficacy.
  3. 1 2 3 "J" here is the symbol for the dimension of luminous intensity, not the symbol for the unit joules.

See also


  1. 1 2 3 Defined such that the maximum value possible is 100%.
  2. most efficient source you can do that mimics solar spectrum only within range of visual sensitivity
  3. 1 2 Integral of truncated Planck function times photopic luminosity function times 683 W/sr, according to the definition of the candela.
  4. 1 2 Truncates the very poor sensitivity (≤2% of the peak) and as such insignificant parts of the visible spectrum
  5. 1 2 Truncates the very poor sensitivity (≤5% of the peak) and as such insignificant parts of the visible spectrum
  6. 1 candela*4π steradians/40 W


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