| Luminous efficacy | |
| Unit: | lm⋅W |
| Symbols: | K |
| Baseunits: | cd⋅s⋅kg⋅m |
| Dimension: | JT3M-1L-2 |
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 the International System of Units (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,[4] and the latter luminous efficacy of a light source[5] or overall luminous efficacy.[6] [7]
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.
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.
By definition, light outside the visible spectrum cannot be seen by the standard human vision system, and therefore does not contribute to, and indeed can subtract from, luminous efficacy.
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 wavelengths outside the visible spectrum reduce luminous efficacy, because they contribute 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.
Wavelengths of light outside of the visible spectrum are not useful for general illumination. Furthermore, human vision responds more to some wavelengths of light than others. This response of the eye is represented by the luminous efficiency function. This is a standardized function representing photopic vision, which models the response of the eye's cone cells, that are active under typical daylight conditions. A separate curve can be defined for dark/night conditions, modeling the response of rod cells without cones, known as scotopic vision. (Mesopic vision describes the transition zone in dim conditions, between photopic and scotopic, where both cones and rods are active.)
Photopic luminous efficacy of radiation has a maximum possible value of, for the case of monochromatic light at a wavelength of . Scotopic luminous efficacy of radiation reaches a maximum of for monochromatic light at a wavelength of .
Luminous efficacy (of radiation), denoted K, is defined as
K=
| \Phiv | |
| \Phie |
=
| ||||||||||
|
,
where
| Type | Luminous efficacy of radiation (lm/W) | Luminous efficiency | |
|---|---|---|---|
| Tungsten light bulb, typical, 2800 K | 15[8] | 2% | |
| Class M star (Antares, Betelgeuse), 3300K | 30 | 4% | |
| Black body, 4000 K, ideal | 54.7 | 8% | |
| Class G star (Sun, Capella), 5800K | 93 | 13.6% | |
| Black-body, 7000 K, ideal | 95 | 14% | |
| Black-body, 5800 K, truncated to 400–700 nm (ideal "white" source) | 251 | 37% | |
| Black-body, 5800 K, truncated to ≥ 2% photopic sensitivity range | 292[9] | 43% | |
| Black-body, 2800 K, truncated to ≥ 2% photopic sensitivity range | 299 | 44% | |
| Black-body, 2800 K, truncated to ≥ 5% photopic sensitivity range | 343 | 50% | |
| Black-body, 5800 K, truncated to ≥ 5% photopic sensitivity range | 348 | 51% | |
| Monochromatic source at | 683 (exact) | 99.9997% | |
| Ideal monochromatic source: (in air) | 683.002[10] | 100% |
See 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.
| Category | Type | data-sort-type="number" | Overall luminous efficacy (lm/W) | data-sort-type="number" | Overall luminous efficiency |
|---|---|---|---|---|---|
| Combustion | Gas mantle | 1–2[13] | 0.15–0.3% | ||
| Incandescent | 15, 40, 100W tungsten incandescent (230 V) | 8.0, 10.4, 13.8[14] [15] [16] [17] | 1.2, 1.5, 2.0% | ||
| 5, 40, 100W tungsten incandescent (120 V) | 5.0, 12.6, 17.5[18] | 0.7, 1.8, 2.6% | |||
| Halogen incandescent | 100, 200, 500W tungsten halogen (230 V) | 16.7, 17.6, 19.8[19] | 2.4, 2.6, 2.9% | ||
| 2.6W tungsten halogen (5.2 V) | 19.2[20] | 2.8% | |||
| Halogen-IR (120 V) | 17.7–24.5[21] | 2.6–3.5% | |||
| Tungsten quartz halogen (12–24 V) | 24 | 3.5% | |||
| Photographic and projection lamps | 35[22] | 5.1% | |||
| Light-emitting diode | LED screw base lamp (120 V) | [23] [24] [25] | % | ||
| 5–16W LED screw base lamp (230V) | 75–212[26] [27] [28] | 11–31% | |||
| 21.5W LED retrofit for T8 fluorescent tube (230V) | 172[29] | 25% | |||
| Theoretical limit for a white LED with phosphorescence color mixing | –[30] | –% | |||
| Arc lamp | Carbon arc lamp | 2–7[31] | 0.29–1.0% | ||
| Xenon arc lamp | 30–90[32] [33] [34] | 4.4–13.5% | |||
| Mercury-xenon arc lamp | 50–55 | 7.3–8% | |||
| Ultra-high-pressure (UHP) mercury-vapor arc lamp, free mounted | 58–78[35] | 8.5–11.4% | |||
| Ultra-high-pressure (UHP) mercury-vapor arc lamp, with reflector for projectors | 30–50[36] | 4.4–7.3% | |||
| Fluorescent | 32W T12 tube with magnetic ballast | 60[37] | 9% | ||
| 9–32W compact fluorescent (with ballast) | 46–75[38] [39] | 8–11.45%[40] | |||
| T8 tube with electronic ballast | 80–100 | 12–15% | |||
| PL-S 11W U-tube, excluding ballast loss | 82[41] | 12% | |||
| T5 tube | 70–104.2[42] [43] | 10–15.63% | |||
| 70–150W inductively-coupled electrodeless lighting system | 71–84[44] | 10–12% | |||
| Gas discharge | 1400W sulfur lamp | 100[45] | 15% | ||
| Metal-halide lamp | 65–115[46] | 9.5–17% | |||
| High-pressure sodium lamp | 85–150 | 12–22% | |||
| Low-pressure sodium lamp | 100–200[47] [48] [49] | 15–29% | |||
| Plasma display panel | 2–10[50] | 0.3–1.5% | |||
| Cathodoluminescence | Electron-stimulated luminescence | 30–110[51] [52] | 15% | ||
| Ideal sources | Truncated 5800 K black-body | 251 | 37% | ||
| Green light at (maximum possible luminous efficacy by definition) | 683.002[53] | 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." At temperatures where the tungsten filament of an ordinary light bulb remains solid (below 3683 kelvin), most of its emission is in the infrared.