Luminous efficacy 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 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 reaches a maximum of 1700 lm/W for narrowband light of wavelength 507 nm.
In some other systems of units, luminous flux has the same units as radiant flux. The luminous efficacy is then dimensionless. In this case, it is often instead called the luminous efficiency or luminous coefficient and may be expressed as a percentage. For example, it is common to express the luminous efficiency in units where 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 unity for a narrow band of wavelengths at 555 nanometres.
|Type|| Luminous efficacy|
| Luminous efficiency|
|Class M star (Antares, Betelgeuse), 3000 K||30||4%|
|ideal black-body radiator at 4000 K||47.5||7.0%|
|Class G star (Sun, Capella), 5800 K||80||12%|
|ideal black-body radiator at 7000 K||95||14%|
|ideal white light source||242.5||35.5%|
|ideal monochromatic 555 nm source||683||100%|
The main difference between the regular and “overall” efficacies is that the latter account for input energy that is lost as heat or otherwise exits the source as something other than electromagnetic radiation. True luminous efficacy is a property of the radiation emitted by a source. Overall luminous efficacy is a property of the source as a whole.
The following table lists overall luminous efficacy and efficiency for various light sources:
|Category|| Type|| Overall|
luminous efficacy (lm/W)
|Incandescent||5 W tungsten incandescent (120 V)||5||0.7%|
|40 W tungsten incandescent (120 V)||12.6||1.9%|
|100 W tungsten incandescent (120 V)||16.8||2.5%|
|100 W tungsten incandescent (220 V)||13.8||2.0%|
|100 W tungsten glass halogen (220 V)||16.7||2.4%|
|2.6 W tungsten glass halogen (5.2 V)||19.2||2.8%|
|quartz halogen (12–24 V)||24||3.5%|
|Fluorescent||5–24 W compact fluorescent||45–60||6.6–8.8%|
|T12 tube with magnetic ballast||60||9%|
|T8 tube with electronic ballast||80–100||12–15%|
|Light-emitting diode||white LED||10 to 90||1.5–13%|
|Prototype LEDs||up to 150||up to 22%|
|Arc lamp||xenon arc lamp||30–50||4.4–7.3%|
|mercury-xenon arc lamp||50–55||7.3–8.0%|
|Gas discharge||high pressure sodium lamp||150||22%|
|low pressure sodium lamp||183 up to 200||27–29%|
|1400 W sulfur lamp||100||15%|
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. 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 filament of an ordinary light bulb remains solid (below 3683 kelvins), most of its emission is in the infrared.
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