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Luminous efficacy
Luminous efficacy is a
property of light sources, which indicates what portion
of the emitted electromagnetic radiation is usable for
human vision. It is the ratio of emitted luminous flux
to radiant flux. Luminous efficacy is related to the overall
efficiency of a light source for illumination, but the
overall lighting efficiency also depends on how much of
the input energy is converted into electromagnetic waves
(whether visible or not).
Explanation
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 not specified, photopic conditions are generally
assumed. |
The response of a typical human
eye to light. The horizontal axis is wavelength in nm.
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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 does not contribute to the efficacy
at all, because 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.
Efficacy and efficiency
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.
Examples
| Type |
Luminous efficacy
(lm/W) |
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% |
| natural sunlight |
93 |
14% |
| ideal black-body radiator at 7000
K |
95 |
14% |
| ideal white light source |
242.5 |
35.5% |
| ideal monochromatic 555 nm source |
683 |
100% |
Lighting efficiency
Artificial light sources are usually evaluated in terms of
a related quantity, the 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. This is often simply called “luminous efficacy”,
which can be confusing as it also has units of lm/W. 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 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.
Examples
The following table lists overall luminous efficacy and efficiency
for various light sources:
| Category |
Type
|
Overall
luminous efficacy (lm/W)
|
Overall
luminous efficiency
|
 |
| Combustion |
candle |
0.3 |
0.04% |
| Incandescent |
5 W tungsten incandescent |
5 |
0.7% |
| |
40 W tungsten incandescent |
12.6 |
1.9% |
| |
100 W tungsten incandescent |
17.5 |
2.6% |
| |
glass halogen |
16 |
2.3% |
| |
quartz halogen |
24 |
3.5% |
| |
high-temperature incandescent |
35 |
5.1% |
| Fluorescent |
5–24 W compact
fluorescent |
45–60 |
6.6%–8.8% |
| |
34 W fluorescent tube
(T12) |
50 |
7% |
| |
32 W fluorescent tube
(T8) |
60 |
9% |
| |
36 W fluorescent tube
(T8) |
up to 93 |
up to 14% |
| |
28 W fluorescent tube
(T5) |
104 |
15.2% |
| Light-emitting diode |
white LED |
26–100 |
3.8%–15% |
| |
white LED (prototypes) |
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% |
| |
1400 W sulfur lamp |
100 |
15% |
Theoretical maximum
(monochromatic 540x1012 Hz,
approx. 555 nm, green) |
|
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 efficiency [sic] 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 kelvin), most of its
emission is in the infrared.
SI photometry units
| Quantity |
Symbol |
SI unit |
Abbr. |
Notes |
| Luminous energy |
Qv |
lumen second |
lm·s |
units are sometimes called talbots |
| Luminous flux |
F |
lumen (= cd·sr) |
lm |
also called luminous power |
| Luminous intensity |
Iv |
candela (= lm/sr) |
cd |
an SI base unit |
| Luminance |
Lv |
candela per square metre |
cd·m–2 |
units are sometimes called nits |
| Illuminance |
Ev |
lux (= lm·m–2) |
lx |
Used for light incident on a surface |
| Luminous emittance |
Mv |
lux (= lm·m–2) |
lx |
Used for light emitted from a surface |
| Luminous efficacy |
|
lumen per watt |
lm·W–1 |
ratio of luminous flux to radiant
flux; maximum possible is 683.002 |
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