Luminous efficacy
Efficacy and efficiency
In some other 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 or luminous coefficient 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.
Luminous efficacy of radiation
Explanation
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 LER, 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
where
y is the standard luminosity function,
J is the spectral power distribution of the radiant intensity.
The luminous coefficient is unity for a narrow band of wavelengths at 555 nanometres.
Note that is an inner product between y and J and that is the one-norm of J.
Examples
Spectral radiance of a black body. Energy outside the visible wavelength range (~380750 nm, shown by grey dotted lines) reduces the luminous efficiency.
Type
Luminous efficacy of radiation
(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 5800 K black-body, truncated to 400700 nm (ideal “white” source)
251
37%
ideal monochromatic 555 nm source
683
100%
Lighting efficiency
Artificial light sources are usually evaluated in terms 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 uminosity 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.
Examples
The following table lists luminous efficacy of a source and efficiency for various light sources:
Category
Type
Overall
luminous efficacy (lm/W)
Overall
luminous efficiency
Combustion
candle
0.3
0.04%
gas mantle
12
0.150.3%
Incandescent
100200 W tungsten incandescent (220 V)
13.815.2
2.02.2%
100200500 W tungsten glass halogen (220 V)
16.717.619.8
2.42.62.9%
540100 W tungsten incandescent (120 V)
512.617.5
0.71.82.6%
2.6 W tungsten glass halogen (5.2 V)
19.2
2.8%
tungsten quartz halogen (1224 V)
24
3.5%
photographic and projection lamps
35
5.1%
Light-emitting diode
white LED (raw, without power supply)
4.5150
0.6622.0%
4.1 W LED screw base lamp (120 V)
58.582.9
8.612.1%
6.9 W LED screw base lamp (120 V)
55.181.9
8.112.0%
7 W LED PAR20 (120 V)
28.6
4.2%
8.7 W LED screw base lamp (120 V)
69.093.1
10.113.6%
Arc lamp
xenon arc lamp
3050
4.47.3%
mercury-xenon arc lamp
5055
7.38.0%
Fluorescent
T12 tube with magnetic ballast
60
9%
932 W compact fluorescent
4675
811.45%
T8 tube with electronic ballast
80100
1215%
T5 tube
70104.2
1015.63%
Gas discharge
1400 W sulfur lamp
100
15%
metal halide lamp
65115
9.517%
high pressure sodium lamp
85150
1222%
low pressure sodium lamp
100200
1529%
Ideal sources
Truncated 5800 K blackbody
251[citation needed]
37%
Green light at 555 nm (maximum possible LER)
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, n 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.17] 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
SI photometry units
v d e
Quantity
Symbol
SI unit
Abbr.
Notes
Luminous energy
Qv
lumen second
lms
units are sometimes called talbots
Luminous flux
F
lumen (= cdsr)
lm
also called luminous power
Luminous intensity
Iv
candela (= lm/sr)
cd
an SI base unit
Luminance
Lv
candela per square metre
cd/m2
units are sometimes called “nits”
Illuminance
Ev
lux (= lm/m2)
lx
Used for light incident on a surface
Luminous emittance
Mv
lux (= lm/m2)
lx
Used for light emitted from a surface
Luminous efficacy
lumen per watt
lm/W
ratio of luminous flux to radiant flux
See also SI Photometry
See also
Luminous coefficient
Photometry
Light pollution
Wall-plug efficiency – a related principle, but slightly different
References
^ a b Ohno, Yoshi (2004), “Color Rendering and Luminous Efficacy of White LED Spectra”, Proc. of SPIE (Fourth International Conference on Solid State Lighting), 5530, SPIE, Bellingham, WA, pp. 88, doi:10.1117/12.565757, http://physics.nist.gov/Divisions/Div844/facilities/photo/Publications/OhnoSPIE2004.pdf
^ Stimson, Allen (1974). Photometry and Radiometry for Engineers. New York: Wiley and Son.
^ Grum, Franc and Becherer, Richard (1979). Optical Radiation Measurements Vol 1. New York: Academic Press.
^ Boyd, Robert (1983). Radiometry and the Detection of Optical Radiation. New York: Wiley and Son.
^ Van Nostrand’s Scientific Encyclopedia, 3rd Edition. Princeton, New Jersey, Toronto, London, New York: D. Van Nostrand Company, Inc.. January 1958.
^ a b Defined such that the maximum value possible is 100%.
^ a b Black body visible spectrum
^ a b Integral of truncated Planck function times photopic luminosity function times 683 W/sr, according to the definition of the candela.[original research?]
^ a b See luminosity function.
^ 1 candela*4 steradians/40 W
^ Westermaier, F. V. (1920). “Recent Developments in Gas Street Lighting”. The American City (New York: Civic Press) 22 (5): 490. http://books.google.com/books?id=rWxLAAAAMAAJ&dq=mantle%20lamp&pg=PA490#v=onepage&q=mantle%20lamp&f=false.
^ Bulbs: Gluehbirne.ch: Philips Standard Lamps (German)
^ a b c d e f Philips Product Catalog (German)
^ “Osram halogen” (in German) (PDF). www.osram.de. http://www.osram.de/_global/pdf/osram_de/tools_services/downloads/allgemeinbeleuchtung/halogenlampen/haloluxhalopar.pdf. Retrieved 2008-01-28. [dead link]
^ a b Keefe, T.J. (2007). “The Nature of Light”. http://www.ccri.edu/physics/keefe/light.htm. Retrieved 2007-11-05.
^ “Osram Miniwatt-Halogen”. www.ts-audio.biz. http://www.ts-audio.biz/tsshop/WGS/411/PRD/LFH0324408/Osram_6406330_500mA_52V_E10_BLK1_MINIWATT-Halogen-Gluehlampe_f.Taschenl..htm. Retrieved 2008-01-28. [dead link]
^ a b c Klipstein, Donald L. (1996). “The Great Internet Light Bulb Book, Part I”. http://freespace.virgin.net/tom.baldwin/bulbguide.html. Retrieved 2006-04-16.
^ White LED Offers Broad Temp Range And Color Yield Electronicdesign (HTTP cookies required) Otherwise see:Google Cache
^ “Nichia NSPWR70CSS-K1 specifications” (pdf). Nichia Corp.. http://www.nichia.co.jp/specification/led_09/NSPWR70CSS-K1-E.pdf. Retrieved April 26, 2009.
^ Klipstein, Donald L.. “The Brightest and Most Efficient LEDs and where to get them”. Don Klipstein’s Web Site. http://members.misty.com/don/led.html#ln. Retrieved 2008-01-15.
^ “Cree XLamp XP-G LEDs Data Sheet”. http://www.cree.com/Products/pdf/XLampXP-G.pdf. Claims 132 lm/W.
^ a b c Toshiba E-CORE LED Lamp
^ GE 73716 7-Watt Energy Smart PAR20 LED Light Bulb
^ Toshiba to release 93 lm/W LED bulb Ledrevie
^ a b “Technical Information on Lamps” (pdf). Optical Building Blocks. http://www.pti-nj.com/UVvis/TechNotes/TechnicalInformationLamps.pdf. Retrieved 2007-10-14. Note that the figure of 150 lm/W given for xenon lamps appears to be a typo. The page contains other useful information.
^ OSRAM Sylvania Lamp and Ballast Catalog. 2007.
^ a b Federal Energy Management Program (December 2000). How to buy an energy-efficient fluorescent tube lamp. U.S. Department of Energy. http://www1.eere.energy.gov/femp/procurement/eep_fluortube_lamp.html.
^ “Low Mercury CFLs”. Energy Federation Incorporated. http://www.energyfederation.org/consumer/default.php/cPath/25_44_3006. Retrieved 2008-12-23.
^ “Conventional CFLs”. Energy Federation Incorporated. http://www.energyfederation.org/consumer/default.php/cPath/25_44_784. Retrieved 2008-12-23.
^ “Global bulbs”. 1000Bulbs.com accessdate=2010-2-20. http://www.1000bulbs.com/32-Watt-Compact-Fluorescents/37889/. |
^ Department of the Environment, Water, Heritage and the Arts, Australia. “Energy Labellingamps”. http://www.energyrating.gov.au/appsearch/download.asp. Retrieved 2008-08-14.
^ {{cite web | url=http://www.1000bulbs.com/F35T5-6500K/39598/ | publisher=1000Bulbs.com accessdate=2010-2-20
^ “1000-watt sulfur lamp now ready”. IAEEL newsletter (IAEEL) (1). 1996. Archived from the original on Aug. 18, 2003. http://web.archive.org/web/20030818061414/195.178.164.205/IAEEL/iaeel/newsl/1996/ett1996/LiTech_b_1_96.html.
^ “The Metal Halide Advantage”. Venture Lighting. 2007. http://www.venturelighting.com/TechCenter/Metal-Halide-TechIntro.html. Retrieved 2008-08-10.
^ a b “LED or Neon? A scientific comparison”. http://www.signweb.com/index.php/channel/12/id/138/.
^ “Why is lightning coloured? (gas excitations)”. http://webexhibits.org/causesofcolor/4.html.
External links
Hyperphysics has these graphs of efficacy that do not quite comply with the standard definition
Energy Efficient Light Bulbs
Other Power
CIPCO Energy Library
Categories: Photometry | Physical quantities | Lighting | Energy economicsHidden categories: All articles that may contain original research | Articles that may contain original research from November 2009 | All articles with dead external links | Articles with dead external links from June 2008 | All articles with unsourced statements | Articles with unsourced statements from November 2009
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