The lux (symbol: lx) is the SI unit of illuminance and luminous emittance measuring luminous power per area. It is used in photometry as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface. It is analogous to the radiometric unit watts per square metre, but with the power at each wavelength weighted according to the luminosity function, a standardized model of human visual brightness perception. In English, "lux" is used in both singular and plural.
Illuminance is a measure of how much luminous flux is spread over a given area. One can think of luminous flux (measured in lumens) as a measure of the total "amount" of visible light present, and the illuminance as a measure of the intensity of illumination on a surface. A given amount of light will illuminate a surface more dimly if it is spread over a larger area, so illuminance is inversely proportional to area.
One lux is equal to one lumen per square metre:
A flux of 1,000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1,000 lux. However, the same 1,000 lumens, spread out over ten square metres, produces a dimmer illuminance of only 100 lux.
Achieving an illuminance of 500 lux might be possible in a home kitchen with a single fluorescent light fixture with an output of 12,000 lumens. To light a factory floor with dozens of times the area of the kitchen would require dozens of such fixtures. Thus, lighting a larger area to the same level of lux requires a greater number of lumens.
As with other SI units, SI prefixes can be used, for example a kilolux (klx) is 1,000 lux.
Illuminance Example 10−4 lux Total starlight, overcast sky 0.002 lux Moonless clear night sky with airglow 0.01 lux Quarter moon 0.27 lux Full moon on a clear night 1 lux Full moon overhead at tropical latitudes 3.4 lux Dark limit of civil twilight under a clear sky 50 lux Family living room 80 lux Hallway/toilet 100 lux Very dark overcast day 320–500 lux Office lighting 400 lux Sunrise or sunset on a clear day. 1,000 lux Overcast day; typical TV studio lighting 10,000–25,000 lux Full daylight (not direct sun) 32,000–130,000 lux Direct sunlight
Relationship between illuminance and irradiance
Like all photometric units, the lux has a corresponding "radiometric" unit. The difference between any photometric unit and its corresponding radiometric unit is that radiometric units are based on physical power, with all wavelengths being weighted equally, while photometric units take into account the fact that the human eye's visual system is more sensitive to some wavelengths than others, and accordingly every wavelength is given a different weight. The weighting factor is known as the luminosity function.
The lux is one lumen/metre2, and the corresponding radiometric unit, which measures irradiance, is the watt/metre2. There is no single conversion factor between lux and watt/metre2; there is a different conversion factor for every wavelength, and it is not possible to make a conversion unless one knows the spectral composition of the light.
The peak of the luminosity function is at 555 nm (green); the eye's visual system is more sensitive to light of this wavelength than any other. For monochromatic light of this wavelength, the irradiance needed to make one lux is minimum, at 1.464 mW/m2. That is, one obtains 683.002 lux per W/m2 (or lumens per watt) at this wavelength. Other wavelengths of visible light produce fewer lumens per watt. The luminosity function falls to zero for wavelengths outside the visible spectrum.
For a light source with mixed wavelengths, the number of lumens per watt can be calculated by means of the luminosity function. In order to appear reasonably "white," a light source cannot consist solely of the green light to which the eye's visual photoreceptors are most sensitive, but must include a generous mixture of red and blue wavelengths to which they are much less sensitive.
This means that white (or whitish) light sources produce far fewer lumens per watt than the theoretical maximum of 683 lumens per watt. The ratio between the actual number of lumens per watt and the theoretical maximum is expressed as a percentage known as the luminous efficiency. For example, a typical incandescent light bulb has a luminous efficiency of only about 2%.
In reality, individual eyes vary slightly in their luminosity functions. However, photometric units are precisely defined and precisely measurable. They are based on an agreed-upon standard luminosity function which is based on measurements of the spectral characteristics of visual photoreception in many individual human eyes.
Use in video camera specifications
Specifications for video cameras such as camcorders and surveillance cameras often include a minimum illuminance level in lux at which the camera will record a satisfactory image. A camera with good low-light capability will have a lower lux rating. Still cameras do not use such a specification, since longer exposure times can generally be used to make pictures at very low illuminance levels, as opposed to the case in video cameras where a maximum exposure time is generally set by the frame rate.
Non-SI units of illuminance
The corresponding unit in English and American traditional units is the foot-candle. One foot candle is about 10.764 lux. Since one foot-candle is the illuminance cast on a surface by a one-candela source one foot away., a lux could be thought of as a "metre-candle", although this term is discouraged because it does not conform to SI standards for unit names.
One phot (ph) equals 10 kilolux.
One nox (nx) equals 1 millilux.
SI photometry unitsSI photometry units
Quantity Symbol[nb 1] SI unit Symbol Dimension Notes Luminous energy Qv [nb 2] lumen second lm⋅s T⋅J units are sometimes called talbots Luminous flux Φv [nb 2] lumen (= cd⋅sr) lm J also called luminous power Luminous intensity Iv candela (= lm/sr) cd J [nb 3] an SI base unit, luminous flux per unit solid angle Luminance Lv candela per square metre cd/m2 L−2⋅J units are sometimes called nits Illuminance Ev lux (= lm/m2) lx L−2⋅J used for light incident on a surface Luminous emittance Mv lux (= lm/m2) lx L−2⋅J used for light emitted from a surface Luminous exposure Hv lux second lx⋅s L−2⋅T⋅J Luminous energy density ωv lumen second per metre3 lm⋅s⋅m−3 L−3⋅T⋅J Luminous efficacy η [nb 2] lumen per watt lm/W M−1⋅L−2⋅T3⋅J ratio of luminous flux to radiant flux Luminous efficiency V 1 also called luminous coefficient See also: SI · Photometry · Radiometry
- ^ Standards organizations recommend that photometric quantities be denoted with a suffix "v" (for "visual") to avoid confusion with radiometric or photon quantities.
- ^ a b c Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ or K for luminous efficacy.
- ^ "J" is the recommended symbol for the dimension of luminous intensity in the International System of Units.
- ^ NIST Guide to SI Units - 9 Rules and Style Conventions for Spelling Unit Names, National Institute of Standards and Technology
- ^ a b c d e f Paul Schlyter, Radiometry and photometry in astronomy FAQ (2006)
- ^ "Petzl reference system for lighting performance". http://en.petzl.com/petzl/frontoffice/Lampes/static/referentiel/present_referentiel_en.jsp. Retrieved 2007-04-24.
- ^ Bunning, Erwin; and Moser, Ilse (April 1969). "Interference of moonlight with the photoperiodic measurement of time by plants, and their adaptive reaction". Proceedings of the National Academy of Sciences of the United States of America 62 (4): 1018–1022. Bibcode 1969PNAS...62.1018B. doi:10.1073/pnas.62.4.1018. PMC 223607. PMID 16591742. http://www.pnas.org/cgi/reprint/62/4/1018. Retrieved 2006-11-10.
- ^ "Electro-Optics Handbook" (pdf). burle.com. p. 63. http://www.burle.com/cgi/byteserver.pl/pdf/Electro_Optics.pdf.
- ^ Pears, Alan (June, 1998). "Chapter 7: Appliance technologies and scope for emission reduction" (PDF). Strategic Study of Household Energy and Greenhouse Issues. Australian Greenhouse Office. p. 61. http://www.energyrating.gov.au/library/pubs/pears-ago1998.pdf. Retrieved 2008-06-26 .
- ^ Australian Greenhouse Office (May, 2005). "Chapter 5: Assessing lighting savings". Working Energy Resource and Training Kit: Lighting. Archived from the original on April 15, 2007. http://web.archive.org/web/20070415151053/www.greenhouse.gov.au/lgmodules/wep/lights/training/training9.html. Retrieved March 13, 2007 .
- ^ "Low-Light Performance Calculator". http://www.scopecalc.com/.
- ^ "How to use a lux meter (Australian recommendation)". http://www.resourcesmart.vic.gov.au/documents/lux_meter.pdf.
- ^ "Illumination". Regulations (Standards - 29 CFR). Occupational Safety and Health Administration, US Dept. of Labor. http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10630.
- ^ European law UNI EN 12464
SI units Base units Derived units Accepted for use
See alsoBook:International System of Units · Category:SI base units
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