Lux: Difference between revisions

lux
lux
	A lux meter for measuring illuminance
General information
Unit system	SI
Unit of	illuminance
Symbol	lx
Conversions
1 lx in ...	... is equal to ...
SI base units	cd⋅sr⋅m−2
US customary units	0.0929 fc
CGS units	10−4 ph

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The lux (symbol: lx) is the unit of illuminance, or luminous flux per unit area, in the International System of Units (SI).^[1]^[2] It is equal to one lumen per square metre. In photometry, this is used 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 watt per square metre, but with the power at each wavelength weighted according to the luminosity function, a model of human visual brightness perception, standardized by the CIE and ISO.^[3] In English, "lux" is used as both the singular and plural form.^[4] The word is derived from the Latin word for "light", lux.

Explanation

Illuminance

Illuminance is a measure of how much luminous flux is spread over a given area. One can think of luminous flux (with the unit lumen) 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 when the luminous flux is held constant.

One lux is equal to one lumen per square metre:

1 lx = 1 lm/m² = 1 cd·sr/m².

A flux of 1000 lumens, spread uniformly over an area of 1 square metre, lights up that square metre with an illuminance of 1000 lux. However, the same 1000 lumens spread out over 10 square metres produces a dimmer illuminance of only 100 lux.

Achieving an illuminance of 500 lx might be possible in a home kitchen with a single fluorescent light fixture with an output of 12000 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 illuminance (lux) requires a greater luminous flux (lumen).

As with other named SI units, SI prefixes can be used. For example, 1 kilolux (klx) is 1000 lx.

Here are some examples of the illuminance provided under various conditions:

Illuminance (lux)	Surfaces illuminated by
0.0001	Moonless, overcast night sky (starlight)^[5]
0.002	Moonless clear night sky with airglow^[5]
0.05–0.3	Full moon on a clear night^[6]
3.4	Dark limit of civil twilight under a clear sky^[7]
20–50	Public areas with dark surroundings^[8]
50	Family living room lights (Australia, 1998)^[9]
80	Office building hallway/toilet lighting^[10]^[11]
100	Very dark overcast day^[5]
150	Train station platforms^[12]
320–500	Office lighting^[9]^[13]^[14]^[15]
400	Sunrise or sunset on a clear day.
1000	Overcast day;^[5] typical TV studio lighting
10,000–25,000	Full daylight (not direct sun)^[5]
32,000–100,000	Direct sunlight

The illuminance provided by a light source on a surface perpendicular to the direction to the source is a measure of the strength of that source as perceived from that location. For instance, a star of apparent magnitude 0 provides 2.08 microlux (μlx) at the Earth's surface.^[16] A barely perceptible magnitude 6 star provides 8 nanolux (nlx).^[17] The unobscured Sun provides an illumination of up to 100 kilolux (klx) on the Earth's surface, the exact value depending on time of year and atmospheric conditions. This direct normal illuminance is related to the solar illuminance constant E_sc, equal to 128000 lux (see Sunlight and Solar constant).

The illuminance on a surface depends on how the surface is tilted with respect to the source. For example, a pocket flashlight aimed at a wall will produce a given level of illumination if aimed perpendicular to the wall, but if the flashlight is aimed at increasing angles to the perpendicular (maintaining the same distance), the illuminated spot becomes larger and so is less highly illuminated. When a surface is tilted at an angle to a source, the illumination provided on the surface is reduced because the tilted surface subtends a smaller solid angle from the source, and therefore it receives less light. For a point source, the illumination on the tilted surface is reduced by a factor equal to the cosine of the angle between a ray coming from the source and the normal to the surface.^[18] In practical lighting problems, given information on the way light is emitted from each source and the distance and geometry of the lighted area, a numerical calculation can be made of the illumination on a surface by adding the contributions of every point on every light source.

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 image-forming 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 per square metre (lm/m²), and the corresponding radiometric unit, which measures irradiance, is the watt per square metre (W/m²). There is no single conversion factor between lux and W/m²; 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 image-forming visual system is more sensitive to light of this wavelength than any other. For monochromatic light of this wavelength, the amount of illuminance for a given amount of irradiance is maximum: 683.002 lx per 1 W/m²; the irradiance needed to make 1 lx at this wavelength is about 1.464 mW/m². Other wavelengths of visible light produce fewer lux per watt-per-meter-squared. 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 image-forming 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 683.002 lm/W. 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 based on measurements of the spectral characteristics of image-forming 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 minimal illuminance level in lux at which the camera will record a satisfactory image.^{[citation needed]} 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 maximal 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 lx. 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 (10 klx).

One nox (nx) equals 1 millilux (1 mlx) at light color 2042 K or 2046 K (formerly 2360 K).^[19]^[20]^[21]^[22]

In astronomy, apparent magnitude is a measure of the illuminance of a star on the Earth's atmosphere. A star with apparent magnitude 0 is 2.54 microlux outside the earth's atmosphere, and 82% of that (2.08 microlux) under clear skies.^[16] A magnitude 6 star (just barely visible under good conditions) would be 8.3 nanolux. A standard candle (one candela) a kilometre away would provide an illuminance of 1 microlux—about the same as a magnitude 1 star.

Legacy Unicode symbol

Unicode includes a symbol for "lx": U+33D3 ㏓ SQUARE LX. It is a legacy code to accommodate old code pages in some Asian languages. Use of this code is not recommended in new documents.

SI photometry units

SI photometry quantities
v
t
e
Quantity		Unit		Dimension ^{[nb 1]}	Notes
Name	Symbol^{[nb 2]}	Name	Symbol	Dimension ^{[nb 1]}	Notes
Luminous energy	$Q v$ ^{[nb 3]}	lumen second	lm⋅s	T⋅J	The lumen second is sometimes called the talbot.
Luminous flux, luminous power	$Φ v$ ^{[nb 3]}	lumen (= candela steradian)	lm (= cd⋅sr)	J	Luminous energy per unit time
Luminous intensity	$I v$	candela (= lumen per steradian)	cd (= lm/sr)	J	Luminous flux per unit solid angle
Luminance	$L v$	candela per square metre	cd/m² (= lm/(sr⋅m²))	L⁻²⋅J	Luminous flux per unit solid angle per unit projected source area. The candela per square metre is sometimes called the nit.
Illuminance	$E v$	lux (= lumen per square metre)	lx (= lm/m²)	L⁻²⋅J	Luminous flux incident on a surface
Luminous exitance, luminous emittance	$M v$	lumen per square metre	lm/m²	L⁻²⋅J	Luminous flux emitted from a surface
Luminous exposure	$H v$	lux second	lx⋅s	L⁻²⋅T⋅J	Time-integrated illuminance
Luminous energy density	$ω v$	lumen second per cubic metre	lm⋅s/m³	L⁻³⋅T⋅J
Luminous efficacy (of radiation)	$K$	lumen per watt	lm/W	M⁻¹⋅L⁻²⋅T³⋅J	Ratio of luminous flux to radiant flux
Luminous efficacy (of a source)	$η$ ^{[nb 3]}	lumen per watt	lm/W	M⁻¹⋅L⁻²⋅T³⋅J	Ratio of luminous flux to power consumption
Luminous efficiency, luminous coefficient	$V$			1	Luminous efficacy normalized by the maximum possible efficacy
See also: SI Photometry Radiometry

^ The symbols in this column denote dimensions; "L", "T" and "J" are for length, time and luminous intensity respectively, not the symbols for the units litre, tesla and joule.
^ Standards organizations recommend that photometric quantities be denoted with a subscript "v" (for "visual") to avoid confusion with radiometric or photon quantities. For example: USA Standard Letter Symbols for Illuminating Engineering USAS Z7.1-1967, Y10.18-1967
^ ^a ^b ^c Alternative symbols sometimes seen: $W$ for luminous energy, $P$ or $F$ for luminous flux, and $ρ$ for luminous efficacy of a source.

References

^ International Bureau of Weights and Measures (2019-05-20), The International System of Units (SI) (PDF) (9th ed.), ISBN 978-92-822-2272-0, archived from the original on 2021-10-18
^ CIE (2020). CIE S 017:2020 ILV: International Lighting Vocabulary, 2nd edition (2 ed.). CIE.
^ ISO/CIE 23539:2023 CIE TC 2-93 Photometry — The CIE system of physical photometry. ISO/CIE. 2023. doi:10.25039/IS0.CIE.23539.2023.
^ NIST Guide to SI Units. Chapter 9 – Rules and Style Conventions for Spelling Unit Names, National Institute of Standards and Technology.
^ ^a ^b ^c ^d ^e Schlyter, Paul (1997–2009). "Radiometry and photometry in astronomy".
Starlight illuminance coincides with the human eye's minimum illuminance while moonlight coincides with the human eye's minimum colour vision illuminance (IEE Reviews, 1972, page 1183).
^ Kyba, Christopher C. M.; Mohar, Andrej; Posch, Thomas (2017-02-01). "How bright is moonlight?" (PDF). Astronomy & Geophysics. 58 (1): 1.31–1.32. doi:10.1093/astrogeo/atx025.
^ "Electro-Optics Handbook" (pdf). photonis.com. p. 63. Retrieved 2012-04-02.^{[permanent dead link]}
^ "NOAO Common and Recommended Light Levels Indoor" (PDF). Archived from the original (PDF) on 2021-07-06. Retrieved 2016-11-13.
^ ^a ^b Pears, Alan (June 1998). "Chapter 7: Appliance technologies and scope for emission reduction". Strategic Study of Household Energy and Greenhouse Issues: A report for Environment Australia (PDF). Department of Industry and Science, Commonwealth of Australia. p. 61. Archived from the original on 2011-03-02. Retrieved 2008-06-26.{{cite book}}: CS1 maint: unfit URL (link)
^ Australian Greenhouse Office (May 2005). "Chapter 5: Assessing lighting savings". Working Energy Resource and Training Kit: Lighting. Archived from the original on 2007-04-15. Retrieved 2007-03-17.
^ "Low-Light Performance Calculator". Archived from the original on 2013-06-15. Retrieved 2010-09-27.
^ Darlington, Paul (2017-12-05). "London Underground: Keeping the lights on". Rail Engineer. Archived from the original on 2018-11-16. Retrieved 2017-12-20.
^ "How to use a lux meter (Australian recommendation)" (PDF). Sustainability Victoria. April 2010. Archived from the original (PDF) on 2011-07-07.
^ "Illumination. - 1926.56". Regulations (Standards - 29 CFR). Occupational Safety and Health Administration, US Dept. of Labor. Archived from the original on 2009-05-08.
^ European law UNI EN 12464
^ ^a ^b Schlyter, Section 7.
^ Schlyter, Section 14.
^ Jack L. Lindsey, Applied Illumination Engineering, The Fairmont Press, Inc., 1997 ISBN 0881732125 page 218
^ Lohse, Bernhard; Stille, Ulrich [in German] (January 1948) [1947-08-19]. Written at Braunschweig, Germany. Deutsche Physikalische Gesellschaft (ed.). "Einführung und Bestimmung des Lichtäquivalents". Zeitschrift für Physik (in German). 125 (1–3). Berlin / Göttingen / Heidelberg, Germany: Springer-Verlag: 133–158. Bibcode:1948ZPhy..125..133L. doi:10.1007/BF01337623. ISSN 0044-3328. S2CID 125512557. Retrieved 2023-03-19.
^ Westphal, Wilhelm Heinrich (1952). "Nox, Dunkelleuchtdichte, Skot". In Westphal, Wilhelm H. (ed.). Physikalisches Wörterbuch (in German) (1 ed.). Berlin / Göttingen / Heidelberg, Germany: Springer-Verlag OHG. pp. 125, 271, 389. doi:10.1007/978-3-662-12706-3. ISBN 978-3-662-12707-0. Retrieved 2023-03-16. pp. 125, 271: Nox, abgek[ürzt] nx, Einheit der Dunkelbeleuchtungsstärke (Dunkelleuchtdichte), welche für zahlenmäßige Angaben und zum Anschluß der Dunkelbeleuchtungsstärke an die normale Beleuchtungsstärke 1940 von der Deutschen Lichttechnischen Gesellschaft [de] geschaffen wurde. Bezüglich der Farbtemperatur der Strahlung und des Anschlusses von Zahlenwerten der Beleuchtungsstärke E und der Dunkelbeleuchtungsstärke E gelten analog die gleichen Festlegungen wie bei der Dunkelleuchtdichte und dem Skot (sk). Für eine Strahlung der Farbtemperatur T₁ = 2360 K gilt: 1 nx = 10⁻³ lx (Lux). Für eine beliebige Strahlung bekannter spektraler Strahlungsleistung S₁ lautet die Verknüpfungsbeziehung zwischen in 10⁻³ lx gemessenem Zahlenwert {E} der Beleuchtungsstärke und in nx gemessenem Zahlenwert {E} der Dunkelbeleuchtungsstärke: {E}_nx = (2,161 ± 0,001) · {E}_10⁻³ lx · ∫ S_λ V_λ,W dλ / ∫ S_λ V_λ dλ, wobei V_λ die relative spektrale Hellempfindlichkeit und V_λ,W die relative spektrale Dämmerungsempfindlichkeit des menschlichen Auges nach Weaver^[A] bedeuten. [...] Dunkelleuchtdichte. [...] Ist das Auge dunkeladaptiert, d.h. einer Leuchtdichte von weniger als 0,01 asb ausgesetzt, so gilt infolge des Purkinje-Phänomens eine von der spektralen Hellempfindlichkeitskurve abweichende, nach dem kurzwelligen Ende des Spektrums hin verschobene Empfindlichkeitskurve des Auges, die Stäbchenkurve des Dämmerungssehens. Unter Zugrundelegung dieser Empfindlichkeitskurve hat man 1940 in Deutschland die Dunkelleuchtdichte mit der Einheit Skot (sk) so festgesetzt, daß bei einem Licht der Farbtemperatur 2360 °K 1 sk = 10⁻³ asb gilt. 1948 ist von der Internationalen Beleuchtungskommission (IBK) die Bezugstemperatur auf 2046 K, die Erstarrungstemperatur des Platins, festgesetzt worden. Die Bezeichnung Skot wurde von der IBK nicht übernommen, dafür soll "skotopisches Stilb" gesagt werden. Als höchstzulässiger Grenzwert für die Dunkelleuchtdichte ist in Deutschland 10 Skot festgesetzt worden, um eine Verwendung der Dunkelleuchtdichte im Gebiet des gemischten Zapfen- und Stäbchensehens zu vermeiden, da in diesem Bereich die photometrischen Maßgrößen wegen der allmählich gleitenden Augenempfindlichkeitskurve ihren Sinn verlieren.
^ Grimsehl, Ernst [in German]; Schallreuter, Walter [in German] (1988) [1976]. "1. Licht: 1.4. Photometrie: 1.4.1. Grundbegriffe". In Haferkorn, Heinz (ed.). Lehrbuch der Physik: Optik (in German). Vol. 3 (19 ed.). Leipzig, Germany: BSB BG Teubner Verlagsgesellschaft. pp. 33–38 [37–38]. doi:10.1007/978-3-322-96431-1. ISBN 978-3-322-96432-8. Order No. 6666211, VLN 294-375/84/88, LSV 1164. Retrieved 2023-03-16. pp. 37–38: Dunkelsehen [...] Für das Dunkelsehen, bei dem nur die Stäbchen angeregt werden, definiert man die Dunkelleuchtdichte mit der Einheit Skot (sk) und die Dunkelbeleuchtungsstärke mit der Einheit Nox (nx). Die Umrechnungsfaktoren zwischen den Hell- und Dunkelgrößen hängen von der spektralen Zusammensetzung des Lichtes ab. Sie werden deshalb für die Farbtemperatur 2042 K (früher 2360 K) festgelegt. Bei dieser ist 1 sk = 10⁻³ asb und 1 nx = 10⁻³ lx.
^ Keplinger, Thomas (2021-03-29). "1939 bis 1945 – Im Keller glüht das Lumogen". Worte im Dunkel (in Austrian German). Vienna, Austria. Archived from the original on 2023-03-16. Retrieved 2023-03-16. Skot und Nox [...] Interessant ist in diesem Zusammenhang die Einführung neuer Messeinheiten. Die Voraussetzungen der Forschung beziehungsweise die Erfordernisse an die Leuchtfarben unterschieden sich so stark von allen bis dahin erforschten Gebieten, dass die Deutsche Lichttechnische Gesellschaft [de] 1940 eigene Einheiten ins Leben rief: Die Dunkelleuchtdichte wurde in Skot und die Dunkelbeleuchtungsstärke in Nox gemessen.^[B] Diese Einheiten grenzten an die bereits bestehenden Größen der Leuchtdichte und Beleuchtungsstärke an und dienten der zahlenmäßigen Erfassung geringster Lichtwerte. So entsprach etwa ein Nox 10⁻³ Lux.

External links

Radiometry and photometry FAQ Professor Jim Palmer's Radiometry FAQ page (University of Arizona).

[note-dimension-symbol-23] The symbols in this column denote dimensions; "L", "T" and "J" are for length, time and luminous intensity respectively, not the symbols for the units litre, tesla and joule.

[note-suffix-v-24] Standards organizations recommend that photometric quantities be denoted with a subscript "v" (for "visual") to avoid confusion with radiometric or photon quantities. For example: USA Standard Letter Symbols for Illuminating Engineering USAS Z7.1-1967, Y10.18-1967

[note-alternative-symbol-photometric-25] Alternative symbols sometimes seen: $W$ for luminous energy, $P$ or $F$ for luminous flux, and $ρ$ for luminous efficacy of a source.

[SIBrochure9thEd-1] International Bureau of Weights and Measures (2019-05-20), The International System of Units (SI) (PDF) (9th ed.), ISBN 978-92-822-2272-0, archived from the original on 2021-10-18

[2] CIE (2020). CIE S 017:2020 ILV: International Lighting Vocabulary, 2nd edition (2 ed.). CIE.

[3] ISO/CIE 23539:2023 CIE TC 2-93 Photometry — The CIE system of physical photometry. ISO/CIE. 2023. doi:10.25039/IS0.CIE.23539.2023.

[4] NIST Guide to SI Units. Chapter 9 – Rules and Style Conventions for Spelling Unit Names, National Institute of Standards and Technology.

[radfaq-5] Schlyter, Paul (1997–2009). "Radiometry and photometry in astronomy".
Starlight illuminance coincides with the human eye's minimum illuminance while moonlight coincides with the human eye's minimum colour vision illuminance (IEE Reviews, 1972, page 1183).

[6] Kyba, Christopher C. M.; Mohar, Andrej; Posch, Thomas (2017-02-01). "How bright is moonlight?" (PDF). Astronomy & Geophysics. 58 (1): 1.31–1.32. doi:10.1093/astrogeo/atx025.

[7] "Electro-Optics Handbook" (pdf). photonis.com. p. 63. Retrieved 2012-04-02.^{[permanent dead link]}

[NOAO_CaRLLI-8] "NOAO Common and Recommended Light Levels Indoor" (PDF). Archived from the original (PDF) on 2021-07-06. Retrieved 2016-11-13.

[energyrating-9] Pears, Alan (June 1998). "Chapter 7: Appliance technologies and scope for emission reduction". Strategic Study of Household Energy and Greenhouse Issues: A report for Environment Australia (PDF). Department of Industry and Science, Commonwealth of Australia. p. 61. Archived from the original on 2011-03-02. Retrieved 2008-06-26.{{cite book}}: CS1 maint: unfit URL (link)

[10] Australian Greenhouse Office (May 2005). "Chapter 5: Assessing lighting savings". Working Energy Resource and Training Kit: Lighting. Archived from the original on 2007-04-15. Retrieved 2007-03-17.

[11] "Low-Light Performance Calculator". Archived from the original on 2013-06-15. Retrieved 2010-09-27.

[12] Darlington, Paul (2017-12-05). "London Underground: Keeping the lights on". Rail Engineer. Archived from the original on 2018-11-16. Retrieved 2017-12-20.

[13] "How to use a lux meter (Australian recommendation)" (PDF). Sustainability Victoria. April 2010. Archived from the original (PDF) on 2011-07-07.

[14] "Illumination. - 1926.56". Regulations (Standards - 29 CFR). Occupational Safety and Health Administration, US Dept. of Labor. Archived from the original on 2009-05-08.

[15] European law UNI EN 12464

[Schlyter7-16] Schlyter, Section 7.

[17] Schlyter, Section 14.

[18] Jack L. Lindsey, Applied Illumination Engineering, The Fairmont Press, Inc., 1997 ISBN 0881732125 page 218

[Lohse-Stille_1948-19] Lohse, Bernhard; Stille, Ulrich [in German] (January 1948) [1947-08-19]. Written at Braunschweig, Germany. Deutsche Physikalische Gesellschaft (ed.). "Einführung und Bestimmung des Lichtäquivalents". Zeitschrift für Physik (in German). 125 (1–3). Berlin / Göttingen / Heidelberg, Germany: Springer-Verlag: 133–158. Bibcode:1948ZPhy..125..133L. doi:10.1007/BF01337623. ISSN 0044-3328. S2CID 125512557. Retrieved 2023-03-19.

[Westphal_1952-20] Westphal, Wilhelm Heinrich (1952). "Nox, Dunkelleuchtdichte, Skot". In Westphal, Wilhelm H. (ed.). Physikalisches Wörterbuch (in German) (1 ed.). Berlin / Göttingen / Heidelberg, Germany: Springer-Verlag OHG. pp. 125, 271, 389. doi:10.1007/978-3-662-12706-3. ISBN 978-3-662-12707-0. Retrieved 2023-03-16. pp. 125, 271: Nox, abgek[ürzt] nx, Einheit der Dunkelbeleuchtungsstärke (Dunkelleuchtdichte), welche für zahlenmäßige Angaben und zum Anschluß der Dunkelbeleuchtungsstärke an die normale Beleuchtungsstärke 1940 von der Deutschen Lichttechnischen Gesellschaft [de] geschaffen wurde. Bezüglich der Farbtemperatur der Strahlung und des Anschlusses von Zahlenwerten der Beleuchtungsstärke E und der Dunkelbeleuchtungsstärke E gelten analog die gleichen Festlegungen wie bei der Dunkelleuchtdichte und dem Skot (sk). Für eine Strahlung der Farbtemperatur T₁ = 2360 K gilt: 1 nx = 10⁻³ lx (Lux). Für eine beliebige Strahlung bekannter spektraler Strahlungsleistung S₁ lautet die Verknüpfungsbeziehung zwischen in 10⁻³ lx gemessenem Zahlenwert {E} der Beleuchtungsstärke und in nx gemessenem Zahlenwert {E} der Dunkelbeleuchtungsstärke: {E}_nx = (2,161 ± 0,001) · {E}_10⁻³ lx · ∫ S_λ V_λ,W dλ / ∫ S_λ V_λ dλ, wobei V_λ die relative spektrale Hellempfindlichkeit und V_λ,W die relative spektrale Dämmerungsempfindlichkeit des menschlichen Auges nach Weaver^[A] bedeuten. [...] Dunkelleuchtdichte. [...] Ist das Auge dunkeladaptiert, d.h. einer Leuchtdichte von weniger als 0,01 asb ausgesetzt, so gilt infolge des Purkinje-Phänomens eine von der spektralen Hellempfindlichkeitskurve abweichende, nach dem kurzwelligen Ende des Spektrums hin verschobene Empfindlichkeitskurve des Auges, die Stäbchenkurve des Dämmerungssehens. Unter Zugrundelegung dieser Empfindlichkeitskurve hat man 1940 in Deutschland die Dunkelleuchtdichte mit der Einheit Skot (sk) so festgesetzt, daß bei einem Licht der Farbtemperatur 2360 °K 1 sk = 10⁻³ asb gilt. 1948 ist von der Internationalen Beleuchtungskommission (IBK) die Bezugstemperatur auf 2046 K, die Erstarrungstemperatur des Platins, festgesetzt worden. Die Bezeichnung Skot wurde von der IBK nicht übernommen, dafür soll "skotopisches Stilb" gesagt werden. Als höchstzulässiger Grenzwert für die Dunkelleuchtdichte ist in Deutschland 10 Skot festgesetzt worden, um eine Verwendung der Dunkelleuchtdichte im Gebiet des gemischten Zapfen- und Stäbchensehens zu vermeiden, da in diesem Bereich die photometrischen Maßgrößen wegen der allmählich gleitenden Augenempfindlichkeitskurve ihren Sinn verlieren.

[Grimsehl_1988-21] Grimsehl, Ernst [in German]; Schallreuter, Walter [in German] (1988) [1976]. "1. Licht: 1.4. Photometrie: 1.4.1. Grundbegriffe". In Haferkorn, Heinz (ed.). Lehrbuch der Physik: Optik (in German). Vol. 3 (19 ed.). Leipzig, Germany: BSB BG Teubner Verlagsgesellschaft. pp. 33–38 [37–38]. doi:10.1007/978-3-322-96431-1. ISBN 978-3-322-96432-8. Order No. 6666211, VLN 294-375/84/88, LSV 1164. Retrieved 2023-03-16. pp. 37–38: Dunkelsehen [...] Für das Dunkelsehen, bei dem nur die Stäbchen angeregt werden, definiert man die Dunkelleuchtdichte mit der Einheit Skot (sk) und die Dunkelbeleuchtungsstärke mit der Einheit Nox (nx). Die Umrechnungsfaktoren zwischen den Hell- und Dunkelgrößen hängen von der spektralen Zusammensetzung des Lichtes ab. Sie werden deshalb für die Farbtemperatur 2042 K (früher 2360 K) festgelegt. Bei dieser ist 1 sk = 10⁻³ asb und 1 nx = 10⁻³ lx.

[Keplinger_2021-22] Keplinger, Thomas (2021-03-29). "1939 bis 1945 – Im Keller glüht das Lumogen". Worte im Dunkel (in Austrian German). Vienna, Austria. Archived from the original on 2023-03-16. Retrieved 2023-03-16. Skot und Nox [...] Interessant ist in diesem Zusammenhang die Einführung neuer Messeinheiten. Die Voraussetzungen der Forschung beziehungsweise die Erfordernisse an die Leuchtfarben unterschieden sich so stark von allen bis dahin erforschten Gebieten, dass die Deutsche Lichttechnische Gesellschaft [de] 1940 eigene Einheiten ins Leben rief: Die Dunkelleuchtdichte wurde in Skot und die Dunkelbeleuchtungsstärke in Nox gemessen.^[B] Diese Einheiten grenzten an die bereits bestehenden Größen der Leuchtdichte und Beleuchtungsstärke an und dienten der zahlenmäßigen Erfassung geringster Lichtwerte. So entsprach etwa ein Nox 10⁻³ Lux.

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[10]

[11]

[12]

[13]

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@@ Line 1: / Line 1: @@
 {{short description|SI derived unit of illuminance}}
 {{Other uses|Lux (disambiguation)}}
+{{Use dmy dates|date=March 2023|cs1-dates=y}}
 {{Infobox unit
 | bgcolor    =
@@ Line 11: / Line 12: @@
 | units1     = [[SI base unit]]s
 | inunits1   = [[candela|cd]]⋅[[steradian|sr]]⋅[[metre|m]]<sup>−2</sup>
-| units2     = [[United States customary units|American customary]] units
+| units2     = [[United States customary units|US customary units]]
 | inunits2   = 0.0929 [[Foot-candle|fc]]
 | units3     = [[Centimetre–gram–second system of units|CGS]] units
-| inunits3   = {{val|e=-4|u=[[phot|phots]]}}
+| inunits3   = {{val|e=-4|u=[[phot|ph]]}}
 }}
-The '''lux''' (symbol: '''lx''') is the unit of [[illuminance]], or [[luminous flux]] per unit area, in the [[International System of Units]] (SI).<ref>[http://physics.nist.gov/cuu/Units/units.html SI Derived Units], National Institute of Standards and Technology.</ref><ref name="iec-845-01-52">{{cite web |url=http://www.electropedia.org/iev/iev.nsf/display?openform&ievref=845-01-52 |work=	Lighting / Radiation, quantities and units |publisher=International Electrotechnical Commission |title=Lux |year=1987 |access-date=2019-11-30}}</ref> It is equal to one [[lumen (unit)|lumen]] per square metre. In [[photometry (optics)|photometry]], this is used 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 [[radiometry|radiometric]] unit [[Irradiance|watt 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 as both the singular and plural form.<ref>[https://www.nist.gov/pml/special-publication-811/nist-guide-si-chapter-9-rules-and-style-conventions-spelling-unit-names NIST Guide to SI Units. Chapter 9 – Rules and Style Conventions for Spelling Unit Names], National Institute of Standards and Technology.</ref>
+The '''lux''' (symbol: '''lx''') is the unit of [[illuminance]], or [[luminous flux]] per unit area, in the [[International System of Units]] (SI).<ref name="SIBrochure9thEd">{{citation |title=The International System of Units (SI) |author=International Bureau of Weights and Measures |author-link=New SI |date=20 May 2019  |edition=9th |isbn=978-92-822-2272-0 |url=https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9.pdf| archive-url = https://web.archive.org/web/20211018184555/https://www.bipm.org/documents/20126/41483022/SI-Brochure-9.pdf/fcf090b2-04e6-88cc-1149-c3e029ad8232 |archive-date=18 October 2021 |url-status=live}}</ref><ref>{{cite book |last1=CIE |title=CIE S 017:2020 ILV: International Lighting Vocabulary, 2nd edition. |date=2020 |publisher=CIE |edition=2 |url=https://cie.co.at/eilvterm/17-21-085}}</ref> It is equal to one [[lumen (unit)|lumen]] per square metre. In [[photometry (optics)|photometry]], this is used 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 [[radiometry|radiometric]] unit [[Irradiance|watt per square metre]], but with the power at each [[wavelength]] weighted according to the [[luminosity function]], a model of [[human]] visual brightness perception, standardized by the CIE and [[International Organization for Standardization|ISO]].<ref>{{cite book | url=https://www.iso.org/standard/83178.html | title=ISO/CIE 23539:2023 CIE TC 2-93 Photometry — The CIE system of physical photometry | publisher=ISO/CIE | date=2023 | language=en | doi=10.25039/IS0.CIE.23539.2023}}</ref> In English, "lux" is used as both the singular and plural form.<ref>[https://www.nist.gov/pml/special-publication-811/nist-guide-si-chapter-9-rules-and-style-conventions-spelling-unit-names NIST Guide to SI Units. Chapter 9 – Rules and Style Conventions for Spelling Unit Names], National Institute of Standards and Technology.</ref>
 The word is derived from the [[Latin]] word for "light", [[wikt:lux|lux]].
 ==Explanation==
 ===Illuminance===
-Illuminance is a measure of how much [[luminous flux]] is spread over a given area.  One can think of luminous flux (measured in [[lumen (unit)|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 when the luminous flux is held constant.
+Illuminance is a measure of how much [[luminous flux]] is spread over a given area.  One can think of luminous flux (with the unit [[lumen (unit)|lumen]]) 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 when the luminous flux is held constant.
 One lux is equal to one lumen per [[square metre]]:
@@ Line 29: / Line 29: @@
 A flux of 1000&nbsp;lumens, spread uniformly over an area of 1 square metre, lights up that square metre with an illuminance of 1000&nbsp;lux. However, the same 1000&nbsp;lumens spread out over 10 square metres produces a dimmer illuminance of only 100&nbsp;lux.
-Achieving an illuminance of 500&nbsp;lx might be possible in a home kitchen with a single [[fluorescent light]] fixture with an output of {{val|12000|u=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.
+Achieving an illuminance of 500&nbsp;lx might be possible in a home kitchen with a single [[fluorescent light]] fixture with an output of {{val|12000|u=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 illuminance (lux) requires a greater luminous flux (lumen).
-As with other SI units, [[SI prefixes]] can be used. For example, 1&nbsp;kilolux (klx) is 1000&nbsp;lx.
+As with other named SI units, [[SI prefixes]] can be used. For example, 1&nbsp;kilolux (klx) is 1000&nbsp;lx.
 Here are some examples of the illuminance provided under various conditions:
@@ Line 38: / Line 38: @@
 ! Illuminance (lux) !! style="text-align:left;"|Surfaces illuminated by
 |-
-|0.0001||Moonless, overcast night sky ([[starlight]])<ref name="radfaq">{{cite web |title=Radiometry and photometry in astronomy |url=http://stjarnhimlen.se/comp/radfaq.html#10 |first=Paul |last=Schlyter |year=1997–2009 }}<br />Starlight illuminance coincides with the human eye's minimum illuminance while moonlight coincides with the human eye's minimum colour vision illuminance (IEE Reviews, 1972, [https://books.google.ca/books?id=00dJAQAAIAAJ&q=minimum+illumination+for+human+colour+vision+lx&sa=X&ved=0ahUKEwjPjuuN88_XAhXLQ98KHZsfBNMQ6AEIOjAD page 1183]).</ref>
+|0.0001||Moonless, overcast night sky ([[starlight]])<ref name="radfaq">{{cite web |title=Radiometry and photometry in astronomy |url=http://stjarnhimlen.se/comp/radfaq.html#10 |author-first=Paul |author-last=Schlyter |date=1997–2009}}<br />Starlight illuminance coincides with the human eye's minimum illuminance while moonlight coincides with the human eye's minimum colour vision illuminance (IEE Reviews, 1972, [https://books.google.com/books?id=00dJAQAAIAAJ&q=minimum+illumination+for+human+colour+vision+lx page 1183]).</ref>
 |-
 |0.002||Moonless clear night sky with [[airglow]]<ref name="radfaq" />
 |-
-|0.05–0.3||Full moon on a clear night<ref>{{cite journal |last1=Kyba |first1=Christopher C. M. |last2=Mohar |first2=Andrej |last3=Posch |first3=Thomas |title=How bright is moonlight? |journal=Astronomy & Geophysics |date=1 February 2017 |volume=58 |issue=1 |pages=1.31–1.32 |doi=10.1093/astrogeo/atx025 |url=https://gfzpublic.gfz-potsdam.de/pubman/item/item_2022891_4/component/file_2029888/2022891.pdf }}</ref>
+|0.05–0.3||Full moon on a clear night<ref>{{cite journal |author-last1=Kyba |author-first1=Christopher C. M. |author-last2=Mohar |author-first2=Andrej |author-last3=Posch |author-first3=Thomas |title=How bright is moonlight? |journal=Astronomy & Geophysics |date=1 February 2017 |volume=58 |issue=1 |pages=1.31–1.32 |doi=10.1093/astrogeo/atx025 |url=https://gfzpublic.gfz-potsdam.de/pubman/item/item_2022891_4/component/file_2029888/2022891.pdf }}</ref>
 |-
 |3.4|| Dark limit of [[Twilight#Civil_twilight|civil twilight]] under a clear sky<ref>{{cite web |url=http://www.photonis.com/attachment.php?id_attachment=95 |title=Electro-Optics Handbook |format=pdf |work=photonis.com |page=63 |access-date=2012-04-02}}{{dead link|date=September 2018|fix-attempted=yes}}</ref>
 |-
-|20–50||Public areas with dark surroundings<ref name="NOAO_CaRLLI">{{cite web |url=https://www.noao.edu/education/QLTkit/ACTIVITY_Documents/Safety/LightLevels_outdoor+indoor.pdf |title=NOAO Common and Recommended Light Levels Indoor |access-date=13 November 2016 |archive-date=6 July 2021 |archive-url=https://web.archive.org/web/20210706034730/https://www.noao.edu/education/QLTkit/ACTIVITY_Documents/Safety/LightLevels_outdoor+indoor.pdf |url-status=dead }}</ref>
+|20–50||Public areas with dark surroundings<ref name="NOAO_CaRLLI">{{cite web |url=https://www.noao.edu/education/QLTkit/ACTIVITY_Documents/Safety/LightLevels_outdoor+indoor.pdf |title=NOAO Common and Recommended Light Levels Indoor |access-date=13 November 2016 |archive-date=6 July 2021 |archive-url=https://web.archive.org/web/20210706034730/https://www.noao.edu/education/QLTkit/ACTIVITY_Documents/Safety/LightLevels_outdoor+indoor.pdf |url-status=dead}}</ref>
 |-
-|50||Family living room lights (Australia, 1998)<ref name="energyrating">{{Cite book|first=Alan |last=Pears |publisher=Department of Industry and Science, Commonwealth of Australia|work=Sustainable Solutions Pty Ltd|title=Strategic Study of Household Energy and Greenhouse Issues|date=June 1998|url=http://www.energyrating.com.au/library/pubs/pears-ago1998.pdf|chapter=Chapter 7: Appliance technologies and scope for emission reduction |page=61 |access-date =2008-06-26| archive-url =https://web.archive.org/web/20110302110649/http://www.energyrating.gov.au/library/pubs/pears-ago1998.pdf| archive-date=2 March 2011}}</ref>
+|50||Family living room lights (Australia, 1998)<ref name="energyrating">{{Cite book |author-first=Alan |author-last=Pears |publisher=Department of Industry and Science, Commonwealth of Australia |title=Strategic Study of Household Energy and Greenhouse Issues: A report for Environment Australia |date=June 1998 |url=http://www.energyrating.com.au/library/pubs/pears-ago1998.pdf |chapter=Chapter 7: Appliance technologies and scope for emission reduction |page=61 |access-date=2008-06-26 |archive-url=https://web.archive.org/web/20110302110649/http://www.energyrating.gov.au/library/pubs/pears-ago1998.pdf |archive-date=2 March 2011 |url-status= usurped}}</ref>
 |-
-|80||Office building hallway/[[Toilet (room)|toilet]] lighting<ref>{{Cite book|author=Australian Greenhouse Office |title=Working Energy Resource and Training Kit: Lighting |date=May 2005 |url=http://www.greenhouse.gov.au/lgmodules/wep/lights/index.html |chapter=Chapter 5: Assessing lighting savings |access-date=2007-03-17 |archive-url=https://web.archive.org/web/20070415151053/http://www.greenhouse.gov.au/lgmodules/wep/lights/training/training9.html |archive-date=2007-04-15 |url-status=dead }}</ref><ref>{{cite web |url=http://www.scopecalc.com/ |title=Low-Light Performance Calculator |access-date=27 September 2010 |archive-url=https://web.archive.org/web/20130615074318/http://scopecalc.com/ |archive-date=15 June 2013 |url-status=dead }}</ref>
+|80||Office building hallway/[[Toilet (room)|toilet]] lighting<ref>{{Cite book |author=Australian Greenhouse Office |title=Working Energy Resource and Training Kit: Lighting |date=May 2005 |url=http://www.greenhouse.gov.au/lgmodules/wep/lights/index.html |chapter=Chapter 5: Assessing lighting savings |access-date=2007-03-17 |archive-url=https://web.archive.org/web/20070415151053/http://www.greenhouse.gov.au/lgmodules/wep/lights/training/training9.html |archive-date=2007-04-15 |url-status=dead}}</ref><ref>{{cite web |url=http://www.scopecalc.com/ |title=Low-Light Performance Calculator |access-date=27 September 2010 |archive-url=https://web.archive.org/web/20130615074318/http://scopecalc.com/ |archive-date=15 June 2013 |url-status=dead}}</ref>
 |-
 |100||Very dark overcast day<ref name="radfaq" />
 |-
-|150||Train station platforms<ref>{{cite web|last1=Darlington|first1=Paul|title=London Underground: Keeping the lights on|url=https://www.railengineer.uk/2017/12/05/london-underground-keeping-the-lights-on/|website=Rail Engineer|access-date=20 December 2017|date=5 December 2017|archive-date=16 November 2018|archive-url=https://web.archive.org/web/20181116020106/https://www.railengineer.uk/2017/12/05/london-underground-keeping-the-lights-on/|url-status=dead}}</ref>
+|150||Train station platforms<ref>{{cite web |author-last=Darlington |author-first=Paul |title=London Underground: Keeping the lights on |url=https://www.railengineer.uk/2017/12/05/london-underground-keeping-the-lights-on/ |website=Rail Engineer |access-date=20 December 2017 |date=5 December 2017 |archive-date=16 November 2018 |archive-url=https://web.archive.org/web/20181116020106/https://www.railengineer.uk/2017/12/05/london-underground-keeping-the-lights-on/ |url-status=dead}}</ref>
 |-
-|320–500|| Office lighting<ref name=energyrating/><ref>{{cite web |url=http://www.resourcesmart.vic.gov.au/documents/lux_meter.pdf|title=How to use a lux meter (Australian recommendation)|publisher=Sustainability Victoria |date=April 2010| archive-url =https://web.archive.org/web/20110707054658/http://www.resourcesmart.vic.gov.au/documents/lux_meter.pdf| archive-date=7 July 2011}}</ref><ref>{{cite web |url=https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10630 |title=Illumination. - 1926.56 |work=Regulations (Standards - 29 CFR) |publisher=Occupational Safety and Health Administration, US Dept. of Labor| archive-url =https://web.archive.org/web/20090508051301/https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10630| archive-date=8 May 2009}}</ref><ref>European law UNI EN 12464</ref>
+|320–500|| Office lighting<ref name=energyrating/><ref>{{cite web |url=http://www.resourcesmart.vic.gov.au/documents/lux_meter.pdf |title=How to use a lux meter (Australian recommendation) |publisher=Sustainability Victoria |date=April 2010 |archive-url=https://web.archive.org/web/20110707054658/http://www.resourcesmart.vic.gov.au/documents/lux_meter.pdf |archive-date=7 July 2011}}</ref><ref>{{cite web |url=https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10630 |title=Illumination. - 1926.56 |work=Regulations (Standards - 29 CFR) |publisher=Occupational Safety and Health Administration, US Dept. of Labor |archive-url =https://web.archive.org/web/20090508051301/https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10630 |archive-date=8 May 2009}}</ref><ref>European law UNI EN 12464</ref>
 |-
 |400||[[Sunrise]] or [[sunset]] on a clear day.
@@ Line 67: / Line 67: @@
 |}
-The illuminance provided by a light source on a surface perpendicular to the direction to the source is a measure of the strength of that source as perceived from that location. For instance, a star of [[apparent magnitude]] 0 provides 2.08 microlux (μlx) at the Earth's surface.<ref name=Schlyter7>[http://stjarnhimlen.se/comp/radfaq.html#7 Schlyter, Section 7].</ref> A barely perceptible magnitude 6 star provides 8 nanolux (nlx).<ref>[http://stjarnhimlen.se/comp/radfaq.html#14 Schlyter, Section 14].</ref> The unobscured Sun provides an illumination of up to 100&nbsp;kilolux (klx) on the Earth's surface, the exact value depending on time of year and atmospheric conditions. This direct normal illuminance is related to the [[solar illuminance constant]] ''E''<sub>sc</sub>, equal to {{val|128000|u=lux}} (see [[Sunlight]] and [[Solar constant]]).
+The illuminance provided by a light source on a surface perpendicular to the direction to the source is a measure of the strength of that source as perceived from that location. For instance, a star of [[apparent magnitude]] 0 provides 2.08 microlux (μlx) at the Earth's surface.<ref name="Schlyter7">[http://stjarnhimlen.se/comp/radfaq.html#7 Schlyter, Section 7].</ref> A barely perceptible magnitude 6 star provides 8 nanolux (nlx).<ref>[http://stjarnhimlen.se/comp/radfaq.html#14 Schlyter, Section 14].</ref> The unobscured Sun provides an illumination of up to 100&nbsp;kilolux (klx) on the Earth's surface, the exact value depending on time of year and atmospheric conditions. This direct normal illuminance is related to the [[solar illuminance constant]] ''E''<sub>sc</sub>, equal to {{val|128000|u=lux}} (see [[Sunlight]] and [[Solar constant]]).
 The illuminance on a surface depends on how the surface is tilted with respect to the source. For example, a pocket flashlight aimed at a wall will produce a given level of illumination if aimed perpendicular to the wall, but if the flashlight is aimed at increasing angles to the perpendicular (maintaining the same distance), the illuminated spot becomes larger and so is less highly illuminated. When a surface is tilted at an angle to a source, the illumination provided on the surface is reduced because the tilted surface subtends a smaller solid angle from the source, and therefore it receives less light. For a point source, the illumination on the tilted surface is reduced by a factor equal to the cosine of the angle between a ray coming from the source and the [[Surface normal|normal]] to the surface.<ref>Jack L. Lindsey, ''Applied Illumination Engineering'', The Fairmont Press, Inc., 1997 {{ISBN|0881732125}} page 218</ref> In practical lighting problems, given information on the way light is emitted from each source and the distance and geometry of the lighted area, a numerical calculation can be made of the illumination on a surface by adding the contributions of every point on every light source.
@@ Line 87: / Line 87: @@
 Specifications for [[video camera]]s such as [[camcorder]]s and [[surveillance camera]]s often include a minimal illuminance level in lux at which the camera will record a satisfactory image.{{citation needed|date=February 2013}} A camera with good low-light capability will have a lower lux rating. [[Still camera]]s do not use such a specification, since longer [[exposure time]]s can generally be used to make pictures at very low illuminance levels, as opposed to the case in video cameras, where a maximal exposure time is generally set by the [[frame rate]].
-=={{Anchor|Lux versus footcandle}}Non-SI units of illuminance==
+{{Anchor|Lux versus footcandle}}
+==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&nbsp;lx. 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]]&nbsp;(ph) equals 10 kilolux (10 klx).
+{{anchor|Nox}}One nox&nbsp;(nx) equals 1 millilux (1 mlx) at light color 2042&nbsp;K or 2046&nbsp;K (formerly 2360&nbsp;K).<ref name="Lohse-Stille_1948"/><ref name="Westphal_1952"/><ref name="Grimsehl_1988"/><ref name="Keplinger_2021"/>
-One nox&nbsp;(nx) equals 1 millilux (1 mlx).
-In [[astronomy]], [[apparent magnitude]] is a measure of the illuminance of a star on the Earth's atmosphere. A star with apparent magnitude 0 is 2.54 microlux outside the earth's atmosphere, and 82% of that (2.08 microlux) under clear skies.<ref name=Schlyter7/> A magnitude 6 star (just barely visible under good conditions) would be 8.3 nanolux. A standard candle (one candela) a kilometre away would provide an illuminance of 1 microlux—about the same as a magnitude 1 star.
+In [[astronomy]], [[apparent magnitude]] is a measure of the illuminance of a star on the Earth's atmosphere. A star with apparent magnitude 0 is 2.54 microlux outside the earth's atmosphere, and 82% of that (2.08 microlux) under clear skies.<ref name="Schlyter7"/> A magnitude 6 star (just barely visible under good conditions) would be 8.3 nanolux. A standard candle (one candela) a kilometre away would provide an illuminance of 1 microlux—about the same as a magnitude 1 star.
 ==Legacy Unicode symbol==
@@ Line 103: / Line 104: @@
 ==See also==
-*[[Exposure value]]
+* [[Exposure value]]
-== Notes and references ==
+== References ==
-{{Reflist|2}}
+{{Reflist|refs=
+<ref name="Lohse-Stille_1948">{{cite journal |title=Einführung und Bestimmung des Lichtäquivalents |language=de |trans-title= |author-first1=Bernhard |author-last1=Lohse<!-- *Schöningen 1911-05-22 --> |author-first2=Ulrich |author-link2=:de:Ulrich Stille |author-last2=Stille |editor=Deutsche Physikalische Gesellschaft |editor-link=Deutsche Physikalische Gesellschaft |journal=[[Zeitschrift für Physik]] |issn=0044-3328<!-- 0939-7922 --> |publisher=[[Springer-Verlag]] |publication-place=Berlin / Göttingen / Heidelberg, Germany |location=Braunschweig, Germany |volume=125 |issue=1–3 |date=January 1948 |orig-date=1947-08-19 |doi=10.1007/BF01337623 |pages=133–158 |bibcode=1948ZPhy..125..133L |s2cid=125512557 |url=https://books.google.com/books?id=fxJXDwKAjX8C |access-date=2023-03-19 |url-status= |archive-url= |archive-date=}}</ref>
+<ref name="Westphal_1952">{{cite book |title=Physikalisches Wörterbuch |language=de |trans-title= |chapter=Nox, Dunkelleuchtdichte, Skot |author-first=Wilhelm Heinrich |author-last=Westphal |editor-first1=Wilhelm H. |editor-last1=Westphal |author-link=Wilhelm Heinrich Westphal |date=1952 |edition=1 |publisher=[[Springer-Verlag OHG]] |publication-place=Berlin / Göttingen / Heidelberg, Germany |isbn=978-3-662-12707-0 |doi=10.1007/978-3-662-12706-3 |pages=125, 271, 389 |chapter-url=https://books.google.com/books?id=QaCFBwAAQBAJ&pg=RA1-PA125 |access-date=2023-03-16 <!-- |url-status=live --> |quote-pages=125, 271 |quote=Nox, abgek[ürzt] nx, Einheit der Dunkelbeleuchtungsstärke (<!-- > -->Dunkelleuchtdichte), welche für zahlenmäßige Angaben und zum Anschluß der Dunkelbeleuchtungsstärke an die normale <!-- > -->Beleuchtungsstärke 1940 von der {{ill|German Lighting Society|de|Deutsche Lichttechnische Gesellschaft|lt=Deutschen Lichttechnischen Gesellschaft}} geschaffen wurde. Bezüglich der Farbtemperatur der Strahlung und des Anschlusses von Zahlenwerten der Beleuchtungsstärke ''E'' und der Dunkelbeleuchtungsstärke ''{{overline|E}}'' gelten analog die gleichen Festlegungen wie bei der <!-- > -->Dunkelleuchtdichte und dem <!-- > -->Skot (sk). Für eine Strahlung der Farbtemperatur ''T''<sub>1</sub>&nbsp;= 2360&nbsp;K gilt: 1&nbsp;nx&nbsp;= 10<sup>−3</sup>&nbsp;lx (<!-- > -->Lux). Für eine beliebige Strahlung ''bekannter'' spektraler <!-- > -->Strahlungsleistung ''S''<sub>1</sub> lautet die Verknüpfungsbeziehung zwischen in 10<sup>−3</sup>&nbsp;lx gemessenem Zahlenwert {''E''} der Beleuchtungsstärke und in nx gemessenem Zahlenwert {''{{overline|E}}''} der Dunkelbeleuchtungsstärke: {''{{overline|E}}''}<sub>nx</sub>&nbsp;= (2,161&nbsp;± 0,001)&nbsp;· {''E''}<sub>10<sup>−3</sup>&nbsp;lx</sub>&nbsp;· ∫&nbsp;''S''<sub>λ</sub> ''{{overline|V}}''<sub>λ,W</sub> ''dλ''&nbsp;/ ∫&nbsp;''S''<sub>λ</sub> ''V''<sub>λ</sub> ''dλ'', wobei ''V''<sub>λ</sub> die relative spektrale <!-- > -->Hellempfindlichkeit und ''{{overline|V}}''<sub>λ,W</sub> die relative spektrale <!-- > -->Dämmerungsempfindlichkeit des menschlichen Auges nach Weaver{{citeref|Lohse|Stille|1948|A}} bedeuten. [...] Dunkelleuchtdichte. [...] Ist das Auge dunkeladaptiert, d.h. einer <!-- > -->Leuchtdichte von weniger als 0,01&nbsp;[[apostilb|asb]] ausgesetzt, so gilt infolge des <!-- > -->[[Purkinje effect|Purkinje-Phänomen]]s eine von der <!-- > -->spektralen Hellempfindlichkeitskurve abweichende, nach dem kurzwelligen Ende des Spektrums hin verschobene Empfindlichkeitskurve des Auges, die Stäbchenkurve des [[Scotopic vision|Dämmerungssehen]]s. Unter Zugrundelegung dieser Empfindlichkeitskurve hat man 1940 in Deutschland die Dunkelleuchtdichte mit der Einheit <!-- >-->[[Skot (unit)|Skot]] (sk) so festgesetzt, daß bei einem Licht der Farbtemperatur 2360&nbsp;°[[Kelvin (unit)|K]] 1&nbsp;sk&nbsp;= 10<sup>−3</sup>&nbsp;asb gilt. 1948 ist von der [[Internationale Beleuchtungskommission|Internationalen Beleuchtungskommission]] (IBK) die Bezugstemperatur auf 2046&nbsp;K, die Erstarrungstemperatur des [[Platinum|Platin]]s, festgesetzt worden. Die Bezeichnung Skot wurde von der IBK nicht übernommen, dafür soll "[[skotopisches Stilb]]" gesagt werden. Als höchstzulässiger Grenzwert für die Dunkelleuchtdichte ist in Deutschland 10&nbsp;Skot festgesetzt worden, um eine Verwendung der Dunkelleuchtdichte im Gebiet des gemischten [[Photopic vision|Zapfen]]- und [[Scotopic vision|Stäbchensehen]]s zu vermeiden, da in diesem Bereich die photometrischen Maßgrößen wegen der allmählich gleitenden Augenempfindlichkeitskurve ihren Sinn verlieren. |trans-quote=}}</ref>
+<ref name="Grimsehl_1988">{{cite book |title=Lehrbuch der Physik: Optik |volume=3 |language=de |trans-title= |chapter=1. Licht: 1.4. Photometrie: 1.4.1. Grundbegriffe |author-first1=Ernst |author-last1=Grimsehl |author-link1=:de:Ernst Grimsehl |author-first2=Walter |author-last2=Schallreuter |author-link2=:de:Walter Schallreuter |editor-first=Heinz |editor-last=Haferkorn |date=1988 |orig-date=1976 |edition=19 |publisher=[[BSB BG Teubner Verlagsgesellschaft]] |publication-place=Leipzig, Germany |isbn=978-3-322-96432-8 |doi=10.1007/978-3-322-96431-1 |id=Order No. 6666211, VLN 294-375/84/88, LSV 1164 |pages=33–38 [37–38] |chapter-url=https://books.google.com/books?id=pkbQBgAAQBAJ&pg=PA37 |access-date=2023-03-16 <!-- |url-status=live --> |quote-pages=37–38 |quote=Dunkelsehen [...] Für das Dunkelsehen, bei dem nur die [[Rod cell|Stäbchen]] angeregt werden, definiert man die Dunkelleuchtdichte mit der Einheit ''[[Skot (unit)|Skot]]'' (sk) und die Dunkelbeleuchtungsstärke mit der Einheit ''Nox'' (nx). Die Umrechnungsfaktoren zwischen den Hell- und Dunkelgrößen hängen von der spektralen Zusammensetzung des Lichtes ab. Sie werden deshalb für die Farbtemperatur 2042&nbsp;K (früher 2360&nbsp;K) festgelegt. Bei dieser ist 1&nbsp;sk&nbsp;= 10<sup>−3</sup>&nbsp;asb und 1&nbsp;nx&nbsp;= 10<sup>−3</sup>&nbsp;lx. |trans-quote=}}</ref>
+<ref name="Keplinger_2021">{{cite web |title=1939 bis 1945 – Im Keller glüht das Lumogen |language=de-AT |trans-title= |author-first=Thomas |author-last=Keplinger |website=Worte im Dunkel |location=Vienna, Austria |date=2021-03-29 |url=https://www.worteimdunkel.at/?p=14007 |access-date=2023-03-16 |url-status=live |archive-url=https://web.archive.org/web/20230316194909/https://www.worteimdunkel.at/?p=14007 |archive-date=2023-03-16 |quote=Skot und Nox [...] Interessant ist in diesem Zusammenhang die Einführung neuer Messeinheiten. Die Voraussetzungen der Forschung beziehungsweise die Erfordernisse an die Leuchtfarben unterschieden sich so stark von allen bis dahin erforschten Gebieten, dass die {{ill|German Lighting Society|de|Deutsche Lichttechnische Gesellschaft|lt=Deutsche Lichttechnische Gesellschaft}} 1940 eigene Einheiten ins Leben rief: Die Dunkelleuchtdichte wurde in [[Skot (unit)|Skot]] und die Dunkelbeleuchtungsstärke in Nox gemessen.{{citeref|Westphal|1952|B}} Diese Einheiten grenzten an die bereits bestehenden Größen der [[luminance|Leuchtdichte]] und [[illuminance|Beleuchtungsstärke]] an und dienten der zahlenmäßigen Erfassung geringster Lichtwerte. So entsprach etwa ein Nox 10<sup>−3</sup>&nbsp;Lux. |trans-quote=}}</ref>
+}}
 ==External links==
-*[https://web.archive.org/web/20130313095139/http://fp.optics.arizona.edu/Palmer/rpfaq/rpfaq.htm Radiometry and photometry FAQ] Professor Jim Palmer's Radiometry FAQ page ([[University of Arizona]]).
+* [https://web.archive.org/web/20130313095139/http://fp.optics.arizona.edu/Palmer/rpfaq/rpfaq.htm Radiometry and photometry FAQ] Professor Jim Palmer's Radiometry FAQ page ([[University of Arizona]]).
 {{SI units}}
 {{Portal bar|Physics|Energy}}
-{{Use dmy dates|date=February 2020}}
 [[Category:SI derived units]]

v t e SI units
Base units	ampere candela kelvin kilogram metre mole second
Derived units with special names	becquerel coulomb degree Celsius farad gray henry hertz joule katal lumen lux newton ohm pascal radian siemens sievert steradian tesla volt watt weber
Other accepted units	astronomical unit dalton day decibel degree of arc electronvolt hectare hour litre minute minute and second of arc neper tonne
See also	Conversion of units Metric prefixes Historical definitions of the SI base units 2019 revision of the SI System of units of measurement
Category