Lux

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lux
Lux meter.jpg
A lux meter for measuring illuminance
General information
Unit system SI
Unit of illuminance
Symbollx
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

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.

Contents

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
0.0001Moonless, overcast night sky (starlight) [5]
0.002Moonless clear night sky with airglow [5]
0.05–0.3Full moon on a clear night [6]
3.4Dark limit of civil twilight under a clear sky [7]
20–50Public areas with dark surroundings [8]
50Family living room lights (Australia, 1998) [9]
80Office building hallway/toilet lighting [10] [11]
100Very dark overcast day [5]
150Train station platforms [12]
320–500Office lighting [9] [13] [14] [15]
400 Sunrise or sunset on a clear day.
1000Overcast day; [5] typical TV studio lighting
10,000–25,000Full daylight (not direct sun) [5]
32,000–100,000Direct 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 Esc, 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
QuantityUnitDimension
[nb 1] 		Notes
NameSymbol [nb 2] NameSymbol
Luminous energy Qv [nb 3] lumen second lm⋅sT⋅JThe lumen second is sometimes called the talbot.
Luminous flux, luminous powerΦ v [nb 3] lumen (= candela steradian)lm (= cd⋅sr)JLuminous energy per unit time
Luminous intensity Iv candela (= lumen per steradian) cd (= lm/sr)JLuminous flux per unit solid angle
Luminance Lv candela per square metre cd/m2 (= lm/(sr⋅m2))L−2⋅JLuminous flux per unit solid angle per unit projected source area. The candela per square metre is sometimes called the nit .
Illuminance Ev lux (= lumen per square metre) lx (= lm/m2)L−2⋅JLuminous flux incident on a surface
Luminous exitance, luminous emittanceMvlumen per square metrelm/m2L−2⋅JLuminous flux emitted from a surface
Luminous exposure Hv lux second lx⋅sL−2⋅T⋅JTime-integrated illuminance
Luminous energy densityωvlumen second per cubic metrelm⋅s/m3L−3⋅T⋅J
Luminous efficacy (of radiation)Klumen per watt lm/W M−1⋅L−2⋅T3⋅JRatio of luminous flux to radiant flux
Luminous efficacy (of a source)η [nb 3] lumen per watt lm/W M−1⋅L−2⋅T3⋅JRatio of luminous flux to power consumption
Luminous efficiency, luminous coefficientV1Luminous efficacy normalized by the maximum possible efficacy
See also:
  1. ↑ 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.
  2. ↑ 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
  3. 1 2 3 Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ for luminous efficacy of a source.

See also

Related Research Articles

<span class="mw-page-title-main">Candela</span> SI unit of luminous intensity

The candela is the unit of luminous intensity in the International System of Units (SI). It measures luminous power per unit solid angle emitted by a light source in a particular direction. Luminous intensity is analogous to radiant intensity, but instead of simply adding up the contributions of every wavelength of light in the source's spectrum, the contribution of each wavelength is weighted by the luminous efficiency function, the model of the sensitivity of the human eye to different wavelengths, standardized by the CIE and ISO. A common wax candle emits light with a luminous intensity of roughly one candela. If emission in some directions is blocked by an opaque barrier, the emission would still be approximately one candela in the directions that are not obscured.

<span class="mw-page-title-main">Luminance</span> Photometric measure

Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls within a given solid angle.

<span class="mw-page-title-main">Exposure (photography)</span> Amount of light captured by a camera

In photography, exposure is the amount of light per unit area reaching a frame of photographic film or the surface of an electronic image sensor. It is determined by shutter speed, lens F-number, and scene luminance. Exposure is measured in units of lux-seconds, and can be computed from exposure value (EV) and scene luminance in a specified region.

<span class="mw-page-title-main">Luminous efficiency function</span> Description of the average spectral sensitivity of human visual perception of brightness

A luminous efficiency function or luminosity function represents the average spectral sensitivity of human visual perception of light. It is based on subjective judgements of which of a pair of different-colored lights is brighter, to describe relative sensitivity to light of different wavelengths. It is not an absolute reference to any particular individual, but is a standard observer representation of visual sensitivity of theoretical human eye. It is valuable as a baseline for experimental purposes, and in colorimetry. Different luminous efficiency functions apply under different lighting conditions, varying from photopic in brightly lit conditions through mesopic to scotopic under low lighting conditions. When not specified, the luminous efficiency function generally refers to the photopic luminous efficiency function.

In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the human eye. The SI unit of luminous intensity is the candela (cd), an SI base unit.

<span class="mw-page-title-main">Photometry (optics)</span> Science of the measurement of visible light

Photometry is the science of the measurement of light, in terms of its perceived brightness to the human eye. It is distinct from radiometry, which is the science of measurement of radiant energy in terms of absolute power. In modern photometry, the radiant power at each wavelength is weighted by a luminosity function that models human brightness sensitivity. Typically, this weighting function is the photopic sensitivity function, although the scotopic function or other functions may also be applied in the same way. The weightings are standardized by the CIE and ISO.

<span class="mw-page-title-main">Foot-candle</span>

A foot-candle is a non-SI unit of illuminance or light intensity. The foot-candle is defined as one lumen per square foot. This unit is commonly used in lighting layouts in parts of the world where United States customary units are used, mainly the United States. Nearly all of the world uses the corresponding SI derived unit lux, defined as one lumen per square meter.

<span class="mw-page-title-main">Luminous flux</span> Perceived luminous power

In photometry, luminous flux or luminous power is the measure of the perceived power of light. It differs from radiant flux, the measure of the total power of electromagnetic radiation, in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light.

<span class="mw-page-title-main">Illuminance</span> Luminous flux incident on a surface per area

In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate with human brightness perception. Similarly, luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as luminous exitance.

A phot (ph) is a photometric unit of illuminance, or luminous flux through an area. It is not an SI unit but rather is associated with the older centimetre–gram–second system of units. The name was coined by André Blondel in 1921.

<span class="mw-page-title-main">Photometer</span> Instrument to measure light intensity

A photometer is an instrument that measures the strength of electromagnetic radiation in the range from ultraviolet to infrared and including the visible spectrum. Most photometers convert light into an electric current using a photoresistor, photodiode, or photomultiplier.

The lumen is the unit of luminous flux, a measure of the total quantity of visible light emitted by a source per unit of time, in the International System of Units (SI). Luminous flux differs from power in that radiant flux includes all electromagnetic waves emitted, while luminous flux is weighted according to a model of the human eye's sensitivity to various wavelengths, this weighting is standardized by the CIE and ISO. One lux is one lumen per square metre.

<span class="mw-page-title-main">Spectral power distribution</span>

In radiometry, photometry, and color science, a spectral power distribution (SPD) measurement describes the power per unit area per unit wavelength of an illumination. More generally, the term spectral power distribution can refer to the concentration, as a function of wavelength, of any radiometric or photometric quantity.

Luminous efficacy is a measure of how well a light source produces visible light. It is the ratio of luminous flux to power, measured in lumens per watt in the International System of Units (SI). Depending on context, the power can be either the radiant flux of the source's output, or it can be the total power consumed by the source. Which sense of the term is intended must usually be inferred from the context, and is sometimes unclear. The former sense is sometimes called luminous efficacy of radiation, and the latter luminous efficacy of a light source or overall luminous efficacy.

A foot-lambert or footlambert is a unit of luminance in United States customary units and some other unit systems. A foot-lambert equals 1/π or 0.3183 candela per square foot, or 3.426 candela per square meter. The foot-lambert is named after Johann Heinrich Lambert (1728–1777), a Swiss-German mathematician, physicist and astronomer. It is rarely used by electrical and lighting engineers, who prefer the candela per square foot or candela per square meter units.

In photometry, luminous energy is the perceived energy of light. This is sometimes called the quantity of light. Luminous energy is not the same as radiant energy, the corresponding objective physical quantity. This is because the human eye can only see light in the visible spectrum and has different sensitivities to light of different wavelengths within the spectrum. When adapted for bright conditions, the eye is most sensitive to light at a wavelength of 555 nm. Light with a given amount of radiant energy will have more luminous energy if the wavelength is 555 nm than if the wavelength is longer or shorter. Light whose wavelength is well outside the visible spectrum has a luminous energy of zero, regardless of the amount of radiant energy present.

Several measures of light are commonly known as intensity:

The apostilb is an obsolete unit of luminance. The SI unit of luminance is the candela per square metre (cd/m2). In 1942 Parry Moon proposed to rename the apostilb the blondel, after the French physicist André Blondel. The symbol for the apostilb is asb.

Skot is an old and deprecated measurement unit of luminance, used for self-luminous objects. The term comes from Greek skotos, meaning "darkness".

<i>Photometria</i> Book by Johann Heinrich Lambert

Photometria is a book on the measurement of light by Johann Heinrich Lambert published in 1760. It established a complete system of photometric quantities and principles; using them to measure the optical properties of materials, quantify aspects of vision, and calculate illumination.

References

  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.
  5. 1 2 3 4 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 ]
  8. ↑ "NOAO Common and Recommended Light Levels Indoor" (PDF). Archived from the original (PDF) on 2021-07-06. Retrieved 2016-11-13.
  9. 1 2 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
  16. 1 2 Schlyter, Section 7.
  17. ↑ Schlyter, Section 14.
  18. ↑ Jack L. Lindsey, Applied Illumination Engineering, The Fairmont Press, Inc., 1997 ISBN   0881732125 page 218
  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). Berlin / Göttingen / Heidelberg, Germany: Springer-Verlag. 125 (1–3): 133–158. Bibcode:1948ZPhy..125..133L. doi:10.1007/BF01337623. ISSN   0044-3328. S2CID   125512557 . Retrieved 2023-03-19.
  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 · ∫ SλVλ,Wdλ / ∫ SλVλdλ, wobei Vλ die relative spektrale Hellempfindlichkeit und Vλ,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.
  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.
  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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