Lux

Source: Wikipedia, the free encyclopedia.
For other uses, see Lux (disambiguation).

lux
SI
Unit ofilluminance
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

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;
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

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

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 

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

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
Quantity Unit Dimension
[nb 1] 		Notes
Name Symbol[nb 2] Name Symbol
Luminous energy Qv[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 Iv candela (= lumen per steradian) cd (= lm/sr) J Luminous flux per unit solid angle
Luminance Lv 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 Ev lux (= lumen per square metre) lx (= lm/m2) L−2⋅J Luminous flux incident on a surface
Luminous exitance
, luminous emittance 		Mv lumen per square metre lm/m2 L−2⋅J Luminous flux emitted from a surface
Luminous exposure
 Hv
lux second
 lx⋅s L−2⋅T⋅J Time-integrated illuminance
Luminous energy density ωv 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:
  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. ^ 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.

See also

References

  1. 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. 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. ^ 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).
  6. 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. ^ 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)
  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. ^ a b Schlyter, Section 7.
  17. ^ Schlyter, Section 14.
  18. ISBN 0881732125
    page 218
  19. S2CID 125512557
    . Retrieved 2023-03-19.
  20. 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. 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.

External links

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