Editing Lux (section)

===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 (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&nbsp;lx&nbsp;= 1&nbsp;lm/m<sup>2</sup> = 1&nbsp;[[Candela|cd]]·[[Steradian|sr]]/m<sup>2</sup>.

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 illuminance (lux) requires a greater luminous flux (lumen).

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:

{| class="wikitable"
! 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 |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" />
|-
|0.01
|Quarter moon on a clear night
|-
|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>
|-
|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>
|-
|100||Very dark overcast day<ref name="radfaq" />
|-
|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>
|-
|400||[[Sunrise]] or [[sunset]] on a clear day.
|-
|1000||Overcast day;<ref name="radfaq" /> typical [[TV studio]] lighting
|-
|10,000–25,000||Full [[daylight]] (not direct sun)<ref name="radfaq" />
|-
|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.<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.