Illuminance is a measure of how muchluminous flux is spread over a given area. One can think of luminous flux (with the unitlumen) 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.
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 singlefluorescent light fixture with an output of12000 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:
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 ofapparent 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 thesolar illuminance constantEsc, equal to128000 lux (seeSunlight andSolar 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 thenormal 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.
Like allphotometric 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 theluminosity function.
The lux is one lumen per square metre (lm/m2), and the corresponding radiometric unit, which measuresirradiance, 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 lightof 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 thevisible 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 theluminous efficiency. For example, a typicalincandescent 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-formingvisual photoreception in many individual human eyes.
Specifications forvideo cameras such ascamcorders andsurveillance 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 longerexposure 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 theframe rate.
The corresponding unit in English and American traditional units is thefoot-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 nox (nx) equals 1 millilux (1 mlx) at light color 2042 K or 2046 K (formerly 2360 K).[19][20][21][22]
Inastronomy,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.
Unicode includes a symbol for "lx":U+33D3㏓SQUARE LX. It is a legacy code to accommodate oldcode pages in someAsian languages. Use of this code is not recommended in new documents.
^The symbols in this column denotedimensions; "L", "T" and "J" are for length, time and luminous intensity respectively, not the symbols for theunits litre, tesla and joule.
^Standards organizations recommend that photometric quantities be denoted with a subscript "v" (for "visual") to avoid confusion with radiometric orphoton quantities. For example:USA Standard Letter Symbols for Illuminating Engineering USAS Z7.1-1967, Y10.18-1967
^abcAlternative symbols sometimes seen:W for luminous energy,P orF for luminous flux, andρ for luminous efficacy of a source.
^abcdeSchlyter, 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).
^"Illumination. - 1926.56".Regulations (Standards - 29 CFR). Occupational Safety and Health Administration, US Dept. of Labor. Archived fromthe original on 2009-05-08.
^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.ISBN978-3-662-12707-0. Retrieved2023-03-16. 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 derDeutschen Lichttechnischen Gesellschaft [de] geschaffen wurde. Bezüglich der Farbtemperatur der Strahlung und des Anschlusses von Zahlenwerten der BeleuchtungsstärkeE und der DunkelbeleuchtungsstärkeE gelten analog die gleichen Festlegungen wie bei der Dunkelleuchtdichte und dem Skot (sk). Für eine Strahlung der FarbtemperaturT1 = 2360 K gilt: 1 nx = 10−3 lx (Lux). Für eine beliebige Strahlungbekannter spektraler StrahlungsleistungS1 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, wobeiVλ die relative spektrale Hellempfindlichkeit undVλ,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 desPurkinje-Phänomens eine von der spektralen Hellempfindlichkeitskurve abweichende, nach dem kurzwelligen Ende des Spektrums hin verschobene Empfindlichkeitskurve des Auges, die Stäbchenkurve desDämmerungssehens. Unter Zugrundelegung dieser Empfindlichkeitskurve hat man 1940 in Deutschland die Dunkelleuchtdichte mit der EinheitSkot (sk) so festgesetzt, daß bei einem Licht der Farbtemperatur 2360 °K 1 sk = 10−3 asb gilt. 1948 ist von derInternationalen Beleuchtungskommission (IBK) die Bezugstemperatur auf 2046 K, die Erstarrungstemperatur desPlatins, 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 gemischtenZapfen- undStäbchensehens zu vermeiden, da in diesem Bereich die photometrischen Maßgrößen wegen der allmählich gleitenden Augenempfindlichkeitskurve ihren Sinn verlieren.{{cite book}}:ISBN / Date incompatibility (help)
^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.ISBN978-3-322-96432-8. Order No. 6666211, VLN 294-375/84/88, LSV 1164. Retrieved2023-03-16. pp. 37–38:Dunkelsehen [...] Für das Dunkelsehen, bei dem nur dieStäbchen angeregt werden, definiert man die Dunkelleuchtdichte mit der EinheitSkot (sk) und die Dunkelbeleuchtungsstärke mit der EinheitNox (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. Retrieved2023-03-16.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 dieDeutsche Lichttechnische Gesellschaft [de] 1940 eigene Einheiten ins Leben rief: Die Dunkelleuchtdichte wurde inSkot und die Dunkelbeleuchtungsstärke in Nox gemessen.[B] Diese Einheiten grenzten an die bereits bestehenden Größen derLeuchtdichte undBeleuchtungsstärke an und dienten der zahlenmäßigen Erfassung geringster Lichtwerte. So entsprach etwa ein Nox 10−3 Lux.
