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The lux (symbol: lx) is the
SI derived unit SI derived units are units of measurement derived from the seven SI base unit, base units specified by the International System of Units (SI). They are either dimensionless quantity, dimensionless or can be expressed as a product of one or more o ...
of
illuminance In photometryPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical ob ...

illuminance
, measuring
luminous flux In photometryPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical ...

luminous flux
per unit area. It is equal to one lumen per square metre. In
photometryPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical object's electroma ...
, this is used as a measure of the intensity, as perceived by the human eye, of
light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nan ...

light
that hits or passes through a surface. It is analogous to the
radiometric Radiometry is a set of techniques for measuring Measurement is the quantification of attributes of an object or event, which can be used to compare with other objects or events. The scope and application of measurement are dependent on the ...
unit
watt per square metre
watt per square metre
, but with the power at each
wavelength In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...

wavelength
weighted according to the
luminosity function (black) and scotopic (green) luminous efficiency functions. The photopic includes the CIE 1931 standard (solid), the Judd–Vos 1978 modified data (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data (dotted). The horizontal axis is wavel ...
, a standardized model of
human Humans (''Homo sapiens'') are the most abundant and widespread species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A speci ...

human
visual brightness perception. In English, "lux" is used as both the singular and plural form. The word is derived from the
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it became ...

Latin
word for "light",
lux The lux (symbol: lx) is the SI derived unit SI derived units are units of measurement ' Measurement is the number, numerical quantification (science), quantification of the variable and attribute (research), attributes of an object or event, wh ...
.


Explanation


Illuminance

Illuminance is a measure of how much
luminous flux In photometryPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical ...

luminous flux
is spread over a given area. One can think of luminous flux (measured in
lumens The lumen (symbol: lm) is the SI derived unit SI derived units are units of measurement ' Measurement is the number, numerical quantification (science), quantification of the variable and attribute (research), attributes of an object or event, wh ...
) 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 The square metre ( international spelling as used by the International Bureau of Weights and Measures The International Bureau of Weights and Measures (french: Bureau international des poids et mesures, BIPM) is an intergovernmental organi ...
: :1 lx = 1 lm/m2 = 1 
cd
cd
·
sr
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 lux might be possible in a home kitchen with a single
fluorescent light light. Fluorescence is the emission of light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that can be visual perception, perceived by the human eye. Visible light is usually defi ...

fluorescent light
fixture with an output of . 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. As with other SI units,
SI prefixes The International System of Units, known by the international abbreviation SI in all languages and sometimes Pleonasm#Acronyms_and_initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wi ...
can be used, for example a kilolux (klx) is 1000 lux. 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 of
apparent magnitude
apparent magnitude
0 provides 2.08 microlux (μlx) at the Earth's surface.Schlyter, Section 7
A barely perceptible magnitude 6 star provides 8 nanolux (nlx). 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 The solar constant (''GSC'') is a flux density measuring mean solar electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature') ...
''E''sc, equal to (see
Sunlight Sunlight is a portion of the given off by the , in particular , , and light. On , sunlight is and through , and is obvious as when the Sun is above the . When direct is not blocked by s, it is experienced as sunshine, a combination of b ...

Sunlight
and
Solar constant The solar constant (''GSC'') is a flux density measuring mean solar electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature') ...
). 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
normal
to the surface.Jack L. Lindsey, ''Applied Illumination Engineering'', The Fairmont Press, Inc., 1997 page 218 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 Radiometry is a set of techniques for measuring Measurement is the quantification of attributes of an object or event, which can be used to compare with other objects or events. The scope and application of measurement are dependent on the ...
" 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 (black) and scotopic (green) luminous efficiency functions. The photopic includes the CIE 1931 standard (solid), the Judd–Vos 1978 modified data (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data (dotted). The horizontal axis is wavel ...
. The lux is one lumen per square metre (lm/m2), and the corresponding radiometric unit, which measures
irradiance In radiometry Radiometry is a set of techniques for measuring ' Measurement is the numerical quantification of the attributes of an object or event, which can be used to compare with other objects or events. The scope and application of measure ...

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 lux per 1 W/m2; the irradiance needed to make 1 lux 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 The visible spectrum is the portion of the that is to the . in this range of s is called ' or simply . A typical will respond to wavelengths from about 380 to about 750 . In terms of frequency, this corresponds to a band in the vicinity of ...
. 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 Luminous efficacy is a measure of how well a light source produces visible light. It is the ratio of luminous flux up Integrating sphere used for measuring the luminous flux of a light source. In photometryPhotometry can refer to: * Photometry ...
. For example, a typical
incandescent light bulb An incandescent light bulb, incandescent lamp or incandescent light globe is an electric light An electric light is a device that produces visible light Light or visible light is electromagnetic radiation within the portion of the ...

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 camera A video camera is a camera A camera is an optical Optics is the branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural ...

video camera
s such as
camcorder A camcorder is an electronic device originally combining a video camera A video camera is a camera A camera is an optical Optics is the branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗ ...

camcorder
s and
surveillance camera A closed-circuit television camera can produce images or recordings for surveillance or other private purposes. Cameras can be either video cameras, or Digital camera, digital stills cameras. Walter Bruch was the inventor of the CCTV camera. The ...

surveillance camera
s often include a minimal illuminance level in lux at which the camera will record a satisfactory image. A camera with good low-light capability will have a lower lux rating.
Still camera A camera is an optical Optics is the branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, i ...
s do not use such a specification, since longer
exposure time Exposure or Exposures may refer to: People * The Exposures, a pseudonym for German electronic musician Jan Jeline Arts, entertainment, and media Films * ''Exposure'' (film), a 1932 American film * ''Exposure'', another name for the 1991 movie ...

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 Frame rate (expressed in or FPS) is the frequency Frequency is the number of occurrences of a repeating event per unit of time A unit of time is any particular time Time is the indefinite continued sequence, progress of existence an ...
.


Non-SI units of illuminance

The corresponding unit in English and American traditional units is the
foot-candle A foot-candle (sometimes foot candle; abbreviated fc, lm/ft2, or sometimes ft-c) is a non-SI unit of illuminance In photometry, illuminance is the total luminous flux incident on a surface, per unit area Area is the quantity that expresse ...
. One foot candle is about 10.764 lux. 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 A phot (ph) is a photometricPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an ...
 (ph) equals 10 kilolux (10 klx). One nox (nx) equals 1 millilux (1 mlx). In
astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses mathematics, phys ...
,
apparent magnitude
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. 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 Unicode, formally the Unicode Standard, is an information technology Technical standard, standard for the consistent character encoding, encoding, representation, and handling of Character (computing), text expressed in most of the world's wri ...

Unicode
includes a symbol for "lx": . It is a legacy code to accommodate old
code page In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithm of an algorithm (Euclid's algorithm) for calculating the greatest common ...
s in some
Asia Asia () is Earth's largest and most populous continent, located primarily in the Eastern Hemisphere, Eastern and Northern Hemisphere, Northern Hemisphere of the Earth, Hemispheres. It shares the continental landmass of Eurasia with the cont ...

Asia
n languages. Use of this code is not recommended in new documents.


SI photometry units


See also

*
Exposure value In photography Photography is the art Art is a diverse range of (products of) human activities involving creative imagination to express technical proficiency, beauty, emotional power, or conceptual ideas. There is no generally agr ...


Notes and references


External links


Radiometry and photometry FAQ
Professor Jim Palmer's Radiometry FAQ page (
University of Arizona The University of Arizona (Arizona, U of A, UArizona, or UA) is a Public university, public Land-grant university, land-grant research university in Tucson, Arizona. Founded in 1885 by the 13th Arizona Territorial Legislature, it was the first ...
). {{Use dmy dates, date=February 2020 SI derived units Units of illuminance