Lux

The lux (symbol: lx) is the SI derived unit of illuminance and luminous emittance, measuring luminous flux per unit area.[1] 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 standardized model of human visual brightness perception. In English, "lux" is used as both the singular and plural form.[2]

Lux
Lux meter
A lux meter for measuring illuminances in work places
General information
Unit systemSI derived unit
Unit ofilluminance
Symbollx 
Conversions
1 lx in ...... is equal to ...
   SAE units   0.0929 fc
   CGS units   10−4 phots

Explanation

Illuminance

1 luminous
Luminous energy and power
2 exitance
Luminous exitance
3 illuminance
Illuminance
4 intensity
Luminous intensity
5 luminance
Luminance

Illuminance is a measure of how much luminous flux is spread over a given area. One can think of luminous flux (measured in lumens) 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, concentrated into 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 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 level of lux requires a greater number of lumens.

As with other SI units, SI prefixes can be used, for example a kilolux (klx) is 1000 lux.

Here are some examples of the illuminance provided under various conditions:

Illuminance (lux) Surfaces illuminated by
0.0001 Moonless, overcast night sky (starlight)[3]
0.002 Moonless clear night sky with airglow[3]
0.05–0.3 Full moon on a clear night[4]
3.4 Dark limit of civil twilight under a clear sky[5]
20–50 Public areas with dark surroundings[6]
50 Family living room lights (Australia, 1998)[7]
80 Office building hallway/toilet lighting[8][9]
100 Very dark overcast day[3]
150 Train station platforms[10]
320–500 Office lighting[7][11][12][13]
400 Sunrise or sunset on a clear day.
1000 Overcast day;[3] typical TV studio lighting
10,000–25,000 Full daylight (not direct sun)[3]
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 at the Earth's surface.[14] A barely perceptible magnitude 6 star provides 8 nanolux.[15] The unobscured Sun provides an illumination of up to 100 kilolux 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.[16] 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.

Luminous emittance

The lux is also a measure of luminous emittance, the number of lumens given off by a surface per square metre, regardless of how that light is distributed in terms of the directions in which it is emitted. It differs from the luminance (measured in candela per square metre or "nits"), which does depend on the angular distribution of the emission. A perfectly white surface with one lux falling on it will emit one lux.

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 lx 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.

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. 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 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 (ph) equals 10 kilolux.

One nox (nx) equals 1 millilux.

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.[14] 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 has a symbol for "lx": (㏓). It is a legacy code to accommodate old code pages in some Asian languages. Use of this code is not recommended.

SI photometry units

SI photometry quantities
Quantity Unit Dimension Notes
Name Symbol[nb 1] Name Symbol Symbol[nb 2]
Luminous energy Qv[nb 3] lumen second lm⋅s TJ The lumen second is sometimes called the talbot.
Luminous flux, luminous power Φv[nb 3] lumen (= candela steradians) 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 L−2J 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−2J Luminous flux incident on a surface
Luminous exitance, luminous emittance Mv lux lx L−2J Luminous flux emitted from a surface
Luminous exposure Hv lux second lx⋅s L−2TJ Time-integrated illuminance
Luminous energy density ωv lumen second per cubic metre lm⋅s/m3 L−3TJ
Luminous efficacy η[nb 3] lumen per watt lm/W M−1L−2T3J Ratio of luminous flux to radiant flux or power consumption, depending on context
Luminous efficiency, luminous coefficient V 1 Luminous efficacy normalized by the maximum possible efficacy
See also: SI · Photometry · Radiometry
  1. ^ Standards organizations recommend that photometric quantities be denoted with a suffix "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
  2. ^ 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.
  3. ^ a b c Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ or K for luminous efficacy.

See also

Notes and references

  1. ^ SI Derived Units, National Institute of Standards and Technology.
  2. ^ NIST Guide to SI Units. Chapter 9 – Rules and Style Conventions for Spelling Unit Names, National Institute of Standards and Technology.
  3. ^ 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).
  4. ^ Kyba, Christopher C. M.; Mohar, Andrej; Posch, Thomas (1 February 2017). "How bright is moonlight?". Astronomy & Geophysics. 58 (1): 1.31–1.32. doi:10.1093/astrogeo/atx025.
  5. ^ "Electro-Optics Handbook" (pdf). photonis.com. p. 63. Retrieved 2012-04-02.
  6. ^ "NOAO Commen and Recommended Light Levels Indoor" (PDF).
  7. ^ a b Pears, Alan (June 1998). "Chapter 7: Appliance technologies and scope for emission reduction". Strategic Study of Household Energy and Greenhouse Issues (PDF). Sustainable Solutions Pty Ltd. Department of Industry and Science, Commonwealth of Australia. p. 61. Archived from the original (pdf) on 2 March 2011. Retrieved 2008-06-26.
  8. ^ 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.
  9. ^ "Low-Light Performance Calculator".
  10. ^ Darlington, Paul (5 December 2017). "London Underground: Keeping the lights on". Rail Engineer. Retrieved 20 December 2017.
  11. ^ "How to use a lux meter (Australian recommendation)" (PDF). Sustainability Victoria (sustainability.vic.gov.au). April 2010. Archived from the original (pdf) on 7 July 2011. External link in |publisher= (help)
  12. ^ "Illumination. - 1926.56". Regulations (Standards - 29 CFR). Occupational Safety and Health Administration, US Dept. of Labor. Archived from the original on 8 May 2009.
  13. ^ European law UNI EN 12464
  14. ^ a b Schlyter, Section 7.
  15. ^ Schlyter, Section 14.
  16. ^ Jack L. Lindsey, Applied Illumination Engineering, The Fairmont Press, Inc., 1997 ISBN 0881732125 page 218

External links

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Their importance becomes apparent in analyzing acid–base reactions for gaseous or liquid species, or when acid or base character may be somewhat less apparent. The first of these concepts was provided by the French chemist Antoine Lavoisier, around 1776.

It is important to think of the acid-base reaction models as theories that complement each other. For example, the current Lewis model has the broadest definition of what an acid and base are, with the Bronsted-Lowry theory being a subset of what acids and bases are, and the Arrhenius theory being the most restrictive.

Filmfare Award for Best Female Debut

The Filmfare Award for Best Female Debut (previously known as "Filmfare Award for Lux New Face of the Year") is given by Filmfare as part of its annual Filmfare Awards for Hindi films to recognise a performance by a female actor in a debut role.

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Lumen (unit)

The lumen (symbol: lm) is the SI derived unit of luminous flux, a measure of the total quantity of visible light emitted by a source per unit of time. Luminous flux differs from power (radiant flux) in that radiant flux includes all electromagnetic waves emitted, while luminous flux is weighted according to a model (a "luminosity function") of the human eye's sensitivity to various wavelengths. Lumens are related to lux in that one lux is one lumen per square meter.

The lumen is defined in relation to the candela as

1 lm = 1 cd ⋅ sr.A full sphere has a solid angle of 4π steradians, so a light source that uniformly radiates one candela in all directions has a total luminous flux of 1 cd × 4π sr = 4π cd⋅sr ≈ 12.57 lumens.

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Lux, Haute-Garonne

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In 1925, it became the first mass-market toilet soap in the world. It is noted as a brand that pioneered female celebrity endorsements.

As of 2009, Lux revenue was estimated at €1 billion, with market shares spread out to more than 100 countries around the globe.

Today, Lux is the market leader in countries like Bangladesh, Brazil, India, Pakistan, South Africa and Thailand.Developed by Unilever, Lux (soap) is now headquartered in Singapore. It celebrated its 90th anniversary in 2018.

Lux Radio Theatre

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Lux Radio Theatre strove to feature as many of the original stars of the original stage and film productions as possible, usually paying them $5,000 an appearance. In 1936, when sponsor manufacturer Lever Brothers (who made Lux brand soap and detergent) moved the show from New York City to Hollywood, the program began to emphasize adaptations of films rather than plays. The first Lux film adaptation was The Legionnaire and the Lady, with Marlene Dietrich and Clark Gable, based on the film Morocco. That was followed by a Lux adaptation of The Thin Man, featuring the movie's actual stars, Myrna Loy and William Powell.

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Rosemary Clooney

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The crimes of sexual abuse offend Our Lord, cause physical, psychological and spiritual damage to the victims and harm the community of the faithful. In order that these phenomena, in all their forms, never happen again, a continuous and profound conversion of hearts is needed, attested by concrete and effective actions that involve everyone in the Church, ... Therefore, it is good that procedures be universally adopted to prevent and combat these crimes that betray the trust of the faithful.

The document was issued three months after the sexual abuse summit convened by Pope Francis at the Vatican in February 2019.

Base units
Derived units
with special names
Other accepted units
See also

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