The lux (symbol: lx) is the SI unit of illuminance and luminous emittance, measuring luminous flux per unit area. It is used in photometry 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 watts 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 in both singular and plural.[1]
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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.
One lux is equal to one lumen per square metre:
A flux of 1,000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1,000 lux. However, the same 1,000 lumens, spread out over ten 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 12,000 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 1,000 lux.
Lux can be used as a measure of the brightness of a light source, for instance of a star or a distant light bulb. 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.[2] A magnitude 6 star (just barely visible) would be 8.3 nanolux. A standard candle (one candela) at a kilometre would be 1 microlux, about the same as a magnitude 1 star.
Here are some examples of the illuminance provided under various conditions, or by heavenly sources:
Examples | |
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Illuminance | Surfaces illuminated by: |
10−4 lux | Total starlight, overcast sky[3] |
0.002 lux | Moonless clear night sky with airglow[3] |
0.01 lux | Quarter moon |
0.27 lux | Full moon on a clear night[3][4] |
1 lux | Full moon overhead at tropical latitudes[5] |
3.4 lux | Dark limit of civil twilight under a clear sky[6] |
50 lux | Family living room lights (Australia, 1998)[7] |
80 lux | Office building lights in hallway/toilet[8][9] |
100 lux | Very dark overcast day[3] |
320–500 lux | Office lighting[10][11][12] |
400 lux | Sunrise or sunset on a clear day. |
1,000 lux | Overcast day;[3] typical TV studio lighting |
10,000–25,000 lux | Full daylight (not direct sun)[3] |
32,000–130,000 lux | Direct sunlight |
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.
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 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 visual system is more sensitive to light of this wavelength than any other. For monochromatic light of this wavelength, the irradiance needed to make one lux is minimum, at 1.464 mW/m2. That is, one obtains 683.002 lux per W/m2 (or lumens per watt) at this wavelength. Other wavelengths of visible light produce fewer lumens per watt. 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 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 lumens per watt. 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 which is based on measurements of the spectral characteristics of visual photoreception in many individual human eyes.
Specifications for video cameras such as camcorders and surveillance cameras often include a minimum 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 maximum exposure time is generally set by the frame rate.
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.
Quantity | Symbol[nb 1] | SI unit | Symbol | Dimension | Notes | |||
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Luminous energy | Qv [nb 2] | lumen second | lm⋅s | T⋅J [nb 3] | units are sometimes called talbots | |||
Luminous flux | Φv [nb 2] | lumen (= cd⋅sr) | lm | J | also called luminous power | |||
Luminous intensity | Iv | candela (= lm/sr) | cd | J | an SI base unit, luminous flux per unit solid angle | |||
Luminance | Lv | candela per square metre | cd/m2 | L−2⋅J | units are sometimes called nits | |||
Illuminance | Ev | lux (= lm/m2) | lx | L−2⋅J | used for light incident on a surface | |||
Luminous emittance | Mv | lux (= lm/m2) | lx | L−2⋅J | used for light emitted from a surface | |||
Luminous exposure | Hv | lux second | lx⋅s | L−2⋅T⋅J | ||||
Luminous energy density | ωv | lumen second per metre3 | lm⋅s⋅m−3 | L−3⋅T⋅J | ||||
Luminous efficacy | η [nb 2] | lumen per watt | lm/W | M−1⋅L−2⋅T3⋅J | ratio of luminous flux to radiant flux | |||
Luminous efficiency | V | 1 | also called luminous coefficient | |||||
See also: SI · Photometry · Radiometry |
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Luxé is a commune in the Charente department in southwestern France.
Lux or LUX may refer to:
A falcon (/ˈfɔːlkən, ˈfæl-/) is any one of 37 species of raptors in the genus Falco, widely distributed on all continents of the world except Antarctica.
Adult falcons have thin, tapered wings, which enable them to fly at high speed and to change direction rapidly. Fledgling falcons, in their first year of flying, have longer flight feathers, which make their configuration more like that of a general-purpose bird such as a broadwing. This makes it easier to fly while learning the exceptional skills required to be effective hunters as adults.
The falcons are the largest genus in the Falconinae subfamily of Falconidae, which itself also includes another subfamily comprising caracaras and a few other species. All these birds kill with their beaks, using a "tooth" on the side of their beaks — unlike the hawks, eagles, and other birds of prey in Accipitridae, which use their feet.
Peregrine falcons have been recorded diving at speeds of 200 miles per hour (320 km/h), making them the fastest-moving creatures on Earth. Other falcons include the gyrfalcon, lanner falcon, and merlin. Some small falcons with long, narrow wings are called hobbies, and some which hover while hunting are called kestrels.
Falcons are a genus of raptor (bird of prey).
Falcons may also refer to:
The Falcons – better known by their Afrikaans name the Valke and currently known as the Hino Valke for sponsorship reasons – are a South African rugby union team in Gauteng province that participates in the annual Currie Cup and Vodacom Cup tournaments.
Their home ground is currently Barnard Stadium in Kempton Park, to which they have returned in 2009. The Falcons have operated out of both Bosman Stadium in Brakpan and Pam Brink Stadium in Springs. They occasionally still host matches at Bosman Stadium. The Falcons draw players from Ekurhuleni and other municipalities to the east and south of Johannesburg.
The Falcons Rugby Union was originally founded in 1947, and was previously known as the Eastern Transvaal Rugby Football Union. After the 1995 World Cup, rugby was declared a professional sport in South Africa, after which the Eastern Transvaal Rugby Football Union merged with the Vaal Triangle Rugby Union to form the Gauteng Falcons Rugby Union and are one of only fourteen Provincial Unions in the country.
Favorites is a compilation album by the American country music singer Crystal Gayle. It was released in 1980, only a short time after another compilation album, Classic Crystal, though consisted of her lesser known songs from the period she was signed to United Artists Records.
The album peaked at #37 on the Billboard Country Albums chart, and #149 on the main Billboard 200 chart.
Pardon me I don't know what made me break down
Don't apologize for bringing up her name
There was a time it didn't hurt to talk about it
But lately the thought of her drives me insane.
I guess I'm not as strong as I used to be
It gets harder all the time to fight the tears
I guess I'm not a strong as I used to be
Losing her has made me weaken through the years.
--- Instrumental ---
Just forget you saw me in this condition
It's not like me to lose my self-control
It still hurts me even though it's been a long time
The hardest thing I ever did was let her go.
I guess I'm not as strong as I used to be
It gets harder all the time to fight the tears
I guess I'm not a strong as I used to be
Losing her has made me weaken through the years.
Losing her has made me weaken through the years...