lux

LUX is the principal centre for the promotion and distribution of experimental film and video works in the UK.

It has one of the largest collections of experimental film and video art and houses works of over 1000 artists. It was formed in the 1990s in the merger of the London Filmmakers' Co-op and the original London Video Arts (later variously named London Video Access and London Electronic Arts).

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The lux (symbol: lx) is the SI unit of illuminance and luminous emittance. It is used in photometry as a measure of the intensity of light, with wavelengths weighted according to the luminosity function, a standardized model of human brightness perception. In English, "lux" is used in both singular and plural.[1]

Definition

1 lx = 1 lm/m2 = 1 cd· sr·m–2

Explanation

Lux is a derived unit based on lumen, and lumen is a derived unit based on candela.

One lux is equal to one lumen per square metre, where 4π lumens is the total luminous flux of a light source of one candela of luminous intensity.

Illuminance Abbr. Example
0.00005 lux50 µlxStarlight
0.0001 lux100 µlxMoonless overcast night sky
0.001 lux1 mlxMoonless clear night sky
0.01 lux10 mlxQuarter Moon
0.25 lux250 mlxFull Moon on a clear night[2]
1 luxMoonlight at high altitude at tropical latitudes[3]
10 luxCandle at a distance of 30 cm (1 ft)
50 luxFamily living room[4]
80 luxHallway/Toilet[5]
400 luxA brightly lit office
400 luxSunrise or sunset on a clear day.
1000 lux1 klxTypical TV studio lighting
32000 lux32 klxSunlight on an average day (min.)
100000 lux100 klxSunlight on an average day (max.)


Unicode has a symbol for "lx": (㏓), but this is just a legacy code to accommodate old code pages in certain Asian languages, and it is not recommended for use in any language today.

Lux versus lumen

The difference between the lux and the lumen is that the lux takes into account the area over which the luminous flux is spread. 1000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1000 lux. The same 1000 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 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.

Lux versus footcandle

One footcandle ≈ 10.764 lux. The footcandle (or lumen per square foot) is a non-SI unit of illuminance. Like the BTU, it is obsolete but it is still in fairly common use in the United States, particularly in construction-related engineering and in building codes. Because lux and footcandles are different units of the same quantity, it is perfectly valid to convert footcandles to lux and vice versa.

The name "footcandle" conveys "the illuminance cast on a surface by a one-candela source one foot away." As natural as this sounds, this style of name is now frowned upon, because the dimensional formula for the unit is not foot · candela, but lumen/sq ft. Some sources do however note that the "lux" can be thought of as a "metre-candle" (i.e. the illuminance cast on a surface by a one-candela source one meter away). A source that is farther away casts less illumination than one that is close, so one lux is less illuminance than one footcandle. Since illuminance follows the inverse-square law, and since one foot = 0.3048 m, one lux = 0.30482 footcandle ≈ 1/10.764 footcandle.

In practical applications, as when measuring room illumination, it is very difficult to measure illuminance more accurately than ±10%, and for many purposes it is quite sufficient to think of one footcandle as about ten lux.

Relationship between illuminance and power

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 eye 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/meter2, and accordingly the corresponding radiometric unit, which has no special name, is the watt/meter2. There is no single conversion factor between lux and watt/meter2; 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.

At a monochromatic light wavelength of 555 nm, the green-colored wavelength to which the eye is most sensitive, the irradiance needed to make one lumen is minimum, at 1.464 mW/m2; that is, the peak of the luminosity function is 683.002 lumens per watt. This means that for green light of this particular wavelength, one lumen = 1/683 watt. The luminosity function falls to zero in the infrared and ultraviolet wavelengths.

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 is most sensitive, but must include a generous mixture of red and blue wavelengths to which it is much less sensitive.

This means that typical 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 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 in fact based on the measurement of eyes and is reasonably close to the sensitivity curve for most eyes.

SI photometry units

SI photometry units
    [ edit]
Quantity Symbol SI unit Abbr. Notes
Luminous energyQvlumen secondlmsunits are sometimes called talbots
Luminous fluxFlumen (= cdsr)lmalso called luminous power
Luminous intensityIvcandela (= lm/sr)cdan SI base unit
LuminanceLvcandela per square metrecdm–2units are sometimes called nits
IlluminanceEvlux (= lmm–2)lxUsed for light on a surface
Luminous emittanceMvlux (= lmm–2)lxUsed for light emitted from a surface
Luminous efficacy lumen per wattlm/Wratio of luminous flux to radiant flux; maximum possible is 683.002

Non-SI units of illuminance

Use in consumer camera specifications

Specifications for camcorders (video 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.

References

1. ^ NIST Guide to SI Units - 9 Rules and Style Conventions for Spelling Unit Names, National Institute of Standards and Technology
2. ^ Petzl reference system for lighting performance (html). Retrieved on 2007-04-24.
3. ^ Bunning, Erwin; and Moser, Ilse (Apr. 1969). "Interference of moonlight with the photoperiodic measurement of time by plants, and their adaptive reaction". Proceedings of the National Academy of Sciences of the United States of America 62 (4): 1018–1022. DOI:10.1073/pnas.62.4.1018. Retrieved on 2006-11-10. 
4. ^ Sustainable Solutions Pty Ltd (June, 1998), "Chapter 7: Appliance technologies and scope for emission reduction", Strategic Study of Household Energy and Greenhouse Issues, Australian Greenhouse Office, <[1] (retrieved on 2007-03-13)
5. ^ Australian Greenhouse Office (May, 2005), "Chapter 5: Assessing lighting savings", Working Energy Resource and training kit: Lighting, <[2] (retrieved on 2007-03-13)

External links

Film is a term that encompasses individual motion pictures, the field of film as an art form, and the motion picture industry. Films are produced by recording images from the world with cameras, or by creating images using animation techniques or special effects.
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Video (Latin for "I see", first person singular present, indicative of videre, "to see") is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.
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Motto
"Dieu et mon droit" [2]   (French)
"God and my right"
Anthem
"God Save the Queen" [3]
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Experimental film, or "experimental cinema," is a term that describes a range of filmmaking styles that are generally quite different from, and often opposed to, the practices of mainstream commercial and documentary filmmaking.
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Video art is a type of art which relies on moving pictures and is comprised of video and/or audio data. (It should not however be confused with television or experimental cinema).
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Si, si, or SI may refer to (all SI unless otherwise stated):

In language:
  • One of two Italian words:
  • (accented) for "yes"
  • si

..... Click the link for more information.
illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception.
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illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception.
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Photometry is the science of measurement of light, in terms of its perceived brightness to the human eye. It is distinct from radiometry, which is the science of measurement of light in terms of absolute power; rather, in photometry, the radiant power at each wavelength is
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luminosity function describes the average sensitivity of the human eye to light of different wavelengths. It should not be considered perfectly accurate in every case, but it is a very good representation of human eye sensitivity and it is valuable as a baseline for experimental
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The lumen (symbol: lm) is the SI unit of luminous flux, a measure of the perceived power of light.
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candela (symbol: cd) is the SI base unit of luminous intensity (that is, power emitted by a light source in a particular direction, with wavelengths weighted by the luminosity function, a standardized model of the sensitivity of the human eye).
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Moonlight is the light that comes to Earth from the Moon. This light does not originate from the Moon, but is actually reflected sunlight. The intensity of moonlight varies greatly depending on the current lunar phase, but even the full moon typically provides only a faint
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In astronomy, a celestial coordinate system is a coordinate system for mapping positions in the sky. There are different celestial coordinate systems each using a coordinate grid projected on the celestial sphere, in analogy to the geographic coordinate system used on the surface
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equator divides the planet into a Northern Hemisphere and a Southern Hemisphere, and has a latitude of 0. Latitude, usually denoted symbolically by the Greek letter phi, , gives the location of a place on Earth north or south of the equator.
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candle is a light source that usually has an internal wick rising through the center of a column of solid fuel.

Prior to the mid 19th century, the candle was made from tallow (a byproduct of beef-fat rendering).
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Sunrise is the time at which the first part of the Sun appears above the horizon in the east. Sunrise should not be confused with dawn, which is the (variously defined) point at which the sky begins to lighten, some time before the sun itself appears, ending twilight.
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Sunset, also called sundown in some American English dialects, is the time at which the Sun disappears below the horizon in the west. It should not be confused with dusk, which is the point at which darkness falls, some time after the beginning of twilight when the Sun
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A television studio is an installation in which television or video productions take place, either for live television, for recording live to tape, or for the acquisition of raw footage for postproduction.
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Sunlight, in the broad sense, is the total spectrum of the electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the atmosphere, and the solar radiation is obvious as daylight when the Sun is above the horizon.
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Unicode is an industry standard allowing computers to consistently represent and manipulate text expressed in any of the world's writing systems. Developed in tandem with the Universal Character Set standard and published in book form as The Unicode Standard
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Code page is the traditional IBM term used for a specific character encoding table: a mapping in which a sequence of bits, usually a single octet representing integer values 0 through 255, is associated with a specific character.
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Asia is the world's largest and most populous continent. It covers 8.6% of the Earth's total surface area (or 29.4% of its land area) and, with almost 4 billion people, it contains more than 60% of the world's current human population.
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Fluorescence is a luminescence that is mostly found as an optical phenomenon in cold bodies, in which the molecular absorption of a photon triggers the emission of another photon with a longer wavelength.
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foot-candle (sometimes footcandle; abbreviated fc, lm/ft², or sometimes ft-c) is a non-SI unit of illuminance or light intensity widely used in photography, film, television, and the lighting industry.
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Si, si, or SI may refer to (all SI unless otherwise stated):

In language:
  • One of two Italian words:
  • (accented) for "yes"
  • si

..... Click the link for more information.
British thermal unit (BTU or Btu) is a unit of energy used in the United States of America, particularly in the power, steam generation and heating and air conditioning industries.
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Photometry is the science of measurement of light, in terms of its perceived brightness to the human eye. It is distinct from radiometry, which is the science of measurement of light in terms of absolute power; rather, in photometry, the radiant power at each wavelength is
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In optics, radiometry is the field that studies the measurement of electromagnetic radiation, including visible light. Note that light is also measured using the techniques of photometry, which deal with brightness as perceived by the human eye, rather than absolute power.
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luminosity function describes the average sensitivity of the human eye to light of different wavelengths. It should not be considered perfectly accurate in every case, but it is a very good representation of human eye sensitivity and it is valuable as a baseline for experimental
..... Click the link for more information.


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