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The lux (symbol: lx) is the unit of illuminance, or luminous flux per unit area, in the International System of Units (SI). It is equal to one lumen per square metre. In photometry, this is used as a measure of the irradiance, as perceived by the spectrally unequally responding human eye, of light that hits or passes through a surface. It is analogous to the radiometry unit Irradiance, but with the power at each wavelength weighted according to the luminosity function, a model of human visual brightness perception, standardized by the CIE and ISO. In English, "lux" is used as both the singular and plural form. NIST Guide to SI Units. Chapter 9 – Rules and Style Conventions for Spelling Unit Names, National Institute of Standards and Technology. The word is derived from the Latin word for "light", .
One lux is equal to one lumen per square metre:
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 single 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 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:
| Moonless, overcast night sky (starlight) 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). | |
| Moonless clear night sky with airglow | |
| 0.01 | Quarter moon on a clear night |
| Full moon on a clear night | |
| Dark limit of civil twilight under a clear sky | |
| Public areas with dark surroundings | |
| Family living room lights (Australia, 1998) | |
| Office building hallway/toilet lighting | |
| Very dark overcast day | |
| Train station platforms | |
| Office lightingEuropean law UNI EN 12464 | |
| Sunrise or sunset on a clear day. | |
| Overcast day; typical TV studio lighting | |
| Full daylight (not direct sun) | |
| 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 (μlx) at the Earth's surface. Schlyter, Section 7. A barely perceptible magnitude 6 star provides 8 nanolux (nlx). Schlyter, Section 14. 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 Esc, equal to (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 Surface 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.
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 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 nanometre (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 lx per 1 W/m2; the irradiance needed to make 1 lx at this wavelength is about 1.464 milliwatt/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.
One phot (ph) equals 10 kilolux (10 klx).
One nox (nx) equals 1 millilux (1 mlx) at light color 2042 K or 2046 K (formerly 2360 K).
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. 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.
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