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36-502: 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 radiometric unit watt per square metre , but with

72-585: A function known as CIE V M ( λ ). More recently, Sharpe, Stockman, Jagla & Jägle (2005) developed a function consistent with the Stockman & Sharpe cone fundamentals ; their curves are plotted in the figure above. Stockman & Sharpe has subsequently produced an improved function in 2011, taking into account the effects of chromatic adaptation under daylight . Their work in 2008 has revealed that "luminous efficiency or V(l) functions change dramatically with chromatic adaptation". The ISO standard

108-412: 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. In practical lighting problems, given information on

144-451: A standard air wavelength of 555.016 nm rather than 555 nm , which is the peak of the luminosity curve. The value of y ( λ ) is 0.999 997 at 555.016 nm , so that a value of 683/ 0.999 997 = 683.002 is the multiplicative constant. The number 683 is connected to the modern (1979) definition of the candela , the unit of luminous intensity . This arbitrary number made the new definition give numbers equivalent to those from

180-549: A star of apparent magnitude 0 provides 2.08 microlux (μlx) at the Earth's surface. 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 E sc , equal to 128 000  lux (see Sunlight and Solar constant ). The illuminance on

216-419: 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

252-455: A symbol for "lx": U+33D3 ㏓ SQUARE LX . It is a legacy code to accommodate old code pages in some Asian languages. Use of this code is not recommended in new documents. Illuminance In SI units illuminance is measured in lux (lx), or equivalently in lumens per square metre ( lm · m ). Luminous exitance is measured in lm·m only, not lux. In the CGS system,

288-525: Is ISO/CIE FDIS 11664-1. The standard provides an incremental table by nm of each value in the visible range for the CIE 1924 function. For very low levels of intensity ( scotopic vision ), the sensitivity of the eye is mediated by rods, not cones, and shifts toward the violet , peaking around 507 nm for young eyes; the sensitivity is equivalent to 1699 lm/W or 1700 lm/W at this peak. The standard scotopic luminous efficiency function or V ′ ( λ )

324-426: 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

360-437: 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. Unicode includes

396-623: Is a standard function established by the Commission Internationale de l'Éclairage (CIE) and standardized in collaboration with the ISO , and may be used to convert radiant energy into luminous (i.e., visible) energy. It also forms the central color matching function in the CIE 1931 color space . There are two luminous efficiency functions in common use. For everyday light levels, the photopic luminosity function best approximates

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432-402: Is capable of seeing somewhat more than a 2 trillion-fold range. The presence of white objects is somewhat discernible under starlight, at 5 × 10  lux (50 μlx), while at the bright end, it is possible to read large text at 10 lux (100 Mlx), or about 1000 times that of direct sunlight , although this can be very uncomfortable and cause long-lasting afterimages . In astronomy ,

468-402: Is maximum: 683.002 lx per 1 W/m; the irradiance needed to make 1 lx at this wavelength is about 1.464  mW /m. 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

504-433: Is normalized to a peak value of unity at 555 nm (see luminous coefficient ). The value of the constant in front of the integral is usually rounded off to 683 lm/W . The small excess fractional value comes from the slight mismatch between the definition of the lumen and the peak of the luminosity function. The lumen is defined to be unity for a radiant energy of 1/683 W at a frequency of 540 THz , which corresponds to

540-795: Is the illuminance in lux, and m v is the apparent magnitude. The reverse conversion is m v = − 14.18 − 2.5 log ⁡ ( E v ) . {\displaystyle m_{\mathrm {v} }=-14.18-2.5\log(E_{\mathrm {v} }).} The luminance of a reflecting surface is related to the illuminance it receives: ∫ Ω Σ L v d Ω Σ cos ⁡ θ Σ = M v = E v R {\displaystyle \int _{\Omega _{\Sigma }}L_{\mathrm {v} }\mathrm {d} \Omega _{\Sigma }\cos \theta _{\Sigma }=M_{\mathrm {v} }=E_{\mathrm {v} }R} where

576-460: Is valuable as a baseline for experimental purposes, and in colorimetry . Different luminous efficiency functions apply under different lighting conditions, varying from photopic in brightly lit conditions through mesopic to scotopic under low lighting conditions. When not specified, the luminous efficiency function generally refers to the photopic luminous efficiency function. The CIE photopic luminous efficiency function y (λ) or V (λ)

612-418: The integral is the inner product of the luminosity function with the spectral power distribution . In practice, the integral is replaced by a sum over discrete wavelengths for which tabulated values of the luminous efficiency function are available. The CIE distributes standard tables with luminosity function values at 5 nm intervals from 380 nm to 780 nm . The standard luminous efficiency function

648-459: 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

684-402: The average spectral sensitivity of human visual perception of light . It is based on subjective judgements of which of a pair of different-colored lights is brighter, to describe relative sensitivity to light of different wavelengths . It is not an absolute reference to any particular individual, but is a standard observer representation of visual sensitivity of a theoretical human eye . It

720-723: The case in video cameras, where a maximal 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 lx. 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 (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

756-447: 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/m), and the corresponding radiometric unit, which measures irradiance , is the watt per square metre (W/m). There is no single conversion factor between lux and W/m; there

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792-448: The illuminance stars cast on the Earth's atmosphere is used as a measure of their brightness. The usual units are apparent magnitudes in the visible band. V-magnitudes can be converted to lux using the formula E v = 10 ( − 14.18 − m v ) / 2.5 , {\displaystyle E_{\mathrm {v} }=10^{(-14.18-m_{\mathrm {v} })/2.5},} where E v

828-512: The integral covers all the directions of emission Ω Σ , and In the case of a perfectly diffuse reflector (also called a Lambertian reflector ), the luminance is isotropic, per Lambert's cosine law . Then the relationship is simply L v = E v R π {\displaystyle L_{\mathrm {v} }={\frac {E_{\mathrm {v} }R}{\pi }}} Luminosity function A luminous efficiency function or luminosity function represents

864-452: 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

900-462: The old definition of the candela. The CIE 1924 photopic V ( λ ) luminosity function, which is included in the CIE 1931 color-matching functions as the y ( λ ) function, has long been acknowledged to underestimate the contribution of the blue end of the spectrum to perceived luminance. There have been numerous attempts to improve the standard function, to make it more representative of human vision. Judd in 1951, improved by Vos in 1978, resulted in

936-462: 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. The word is derived from the Latin word for "light", lux . Illuminance is a measure of how much luminous flux is spread over a given area. One can think of luminous flux (with

972-409: The response of the human eye. For low light levels, the response of the human eye changes, and the scotopic curve applies. The photopic curve is the CIE standard curve used in the CIE 1931 color space. The luminous flux (or visible power) in a light source is defined by the photopic luminosity function. The following equation calculates the total luminous flux in a source of light: where Formally,

1008-412: 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 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

1044-451: 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: 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,

1080-402: The same luminous efficiency function as people with protanopia. Their insensitivity to long-wavelength red light makes it possible to use such illumination while studying the nocturnal life of animals. For older people with normal color vision, the crystalline lens may become slightly yellow due to cataracts , which moves the maximum of sensitivity to the red part of the spectrum and narrows

1116-451: The sensitivity of the eye as a function of wavelength. For people with protanopia , the peak of the eye's response is shifted toward the short-wave part of the spectrum (approximately 540 nm), while for people with deuteranopia , there is a slight shift in the peak of the spectrum, to about 560 nm. People with protanopia have essentially no sensitivity to light of wavelengths more than 670 nm. Most non- primate mammals have

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1152-522: The spectral characteristics of image-forming visual photoreception in many individual human eyes. 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

1188-552: The unit lumen ) 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 : 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,

1224-476: The unit of illuminance is the phot , which is equal to 10 000  lux . The foot-candle is a non-metric unit of illuminance that is used in photography . Illuminance was formerly often called brightness , but this leads to confusion with other uses of the word, such as to mean luminance . "Brightness" should never be used for quantitative description, but only for nonquantitative references to physiological sensations and perceptions of light. The human eye

1260-534: 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. 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

1296-437: Was adopted by the CIE in 1951, based on measurements by Wald (1945) and by Crawford (1949). Luminosity for mesopic vision , a wide transitioning band between scotopic and phototic vision, is more poorly standardized. The consensus is that this luminous efficiency can be written as a weighted average of scotopic and mesopic luminosities, but different organizations provide different weighting factors. Color blindness changes

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