In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate with human brightness perception. Similarly, luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as luminous exitance.
In SI units illuminance is measured in lux (lx), or equivalently in lumens per square metre (lm·m−2). Luminous exitance is measured in lm·m−2 only, not lux. In the CGS system, the unit of illuminance is the phot, which is equal to 10 000 lux 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 is capable of seeing somewhat more than a 2 trillion-fold range. The presence of white objects is somewhat discernible under starlight, at 5e-5lux, while at the bright end, it is possible to read large text at 108 lux, or about 1000 times that of direct sunlight, although this can be very uncomfortable and cause long-lasting s.
In astronomy, 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
where Ev is the illuminance in lux, and mv is the apparent magnitude. The reverse conversion is
The luminance of a reflecting surface is related to the illuminance it receives:
where the integral covers all the directions of emission ΩΣ, and
v is the surface's luminous exitance
v is the received illuminance, and
is the reflectance.
In the case of a perfectly diffuse reflector (also called a Lambertian reflector), the luminance is isotropic, per Lambert's cosine law.
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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 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 model of human visual brightness perception, standardized by the CIE and ISO.
Photometry is the science of the 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 radiant energy (including light) in terms of absolute power. In modern photometry, the radiant power at each wavelength is weighted by a luminosity function that models human brightness sensitivity. Typically, this weighting function is the photopic sensitivity function, although the scotopic function or other functions may also be applied in the same way.
Diffuse reflection is the reflection of light or other waves or particles from a surface such that a ray incident on the surface is scattered at many angles rather than at just one angle as in the case of specular reflection. An ideal diffuse reflecting surface is said to exhibit Lambertian reflection, meaning that there is equal luminance when viewed from all directions lying in the half-space adjacent to the surface.
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