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The lux (symbol: lx) is the SI unit of illuminance and luminous emittance, measuring luminous flux per unit area.
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 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.