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In photometry, 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. Similarly, luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as luminous exitance. In SI derived units, these are both measured in lux (lx) or lumens per square metre (cd·sr·m-2). In the CGS system, the unit of illuminance is the phot. One phot is equal to 10000 lux. Illuminance was formerly often called brightness, but this leads to confusion with other uses of the word. "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 over more than a 2 trillion–fold range: The presence of white objects is somewhat discernible under starlight, at 5Ă—10-5 lux, 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 afterimages.
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