What is the unit for measuring brightness of light

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william1941 | College Teacher | (Level 3) Valedictorian

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The SI unit for measuring the intensity of light as perceived by the human eye is the lux (lx). This is technically the unit for illuminance or luminous emittance. The total amount of visible light is measured in units of lumens. How bright it appears when it falls on a surface area is illuminance. When the same luminous flux falls over a larger area it will appear to be less bright. 1 lx = 1 lm/m^2 which in terms of the basic SI units is 1 cd*sr*m^-2.  For example a total star lit sky has a measure of 10^-4 lux, whereas direct sunlight has a measure of over 32000 lux.

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krishna-agrawala | College Teacher | (Level 3) Valedictorian

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The unit of measuring the brightness - or to use the precise scientific terminology, illuminance - in SI system of measurement is lux or lumen per square meter. Illuminance is measured in foot candles. One foot-candle is equal to 10.76 lux.

Please note that lumen is the measure of total light emitted by a source of light. While the lux is the intensity of light received by a surface. The illuminance of a surface from a given source of light is inversely proportional to the distance of the surface from the source. Thus, as we move away from a given source of light the illuminance from that source is proportion to square of the total distance.