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A lumen (lm) is the International System of Units (SI) measurement for luminous flux (brightness). It is defined as: the amount of light emitted when considering a solid angle of one steradian (sr), from a source that discharges to an equal light in all directions, with the strength of one candela (cd). The unit is defined in terms of steradian because light spreads in all three dimensions, hence the angle must be observed.
The lumen is often misinterpreted as the watt, which is inaccurate. The watt is the measure energy which, when supplied to a bulb, produces luminous light. The brightness of this light is measured in lumens. European Union legislation has set the criteria for light to be measured in lumens, supplied by a primary source. The amount of light emitted (lumens) depends on the type of source and spectrum of light.
In terms of technology, lumens are used to measure the brightness of devices such as monitors, televisions and projectors.