One lumen is the amount of light emitted by a light source with a luminous intensity of one candela (cd) into the spheridian unit of one steradian (sr – quotient of the superficial content of a segment of a spherical surface and the square of the associated radius of the sphere). The lumen unit is now mainly used in a form defined by the American National Standards Institute (called the “ANSI lumen”). To this end, the average of the brightness values measured at nine points on an illuminated surface is taken and the luminous flux determined on the basis of a table published by the Institute.

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