Lumen - definition
Lumen - definition
The Lumen is the unit of measurement of luminous flux in SI system, denoted by the symbol lm. It defines the amount of light emitted by a source per unit time, taking into account the sensitivity of the human eye to different wavelengths of visible radiation. One lumen corresponds to the luminous flux emitted at a solid angle of one steradian by a point light source with a luminous intensity of one candela (cd).
The value expressed in lumens indicates the total visible radiant power emitted by the source, regardless of the direction of distribution. Lumens is therefore a measure of the 'visible brightness' of light, distinct from units relating to total radiation, which may also include ranges that are invisible to humans, such as infrared or ultraviolet.
This unit is commonly used in lighting engineering, the design of illumination systems and the evaluation of the performance of light sources. This parameter allows an objective comparison of different lighting technologies - for example, incandescent lamps, compact fluorescent lamps or LEDs - regardless of their electrical output. The luminous efficacy of a source, expressed in lumens per watt (lm/W), is used to assess the efficiency of the conversion of electrical energy into visible light.
In practice, lumens are also used to assess the brightness of luminaires, projectors, screens and other light-emitting devices. Due to the dependence of brightness perception on ambient conditions and individual visual characteristics, lumens provide a standardised reference point for the design of ergonomic and energy-efficient lighting systems.
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