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Amount of light in lights (Lumen)

The lumen (symbol: lm) is a unit for light current. It measures the total amount of light a light source emits in every single direction. Light current differs from radiation current because light current takes the sensitivity of the human eye for the visible part of the electromagnetic radiation into account. Lumen is also the unit to measure the clarity of a light source, no matter what the direction of the light ray is.

Technical information lumen

Lumen is a unit that stems from the SI system. One lumen is the light current of a pointed light source of which the light is bundled into one steradian and emits one candela. In general, this means the light current in lumen is equal to the multiplication of the size of the light strength of this source (in candela) and the size of the spacial angle of the emitted light (in steradian). One lumen is equal to one cd sr. In other words, because a round sphere is 4π steradian, a light source that emits a uniform light strength of one candela in all directions will possess a total light current of 4π lumen; 1 lm = 1 cd·sr

Lumen and observation

Normally the intensity of light (as an electromagnetic wave) is reflected by the Poynting vector in W/m². Because the human eye is only able to observe part of the electromagnetic spectrum (most sensitive for 550-nm waves, ergo green), the lumen is also defined as being the equivalent of 1/683 watt of the 550-nm light.

LED lumen and other light sources

Light current is a measure for the total amount of light in a light stream. The light strength is a measure for the density of said light. If one observes part of the light stream, that part will have a smaller light stream (in lumen), but -principally- the same light strength (in candela) as the entire stream. The light stream depends on the performance of the source and the colour of the light. A light bulb has a light current of approximately twelve lumen per watt. LED and lumen show that a limited amount of current is still capable of emitting enough light. Mag Lite and lumen go very well together, because these magnificent torchlights have a strong light performance and therefore are great to use in the dark.