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The SI unit of luminous flux, symbol lm, equal to one candela steradian. (Thanks Gritchka!)
The luminous flux, or lumen dF is given by :

dF = IdR

I being the luminous intensity, or candela, dR describes 'an element of solid angle', the steradian. The lumens tells you the total flux of energy emitted by, say a lamp, over all possible directions. This is different from the candela, which just tells you brightness of an object, from any given direction.

To get an accurate idea of how a light will illuminate a room, you need to know its output in lumens, and have a diagram showing its brightness in candelas over the range of possible angles it can shine. These two facts allow you to calculate the actual energy falling on an object at any given location in a room, this value is called the lux.

In anatomy, the lumen is the inner open space or cavity of a tubular structure, such as a blood vessel or the gastrointestinal tract.

Similarly in botany, the lumen is a cavity surrounded by plant cell walls (as in the phloem or xylem).

Lu"men (?), n.; pl. L. Lumina (#), E. Lumens (#). [L., light, an opening for light.]

1. (Photom.)

(a)

A unit of illumination, being the amount of illumination of a unit area of spherical surface, due to a light of unit intensity placed at the center of the sphere.

(b)

A unit of light flux, being the flux through one square meter of surface the illumination of which is uniform and of unit brightness.

2. (Biol.)

An opening, space, or cavity, esp. a tubular cavity; a vacuole.

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