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From: Webster's Revised Unabridged Dictionary (1913)

 Lu·men n.; pl. L. Lumina E. Lumens .
 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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