GNU Collaborative International Dictionary of English

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Found one definition

1.       
   ‖Lumen (?), 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.
   
   [Webster 1913 Suppl.]
   
   
   2.  (Biol.) An opening, space, or cavity, esp. a tubular cavity; a vacuole.
   
   [Webster 1913 Suppl.]