Lumen

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Lumen

(n.) 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.
(n.) A unit of light flux, being the flux through one square meter of surface the illumination of which is uniform and of unit brightness.
(n.) An opening, space, or cavity, esp. a tubular cavity; a vacuole.