Lumen

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

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

III. Lumen ·add. ·noun 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.