lumen

lumen is a   Noun

Definition:

[1] 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.

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

[3] (Biol.) An opening, space, or cavity, esp. a tubular cavity; a vacuole.

Synonyms: