Lumen

n. pl. L. Lumina (#), E. Lumens (#)

1. (Photom.) An opening, space, or cavity, esp. a tubular cavity; a vacuole.
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.