Lumen /(?)/

Lu·men

Lumen

n.

pl. Lumina, Lumens

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