from
The Collaborative International Dictionary of English v.0.48
Lumen \Lu"men\, n.; pl. L. {Lumina}, E. {Lumens}. [L., light, an
opening for light.]
1. (Photom.)
(a) 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.
(b) A unit of light flux, being the flux through one
square meter of surface the illumination of which is
uniform and of unit brightness.
[Webster 1913 Suppl.]
2. (Biol.) An opening, space, or cavity, esp. a tubular
cavity; a vacuole.
[Webster 1913 Suppl.]