lumen

The inner open space or cavity of a tubular organ, as of a blood vessel or an intestine.

The interior of a membrane-bound compartment or organelle in a cell.

The SI unit of luminous flux, equal to the amount of light passing through a solid angle per unit time from a light source of one candela intensity radiating equally in all directions.

An opening or passageway, as, in anatomy, of a hollow tubular organ: as, the lumen of the intestine or of a blood-vessel.

In botany, the internal cavity, or space within the wall, of a cell.

The unit of flux of light; the flux of light in a beam subtending unit solid angle where the source has an intensity of one hefner. See illumination, 1, and light flux.

The hollow tube of an operating-needle or of a hypodermic syringe.

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.

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