Lumen, straight from the Latin for light, lumen. The Romans also used the term for the aperture of windows, particularly round ones, and it once was the name given to the pupil of the eye.
Its anatomical usage now is reserved for the space inside a tube, e.g. an arterial lumen, a definition probably stemming from a tube's round opening when seen in cross section, or perhaps from the light seen through a tube if opened on both ends.
In physics, a lumen is a unit of brightness. A 100-watt incandescent light bulb generates about 1600 lumens.
Other words from lumen include luminary, luminescence, illuminate.