Yes, LUMEN is a Scrabble word!

Letter values in Scrabble US: L 1, U 1, M 3, E 1, N 1.

Is it a valid word?

LUMEN is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a unit of luminous flux equal to the amount of light given out through a solid angle of 1 steradian by a point source of 1 candela intensity radiating uniformly in all directions
a cavity or passage in a tubular organ

Not playable in Scrabble US: LM.