Yes, LAMBERTS is a Scrabble word!

Letter values in Scrabble US: L 1, A 1, M 3, B 3, E 1, R 1, T 1, S 1.

Is it a valid word?

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

a cgs unit of illumination equal to the brightness of a perfectly diffusing surface that emits or reflects one lumen per square centimeter
English composer and conductor (1905-1951)

Also: l, constant lambert, leonard constant lambert.