Luminal (n.)

trade name of phenobarbitone, used as a sedative and hypnotic, coined 1912 in German from Latin lumen "light" (from suffixed form of PIE root *leuk- "light, brightness") + -al (3), "the root here being used, very irregularly, as an equivalent of pheno-" [Flood].

1897, "of or pertaining to a lumen," with -al (1).