lumen (n.)

Proto-Indo-European root meaning "light, brightness."

It forms all or part of: allumette; elucidate; illumination; illustration; lea; leukemia; leuko-; light (n.) "brightness, radiant energy;" lightning; limn; link (n.2) "torch of pitch, tow, etc.;" lucent; lucid; Lucifer; luciferase; luciferous; lucifugous; lucubrate; lucubration; luculent; lumen; Luminal; luminary; luminate; luminescence; luminous; luna; lunacy; lunar; Lunarian; lunate; lunation; lunatic; lune; lunette; luni-; luster; lustrum; lux; pellucid; sublunary; translucent.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit rocate "shines;" Armenian lois "light," lusin "moon;" Greek leukos "bright, shining, white;" Latin lucere "to shine," lux "light," lucidus "clear;" Old Church Slavonic luci "light;" Lithuanian laukas "pale;" Welsh llug "gleam, glimmer;" Old Irish loche "lightning," luchair "brightness;" Hittite lukezi "is bright;" Old English leht, leoht "light, daylight; spiritual illumination," German Licht, Gothic liuhaþ "light."

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