Etymology
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lux (n.)
unit of illumination, 1889, from Latin lux "light," from PIE root *leuk- "light, brightness."
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link (n.2)
"torch of tow, pitch, etc.," 1520s, of uncertain origin, possibly from Medieval Latin linchinus, from lichinus "wick," from Greek lykhnos "portable light, lamp," from PIE root *leuk- "light, brightness."
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*leuk- 

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."

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phillumenist (n.)

"a collector of match-boxes or match-books," by 1949, from philo- + Latin lumen"light" (n.) (from suffixed form of PIE root *leuk- "light, brightness") + -ist.  

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lucidity (n.)
1650s, "brightness," from French lucidité, from Late Latin luciditas, from Latin lucidus "light, bright, clear," from lucere "to shine," from PIE *louk-eyo-, suffixed (iterative) form of root *leuk- "light, brightness." Meaning "intellectual clarity, transparency of expression" is by 1851.
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luculent (adj.)

early 15c., "luminous, bright;" 1590s, "evident, lucid," from Latin luculentus "full of light, bright, splendid," from the stem of lux "light" (from PIE root *leuk- "light, brightness").

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leukocyte (n.)
also leucocyte, "white blood cell, white or colorless corpuscle of the blood or lymph," 1860, via French leucocyte, from leuco-, a Latinized combining form of Greek leukos "white, clear," from PIE root *leuk- "light, brightness" + -cyte "cell."
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lucent (adj.)
mid-15c., "shining, bright, luminous," from Latin lucentem (nominative lucens), present participle of lucere "to shine, glow, be bright," from PIE root *leuk- "light, brightness." Meaning "translucent, clear" is from 1820. Related: Lucently.
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leuko- 
before vowels leuk-, also sometimes in Latinized form leuco-/leuc-, word-forming element used from 19c. and meaning "white" (or, in medicine, "leukocyte"), from Greek leukos "clear, white," from PIE *leuko-, suffixed form of root *leuk- "light, brightness."
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leukemia (n.)
progressive blood disease characterized by abnormal accumulation of leucocytes, 1851, on model of German Leukämie (1848), coined by R. Virchow from Greek leukos "clear, white" (from PIE root *leuk- "light, brightness") + haima "blood" (see -emia). Formerly also leucemia.
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