c. 1600, "brilliant light, brightness shooting in diverging rays or beams," from radiant (adj.) or else from Medieval Latin radiantia "brightness," from radiare "to beam, shine" (see radiation). Figurative use, of beauty, joy, etc., is by 1761. Related: Radiancy.
Radiance ... is generally a light that is agreeable to the eyes; hence the word is often chosen for corresponding figurative expressions: as, the radiance of his cheerfulness; the radiance of the gospel. Brilliance represents a light that is strong, often too strong to be agreeable, and marked by variation or play and penetration : as, the brilliance of a diamond or of fireworks. [Century Dictionary]