"radiating or reflecting light," Old English bryht, metathesis of beorht "bright; splendid; clear-sounding; beautiful; divine," from Proto-Germanic *berhtaz "bright" (source also of Old Saxon berht, Old Norse bjartr, Old High German beraht, Gothic bairhts "bright"), from PIE root *bhereg- "to shine; bright, white." The meaning "quick-witted, having brilliant mental qualities" is from 1741.
word-forming element denoting action, quality, or state, attached to an adjective or past participle to form an abstract noun, from Old English -nes(s), from Proto-Germanic *in-assu- (cognates: Old Saxon -nissi, Middle Dutch -nisse, Dutch -nis, Old High German -nissa, German -nis, Gothic -inassus), from *-in-, originally belonging to the noun stem, + *-assu-, abstract noun suffix, probably from the same root as Latin -tudo (see -tude).
<a href="https://www.etymonline.com/word/brightness">Etymology of brightness by etymonline</a>
Harper, D. (n.d.). Etymology of brightness. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/brightness