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"brightness" is also used as a crossword clue. If that's what you saw, the answer is often luminosity (10 letters).
Definitions
the location of a visual perception along a continuum from black to white
Word origin
From Middle English bryghtnes, brightnesse, from Old English beorhtnes (“brightness, splendor”), from Proto-West Germanic *berhtnassī, equivalent to bright + -ness. Cognate with Old High German berahtnessī, berahtnessi (“brightness”).
Source: Wiktionary, CC BY-SA 4.0
Used as a crossword answer2 curated clues
01“Glow strength”10 letters
02“Luminance”10 letters
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