The most common and most likely answer to this crossword clue is the 9 letter word LUMINANCE
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More clues leading to the result LUMINANCE
Buddy explains!
The straight part
We know that "brightness" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case brightness = luminance.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does LUMINANCE mean?
LUMINANCE noun- The quality of being luminous.
- The amount of light that passes through, is emitted, or is reflected from a particular area, and falls within a given solid angle.
- The luminous flux emitted in a given direction divided by the product of the projected area of the source element perpendicular to the direction and the solid angle containing that direction (i.e. luminous intensity divided by unit area), measured in stilbs or apostilbs.