Brightness is mainly a perceptual term: it describes how bright something appears to our eyes. That’s why definitions often say it is the perception caused by luminance.

It helps to separate a few related ideas:

• Color: light blue and dark blue are not just different “brightnesses”; they can also differ in tone and amount of white/black mixed in.
• Pixel/image values: in digital images, each RGB channel has a numeric value (for example 0–255 in 8-bit images). Higher values generally appear brighter, and 0 is black.
• Physical light measurement: light from a source is measured with photometric units such as lumens (total light output), but that is not the same as perceived brightness of a color or surface.

So, strictly speaking, brightness itself doesn’t have one simple universal unit in everyday photographic use. In editing software it may be represented as channel values or percentages, while physical light output uses units like lumens. If you’re comparing “light blue” and “dark blue” in an image, you’re usually talking about relative tonal/lightness values rather than a single standalone brightness unit.