How much light an area appears to give out or reflect — in everyday use, also a loose word for both value and lightness.
The CIE defines brightness as the attribute of a visual perception by which an area appears to emit, transmit or reflect more or less light. It is an absolute impression that rises with the light level, unlike lightness, which is judged relative to white. Software uses the word differently: the 'B' in HSB and most brightness sliders are HSV value.
'Brighter' in Adjust My Colour specifically means an HSV-value increase, not a saturation increase — 'brighter' and 'more vivid' are not the same adjustment.
Adjust My Colour's 'brighter' option raises HSV value; it will not necessarily make a colour look more saturated.
Knowing which technical meaning is in play avoids confusion when a tool's 'brighter' result looks different from what was expected.
The CIE defines brightness as the attribute by which an area appears to emit, transmit or reflect more or less light. It is a perception, so it has no instrument reading of its own. The physical quantity that drives it is luminance, measured in candelas per square metre, and colour-appearance models such as CIECAM16 predict brightness from luminance together with the surroundings and the eye's state of adaptation.
| Screen white | Grey's luminance | Grey's CIELAB L* | Contrast with white |
|---|---|---|---|
| 100 cd/m² | 21.6 cd/m² | 53.6 | 3.95:1 |
| 400 cd/m² | 86.3 cd/m² | 53.6 | 3.95:1 |
Turning the screen up multiplies the luminance of Grey #808080 from 21.6 to 86.3 cd/m², and it looks brighter. Its lightness is still L* 53.6 and its contrast with white is still 3.95:1, because both are judged against the screen's own white, which rose by the same factor.
How this was calculated: Relative luminance of the HEX value (0.216) multiplied by two illustrative peak-white settings; the two settings are examples, not measurements of any display.
Judging a colour scheme on a phone at full brightness, then seeing it on an office monitor or in print.
Nothing in the file changed, but every colour looked more colourful and more contrasting on the brighter screen. Decisions about subtle greys and dark themes need checking at the dimmest setting they will meet.