Brightness

Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light.

The word is from a Proto-Germanic *berhtaz, ultimately from a PIE root with a closely related meaning, *bhereg- "white, bright".

As defined by the US Federal Glossary of Telecommunication Terms (FS-1037C), "brightness" should now be used only for non-quantitative references to physiological sensations and perceptions of light.

[4] The United States Federal Trade Commission (FTC) has assigned an unconventional meaning to brightness when applied to lamps.

Luminance, the original meaning of brightness, is the amount of light per solid angle coming from an area, such as the sky.