As we now know all perceived colors can be represented by three independent variables, either the LMS responses, or the tristimulus values under the primaries of a given color system, such as RGB or XYZ. In many situations (e.g., computer image processing), it is more convenient to represent a color by a different set of three independent variables HSL (or HSI, HSV). These are defined as
Brightness: for measuring self-luminous objects that emits light (CRT, etc.)
Lightness: for measuring reflected light.
The hue, saturation and luminance can be obtained given the tristimulus values of a color under any color system.