Brightness

Brightness is the sensation of how much light an area emits or reflects. It is an absolute measure: the same sheet of paper looks brighter in the sun than indoors, although in both cases it is "white".

Brightness versus lightness

Colorimetry separates two things that everyday language often blurs:

• brightness: how much light is seen, absolutely. It rises when the illumination rises.
• lightness: how light a color is compared to white in the same conditions. It does not change when the illumination of the whole scene rises.

A gray card in the sun has high brightness, in shade low. Its lightness is similar in both, because the card is just as gray compared with a white card next to it. The eye and brain judge lightness very well and brightness much worse, which is why it is hard to guess by eye how many lux a room has.

Brightness versus luminance

Luminance is a physical quantity, measured with a photometer in candelas per square meter. Brightness is a sensation. They rise together, but not proportionally: twice the luminance does not look twice as bright. The relation resembles a square root or a logarithm and depends on the surround. The same screen luminance seems blinding in a dark room and pale in sunlight.

In color appearance models such as CIECAM02 and CAM16, brightness is written Q and lightness J. Q depends on the luminance of white and on the surround; J does not.

B in the HSB model

In Adobe software the third component of the HSV model is called brightness. Despite the name, it is not brightness in the sense above, just the largest RGB component:

This "brightness" has two flaws. First, it ignores the two smaller components: red (255, 0, 0) and white (255, 255, 255) both have B = 100%. Second, it ignores the eye's sensitivity: pure yellow and pure blue both have B = 100%, although yellow looks far brighter.

When you need a brightness measure that matches what people see, use L* from CIELAB, not B from HSB.