Color measures

In everyday speech "bright" and "saturated" are enough. In colorimetry each of these words has several precisely defined counterparts, and graphics software uses them inconsistently. This page sorts out the terms; the following pages cover them one by one.

Two families of measures

Color measures fall into two groups:

measures of the amount of light: how much light there is or seems to be,
measures of "colorness": how much a color differs from gray.

Each group has absolute measures (depending on how much light there is overall) and relative ones (comparing the color to white or to its own brightness).

Measures of the amount of light

Measure	What it measures	Where it appears
luminance	physical amount of light weighted by eye sensitivity, in cd/m²	Y in CIE XYZ, photometry, WCAG
brightness	perceived amount of light an area emits, absolute	B in HSB, Q in CIECAM
lightness	perceived brightness relative to white	L* in CIELAB and CIELUV, L in HSL (in name only)
intensity	in HSI the mean of RGB components, in physics radiant power	I in HSI

Measures of "colorness"

Measure	What it measures	Where it appears
colorfulness	how strongly a hue is seen, absolute (grows with the amount of light)	M in CIECAM
chroma	colorfulness relative to a white of the same illumination	C* in LCh, C in Munsell
saturation	colorfulness relative to the color's own brightness	S in HSV, HSL, HSI (each different), s_uv in CIELUV

The difference between chroma and saturation shows in a simple example: a red apple in sunlight and in shadow. In shadow it reflects less light, so its chroma is lower (it differs less from the white paper next to it), but its saturation stays roughly the same, because its color relative to its own brightness does not change.

One color, many numbers

RGB (200, 80, 40), #c85028, by the different measures:

V (HSV) = 78.4% largest component L (HSL) = 47.1% mean of max and min I (HSI) = 41.8% mean of three components Y (luminance) = 18.2% light weighted by eye sensitivity L* (CIELAB) = 49.7 perceived lightness relative to white S (HSV) = 80.0% S (HSL) = 66.7% S (HSI) = 62.5% C* (LCh) = 65.1 s_uv (CIELUV) = 2.07

All of them are correct; each answers a different question. Trouble starts when a number from one program goes into another that means something else by the same name.