Editing Colorfulness (section)

== Chroma ==
{{See also|Psychophysics}}

The naïve definition of saturation does not specify its response function. In the CIE XYZ and RGB color spaces, the saturation is defined in terms of additive color mixing, and has the property of being proportional to any scaling centered at white or the white point illuminant. However, both color spaces are non-linear in terms of psychovisually perceived [[color difference]]s. It is also possible — and sometimes desirable — to define a saturation-like quantity that is linearized in term of the psychovisual perception.

In the [[CIELAB color space|CIE 1976 LAB and LUV color spaces]], the unnormalized '''chroma''' is the radial component of the cylindrical coordinate [[HCL color space|CIE LCh]] (lightness, chroma, hue) representation of the LAB and LUV color spaces, also denoted as CIE LCh(ab) or CIE LCh for short, and CIE LCh(uv). The transformation of <math>(a, b)</math> to <math>\left(C_{ab}, h_{ab}\right)</math> is given by:
<math display=block>C_{ab}^* = \sqrt{a^{*2} + b^{*2}}</math>
<math display=block>h_{ab} = \operatorname{atan2}\left({b^\star},{a^\star}\right)</math>
and analogously for CIE LCh(uv).

The chroma in the CIE LCh(ab) and CIE LCh(uv) coordinates has the advantage of being more psychovisually linear, yet they are non-linear in terms of linear component color mixing. And therefore, chroma in CIE 1976 Lab and LUV color spaces is very much different from the traditional sense of "saturation".

===In color appearance models===

Another, psychovisually even more accurate, but also more complex method to obtain or specify the saturation is to use a [[color appearance model]] like CIECAM02. Here, the '''chroma''' color appearance parameter might (depending on the color appearance model) be intertwined with e.g. the physical brightness of the illumination or the characteristics of the emitting/reflecting surface, which is more sensible psychovisually.

The CIECAM02 chroma <math>C,</math> for example, is computed from a lightness <math>J</math> in addition to a naively evaluated color magnitude <math>t.</math> In addition, a colorfulness <math>M</math> parameter exists alongside the chroma <math>C.</math> It is defined as <math>M = CF_B^{0.25},</math> where <math>F_L</math> is dependent on the viewing condition.<ref name=ciecam02/>