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 differences. It is also possible — and sometimes desirable — to define a saturation-like quantity that is linearized in term of the psychovisual perception.