Tristimulus

The amounts of red, green, and blue needed to form any particular color are called the tristimulus values and are denoted X, Y, and Z, respectively. A color is then specified by its trichromatic coefficients, defined as

$$ x=\frac{X}{X+Y+Z} $$ and $$ y=\frac{Y}{X+Y+Z} $$ and $$ z=\frac{Z}{X+Y+Z} $$

It is noted from these equations that

$$x+y+z=1$$.