public function cie76($color1, $color2)
    {
        $f1 = $this->toLab($color1);
        $f2 = $this->toLab($color2);

        $deltaL = $f2->l - $f1->l;
        $deltaA = $f2->a - $f1->a;
        $deltaB = $f2->b - $f1->b;

        $deltaE = $deltaL * $deltaL + $deltaA * $deltaA + $deltaB * $deltaB;

        return $deltaE < 0 ? 0 : sqrt($deltaE);
    }