surface
glow( float	attenuation = 2 )
{
	normal n = faceforward(normalize(N), I);
	vector i = normalize(I);
	float behind = i.n;

	if (behind < 0) {
		behind = behind * behind / (i.i * n.n) ;
		Ci = Cs * behind;
		Oi = Os * behind * 0.3;
	} else
		Oi = 0.0;
}