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

	Oi = 0.0;
	Ci = 0.0;
	if (behind > 0) { //don't cover filament
		Oi = 0.1;
		Ci = Cs*0.25;
	}
}