public double getThickness(Vector3D pt){
		//compute by pt.y & central.y
		//300nm ~ 2000 nm
		double min = 300;
		double max = 2000;
		double relative = (pt.y - (central.y - radius)) / (radius * 2);
		if(relative < 0) relative = 0;
		if(relative > 1) relative = 1;
		double thickness = (max - min) * relative + min;
		return thickness;
	}