@Override
    public void write(JmeExporter ex) throws IOException {
        super.write(ex);
        OutputCapsule oc = ex.getCapsule(this);
        oc.write(position, "position", null);
        oc.write(radius, "radius", 0f);
    }
for internal use only
@return the inverse of the radius
jme3-core/src/main/java/com/jme3/light/PointLight.java