toJSON(obj) {
        var json = LightSerializer.prototype.toJSON.call(this, obj);

        json.isHemisphereLight = obj.isHemisphereLight;
        json.skyColor = obj.skyColor;
        json.groundColor = obj.groundColor;

        return json;
    }
HemisphereLightSerializer
@author tengge / https://github.com/tengge1