The Shape and Material of the collector differ from different designs as well. The collector is generally composed of one bent glass mirror, with either silver or aluminum coated on the backside of the glass. The glass is about four-millimeter thick and low in iron, maximizing the reflectance of incoming sunlight (about 93.5% with silver coating protected by multilayer paint). Although National Renewable Energy Lab (NREL) uses silver for its collector and it has a higher reflectance, aluminum is also adopted by others for its cheaper cost and stronger resistance to erodent environment.