A layer of polycarbonate material is sandwiched between ordinary glass layers, a process known as lamination. In this process, a material similar to ordinary glass but thicker than ordinary glass is formed. Polycarbonate is a hard, transparent plastic - people usually use its brand (Lexan, Tuffak or Cyrolon) to call it. The thickness of the bulletproof glass is between 7mm and 75mm. Bullets fired on bullet-proof glass will break through the outer glass, but the polycarbonate glass material layer can absorb the energy of the bullet, thus preventing it from penetrating the inner layer of glass. These large pieces of bullet-proof glass have been used for public purposes during the Second World War. They are usually as thick as 10.0 cm to 12.0 cm and are extremely heavy.