A: Polycarbonate is a type of plastic , the housings made from this material, by Ikelite, Nimar or some Sea and Sea housings are usually bigger , lighter and considered less reliable in heavy use for more than 3 years. Aluminum housings are made from a solid block of Aluminum , milled by using a CNC machine. These housings are usually accurately contoured to the camera , very sturdy, heavy duty and well built. They are the professional choice as they are more ergonomic and more reliable.