Bullet proof glass refers to any type of glass that is built to stand up against being penetrated by most bullets. In the industry itself, this glass is referred to as bullet-resistant glass, because there is no feasible way to create consumer-level glass that can truly be proof against bullets. There are two main types of bullet proof glass: that which uses laminated glass layered on top of itself, and that which uses a polycarbonate thermoplastic.
Laminated bullet proof glass has been used since World War II, and although quite effective, it is also relatively heavy and thick. Generally it is about four inches (100 mm) thick, although it may be as thick as eight inches (200 mm) for some applications. Thin sheets of glass are laminated together using a bonding agent, usually polyurethane or ethylene-vinyl.
The more common type of modern bullet resistant glass uses a polycarbonate thermoplastic layered between glass. This layer, which comes under many trade names, including Lexan, Armormax, and Makroclear, is built to withstand blunt force. It distributes the force relatively evenly, so that if it is hit with a hammer, a fist, or an axe, it would repel the object easily.
By itself, this polycarbonate layer could easily be penetrated by a streamlined bullet. When layered with glass, however, it becomes much more effective. As the bullet hits the glass layer, it flattens out as it breaks the glass, which by itself would not stop it from passing through. When it encounters the polycarbonate layer, it has been flattened such that the plastic is able to repel it. Multiple layers of glass and plastic are layered on top of each other to ensure that the bullet is adequately flattened and slowed to be completely stopped.
In recent years, a type of bullet proof glass has become popular in deployments such as armored cars, where it is desirable for those on the inside to be able to fire at those on the outside, while still being protected from incoming bullets. One-way bullet resistant glass uses a two-layered approach to ensure bullets can only penetrate from one direction. The outside of the window is made up of brittle glass, which shatters on impact, while the inside of the window is made up of a more flexible layer that gives under direct force. When a bullet is fired from the inside, it penetrates the flexible layer, then shatters a small area of the outer layer and makes its way through. When a bullet is fired from the outside, however, it hits the brittle layer first, which shatters and distributes its force, so that it is insufficient to penetrate the flexible layer.
The field of bullet proof glass is constantly developing, and there are a number of military projects underway to create lighter-weight, more defensive forms of bullet proof glass. One of the most promising is the use of aluminum oxynitride in the outer layer, in place of a polymer layer. This allows for much lighter, more resistant glass that can be used in military assault vehicles and aircraft.