thumb|Bulletproof glass of a jeweler's window after a burglary attempt.
thumb|The [[Mona Lisa behind bulletproof glass at the Louvre Museum]]
Bulletproof glass, ballistic glass, transparent armor, or bullet-resistant glass is a strong and optically transparent material that is particularly resistant to penetration by projectiles, although, like any other material, it is not completely impenetrable. It is usually made from a combination of two or more types of glass, one hard and one soft. The softer layer makes the glass more elastic, so that it can flex instead of shatter. The index of refraction for all of the glasses used in the bulletproof layers must be almost the same to keep the glass transparent and allow a clear, undistorted view through the glass. Bulletproof glass varies in thickness from .
Bulletproof glass is used in windows of buildings that require such security, such as jewelry stores and embassies, and of military and private vehicles.
Construction
thumb|A rough visualisation of bulletproof glass, composed of layers of plastic sheeting (grey) and layers of glass (blue)
Bullet-resistant glass is constructed using layers of laminated glass. The more layers there are, the more protection the glass offers. When weight reduction is needed, polycarbonate (a thermoplastic) is laminated onto the safe side to stop spall. The aim is to make a material with the appearance and clarity of standard glass but with effective protection from small arms. Polycarbonate designs usually consist of products such as Armormax, Makroclear, Cyrolon: a soft coating that heals after being scratched (such as elastomeric carbon-based polymers) or a hard coating that prevents scratching (such as silicon-based polymers).
The plastic in laminate designs also provides resistance to impact from physical assault from blunt and sharp objects. The plastic provides little in the way of bullet-resistance. The glass, which is much harder than plastic, flattens the bullet, and the plastic deforms, with the aim of absorbing the rest of the energy and preventing penetration. The ability of the polycarbonate layer to stop projectiles with varying energy is directly proportional to its thickness, and bulletproof glass of this design may be up to 3.5 inches thick. As noted above, laminated glass is usually backed with one or more polycarbonate layers to prevent spalling.
In the table below, you may note that UL 752 Level 6 is missing; this is because Level 6 is easier to resist than Level 4, so it has been left out to prevent confusion.
{| class="wikitable"
! colspan="22"|Sample thickness and weight for bullet-resistant glass materials
|-
! rowspan=3 | Protection Level
! rowspan=3 | Example Threat Stopped || colspan="4"|Glass Laminate||colspan="4"|Polycarbonate||colspan="4"|Acrylic||colspan="4"|Glass-Clad Polycarbonate||colspan="4"|Aluminum oxynitride
|-
! colspan="2"| Thickness || colspan="2" | Weight || colspan="2" | Thickness || colspan="2" | Weight || colspan="2" | Thickness || colspan="2" | Weight || colspan="2" |Thickness || colspan="2" | Weight || colspan="2" |Thickness || colspan="2" | Weight
|-
! in. || mm || !! || in. || mm || !! || in. || mm || !! || in. || mm || || || in. || mm || !!
|-
| UL 752 Level 1 || 9mm HydraShok JHP +P+ 3 shots || 1.185 || 30.09 || 15.25 || 74.46 || 0.75 || 19.05 || 4.6 || 22.46 || 1.25 || 31.75 || 7.7 || 37.6
|
|
| ) || .44 Magnum 3 shots (5 shots for NIJ IIIa) || 1.59 || 40.38 || 20.94 || 102.24 || 1.25 || 31.75 || 7.7 || 37.6 || || || ||
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| When projectiles do not penetrate, the depth of the dent left by the impact can be measured and related to the projectile’s velocity and thickness of the material.
Environmental effects
The properties of bullet-resistant glass can be affected by temperature and by exposure to solvents or UV radiation, usually from sunlight. If the polycarbonate layer is below a glass layer, it has some protection from UV radiation due to the glass and bonding layer. Over time the polycarbonate becomes more brittle because it is an amorphous polymer (which is necessary for it to be transparent) that moves toward thermodynamic equilibrium. ALON is much lighter and performs much better than traditional glass/polymer laminates. Aluminum oxynitride "glass" can defeat threats like the .50 caliber armor-piercing rounds using material that is not prohibitively heavy.
Spinel ceramics
Certain types of ceramics can also be used for transparent armor due to their properties of increased density and hardness when compared to traditional glass. These types of synthetic ceramic transparent armors can allow for thinner armor with equivalent stopping power to traditional laminated glass.
See also
- Transparent Armor Gun Shield
- Prince Rupert's Drop