Ballistic helmets have to be tested before they are assigned an NIJ protection level. Level IIIA is the least level of protection that a ballistic helmet can have. Two parameters are used to determine the protection level of a ballistic helmet. They are Penetration Depth and Back Face Deformation.
Penetration depth, as the name suggests, is the measure of how deep a bullet penetrates under the surface of a ballistic helmet. Why would anyone care about the penetration depth as long as the helmet can stop the bullet? You might wonder. A gunshot is not injurious only when it penetrates into your body. Blunt trauma can cause injuries as well, even if the bullet does not penetrate through. Blunt trauma to the head is particularly very hazardous. Penetration depth helps ensure the helmet can offer as much protection to your head as needed.
Back Face Deformation is a measure of depth as well but on a clay head form. During testing, the ballistic helmet is placed on a clay head form and secured in place. The clay will take a perfect shape of the helmet. The helmet is then removed, and a laser scan of the surface of the clay head form is taken. The helmet is put back onto the head form and a shot is taken. The helmet is then removed, and another scan of the head-form's surface is taken. The two scans are compared to determine if there's been any impact on the head-form's surface and if there is, how deep of a hole was formed. This test is a good indicator of how protective of your head a helmet would be.
