As the "optical shield" for the future warfare, transparent armors exhibit high optical transparency, high hardness, high strength, and resistance to ballistic and explosive debris impacts. They are widely used on the equipment platforms such as armored vehicles, armed helicopters, and ship carriers. At present, the transparent armor with a superior elastic resistance is developed. However, the improvement of armor protection performance and lightweight armor design becomes a challenge, and it is manifested in the difficulty of balancing its protection performance and optical properties. Based on the research status and existing problems of transparent armor in the field of protective materials, this review represented the following aspects:

1) The current development status of transparent protective materials are reviewed. It is important to introduce the physical and chemical properties, mechanism of action, and industrial status of hard protective materials and soft energy absorbing materials for in-service armor. The research status indicates that sapphire single crystal is the most suitable material for mass production of new armored projectile surface, and its mechanical properties and industrial advantages do not possess, compared to other hard and transparent protective materials such as float glass, AlON and MgAl₂O₄ ceramics. However, the cleavage fracture of sapphire single crystals restricts its application potential in transparent protection. Polyurethane adhesive can achieve the high-strength bonding of inorganic and organic materials in the armor fields, but its poor environmental weather resistance affects the service life of transparent armor. Polycarbonate board is an ideal soft energy absorbing material with superior optical properties, impact resistance, and creep resistance. However, it is prone to moisture absorption and fogging, affecting the observation field of transparent armor.

2) The structural design strategy and the analysis of failure behavior are explored in transparent armor. A comparison is made between the design principles, performance differences, and failure mechanisms of conventional laminated glass structure (or similar-laminated glass structure) armors and the new structural armors with three functional layers. At present, the core of structure design is the synergistic matching of hard protective materials and soft energy-absorbing materials in optical properties and protective performance in the new structural transparent armor. The comparison results of different structural armors indicate that the weight of new structural armor is reduced by more than 30%, compared with that of conventional glass transparent armor under the same protection level. The conventional armor has a large damage surface with the first layer and the second layer being crushed and cracked, after being impacted by the projectile, resulting in the loss of its observation and combat capabilities. The damage surface of new structural armor shows granular crushing around the impact point, leading to the local area of the armor losing its observation and combat capabilities. The main causes of failure are the interface delamination cracking and projectile penetration damage in the optical application field of transparent armor.

3) The evaluation methods of transparent armor are summarized. The comparison results of key evaluation items of transparent armor procurement indicate that European and American countries focus on the optical properties of armor, while China pays more attention to the protective performance of armor. The United States establishs a relatively complete transparent armor procurement specification, and incorporates the optical properties of armor such as transmittance, haze, and light distortion into the procurement standards. For special night vision application scenarios, the U.S. military focuses on the optical properties of transparent armor rather than protective performance. However, there is no scientific and effective normative document for armor evaluation methods in China, and their test and characterization are often based on the evaluation methods of non-transparent protective materials. In addition, the existing test standards for transparent bulletproof materials do not pay attention to the balanced evaluation between the optical properties and protective performance of the armor, as well as the adaptability of transparent armor to the environment.