Sapphire is a widely used for Forward-Looking Infra-Red (FLIR) window material.
Compared to other window materials, it is;
(1) Almost no deterioration over time (ex. devitrification)
(2) Resistant to cracking
(3) Scratch-resistant
(4) Heat-resistant
(5) High homogeneity
(6) Low toxicity
These characteristics make it widely used for windows that are used under harsh conditions and where reliability is required.
