Custom-engineered sapphire optical domes for deep-water UAVs extend the viewing angle of cameras by up to 160° and can withstand pressures up to 10,000 psi. They have Mohs 9 hardness and resist sand, saltwater, and most chemicals. They provide up to 85% transmission uncoated in the UV to IR, with up to 99% when AR-coated.

(From:https://www.laserfocusworld.com/articles/2018/02/meller-optics-sapphire-domes-have-utility-in-deep-water-uavs.html)