The US Office of Naval Research says it designed the new Fast-Tint Protective Eyewear (FTPE) lenses to the requirements of the famous yet reclusive US Navy SEAL frogman-commandos. In common with other Western troops nowadays, the SEALs are encouraged more and more to wear protective eyewear in combat - potentially saving their sight from damage by flying fragments, grit, stones, blast etc.
As with the other troops, the SEALs have been issued a range of different-coloured interchangeable lenses for different light conditions. But this is too fiddly for situations such as assaulting buildings where operators will move swiftly from blazing outdoor sunlight to dark or dim interiors. SEALs on such an operation tend to simply take their sunglasses off as they go through the door - just before the guns and grenades start going off and the air fills with flying bits and pieces.
A possible solution might be so-called "photochromic" lenses (phototropic would seem more like the right word), well known under brand names such as Reactolite. But these take too long to change tint, and also they actually react to ultraviolet rather than visible light - which means that they won't necessarily darken in response to heavy sunlight through a window (as the UV doesn't penetrate the glass). Nor is there any way to manually select a particular tint or colour.
Hence the FTPE lenses, which are effectively basic liquid-crystal display screens. They can turn amber, blue, grey or clear when a small electric charge is applied - either by a miniature light-sensing automatic unit, or at the touch of a button.