They are an LCD display like your computer that is clear to look through until you strike an ark on the welder then they tint very dark to protect your vision. The tech was originally developed by the Military to protect pilots vision from the effect of a nuclear explosion.
The idea is that the pilot might be on a boming run about to shoot the shit out of someone, so the last thing he (or she) needs at Mach 3 with a wing full of nukes is to go blind from a nuke flash.
So the lenses are actually LCD screens that will go dark (maybe black) in the event of a flash.
Good quality welding shields respond in 1bout 1/25,000 of a second.
So i bet military spec ones might be way faster.
I am also betting they have a data overlay like these with tactical information.