An auto-darkening helmet contains an electronic shutter module.
This module contains fixed/static IR (Infra-Red) and UV (Ultraviolet) filters that require no electricity to operate along with an electrically operated LC (Liquid Crystal) light valve or shutter.
The LC valve/shutter darkens/lightens when a voltage is applied that re-orients the crystals that are floating in a liquid which is separated by two sheets of glass.
The shutter module also contains photocells (light-dependent resistors) or other photosensor that is wired into an electronic control module.
The control module detects the bright light emitted by the welding arc and closes (darkens) the LC valve/shutter.
A timer circuit lightens the screen again some time (perhaps 100 milliseconds) after the arc goes out and ambient light levels return to normal.