Electro-optic filters with a transmittance variable in time are used for eye protection in electric welding applications. These filters change their luminous transmittance automatically from an initial maximum value to a minimum value when the welding arc is ignited. In the light state, the work-piece and thus the location where the electrode is to be set can be observed. After the ignition of the arc, within a certain switching time, the filter changes to its dark state in order to make the welding process visible.
E. Buhr, E. Buhr,
"Dynamic filters for protection devices", Proc. SPIE 10305, Dosimetry of Laser Radiation in Medicine and Biology, 103050G (10 January 1989); doi: 10.1117/12.2283597; https://doi.org/10.1117/12.2283597