Before auto-darkening helmets, welding helmets just had standard glass lenses that were coated to a fixed dark shade. These require the welder to flip or keep adjusting the helmet to see clearly while still keeping his eyes protected. An auto-darkening helmet makes this unnecessary because it automatically adjusts to the work being done.
Lens reaction time varies with auto-darkening helmets. The faster the reaction time, the more expensive the helmet. Reaction time ranges from about 1/25,000 of a second for the fastest to 1/3,600 of a second for the slowest. Faster helmets can be of particular benefit to professional welders.
Even small delays in lens reaction times can result in UV damage to the eyes over time. Welders may not realize damage is occurring until after the fact. Buyers can also choose helmets that allow them to adjust the length of time the lenses stay darker and how sensitive they are to light.