Describes requirements for automatic welding filters which switch their luminous transmittance to a lower predetermined value when a welding arc is ignited (referred to as welding filters with switchable scale numbers). Also ascertains requirements for automatic welding filters which switch their luminous transmittance to a lower value, where the lower value of luminous transmittance is set automatically in dependence on the illuminance generated by the welding arc (referred to as welding filters with automatic scale number setting).