Protection level according to DIN EN 169

The shade number of your welding filter tells you how much light gets through – the higher the number, the darker the glass. If the lens is too light, you risk flash burns, and that's no joke – it really damages your eyes. The right shade depends on your process and amperage, so the hotter you weld, the darker the filter needs to be. And if the lens has scratches or is worn out, you can kick it in the trash, because it won't protect reliably anymore.