A welding helmet lens has a clear outer cover to help protect it from damage. This cover can become dirty from smoke and spatter, making it difficult to see the weld. They are available from a welding supplier as replacement parts, but cleaning them can extend their life.
Toothpaste is a mild abrasive that works great for cleaning this outer cover. Remove the cover from the helmet. Wet a soft cloth, apply a little toothpaste, and rub it in a circular motion. Rinse with clean water.