High Quality Content by WIKIPEDIA articles! Didymium (Greek: twin element) is a mixture of the elements praseodymium and neodymium. It is used in safety glasses for glassblowing and blacksmithing, especially when a gas (propane) powered forge is used, where it provides a filter which selectively blocks the yellowish light at 589 nm emitted by the hot sodium in the glass, without having a detrimental effect on general vision, unlike dark welder`s glasses. Blocked also is the strong ultraviolet light emitted by the superheated forge gases and insulation lining the forge walls thereby saving the crafters` eyes from serious cumulative damage. (See also arc eye, also known as welder`s flash or photokeratitis.)