didymium

thumb|Didymium glasses

Didymium () is a mixture of the elements praseodymium and neodymium. It is used in safety glasses for glassblowing and blacksmithing and filter lenses for flame testing, especially with a gas (propane)-powered forge, where it provides a filter that 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 and cobalt glasses. The usefulness of didymium glass for eye protection of this sort was discovered by Sir William Crookes.