Didymium

Mixture of praseodymium and neodymium

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.

This keyword could refer to multiple things. Here are some suggestions: