Didymium

Didymium (Greek: twin element) is a mixture of the elements praseodymium and neodymium. Ever since the invention by Sir William Crookes, 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.)

Didymium photographic filters are often used to enhance fall scenery by making leaves appear more vibrant. It does this by removing part of the orange region of the color spectrum; it's an optical band-stop filter. Unfiltered, this group of colors tends to make certain elements of a picture appear "muddy". The "Sodium Vapor Process" used in motion picture matte work included a didymium filtering prism in the camera.

Didymium is also used in calibration materials.