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 blocks the yellowish light 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.
Didymium photographic filters are often used to enhance fall scenery by making leaves appear more vibrant. This is accomplished via the special properties of the substance which causes the removal of part of the orange region of the color spectrum. When present, this group of colors tends to make certain elements of a picture appear "muddy".
Didymium is also used in calibration materials.