The Collaborative International Dictionary
Didymium \Di*dym"i*um\, n. [NL., fr. Gr. ? twin.] (Chem.) A rare metallic substance usually associated with the metal cerium; -- hence its name. It was formerly supposed to be an element, but has since been found to consist of two simpler elementary substances, neodymium and praseodymium. See Neodymium, and Praseodymium.
Wiktionary
n. A mixture of praseodymium and neodymium once thought to be an element (symbol ''Di'').
Wikipedia
Didymium 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. The strong infrared light emitted by the superheated forge gases and insulation lining the forge walls is also blocked thereby saving the crafters' eyes from serious cumulative damage such as Glassblower's cataract. The usefulness of didymium glass for eye protection of this sort was discovered by Sir William Crookes.
Didymium photographic filters are often used to enhance autumn scenery by making leaves appear more vibrant. It does this by removing part of the orange region of the color spectrum, acting as 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 for spectroscopy.
Usage examples of "didymium".
Now, wearing a pair of goggles with didymium lenses, Toby was in his work chair at the glassblowing table, in front of the Fisher Multi-Flame burner.
Yet on his high work chair, turning the glass in the flame, adjusting the oxygen flow with intuitive precision, face shimmering with reflected light, eyes concealed behind didymium goggles, Toby did not in any way seem below average, did not in any way impress me as being diminished by his condition.