Didymium

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 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.

Didymium was discovered by Carl Mosander in 1841 and was so named because it is very similar to lanthanum, with which it was found. Mosander wrongly believed didymium to be an element, under the impression that "ceria" (sometimes called cerite) isolated by Jöns Jakob Berzelius in 1803 was really a mixture of cerium, lanthanum and didymium. He was right about lanthanum's being an element, but not about didymium. Mosander did as well as could be expected at the time, since spectroscopy had not yet been invented. His three "elements" accounted for at least 95% of the rare earths in the original cerite from Bastnäs, Sweden. Didymium had not been difficult to find, since it was providing the pinkish tinge to the salts of ceria when in trivalent form. During the period when didymium was believed to be an element, the symbol Di was used for it. In the illustration of Mendeleev's first attempt at a periodic table, shown on the right, it will be noted that the atomic weights assigned to the various lanthanides, including didymium, reflect the original belief that they were divalent. Their actual trivalency meant that his atomic weights for them were only about 67% of their true values.

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