Didymium

Didymium (δίδυμο, twin) is a mixture of the elements praseodymium and neodymium. It is used in safety glasses for glassblowing and blacksmithing, 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. 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". These photographic filters are also used by nightscape photographers, as they absorb part of the light pollution caused by sodium street lights. Didymium was also used in the sodium vapor process for matte work due to its ability to absorb the yellow color produced by its eponymous sodium lighting. Didymium is also used in calibration materials for spectroscopy. Didymium was discovered by Carl Mosander in 1841. It was named after the Greek word δίδυμο ("twin") because it is very similar to lanthanum and cerium, 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. Since spectroscopy had not yet been invented, Mosander did as well as could be expected at the time. 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 in trivalent form it tinged the salts of ceria pink. During the period when didymium was believed to be an element, the symbol Di was used for it.