Lanthanoids are a group of fifteen elements in the periodic table. All lanthanoids are relatively soft and reactive metals. They are counted among the so-called "rare earths" - a somewhat elusive term, as lanthanoids are not really that rare: cerium, for instance, is more abundant in nature than arsenic or lead.
In 1794, the first natural lanthanoid compounds were discovered in Sweden. By 1907, a total of thirteen non-radioactive lanthanoids were known. From the mid-1960s the fluorescence of lanthanoids upon stimulation with ultraviolet light was the subject of intense research. Since then, "rare earth" metals have been used for various industrial purposes, including luminescent materials in TV sets, steel alloy ingredients, permanent magnets, flints, and catalyzers. Inks containing customary lanthanoid compounds have been used since 1998 in the printing of banknotes and revenue stamps, as they permit the addition of hidden images and seals.
In 2005, the Swiss Authentication research team started working on an innovative lanthanoid-based security solution. One year later, patent protection was requested for the selective alteration of the molecular structure of lanthanoids to create unique emission spectrums, as well as for the corresponding detection technology.
Lanthanum - the first of the Lanthanides (Wikipedia.org image)
At this depiction of the Periodic table: www.radiochemistry.org The lanthanide series is the second row from the bottom elements 58 - 71 (separate from the main chart)
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