Abstract: The optical properties of rare earth (RE) ions and theirs application for identification and protection of documents will be discussed. RE ions doped powders e.g. fluoride crystals, oxychlorides, oxyfluorides can be used for security printing as components of applied printing dyes. In described above compounds, the anti-Stokes emission occurs. It means that the conversion from infrared-to-visible has place. When excited with an IR diode laser with $lambda equals 980 nm, the emission in visible is observed. The are many advantages of such security mean. The RE dyes can be invisible and the emission is induced by 'invisible' light. In order to evoke the emission, the matching between used chemical compound and laser light is necessary. The up-conversion phenomena are very rare and can be observed only in some special compounds made by specialized laboratories. !4
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