Security paper (e.g. passport paper): Does not contain optical brighteners and thus remains dark under UV light. The use of paper substrate with optical brighteners in passports is, however, also possible (but they are not used very often): Optical brighteners are substances which are incorporated during manufacture into the paper pulp (which consists mainly of wood fibres), in order to make the paper appear whiter. The presence of optical brighteners can be detected by their bluish fluorescence under UV light.
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