Techniques
Various techniques exist to implement security paper, particularly for reducing copying. These include:
- Single or multi-tone watermarks – thus the same paper stock must be used in copies
- A colored or patterned background, so erasures or alterations are visible
- Dithered patterns, notably a finely dithered background “VOID” word, which on original is largely indistinguishable from the rest of ground (due to having same overall density), but when copied becomes visible, due to either the scanner or the printer not being able to reproduce the required resolution (effectively a high frequency signal that is smeared out by copying).
- Microprinting
- Phosphorescent fibres in the paper that can only be seen in UV-light, or fibres visible in daylight
- Metallised threads
- (Prolonged) exposure to UV or oxygen causes a pattern to appear on the paper (e.g. to indicate expiry)
- Holographic images
- Scratchable surface, e.g. on lottery scratchcards, to indicate when information hs been revealed
- Chemical sensitisation of the paper, e.g. ink erasers or sensitivity to acids/alkalines
- Taggants added in the coating or in the base paper; chemical "DNA" that can only be decoded by special means and acts as a unique identifier
- A note on the paper describing the security measures, so they can be checked
The most sophisticated techniques are used with paper money – see Counterfeit money: Anti-counterfeiting measures for discussion.
Read more about this topic: Security Paper
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