Security paper is a paper that incorporates features that can be used to identify or authenticate a document as original, e.g., watermarks or invisible fibers in paper, or features that demonstrate tamper evidence when fraud is attempted, e.g., to remove or alter print such as amounts or signatures on a cheque. Examples of this kind of papers are those used for identification such as passports and certificates, such as birth certificates and different types of academic and qualification certificates, as well as government documents, e.g., voting ballots and tax strips.
Various techniques exist to implement security paper, particularly for reducing copying. These include:
- Hologram hot stamping Bar or logo
- 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 “COPY ALERT” message, which on original is largely indistinguishable from the rest of the background (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).
- Thermos chromatic ink that fade when rubbed.
- Micro printing
- Phosphorescent fibers in the paper that can only be seen in UV-light, or fibers visible in daylight
- Metallized threads
- (Prolonged) exposure to UV or oxygen causes a pattern to appear on the paper (e.g. to indicate expiry)
- Holographic images
- Scratch able surface, e.g. on lottery scratch cards, to indicate when information has been revealed
- Chemical sensitization of the paper, e.g. ink erasers or sensitivity to acids/alkaline
- Tangents 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