1. Optical pattern recognition for validation and security verification
2. Holographic counterfeit protection
3. R.N. Goldman, Non-counterfeitable document system, US Patent 4785290 (1988) and US Patent 4663622 (1987).
4. J. Samyn, Method and apparatus for checking the authenticity of documents, US Patent 4820912 (1989).
5. H. Hoshino, M. Yoda, I. Takeuchi, T. Kurihara, Method and apparatus for checking objects to be checked for authenticity, US Patent 5473147.