The present invention is directed to a scratch-off type lottery ticket in which a layer comprised of a non-conductive ink region and a conductive ink region are printed below an opaque scratch-off layer. The conductive ink region is detectable through the opaque layer by conventional lottery ticket authentication/validation...http://www.google.co.uk/patents/US6142533?utm_source=gb-gplus-sharePatent US6142533 - Lottery ticket with hidden conductive ink security means
