SECURITY DOCUMENT WITH SECURITY MARKING FORMED OF TRANSPARENT WINDOWS

Security document (1) with security indicia (21) for preventing unauthorised reproduction, the security indicia being detectable in transmitted light, wherein the security indicia are located within the bounds of a security pattern (20) acting to visually conceal the security pattern in reflected light. The security pattern may be formed of one or more elements (25) and may have a complexity selected to enable the concealment of the security indicia (21).