Abstract: The invention relates to a security device for security substrates, such as paper, used for making security documents, such as bank notes, having anti-counterfeitable features. The security device comprising a carrier (11) of an at least partially light transmitting polymeric material, said carrier bearing a plurality of fine opaque lines (13) separated by gaps (14). The appearance of the lines is varied at regions, said regions in neighbouring lines combining to provide identifying indicia which are visible to the human eye.