A security sheet comprising: a data sheet for displaying data, wherein the data sheet comprises: an outer surface; at least one transparent region extending from the outer surface; and a camouflage pattern; and an attachment layer for attaching the data sheet into a security document, wherein the attachment layer comprises a connection portion at least partially embedded in the at least one transparent region, wherein: the camouflage pattern at least partially overlaps the at least one transparent region and connection portion and the camouflage pattern is for interfering with the visibility of the connection portion when the outer surface is viewed by the naked eye in reflected visible light incident upon the outer surface; the camouflage pattern extends across the data sheet beyond the boundaries of the connection portion and comprises at least one pattern element overlapping and covering at least part of the connection portion; and the camouflage pattern comprises a camouflage pattern area, which is the area defined within the outermost boundaries of the pattern element(s), extends over at least 25% of a connection portion area, which is the area defined by the outer boundaries of the connection portion; characterised in that the area covered by the pattern element(s) extends over at least 25% of the camouflage pattern area. a data sheet for displaying data, wherein the data sheet comprises: an outer surface; at least one transparent region extending from the outer surface; and a camouflage pattern; and an attachment layer for attaching the data sheet into a security document, wherein the attachment layer comprises a connection portion at least partially embedded in the at least one transparent region, wherein: the camouflage pattern at least partially overlaps the at least one transparent region and connection portion and the camouflage pattern is for interfering with the visibility of the connection portion when the outer surface is viewed by the naked eye in reflected visible light incident upon the outer surface; the camouflage pattern extends across the data sheet beyond the boundaries of the connection portion and comprises at least one pattern element overlapping and covering at least part of the connection portion; and the camouflage pattern comprises a camouflage pattern area, which is the area defined within the outermost boundaries of the pattern element(s), extends over at least 25% of a connection portion area, which is the area defined by the outer boundaries of the connection portion; characterised in that the area covered by the pattern element(s) extends over at least 25% of the camouflage pattern area. the camouflage pattern at least partially overlaps the at least one transparent region and connection portion and the camouflage pattern is for interfering with the visibility of the connection portion when the outer surface is viewed by the naked eye in reflected visible light incident upon the outer surface; the camouflage pattern extends across the data sheet beyond the boundaries of the connection portion and comprises at least one pattern element overlapping and covering at least part of the connection portion; and the camouflage pattern comprises a camouflage pattern area, which is the area defined within the outermost boundaries of the pattern element(s), extends over at least 25% of a connection portion area, which is the area defined by the outer boundaries of the connection portion; characterised in that the area covered by the pattern element(s) extends over at least 25% of the camouflage pattern area.