Sheet-like product and method for authenticating a security tag

#The present teaching relates to a sheet-like product that can be used as a raw material for producing security tags, which when arranged on products or product packaging provide a means for optical authentication of that product or packaging. Specifically, the present teaching relates to a sheet-like product comprising: at least one security feature having optical properties that change with the viewing angle and at least one marker, wherein each marker is uniquely attributable to a position on the sheet-like product, wherein the position of the at least one security feature on the sheet-like product is predetermined relative to the position of the at least one marker on the sheet-like product. Such security tags are generally used to combat product counterfeiting by making forgings detectable. The present teaching further relates to a method for determining an optical property of a partial or complete security feature comprised in a section of such a sheet-like product.

#In WO 2015/079014 A1 a system for producing security tags has been described. Basically, they are produced by cutting patches from a raw material and randomly scattering those on a substrate material and/or the object itself. The raw material feed is subject to manufacturing tolerances and the cutting and/or scattering therefore results in a truly random (stochastic) shape and distribution of the cut patches. These random properties can be used as unambiguous markings for objects or products. The method described in WO 2013/188897 A1 exploits these properties and stores the random features together with an object identifier in a database or on the object itself (with a cryptographic signature). This duple can later be used for visually authenticating such an object marking comprising an identifier and a random feature with a programmable device comprising a camera. However, marking of this type is relatively expensive and additional effort is required for registering each marking by producing a digital signature or initializing a database.

#Therefore WO 2016/034555 A1 proposes a system to register such random security features through performing a plausibility check during the first authentication attempt. If the plausibility check is passed, the random features are recorded and enrolled in the database. This removes the necessity of registering each marking by the producer in exchange for a lower security during the first authentication attempt. One disadvantage of this method is that the recording of features during the authentication attempt may be inaccurate. When such inaccurate features are enrolled, they may compromise later authentication attempts, which do not record the same inaccuracies and thus fail the authentication.

#In the following, the present teaching will be explained further by means of particularly preferred embodiments to which it is not to be restricted, though, and with reference to the drawings. The drawings show in detail:

#FIG. 1 schematically a view of one side of a sheet-like product according to the present teaching;

#FIG. 2 a detailed view of a section cut out of the sheet-like product according to FIG. 1; and

#FIG. 1 shows a sheet-like product 1 forming a raw material, e.g. a sheeting, a film or a foil, or being part thereof. The sheet-like product 1 comprises twenty-eight security features 2 having optical properties that change with the viewing angle. The sheet-like product 1 also comprises twenty-eight markers 3. In this example the security features 2 and markers are arranged in a grid in an alternating manner. However in different embodiments the security features 2 may also overlap with the markers 3 or be part of the markers 3.

#Each marker 3 is uniquely identifiable and uniquely attributable to a position on the sheet-like product 1. Each marker 3 should be readable by a programmable device comprising a camera (e.g. a smartphone or tablet). The markers 3 in this example are two-dimensional codes comprising an encoded identifier. Of course any codes similar to common 1D or 2D barcodes, i.e. Data Matrices, can be used for the markers 3. Each encoded identifier is unique across the sheet-like product 1, i.e. for each marker 3. The encoding of the encoded identifier comprises information for error detection or error correction, e.g. for using encoding strategies which employ error-corrective codes (ECC), e.g. Reed-Solomon. The use of such codes aids robust/reliable decoding and furthermore ensures that if a marker 3 is decoded, it has the correct content. There is therefore a close to 100% probability that if a marker 3 is decoded, the identifier resulting from the decoding is correct, i.e. it is equal to the identifier encoded during production or design of the sheet-like product 1.

#Each security feature 2 comprises two or more optical variable devices 4, 5 placed in an adjacent manner (e.g. side by side). The two or more optical variable devices (OVDs) 4, 5 can either have distinct optical characteristic or share the same principal optical characteristics but have different diffraction starting angles and therefore appear differently from any one viewing angle, which is indicated by different shades in FIG. 1. FIG. 1 shows an example of a security feature comprising two different OVDs. For simplicity we will subsequently use examples referring to so-structured security features with two OVDs. All examples can trivially be extend to security features comprising multiple OVDs. For evaluation with a programmable device's camera we can independently look at the OVD left 5 and the OVD right 4 within a security feature 2 and examine differences in their appearance. If they are not differing or show unexpected behavior, verification fails and we can hence conclude that somebody tried to tamper the security tag, e.g. a plausibility check fails and the enrollment is interrupted. To provide the optical variable devices, the sheet-like product 1 may be a holographic sheet or a holographic film. The positions of the security features 2 on the sheet-like product 1 are predetermined relative to the positions of the markers 3 on the sheet-like product 1.