Provided herein is a label structure for tagging a product item. The label structure includes a first graphic pattern having a first number of pixels and printed on a first portion of the label structure; a second graphic pattern having a second number of pixels and printed on a second portion of the label structure, the second graphic pattern forming an optical code readable by an optical decoding device; and shielding means capable of hiding or making visible the first graphic pattern depending on the values taken by a physical quantity to which the label structure is exposed. The first and second graphic patterns can be observed in combination, when the first graphic pattern is made visible by the shielding means.