A reading apparatus of an optical data carrier bearing a first diffractive structure and a second diffractive structure obtained on overlapped levels, wherein the second structure bears an information diffractive code and the first structure bears a control diffractive code apt to act as a read key for said information code, the construction being such that, when the data carrier is exposed to a substantially monochromatic beam of electromagnetic waves, each of the first diffractive structure and the second diffractive structure deviates such incident beam into one or more diffracted beams spatially independent from the one or more diffracted beams originated by the other structure, which apparatus comprises: - an emitting unit, apt to emit a substantially monochromatic beam of electromagnetic waves apt to be deviated by the first diffractive structure and the second diffractive structure; - a first detecting unit, for detecting the control code of the first diffractive structure of the data carrier from the one or more diffracted beams originated by the first diffractive structure, comprising first detecting means; - a second detecting unit, for detecting the information code of the other diffractive structure of the data carrier from the one or more diffracted beams originated by the second diffractive structure, comprising second detecting means arranged at a position distinct from that of the first detecting means; and - a processing unit, apt to verify the authenticity of the data carrier according to the detections of said detecting units (12, 13), characterised in that it further comprises - programming means selectively programmable with and apt to store at least one application parameter representative of the specific application of the data carrier bearing the diffractive structures (2, 3), and having preset at least one position parameter related to a linear and/or angular position of the first diffractive structure; wherein said at least one application parameter defines a discrimination threshold of at least one of the optical codes of the diffractive structures, wherein said position parameter is representative of the linear and/or angular position of the first diffractive structure on the data carrier, and wherein, in use, the first detecting unit reads the control diffractive code of the first diffractive structure only in correspondence of one or more positions defined by said at least one position parameter preset in said programming means. - an emitting unit, apt to emit a substantially monochromatic beam of electromagnetic waves apt to be deviated by the first diffractive structure and the second diffractive structure; - a first detecting unit, for detecting the control code of the first diffractive structure of the data carrier from the one or more diffracted beams originated by the first diffractive structure, comprising first detecting means; - a second detecting unit, for detecting the information code of the other diffractive structure of the data carrier from the one or more diffracted beams originated by the second diffractive structure, comprising second detecting means arranged at a position distinct from that of the first detecting means; and - a processing unit, apt to verify the authenticity of the data carrier according to the detections of said detecting units (12, 13), characterised in that it further comprises - programming means selectively programmable with and apt to store at least one application parameter representative of the specific application of the data carrier bearing the diffractive structures (2, 3), and having preset at least one position parameter related to a linear and/or angular position of the first diffractive structure;