Method of verifying an authenticity of a printed item and data processing terminal

A method of copy protecting a printed item, the method comprising:

• a) providing a printing pattern master file of the printed item with a noise pattern in at least one colour channel,
• b) printing the printed item according to the printing pattern master file including the noise pattern,
• c) providing a reference image of the printed item, the reference image comprising the noise pattern in the at least one colour channel,
• d) photographing a specimen of the printed item to obtain a photographic image of the specimen of the printed item,
• d) determining a test noise parameter from the photographic image of the specimen of the printed item, wherein the test noise parameter reflects a noise pattern in at least one colour channel of the photographic image of the specimen of the printed item,
• e) determining a reference noise parameter from the reference image, wherein the reference noise parameter reflects the noise pattern in the at least one colour channel of the reference image,
• wherein the test noise parameter from the photographic image of the printed item is a test signal to noise ratio from the photographic image of the printed item based on the at least one colour channel and the noise pattern in the at least one colour channel of the photographic image of the printed item,
• wherein the reference noise parameter from the reference image is a reference signal to noise ratio from the reference image of the printed item based on the at least one colour channel and the noise pattern in the at least one colour channel of the reference image, and
• wherein the method further comprises:
• f) comparing the test signal to noise ratio to the reference signal to noise ratio thereby deriving, from a difference between the test signal to noise ratio and the reference signal to noise ratio, a colour modulation change between the at least one colour channel of the photographic image of the specimen of the printed item and the at least one colour channel of the reference image of the printed item, and
• g) determining an authenticity of the printed item from a result of the comparing, thereby determining the authenticity from the colour modulation change between the at least one colour channel of the photographic image of the specimen of the printed item and the at least one colour channel of the reference image of the printed item.