Method for printing and identifying authentication marks by means of an amplitude-modulated raster print

A printing method and authentication method for a print to be made of a digital image, comprising:

• a method of printing authentication marks by applying an amplitude modulated halftone print in a detection zone to an object, wherein the detection zone has a printed area comprising contiguous halftone cells in each of which a halftone dot is printed from a matrix of printable halftone elements, wherein in a predetermined manner for a plurality of tone values of halftone dots to be printed, with a constant tone value of the print, a predetermined asymmetrical matrix image is assigned for a Re print image arising from print data of the halftone elements to be printed, wherein at least two finder edges, which are not parallel to one another, from at least one finder zone are printed to determine a position, boundary and orientation of the detection zone, and
• a method of authenticating a print on a printed article, comprising providing a portable image capture device having a microprocessor for executing an authentication program, providing the print image predetermined from the print data and resulting therefrom for a predetermined number of halftone dots of the printed article from a detection zone, and providing a computer program for comparing the print image predetermined from the print data of the halftone dots;
• said method comprising the further steps of capturing an image of the printed article providing a captured image;
• detecting the at least two finder edges from the at least one finder zone to define the detection zone with halftone dot accuracy from the captured image of the printed article;
• comparing the captured image of the detection zone with the print image predetermined from and resulting from the print data;
• and deciding on the basis of the comparison whether or not there is an original print on the printed article.