The invention concerns a method for printing an authentication pattern (71) wherein, using a multi-deflection continuous inkjet printer, the pattern is printed on a substrate (40) and only contains a small number of black pixels per raster. The resolution in the travel direction X of the substrate and in direction Y of the rasters is thereby largely improved compared with printing in dot-matrix mode, which makes reproduction of the pattern difficult for an infringer not having means to determine the electric charge to be applied to each of the droplets of a train of droplets needed to print each raster of the pattern.