A magnetic assembly for producing an optical effect layer on a substrate, said magnetic assembly being configured for receiving the substrate in an orientation at least partially parallel to a first plane and above the first plane and further comprising: a) a first magnetic-field generating device comprising at least four first dipole magnets having their magnetic axes oriented to be substantially perpendicular to the first plane, wherein each of the first dipole magnets is arranged on an intersection of at least two substantially parallel straight lines α i (i = 1, 2,...) and at least two substantially parallel straight lines β j (j = 1, 2,...), the straight lines α i and β j forming a grid, wherein at least two first dipole magnets are disposed on one of the straight lines α i and at least two other first dipole magnets are disposed on another one of the straight lines α i, wherein the first dipole magnets of said first magnetic-field generating device are partially or fully embedded in a first supporting matrix; and b) a second magnetic-field generating device comprising one or more second dipole magnets having their magnetic axes oriented to be substantially parallel to the first plane and wherein the one or more second dipole magnets are partially or fully embedded in a second supporting matrix; wherein the second magnetic-field generating device is disposed below the first magnetic-field generating device, and wherein each straight line α i and a vector sum H of the magnetic axes of the one or more second dipole magnets are substantially non-parallel and substantially non-perpendicular with respect to each other, characterized in that, on each straight line α i, and on each straight line β j, neighboring first bar dipole magnets have their North pole pointing in an opposite direction. a) a first magnetic-field generating device comprising at least four first dipole magnets having their magnetic axes oriented to be substantially perpendicular to the first plane, wherein each of the first dipole magnets is arranged on an intersection of at least two substantially parallel straight lines α i (i = 1, 2,...) and at least two substantially parallel straight lines β j (j = 1, 2,...), the straight lines α i and β j forming a grid, wherein at least two first dipole magnets are disposed on one of the straight lines α i and at least two other first dipole magnets are disposed on another one of the straight lines α i, wherein the first dipole magnets of said first magnetic-field generating device are partially or fully embedded in a first supporting matrix; and wherein at least two first dipole magnets are disposed on one of the straight lines α i and at least two other first dipole magnets are disposed on another one of the straight lines α i, wherein the first dipole magnets of said first magnetic-field generating device are partially or fully embedded in a first supporting matrix; and b) a second magnetic-field generating device comprising one or more second dipole magnets having their magnetic axes oriented to be substantially parallel to the first plane and wherein the one or more second dipole magnets are partially or fully embedded in a second supporting matrix; wherein the second magnetic-field generating device is disposed below the first magnetic-field generating device, and wherein each straight line α i and a vector sum H of the magnetic axes of the one or more second dipole magnets are substantially non-parallel and substantially non-perpendicular with respect to each other, characterized in that, on each straight line α i, and on each straight line β j, neighboring first bar dipole magnets have their North pole pointing in an opposite direction. wherein the second magnetic-field generating device is disposed below the first magnetic-field generating device, and wherein each straight line α i and a vector sum H of the magnetic axes of the one or more second dipole magnets are substantially non-parallel and substantially non-perpendicular with respect to each other,