A process for producing an optical effect layer on a substrate, said process comprising the steps of: a) applying on a substrate surface a radiation curable coating composition comprising non-spherical magnetic or magnetizable pigment particles, said radiation curable coating composition being in a first state, b) exposing the radiation curable coating composition to a magnetic field of a magnetic assembly comprising i) a loop-shaped magnetic-field generating device being either a single loop-shaped dipole magnet having a North-South magnetic axis substantially perpendicular to the substrate surface or a combination of two or more dipole magnets disposed in a loop-shaped arrangement and having a resulting North-South magnetic axis substantially perpendicular to the substrate surface, and ii) a magnetic-field generating device being either a single bar dipole magnet having a North-South magnetic axis substantially parallel to the substrate surface or a combination of two or more bar dipole magnets having a resulting North-South magnetic axis substantially parallel to the substrate surface, so as to orient at least a part of the non-spherical magnetic or magnetizable pigment particles, and c) at least partially curing the radiation curable coating composition of step b) to a second state so as to fix the non-spherical magnetic or magnetizable pigment particles in their adopted positions and orientations, wherein the optical effect layer provides an optical impression of a loop-shaped body having a size that varies upon tilting the optical effect layer. a) applying on a substrate surface a radiation curable coating composition comprising non-spherical magnetic or magnetizable pigment particles, said radiation curable coating composition being in a first state, b) exposing the radiation curable coating composition to a magnetic field of a magnetic assembly comprising i) a loop-shaped magnetic-field generating device being either a single loop-shaped dipole magnet having a North-South magnetic axis substantially perpendicular to the substrate surface or a combination of two or more dipole magnets disposed in a loop-shaped arrangement and having a resulting North-South magnetic axis substantially perpendicular to the substrate surface, and ii) a magnetic-field generating device being either a single bar dipole magnet having a North-South magnetic axis substantially parallel to the substrate surface or a combination of two or more bar dipole magnets having a resulting North-South magnetic axis substantially parallel to the substrate surface, so as to orient at least a part of the non-spherical magnetic or magnetizable pigment particles, and i) a loop-shaped magnetic-field generating device being either a single loop-shaped dipole magnet having a North-South magnetic axis substantially perpendicular to the substrate surface or a combination of two or more dipole magnets disposed in a loop-shaped arrangement and having a resulting North-South magnetic axis substantially perpendicular to the substrate surface, and ii) a magnetic-field generating device being either a single bar dipole magnet having a North-South magnetic axis substantially parallel to the substrate surface or a combination of two or more bar dipole magnets having a resulting North-South magnetic axis substantially parallel to the substrate surface, so as to orient at least a part of the non-spherical magnetic or magnetizable pigment particles, and c) at least partially curing the radiation curable coating composition of step b) to a second state so as to fix the non-spherical magnetic or magnetizable pigment particles in their adopted positions and orientations,