A process for producing an optical effect layer on a substrate, said process comprising the steps of: i) 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, ii) exposing the radiation curable coating composition to a magnetic field of an apparatus comprising: a) a magnetic assembly comprising a supporting matrix and: a1) a loop-shaped magnetic-field generating device being either a single loop-shaped dipole magnet having a magnetic axis substantially perpendicular to the substrate surface or a combination of two or more dipole magnets disposed in a loop-shaped arrangement, each of the two or more dipole magnets having a magnetic axis substantially perpendicular to the substrate surface and having a same magnetic field direction, and a2) a single dipole magnet having a magnetic axis substantially perpendicular to the substrate surface or two or more dipole magnets having a magnetic axis substantially perpendicular to the substrate surface and having a same magnetic field direction and/or one or more pole pieces, b) a magnetic-field generating device being either a single bar dipole magnet having a magnetic axis substantially parallel to the substrate surface or a combination of two or more bar dipole magnets, each of the two or more bar dipole magnets having a magnetic axis substantially parallel to the substrate surface and having a same magnetic field direction, so as to orient at least a part of the non-spherical magnetic or magnetizable pigment particles, and iii) at least partially curing the radiation curable coating composition of step ii) 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 one or more loop-shaped bodies having a size that varies upon tilting the optical effect layer. i) 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, ii) exposing the radiation curable coating composition to a magnetic field of an apparatus comprising: a) a magnetic assembly comprising a supporting matrix and: a1) a loop-shaped magnetic-field generating device being either a single loop-shaped dipole magnet having a magnetic axis substantially perpendicular to the substrate surface or a combination of two or more dipole magnets disposed in a loop-shaped arrangement, each of the two or more dipole magnets having a magnetic axis substantially perpendicular to the substrate surface and having a same magnetic field direction, and a2) a single dipole magnet having a magnetic axis substantially perpendicular to the substrate surface or two or more dipole magnets having a magnetic axis substantially perpendicular to the substrate surface and having a same magnetic field direction and/or one or more pole pieces, b) a magnetic-field generating device being either a single bar dipole magnet having a magnetic axis substantially parallel to the substrate surface or a combination of two or more bar dipole magnets, each of the two or more bar dipole magnets having a magnetic axis substantially parallel to the substrate surface and having a same magnetic field direction, so as to orient at least a part of the non-spherical magnetic or magnetizable pigment particles, and a) a magnetic assembly comprising a supporting matrix and: a1) a loop-shaped magnetic-field generating device being either a single loop-shaped dipole magnet having a magnetic axis substantially perpendicular to the substrate surface or a combination of two or more dipole magnets disposed in a loop-shaped arrangement, each of the two or more dipole magnets having a magnetic axis substantially perpendicular to the substrate surface and having a same magnetic field direction, and a2) a single dipole magnet having a magnetic axis substantially perpendicular to the substrate surface or two or more dipole magnets having a magnetic axis substantially perpendicular to the substrate surface and having a same magnetic field direction and/or one or more pole pieces, a1) a loop-shaped magnetic-field generating device being either a single loop-shaped dipole magnet having a magnetic axis substantially perpendicular to the substrate surface or a combination of two or more dipole magnets disposed in a loop-shaped arrangement, each of the two or more dipole magnets having a magnetic axis substantially perpendicular to the substrate surface and having a same magnetic field direction, and a2) a single dipole magnet having a magnetic axis substantially perpendicular to the substrate surface or two or more dipole magnets having a magnetic axis substantially perpendicular to the substrate surface and having a same magnetic field direction and/or one or more pole pieces, b) a magnetic-field generating device being either a single bar dipole magnet having a magnetic axis substantially parallel to the substrate surface or a combination of two or more bar dipole magnets, each of the two or more bar dipole magnets having a magnetic axis substantially parallel to the substrate surface and having a same magnetic field direction, iii) at least partially curing the radiation curable coating composition of step ii) to a second state so as to fix the non-spherical magnetic or magnetizable pigment particles in their adopted positions and orientations,