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, said first state being a liquid state; ii) exposing the radiation curable coating composition to a magnetic field of a magnetic assembly comprising: a) a first magnetic-field generating device having its North-South magnetic axis substantially perpendicular to the substrate surface and having length L1; b) a second magnetic-field generating device having its North-South magnetic axis substantially perpendicular to the substrate surface and having a length L3, c) a flat pole piece lacking any protrusions or projections extending outside the surface of said pole piece and having a length L5, wherein the first magnetic-field generating device and the second magnetic-field generating device have a same magnetic field direction, wherein the first magnetic-field generating device faces the substrate and is disposed on top of the pole piece, wherein the second magnetic-field generating device faces the environment and is disposed below the flat pole piece, wherein the length L1 of the first magnetic-field generating device is smaller than the length L3 of the second magnetic-field generating device, wherein the length L1 of the first magnetic-field generating device is smaller than the length L5 of the flat pole piece, and wherein the length L3 of the second magnetic-field generating device is smaller than the length L5 of the pole piece, 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 a moon crescent moving and rotating upon tilting the substrate comprising 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, said first state being a liquid state; ii) exposing the radiation curable coating composition to a magnetic field of a magnetic assembly comprising: a) a first magnetic-field generating device having its North-South magnetic axis substantially perpendicular to the substrate surface and having length L1; b) a second magnetic-field generating device having its North-South magnetic axis substantially perpendicular to the substrate surface and having a length L3, c) a flat pole piece lacking any protrusions or projections extending outside the surface of said pole piece and having a length L5, wherein the first magnetic-field generating device and the second magnetic-field generating device have a same magnetic field direction, wherein the first magnetic-field generating device faces the substrate and is disposed on top of the pole piece, wherein the second magnetic-field generating device faces the environment and is disposed below the flat pole piece, wherein the length L1 of the first magnetic-field generating device is smaller than the length L3 of the second magnetic-field generating device, wherein the length L1 of the first magnetic-field generating device is smaller than the length L5 of the flat pole piece, and wherein the length L3 of the second magnetic-field generating device is smaller than the length L5 of the pole piece, so as to orient at least a part of the non-spherical magnetic or magnetizable pigment particles; and a) a first magnetic-field generating device having its North-South magnetic axis substantially perpendicular to the substrate surface and having length L1; b) a second magnetic-field generating device having its North-South magnetic axis substantially perpendicular to the substrate surface and having a length L3, c) a flat pole piece lacking any protrusions or projections extending outside the surface of said pole piece and having a length L5, wherein the first magnetic-field generating device and the second magnetic-field generating device have a same magnetic field direction, wherein the first magnetic-field generating device faces the substrate and is disposed on top of the pole piece, wherein the second magnetic-field generating device faces the environment and is disposed below the flat pole piece, wherein the length L1 of the first magnetic-field generating device is smaller than the length L3 of the second magnetic-field generating device, wherein the length L1 of the first magnetic-field generating device is smaller than the length L5 of the flat pole piece, and wherein the length L3 of the second magnetic-field generating device is smaller than the length L5 of the pole piece, 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,