A method of manufacturing a security device component comprising: a) providing a dielectric substrate, the substrate having a first surface defining a relief structure (20,30) wherein the relief structure comprises an array of plasmonic nanostructure elements, the array of plasmonic nanostructure elements defining raised portions and recessed portions of the first surface, and an array of reduced-metallising relief elements (31(), the array of reduced-metallising relief elements defining inclined portions of the first surface having an inclination relative to the plane of the substrate greater than the inclination of the raised portions and/or the recessed portions of the first surface; b) directionally coating the first surface of the substrate with a metal layer such that the metal layer coating the raised portions and/or the recessed portions of the first surface defined by the array of plasmonic nanostructure elements is thicker than any metal layer coating the inclined portions of the first surface defined by the array of reduced-metallising relief elements, wherein the plasmonic nanostructure elements and the metal layer, in combination, provide an array of functioning plasmonic nanostructures; and characterised in that the inclined portions of the first surface and the directional coating are such that any metal layer coating the inclined portions of the first surface has a thickness of less than 15 nm; and/or further comprising c) etching the metal layer coated on the first surface such that any metal layer is substantially removed from the inclined portions of the first surface and remains on the raised portions and/or the recessed portions, thereby maintaining the array of functioning plasmonic nanostructures. a) providing a dielectric substrate, the substrate having a first surface defining a relief structure (20,30) wherein the relief structure comprises an array of plasmonic nanostructure elements, the array of plasmonic nanostructure elements defining raised portions and recessed portions of the first surface, and an array of reduced-metallising relief elements (31(), the array of reduced-metallising relief elements defining inclined portions of the first surface having an inclination relative to the plane of the substrate greater than the inclination of the raised portions and/or the recessed portions of the first surface; b) directionally coating the first surface of the substrate with a metal layer such that the metal layer coating the raised portions and/or the recessed portions of the first surface defined by the array of plasmonic nanostructure elements is thicker than any metal layer coating the inclined portions of the first surface defined by the array of reduced-metallising relief elements, wherein the plasmonic nanostructure elements and the metal layer, in combination, provide an array of functioning plasmonic nanostructures; and characterised in that the inclined portions of the first surface and the directional coating are such that any metal layer coating the inclined portions of the first surface has a thickness of less than 15 nm; and/or further comprising c) etching the metal layer coated on the first surface such that any metal layer is substantially removed from the inclined portions of the first surface and remains on the raised portions and/or the recessed portions, thereby maintaining the array of functioning plasmonic nanostructures.