Methods of producing diffractive structures in security documents
Various methods of producing a security document or article including a diffractive optical microstructure are disclosed. One method includes applying an opacifying layer ( 102 ) to at least one surface of a transparent substrate ( 100 ). An area of the opacifying layer ( 102 ) is exposed to laser radiation ( 108 ) to ablate apertures ( 110 ) in selected portions of the opacifying layer ( 102 ), thereby forming a diffractive optical microstructure ( 112 ) on the surface of the substrate ( 100 ). The laser radiation may be patterned prior to exposing the opacifying layer ( 102 ), for example by passing the radiation through a mask ( 104 ). Alternatively, a focussed or collimated laser beam ( 206 ) may be directed onto the selected portions of the opacifying layer ( 102 ). Laser radiation may be directed onto the opacifying layer ( 102 ) either directly, or through the transparent substrate ( 100 ). Security documents or articles made in accordance with the method are also provided.
1. A method of producing a security document or device comprising a substrate and an optically diffractive device, the method comprising the steps of:
applying a coating to an area of a surface on at least one side of the substrate; and
irradiating an area of the coating with a patterned laser beam bearing the pattern of a desired diffractive device to ablate selected portions of the coating and thereby form an optically diffractive structure in said coating,
wherein the substrate comprises a transparent plastic film and the coating comprises a transparent coating, in the absence of a reflective or opacifying layer at least during said step of irradiating,
wherein said step of irradiating comprises directly exposing the coating to said patterned laser beam at a surface thereof so that at least one portion of the transparent coating is ablated its full thickness without ablating the substrate, and
wherein laser ablation of the transparent coating with the patterned laser beam forms a diffractive microstructure in the transparent coating which is encoded with complex data, and the resulting optically diffractive device is a numerical-type diffractive optical element.
2. A method of producing a security document or device according to claim 1 , the method further comprising the step of:
placing a mask in the path of laser radiation to create said patterned laser beam bearing the pattern of the desired diffractive device.
3. A method of producing a security document or device according to claim 2 wherein the mask is Fourier-encoded with the complex data required to form the numerical-type diffractive optical element.
4. A method of producing a security document or device according to claim 1 further comprising a subsequent step of applying a reflective coating to the transparent coating.
5. A method of producing a security document or device according to claim 4 wherein the reflective coating is formed from polymeric material containing metallic pigment particles.
6. A method of producing a security document or device according to claim 1 further comprising a subsequent step of applying a transparent layer to the transparent coating.
7. A method of producing a security document or device according to claim 1 , the method further comprising the step of:
applying at least one opacifying layer to the substrate, said at least one opacifying layer only partly covering a surface of the substrate to leave at least said optically diffractive device uncovered by said opacifying layer.