WO2016154668A1

WO2016154668A1 - Diffractive optical element including two selectable images

Info

Publication number: WO2016154668A1
Application number: PCT/AU2016/050221
Authority: WO
Inventors: Michael Hardwick
Original assignee: Innovia Security Pty Ltd
Priority date: 2015-03-27
Filing date: 2016-03-24
Publication date: 2016-10-06
Also published as: AU2016240395A1; MX2017012382A; GB201714212D0; GB2550821A

Abstract

An optical device including a numerical-type diffractive optical element (DOE) and a birefringent layer, the DOE including a grating including a first grating region associated with a first image and a second grating region associated with a second image, wherein the first grating region includes grating elements with a first depth and wherein the second grating region includes grating elements with a second depth, such that the first depth is selected based on a first refractive index of the birefringent layer and wherein the second depth is selected based on a second refractive index of the birefringent layer.

Description

DIFFRACTIVE OPTICAL ELEMENT INCLUDING TWO SELECTABLE IMAGES FIELD OF THE INVENTION

The invention generally relates to optical devices for documents, in particular optical devices providing a security function.

BACKGROUND TO THE INVENTION

Banknotes (and other security documents) include visual security features that are difficult to reproduce (and, therefore, counterfeit) using conventional means (for example, photocopiers). It is common for such visual security features to include an effect such that the visual security features take on a different appearance when viewed from different positions. When a counterfeit copy of the security document is made, it is difficult for the counterfeiters to reproduce the effect, and, therefore, it is difficult for a passable copy of the security document to be produced.

However, as the sophistication of counterfeiters increases, the ability to reproduce or mimic complicated security features increases. Therefore, it is possible to produce a counterfeit security document including a passable, if not identical, copy of the visual security features incorporated into the security document.

In the example of banknotes, often members of the public lack the requisite sophistication and/or time to adequately inspect the security features of the banknotes to ensure that the banknotes are legitimate and not counterfeit. This makes it easier for counterfeiters to produce passable counterfeit versions of the banknotes with visual effects close enough to the visual security features of authentic banknotes to dupe, or at least confuse, members of the public.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided an optical device including a diffractive optical element (DOE) and a birefringent layer, the DOE including a grating including a first grating region associated with a first image and a second grating region associated with a second image, wherein the first grating region includes grating elements with a first depth and wherein the second grating region includes grating elements with a second depth, such that the first depth is selected based on a first refractive index of the birefringent layer and wherein the second depth is selected based on a second refractive index of the birefringent layer.

The first depth is preferably selected based on an optimal diffraction intensity corresponding to the first refractive index, and similarly the second depth is preferably selected based on an optimal diffraction intensity corresponding to the second refractive index.

Typically, the birefringent layer is configured such that the first refractive index and the second refractive index are substantially different, such that the differences in optimal diffraction intensity are such that one image is substantially removed upon application of a polariser to a side of the DOE, where the polarising direction of the polariser is parallel to one of the refractive indices.

In an embodiment, the DOE is formed from an embossed and cured ink, preferably a radiation curable ink, applied to a surface of a substrate. Also, the birefringent layer may completely cover the grating. The DOE may be a reflective DOE or alternatively a transmissive DOE.

Preferably, the birefringent layer includes a planar, or substantially planar, outward facing surface.

In an embodiment, the birefringent layer is a liquid crystal layer. Typically, the liquid crystal layer includes cross-linked liquid crystal polymers. The grating may include an arrangement of elongated grating elements, where the grating elements are substantially parallel such as to provide for alignment of the liquid crystal layer.

Optionally, the grating includes a plurality of grating elements, where the grating elements corresponding to the first grating region are arranged to produce the first image upon diffraction of incident light, and where the grating elements corresponding to the second grating region are arranged to produce the second image upon diffraction of incident light.

In an embodiment, the first image and the second image are complementary. In an alternative embodiment, the first image and the second image are unrelated. In yet another alternative embodiment, the first image and the second image are cancelling. According to another aspect of the present invention, there is provide a document, such as a banknote, including an optical device of the first aspect, and optionally including one or more further security features.

The document optionally includes a first polariser located separately to the optical device, where the first polariser is configured to have a polarising direction parallel to the first refractive index when the first polariser is overlayed onto the optical device. The document may also include a second polariser located separately to the optical device and the first polariser, where the second polariser is configured to have a polarising direction parallel to the second refractive index when the second polariser is overlayed onto the optical device.

The optical device may be located within a window region of the security document. Alternatively, the optical device may be located within a half-window region of the security document.

According to yet another aspect of the present invention, there is provided a method for producing an optical device, including the steps of: forming a grating onto a first side of a substrate, the grating having a surface profile including a first grating region and a second grating region, wherein the first grating region has a grating depth different to the second grating region; and applying a birefringent layer, preferably a liquid crystal layer, to the grating, wherein the first grating region is associated with a first image, and wherein the second grating region is associated with a second image, and wherein the first image and the second image are configured to be viewable upon diffraction of incident light by the grating.

Optionally, the step of incorporating a grating onto a first side of the substrate includes the steps of: providing a shim having a surface profile inverse to the surface profile of the grating; applying a curable ink, preferably a radiation curable ink, to the first side; embossing the first side of the substrate with the embossing tool; and curing the curable ink. Where the birefringent layer is a liquid crystal layer, the method may include the step of cross-linking the liquid crystal layer. Security Document or Token

As used herein the term security documents and tokens includes all types of documents and tokens of value and identification documents including, but not limited to the following: items of currency such as banknotes and coins, credit cards, cheques, passports, identity cards, securities and share certificates, driver's licenses, deeds of title, travel documents such as airline and train tickets, entrance cards and tickets, birth, death and marriage certificates, and academic transcripts.

The invention is particularly, but not exclusively, applicable to security documents or tokens such as banknotes or identification documents such as identity cards or passports formed from a substrate to which one or more layers of printing are applied. The diffraction gratings and optically variable devices described herein may also have application in other products, such as packaging.

Security Device or Feature

As used herein the term security device or feature includes any one of a large number of security devices, elements or features intended to protect the security document or token from counterfeiting, copying, alteration or tampering. Security devices or features may be provided in or on the substrate of the security document or in or on one or more layers applied to the base substrate, and may take a wide variety of forms, such as security threads embedded in layers of the security document; security inks such as fluorescent, luminescent and phosphorescent inks, metallic inks, iridescent inks, photochromic, thermochromic, hydrochromic or piezochromic inks; printed and embossed features, including relief structures; interference layers; liquid crystal devices; lenses and lenticular structures; optically variable devices (OVDs) such as diffractive devices including diffraction gratings, holograms and diffractive optical elements (DOEs).

Substrate

As used herein, the term substrate refers to the base material from which the security document or token is formed. The base material may be paper or other fibrous material such as cellulose; a plastic or polymeric material including but not limited to polypropylene (PP), polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC), polyethylene terephthalate (PET); or a composite material of two or more materials, such as a laminate of paper and at least one plastic material, or of two or more polymeric materials.

Transparent Windows and Half Windows

As used herein the term window refers to a transparent or translucent area in the security document compared to the substantially opaque region to which printing is applied. The window may be fully transparent so that it allows the transmission of light substantially unaffected, or it may be partly transparent or translucent partially allowing the transmission of light but without allowing objects to be seen clearly through the window area.

A window area may be formed in a polymeric security document which has at least one layer of transparent polymeric material and one or more opacifying layers applied to at least one side of a transparent polymeric substrate, by omitting least one opacifying layer in the region forming the window area. If opacifying layers are applied to both sides of a transparent substrate a fully transparent window may be formed by omitting the opacifying layers on both sides of the transparent substrate in the window area.

A partly transparent or translucent area, hereinafter referred to as a "half- window", may be formed in a polymeric security document which has opacifying layers on both sides by omitting the opacifying layers on one side only of the security document in the window area so that the "half-window" is not fully transparent, but allows some light to pass through without allowing objects to be viewed clearly through the half-window.

Alternatively, it is possible for the substrates to be formed from an substantially opaque material, such as paper or fibrous material, with an insert of transparent plastics material inserted into a cut-out, or recess in the paper or fibrous substrate to form a transparent window or a translucent half-window area.

Opacifying layers

One or more opacifying layers may be applied to a transparent substrate to increase the opacity of the security document. An opacifying layer is such that I_T < L₀, where L₀ is the amount of light incident on the document, and LT is the amount of light transmitted through the document. An opacifying layer may comprise any one or more of a variety of opacifying coatings. For example, the opacifying coatings may comprise a pigment, such as titanium dioxide, dispersed within a binder or carrier of heat-activated cross-linkable polymeric material. Alternatively, a substrate of transparent plastic material could be sandwiched between opacifying layers of paper or other partially or substantially opaque material to which indicia may be subsequently printed or otherwise applied.

Diffractive Optical Elements (DOEs)

As used herein, the term diffractive optical element refers to a numerical- type diffractive optical element (DOE). Numerical-type diffractive optical elements (DOEs) rely on the mapping of complex data that reconstruct in the far field (or reconstruction plane) a two-dimensional intensity pattern. Thus, when substantially collimated light, e.g. from a point light source or a laser, is incident upon the DOE, an interference pattern is generated that produces a projected image in the reconstruction plane that is visible when a suitable viewing surface is located in the reconstruction plane, or when the DOE is viewed in transmission at the reconstruction plane. The transformation between the two planes can be approximated by a fast Fourier transform (FFT). Thus, complex data including amplitude and phase information has to be physically encoded in the micro- structure of the DOE. This DOE data can be calculated by performing an inverse FFT transformation of the desired reconstruction (i.e. the desired intensity pattern in the far field).

DOEs are sometimes referred to as computer-generated holograms, but they differ from other types of holograms, such as rainbow holograms, Fresnel holograms and volume reflection holograms.

Embossable Radiation Curable Ink

The term embossable radiation curable ink used herein refers to any ink, lacquer or other coating which may be applied to the substrate in a printing process, and which can be embossed while soft to form a relief structure and cured by radiation to fix the embossed relief structure. The curing process does not take place before the radiation curable ink is embossed, but it is possible for the curing process to take place either after embossing or at substantially the same time as the embossing step. The radiation curable ink is preferably curable by ultraviolet (UV) radiation. Alternatively, the radiation curable ink may be cured by other forms of radiation, such as electron beams or X-rays.

The radiation curable ink is preferably a transparent or translucent ink formed from a clear resin material. Such a transparent or translucent ink is particularly suitable for printing light-transmissive security elements such as sub- wavelength gratings, transmissive diffractive gratings and lens structures.

The transparent or translucent ink preferably comprises an acrylic based UV curable clear embossable lacquer or coating.

Such UV curable lacquers can be obtained from various manufacturers, including Kingfisher Ink Limited, product ultraviolet type UVF-203 or similar. Alternatively, the radiation curable embossable coatings may be based on other compounds, eg nitro-cellulose.

The radiation curable inks and lacquers used herein have been found to be particularly suitable for embossing microstructures, including diffractive structures such as diffraction gratings and holograms, and microlenses and lens arrays. However, they may also be embossed with larger relief structures, such as non- diffractive optically variable devices.

The ink is preferably embossed and cured by ultraviolet (UV) radiation at substantially the same time. In a particularly preferred embodiment, the radiation curable ink is applied and embossed at substantially the same time in a Gravure printing process.

Preferably, in order to be suitable for Gravure printing, the radiation curable ink has a viscosity falling substantially in the range from about 20 to about 175 centipoise, and more preferably from about 30 to about 150 centipoise. The viscosity may be determined by measuring the time to drain the lacquer from a Zahn Cup #2. A sample which drains in 20 seconds has a viscosity of 30 centipoise, and a sample which drains in 63 seconds has a viscosity of 150 centipoise.

DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to the accompanying drawings. It is to be appreciated that the embodiments are given by way of illustration only and the invention is not limited by this illustration. In the drawings:

Figure 1 shows a document including an optical device;

Figure 2 shows an optical device including an embossing layer including a grating;

Figure 3a shows a DOE operating in a transmission mode;

Figure 3b shows a DOE operating in reflection mode;

Figure 4a shows a DOE covered by a liquid crystal layer;

Figure 4b shows a surface profile of an example DOE;

Figure 5a shows two complementary images;

Figure 5b shows two unrelated images;

Figure 5c shows two cancelling images;

Figure 6a shows an arrangement including a DOE operating in transmission mode and a polariser;

Figure 6b shows another arrangement including a DOE operating in transmission mode and a polariser;

Figure 6c shows an arrangement including a DOE operating in reflection mode and a polariser;

Figure 7a shows a polariser aligned with a first refractive index;

Figure 7b shows a polariser aligned with a second refractive index;

Figure 8 shows a security document including an optical device, a first polariser, and a second polariser;

Figure 9a shows an embossing arrangement;

Figure 9b shows an embossing step;

Figure 10a shows two continuous grating regions; and

Figure 10b shows a plurality of discontinuous grating regions.

DETAILED DESCRIPTION OF THE EMBODIMENTS

For a diffractive grating covered by a high refractive index (HRI) material, there is, in general, an optimal grating depth of the diffractive grating to allow for maximum perceived diffraction intensity, whether viewed in a reflection or transmission mode, for a wavelength or range of wavelengths of interest. The optimal depth can be determined experimentally and/or through calculation based on the refractive index and wavelength of interest. Referring to Figure 1 , there is provided a document 2 including an optical device 4. The document 2 typically requires protection from counterfeiting and may as such be referred to as a "security document". Examples of such security documents include banknotes, credit cards, passports, etc. The document 2 can include one or more further security features 6, though this is optional. The further security features can, for example, be selected from: micromirror features, holographic features, and other optically variable features. The document 2 and optical device 4 can each include a substrate 8, such that the substrate 8 of each of the document 2 and optical device 4 can be the same or different.

Referring to Figure 2, the optical device 4 includes a diffractive optical element (DOE) 10. The DOE 10 can be formed on an embossable surface 12 of the optical device 4, and the DOE 10 can operate as either a reflective or transmissive DOE 10. The substrate 8 can, for example, include biaxial polypropylene (BOPP) or other suitable transparent material, or alternatively the substrate 8 can be a paper or paper composite substrate 8. The embossable surface 12 can be the substrate 8, such that a surface of the substrate 8 is directly embossed during an embossing process. Alternatively, the embossable surface 12 can correspond to an embossable layer 14 applied to the substrate 8, as shown in Figure 2. For the purposes of this disclosure, it will be assumed the embossable surface 12 corresponds to a separate embossable layer 14, in particular it will be assumed that a radiation curable ink is applied to the substrate 8 before the DOE 10 is embossed onto the optical device 4. The DOE 10 includes a grating 26 including two grating regions 28, 30; the grating of the first grating region 28 having a grating depth different to the grating depth of the grating elements of the second region 30 (smaller as shown in Figure 2).

Referring to Figures 3a and 3b, the optical device 4 includes an input side 16 and an output side 18. Incident light 20 from a point or collimated light source is incident on the input side 16 of the optical device 4. After interacting with the DOE 10, the light is emitted from the output side 18 as emitted light 22. The input side 16 and the output side 18 can correspond to opposite sides of the optical device 4 (as shown in Figure 3a) or to the same side of the optical device 4 (as shown in Figure 3b). The emitted light 22 is then viewed by a viewer, which may involve projecting the emitted light 22 onto a screen, or viewing the emitted light 22 directly. There can be an ideal separation distance between the viewer and the DOE 10.

Referring to Figure 4, a birefringent layer 24 is located covering the DOE 10, preferably such that it completely covers the DOE 10. In the embodiments described herein, the birefringent layer 24 is a liquid crystal layer 24, and the two terms are used interchangeably. The liquid crystals molecules of the liquid crystal layer 24 are aligned with the grating elements of the grating regions 28, 30, which are substantially parallel, and therefore will be in a substantially aligned state (generally known as a nematic phase). In this state, the liquid crystal layer 24 will act as a birefringent substantially transparent layer, with a first refractive index and a different, second refractive index (for example, the first refractive index can be the ordinary refractive index of the liquid crystal layer 24 whereas the second refractive index can be the extraordinary refractive index of the liquid crystal layer 24).

The arrangement of grating elements is described herein is being "parallel" or "substantially parallel". In general, the surface profile of a DOE does not correspond to an arrangement of perfectly parallel identical grating elements. Figure 4b shows a sample surface profile of a DOE 10 suitable for use in the embodiments herein described. As can be seen, the surface profile corresponds to reasonably parallel, but not identical, grating elements. For the purposes of the invention, the DOE should be designed such that its surface profile is composed of sufficiently parallel regions as to provide for alignment of the applied liquid crystals.

The presence of two refractive indices associated with the liquid crystal layer 24 results in two different optimal grating heights for the grating 26 covered by the liquid crystal layer 24. The first grating region 28 of the DOE 10 has a first grating depth selected to provide an optimal diffractive effect in light of the first refractive index 34, and the second grating region 30 of the DOE 10 has a second grating depth selected to provide an optimal diffractive effect in light of the second refractive index.

Referring to Figures 5a to 5c, under normal viewing conditions, where the DOE 10 is illuminated by non-polarised light, two images 42, 44 will be viewable corresponding to each of the grating regions. The two images 42, 44 can be selected to be:

a) complementary, such that the images 42, 44 complement each other, for example in Figure 5a, the first image (One Hundred") 42 and the second image ("Dollars") 44 are combined to create a third composite image 62 (One Hundred Dollars");

b) unrelated, such that the images 42, 44 have no relationship to each other, for example in Figure 5b, the first image (One Hundred") 42 and the second image ("Dollars") 44 overlap to form an unrecognisable third image 64 (a superposition of "One Hundred" and "Dollars"); or

c) cancelling, such that the images 42, 44 cancel each other out, which can be a special case of the complementary arrangement, for example in Figure 5c, the first image ("One Hundred Dollars") 42 and the second image (the background excluding "One Hundred Dollars") 44 combine to form a composite image with no easily identifiable detail 66.

Referring to Figures 6a to 6c, a polarising filter 46 can be used to selectively display one of the two images 42, 44 (for example, of Figure 5a). There are two configurations associated with a transmissive device that can be employed, the first configuration (as shown in Figure 6a) corresponds to the polarising filter 46 being located covering the input side 16, whereas the second configuration (as shown in Figure 6b) corresponds to the polarising filter covering the output side 18. For a reflective device, there is one configuration (as shown in Figure 6c), where the polarising filter 46 effectively covers the input side 16 and the output side 18, as they correspond to the same side of the optical device 4.

Considering the first transmissive configuration, referring to Figures 7a, to select the first image 42, the polarising filter 46 is placed against the input side 16 of the optical device 4, such that the incident light 20 is polarised before interacting with the DOE 10. The polarising filter 46 is selected to have a polarising direction P1 parallel to the first refractive index (R1 ). The incident light 20 interacts with both the first grating region 28 and the second grating region 30; however due to the effective refractive index being equal to the first refractive index, a diffraction effect is strongest corresponding to the first grating region 28, and therefore the first image 42 will be dominantly viewable, or at least the first image 42 will be substantially brighter than the second image 44. Referring to Figure 7b, to select the second image 44, the polarising filter 46 is selected to have a polarising direction P2 parallel to the second refractive index (R2). Now, the effective refractive index is equal to the second refractive index, and therefore only the second image 44 will be dominantly viewable, or at least the second image 44 will be substantially brighter than the first image 42.

The second transmissive configuration and the reflective configuration operate in a similar manner to the first transmissive configuration, in that the polarising filter 46 is selected to have a polarising direction parallel one of the refractive indices, thereby selecting one of the images 42, 44.

Referring to Figure 8, in order to ensure that the polarising filter 46 is correctly aligned (either with the first or the second refractive index), the polarising filter can be incorporated into the document 2 as a first polarising filter 50. The appropriate image 42, 44 can therefore be viewed by folding the document 2 in order to overlay the first polarising filter 50 and the optical device 4. Optionally, the document 2 can include a second polarising filter 54 with a polarising direction 56 perpendicular to the first polarising filter 50, and therefore parallel to the other refractive index. This way, the polarising filters 50, 54 act as verification elements, with each causing a different image 42, 44 to be viewed when applied to the optical device 4.

Referring to Figures 9a and 9b, an embossing process can be used to provide a grating 26 with two grating regions 28, 30, each with different grating depths. An embossing tool 60 can be created incorporating a shim 62 having a surface profile which is the inverse of the required surface profile of the grating 26.

The embossing tool 60 is utilised to emboss an embossable layer 14, for example a curable ink (such as a radiation curable ink) which has previously been applied, for example through printing, to a surface of the substrate 8, thereby forming the desired grating 26 on the surface of the embossable layer 14. When the embossable layer 14 includes a radiation curable ink, the embossable layer 14 can be irradiated with UV radiation, for example from the opposite side of the transparent substrate 8. After the grating 26 is formed, the liquid crystal layer 24 is applied to the outwards facing side of the grating 26 (not shown). Preferably, the liquid crystal layer 24 is applied such that the outward facing surface of the layer 24 is planar, or at least substantially planar. The liquid crystal layer 24 can include liquid crystal polymers (LCP), which can then be cured such that the LCPs are cross- linked and fixed in place. Other techniques can be employed to cause the liquid crystal layer 24 to be fixed in place, for example a sealing layer can be applied over the liquid crystal layer 24, sealing the liquid crystal layer 24 in place.

In general, referring to Figures 10a and 10b, the grating 26 can be designed such that each grating region 28, 30 corresponds to one continuous region of the grating 26 (as shown in Figure 10a), or alternatively each grating region 28, 30 can correspond to a plurality of discontinuous regions of the grating 26 (such as a pixel configuration as shown in Figure 10b).

Further modifications and improvements may be made without departing from the scope of the present invention. For example, rather than embossing the composite grating onto the embossable layer, another technique for providing a grating on a substrate can be used, for example laser ablation or etching.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1 . An optical device including a numerical-type diffractive optical element (DOE) and a birefringent layer, the DOE including a grating including a first grating region associated with a first image and a second grating region associated with a second image, wherein the first grating region includes grating elements with a first depth and wherein the second grating region includes grating elements with a second depth, such that the first depth is selected based on a first refractive index of the birefringent layer and wherein the second depth is selected based on a second refractive index of the birefringent layer.

2. An optical device as claimed in claim 1 , wherein the first depth is selected based on an optimal diffraction intensity corresponding to the first refractive index, and wherein the second depth is selected based on an optimal diffraction intensity corresponding to the second refractive index.

3. An optical device as claimed in claim 2, wherein the birefringent layer is configured such that the first refractive index and the second refractive index are substantially different, such that the differences in optimal diffraction intensity are such that one image is substantially removed upon application of a polariser to a side of the DOE, where the polarising direction of the polariser is parallel to one of the refractive indices.

4. An optical device as claimed in any one of the previous claims, wherein the DOE is formed from an embossed and cured ink, preferably a radiation curable ink, applied to a surface of a substrate,

5. An optical device as claimed in any one of the previous claims, wherein the birefringent layer completely covers the grating.

6. An optical device as claimed in any one of the previous claims, wherein the birefringent layer includes a planar, or substantially planar, outward facing surface.

7. An optical device as claimed in any one of the previous claims, wherein the birefringent layer is a liquid crystal layer, and preferably includes cross-linked liquid crystal polymers.

8. An optical device as claimed in claim 7, wherein the grating includes an arrangement of elongated grating elements, and wherein the grating elements are substantially parallel such as to provide for alignment of the liquid crystal layer.

9. An optical device as claimed in any one of the previous claims, wherein the grating includes a plurality of grating elements, and wherein the grating elements corresponding to the first grating region are arranged to produce the first image upon diffraction of incident light, and wherein the grating elements corresponding to the second grating region are arranged to produce the second image upon diffraction of incident light.

10. An optical device as claimed in any one of the previous claims, wherein the first image and the second image are complementary.

1 1 . An optical device as claimed in any one of claims 1 to 9, wherein the first image and the second image are unrelated.

12. An optical device as claimed in any one of claims 1 to 9, wherein the first image and the second image are cancelling.

13. An optical device as claimed in any one of the previous claims, wherein the DOE is a reflective DOE.

14. An optical device as claimed in any one of claims 1 to 12, wherein the DOE is a transmissive DOE.

15. A document, such as a banknote, including an optical device according to any one of the previous claims, and optionally including one or more further security features.

16. A document as claimed in claim 15, including a first polariser located separately to the optical device, wherein the first polariser is configured to have a polarising direction parallel to the first refractive index when the first polariser is overlayed onto the optical device.

17. A document as claimed in claim 16, including a second polariser located separately to the optical device and the first polariser, wherein the second polariser is configured to have a polarising direction parallel to the second refractive index when the second polariser is overlayed onto the optical device.

18. A document as claimed in any one of claims 15 to 17, wherein the optical device is located within a window region of the security document.

19. A document as claimed in any one of claims 15 to 17, wherein the optical device is located within a half-window region of the security document.

20. A method for producing an optical device, including the steps of:

a) forming a grating onto a first side of a substrate, the grating having a surface profile including a first grating region and a second grating region, wherein the first grating region has a grating depth different to the second grating region, wherein the grating defines a numerical-type diffractive optical element (DOE); and

b) applying a birefringent layer to the grating,

wherein the first grating region is associated with a first image, and wherein the second grating region is associated with a second image, and wherein the first image and the second image are configured to be viewable upon diffraction of incident light by the grating.

21 . A method as claimed in claim 20, wherein the step of incorporating a grating onto a first side of the substrate includes the steps of: providing a shim having a surface profile inverse to the surface profile of the grating; applying a curable ink, preferably a radiation curable ink, to the first side; embossing the first side of the substrate with the embossing tool; and curing the curable ink.

22. A method as claimed in any one of claims 20 to 21 , wherein the birefringent layer is a liquid crystal layer and the method including the step of cross-linking the liquid crystal layer.