FR2990525A1 - OPTICAL SECURITY DEVICE PRODUCING DIFFERENT IMAGES IN REFLECTION AND TRANSMISSION. - Google Patents

Abstract

An optical security device 200 which has a relief structure 204 provided on a transparent or partially transparent substrate 202, a coating 206 provided on the relief structure 204, where under the incident light, the relief structure 204 produces a first image 102 and a second image 104 such that when the device 200 is viewed in transmission, the first image 102 and the second image 104 are visible, and when the device 200 is viewed in reflection, the first image 102 is visible and the second image 104 is weakly visible or is not visible. This can be achieved by providing a coating 206 with a high refractive index such that the depth of the portion 210 of the relief structure 204 that produces the second image 104 is insufficient for the second image 104 to be visible in reflection.

Description

The present invention relates to optical security devices having relief structures such that they can be used to produce optical effects for verification of secure documents. A wide variety of diffractive structures are available to add security to a document. Such diffractive structures may include, for example, holograms, diffractive lenses (such as NanogravureTM characteristics) and diffractive / refractive structures. A diffractive structure may be provided on a transparent substrate, and coated with a high refractive index coating, thereby producing a diffraction effect that can be visualized in both reflection and transmission. Such diffractive structures offer some degree of protection against counterfeiting. However, it is desirable to develop improved diffractive structures to provide additional security features. According to one aspect of the present invention, there is provided an optical security device comprising a relief structure provided on a transparent or partially transparent substrate, the relief structure having a first portion which, under incident light, produces a first image, and a second portion which, under the incident light, produces a second image; and a coating provided on the relief structure; wherein when the device is viewed in transmission, the first image and the second image are visible, and when the device is viewed in reflection, the first image is visible and the second image is substantially non-visible. Preferably, the first portion of the relief structure has a greater depth than the second portion of the relief structure.

Preferably, the refractive index of the coating is sufficiently high so that the depth of the second portion of the relief structure is insufficient so that the second image is clearly visible in reflection.

The relief structure may be a binary diffraction grating. As an alternative, the relief structure is a multilevel diffractive structure. If the relief structure is either a binary diffractive grating or a multi-level diffractive structure, the first portion of the relief structure preferably has a depth which is approximately twice the depth of the second portion of the relief structure . More preferably, when the relief structure is a binary diffraction grating, the first portion of the relief structure has a depth of approximately equal to. 1000 nm, and the second portion of the relief structure has a depth of approximately 450 nm. Alternatively, when the relief structure is a multi-level diffractive structure, it is further preferable that the first portion of the relief structure has a depth of approximately 2400 nm, and the second portion of the relief structure. has a depth of approximately 1,200 nm.

In another embodiment, the relief structure may be a diffractive structure. If the relief structure is a diffractive structure, it is preferable that the first portion of the relief structure has a depth of approximately seven times that of the second portion of the relief structure. It is further preferable that the first portion of the relief structure has a depth of approximately 501.1,1ri and that the second portion of the relief structure has a depth of approximately 7 μm. The relief structure can be formed by embossing a radiation curable embossable ink.

A device according to the invention may be provided in or on a secure document, and may further be provided in a secure document window. The secure document can be a bank note. According to another aspect of the invention, there is provided a method for producing an optical security device, comprising the steps of providing a relief structure on a transparent or partially transparent substrate, the relief structure having a first portion which, under incident light, produces a first image, and a second portion which, under the incident light, produces a second image; and providing a coating on the relief structure; such that when the device is viewed in transmission, the first image and the second image are visible, and when the device is viewed in reflection, the first image is visible and the second image is substantially non-visible. As used herein, the term secure document includes all 15 types of valuable documents and tokens and identification documents, including, but not limited to: currency, such as banknotes and coins, cards credit, checks, passports, identity cards, securities certificates and share certificates, driving licenses, deeds, travel documents such as 20 air and train tickets, cards and tickets entry, as well as birth, marriage and death certificates, and transcripts. As used herein, the window term refers to a transparent or translucent area in the secure document, as compared to the substantially opaque region on which printing is applied. The window may be entirely transparent so as to allow light transmission substantially unaffected, or may be partially transparent or partially translucent, permitting transmission of light but without allowing objects to be clearly seen through the window area.

A window area may be formed in a polymeric secure 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, omitting at least one opacifying layer in the region forming the window area. If opacifying layers are applied on both sides of a transparent substrate, a fully transparent window can be formed by omitting the opacifying layers on both sides of the transparent substrate in the window area.

A partially transparent or translucent zone, hereinafter referred to as a "half-window", may be formed in a polymeric secure document which has opacifying layers on both sides by omitting the opacifying layers on only one side of the secure document in the area of the window, so that the half-window is not completely transparent, but that light passes through without the objects clearly visible through the half-window. Alternatively, the substrates may be formed from a substantially opaque material, such as paper or fibrous material, with a transparent plastic material insert inserted into a blank or trough of the fibrous paper or substrate to to form a transparent window area or translucent half-window. As used herein, the term diffractive optical element refers to a digital type diffractive optical element (DOD). Digital-type diffractive optical elements (DODs) are based on the mapping of complex data that reconstructs a two-dimensional intensity model in the far field (or reconstruction plane). Thus, when substantially collimated light, e.g. from a point light source or a laser, is incident on the EOD, an interference pattern is generated which produces a projected image in the reconstruction plane which is visible when an appropriate viewing surface is located in the reconstruction plane, or when the EOD is seen in transmission to the reconstruction plane. The transformation between the two planes can be approximated by a Fast Fourier Transformation (FFT). Thus, complex data with amplitude and phase information must be physically encoded in the EOD microstructure. These EOD data can be calculated by performing an inverse FFT transformation of the desired reconstruction (i.e. the desired intensity pattern in the far field).

EODs are sometimes called computer-generated holograms, but they differ from other types of holograms, such as rainbow holograms, Fresnel holograms, and reflective volume holograms. The refractive index n of a medium corresponds to the ratio between the speed of light in the vacuum and the speed of light in the medium. The refractive index n of a lens determines the importance according to which the light rays that reach the surface of the lens will be refracted, according to Snell's law: ni * Sin (a) = n * Sin (0), where a corresponds to the angle between an incident ray and the normal at the point of incidence on the surface of the lens, 0 is the angle between the refracted ray and the normal at the point of incidence, and n1 is the refractive index of the air (it can be considered approximately that n1 is 1). The term radiation-curable embossable ink used herein refers to any ink, lacquer or other coating that can be applied to the substrate during a printing process and which can be embossed then it is soft (soft) to form a relief structure and cured by radiation to fix the embossed relief structure.

The curing process does not occur until the radiation curable ink is embossed, but it is possible that the curing process occurs after embossing or substantially simultaneously with 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 clear or translucent ink formed from a transparent resin material. Such transparent or translucent ink is particularly suitable for printing light-transmitting security elements such as sub-wavelength gratings, transmissive diffraction gratings and lens structures.

In a particularly preferred embodiment, the transparent or translucent ink preferably comprises a lacquer or an acrylic-based, UV-curable transparent embossable coating. Such UV curable lacquers can be obtained from various manufacturers, including Kingfisher Jnk Limited, UVF-203 ultraviolet type or equivalent. As an alternative, the radiation curable embossable coatings may be based on other compounds, for example nitrocellulose. The radiation curable inks and varnishes used in the invention have proved particularly suitable for embossing microstructures, including diffractive structures such as diffraction gratings and holograms, as well as microlenses and lens arrays. However, they can 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 essentially simultaneously. In a particularly preferred embodiment, the radiation curable ink is applied and embossed substantially at the same time during a gravure process. Preferably, and to be suitable for gravure printing, the radiation curable ink has a viscosity substantially in the range of about 20 to about 175 centipoise, and more preferably about 30 to about 150 centipoise. mPa.s (centipoises). The viscosity can be determined by measuring the time required to dispense the lacquer of a Zahn # 2 cut. A sample that flows in 20 seconds has a viscosity of 30 mPa.s (centipoise), and a sample that flows in 63 seconds a viscosity of 150 mPa.s (centipoise). With some polymeric substrates, it may be necessary to apply an interlayer to the substrate prior to applying the radiation curable ink to improve adhesion to the substrate of the embossed ink structure. The intermediate layer preferably comprises a primer layer, and more preferably, the primer layer comprises a polyethylene imine. The primer layer may also include a crosslinking agent, for example a multifunctional isocyanate. Examples of other suitable finishes for use in the invention include: polymers terminated with a hydroxyl group; hydroxyl terminated polyester copolymers; crosslinked or uncrosslinked hydroxyl acrylates; Polyurethanes; and anionic or cationic UV curing acrylates. Examples of suitable crosslinking agents include: isocyanates; polyaziridines; zirconium complexes; aluminum acetylacetone; melamines; and carbodiimides.

The type of primer may vary for different substrates and embossed ink structures. A primer is preferably selected which does not substantially affect the optical properties of the embossed ink structure. Figure la is a representation of a combined image produced by a relief structure. Fig. 1b shows the image components separated from the image of Fig. 1a. Fig. 2 is a schematic sectional view of an optical safety device having a relief structure in the form of a hologram having varying depths. Figure 3 is a schematic sectional view of a security optical device having a relief structure in the form of a diffractive lens or a multi-level structure with varying depths. Figure 4 is a schematic sectional view of a security optical device having a relief structure in the form of a reflective / refractive structure with varying depths. FIGS. 5a and 511 are representations of the visible image when the optical security devices of FIGS. 2 to 4 are seen in reflection and in transmission, respectively.

Referring to Figs. 1a and 1b, according to a preferred embodiment of the invention, an optical security device has a relief structure which, under the incident light, produces the combined image 100. The combined image 100 consists of a first image 102, which is an image of the letter "A", and a second image 104, which is an image of a sun. The relief structure may be any of a variety of relief structures known in the art that produce an image under incident light, including for example holograms, diffraction gratings, diffractive lenses, optical elements digital-type diffractive (EOD), multi-level EODs, and reflective / refractive structures, or a combination of such structures. In one embodiment of the invention, as shown in FIG. 2, the security optical device 200 comprises a substrate 202, a relief structure in the form of a holographic structure 204, and a coating 206. portion of the holographic structure 208 has a greater depth than the other portion 210 of the holographic structure. The portion 208 of the holographic structure having a greater depth is responsible for producing the first image 102, while the shallower portion 210 is responsible for producing the second image 104. The substrate 202 is transparent or essentially transparent, so that under the incident light, the first image 102 and the second image 104 are visible in transmission. The relief structure may be produced by any suitable method, such as embossing the relief structure 204 in the substrate 202 using a radiation curable embossable ink. The coating 206 is selected with a refractive index which is substantially greater than the refractive index of the substrate 202. It has been found that as the refractive index of the coating on a relief structure increases, the depth of the relief structure must also increase to allow to see the effect of the relief structure in reflection. If the relief structure is not deep enough for the refractive index of the coating, the optical effect produced by the relief structure is decreased, and in some cases not at all visible. Thus, the refractive index of the coating 206 and the depth of the portion 210 of the holographic structure which is responsible for the second image 104 are selected so that the second image 104 is not visible or is only faintly visible when the optical security device 200 is seen in reflection, because of the insufficient depth of the portion 210 of the structure with respect to the refractive index of the coating 206. For a binary diffraction grating, for which the index of 5 refraction of the coating 206 is approximately equal to 1.7, the preceding effect can be obtained by providing the portion 208 with a depth of approximately 1000 nm, while providing the portion 210 with a depth of approximately 450 nm. Accordingly, when the device 200 is seen in reflection, the first image 102 is visible, but yet the second image 104 is not visible or only faintly visible, and the observer sees only the image 10 2, as shown in Figure 5a. However, when the same device is seen in transmission, the first image 102 and the second image 104 are both visible, and an observer will see the combined image shown in Figure 5b. In another embodiment of the invention, as seen in FIG. 3, the relief structure is a diffractive lens or a multi-level structure 304. The security optical device 300, the substrate 302 and the coating 306 are analogous to references 200, 202 and 206, respectively. The structure 304 has a portion 308 which has a depth greater than that of the other portion 310 of the structure. The portion 308 of the deeper structure is responsible for the production of the first image 102, while the shallower portion 310 is responsible for producing the second image 104. The refractive index of the coating 306 and the depth of the portion 310 of the structure that is responsible for the second image 104 are selected such that the second image 104 is not visible or is only faintly visible when the security optical device 300 is viewed in reflection due to the insufficient depth of the portion 310 of the structure with respect to the refractive index of the coating 306. For a multi-level diffractive structure, for which the refractive index of the coating 306 is about 1, 7, the preceding effect can be obtained by providing the portion 308 with a depth of approximately 2400 nm while the portion 310 is provided with a depth of approximately 1 200 nm. Consequently, when a device 300 is seen in reflection, the first image 102 is visible, but yet the second image 104 is not visible or only faintly visible, and an observer will see only the image 102 as than shown in Figure 5a. However, when the same device is seen in transmission, both the first image 102 and the second image 104 are visible, and an observer will see the combined image shown in Figure 5b.

In another embodiment of the invention, as shown in FIG. 4, the relief structure is a reflective / refractive structure 404. The security optical device 400, the substrate 402 and the coating 406 are analogous to the references 200. , 202 and 206, respectively. The structure 404 has a portion 408 which has a greater depth than the portion 410 of the structure. The portion of the structure which has a greater depth 408 is responsible for the production of the first image 102, while the lower thickness portion 410 is responsible for producing the second image 104. The refractive index of the coating 406 and the depth of the portion 410 of the structure that is responsible for the second image 104 are selected such that the second image 104 is essentially non-visible, namely that the second image is not visible at all or is only weakly visible when the optical security device 400 is seen in reflection, due to the insufficient depth of the portion 410 of the structure with respect to the refractive index of the coating 406. For a refractive structure, for which the index of The refraction of the coating 406 is about 1.7, the foregoing effect can be achieved by providing the portion 408 with a depth of about 50 μm while providing for the portion 410 a depth of approximately 7 gm. Accordingly, when the device 400 is seen in reflection, the first image 102 is visible, but yet the second image 104 is not visible or only faintly visible, and an observer will see only the image 102 as shown in Figure 5a. However, when the same device is seen in transmission, both the first image 102 and the second image 104 are visible, and an observer will see the combined image shown in Figure 5b.

The security optical device may be provided in the form of a security feature on a secure document, for example a banknote. The substrate may be formed of any material, and may for example be a polymeric substrate.

Claims (27)

REVENDICATIONS1. An optical security device (200), comprising: a) a relief structure (204) provided on a transparent or partially transparent substrate (202), the relief structure (204) having a first portion (208) which under the light incident, produces a first image (102), and a second portion (210) which, under the incident light, produces a second image (104); and b) a coating (206) provided on the relief structure (204); where when the device (200) is seen in transmission, the first image (102) and the second image (104) are visible, and when the device (200) is seen in reflection, the first image (102) is visible and the second image (104) is essentially non-visible. 2. Device according to claim 1, wherein the first portion (208) of the relief structure (204) has a depth greater than the second portion (210) of the relief structure (204). 15 3. Device according to claim 2, wherein the refractive index of the coating (206) is sufficiently high so that the depth of the second portion (210) of the relief structure (204) is insufficient for the second image ( 104) is clearly visible in reflection. The device of any one of claims 1 to 3, wherein the relief structure (204) is a binary diffraction grating. 5. Device according to any one of claims 1 to 3, wherein the relief structure (204) is a multi-level diffractive structure. The device of claim 4 or 5, wherein the first portion (208) of the relief structure (204) has a depth which is approximately twice the depth of the second portion (210) of the embossed structure. (204). The device of claim 4, wherein the first portion (208) of the relief structure (204) has a depth of approximately 1000 nm, and the second portion (210) of the relief structure (204) has a depth of approximately 450 nm. 30 The device according to claim 5, wherein the first portion (208) of the relief structure (204) has a depth of approximately 2400 nm, and the second portion (210) of the relief structure (204) has a depth of approximately 1,200 nm. 9. Device according to any one of claims 1 to 3, wherein the relief structure (204) is a di ffactive structure. The device of claim 9, wherein the first portion (208) of the relief structure (204) has a depth of approximately seven times the depth of the second portion (210) of the embossed structure. Apparatus according to claim 9, wherein the first portion (208) of the relief structure (204) has a depth of approximately 50 μm, and the second portion (210) of the relief structure (204) has a depth of approximately 7 days. 12. Device according to any one of the preceding claims, wherein the relief structure (204) is an embossed embossed structure. 13. Apparatus according to any one of the preceding claims provided in 15 or on a secure document. 14. Device according to claim 13, wherein the device (200) is provided in a window of the secure document. The device of claim 13 or claim 14, wherein the secure document is a bank note. 20 A method of producing a security optical device (200), comprising the following steps: a) providing a relief structure (204) provided on a transparent or partially transparent substrate (202), the relief structure ( 204) having a first portion (208) which, under the incident light, produces a first image (102), and a second portion (210) which, under the incident light, produces a second image (104); and b) providing a coating (206) on the relief structure (204); so that when the device (200) is viewed in transmission, the first image (102) and the second image (104) are visible, and when the device (200) is viewed in reflection, the first image (102) is ) is visible and the second image (104) is substantially non-visible. The method of claim 16, wherein the first portion (208) of the relief structure (204) has a depth greater than the second portion (210) of the relief structure (204). 18. The method of claim 16 or 17, wherein the refractive index of the coating (206) is sufficiently high so that the depth of the second portion (210) of the relief structure (204) is insufficient for the second image (104) is clearly visible in reflection. The method of any one of claims 16 to 18, wherein the relief structure (204) is a binary diffraction grating. 10 The method of any one of claims 16 to 18, wherein the relief structure (204) is a multi-level diffractive structure (304). The method of claim 19 or 20, wherein the first portion (208) of the relief structure (204) has a depth which is approximately twice the depth of the second portion (210) of the relief structure ( 204). 15 The method of claim 19, wherein the first portion (208) of the relief structure (204) has a depth of approximately 1,000 nm, and the second portion (210) of the relief structure (204) has a depth of approximately 450 nm. The method of claim 20, wherein the first portion (208) of the relief structure (204) has a depth of approximately 2400 nm, and the second portion (210) of the relief structure (204). has a depth of approximately 1,200 nm. 24. The method of any one of claims 16 to 18, wherein the relief structure (204) is a diffractive structure. 25 The method of claim 24, wherein the first portion (208) of the relief structure (204) has a depth of approximately seven times the depth of the second portion (210) of the relief structure (204). The method of claim 24, wherein the first portion (208) of the relief structure (204) has a depth of approximately 50 minutes, and the second portion (210) of the relief structure (204) has a depth of approximately 7 The method of any one of claims 16 to 26, wherein the relief structure (204) is formed by embossing a radiation curable embossable ink.