EP-4738018-A2 - OPTICAL DEVICE AND METHOD OF MANUFACTURE THEREOF

EP4738018A2EP 4738018 A2EP4738018 A2EP 4738018A2EP-4738018-A2

Abstract

An optical device is disclosed. Upon illumination, the optical device exhibits one or more diffractive images dependent upon viewing angle, said optical device having a diffractive structure comprising a plurality of grating regions, each grating region corresponding to a component of a respective diffractive image, wherein: each grating region of the diffractive structure comprises a plurality of grating elements arranged along a respective first direction, each grating element having a principal component of orientation within the plane of the device that is substantially orthogonal to said respective first direction; wherein, the grating elements within each grating region have a constant pitch and substantially the same orientation such that each grating region, upon illumination, exhibits a diffractive colour such that the corresponding diffractive image is exhibited; wherein, the diffractive structure comprises first and second grating regions that are both elongate along a common first direction, said first and second grating regions being adjacent along said common first direction, and wherein the pitch and/or orientation of the grating elements of the first and second grating regions are different. A method of forming the optical device is also disclosed.

Inventors

HOLMES, BRIAN
FOURNIER, FREDERIC

Assignees

De La Rue International Limited

Dates

Publication Date: 20260506
Application Date: 20201009

Claims (15)

An optical device that, upon illumination, exhibits one or more diffractive images dependent upon viewing angle, said optical device having a diffractive structure comprising a plurality of grating regions, each grating region corresponding to a component of a respective diffractive image; wherein each grating region comprises a plurality of grating elements arranged along a respective first direction such that each grating region, upon illumination, exhibits a diffractive colour such that the corresponding diffractive image is exhibited, and wherein, for at least one diffractive image: the grating regions are arranged in accordance with a plurality of diffractive pixels of the corresponding diffractive image, each diffractive pixel being assigned a uniform colour, and each grating region covers a proportion of a diffractive pixel corresponding to the proportion of the diffractive colour of the grating region to be exhibited by the diffractive pixel such that the corresponding diffractive image is exhibited; and wherein, at least one grating region covers the proportion of its respective diffractive pixel in accordance with a dithering arrangement.
The optical device of claim 1, wherein for at least one diffractive image; the diffractive pixels are arranged as an arrangement of image elements, wherein each image element comprises at least one diffractive pixel corresponding to a first colour channel and at least one diffractive pixel corresponding to a second colour channel; wherein, the diffractive structure comprises a plurality of first grating regions that upon illumination exhibit a diffractive colour corresponding to the first colour channel, and a plurality of second grating regions that upon illumination exhibit a diffractive colour corresponding to the second colour channel; wherein, at least one first grating region covers a proportion of a first diffractive pixel corresponding to the proportion of the first colour channel to be exhibited by the associated image element in accordance with a dithering arrangement, and at least one second grating region covers a proportion of a second diffractive pixel corresponding to the proportion of the second colour channel to be exhibited by the associated image element in accordance with a dithering arrangement, such that each image element exhibits assigned uniform colour.
The optical device of claim 2, wherein for at least one diffractive image, each image element further comprises at least one third diffractive pixel corresponding to a third colour channel; wherein. the diffractive structure further comprises a plurality of third grating regions that upon illumination exhibit a diffractive colour corresponding to the third colour channel; wherein, at least one third grating region covers a proportion of a third diffractive pixel corresponding to the proportion of the third colour channel to be exhibited by the associated image element in accordance with a dithering arrangement such that the image element exhibits a uniform colour; preferably wherein the first, second and third colour channels correspond to red, green and blue colour channels.
The optical device of any of claims 1 to 3, wherein the dithering arrangement comprises a plurality of dithering elements within which no grating elements are present.
The optical device of claim 4, wherein each dithering element has a dimension in a cross direction orthogonal to the arrangement of grating elements of the respective diffractive pixel of at least two times smaller than the dimension of the respective diffractive pixel along said cross direction.
The optical device of claim 4 or claim 5, wherein the density of the dithering elements varies within a diffractive pixel.
The optical device of any of the preceding claims, wherein the optical device does not comprise any focussing elements,
A method of forming an optical device that, upon illumination, exhibits one or more diffractive images dependent on viewing angle, the method comprising: forming a diffractive structure in a carrier layer, said diffractive structure comprising a plurality of grating regions, each grating region corresponding to a component of a respective diffractive image; wherein each grating region comprises a plurality of grating elements arranged along a respective first direction such that each grating region, upon illumination, exhibits a diffractive colour such that the corresponding diffractive image is exhibited, and wherein, for at least one diffractive image: the grating regions are arranged in accordance with a plurality of diffractive pixels of the corresponding diffractive image, each diffractive pixel being assigned a colour, and each grating region covers a proportion of a diffractive pixel corresponding to the proportion of the diffractive colour of the grating region to be exhibited by the diffractive pixel such that the corresponding diffractive image is exhibited; and wherein, at least one grating region covers the proportion of its respective diffractive pixel in accordance with a dithering arrangement.
The method of claim 8, wherein the diffractive structure is formed in accordance with a template defining a plurality of diffractive pixels of the one or more diffractive images to be exhibited by the device.
The method of claim 8, wherein the template is generated by, for each of the one or more diffractive images: providing an arrangement of grid pixels; providing a source colour image comprising a plurality of image pixels, each image pixel being assigned a colour; based on the plurality of image pixels of the source image, generating a mask defining masked regions and non-masked regions of the grid pixels, wherein the non-masked regions define proportions of the grid pixels required to generate a version of the source image; and applying the mask to the arrangement of grid pixels so as to generate the template.
The method of claim 10, wherein the arrangement of grid pixels comprises first grid pixels corresponding to a first colour channel and second grid pixels corresponding to a second colour channel, and wherein preferably the arrangement of grid pixels further comprises third grid pixels corresponding to a third colour channel, said first, second and third colour channels correspond to red, green and blue colour channels.
The method of any of claims 8 to 11, wherein the dithering arrangement comprises a plurality of dithering elements within which no grating elements are present, preferably wherein the density of the dithering elements varies within a diffractive pixel.
The optical device of any of claims 1 to 7, wherein the optical device is a security device.
A security article comprising a security device according to claim 13, wherein the security article is preferably formed as a security thread, strip, foil, insert, label, patch, or a substrate for a security document.
A security document comprising a security device according to claim 13, or a security article according to claim 14, the security document preferably comprising a banknote, cheque, passport, identity card, certificate of authenticity, fiscal stamp or another document for securing value or personal identity.

Description

FIELD OF THE INVENTION The present invention is directed to optical devices and their method of manufacture. The invention is particularly applicable to the field of security devices, where such optical devices may be used as means of determining the authenticity of a security document such as a banknote or passport. BACKGROUND TO THE INVENTION Articles of value, and particularly documents of value such as banknotes, cheques, passports, identification documents, certificates and licences, are frequently the target of counterfeiters and persons wishing to make fraudulent copies thereof and/or changes to any data contained therein. Typically such documents are provided with a number of visible optical devices acting as security devices for checking the authenticity of the object. By "security device" we mean a feature which is not possible to reproduce accurately by taking a visible light copy, e.g. through the use of standardly available photocopying or scanning equipment. Examples include features based on one or more patterns such as microtext, fine line patterns, latent images, venetian blind devices, lenticular devices, moiré interference devices and moiré magnification devices, each of which generates a secure visual effect. Other known security devices include holograms, watermarks, embossings, perforations and the use of colour-shifting or luminescent / fluorescent inks. Common to all such devices is that the visual effect exhibited by the device is extremely difficult, or impossible, to copy using available reproduction techniques such as photocopying. Security devices exhibiting non-visible effects such as magnetic materials may also be employed. One class of optical devices are those which produce an optically variable effect, meaning that the appearance of the device is different at different angles of view and/or illumination. Such devices are particularly effective as security devices since direct copies (e.g. photocopies) will not produce the optically variable effect and hence can be readily distinguished from genuine devices. Optically variable effects can be generated based on various different mechanisms, including holograms and other diffractive devices, moire interference and other mechanisms relying on parallax such as venetian blind devices, and also devices which make use of focusing elements such as lenses, including moiré magnifier devices, integral imaging devices and so-called lenticular devices. The present invention is directed to diffractive devices that produce an optically variable effect. When white light is incident upon a diffraction grating, it is split into its constituent wavelengths (i.e. colours) according to the diffraction grating equation: Psinθi±sinθd=mλ, where P is the pitch of the grating elements of the diffraction grating, m is the diffraction order, λ is the wavelength of light, and θi and θd are the angle of incidence and diffraction, respectively, of light measured relative to the normal of the diffraction grating. When light is incident on the diffraction grating along a direction that is parallel to the diffraction grating normal, the equation simplifies to: Psinθd=mλ, This effect can be used to generate optical devices that exhibit colour images at particular viewing angles (corresponding to the angle of diffraction), with diffraction gratings of differing pitch being formed as an array of pixels such that, when illuminated, the device exhibits an image in accordance with the array of pixels. As discussed above, the wavelength of light (and hence colour) exhibited by a diffraction grating upon illumination is dependent on the pitch, P, of the grating elements of the grating. The perceived quality (saturation and brightness) of the colour exhibited by a diffraction grating is dependent on the number of grating elements within that grating. Firstly, the resolving power of a grating is a function of the number of grating elements within that grating. In more detail, when a diffraction grating is illuminated by polychromatic incident light, for a given wavelength λ, the number of grating elements illuminated by the incident light determines the closest resolvable wavelength, λ+Δλ, with the resolving power R of the grating given by R = λ/Δλ = mN, where m is the order of diffraction and N is the number of illuminated grating elements. Hence, the resolving power of a diffraction grating increases with the number of grating elements. Therefore, an increased number of grating elements within a diffraction grating improves the ability of the viewer to resolve different colours in the replayed image. For the purposes of this specification, we will refer to an increase in the resolving power of a grating region as providing an increase in the "saturation" of the exhibited diffractive colour. Secondly, the brightness of the light diffracted from a diffraction grating is also dependent on the number of grating elements present. When a diffraction grating is illuminated