An optical security device and method of manufacture

Abstract

A security device is disclosed including a substrate and one or more focusing elements or lens structures located on one side of the substrate. The security device includes a plurality of image elements associated with each focusing element wherein the image elements include at least first and second groups of image elements. Each image element may be composed of pixels located in an object plane to be viewable through the associated focusing element. Each image element comprises a diffractive grating element or sub-wavelength grating element which when illuminated by a light source generates a diffraction image observable at a range of viewing angles around the device. Image elements of the first group are visible in a first range of viewing angles and image elements of the second group are visible in a second range of viewing angles. The security device is particularly suitable for use on security documents, such as banknotes. A method of forming a security device is also disclosed.

1 . A security device including:
 a plurality of focusing elements;   a plurality of image elements associated with each focusing element wherein said image elements include at least a first and a second group of image elements,   wherein each image element is located in an object plane to be viewable through the associated focusing element,   wherein each image element comprises a diffractive grating element or sub-wavelength grating element which when illuminated by a light source generates a diffraction image observable at a range of viewing angles around the device, and image elements of the first group are visible in a first range of viewing angles and image elements of the second group are visible in a second range of viewing angles.   
     
     
         2 . A security device according to  claim 1 , wherein said image elements include three or more groups of image elements to represent an image observable from different viewing angles. 
     
     
         3 . A security device according to  claim 1  wherein the diffraction image is a greyscale or monochromatic image that includes a plurality of brightness levels across the image, and/or a color image that includes a plurality of colors. 
     
     
         4 . A security device according to  claim 1 , wherein each image element comprises red, green and blue sub-elements, and the red, green and blue sub-elements each include its own diffractive grating element or sub-wavelength grating element, wherein the frequency and/or the pitch of the diffractive grating elements or sub-wavelength grating elements are different in the red, green and blue sub-elements so that each sub-element produces a predetermined primary color upon illumination. 
     
     
         5 . (canceled) 
     
     
         6 . A security device according to  claim 4 , wherein the red, green and blue sub-elements are vertically arranged as a strip, within the red sub-element being located at the top of the vertical strip, the green sub-element being located in the middle, and the blue sub-element being located at the bottom of the strip. 
     
     
         7 . (canceled) 
     
     
         8 . A security device according to  claim 6 , wherein the red, green and blue sub-elements are of a same physical size, with the grating elements of the red, green and blue sub-elements comprising a size distribution and/or a spatial distribution corresponding to grey levels or brightness levels associated with the sub-element. 
     
     
         9 . (canceled) 
     
     
         10 . A security device according to  claim 4 , wherein each of the sub-elements includes an effective grating area that includes the diffraction grating element or the sub-wavelength grating element, and a non-diffractive area that does not include any grating elements, and a brightness value of each sub-element is varied by changing the effective grating area within each sub-element and/or the non-diffractive area of the sub-element. 
     
     
         11 . A security device according to  claim 10 , wherein the non-diffractive area within each of the red, green and blue sub-elements are of a same size for a given image element, to thereby generate a greyscale image, or that the non-diffractive area within each of the red, green and blue sub-elements are of a different size for a given image element, to thereby generate a color image. 
     
     
         12 . A security device according to  claim 1 , wherein each image element includes a randomized diffraction grating with a randomized grating pitch and/or width to diffract incident light at different angles, such that incident light diffracted from the diffraction grating is diffused and the diffraction image observable is a greyscale image. 
     
     
         13 . A security device according to  claim 12 , wherein each image element includes an effective grating area that includes the diffraction grating element or the sub wavelength grating element, and a non-diffractive area that does not include any grating element, and a brightness value of each image element is varied by changing the effective grating area and/or the non-diffractive area of the image element. 
     
     
         14 . A security device according to  claim 10 , wherein the non-diffractive area includes a microstructured surface with light traps for creating internal reflections of most incident light that then fails to reflect out and away from the light traps; or the non-diffractive area has a reflective coating for specular reflection of incident light; or the non-diffractive area has a light absorbing coating. 
     
     
         15 .- 18 . (canceled) 
     
     
         19 . A security device according to  claim 1 , wherein the diffractive grating element or sub-wavelength grating element is formed from a surface relief structure formed in a radiation curable ink, and wherein the focusing elements are formed from a radiation curable ink by printing and/or embossing. 
     
     
         20 .- 21 . (canceled) 
     
     
         22 . A security device according to  claim 1 , wherein the focusing elements include refractive or diffractive part-cylindrical lenses or zone plates. 
     
     
         23 . (canceled) 
     
     
         24 . A security device including:
 a plurality of focusing elements;   a plurality of image elements located in an object plane to be viewable through the focusing elements, said image elements including at least first and second groups of image elements,
 wherein each image element includes a surface structured for causing diffuse scattering of incident light, wherein said plurality of image elements are arranged to generate an image observable when illuminated by the incident light, and image elements of the first group are observable in a first range of viewing angles and image elements of the second group are observable in a second range of viewing angles. 
   
     
     
         25 . A security device according to  claim 24 , wherein the surface structured for causing diffuse scattering of incident light includes a randomized diffraction grating with a random grating pitch to diffract light of a given wavelength at different angles such that white incident light diffracted from the random diffractive grating is diffused and the image observable is a greyscale image. 
     
     
         26 . A security device according to  claim 24 , wherein the surface structured for causing diffuse scattering of incident light may include an array of randomly arranged micromirrors which cause incident light to be diffuse scattered in different directions. 
     
     
         27 . A security device according to  claim 25 , wherein each image element includes an effective grating area that includes the randomized diffraction grating element, and a non-diffractive area that does not include any grating element, and a brightness value of each image element is varied by changing the effective grating area and/or the non-diffractive area of the image element. 
     
     
         28 . A security device according to  claim 27 , wherein the non-diffractive area includes any one or more of:
 a microstructured surface with light traps for creating internal reflections of most incident light that then fails to reflect out and away from the light traps,   a reflective coating for specular reflection of incident light, and   a light absorbing coating.   
     
     
         29 . A security device according  claim 1 , wherein the image elements are anamorphic, and more preferably the image elements are configured to have a rectangular shape, such that the lengths of the image elements are greater than the widths. 
     
     
         30 . (canceled) 
     
     
         31 . A security device according to  claim 1 , wherein the image observable is a three dimensional image of a scene, object and/or a person. 
     
     
         32 .- 37 . (canceled)