OPTICALLY VARIABLE DEVICE
FIELD OF THE INVENTION
The present invention relates to optically variable devices. The devices are particularly suitable for use as security features in banknotes and other documents of value, but may also have other application, for example as decorative features.
BACKGROUND TO THE INVENTION
Optically variable devices are commonly used as security features. Such devices often take the form of a diffractive optically variable device (DOVD), which has a distinctive coloured and shimmering or glittering appearance due to the appearance of different diffraction orders for different wavelengths of incident light as the observation or illumination angle is changed.
Other commonly employed devices rely instead on refraction. For example, an array of interlaced portions of a plurality of two-dimensional images located at the foci of an array of lenticular or spherical microlenses can display apparent motion effects as the observation angle is varied. The portions corresponding to each image are visible at different respective observation angles so that an illusion of motion is produced. Refractive devices of this type can also in some cases be made to produce an illusion of depth, by adjusting the focal length so that the resulting image lies outside the plane of the device.
Purely refractive devices of this type have the disadvantage of adding significantly to the thickness of a security document to which they are applied, since the typical thickness of the lenses required to produce the desired effects can be of the order of 50 microns or more, which is also the typical thickness of, for example, a banknote substrate.
SUMMARY OF THE INVENTION
It is therefore desirable to provide an alternative optically variable device which gives an impression of depth to a person viewing the device, but which does not add significantly to the thickness of a document or other article to which the device is applied.
Accordingly, there is provided an optically variable device including a plurality of relief elements applied to a surface, wherein the relief elements
collectively generate, by diffraction and/or refraction and/or reflection, an image of a three-dimensional object.
Preferably, the relief elements lie on or near contour lines on the surface, the contour lines corresponding to height and/or depth levels of the object. The contour lines may include open contours corresponding to a two-dimensional surface of the object, the open contours lying on a plane which is at an angle to the surface of the device, so that the image is viewable off-axis. This has the particular benefit of providing an additional immediately recognisable security feature, since a person viewing the device would ordinarily expect the three- dimensional image to appear on-axis, but will not see the image until the device is tilted away from the on-axis position. There may also be increased ease of application of relief elements which follow open rather than closed contours.
The relief elements may include a first group of relief elements which are visible in a first range of observation angles, and a second group of relief elements which are visible in a second, different, range of observation angles.
Preferably, the first and second ranges are sufficiently close together to produce an autostereoscopic effect to the observer.
In one particularly preferred embodiment, the relief elements are segments of diffractive grooves which substantially follow the contours. This has substantial advantages over known diffractive structures, in that when it is desired to produce a three-dimensional effect which is achromatic or which is of a single colour, the segments of the grooves can be modulated so as to reduce or eliminate spurious diffraction effects.
Each segment may have a projected shape on the surface which includes at least one straight edge. The projected shape is preferably substantially polygonal. In some embodiments, the projected shape is a regular polygon.
Alternatively, each segment may have a projected shape on the surface which includes at least one arcuate section. The shape may be substantially circular, part-circular, elliptical or part-elliptical.
The maximum dimension of each relief element is preferably less than 60 microns, and even more preferably less than 1 0 microns. This provides a low- profile relief structure which appears three-dimensional but is two-dimensional to
the touch, thereby increasing the recognisability of the device as a security feature.
The projected shapes of the first group of relief elements and the projected shapes of the second group of relief elements may differ. Thus, for example, the two groups may produce different diffractive and/or reflective and/or refractive effects so that a first three-dimensional image appears at one viewing angle whilst a second three-dimensional image appears at another viewing angle.
The spacings between adjacent segments, and/or the dimensions of individual segments, may be modulated in random or pseudo-random fashion in order to ensure that unwanted diffraction orders are not present in the image seen by the observer. Alternatively, the spacings and/or dimensions of the segments may be varied according to a particular predetermined function of the spatial coordinates of the segments, for example a harmonic or periodic function.
As an alternative (or in addition) to modulation of the spacing and/or dimensions of the segments, their positions may be offset from the contours of the image surface by a predetermined distance. The predetermined distance may be modulated in random or pseudo-random fashion, or may vary according to a harmonic or periodic function.
In another preferred embodiment, the spacing between segments and/or the dimensions of the segments may be modulated in at least one region to encode a hidden image. The hidden image may be viewable by transposing a decoding screen on the at least one region, and the decoding screen may be a line screen or dot screen.
In yet another embodiment, the spacing between segments and/or the dimensions of the segments may be modulated so that at least some regions of the device are diffusely scattering. This results in at least part of the image giving a perception of achromatic surface roughness to the observer, and can be advantageous in that it allows for a more realistic impression of a three- dimensional object to be conveyed to an observer.
In one particularly preferred embodiment, the device diffracts light at angles away from the zero-order to produce the image. When the device produces a substantially achromatic image, this provides yet another distinctive security feature, since an observer viewing the security device would ordinarily
assume that the image would be visible at or near the angle of specular reflection from the device surface rather than at an oblique angle.
BRIEF DESCRIPTION OF THE DRAWINGS
Certain embodiments of the invention will now be described by reference to the accompanying drawings, in which:
Figures 1 (a) to 1 (c) show security documents including optical security devices which produce apparently three-dimensional images;
Figure 2 shows a three-dimensional object corresponding to one of the images of Figure 1 , a contour map of the three-dimensional object, and diffractive relief elements of an optical security device distributed along the contours in two different ways;
Figure 3 shows a three-dimensional image which is visible on-axis, and the contours and relief elements of the structure producing the image;
Figure 4 shows an alternative to Figure 3 in which the three-dimensional image is produced off-axis; and
Figure 5 shows an alternative embodiment of an optical security device producing a three-dimensional image by stereoscopy.
DESCRIPTION OF PREFERRED EMBODIMENTS
In Figure 1 (a), there is shown a cross-section through a security document 10 including a substrate 20. The substrate 20 may be a transparent or translucent plastics material such as biaxially oriented polyproplylene (BOPP). Opacifying layers 25 are applied to each side of the substrate 20 to facilitate the application of printing, for example the printed elements 15 which form part of a denominator or other indicia. The opacifying layers 25 are omitted on one or both sides of the substrate 20 in regions 28 in order to form a transparent half-window or window region in the security document.
The window or half-window is itself a security feature, but the security of the document is enhanced by the application of a further security device partly or wholly within the window or half-window. For example, in Figure 1 (b), only the top opacifying layer 35 of security document 1 1 is part-omitted, whilst bottom opacifying layer 36 is applied to the entire surface of the substrate 20, so that a half-window 29 is formed. A security device 50 is then applied to the upper surface 22 of the substrate, for example by an embossing method, within the half-
window 29. The security device may be applied directly into the substrate, or may be applied to a further layer of material, for example a UV-curable ink or lacquer layer which has been applied to substrate 20 in the half-window region.
Upon illumination of the security device 50 in direction 100, light is diffracted, reflected or refracted in directions 102, 103 from the relief elements 60 of the security device 50 and a person viewing the device in reflection mode (i.e., from the same side of the substrate as the light source) at either of those angles sees an image 202, 203 of a three-dimensional object. Images 202 and 203 could correspond to, for example, non-zero diffraction orders of the security device 50. It is also possible to produce an image in direction 1 00', corresponding to the direction of specular reflection, i.e. a zero-order image.
Alternatively, a security device 52 could be formed in a full window 28 as in Figure 1 (c), in which the opacifying layers 45, 46 are part-omitted on both sides of the substrate 20 in order to form the window region 28. A person viewing the security document 12 containing the security device 52 may then view a three- dimensional image 204 in transmission mode (i.e., from the opposite side of the substrate to the light source). An image (not shown) may be formed in the on-axis direction 1 10, or in an off-axis direction 1 12.
Referring now to Figure 2, three-dimensional object 201 is shown divided up into slices 212a-212e corresponding to different height levels of the object 201. Each slice has a predetermined thickness which is preferably of the order of the wavelength of light which is to be used to illuminate the security device 50.
The three-dimensional object may be a real or imagined (e.g., computer- generated) object.
Each contour 222a-222e of the contour map 230 of the two-dimensional surface of the object 202 corresponds to the base of one of the slices 21 2a-212e. The contour map 230 can be used as a template for the placement of the relief elements 60 on the upper surface 22 of substrate 20. The relief elements 60 may be continuous diffractive grooves following contours 222a-222e, or alternatively may be segmented diffractive, reflective or refractive grooves 251 , 261 , 262.
The segments 251 of the device 250 and segments 261 , 262 of the device 260, shown projected into the plane of the substrate surface 22, are of varying size and spacing. Modulation of the segment spacing and width prevents or
greatly reduces the appearance of unwanted diffraction orders in the case where the device is operating by the diffraction process, rather than by reflection or refraction. For example, if a substantially achromatic image appearing in the first order is desired, then it is preferred to ensure that the zero, second, third etc. orders do not contribute to the image to spoil its achromatic appearance.
The segments 251 of device 250 are of substantially polygonal shape (rectangles, rhombi, parallelograms etc), and lie exactly or almost exactly on the contours 222a-222e (contours not shown). In an alternative device 260, the segments 262 may be offset from the contours 222b-222e while other segments 261 lie along contour 222a. The segments may include non-polygonal elements of varying size, for example flattened ellipses 262. The spacings between adjacent segments along a contour may also be modulated as described above.
Referring now to Figures 3(a) to 3(c), there is shown a three-dimensional image 500 lying along a plane 502 and with a surface 501 having the functional form Z = f(x,y) = 0. 125(cos(1.5706x)cos(1.5706y) + 0.667( cos(1.5706x) + cos(1.5706y) ) ) where the x, y and z axes are oriented generally as shown at 510. The surface of the optically variable device is substantially parallel to the x-y plane. A relief structure 520 having a set of relief elements 521 , 522, 523, 524 lying on or near the closed contours 505 of the image produces the on-axis image 500. The relief elements may be diffractive, reflective or refractive and may take various forms, for example part-ellipses 521 , trapezoids 522, part-annuli 523, part-circles 524, etc.
If the surface 501 is rotated through an angle β in the y-z plane as shown in Figure 4(a), the three-dimensional image 550 of the object will appear to the viewer when the device is tilted off-axis by an angle β. The surface now has the functional form Z = f(x,y) = tan$)y +0.125(cos(1.5706x)cos(1.5706y) + 0.667( cos(1.5706x) + cos(i.5706y) ) ). The open contours 555 on the surface of the device along which the relief elements should be placed to produce such an image are shown in Figure 4(b). The corresponding relief elements may be of various shapes and sizes as shown in Figure 4(c), for example trapezoids 561 , parallelograms 562 and so on. Some of the relief elements are offset by a small distance relative to the corresponding contour line in order to reduce the effect of parasitic diffraction
on image brightness and achromaticity. For example, relief elements 571 , 572, 573 and 574 are each offset relative to contour line 570.
Figure 5 shows an alternative embodiment in which optical security device 600 is illuminated by light source 610. A first group of relief elements 621 diffracts, reflects or refracts light in a first direction 61 1 which is seen by the left eye 601 of an observer. Likewise, light is diffracted, reflected or refracted in a second direction 612 by a second group 622 of relief elements to be observed by the right eye 602. The difference in viewing angles 61 1 , 61 2 is small so that the net effect of diffractive structures 621 , 622 is an autostereoscopic one, giving the observer the impression that the portion of the combined image being viewed is located at a point 640 located outside the plane 650 of the device 600. Similarly, other regions of relief elements (not shown) produce a stereoscopic effect at different points to reconstruct the surface of an apparently three-dimensional object 630.
It will be recognised that various modifications to the above-described embodiments are possible. For example, the two groups 621 , 622 could be made to produce images at substantially the same observation angle, but with different polarisations, for example left- and right-handed circular polarisations or two different linear polarisations. A person viewing the device through a pair of glasses, the lenses of which act as polarisation filters for the two different polarisations, could then observe a three-dimensional effect by virtue of stereoscopy (rather than autostereoscopy).