OPTICAL SECURITY DEVICE

Abstract

An optical security device (1) comprising a recorded or encoded representation of an optically variable image (40), the image (40) being reconstructable and viewable for security, authentication or identification purposes, the device (1) comprising: an optical structure (30) in which there is recorded or encoded a representation of the optically variable image (40) in the form of a plurality of discrete portions (e.g. 50) of the complete image (40) to be reconstructed, wherein the respective said plurality of portions (e.g. 50) of the optically variable image (40) can, when reconstructed, only be viewed in their totality by viewing the said image (40) at or over a plurality of different viewing angles or angle ranges (70) relative to the optical structure (30) and/or at or over a plurality of different angles or range of angles of illumination of the optical structure (10), and wherein the size, in at least one dimension, direction, axis or plane, of the complete image (40), when reconstructed, is greater than the corresponding size, in the same, or the respective same, dimension, direction, axis or plane, of a part of the optical structure (30) having the said plurality of portions (e.g. 50) of the image (40) collectively recorded or encoded therein.

Claims

1. An optical security device comprising a recorded or encoded representation of an optically variable image, the image being reconstructable and viewable for security, authentication or identification purposes, the device comprising: an optical structure in which there is recorded or encoded a representation of the optically variable image in the form of a plurality of portions of the complete image to be reconstructed, wherein the respective said plurality of portions of the optically variable image can, when reconstructed, only be viewed in their totality by viewing the said image at or over a plurality of different viewing angles or angle ranges relative to the optical structure and/or at or over a plurality of different angles or ranges of angles of illumination of the optical structure, and wherein the size, in at least one dimension, direction, axis or plane, of the complete image, when reconstructed, is greater than the corresponding size, in the same, or the respective same, dimension, direction, axis or plane, of a part of the optical structure having the said plurality of portions of the image collectively recorded or encoded therein.

2. An optical security device according to claim 1, wherein the optical structure has recorded or encoded therein a representation of the optically variable image in the form of a plurality of portions of the complete image to be reconstructed, wherein each respective one of some or all of the said plurality of portions of the optically variable image can, when reconstructed, only be viewed in full by viewing the said respective image portion at or over a respective range of viewing angles relative to the optical structure and/or at or over a range of angles of illumination of the optical structure.

3. An optical security device according to claim 2, wherein either: (i) each respective one of some or all of the said plurality of portions of the optically variable image can, when reconstructed, only be viewed in full by viewing the said respective image portion at or over a respective range of viewing angles relative to the optical structure and at a respective given angle of illumination of the optical structure; or (ii) each respective one of some or all of the said plurality of portions of the optically variable image can, when reconstructed, only be viewed in full by viewing the said respective image portion at a respective given viewing angle and at or over a respective range of angles of illumination of the optical structure.

4. An optical security device according to claim 1, wherein the optical structure has recorded or encoded therein a representation of the optically variable image in the form of a plurality of discrete portions of the complete image to be reconstructed, wherein each respective one of the said plurality of portions of the optically variable image can, when reconstructed, only be viewed by viewing the respective said image portion at or over a respective viewing angle or angle range relative to the optical structure and/or angle or angle range of illumination of the optical structure which is different from the viewing angle/angle range and/or angle/angle range of illumination at or over which at least one other of the said image portions is viewable when reconstructed.

5. An optical security device according to claim 2 to, wherein either: (i) the respective viewing angle/angle range and/or angle/angle range of illumination at or over which a, or any, given one of the image portions can be viewed, when reconstructed, is substantially the same viewing angle/angle range and/or angle/angle range of illumination at or over which at least one other of the said image portions, but not all of those other said image portions, (optionally or at least some of, but not all of, the remaining reconstructed image portions) is/are viewable when reconstructed; or (ii) the respective viewing angle/angle range and/or angle/angle range of illumination at or over which a, or any, given one of the image portions can be viewed, when reconstructed, is substantially different from the respective viewing angle(s)/angle range(s) and/or angle(s)/angle range(s) of illumination at or over which at least one other of the of the reconstructed image portions (optionally or at least some of the remaining reconstructed image portions) is/are viewable; whereby, in either case (i) or (ii), in order for the complete reconstructed image to be viewable in its totality, such that all the portions of the complete reconstructed image are each respectively reconstructable and viewable, it is necessary for at least two different viewing angles/angle ranges and/or angles/angle ranges of illumination to be employed to view all the reconstructed image portions.

6. An optical security device according to claim 2 to, wherein the respective viewing angle/angle range and/or angle/angle range of illumination at or over which a, or any, given one of the image portions can be viewed, when reconstructed, is substantially different from the respective viewing angles/angle ranges and/or angles/angle ranges of illumination at or over which substantially all the other reconstructed image portions are viewable; whereby in order for the complete reconstructed image to be viewable in its totality, such that all the portions of the complete reconstructed image are each respectively reconstructable and viewable, it is necessary for a plurality of different viewing angles/angle ranges and/or angles/angle ranges of illumination to be employed to view respective ones of the plurality of reconstructed image portions; optionally wherein each respective viewing angle/angle range and/or angle/angle range of illumination at or over which a respective reconstructed image portion is viewable is unique to that respective reconstructed image portion.

7. (canceled)

8. An optical security device according to claim 1, wherein either: (i) the size, in at least one dimension, direction, axis or plane, of one or more given ones of the viewable portions of the reconstructed image is greater than the corresponding size, in the same, or the respective same, dimension, direction, axis or plane, of a part of the optical structure having the said one or more given optically variable image portion(s) recorded or encoded therein; or (ii) the size, in at least one dimension, direction, axis or plane, of a given part of the optical structure having one or more given ones of the optically variable image portion(s) recorded or encoded therein is smaller than the corresponding size, in the same, or the respective same, dimension, direction, axis or plane, of the said one or more respective viewable portion(s) of the reconstructed image; optionally wherein the size differential of the or the respective reconstructed image portion relative to the or the respective recorded or encoded image portion of the structure is such that the respective reconstructed image portion is any of: >1 times, or 2 times, or ≥3 times, or ≥4 times, or ≥5 times, or ≥6 times, or ≥7 times, or ≥8 times, or ≥9 times, or ≥10 times, or ≥12 times or ≥13 times or ≥15 times or ≥18 times or ≥20 times, the size of the respective recorded image portion of the structure (wherein any of said ranges include fractions as well as whole number multiples).

9. (canceled)

10. An optical security device according to claim 1, wherein the angular size of the reconstructed image is >1 times, optionally ≥2 or ≥3 times, further optionally ≥10 times, yet further optionally ≥15 or ≥20 times (which ranges include fractional multiples as well as whole number multiples) the angular size of the optical structure when viewed from a given viewing distance and/or location therefrom.

11. An optical security device according to claim 1, wherein the recorded or encoded representation of the optically variable image is a hologram.

12. An optical security device according to claim 1, wherein either: (i) the recorded or encoded representation of the optically variable image is a two-dimensional (2-D) or a three-dimensional (3-D) hologram, and the optically variable image is an image of one or more 2-D or 3-D objects, pictures, patterns, one or more alphanumeric or other typographical characters, or any combination of any two or more of any of the foregoing things; or (ii) the optical structure having the portion(s) of the optically variable image recorded or encoded therein is or comprises a DOVID (diffractive optically variable image device).

13. An optical security device according to claim 1, wherein the optical structure defines a structure plane, optionally a plane which is contained within the thickness of the optical structure and/or which is non-parallel to light incident on the optical structure during the reconstruction of the recorded or encoded image, and the reconstructed image lies wholly or at least partially to one side of, optionally or to each of both sides of, the said structure plane; optionally wherein the reconstructed image lies out of the structure plane on the side thereof opposite to that from which the illuminating light is incident thereon.

14. (canceled)

15. An optical security device according to claim 1, wherein either: (i) the device takes the form of a security feature which has already been applied to or incorporated into the structure of an item or object whose security, authentication or identification is required; or (ii) the device is provided in the form of a discrete security device or security element for application, affixation or incorporation into the structure of an item or object whose security, authentication or identification is required.

16. An optical security device according to claim 1, wherein the optical structure in which is recorded or encoded the representation of the optically variable image is, or is contained within or on, a body of optically active material, wherein the body of optically active material comprises a sheet or plate or film or layer of the optically active material with a thickness range of from 0.1 or 0.5 or 1 or 5 or 10 up to 100 or 200 or 300 or 400 or 500 or 800 or 1000 μm; optionally wherein a portion of the body of optically active material in which is recorded or encoded the representation of the optically variable image further comprises one or more additional or auxiliary overt or covert security features formed by modulations of the surface, volume or internal optical properties of the said body portion and comprising one or more graphics, holograms, micro- or nano-graphics, diffractive or non-diffractive images and structures, or hidden images.

17. (canceled)

18. A method for the production of an optical security device according to claim 1, the method comprising: recording or encoding in an optical structure a recorded or encoded representation of the optically variable image in the form of a plurality of portions of the complete image to be reconstructed, wherein the respective said plurality of portions of the optically variable image can, when reconstructed, only be viewed in their totality by viewing the said image at or over a plurality of different viewing angles or angle ranges relative to the optical structure and/or at or over a plurality of different angles or ranges of angles of illumination of the optical structure, and wherein the size, in at least one dimension, direction, axis or plane, of the complete image, when reconstructed, is greater than the corresponding size, in the same, or the respective same, dimension, direction, axis or plane, of a part of the optical structure having the said plurality of portions of the image collectively recorded or encoded therein; optionally wherein either: (i) wherein the method comprises forming the optical structure, having the said plurality of portions of the record or encoded representation of the complete optically variable image recorded or encoded therein, as an optical security feature, and either simultaneously or subsequently to that recording or encoding incorporating the said optical security feature into an item or object whose security, authentication or identification is required; or (ii) wherein the method comprises forming the optical structure, having the said plurality of portions of the record or encoded representation of the complete optically variable image recorded or encoded therein, as a discrete optical security device or security element, and the method further comprises a step of applying, affixing or incorporating the optical security device or element onto or into the structure of an item or object whose security, authentication or identification is required.

19. (canceled)

20. (canceled)

21. A method of authenticating or identifying an item or object having applied or affixed thereto or incorporated into the structure thereof an optical security device according to claim 1, the method comprising: (a) reconstructing the said plurality of portions of the complete optically variable image recorded or encoded in the optical structure of the device; and (b) viewing one or more of the said plurality of reconstructed portions of the complete image.

22. A method according to claim 21, wherein the optical security device is an optical security device in which: the optical structure has recorded or encoded therein a representation of the optically variable image in the form of a plurality of discrete portions of the complete image to be reconstructed, and: (i) each respective one of the said plurality of portions of the optically variable image can, when reconstructed, only be viewed by viewing the respective said image portion at or over a respective viewing angle or angle range relative to the optical structure and/or angle or angle range of illumination of the optical structure which is different from the viewing angle/angle range and/or angle/angle range of illumination at or over which at least one other of the said image portions is viewable when reconstructed; or (ii) the respective viewing angle/angle range and/or angle/angle range of illumination at or over which a, or any, given one of the image portions can be viewed, when reconstructed, is substantially the same viewing angle/angle range and/or angle/angle range of illumination at or over which at least one other of the said image portions, but not all of those other said image portions, (optionally or at least some of, but not all of, the remaining reconstructed image portions) is/are viewable when reconstructed; or (iii) the respective viewing angle/angle range and/or angle/angle range of illumination at or over which a, or any, given one of the image portions can be viewed, when reconstructed, is substantially different from the respective viewing angle(s)/angle range(s) and/or angle(s)/angle range(s) of illumination at or over which at least one other of the of the reconstructed image portions (optionally or at least some of the remaining reconstructed image portions) is/are viewable; wherein the viewing step (b) comprises: (bi) viewing the or each respective one of the plurality of reconstructed portions of the complete optically variable image at or over, or only at or over, the or the respective viewing angle or angle range relative to the optical structure, and/or by illuminating the optical structure at or over the respective angle or angle range of illumination, that is associated with that respective reconstructed image portion of the complete image; and/or (bii) viewing respective ones of the plurality of reconstructed portions of the complete optically variable image at or over, or only at or over, respective ones of the plurality of different viewing angles/angle ranges and/or angles/angle ranges of illumination of the optical structure; whereby the complete reconstructed optically variable image is viewable in its totality, such that all the portions of the complete reconstructed image are each respectively reconstructable and viewable, by, or only by: (ci) viewing the or each respective portion thereof at or over, or only at or over, the or the respective viewing angle/angle range and/or angle/angle range of illumination of the optical structure that is associated with that respective image portion, and/or (cii) viewing respective ones of the plurality of reconstructed portions thereof at or over, or only at or over, the respective ones of the plurality of different viewing angles/angle ranges and/or angles/angle ranges of illumination of the optical structure, as the case may be.

23. A method according to claim 21, wherein the recorded or encoded optically variable image, or respective portions thereof, is reconstructed by illumination of the optical structure, or a respective part or portion or region thereof, by electromagnetic radiation, the electromagnetic radiation being of a wavelength/frequency appropriate to the means used to record/encode and/or intended for reconstructing the image, optionally wherein the electromagnetic radiation comprises visible light.

24. A method according to claim 21, wherein one of the following (i), (ii) or (iii) is present or satisfied: (i) the viewing step (b) is carried out by: (ia) visual viewing of at least a or a respective portion of the image by one or more eyes of a human observer, or (ib) detecting at least a or a respective portion of the image using an optically sensitive device, optionally in combination with suitable image processing hardware and/or software; or (ii) the viewing step (b) comprises: viewing a first one of the plurality of portions of the reconstructed image by means of a first viewing step or operation in which just the first portion only of the reconstructed image is viewed; and optionally one or more additional or further viewing steps or operations in each of which a respective additional or further one of the plurality of portions of the reconstructed image is viewed; optionally wherein in each of the said viewing steps or operations the viewing of each respective portion of the reconstructed image is carried out by viewing each respective portion of the image at or over a given, or at or over a unique, viewing angle or angle range relative to the optical structure and/or by illuminating the optical structure at or over a given, or at or over a unique, angle or angle range of illumination that is associated with that respective image portion; or (iii) both of the above (i) and (ii) are present or satisfied.

25. (canceled)

26. (canceled)

27. A method according to claim 21, wherein the overall complete reconstructed image is of such a size, greater than that of the parts of the optical structure in which are recorded or encoded the various portions of the image, that it is only viewable in its entirety upon a plurality of individual or discrete viewing steps or operations being carried out, each such viewing step or operation being such as to view a respective portion only, optionally a respective discrete portion only, of the complete reconstructed image, and further optionally a respective discrete portion only of the complete reconstructed image independently of the viewing of any of the other portion(s) of the complete reconstructed image.

28. A method according to claim 21, wherein: plural portions of the complete reconstructed image are viewable independently of each other, and wherein the viewing of the complete reconstructed image is accomplished by illuminating corresponding respective portions of the recorded or encoded image in a plurality of discrete illumination steps or operations, wherein the viewing of the complete reconstructed image is accomplished by illuminating the recorded or encoded image sequentially in a series of discrete viewing steps or operations, each at or over a respective selected one of a plurality of different angles of incidence or ranges of angles of incidence of the incoming light, such that each respective incident angle or angle range corresponds to and enables the viewing of a respective portion of the reconstructed image at or over a respective different viewing angle/angle range relative to the optical structure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0083] Some embodiments of the invention in its various aspects will now be described in detail, by way of example only, with reference to the accompanying drawings, in which:

[0084] FIG. 1 is a perspective schematic view of an optical arrangement or system incorporating an optical security device according to one embodiment of the invention, showing the general arrangement of the arrangement or system's main features and the manner in which one given portion of the security image is reconstructable and viewable;

[0085] FIGS. 2(a), (b), (c), (d), (e) and (f) are face-on views of various schematic examples of other optical security features containing various different configurations of optical structure for incorporating recorded or encoded holographic security images for reconstruction and viewing, for use in optical security devices according to various embodiments of the invention;

[0086] FIG. 3 is a perspective schematic view of a more advanced optical arrangement or system incorporating an optical security device according to another embodiment of the invention, showing the general arrangement of the arrangement or system's main features and illustrating the manner in which a complete plural-component security image is reconstructable from its component portions, and viewable as the complete reconstructed image, by independent illumination and viewing of the recorded image at different respective illumination and viewing angles; and

[0087] FIG. 4 is a perspective schematic view of another optical arrangement or system embodying an example optical security device of the invention, showing in general terms the manner in which different portions of a given three-dimensional image recorded/encoded in the device are reconstructed and viewed from different viewing angles.

DETAILED DESCRIPTION OF EMBODIMENTS

[0088] Referring to FIG. 1, this shows a schematic representation of an optical system, in accordance with an example embodiment of the invention, for reconstructing a holographic image recorded in an optical structure of a security device, in which the reconstructed image is significantly larger in size than the actual recorded image, i.e. that portion of the optical structure which has the recorded image recorded within it. This FIG. 1 is actually a “snapshot” representation of the optical structure and one portion only of the complete security image—namely the stem portion only of a complete wine glass—in the process of being reconstructed and viewed.

[0089] As shown in FIG. 1, a security device 1 comprises a generally planar body 4, e.g. a sheet or film or other relatively thin layer, of optically active material comprising a recorded or encoded optically variable image within an optical structure 30 contained in an area or region of the body 4 preferably elongated in one (i.e. a side-to-side longitudinal) direction, such as in the form of a stripe 10. The optical structure 30 contains an encoded holographic image recorded therein, which in this illustrative schematic example is a recorded or encoded representation of an image of a wine glass 40. Upon reconstruction of the image encoded in the structure 30 by incident light, the reconstructed image 40 appears to be floating in space out of (i.e. behind) the structure plane of the optical structure-containing stripe 10, and its characteristic size is several times larger than the optical structure-containing stripe 10 in which the optically variable holographic image is recorded, i.e. several times larger than that portion of the stripe 10 in which is recorded the optically variable holographic image, at least in the stripe 10's smaller width dimension 20. As a result, the reconstructed holographic image 40 can be viewed in its entirety only when an observer 60 changes its/their viewing angle 70 typically in the plane of the shorter dimension 20 of the optical structure-containing stripe 10.

[0090] By way of example, the width of the stripe 10 may be relatively narrow, such as of the order of approximately 1 to 3 mm, e.g. around ˜1 mm, whereas the corresponding width, in the same dimension/direction, of the reconstructed image 40 may be relatively wide or tall, such as of the order of approximately 1-5 cm, e.g. around ˜1 cm. This size differential thus enables only a minor proportion 50 of the reconstructed image 40 to be viewable by an observer 60 at any single given viewing position or angle (relative to the device's structure plane). Moreover the size of the reconstructed image 40 may be independent of the size of the hologram 30 itself. Furthermore, it may be noted that the size of the reconstructed image 40 may be independent of the distance between the observer 60 and the plane of the optical structure (stripe) 10, although the size (and/or extent, relative to the whole) of the visible portion 50 of the reconstructed image 40 may be dependent on that viewing distance.

[0091] Thus, and in accordance with embodiments of this invention, under normal viewing conditions by the observer 60 only a relatively small or partial portion—e.g. that represented by the portion 50, that being the stem of the wine glass—of the complete reconstructed image 40 (i.e. the complete wine glass) is viewable by the observer 60 at a single given viewing angle, as represented by the viewer 60's as-drawn current position shown in the drawing.

[0092] Thus, if the observer 60 wishes or needs to view other portions of the complete image 40—i.e. other portions of the wine glass—then they need to do so at other respective different viewing angles (not shown in the drawing), whereby other parts of the wine glass, e.g. its bowl or its base, can only be viewed at such respective other viewing angles.

[0093] The characteristic feature of the invention that the reconstructed image needs to be larger (in a corresponding, or respective corresponding, direction, dimension, axis or plane) than the optical structure in which it is recorded or encoded, so that the observer is able to see only a relatively small or partial portion of the image (for example as described in the three preceding paragraphs, in the context of one illustrative embodiment), also implies that the designer of optical security devices embodying the present invention generally may need to take into consideration an intended or optimum viewing distance when designing the size of the reconstructed image and the size of the optical structure. At the intended or optimum viewing distance from the optical structure (as determined also by the designer), i.e. at the intended or optimum location of the observer, the angular size of the reconstructed image may need to be a multiple (e.g. an integer multiple greater than 1 or a fractional multiple (i.e. other than a whole number) greater than 1) of the angular size of the optical structure in the corresponding direction, dimension, axis or plane. (In general terms, by “angular size” is meant the angle under which an object is viewable or viewed at a specific distance from it.) For example, in many embodiments of the invention the angular size of the reconstructed image may be any of ≥1 times, or ≥2 times, or ≥3 times, or ≥4 times, or ≥5 times, or ≥6 times, or ≥7 times, or ≥8 times, or ≥9 times, or ≥10 times, or perhaps even as much as ≥12 or ≥13 or ≥15 or ≥18 times or even ≥20 times (which ranges include fractional multiples as well as whole number multiples), the angular size of the optical structure when viewed from the same location and/or viewing distance therefrom.

[0094] The basic stripe 10 may be formed as a standard security device as is often incorporated into printed documents or securities, such as banknotes, transportation tickets, event or other tickets, tax stamps, credit and debit cards, passports, visas, ID cards, or authentication features of branded (e.g. “luxury”) goods. Such a stripe 10 may be straight, curved, or formed by a combination of various graphical shapes, or otherwise structured, while its width 20 is characteristically smaller than its longitudinal length. The width of the stripe 10 can be constant or variable along the length of the stripe 10. Alternatively, such a stripe in which the holographic image is recorded or encoded may instead take a different overall geometric form, such as a circular, elliptical, polygonal or other regularly or irregularly shaped patch, land, region or portion of the body 4 of the optically active material.

[0095] Some examples of such various configurations and arrangements of stripes or patches or lands 10 in which the hologram or other recorded or encoded representation of the optically variable image is contained are illustrated in FIGS. 2(a)-(f).

[0096] In addition to the characteristic holographic image 30 recorded or encoded in the optical structure of the stripe or patch or land 10, the stripe or patch or land 10 may further contain any number of, even a large number of, conventional auxiliary security features such as any suitable known overt or covert features, e.g. graphics, holograms, micro- or nano-graphics, diffractive or non-diffractive images and structures, hidden images, holograms, and suchlike. These features may be incorporated into the material of the stripe or patch or land 10 in a conventional manner, e.g. typically in a form of modulation of its surface, volume or optical properties. The presence of these auxiliary security features, at least one or more of which may for example be more readily visible to the naked eye than any portion of the reconstructed image of the characteristic recorded holographic image which characterises embodiments of this invention, may be useful in making it more difficult fora casual observer or viewer to discern the presence in the stripe or patch or land 10 of the characteristic recorded image recorded or encoded in the optical structure 30 central to this invention, especially when viewed under conditions in which a significant proportion of the reconstructed image 40 is not visible and/or its presence may not be suspected.

[0097] Thus, the hologram structure 30 contained in the stripe or patch or land 10 contains a recorded or encoded image which, when reconstructed, is located out of the hologram plane—which is to say, in front of or behind the general plane of the hologram structure 30 within the stripe, patch or land 10 (or the body 4), and at such a distance from that plane that only a minor portion 50 of the complete reconstructed image 40 in the direction of the stripe, patch or land width 20 can be viewed by an observer 60, as illustrated schematically in FIG. 1.

[0098] In practising some embodiments of the invention the observer 60 may be positioned relative to the hologram structure 30 at a suitable optimum distance therefrom to allow a correct viewing of the portion 50 of the reconstructed image 40, which distance may be termed a “standard observing distance”. In some practical example embodiments that distance may be of the order of from around 10 or 15 to around 30 cm, e.g. around 25 cm. By moving the observer 60's position in a direction 70 generally parallel to the stripe width 20, the observer 60 can see different minor portions 50 etc of the overall image 40 and eventually—once the observer's viewing position has covered a sufficient distance—recognise the object the overall reconstructed holographic image 40 represents.

[0099] In further developed forms of some embodiments of the invention, the hologram structure 30 may be designed in such a way that under standard, everyday office or daylight lighting conditions the reconstructed holographic image 40, or any given portion thereof, is itself not clearly recognisable—for example it may be designed to be blurred or otherwise disguised, hidden or camouflaged under such conditions. However, at the same time it may be so designed that only once the hologram has been illuminated by a predetermined “correct” lighting condition, e.g. using a “point” light source—i.e. a source with a real or virtual emitting area significantly smaller (typically at least 10×) relative to the distance from illuminated object, or a source emitting collimated or quasi-collimated light—does the reconstructed image, or any given portion thereof, becomes recognizable, e.g. sharp or focused enough, to be recognisable. Such a “point” light source may be defined as one whose notional source is for most practical purposes able to be considered as being at infinity, i.e. one whose rays are generally approximately parallel to one another.

[0100] In practising such further developed embodiment forms of the invention it may be up to the hologram designer, i.e. the person skilled in the art, to determine the optimum observation conditions—e.g. white vs. monochromatic light, illumination angle, observation angle, “standard” (i.e. optimum) observation distance—and to choose an appropriate object (or objects) to be recorded or encoded into the structure 30, such as whether a 3-D object or a string of 2D characters or text, or even a combination of multiple such objects. The hologram structure 30 may also be designed such that it reveals different object images when illuminated or observed from different specific directions or with the use of monochromatic or quasi-monochromatic (i.e. of a narrow wavelength bandwidth, e.g. typically <20 nm wide, or perhaps even as narrow as <1 nm wide, as is the case with many lasers) light or a combination of plural light sources of different wavelengths (or frequencies).

[0101] Turning to FIG. 3, here there is shown a more advanced optical arrangement or system embodying the present invention, in which plural portions of a holographic image are observed at respective different predetermined configurations of the light source, viewer and optical structure. The FIG. 3 also illustrates the manner in which, in this example embodiment, in contrast with the simpler and more basic embodiment of FIG. 1, the portions of the reconstructed image may be combined into a complete final image using a predetermined key (e.g. an algorithm).

[0102] As shown in FIG. 3, a security device 1 comprises a planar body 4, e.g. a sheet or film or other relatively thin layer, of optically active material comprising an optically variable image-containing optical structure 30 contained in an area or region elongated in one direction, such as in the form of a stripe 10. The optical structure 30 contains an encoded holographic image recorded therein, which in this illustrative schematic example is an image of three letters “ABC” 40. This image cannot be viewed in a simple sequence of viewing steps, for example only by changing the viewing angle as shown in the embodiment of FIG. 2.

[0103] Instead, in this embodiment plural predetermined configurations of light sources 81, 82, 83 and of plural viewers 61, 62, 63, 64 in combination with respective viewer motions 71 and 72 and the hologram structure 30 have to be used to reconstruct respective plural portions of the holographic image 51, 52, 53, 54, which are respective portions of the three letters “ABC” 41.

[0104] In more detail, the first viewing step comprises using a light source 81 configured to illuminate structure 30 and to reconstruct the letter “A” and the top of the letter “C”, a viewer 61 being positioned at a predetermined location so as to be able to view a portion of this letter 51, and using motion 71 (i.e. a predetermined continuous sequence of viewing steps) the viewer eventually views the entire letter.

[0105] The next viewing step comprises using the same configuration of the light source 81 (reconstructing the letter “A” and the top portion of the letter “C”), and the viewer 64 then is re-positioned to a different predetermined location so as to be able to view the top portion of the letter “C” 54 in its entirety.

[0106] The following viewing step comprises using light source 82 configured to illuminate structure 30 to reconstruct the letter “B”, the viewer 63 now being re-positioned again to a different predetermined location so as to be able to view the whole letter “B” 53.

[0107] The last sequence of viewing steps comprises using light source 83 configured to illuminate structure 30 to reconstruct the central and bottom portions of the letter “C”, the viewer 62 now being re-positioned to another predetermined location so as to be able to view a portion of the letter “C” 52, and using motion 72 (i.e. a predetermined continuous sequence of viewing steps) the viewer eventually views the entire central and bottom portions of the letter “C”.

[0108] The complex nature of the viewing process (as described by way of example above) may require certain guidance. In fact, in the case of the described embodiment of the invention being used as a security feature, it is highly desirable that the sequence of viewing steps requires such a guidance, without which the sequence cannot be determined or guessed easily or not at all. The guidance may be provided by a designer in the form of a viewing algorithm comprising a predetermined sequence of viewing steps and viewing configurations (i.e. all necessarily predetermined viewing conditions). Performing the viewing steps according to such an algorithm will thus ensure that all the portions of the holographic image are viewed, and viewed correctly.

[0109] However, as can be also derived from FIG. 3, being able to view all portions of the holographic image may not be enough to recognise the entire object represented by these portions, since in the case of the described embodiment, these portions 41 are “dislocated” or “scrambled” when compared to the proper appearance of the final image 40. According to the viewing steps described above, the portions 41 are viewed in full—however, for the viewer they appear in locations which do not correspond to the locations of the respective portions in the final image 40. Therefore, a further key 9—in addition to the viewing algorithm—is required in order to put viewed portions of the image into a proper order and/or positions and so to obtain the final representation of the complete recorded image. This additional operation may be done either by a mental process of a person performing the viewing, or by a viewing device in which such an operation is programmed, or by a combination of both. Again, the key 9 defining how to compose the viewed portions of the holographic image 51, 52, 53, 54 into the final complete reconstructed image 40 may be provided by the designer.

[0110] Not all algorithms are necessarily complex and/or an additional key 9 may not even be required—for example, as is the case illustrated in FIG. 1, where such an algorithm is very simple and effectively consists only of the steps of configuring a light source to reconstruct the image 40, and of a continuous sequence of viewing steps describing changing the position 70 of the viewer 60 in order to view all the portions 50 of the reconstructed image 40 so that the object represented by the image is recognised. However, the higher complexity of an embodiment of the invention, the higher level of security it may provide in security applications.

[0111] FIG. 4 illustrates schematically the viewing of different portions 150A, 150B, 150C (in this case the bowl, stem and base of a wine glass) of a given complete three-dimensional image 140 (in this case the complete wine glass) from different viewing angles. This complete image 140 may be considered to be the same complete image of a wine glass as recorded in the strip 20 shown in FIG. 1. The image 140 in this example is composed of three discrete portions: namely the bowl 150A, stem 150B and base 150C. In this embodiment all three portions 150A, 150B, 150C are seamlessly connected. To view each portion 150A, 150B, 150C in full, plural viewing angles have to be applied when viewing each said portion: namely at least a range of viewing angles from left to right, corresponding to the illustrated viewing positions 161 and 162, and possibly in combination with a vertical range of viewing angles 170 (which correspond to those 70 as shown schematically in FIG. 1).

[0112] Taking the arrangement of FIG. 1 a developmental stage further, in an alternative example the image of the complete wine glass 140 could for instance be formed from six portions: i.e. left and right sides of the bowl, left and right sides of the stem, and left and right sides of the base. In such a case, a single viewing angle (from left or right) may be sufficient to view each discrete portion in full.

[0113] The above-described embodiments of security features according to the invention thus represent a novel way to provide a covert security feature, yet one which is recognisable by an observer with little in the way of extraneous equipment. For example, in one practical scenario (as in the embodiment of FIG. 1) the stepwise or stagewise viewing and recognition of the complete reconstructed image 40 may be accomplished by means of a simple illumination device such as, for example, a flashlight or a lighting app incorporated into a smartphone. It also enables the incorporation into a given narrow strip or stripe 10 a potentially significantly larger reconstructed image than has hitherto been possible or obvious to do (e.g. easily 5 to 10 times greater than the size of the hologram), thereby leading to improved levels of security or authentication capability through the forcing of the observer to go to greater lengths in viewing and identifying individual and relatively smaller (in comparison with the whole) portions of the overall complete reconstructed image 40.

[0114] Throughout the description and claims of this specification, the words “comprise” and “contain” and linguistic variations of those words, for example “comprising” and “comprises”, mean “including but not limited to”, and are not intended to (and do not) exclude other moieties, additives, components, elements, integers or steps.

[0115] Throughout the description and claims of this specification, the singular encompasses the plural unless expressly stated otherwise or the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless expressly stated otherwise or the context requires otherwise.

[0116] Throughout the description and claims of this specification, features, components, elements, integers, characteristics, properties, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith or expressly stated otherwise.