Optical: systems and elements – Holographic system or element – Authentication
Reexamination Certificate
2001-11-26
2004-11-23
Juba, Jr., John (Department: 2872)
Optical: systems and elements
Holographic system or element
Authentication
C359S024000, C283S086000
Reexamination Certificate
active
06822769
ABSTRACT:
The invention relates to a security device for use in securing documents and other articles of value against fraudulent reproduction, counterfeiting and the like.
Many security devices are based on the use of optically variable effect generating structures which generate holograms and the like since these are difficult to manufacture. Examples of such holographic structures and their manufacturing techniques can be found in EP-A-0548142, EP-A-0632767, and WO-A-99/59036 all owned by De La Rue International Limited and the teaching of which and other patents referred to within these documents are incorporated herewith by reference. WO-A-92/09444 is also incorporated as part of the prior art which teaches how to create an improved durability secure and simple to authenticate optical microstructure image feature for visual authentication of a banknote. Certain visual diffraction grating security devices already exist, for example as described in EP-A-0105099 which describes a security device showing an apparent movement effect consisting of areas of plane diffraction grating orientated at different directions along a track each of which diffracts an incident light beam into one particular direction although it should be noted that these are purely diffractive grating devices, with each segment purely a diffraction grating and thus incapable of forming a focused out of plane image under coherent illumination as a covert feature.
In terms of previous machine readable or coherently viewable holographic security structures we would refer to EP-A-0548142 for techniques on how a hologram may create an out of plane image for authentication, although we would point out that in this case the machine readable structure was designed to be completely hidden from visual view to an observer of the hologram and in fact consisted of superposed weak diffraction gratings which did not form a focussed out of plane image. DE-A-3840037 shows an example of a visual security hologram containing a superposed additional laser transmission hologram designed to form an out of plane image to be revealed under laser light with a machine reader or visualiser and designed to be non-recognisable under normal lighting.
There is a continuing need to enhance the security of such security devices while enabling them to be readily authenticated.
U.S. Pat No. 5,825,478 describes a system and method for determining which of a plurality of visually indistinguishable objects have been marked with a covert indicator in which a portion of a surface of each of a first type of objects are provided with a covert holographic indicator which is exposed to be viewed but which is detectable only when illuminated with a coherent reference light of predetermined wavelength.
In accordance with the present invention, a security device comprises a holographic optically variable effect generating structure having at least two discrete sections which generate In response to white light illumination an optically variable image consisting of at least two defined graphical elements located at or near an image plane either on or adjacent to the plane of the device, and which, in response to coherent illumination, generate at least two discrete covert images, in the form of indicia, whose image planes are located at a distance away from the physical plane of the device, the covert images being reconstructed at different angles to a normal to a substrate supporting the device such that the covert images are spatially separated on their image plane, and the covert images being substantially non-visible under white light illumination.
By holographic optical variable effect generating structure we mean, In this context, any diffractive device with the property of forming a first visual localized graphical image near the real plane of the device for visual observation and which also forms an out of plane covert laser verifiable image (instead of the more usual rainbow slit) for viewing under coherent light. Such a structure can only be created by holographic means, or by computer calculation and direct writing of the required fringe pattern of the structure which represents a slower and much more time consuming method of creating such an element.
A particularly useful form of holographic optically variable effect generating structure is one where the replay directions of the visual optically variable image are designed so as to generate an apparent movement effect on tilting the device around a particular axis.
This new holographic security device comprises a structure which generates at least two defined localized optically variable graphical images in the image plane adjacent the structure in response to white light illumination and at least two covert images spaced from the image plane in response to coherent illumination, and located at different angles to a normal to a substrate supporting the device. The covert images will generally be separated by a distance of the order of half their size. Thus, under normal white light illumination, the observer will see a typical optically variable image such as a hologram or diffractive effect consisting of at least two or more parts but under coherent illumination such as laser light, the observer will see two or more different covert images.
These covert images are preferably in the form of graphical elements, logos or alphanumeric characters which may typically be related to the article or document with which the security device is provided. The visual optically variable images are also preferably in the form of small defined shapes or indicia.
In the preferred case, the covert images are visible to the naked eye when reconstructed under coherent laser illumination of the device, although they could also be constructed at a wavelength outside the visible region for machine detection.
In one embodiment, the optically variable effect generating structure generates two or more graphical images or shapes in response to white light illumination which appear to move as the device is tilted. In this context, we distinguish between the apparent three-dimensional appearance of a hologram or the like with a movement effect such as a lateral movement generated by different diffraction angles between elements.
Conveniently, in this embodiment, each element of the holographic optical variable effect generating structure is formed as a single structure with typically a maximum lateral dimension not exceeding 1.5 mm. Preferably, the maximum lateral dimension is not less than 0.5 mm and most preferably in the range 0.5-0.75 mm, there being at least two such elements, replaying different covert images in different directions within the device.
The reconstruction of the covert images can be enhanced by providing a number of pairs or sets (of three or more) of these sections, each member of a pair or set being arranged to generate the same covert image in response to coherent illumination as the other member(s) of the pair or set. In this case, the individual elements must be located sufficiently close to each other that they fall within the diameter of an illuminating, coherent beam in order for the whole covert message to be reconstructed. Typically, the illuminating beam has a diameter of about 2 or 3 mm using a laser pointer or similar device.
In order to increase security both by increasing the complexity of the overall device and in order to conceal the presence of this new holographic optically variable effect generating structure, the optically variable effect generating structure may be located within a set of further optically variable effect generating structures, designed only to produce white light viewable images or effects, wherein the single and further optically variable effect generating structures, on tilting the substrate under white light illumination, cooperate together to generate a moving image effect. Thus, to the unskilled observer, a moving image effect will be observed under white light illumination and only by illuminating the correct set of optically variable effect generating structures
Drinkwater Kenneth John
Holmes Brian William
De la Rue International Limited
Jr. John Juba
Oliff & Berridg,e PLC
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