Optical: systems and elements – Holographic system or element – Authentication
Reexamination Certificate
2000-11-14
2002-04-09
Henry, Jon (Department: 2872)
Optical: systems and elements
Holographic system or element
Authentication
C359S025000, C359S031000, C283S086000, C283S901000
Reexamination Certificate
active
06369919
ABSTRACT:
Security documents such as bank notes now frequently carry optically variable devices (OVDs) such as diffraction grating or holographic optical microstructures as a security feature against copy and counterfeit. This has been motivated by the progress in the fields of computer based desktop publishing and scanning which renders conventional security print technologies such as intaglio and offset printing increasingly accessible to counterfeit. A particularly good way to strengthen security documents against counterfeit is to combine security print with optically variable diffractive devices whose structures are non-copiable by scanners and which can exhibit optically variable effects such as colour changes by diffraction, apparent runs and movement effects and distinct switches between images. A particularly advantageous effect is where the OVD produces a distinct clear switch between two or more overlapping images providing a clear effect that cannot be simulated by print.
Several such classes of diffractive based security devices exist. Two common types, both based on arrays of surface diffraction gratings, are the “Exelgram” developed by CSIRO (Commonwealth Scientific and Industrial Research Organisation), Australia and the Kinegram, developed by Landis and Gyr, Switzerland. These are described in WO-A-93/18419, WO-A-95/04948 and WO-A-95/02200 for the Exelgram and U.S. Pat. No. 4761253 and EP-A-0105099 for the Kinegram. Both of these techniques use directly written localised surface diffraction gratings, written in the case of the Exelgram by an electron beam direct write process and in the case of a kinegram by the recombining step and repeat process outlined in U.S. Pat. No. 4761253.
Both of these techniques enable one precise diffraction grating to be written into a particular area. In the case of WO-A-95/02200, a device is disclosed displaying two angularly separated but overlapping diffracted images made from two completely overlapping diffraction grating areas while WO-A-95/04948 details a diffraction grating device made from a series of tracks of diffraction grating structures that exhibits a clearly switching image where the separate images can occupy overlapping areas. Both of these devices have been used for applications on security documents such as bank notes. Another type of device that can exhibit optical switching effects is a holographic structure manufactured using older holographic techniques. A typical example of such a device used as a security device on a bank note is the multiply redundant hologram described in EP-A-0558574 where to maintain holographic efficiency the hologram uses spatially separated switching image.
Now for application on a security document such as a bank note, the microscopically rough surface of the paper can have a severely detrimental effect on a diffractive image given that it is typically applied as a thin layer of embossed lacquer applied using the known print process of hot stamping. This is because the roughness of the surface and paper fibre intrusion severely disrupts the integrity of the thin layer of lacquer carrying the diffractive structure, thereby severely degrading its optical efficiency. It is therefore very important that the optical efficiency of the diffractive structure is maximised which has tended to result in the use of diffractive devices, such as the Exelgram, where there is a device to achieve an optically variable effect defined by a switch between two or more overlapping images. This is because the controlled “direct write” style origination technique of an Exelgram or Kinegram allow close control of the areas of diffraction grating allowing switching overlapping images to be created from two sets of interwoven tracks (WO 95/04948) so that each microscopic area of the device only consists of one diffraction grating which, when blocked onto the rough surface of a paper document, maintains its diffraction efficiency reasonably well as it is possible to maximise the single grating modulation whereas a switching device made up of overlapping diffraction grating areas would have a lower overall diffraction efficiency due to the complicated nature of the overlapping microstructures.
In accordance with one aspect of the present invention, a holographic security device comprises first and second holographic image generating structures based on originations prepared using an H
1
/H
2
process, the structures being recorded in respective sets of substantially non-overlapping regions of a record medium, the regions of one set being interleaved with regions of the other set, whereby both interleaved structures are substantially non-visible to the unaided eye, whereby the holographic security device generates two or more holographic images viewed from separate viewing directions around the device and normally seen by tilting the device, and whereby each particular holographic image in a viewing direction is generated in whole or part by the holographic image generating structure associated with one set of interleaved lines.
By holographic structures this description means structures that generate graphical images by the mechanism of diffraction of light where the original pattern has been generated by a holographic process of optical interference, whereby within the manufacturing stage of this origination process at least one component of the image may contain a rainbow hologram and where optionally at least one holographic intermediate hologram or H
1
is used which enables at least one component of the resulting image optionally to contain true holographic depth effects if desired (as associated with 2D/3D or 2D rainbow holograms as known in the art). This description also applies to surface 2D structures generated by the above holographic process but constrained to lie substantially on the image plane of the final device and with the preferred option of being constrained in the range of spatial frequencies contained therein (i.e. viewing angle of replay). This forms in the limiting case of extreme constraint a holographic structure substantially similar in visual performance to a pure diffraction grating structure but subtlely distinct in that on a microscopic level the microstructure will have been formed by a holographic projection process and may contain evidence of recorded laser generated speckle pattern structures.
This development relates to the method of enhancing the visibility and efficiency of a security hologram particularly for application to paper security documents such as bank notes where paper roughness and intrusion from paper fibres severely degrades the efficiency of a conventional hologram. This development also allows the creation of an optical microstructure which upon illumination generates two or more overlapping images which can be observed by eye from at least two separate viewing directions around the device. Although this is possible using conventional holographic techniques by recording overlapping holographic images with the optical microstructure pertaining to each image simply superposed in the area of overlap, the resulting composite microstructure will always replay each component of the image with a reduced efficiency or brightness compared to a single diffractive structure. In fact, the presence of overlapping diffractive microstructures always results in a structure with a reduced optical diffraction efficiency compared to a single diffractive structure due to the presence of overlapping microstructure and always tends to witness the presence of the second “ghost” image in the overlap area due to medium saturation and a reduction in optical efficiency. This is due to the presence in the overlapping areas of two very different holographic structures with different orientations to the carrier grating frequencies. This limits the overall optical efficiency and observed brightness of the holographic image, which is particularly disadvantageous in bank note holograms where there is a severe reduction in perceived brightness after application of the hot stamping foil to the bank
Drinkwater Kenneth John
Holmes Brian William
De La Rue International Limited
Henry Jon
LandOfFree
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