Optical: systems and elements – Holographic system or element – Authentication
Patent
1994-09-07
1997-12-02
Dzierzynski, Paul M.
Optical: systems and elements
Holographic system or element
Authentication
359 32, 283 94, 380 54, G03H 100, G03H 122, B42D 1500, G09C 300
Patent
active
056942290
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a holographic security device and to items carrying such devices.
2. Description of Related Art
Holographic devices such as three-dimensional holograms are widely used as authenticatable devices on security articles such as identification cards, credit cards and the like. Conventional holograms comprise three-dimensional object holograms which project the image of a three-dimensional object which is easily recognisable for authentication purposes. With the increasing sophistication of counterfeiters, simple three-dimensional holograms are no longer as secure as they once were. An attempt was made in the past to produce a more complex hologram as is described in GB-A-2136352. In this case, a stack of transparencies were overlapped and randomly deformed and then used to create a hologram which constituted the moire pattern which resulted from the transparency stack. Although such a resultant pattern is very difficult to counterfeit, the pattern itself is not particularly suitable when the device is to be used for authentication purposes. The pattern will have a random and rather messy form which is not readily authenticatable.
Another example of the use of holograms in the form of moire patterns is described in JP-A-62206582. This document aims to generate as complex a moire pattern as possible which again makes it difficult to authenticate the device.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, we provide a holographic security device in which the holographic effect is generated by the diffraction of light by a surface relief pattern on a film and which, when illuminated, generates a moire pattern produced from a pair of overlapping, regular arrays of lines or dots, wherein each array has a line of symmetry, the lines of symmetry being aligned, and wherein the device exhibits an ordered variation in the form of the observed moire patterns in a direction parallel with the alignment direction of the lines of symmetry and a colour variation but substantially no form variation transverse to the alignment direction.
This aspect of the invention means that on, tilting a hologram containing this feature to observe parallax and depth, a viewer sees a set of consecutive moire fringe patterns generated by the overlap of the two line patterns of very similar form and general arrangement as all these patterns retain a common axis of symmetry. A viewer therefore observes on tilting the hologram a moire pattern that remains substantially uniform in form as the viewing position changes, with moire fringes of consistent shapes moving across the viewing zone in a simple and ordered way. This is in contrast to the previously developed techniques where the intention appears to have been to make the patterns as complex as possible.
Thus in contrast to the previous proposal for utilizing moire patterns in holograms, we have devised a new device in which a particular type of ordered moire pattern is produced. This retains the advantages of reducing the risk of counterfeiting due to the difficulty of re-origination of such moire patterns, but significantly improves the ability of the device to be used for authentication purposes since this ordered moire pattern is easier to recognise and authenticate. By "ordered moire pattern" we mean a relatively simple geometric pattern, having a line of symmetry, which is easily recognisable but very difficult to reproduce.
This invention is primarily intended for use within white light viewable holograms, in which the moire effect is exhibited. These holograms are formed by surface relief patterns which diffract light in order to create the holographic effect. Such surface relief patterns are replicated into polymeric layers in order to mass produce the affect, using a master plate which exhibits the (inverse) surface relief pattern.
Replication can be done by plastic deformation of a plastic film eg under heating, or by curing a polymeric composition under the influence of
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Huang Wei-shi et al.;"Spacial Moire Hologram;" International Conference on Holography Applications; 2-4 Jul. 1986 Beijing, China; SPIE vol. 673, pp. 66-67.
Graham Saxby; Practical Holography; Prentice Hall 1988; pp. 50-52.
Drinkwater Kenneth John
Holmes Brian William
Chang Audrey
De La Rue Holographics Limited
Dzierzynski Paul M.
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