WO2003006261A1

WO2003006261A1 - An optically variable device and a method of producing an optically variable device

Info

Publication number: WO2003006261A1
Application number: PCT/AU2002/000926
Authority: WO
Inventors: Murray Scott; Zoltan Hegedus
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 2001-07-09
Filing date: 2002-07-09
Publication date: 2003-01-23
Also published as: AUPR627201A0

Abstract

A document, in the form of a bank note (1), includes a rectangular polymer sheet (2) having two opposite faces (3, 4) for bearing respective information. A rectangular security window (5) is disposed in sheet (2) for containing an optically variable device (6). Device (6) has a rectangular polarisation element (7) having two opposites faces (9, 10) that have a predetermined first birefringent pattern and a predetermined second birefringent pattern respectively.

Description

TITLE: AN OPTICALLY VARIABLE DEVICE AND A METHOD OF PRODUCING AN OPTICALLY VARIABLE DEVICE

FIELD OF THE INVENTION

The present invention relates to an optically variable device and a method of producing an optically variable device.

The invention has been developed primarily as document security for documents such as bank notes and will be described hereinafter with reference to that application. However, the invention is not limited to that particular field of use and is also suitable as a security or verification device for credit cards, signature capture, or anti-forgery measures and other like arrangements. DISCUSSION OF THE PRIOR ART

Use is presently made of a variety of anti-forgery methods for bank notes. However, with the continuing advancement of analysis technology and techniques, those less scrupulous members of society are also able to unravel what was not long ago considered to be a sophisticated anti-counterfeiting measure. It also becomes possible for those parties to more easily replicate the bank notes.

In an attempt to solve this problem there has been a move to more and more complex solutions on behalf of those who produce the bank notes. While this inevitably adds to the cost that is encountered by a would-be forger in reproducing the notes, it also results in a prohibitive cost of production of the notes in the first place.

This financial barrier has, in turn, led to the consideration of a different approach. That being the inclusion of a number of different security technologies, each of which is relatively difficult to replicate. For example, it is known from United States Patent 6,144,428 to include in a bank note, in addition to other measures, a patterned liquid crystal birefringent layer with an external polarisation analyser. This allows selective observation of the pattern provided by the birefringence. While this approach has some merit, it is aimed for use by an inspector or machine, not by unskilled individuals handling banknotes routinely in retail transactions. Moreover, the liquid crystal birefringent coating is also unproven in durability under banknote service. Other methods of affecting a patterned birefringence are also known, for example, in the field of digital data storage. One such method is disclosed in United States Patent 4,551,819 and includes the modification of polymer sheets through transient heating. This heating is induced by the absorption of electromagnetic radiant energy in selected areas of the birefringent material by carefully choosing the radiant wavelength to match absoφtion resonance characteristics of the sheets. Alternatively, a dye is included as part of the sheet to enhance the absoφtion for desired wavelengths. This method is employed to create data storage media, and is relatively slow and cumbersome to implement in other forms. Accordingly, its use is restricted to high end applications that justify that expense.

Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.

BROAD DISCLOSURE OF THE INVENTION

It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

According to a first aspect of the invention there is provided a method of producing an optically variable device (OVD), the method including: providing a polarising substrate having two opposite faces; thermally modifying a first sheet to include a predetermined first birefringent pattern; and laminating the first sheet to one of the faces of the substrate. Preferably, the step of providing the polarising substrate includes: providing a transparent substrate having two opposite faces; and laminating a polarising layer to one of the faces of the transparent substrate.

Preferably also, the method includes the step of laminating another polarising layer to the other of the faces of the transparent substrate. In other embodiments, the polarising substrate is a unitary structure.

Preferably, the method also includes: thermally modifying a second sheet to include a predetermined second birefringent pattern; and laminating the second sheet to the other of the faces of the substrate. It will be appreciated that the term "pattern" is not intended to be limited to repetitive or regular variations in birefringence, although it clearly includes these within its scope. Preferably also, the method includes disposing the OVD in the window of a document. More preferably, the OVD spans the window. Even more preferably, that document is a bank note. However, in other embodiments the document is another form of sheet material. It is also preferred that the document includes a second window that is spaced apart from the first, wherein predetermined folding of the sheet allows the windows to overlie each other.

In a preferred form, thermally modifying the first sheet and the second sheet includes exposing those sheets to thermal energy that is spatially varied through the use of one or more masks. More preferably, the mask used for the first sheet is different to the mask used for the second sheet. That is, the resultant birefringent pattern of the first sheet is different from that of the second sheet.

In some embodiments each mask includes a glass substrate bearing a lithographically patterned heat resistant absorber. In this case, the first sheet is brought into contact with the patterned absorber and a radiant energy source is disposed on the opposite side of the first sheet to the absorber. The portions of the first sheet that are in contact with the absorber are preferentially treated by the thermal energy to provide a patterned and modified birefringence in the sheet. The same process is undertaken with the second sheet, although use is made of a different mask to provide a different patterned birefringence. Preferably, the energy source is transient radiant energy source.

In other embodiments the glass substrate is patterned with a reflective mask. A separate uniform absorber is placed in contact with the first sheet. In this form, the portions of the first sheet covered by the reflective portions of the mask will be less affected by the application of the thermal energy. That is, it provides a patterned birefringence that is a negative of the pattern that is provided by the absorber.

In a preferred form, the first sheet and the second sheet are a plastic film that is stretched prior to the application of the thermal energy. More preferably, the film is stretched prior to the lamination of the film to the substrate.

Preferably, the stretching is in one direction only. In some embodiments the stretching occurs by rolling the sheet, while in other embodiments the stretching occurs due to a shearing action being applied to the sheet. In some embodiments use is made of a radiant energy source that provides a substantially uniform distribution of energy across the area of the sheet being treated. That is, the entire patterning of the sheet to achieved in a single exposure. In other embodiments, however, use is made of a localised thermal source that scans across the area to be treated. In some cases this is a pixel-by-pixel scan, while in others it is a line-by-line scan. For example, in one embodiment, one axis of the scan is provided by the sheet moving past a line scanning heat source.

Preferably, the first sheet has approximately half- wave retardation for visible light and a first principal axis of birefringence, and the polarising substrate includes a polarising axis that is disposed at an angle of approximately 45° to the first axis. More preferably, the second sheet has approximately half-wave retardation for visible light and a second principal axis of birefringence that is disposed at an angle of approximately 45° to the polarisation axis of the polarising substrate. Even more preferably, the first axis and the second axis are substantially parallel. Preferably also, the step of thermally modifying the first or the second sheet occurs subsequent to the step of laminating the respective sheets to the substrate. More preferably, the step of thermally modifying the first or the second sheet occurs immediately subsequent to the step of laminating the respective sheets to the substrate. Even more preferably, the step of thermally modifying the first or the second sheet and the step of laminating the respective sheets to the substrate occurs in a single pass.

According to a second aspect of the invention there is provided an optically variable device including: a polarising substrate having two opposite faces; a first sheet that is laminated to one of the faces and which is thermally modified to include a predetermined first birefringent pattern; and a second sheet that is laminated to the other face and which is thermally modified to include a predetermined second birefringent pattern.

According to a third aspect of the invention there is provided an optically variable device including: a polarising substrate having two opposite faces; a first image carrier mounted to one face of the polarising substrate and having a predetermined first birefringent pattern; and a second image carrier mounted to the other face of the polarising substrate and having a predetermined second birefringent pattern.

Preferably, the first birefringent pattern and the second birefringent pattern are different. More preferably, the first and the second birefringent patterns are induced through respective thermal modification of the first and second carriers. Even more preferably, the first and second carriers are fixedly mounted to the opposite faces of the polarising substrate and the first birefringent pattern and the second birefringent pattern are induced in the carriers prior to that mounting.

Preferably also, the carriers and the substrate, in combination, are substantially transparent to non-polarised light.

In a preferred form, the device includes a polarising analyser that is moveable between an operative position and an inoperative position such that in the operative position at least one of the first birefringent pattern or the second birefringent pattern is observable by a viewer. In some embodiments, movement of the analyser into the operative position allows the viewer to observe both the first birefringent pattern and the second birefringent pattern. However, for conditions involving non-polarised light the viewer is able to only observe the birefringenct patterns one at a time. That is, as the carriers are disposed on opposite faces of the substrate, the observation of one and then the other birefringent patterns requires relative movement between the device and the viewer.

In other embodiments the analyser is movable between one of two operative positions and an inoperative position such that in one of the operative positions the first birefringent pattern is observable by the viewer, while in the other of the operative positions the second birefringent pattern is observable by the viewer. Preferably, the substrate and the analyser allow the viewer to observe any one of the birefringent patterns when sandwiched between the substrate and the analyser.

According to a fourth aspect of the invention there is provided a method of producing an optically variable device, the method including: providing a polarising substrate having two opposite faces; mounting a first image carrier to one face of the polarising substrate wherein the first image carrier has a predetermined first birefringent pattern; and mounting a second image carrier to the other face of the polarising substrate wherein the second image carrier has a predetermined second birefringent pattern.

Preferably, the first birefringent pattern and the second birefringent pattern are different. More preferably, the method includes the steps of inducing the first and the second birefringent patterns through respective thermal modification of the first and second carriers. Even more preferably, the method includes the steps of inducing the first birefringent pattern and the second birefringent pattern in the carriers and then fixedly mounting the first and second carriers to the opposite faces of the polarising substrate. Preferably also, the carriers and the substrate, in combination, are substantially transparent to non-polarised light.

In a preferred form, the method includes providing a polarising analyser that is moveable between an operative position and an inoperative position such that in the operative position at least one of the first birefringent pattern or the second birefringent pattern is observable by a viewer.

According to a fifth aspect of the invention there is provided a document including: a sheet having two opposite faces for bearing information; a first window in the sheet; and a device of the third aspect wherein the substrate is located within the window.

Preferably, the substrate spans the first window. More preferably, the substrate is integral with the sheet. In some embodiments the document is lamina and the substrate is a layer of the document.

Preferably also, the carriers span the window. However, in some embodiments, one or both of the carriers extend over less than the entirety of the window. Most preferably, the first birefringence and the second birefringence are contained within the window.

In a preferred form, the window is bounded by the sheet. However, in other embodiments, the sheet includes a peripheral edge and the window defines part of that edge.

Preferably, the document includes a second window in the sheet that is spaced apart from the first window and in which is located the analyser. More preferably, the analyser spans the window. Even more preferably, the analyser is integrally formed with the sheet. In other embodiments the analyser and the substrate are integrally formed in a single sheet that is laminated with one or more other sheets to define the document. Preferably also, the spacing between the widows is sufficient to allow the sheet to be folded such that the windows are overlaid for allowing the viewer to selectively observe the first birefringent pattern and the second birefringent pattern. That is, the sheet is folded along a fold line that lies between the windows such that the analyser is located adjacent to and in an overlying relationship with the first carrier for allowing the viewer to observe the first birefringent pattern. The sheet is folded along the same fold line, although in the opposite sense, to locate the analyser adjacent to and in an overlying relationship with the second carrier for allowing the viewer to observe the second birefringent pattern.

In a preferred form the document is a bank note. More preferably, the bank note is formed from one or more polymers.

According to a sixth aspect of the invention there is provided a method of producing a document, the method including: providing a sheet having two opposite faces for bearing information; forming a first window in the sheet; and locating a device of the third aspect within the window.

Preferably, the document is a bank note. More preferably, the bank note is formed form one or more polymers. In other embodiments the document is a credit card. In other embodiments the document is an access card or identification card.

According to a seventh aspect of the invention there is provided an optically variable device including a polarising element having two opposite faces that have a predetermined first birefringent pattern and a predetermined second birefringent pattern respectively.

More preferably, the element includes: a polarising substrate having a first face and a second face opposite the first face; a first lamina carrier having the first birefringent pattern and being fixedly located to the first face; and a second lamina carrier having the second birefringent pattern and being fixedly located to the second face.

According to a eighth aspect of the invention there is provided a method of producing an optically variable device, the method including providing a polarising element having two opposite faces that have a predetermined first birefringence and a predetermined second birefringence respectively.

According to a ninth aspect of the invention there is provided a bank note including: a sheet having two opposite faces for bearing respective information; a security window in the sheet for containing an optically variable device having a polarising element having two opposite faces that have a predetermined first birefringent pattern and a predetermined second birefringent pattern.

Preferably, the bank note includes a second window in the sheet that is spaced apart from the security window for being selectively overlaid with the opposite faces of the device to allow a viewer to respectively observe the first birefringence and the second birefringence.

According to a tenth aspect of the invention there is provided a method of producing a bank note, the method including: providing a sheet having two opposite faces for bearing respective information; forming a security window in the sheet for containing an optically variable device having a polarising element having two opposite faces that have a predetermined first birefringent pattern and a predetermined second birefringent pattern. BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a perspective view of a bank note according to the invention;

Figure 2 is a cross section taken along line 2-2 of Figure 1; Figure 3a is an example of a single sided optical varied device (OVD) without a polarisation analyser;

Figure 3b is an example of a single sided OVD with a polarisation analyser; Figure 3c is a similar example to Figure 3b where the birefringent pattern is not between the polarisation analyser and the polarisation sheet;

Figure 3d is an example of a single sided OVD being viewed without a polarisation analyser in the presence of partially polarised light; Figure 4a is a schematic cross section of the double sided OVD that is used in the bank note of Figure 1 when viewed in non-polarised light and without a polarisation analyser;

Figure 4b is the OVD of Figure 4a when viewed in non-polarised light and with a polarisation analyser that is disposed intermediate face 9 and the viewer; Figure 4c is the OVD of Figure 4a when viewed in non-polarised light and with a polarisation analyser that is disposed intermediate face 10 and the viewer;

Figure 4d is the OVD of Figure 4a when viewed in partially polarised light and without a polarisation analyser;

Figure 4e is the OVD of Figure 4a when viewed in partially polarised light and with a polarisation analyser that is disposed intermediate face 10 and the viewer;

Figure 5a is a schematic representation of a first relative disposition of the windows of the bank note of Figure 1;

Figure 5b is a schematic representation of a second relative disposition of the windows of the bank note of Figure 1; Figure 5c is a schematic representation of a third relative disposition of the windows of the bank note of Figure 1;

Figure 5d is a schematic representation of a fourth relative disposition of the windows of the bank note of Figure 1;

Figure 6 is a schematic cross section of an apparatus for carrying out the method in accordance with one embodiment of the invention; and

Figure 7 is a schematic cross section of an apparatus for carrying out the method in accordance with another embodiment of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to Figure 1 there is illustrated a document in the form of a bank note 1 that includes a flexible rectangular polymer sheet 2 having two opposite faces 3 and 4 for bearing respective information. While in Figure 1 the faces are blank -for the puφoses of clarity - it will be appreciated by the skilled addressee that those faces contain the various indicia, patterns and other markings and images that are required to identify the bank note as being of a particular country or region and of a particular denomination.

A transparent rectangular security window 5 is disposed in sheet 2 for containing an optically variable device 6, as best shown in Figure 2. Turning now to Figure 2, it will be appreciated that device 6 has a rectangular polarisation element 7 having two opposite faces 9 and 10 that have a predetermined first birefringent pattern and a predetermined second birefringent pattern respectively.

In the preferred embodiments, the "pattern" is typically not comprised of repetitive or regular variations in birefringence, although other embodiments use such variations. Instead, the preferred embodiments make use of variations that are spatially arranged such that, in correct lighting conditions, allow a viewer to perceive an image. More preferably, the image is recognisable to the viewer. For example, in some preferred embodiments, the image is of the head or bust of a person. In other preferred embodiments the image is a silhouette of one or more letters.

In other preferred embodiments, the spatial arrangement of the patterns on the each sheet provides an incomplete image, wherein the total image is only viewable when the sheets are overlaid. This functionality will be described further in the following description. Bank note 1 includes two spaced apart and parallel longitudinally extending edges 13 and 14 that are joined by two spaced apart parallel transversely extending edges 15 and 16. In this embodiment, window 5 is located adjacent to edges 14 and 16. In other embodiments, however, the window is disposed elsewhere on sheet 2.

Bank note 1 also includes a second transparent rectangular window 18 that is spaced apart from window 5 and disposed adjacent to edges 14 and 15. Window 18 is formed from sheet material that comprises a polarisation analyser. In alternative embodiments, window 18 is disposed elsewhere on sheet 2.

Windows 5 and 18 are substantially transparent to non-polarised light. Accordingly, in the configuration shown in Figure 1, both windows, when observed by a viewer with the aid of non-polarised light, appear substantially transparent. In some embodiments one or both of windows 5 and 18 carry translucent or opaque images or indicia over at least a portion of their faces. Element 7 spans window 5 and acts as a substrate for opposite thin film plastics layers 19 and 20 that are laminated to element 7. The layers include, respectively, the first birefringent pattern and the second birefringent pattern. In this embodiment the first birefringent pattern differs from the second birefringent pattern. However, in other embodiments, those patterns are the same. The inducing of the birefringent patterns in layers 19 and 20 will be described in more detail below. At this point, all that need be appreciated is that those birefringent patterns, when subject to appropriate viewing conditions, are observable by a viewer.

For strength and ease of production the principal birefringence axes of the birefringent plastic layers 19 and 20 lie square to the edges of bank note 1. This, in turn, requires that the polarising laminate be disposed at 45 ° to edges 14 and 15.

In use, bank note 1 is folded along a fold line that is parallel to and intermediate edges 15 and 16 so that windows 5 and 18 overlie each other. As will be apparent to the addressee, there are four possible overlying configurations, these being where:

1. Surface 9 is facing the viewer and window 18 is adjacent to surface 10, as shown in Figure 5a.

2. Surface 9 is facing the viewer and window 18 is adjacent to surface 9, as shown in Figure 5b. 3. Surface 10 is facing the viewer and window 18 is adjacent to surface 10, as shown in Figure 5c. 4. Surface 10 is facing the viewer and window 18 is adjacent to surface 9, as . shown in Figure 5d. In the presence of natural light the above configurations respectively provide the viewer with:

1. An image corresponding to a positive or a negative of the birefringent pattern on surface 10.

2. An image corresponding to a positive or a negative of the birefringent pattern on surface 9. 3. An image corresponding to a positive or a negative of the birefringent pattern on surface 10. 4. An image corresponding to a positive or a negative of the birefringent pattern on surface 9. To assist in the understanding of the operation of the preferred embodiment, the addressee is now provided with some background on the viewing of birefringent patterns in varying conditions.

Reference is first made to Figure 3 a that illustrates a single sided birefringent OVD 25 that includes a polarisation substrate 26 having two opposite faces. A thin film 29 is abutted to one of the faces and includes an induced birefringent pattern having a predetermined spatial distribution across the film. In this embodiment the birefringent pattern has been heat induced, with the heat treated areas being shown shaded in the Figures. However, it will be appreciated that this is for the puφoses of explaining the working of the preferred embodiments, and that the birefringence will only be observable by a viewer under specific conditions.

In Figure 3a, OVD 25 is disposed intermediate the viewer and a non-polarised light source 30. In the absence of polarised light and a polarisation analyser, as is the case in Figure 3a, no pattern is observable by the viewer. However, if a polarisation analyser 31 is placed between film 29 and the viewer, as shown in Figure 3b, the viewer will observe the pattern created by the birefringent pattern of film 29.

It will be appreciated that the relative orientation of the respective polarisation axes of polarising substrate 26 and analyser 31 is also important. If the polarisation axis of element 7 lies at approximately 45° to the document edges, then folding the document parallel to an edge will result in element 7 and window 18 being in a crossed configuration. In this case unmodified half-wave portions of the birefringent pattern will appear bright, whereas thermally modified portions of the pattern will retain less birefringence and appear correspondingly darker in proportion to the temperature to which the birefringent sheet is heated and the depth to which the thermal modification penetrates. An image pattern thus comprises a binary density pattern of light and dark shading or, as in some embodiments, a continuum of grey shades. If OVD 25 is reversed from the orientation of Figure 3b and into the orientation of Figure 3c, film 29 is now, with respect to the viewer, on the far side of substrate 26. Notwithstanding that analyser 31 is placed between the viewer and polarisation substrate 26, no image or pattern appears.

A further important practical effect is illustrated in Figure 3d. In this case, film 29 lies, with respect to the viewer, on the far side of polarising substrate 26. The physical conditions are such that partially polarised light enters element 25 and provides the viewer with a faint image represented by the birefringent pattern that has been induced in film 29. The image appears with positive or negative contrast dependant on the degree and direction of polarisation of the incident light. That is, under certain conditions, and in the absence of a polarisation analyser, the birefringent pattern is observable. These conditions include where the incident light is partially polarised either by another polariser, or by natural effects such as sky scatter or reflection from dielectric materials such as floor tiles, water, glass or other dielectric reflective surfaces.

Reference is now made to Figures 4a to 4e where corresponding features are denoted by corresponding reference numerals. More particularly, the Figure 4 embodiment is directed toward the double sided OVD 6 used in bank note 1. As mentioned above, device 6 includes a polariser 7 that acts as a substrate for two opposite polymer sheets 37 and 38. These sheets define the respective faces 9 and 10 and bear induced birefringent patterns. Again, in this embodiment, the birefringent patterns for sheets 37 and 38 are different from each other. Sheets 37 and 38 are laminated to the opposite faces of polariser 7 to define the sequential layers of polymer film/polarisation element/polymer film. Each sheet has a birefringence axis and polariser 7 has a polarisation axis, and the sheets are laminated with the polariser such that both birefringence axes are rotationally disposed at 45° to the polarisation axis. In ordinary un-polarised light with no polarisation analyser, no pattern is observable by the viewer. This situation is represented in Figure 4a. However, with a polarisation analyser - that is window 18 - disposed between the viewer and the face 9, an image corresponding to the birefringent pattern of face 9 is observed. That is, the viewer observes an image corresponding to the birefringent pattern in sheet 38, as the birefringence pattern lies between two appropriately orientated polarisation sheets. This image observed by the viewer has either a positive or a negative contrast according to the two possible contrast reversals. If bank note 1 is reversed so that the polarisation analyser - window 18 - is placed between the viewer and the other side of the OVD, such as that shown in Figure 4c, the viewer observes the image provided by the patterned birefringence of film 37. In this case, that patterned birefringence differs from that of film 38 and, as such, the image observed with be different to that observed in the Figure 4b circumstance.

Figure 4c provides an illustration of a circumstance where no polarisation analyser is used. However, as the incident light is partially polarised by reflection from a dielectric material, the viewer observes an image corresponding to the birefringence pattern on that side of the OVD that is facing away from the viewer. The image is observed with a positive or a negative contrast dependant on the degree and direction of polarisation of the incident light. This partial polarisation of light also occurs through other natural effects such as sky scatter and reflection from dielectric surfaces. The present embodiments have concentrated primarily on the application of the invention to a bank note. It will be appreciated, however, that device 6, in other embodiments, is mounted within a window formed in a credit card, access card, security card, a folder for containing paper or other information, a container or other containment means. The OVD of the preferred embodiments provides a greater degree of flexibility as a document security device. Firstly, there are a number of combinations of images that are able to be examined to determine authenticity of bank note 1. Secondly, these images are easily observable simply by exposing the bank note to appropriate conditions - that is, either substantially natural light or to partially polarised light. Thirdly, the incoφoration of the polarisation analyser in the same bank note provides a simple and convenient test device usable by members of the public or point of sale retail operators. In addition to this, the OVD is able to be inexpensively and accurately implemented. This will be described in more detail below.

In its broadest form, the preferred method of producing an optically variable device (OVD) includes: providing a polarisation substrate having two opposite faces; thermally modifying a first sheet in the form of a first film to include a predetermined first birefringent pattern; and laminating the first film to one of the faces of the substrate.

Preferably, as in all embodiments, the substrate has a polarisation axis and the first sheet has a birefringence axis, and the polarisation axis and the birefringence axis are rotationally offset from each other by about 45°.

For the embodiments described above, use is made of a double sided laminate and this involves the additional steps of: thermally modifying a second sheet in the form of a second film to include a predetermined second birefringent pattern; and laminating the second film to the other face of the substrate.

Preferably, the second sheet has a birefringence axis, and the polarisation axis and the birefringence axis of the second sheet are rotationally offset from each other by about 45°. That is, in some embodiments the birefringence axes of the first and the second patterns will be aligned, while in other embodiments they will be rotationally offset from each other by 90°.

In this embodiment, the step of thermally modifying the first and the second sheet occurs subsequent to the step of laminating the respective sheets to the substrate. In fact, the step of thermally modifying the first or the second sheet occurs immediately subsequent to the step of laminating the respective sheets to the substrate and in a single pass.

In the preferred embodiments, the films are polymer sheets formulated for durability in banknote service, as used for example in Commonwealth of Australia banknotes of all denominations. The polymer sheets are rendered suitably birefringent by controlling the normal processes of stretching and rolling conventionally used in the production process. That is, for such polymer bank notes, it is relatively straightforward to introduce the required steps into the pre-existing processing. MODIFYING BIREFRINGENCE BY HEATING.

Polymer materials containing an amoφhous phase have a glass transition temperature marking an origin for the temperature dependence of the speed of viscous deformation under stress. The glass transition is different from the melting point, which is a property of the crystalline phase (a state of molecular arrangement, not to be confused with the phase of light waves) also present in many polymers. While the amoφhous phase remains cooler than its glass transition temperature the internal stresses induced by stretching or rolling processes remain locked in, maintaining the molecular arrangement, which causes birefringence. Heating beyond the glass transition temperature allows internal stresses to relax progressively faster and partially removes the birefringence. This is "plastic" behaviour and does not occur with highly crystalline polymers. Melting the polymer can mar its surface unless refreezing occurs on a submicrosecond timescale. In circumstances were only millisecond timescale heating techniques are available it is still possible to perform the method of the preferred embodiments, although the materials are limited to those having a workable gap between glass transition and melting points.

Amoφhous and crystalline phases also behave differently in the propagation of thermal transients. The glass transition is a 2^nd order phase transition marked by a step change in specific heat with temperature but no latent heat. Melting of a crystalline phase is however a 1^st order transition marked by a spike in the specific heat versus temperature curve, the area of which corresponds to the latent heat of fusion. To propagate a heat pulse in crystalline material thus requires transport of more heat to the melt front than is the case with a predominantly plastic material, reducing the penetration of a transient heating effect. This creates the need for heating to take place by direct absoφtion of radiant energy.

The material used in Australian Commonwealth banknotes is based on polypropylene and is strongly crystalline. Accordingly, it does not afford much plastic behaviour and has to be modified by melting. This, in turn, requires extremely fast thermal transients at a wavelength absorbed directly by the polymer, for example at ~10μm wavelength produced by a Q switched CO₂ laser.

The following table provides some melting and glass transition temperatures for common polymers. (Sourced from http://www.psrc.usm.edu/macrog/pindex.htm)

Polymer Glass transition °C Melting point °C

Polyethylene -130 to -80 °C 137 °C

Polypropylene -17 °C 174 °C

Polycarbonate * 150 °C n.a. (amoφhous) Patterned birefringence is easily demonstrated at millisecond timescales using polyethylene sheet such as that commonly used for packaging material. For example, a 30 micron thick film used to form an envelope for posting magazines has been found to be already birefringent with retardation value less than half wave for visible light. While viewing this material between parallel polarisers, with its birefringence axis at 45° to that of the polarisers so as to appear darkest, the film, when stretched along this axis, gains further enhancement of the existing birefringence. With increasing strain the film attains maximum darkness followed by a light blue colour, then a succession of colours corresponding to higher order halfwave multiples for various wavelengths. For monotonic contrast transfer of thermal birefringence patterns the film is used in the stretched condition giving the first maximum darkening, corresponding to approximately halfwave retardation for visible light. Note that for maximum contrast of the observed birefringence pattern the heat transient should extend through most or all of the film thickness. Other embodiments utilise an alternative film such as a polyethylene film of about 10 microns thickness. Such films are commonly available in rolls as kitchen "cling wrap" films and are sold under a variety of trade marks. These types of films commonly have very little birefringence as supplied, though being mechanically anisotropic. It has been found that if such films are stretched in the "strong" circumferential direction of the roll to enhance the small existing birefringence, the -30% strain required for half-wave retardation in the film is very close to its fracture strain and mild heating of the film during stretching using ~70 °C hot air was helpful. Having greater specific birefringence than a thicker halfwave retarding sheet, thin polyethylene film is more sensitive to thermal modification and affords better pattern contrast when heated by contact with one surface. It also "clings" to the heat source surface for better and more stable thermal contact.

The application of the thermal energy to the respective films to induce the required patterned birefringence involves the use of a master plate. This plate bears the desired pattern, in either a positive or a negative, and is prepared by lithographic patterning of an opaque film. This film is deposited on a heat resisting transparent material, for example, metal on glass. This master pattern is transferred in one of a number of ways, two examples of which are provided below. EXAMPLE 1: CONTACT PRINTING.

A birefringent polyethylene film comprising stretched HOME BRAND™ kitchen cling wrap film is placed in thermal contact with a metallised side of a master plate. The plate includes a 75 mm square, 2 mm thick glass substrate with a low- reflectance chromium layer on which the desired pattern has been lithographically etched. That is, the plate is a HOYA microlithographic mask plate. The polyethylene film is applied with gentle stretching to eliminate wrinkles then pressed firmly onto the surface from the centre outward to expel air bubbles and ensure good thermal contact. An Elinchrom 6000 Micro AS photographic flash unit was set for a flash intensity of 800 units, with the A3000N lamp head spaced by a 50 mm reflector skirt from the work piece. One flash is directed through the polyethylene film which is, in turn, locally heated by energy absorbed in the dark metal portions of the pattern. The birefringence of the polyethylene film is much reduced in areas contacting the metal absorber and little affected in areas contacting the transparent glass. This leaves a birefringence pattern which is an inverse replica of the master pattern. This is called an inverse birefringence pattern. The patterned birefringent film is viewed between polarisation sheets aligned at 45° with respect to the principal strain axis of the polyethylene film. Rotating one of the polarisation sheets by 90° reverses the contrast of the observed pattern. Rotating either polarisation sheet through 45°, that is, so that it is parallel to the birefringence axis, the pattern disappears.

The source of thermal energy is, in other embodiments, disposed on the opposite side of the mask plate.

This example is schematically illustrated in Figure 6. EXAMPLE 2: PROXIMITY PRINTING. Using the contacting technique described in Example 1, a similarly prepared birefringent polyethylene film is placed in thermal contact with a uniformly black absorber plate including a powder coated black enamel on sheet aluminium. The same master plate as used in Example 1 is now used as a mask. It is placed patterned metal side towards this assembly with a small air gap, typically about 0.1 mm. From the glass side of the master plate the metal pattern is highly reflective. An Elinchrom 6000 Micro AS photographic flash unit was set for a flash intensity of 800 units, with the A3000N lamp head spaced by a 50 mm reflector skirt from the work piece. One flash is applied through the glass, and the polymer is heated by areas of the black absorber plate under transparent portions of the master plate. Areas covered by the reflecting metal portions of the mask are unaffected, thus leaving a direct birefringent pattern replicating the master pattern without contrast reversal. If placed in the same orientation as in Example 1 and observed with the same polarisation sheets and orientations, the pattern is observed with contrast reversed relative to that obtained by the previous example.

This example is schematically illustrated in Figure 7. EXAMPLE 3: SCANNING A laser provides a beam that is progressively scanned across the film. The energy of the beam is temporally modulated as the scanning occurs to provide the desired dose of thermal energy at each point of incidence on the film. This eliminates the need to provide a mask and lends itself to electronic control.

The third example is a pixel-by-pixel approach to birefringence modification, while the first two examples are concerned with creating the image as a whole simultaneously. It will be appreciated, however, that the first two examples are, in some cases, used to produce the birefringence modification in a non-simultaneous manner. For example, by exposing the film to the thermal source on a line-by-line basis. Additionally, in other embodiments, an array of masks are simultaneously exposed by a thermal source so that a plurality of birefringences are simultaneously produced.

It will be appreciated from the teaching herein that it is important to control the thermal source to maximise the yield of the production process. Optimal contrast and resolution of the birefringent pattern is obtained by ensuring that the flash duration or beam dwell time of the radiant energy source is short compared with the time for the heat to diffuse laterally between adjacent features of the desired pattern or image.

The preferred embodiments provide an effective security or verification system, particularly for bank notes. Moreover, the technology is both inexpensive to apply but difficult in practice to accurately replicate. This makes it ideal as part of a number of anti-counterfeiting measures incoφorated into a bank note.

The preferred embodiments also provide a robust and easy to use arrangement suitable for handling the demanding physical treatment that has to be tolerated by bank notes. It is also able to be incoφorated with the inks and polypropylene polymer used for example on Commonwealth of Australia banknotes.

The visible nature of the birefringent patterns of the preferred embodiments allows most sighted users to quickly and easily verify the authenticity of a bank note. In addition, some embodiments the pattern includes detail that is not visible to the naked eye and which is intended for machine reading. This provides an additional level of protection.

The method according to the preferred embodiments of the invention is well adapted to mass production and low cost manufacture. That is, it provides for rapid parallel patterning methodologies using a binary or variable density analogue patterned master to control thermal birefringence modification over extended areas. Accordingly, the preferred embodiments provide a cost effective an optically variable device and, in turn, a cost effective security mechanism for documents that are verifiable by members of the public and point-of-sale retail operators for prompt detection of forgeries.

Although the invention has been described with reference to specific examples it will be appreciated by those skilled in the art that it may be embodied in many other forms.

Claims

1. A method of producing an optically variable device (OVD), the method including: providing a polarising substrate having two opposite faces; thermally modifying a first sheet to include a predetermined first birefringent pattern; and laminating the first sheet to one of the faces of the substrate.

2. A method according to claim 1 further including: thermally modifying a second sheet to include a predetermined second birefringent pattern; and laminating the second sheet to the other of the faces of the substrate.

3. A method according to claim 1 or claim 2 including the step of disposing the OVD in the window of a document.

4. A method according to claim 3 wherein the OVD spans the window.

5. A method according to claim 3 wherein the document is a bank note.

6. A method according to claim 3 wherein the document is a sheet material.

7. A method according to claim 3 wherein the document includes a second window that is spaced apart from the first, wherein predetermined folding of the sheet allows the windows to overlie each other.

8. A method according to claim 1 wherein the steps of thermally modifying the first sheet and the second sheet includes exposing those sheets to thermal energy that is spatially varied through the use of one or more masks.

9. A method according to claim 8 wherein the mask used for the first sheet is different to the mask used for the second sheet such that the resultant birefringent pattern of the first sheet is different from that of the second sheet.

10. A method according to claim 8 wherein the or each mask includes a glass substrate bearing a lithographically patterned heat resistant absorber.

11. A method according to claim 10 including the further step of bringing the first sheet is into contact with the patterned absorber and disposing a radiant energy source on the opposite side of the first sheet to the absorber such that the portions of the first sheet that are in contact with the absorber are preferentially treated by the thermal energy to provide a patterned and modified birefringence in the sheet.

12. A method according to claim 10 including the steps of: providing the glass substrate with a patterned reflective mask; and placing a separate uniform absorber in contact with the first sheet whereby the portions of the first sheet covered by the reflective portions of the mask are less affected by the application of the thermal energy to provide a patterned birefringence that is a negative of the pattern that is provided by the absorber.

13. A method according to claim 1 wherein the first sheet and the second sheet are respective plastics films that are stretched prior to the application of the thermal energy.

14. A method according to claim 13 wherein the stretching is in one direction only.

15. A method according to claim 1 wherein the step of thermally modifying the sheets includes the use of a radiant energy source that provides a substantially uniform distribution of energy across the area of the sheet being treated such that the entire patterning of the sheet to achieved in a single exposure.

16. An optically variable device including: a polarising substrate having two opposite faces; a first sheet that is laminated to one of the faces and which is thermally modified to include a predetermined first birefringent pattern; and a second sheet that is laminated to the other face and which is thermally modified to include a predetermined second birefringent pattern.

17. An optically variable device including: a polarising substrate having two opposite faces; a first image carrier mounted to one face of the polarising substrate and having a predetermined first birefringent pattern; and a second image carrier mounted to the other face of the polarising substrate and having a predetermined second birefringent pattern.

18. A device according to claim 16 or claim 17 wherein the first birefringent pattern and the second birefringent pattem are different.

19. A device according to claim 17 wherein the first and second carriers are fixedly mounted to the opposite faces of the polarising substrate and the first birefringent pattern and the second birefringent pattern are induced in the carriers prior to that mounting.

20. A device according to claim 17 wherein the carriers and the substrate, in combination, are substantially transparent to non-polarised light.

21. A device according to claim 17 including a polarising analyser that is moveable between an operative position and an inoperative position such that in the operative position at least one of the first birefringent pattern or the second birefringent pattern is observable by a viewer.

22. A device according to claim 17 including a polarising analyser that is movable between one of two operative positions and an inoperative position such that in one of the operative positions the first birefringent pattern is observable by the viewer, while in the other of the operative positions the second birefringent pattern is observable by the viewer.

23. A method of producing an optically variable device, the method including: providing a polarising substrate having two opposite faces; mounting a first image carrier to one face of the polarising substrate wherein the first image carrier has a predetermined first birefringent pattern; and mounting a second image carrier to the other face of the polarising substrate wherein the second image carrier has a predetermined second birefringent pattern.

24. A method according to claim 23 wherein the first and the second birefringent patterns are visibly discemable by a viewer.

25. A method according to claim 23 wherein the first birefringent pattern and the second birefringent pattern are different.

26. A method according to claim 23 including the steps of inducing the first and the second birefringent patterns through respective thermal modification of the first and second carriers.

27. A method according to claim 26 including the steps of inducing the first birefringent pattem and the second birefringent pattern in the carriers and then fixedly mounting the first and second carriers to the opposite faces of the polarising substrate.

28. A method according to claim 23 wherein the carriers and the substrate, in combination, are substantially transparent to non-polarised light.

29. A method according to claim 23 including the step of providing a polarising analyser that is moveable between an operative position and an inoperative position such that in the operative position at least one of the first birefringent pattern or the second birefringent pattern is observable by a viewer.

30. A document including: a sheet having two opposite faces for bearing information; a first window in the sheet; and a device of the third aspect wherein the substrate is located within the window.

31. A document according to claim 30 wherein the substrate spans the first window.

32. A document according to claim 30 wherein the substrate is integral with the sheet.

33. A document according to claim 30 that is lamina wherein the substrate is a layer of the document.

34. A document according to claim 30 wherein the carriers span the window.

35. A document according to claim 30 wherein the first birefringence and the second birefringence are contained within the window.

36. A document according to claim 30 wherein the window is bounded by the sheet.

37. A document according to claim 30 including a second window in the sheet that is spaced apart from the first window and in which is located a polarising analyser.

38. A document according to claim 37 wherein the analyser spans the window.

39. A document according to claim 37 wherein the analyser is integrally formed with the sheet.

40. A document according to claim 37 wherein the analyser and the substrate are integrally formed in a single sheet that is laminated with one or more other sheets to define the document.

41. A document according to claim 37 wherein the spacing between the widows is sufficient to allow the sheet to be folded such that the windows are overlaid for allowing the viewer to selectively observe the first birefringent pattern and the second birefringent pattern.

42. A document according to claim 30 wherein the document is a bank note formed from one or more polymers.

43. A method of producing a document, the method including: providing a sheet having two opposite faces for bearing information; forming a first window in the sheet; and locating a device of the third aspect within the window.

44. A method according to claim 43 wherein the document is selected from the group including: a bank note; a bank note formed form one or more polymers; a credit card; an access card; and an identification card.

45. An optically variable device including a polarising element having two opposite faces that have a predetermined first birefringent pattern and a predetermined second birefringent pattern respectively.

46. A device according to claim 45 wherein the element includes: a polarising substrate having a first face and a second face opposite the first face; a first lamina carrier having the first birefringent pattern and being fixedly located to the first face; and a second lamina carrier having the second birefringent pattern and being fixedly located to the second face.

47. A method of producing an optically variable device, the method including providing a polarising element having two opposite faces that have a predetermined first birefringence and a predetermined second birefringence respectively.

48. A bank note including: a sheet having two opposite faces for bearing respective information; a security window in the sheet for containing an optically variable device having a polarising element having two opposite faces that have a predetermined first birefringent pattern and a predetermined second birefringent pattern.

49. A bank note according to claim 48 including a second window in the sheet that is spaced apart from the security window for being selectively overlaid with the opposite faces of the device to allow a viewer to respectively observe the first birefringence and the second birefringence.

50. A method of producing a bank note, the method including: providing a sheet having two opposite faces for bearing respective information; forming a security window in the sheet for containing an optically variable device having a polarising element having two opposite faces that have a predetermined first birefringent pattern and a predetermined second birefringent pattern.