GB2456596A

GB2456596A - Holographic security marker

Info

Publication number: GB2456596A
Application number: GB0809649A
Authority: GB
Inventors: Stephen Christopher Brown; Martin John Richardson
Original assignee: De Montfort University
Current assignee: De Montfort University
Priority date: 2008-01-18
Filing date: 2008-05-28
Publication date: 2009-07-22
Also published as: GB0800865D0; GB0809649D0

Abstract

The invention provides a holographic security marker comprising a holographic security symbol containing encoded digital security data recorded on a thin film data carrier that is laminated to a surface protective film. The replay frequency of the recorded volume holographic data is in a band outside of the visible spectrum, preferably into the infra-red or near-infra-red part of the spectrum. The hologram may be recorded in the visible range and then moved into the non-visible range. The data carrier may for example be a polymeric pressure-dependent light-sensitive material that is capable of volume holographic storage, and which has been cross-linked, after recordal of visible holographic data thereon, after exposing the data carrier to an external pressure which moves the replay frequency of the recorded holographic data to a band width outside of the visible spectrum. Alternatively the polymer may be a swellable polymer which is pre-swollen before recordal of the hologram in the visible range and then dried to contract it to move the recorded holographic data into the non-visible range. The recorded holographic image may be a volume hologram or a volume hologram combined with a surface hologram. The digital security data may be a bar code or a matrix code, and the holographic image of the security data recorded in the volume hologram can be replayed only using suitable camera or data capture equipment which is capable of replaying the hologram within that narrow band width, and capable of reading and if necessary decrypting the encoded digital data. The bar code or matrix code is invisible to the human eye.

Description

, 2456596

TITLE

Holographic Security Marker Field of the Invention

The invention relates to the field of holographic security marking, and provides a holographic security marker and system that creates a wide range of potentially valuable novel security marking opportunities.

Background Art

Security codes or product identification codes are known, for example as bar codes which are machine-readable. Bar codes are sometimes referred to as unidimensional codes because they are essentially linear in data content. There are currently 28 different standards or symbologies, and typically bar codes would be read by a scanning laser. Two-dimensional or matrix codes are also known, and can contain far greater amounts of information or data. There are currently 39 different accepted standards for matrix codes, of which QR codes ("Quick Recognition" codes) are but one example. Matrix codes cannot be read by lasers as there is no established sweep pattern that can encompass the entire symbol. They are read by camera capture devices which are responsive to the complete two-dimensional matrix of each coded symbol.

This invention is based in part on the observation that known security codes are visible to the naked eye, and their presence can therefore easily be detected. When bar coded or matrix coded information is incorporated, for example, into a high value, high risk product such as a credit card or identity card then even a casual observer can see that the coded information is present, and it is then relatively easy for a potential criminal to read and decode or duplicate that coded information.

This invention is also based in part on the appreciation that holographic techniques can be used with advantage to record security codes such as those referred to above. The security codes can be recorded as either surface or volume holograms.

Surface or Relief Holograms

Surface holograms, sometimes referred to as relief holograms, are created by the formation of a surface diffraction grating on a substrate. The substrate is generally a metal foil onto which the surface diffraction grating is embossed by pressing onto the foil a master 'negative' formed from a material harder than the metal of the foil. The press used is similar to a printing press. The embossed surface diffraction grating may then be protected by the application, if desired, of a transparent surface protective film.

Volume holograms

There are many ways of categorizing volume holograms. One of the valid criteria is the thickness of the recording layer (as compared to the interference fringe spacing within the layer), i.e. the layer coated on the material substrate. Volume holograms can thus be classified as "thin" or "thick' (sometimes also called respectively "plane" or "volume" holograms, respectively). To distinguish between the two types, the Q-parameter is normally used; it is defined in the following way:

Q = 2fffld/(n02)

where D is the wavelength of the illuminating light, d is the thickness of the layer, n is the refractive index of the emulsion, and 0 the spacing between the recorded fringes. A hologram is considered thick if Q > 10, and thin when Q < 1. Holograms with Q-values between 1 and 10 are sometimes treated as thin and at other times as thick.

The volume hologram represents a very important hologram type since it is here where, at least theoretically, the highest possible diffraction efficiency (100%) can be obtained. A modulated medium is a medium in which the refractive index varies in such a manner that it is approximately periodic in small regions. Typical modulated media are holograms, because holograms record interference fringes, which are always contour surfaces. Bragg's law and the Bragg condition are of importance for thick holograms

2dsinQ = 0a/n

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where D is half the angle between the reference and the object beams at the recording stage (as well as the angle between the illuminating and the diffracted beams and the scattering planes in the emulsion at reconstruction), d is the spacing between the interference planes in the emulsion, 0a is the wavelength in air, and n the average refractive index of the hologram medium.

Volume Hologram Recordal

Volume holograms can be recorded on thin film data carriers that are sensitive to exposure to light to a resolution that enables the creation of information-carrying interference patterns. For example, the thin film data carrier may be a polymer film having distributed therein nano-sized particles of photosensitive material. First, the thin film is exposed to create the information carrying interference pattern. After exposure the information-carrying interference pattern is developed by techniques not dissimilar to known photographic development techniques, and after development the recorded interference pattern is fixed in the thin film data carrier by removing from the thin film all nano-particles of the original unexposed photosensitive material. As a generality, the replay frequency of the recorded hologram is the same as the frequency of the laser which created the initial interference patterns within the thin film data carrier.

Photopolvmers for Volume Hologram Recordal

A photopolymer volume hologram recording material consists of three parts: a photopolymerizable monomer, an initiator system (which initiates polymerization upon exposure to light) and a polymer (the binder). First, an exposure is made to the information-carrying interference pattern. This exposure polymerizes a part of the monomer. Monomer concentration gradients, formed by variation in the amount of polymerization due to the variation in exposures, give rise to diffusion of monomer molecules from the regions of high concentration to the regions of lower concentration. The material is then exposed to regular light of uniform intensity until the remaining monomer is polymerized. A difference in the refractive index within the material is obtained.

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The process is simple and very suitable for machine processing. Generally the holograms are reproduced by contact copying from masters (full beam copying or scanning by a laser beam). The sensitivity of the polymer material restricts the copying speed. The polymer material is more expensive than the materials currently used for creating surface or relief holograms, a reason why volume holograms are not generally used for security documents. Despite that known cost deterrent, however, proposals do exist for making use of the higher security features available from volume holograms. For example, W02006/021102 discloses a possible combination of a volume hologram with a printed motif or even with a surface hologram to create a combined image, visible to the human eye, which indicates whether the item marked (which could be a bank note, a credit card or an identity card for example) is a valid item or not.

Volume holographic data carriers have recently been developed which are both pressure dependent and light sensitive. They are cross-linkable polymers which, if they have a volume hologram recorded on them, can vary the display colour of that hologram, dependent on the external pressure applied to the polymer during or immediately prior to a subsequent cross-linking process. Cross-linking is usually achieved by ultraviolet irradiation. Therefore if the volume hologram is recorded on a thin film of such a polymer before cross-linking, the colour of any display of that hologram can be varied and fixed by varying the external pressure applied during or immediately prior to the ultraviolet radiation which results in cross-linking of the polymer. Such pressure-dependent and light-sensitive polymeric materials have been developed by and are available from, among others, Bayer AG. The variation of the colour of any display of the recorded hologram is a consequence of distortion of the Bragg planes of the data carrier material, which distortion is fixed when the polymeric material is cross-linked. Thus cross-linking fixes the replay frequency, and therefore the colour, of the recorded hologram.

The Bayer photopolymer materials have a coated film layer thickness of about 20 micrometres. The photopolymer film is generally coated in a 12.5 inches (31.75 cm) width on a 14 inches (35.6 cm) wide Mylar polyester base which is .002 inch (0.005 cm)

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thick. The film is protected with a .00092 inch (0.0023 cm) thick Mylar polyester cover sheet.

The recording of a hologram on the above polymers is rather simple. The polymer film has to be laminated to a piece of clean glass or attached to a glass plate (or to some other pressure-resistant substrate) using an index-matching liquid. Holograms can be recorded manually, but in order to produce large quantities of holograms, a special machine is required. For hologram replication a laser line scanning technique can provide the highest production rate. The photopolymer material typically needs an exposure of about 10 mJ/cm2.

The invention makes use of the above cross-linkable pressure-dependent and light-sensitive polymers to provide a novel and useful volume holographic security marker, with widespread and novel commercial potential and immensely improved security implications.

The Invention

The invention provides a holographic security marker as defined in claim 1 herein. Other aspects of the invention are as defined in the other independent claims. Optional but sometimes preferred features of the invention are as defined in the subclaims.

It is often desirable for any security coded marking on an object to be invisible to the naked eye. The invention achieves that by making use of a volume hologram which is readable only within a narrow frequency band which is beyond the visible range. For example the hologram may be visible only in infra-red or near-infra-red light, and preferably only within a predetermined narrow band of the infra-red or near-infra-red spectrum.

According to the invention is it possible to control within precise limits the replay frequency of the recorded holographic data in a volume hologram. That may be

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achieved by controlling the replay frequency, or colour, of the original recorded hologram.

The technique of phase shift of a recorded volume hologram has been reported. For example a hologram recorded with a blue light laser having a wave length of 588 Angstroms has been phase shifted to replay as a red holographic image by pre-swelling the thin film data carrier before holographic recordal, and then drying the thin film data carrier after exposure, developing and fixing the volume holographic image. The drying causes contraction of the thin film data carrier displaces the Bragg planes within the data carrier, and moves the replay frequency of the recorded interference fringes. It has never been suggested that there might be a significant security advantage in moving the replay frequency out of the visible range altogether. The Applicants have found, however, that it is possible to move the replay frequency out of the visible range, preferably into the infra-red or near-infra-red range and possibly establishing a final replay frequency that is selected within a very narrow band width of the non-visible spectrum. If the thin film data carrier is a swellable material such as gelatin, then the phase shift can be obtained by pre-swelling the thin film data carrier before recordal of the volume hologram using light in the visible spectrum, and then phase shifting the recorded hologram by drying the thin film data carrier until it contracts to its original dimensions. Alternatively using a thin film data carrier which is a polymeric pressure-dependent light-sensitive material that is capable of volume holographic storage, the volume hologram can be created initially using light in the visible range, and then compressed at an external pressure which moves the replay frequency of the recorded volume holographic data to a band width outside of the visible spectrum. If the thin film data carrier is then cross-linked in that compressed state, then the replay frequency remains outside of the visible spectrum. Other factors affecting the phase shift can be the temperature at which the cross-linking of the data carrier with the volume hologram recorded thereon is carried out, and of course the pressure applied during or immediately prior to cross-linking. By suitable control of those two parameters, together with the replay frequency, or colour, of the original recorded hologram, and by suitable choice of the material and

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film thickness of the thin film data carrier, the replay frequency can be selected to fall within a very narrow band width of the non-visible spectrum.

If the thin film data carrier is a polymer film having distributed therein nano-sized particles of photo-sensitive material, then of course the initial creation of the recorded volume-holographic data can be achieved using a laser operating in the infra-red or near-infra-red range. Significantly higher security can, however, be achieved by utilising the phase shift techniques discussed above to move the replay frequency of the recorded holographic data into a very narrow frequency range beyond the visible spectrum, so that the recorded data requires very accurate and sophisticated equipment to be able to replay the hologram.

In a preferred aspect of the invention two volume holograms can be recorded in the same polymer film, using lasers operating at different wavelengths. Using the phase shift techniques described above the replay frequencies of both recorded holograms can be moved from the visible replay frequencies at which they were initially recorded, preferably one into a portion of the still visible range of the spectrum and the other into the non-visible part of the spectrum such as the infra-red or near infrared range.

If a swellable polymer film is used as the basis for the thin film data carrier, then that polymer is preferably capable of absorbing an aqueous or non-aqueous solution of fixing agent that is capable of removing from the polymer film unexposed particles of the photo-sensitive material after initial recordal of the volume polographic data. The film may, for example, be a gelatin film which preferably has a thickness of about 40 micro metres. Distributed through the film are nano-particles of a photo-sensitive material such as a silver halide. A particle size of about 40 nano metres has been found to be very effective. Size consistency of the nano particles is important in achieving high resolution, avoiding Raleigh scatter. The pre-swelling of the gelatin film can be achieved by initial wetting of the film with a solution of a swelling agent. Suitable swelling agents are triethylchloromethane or sorbitol, although other swelling agents may be used. The extent of swelling may be controlled accurately by control

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of the concentration of the swelling agent in the initial solution, and of course by controlling the time and temperature of exposure of the thin film to the swelling solution. That exposure may be by immersion. It has been found, for example, that immersion of a 40 micrometre thin film of gelatin containing nano-particles of silver halide in a 1% aqueous solution of triethylchloromethane increases the thickness of the film by about 3 micrometres. Higher concentrations of triethylchloromethane result in correspondingly greater amounts of swelling. After exposing the swollen film to generate the interference fringes of the encoded digital security data volume holographic image, that image can be developed by converting the exposed silver halide nano particles to silver nitrate, and fixed by removing the unexposed silver halide nano particles, for example using a washing solution of potassium dichromate in sulphuric acid. When the image has been suitably developed and fixed, the thin film is dried, which contracts it to its original thickness. The associated relative movement of the Bragg planes causes a phase shift in the recorded volume hologram, so that it is no longer capable of replay in the visible spectrum. If it is desired to maintain the recorded holographic security data permanently at a replay frequency outside the visible spectrum, then the gelatin film is preferably encapsulated in a transparent lamination of a water-impermeable material, so that it is no longer sensitive to atmospheric moisture of the application of swelling agents, either of which might cause swelling of the film to return the recorded holographic data once more into the visible replay spectrum.

If the thin film data carrier is a polymeric pressure-dependent light-sensitive material that is capable of volume holographic storage, then the application of pressure during or immediately prior to cross-linking of the polymer shifts the Bragg planes and alters the frequency of the resulting hologram display, shifting it according to the invention into the non-visible spectrum. The pressure applied during the compressing of the data carrier requires careful control since that pressure, coupled with the initial replay frequency of the recorded holographic data and the temperature and choice of data carrier material, will dictate the precise replay frequency of the recorded holographic data.

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For maximum security, however, the data carrier may have both a surface hologram and a volume hologram recorded thereon. For example, the two holograms may be married together by creating a master surface hologram; creating from that a negative, or 'mother' hologram to be used in a press; creating a volume hologram by exposing the photosensitive thin film data carrier; compressing the 'mother' surface hologram onto the thin film data carrier; and then cross-linking the thin film data carrier. The pressure applied during the pressing of the surface hologram onto the surface of the data carrier requires careful control, since that pressure, coupled with the initial replay frequency of the recorded holographic data and the temperature and choice of data carrier material, will dictate the precise replay frequency of the recorded holographic data on the data carrier.

The coding used for the security coded marking of the invention may be an image, such as a digital photograph, or any one-dimensional bar code or any two-dimensional matrix code such as a QR code. An optical scanner may be used to decode and reproduce either in the visible spectrum or in machine readable form either the digital photograph or a one-dimensional bar code, although a camera capture device would be needed to decode and analyze two-dimensional coded material. Whatever the coding, however, the security data held in the volume hologram data carrier would be invisible to the naked eye or even to scanning or camera equipment which was not tuned to the frequency programmed into the recorded holographic data for its unique display. If a surface hologram were used as the pressing tool then this surface hologram would be visible normally, thus indicating the presence and location of the security data but not revealing its identity. If a blank were used to apply the pressure to the volume hologram then the resulting security hologram would be completely invisible to the naked eye or even to scanning or camera equipment which was not tuned to the frequency programmed into the recorded holographic data for its unique display.

The data embedded within that security coded marking may be data relating to the article marked with the security code or data relating to an authorized or legal owner of the article so marked. For example, if the article marked with the security code is a

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motor vehicle, the data may relate to the vehicle itself, for example its chassis number, any coded manufacturer information that might identify the production details, or any other information as stored in a remote database to confirm the vehicle identity. Particularly in the case of high value top of the range motor vehicles, such encoded data could be a very valuable weapon in the armory of the police trying to trace stolen and possibly disguised motor vehicles.

One example of the creation of a security marker according to the invention is described below with reference to the accompanying drawings, of which:

Figure 1 is a schematic illustration of the recordal of a surface hologram;

Figure 2 is a schematic illustration of the creation of a volume hologram;

Figure 3 is a schematic illustration of a press for embossing the surface hologram relief onto the surface of the thin film data carrier which has the volume hologram recorded thereon; and

Figure 4 is a schematic explanation of the combination of surface and volume holograms onto a single carrier..

Figure 1 illustrates one system for the recording of a surface hologram master. A diode laser 1 directs its beam via a mirror 2 through a beam splitter 3 which splits the beam into object and reference beams. The object part of the split beam is directed via a mirror 4 to a beam spreader 5 which spreads the object beam to illuminate an object 6 (in this example, a motor car or model motor car). Reflected light from the object 6 passes through a lens 7 and reforms an image 8 of the object at the focal length (f) of the lens 7. The reference part of the split beam is deflected via the beam splitter 3 and passes though a rod lens 9 and is reflected by a collimating mirror 10 onto the holographic surface where it meets the object beam to form a standing wave-front. This standing wave-front is recorded on the light sensitive coating of a glass plate 11 which becomes the master of the surface hologram. Duplicates of that master can be prepared by first creating a die which is a negative of the master plate. This die can then be used in a press to emboss further copies of the original.

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Figure 2 illustrates a method for creating a volume type hologram. Light from a diode laser 20 passes through a polymer plate 21, effectively as a reference beam, onto an object 22 below. The polymer plate 21 is made of a photosensitive material and after exposure becomes the volume holographic data carrier. In this example, which creates a volume hologram suitable for further processing into a security marker according to the invention, that photopolymer material is also pressure-sensitive and the object 22 is a digitally encoded matrix code presented on a metal base plate 23. Reflected light from the object 22, which is effectively the object beam, passes back through the polymer plate 21 producing a standing wave. This standing wave produces the volume fringes within the photopolymer which, because of its photosensitivity, is exposed and hardens in the exposed areas, thus creating from the polymer plate 21 the volume holographic data carrier.

Figure 2 also illustrates a vibration-absorbing base 24, such as a plastic sponge base, which isolates the photographic hologram recording equipment from external vibrations.

Figure 3 illustrates the further processing of the volume holographic data carrier 21 of Figure 2 to create a security marker according to the invention. Figure 3 shows an apparatus for applying pressure to the volume holographic data carrier 21, comprising a pair of vice or press jaws 30 which apply pressure though flat metal plates or jaws 31 to the opposite sides of the volume holographic data carrier 21 of Figure 2 which in Figure 3 is mounted on a substrate 32 which is dimensionally stable under the pressures concerned. The pressure applied to the data carrier 21 is controlled so that it compresses the spacing between the Bragg planes of the recorded volume hologram until the replay frequency of that recorded hologram is in the non-visible range. The pressure required to move the replay of the volume hologram into the non-visible part of the spectrum depends, of course, on the replay frequency of the original recorded hologram, on the nature of the photopolymer of the data carrier 21 and on the pressure applied. It also depends on the temperature of the carrier 21 during the compression. Suitable control of the pressure can move the replay frequency into a narrow and controllable part of the non-visible spectrum, and with suitably fine control it is

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possible to tune the replay frequency down to a very narrow frequency band of from one to several wavelengths of light.

Figure 3 also shows, dotted, the insertion of a die 33 between the jaws 31 and the volume holographic data carrier 21. That is an optional refinement of the method described above. The die 33 is a metal die having the surface relief of a negative of the master surface hologram plate 11 of Figure 1, and during that pressing the surface hologram is pressed into the surface of the volume holographic data carrier 21. The result is a combined hologram as illustrated in Figure 4. Figure 4 illustrates, very schematically, the die 33 with an exaggerated surface relief on its right hand side representing the surface hologram negative; the volume holographic data carrier 21 which comprises a substrate layer 32 and a data carrier layer 21; and the data carrier (identified as 41) after compressing, when it carries the combined surface and volume holographic data.

Two examples of the practical use of the invention are given below. Those examples are not limiting. They merely illustrate two completely different applications in order to demonstrate the wide range of commercial possibilities that can be realized using the invention.

Example 1

Motor Car or Site Plant Security

Motor vehicles, particularly high specification cars with a high retail value, and construction site plant and equipment such as diggers and earth-moving machines, are particularly susceptible to theft. The vehicles or plant can then be resprayed, given false number plate identification, and sold on. Tracing the stolen vehicles and plant can be very difficult. According to the invention a holographic security marker can be prepared containing accurate data to identify each individual vehicle or plant, and secured to the vehicle or plant either on a security plate visibly secured to the vehicle or plant and/or a location which cannot readily be identified without use of equipment that will replay the hologram.

Suppose that the security marker is to be incorporated into a security plate visible on the vehicle or plant, such as the vehicle number plate. First, data is collected which is unique to that vehicle. That data could include, for example, the vehicle registration number, the chassis number, the engine number and any other identification data unique to that vehicle. That may be data kept in a remote database such as the name and address of the first recorded user, or it could be information known only to the manufacturer such as a vehicle inspection history during the manufacturing process. Once the data collection is complete, that data may be encrypted if desired, and then recorded in a digital one-dimensional bar code form as a volume hologram on a thin film of a pressure-sensitive cross-linkable photopolymer. Lamination of that thin film beneath a transparent protective layer protects the thin film while still permitting the hologram to be replayed preferably over a wide parallax. There is very little limit on the amount of unique identification data that may be collected and recorded in this way, since the amount of data that can be stored holographically in a machine readable form is very large indeed. Terabytes of encoded digital data can be contained in a single holographic coded symbol.

Because the thin film on which the holographic image is recorded is pressure-sensitive, compressing it between the plates of a press enables the user to vary the Bragg spacing so as to vary the colour of a holographic image reproduced therefrom. According to the invention the pressure applied between those press plates should be sufficient to move the display of the hologram out of the visible part of the spectrum and preferably into the infra-red or near-infra-red part of the spectrum. The pressure should be carefully controlled so as to render the replayed holographic image detectable preferably only in a narrow frequency band in the infra-red or near-infrared range, and while such a pressure is applied the thin film polymer is cross-linked by exposure to ultraviolet radiation, to set the distortion of the Bragg planes of the hologram and to maintain that holographic image in a condition of being detectable only in that narrow frequency band.

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The laminated thin film of cross-linked photopolymer, with the digital hologram recorded thereon, may be affixed either overtly or covertly to the vehicle or plant being protected. An overt fixing would be to a known and recognizable part of the vehicle or plant, such as the registration number plate. Typically it could be laminated between a transparent cover and a visual data carrier bottom layer of that number plate. The advantage of an overt fixing would be that it would be easy to aim an infra-red laser camera at the relevant part of the vehicle or plant to generate the holographic image and read the recorded data. One disadvantage would be that it would be relatively easy for the known and recognizable part of the vehicle or plant, such as the number plate, to be exchanged for one either without the erpbedded holographic data or with substitute encoded data. The replacement of the plate by one without encoded holographic data would in principle be discernible because the very absence of the correct plate would be an indication of something amiss if a whole series were known to be encoded. The replacement of the plate by one with substitute encoded data would be unlikely because the technology to create a substitute hologram which displays in the same narrow band of the non-visible spectrum would be beyond the average criminal. A covert fixing would be to a part of the vehicle where the presence of the holographic recording would be undetectable except when exposed to a laser in the appropriate range of the non-visible spectrum. For example, the holographic recorded laminated thin film could be adhered to a part of the bodywork of the vehicle or plant (preferably a smooth flat part of a body panel) and then covered with a coating of clear lacquer as part of the final spray finish to the body panel in the vehicle spray shop. Alternatively it could be adhered to a part of the windscreen glass of the vehicle or plant. Because the thin film would be transparent, its presence would be undetectable to the human eye. On the other hand a suitable infra-red camera would be able to generate and record the holographic image and capture the embedded digital information, so that the security details of the vehicle or plant could then be checked (optionally after suitable decoding of the algorithm form in which they are recorded). One disadvantage of such a covert fixing of the recorded holographic thin film laminate would be that it would require the hologram to replay at a wide angular range. Wide parallax holograms would therefore be required, but such wide parallax holograms could conceivably be read by road-side cameras, so that

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considerable additional security could be established by capturing the holographic data carried by vehicles on the move past a stationary camera system. Another possible disadvantage would be that it would be easy to paint over the top of a covertly placed film if it were affixed to a painted surface. However once again the very absence of detectable holographic security data would be an indication of something amiss if a whole series were known to be so encoded.

Example 2

Credit Card. Identity Card or Passport Identification

Instead of carrying recorded holographic information relating to the nature of the article being security marked, the invention can be used to record the legal or authorized owner of an article. For example, the ownership details of a credit card, identity card or passport could be recorded. As with the first example, the data must first be collected. Since that would be information unique to the individual owning the card or passport, it may be biometric data, personal data known only to the authorized owner (such as a password, address data or personal history) or even a digital photograph. All of that data may be stored digitally in the form of a matrix code recorded as a volume hologram, but moved according to the invention for replay only in a specific frequency range out of the visible part of the spectrum. Added security can be obtained by encrypting the data before incorporating it into the matrix code and recording the hologram. By careful control of the pressure applied to the recorded thin film hologram during the cross-linking step, replay can be controlled to take place over only a narrow bandwidth of infra-red or near-infra-red light, so that sophisticated hologram recording and polymer cross-linking equipment would be needed to record and set the frequency of subsequent display of the hologram, and equally sophisticated camera equipment would be necessary in order to recapture that information. Indeed the sole restriction to the band width of the resulting display would be the band width needed to accommodate possible temperature variations of the eventual display conditions. The display band width could therefore be reduced to a band width of only a few wavelengths of light, for example from one to five wavelengths, in the infra-red or near-infra-red range.

Because of the narrow bandwidth of data capture, and the vast amount of data that can be recorded in a matrix code hologram, the security of the card or passport may be established at a level far higher than any levels of security currently known. Security card fraud and identity theft could be virtually eliminated. Once the holographic data had been recorded on the thin film of the photosensitive and pressure-sensitive polymer, and the cross-linking under pressure achieved, then lamination of the holographic film into the credit card or identity card could follow known techniques.

The above two examples show that there is virtually no limit to the range of articles that can be security tagged using a security marker according to the invention.

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Claims

1. A holographic security marker comprising a volume holographic security symbol containing encoded digital security data on a thin film data carrier that is laminated to a surface protective film, characterized in that the replay frequency of the recorded volume holographic data is in a band outside of the visible spectrum.

2. A security marker according to claim 1, wherein the thin film data carrier is a polymer film having distributed therein nano-sized particles of photosensitive material.

3. A security marker according to claim 1 or claim 2, wherein the polymer is capable of absorbing an aqueous or non-aqueous solution of a fixing agent capable of removing from the polymer film unexposed particles of the photosensitive material after recordal of the volume holographic data.

4. A security marker according to claim 3 wherein the polymer is gelatin and the photosensitive nano particles are particles of a silver halide.

5 A security marker according to any preceding claim, wherein the volume holographic data has been recorded using a scanning laser operating in the infra-red or near-infra-red range, creating a recorded volume hologram having a replay frequency in the infra-red or near-infra-red range.

6. A security marker according to claim 4, wherein the gelatin thin polymer film has been caused to swell before recordal of the volume hologram by treatment with an aqueous solution of a swelling agent, the volume hologram has been recorded using a scanning laser operating in the visible range, creating a recorded volume hologram having a replay frequency in the visible range, and then the swollen gelatin thin polymer film has been dried to cause it to contract, moving the replay frequency of the recorded volume hologram into the non-visible range.

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7. A security marker according to claim 6, wherein the replay frequency of the recorded volume hologram is moved into the infra-red or near-infra-red range by drying the swollen gelatin thin film.

8. A security marker according to any of claims 6 to 7, wherein the swelling agent is triethylchloromethane.

9. A security marker according to claim 6 or claim 7, wherein the swelling agent is sorbitol.

10. A security marker according to claim 1, wherein the data carrier is a polymeric pressure-dependent light-sensitive material that is capable of volume holographic storage, and which has been cross-linked, after recordal of volume holographic data thereon, at an external pressure which moves the replay frequency of the recorded volume holographic data to a band width outside of the visible spectrum.

11. A security marker according to claim 10, wherein the thin film data carrier has a surface hologram embossed into the surface thereof by the application of a surface hologram die under pressure, before the cross-linking which fixes the replay frequency of the recorded volume holographic data.

12. A security marker according to any preceding claim, wherein the holographic data recorded on the thin film data carrier includes digital volume hologram recorded data.

13. A security marker according to any preceding claim, wherein the replay frequency of the recorded holographic data is in the infra-red or near-infra-red part of the spectrum.

14 A security marker according to claim 13 wherein the replay frequency of the recorded holographic data is in only a narrow and predetermined frequency range within the infra-red or near-infra-red part of the spectrum.

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15. A security marker according to claim 14, wherein the narrow and predetermined frequency range is a range of from 1 to 5 wavelengths of light within the infra-red or near-infra-red part of the spectrum.

16. A security marker according to any preceding claim, wherein the marker is secured to an article and the recorded data is unique to that individual article.

17. A security marker according to any of claims 1 to 15, wherein the marker is secured to an article and the recorded data is a repetition of data contained in a remote database that is unique to that individual article.

18 A security marker according to any of claims 1 to 15, wherein the marker is secured to an article and the recorded data is unique to a lawful or legitimate owner of that article.

19. A security marker according to claim 18, wherein the recorded data includes biometric data or photographic data that is unique to the lawful or legitimate owner of the article.

20 A security marker according to any of claims 16 to 19, wherein the article is an article of commerce.

21. A security marker according to any of claims 16 to 19, wherein the article is a credit card, identity card or passport.

22. A security system which comprises affixing to an article a holographic security marker according to any preceding claim, the recorded holographic data on which has a predetermined replay frequency outside of the visible spectrum, and checking the identity or ownership of that article by reading the data from that hologram while it is illuminated with light with a wavelength corresponding to that predetermined replay frequency.

-20-

23. A security system according to claim 22, wherein the holographic data is recorded on the data carrier with a wide parallax replay range, so that the holographic data can be read while the cuticle moves past a data reader responsive to the predetermined replay frequency.