GB2221550A

GB2221550A - "Holographic data carriers"

Info

Publication number: GB2221550A
Application number: GB8913254A
Authority: GB
Inventors: Kenneth John Drinkwater
Original assignee: Chubb Electronics Ltd
Current assignee: Chubb Electronics Ltd
Priority date: 1988-06-15
Filing date: 1989-06-08
Publication date: 1990-02-07
Also published as: GB8814233D0; GB8913254D0

Abstract

A process for the manufacture of coded arrays of holographic optical elements (HOEs) for use on data carriers to represent e.g. identification data on access-control cards or authentication data for documents or goods. The process accordingly involves the creation of a master hologram (4) which is constructed to diffract incident light (5) into a plurality of different reconstructed beams (6) and the use of the master hologram in a holographic production system to direct its reconstructed beams (6) into a recording medium (2) for interference with a reference beam, the reference (3) and/or said reconstructed (6) beams being spatially modulated by a mask (7) comprising a coded array of discrete transparent areas. In this way a large number of different coded HOE arrays with multiple reconstructed beams can be produced from only one (or a limited number of) master hologram(s) in conjunction with differently coded masks. <IMAGE>

Description

Data Carriers The present invention relates generally to data carriers and more particularly to the storage of data represented by bar codes or other forms of coded arrays of holographic optical elements (hereinafter referred to as HOEs). By "holographic optical element" we mean a holographic diffraction grating structure which is adapted to reconstruct an output light beam in a specific angular direction by diffraction of incident light of a specific wavelength and incidence angle.

The invention is concerned in particular with a process for the manufacture of coded HOE arrays to be used for security purposes, where they may be applied to, for example: access-control cards or the like tokens, where they represent authorisation or identification data necessary e.g. for gaining access to a restricted area, operation of a cash-dispensing machine or the provision of some other service; documents such as credit cards, tickets, passports, licences, identity cards, wills, cheques and the like, as a means of authenticating the same; and goods which are vulnerable to counterfeiting, for example audio/video tapes and discs and various other high-value branded consumer goods, again as a means of distinguishing the genuine goods from unauthorised copies.

However, this list of potential applications for HOE arrays manufactured in accordance with the invention is not intended to be exhaustive and the term "data" as used herein is to be given its widest meaning.

Data carriers of this general character are known e.g.

from International patent application no PCT/GB86/00109 (WO 86/05300) of KJ Hayden, where data is represented by coded arrays of thick-film reflection HOEs and reading or recognition of the data is achieved by detection of light emanating in specific directions from the discrete reflective regions of the HOE array when they are illuminated at specific incidence angles.

We recognise that one way in which to increase still further the security of the data on such carriers from fraudulent imitation or copying is to arrange that the HOEs within a coded array will "reflect" or otherwise diffract light incident from a specific angular direction into a plurality of reconstructed beams of different selected angular directions (or vice versa); consequently to read or authenticate the data correctly it is necessary to detect light emanating from individual HOEs at a plurality of different angles when illuminated at a specific incidence angle (or vice versa).

The present invention proposes a process for the manufacture of such coded HOE arrays which comprises: creating a master hologram which is constructed to diffract incident light into a plurality of reconstructed beams of different selected angular directions; and setting up a holographic production system in which a reference bean is incident upon a holoyraphic recording medium from one side thereof; said master hologram is located on the other side of said medium to diffract said plurality of.reconstructed-beams into said medium, for interference therein with-the reference beam; and a mask comprising a-coded array of discrete transparent areas is placed to one or other side of the recording medium so as to spatially modulate the reference beam and/or said reconstructed beams; whereby the interference pattern recorded in said medium comprises a coded array of discrete HOEs the spatial arrangement of which is determined by the spatial arrangement of said transparent areas and each member of which is adapted to diffract light incident from a specific angular direction into a plurality of reconstructed beams of different angular directions, (or vice versa), as determined by the diffraction characteristics of the master hologram.

In a preferred embodiment the master hologram comprises a transmission hologram which is located to diffract into the recording medium light from a separate object beam which is directed to the master hologram from the opposite side of the recording medium to the reference beam.

It is also within the scope of the invention, however, for the master hologram to comprise a reflection hologram which is located to diffract back into the recording medium light from the reference beam which is transmitted through said medium to the master hologram.

It is to be understood that the use of the term "light herein does not imply a limitation to electromagnetic radiation lying strictly within the visible spectrum and includes in particular radiation within the infra-red and ultra-violet bands.

The invention will be more easily understood from the following description of exemplary embodiments thereof, taken in conjunction with the accompanying drawings, Figures 1-3 of which are respective schematic sectional views through a recording medium, mask and master hologram set up for exposure in a holographic production system in accordance with respective embodiments of the process of the invention.

Referring to Figure 1, the intention is to create a coded array of thick-film reflection HOEs 1 in a suitable holographic recording medium 2, such as a silver halide emulsion, the recorded data being in the form of a bar code where the presence or absence of a HOE at each available position along a track represents a digital "1" or "0". The array could equally be, however, in the form of a two-dimensional matrix, coded by the presence or absence of a HOE at each available position. In any event, it is anticipated that a large number of separate coded arrays will be recorded together at any one exposure, on an emulsion sheet 2 of, say, A4 size, the developed arrays ultimately being separated and affixed to respective access-control cards, passports, labels or other data carriers.For the purpose of this description, however, the production of only a single, simple array is illustrated.

From one side the emulsion 2 is exposed to a reference laser beam 3 incident at a selected angle. At the opposite side of the emulsion is located a master hologram 4. This master comprises a sheet upon which has been created a thick-film transmission HOE extending over the whole of its useful area and constructed so as to diffract light which is incident to any portion of it at the angle of an object beam 5 into a plurality, say three, of reconstructed beams 6 having different specific exit directions.

The master may itself be created by exposure of a sheet of suitable holographic recording medium, such as a silver halide emulsion or dichromated gelatin, to reference and object light beams directed from the same side at respective angles corresponding to the incidence angle of the beam 5 and, successively or simultaneously, the conjugates of the exit angles selected for all of the reconstructed beams 6.

Located next to the master hologram 4 in the path of the object beam 5 is a spatial light modulator or mask 7.

This mask is conveniently in the form of a laser-printed transparent sheet, upon which the code to be transferred to the emulsion 2 is represented by an array of discrete transparent areas left between printed opaque areas 8.

In operation, the mask 7 spatially modulates the object beam 5 so that light reaches only those portions of the master hologram 4 which register with the transparent areas (apertures) of the mask. This light is diffracted by the portions of the master registering with the mask apertures to reconstruct at each position the plurality of reconstructed beams 6 at the selected exit angles, which interfere with the beam 3 in respective corresponding portions of the emulsion 2.From a single exposure there is thus stored in the developed emulsion an array of HOEs 1 each one of which is constructed to "reflect" light from an incident angle corresponding to that of beam 3 into a plurality of directions corresponding to those of the conjugates of the reconstructed beams 6 entering the emulsion from the master hologram 4 (or viceversa), the spatial arrangement of this array corresponding to that of the array of apertures in the mask 7.

In practice, a treatment step may also be included to cause swelling of the emulsion 2, to change the spacing of the interference fringes in the HOEs 1 so that they will "reflect" light of a different wavelength to that from which they were created, e.g. so as to shift the reconstructed wavelength from the visible into the infra red band. Such swelling also causes a tilt in the angle of the interference fringes so that the reconstruction angles will in fact differ somewhat from those of the reconstructed beams from the master hologram 4 by which the HOEs 1 were created, when this additional treatment is employed.

Clearly, a large number of differently-coded arrays of HOEs 1 can be produced using the same master hologram 4 in a method as described above, by substituting masks 7 with differently-coded arrays of apertures. Variation of the angular diffraction characteristics of the HOEs 1 within an array can also be achieved using the same laser rig by substituting master holograms 4 constructed to produce different sets of reconstructed beam directions from the same object beam 5.

The present invention therefore provides a method of producing large numbers of individual coded HOE arrays the members of which are each arranged to diffract incident light into a plurality of different selected angular directions, which is both simple and cheap, since the necessity to make multiple exposures within a complex optical system applies only to the production of the one (or a limited number of) master hologram(s).

In a variation of this arrangement shown in Figure 2 the components of the emulsion/mask/hologram "sandwich" are rearranged so that the mask 7 is instead located to the same side of the emulsion 2 as the reference beam 3, the master hologram 4 still of course being located on the other side of the emulsion. The mask therefore spatially modulates the beam 3 instead of the beam 5 but the effect in limiting the areas of the emulsion 3 in which interference takes slacks to produce the HOES 1 is similar.

In a further variation as shown in Figure 3 -the master transmission hologram is replaced-by a reflective equivalent 4A located on the opposite side of the emulsion 2 to the reference beam 3 and used to create multiple object beams 6A, for interference with the reference beam in the emulsion, from light transmitted to it through the emulsion from the reference beam.

It is of note that although the drawing of necessity illustrate all light beams in a single plane, the hologram 4 or 4A can and generally will be arranged to produce reconstructed beams 6 in different planes so that the "reflected" beams from the HOEs 1 produced in the resultant coded arrays will likewise not be uniplanar.

Finally, although for ease of illustration the layers of the emulsion/mask/hologram "sandwiches" are shown somewhat spaced apart in the drawings, in practice they will be arranged in close contact, in particular to minimise divergence of the beams 6, 6A before interference with the beam 3.

Claims

1. A process for the manufacture of coded arrays of holographic optical elements (HOEs) which comprises: creating a master hologram which is constructed to diffract incident light into a plurality of reconstructed beams of different selected angular directions; and setting up a holographic production system in which a reference beam is incident upon a holographic recording medium from one side thereof; said master hologram is located on the other side of said medium to diffract said plurality of reconstructed beams into said medium, for interference therein with the reference beam; and a mask comprising a coded array of discrete transparent areas is placed to one or other side of the recording medium so as to spatially modulate the reference beam and/or said reconstructed beams; whereby the interference pattern recorded in said medium comprises a coded array of discrete HOEs the spatial arrangement of which is determined by the spatial arrangement of said transparent areas and each member of which is adapted to diffract light incident from a specific angular direction into a plurality of reconstructed beams of different angular directions, (or vice versa), as determined by the diffraction characteristics of the master hologram.

- rz P, t said master holcnrnm comprises a transmission hologram which is located to diffract into the recording medium light from an object beam which is directed to the master hologram from a position to the opposite side of the recording medium to the reference beam.

3. A process according to claim 1 wherein said master hologram comprises a reflection hologram which is located to diffract back into the recording medium light from the reference beam which is transmitted through said medium to the master hologram.

4. A process according to any preceding claim comprising the subsequent step of treating the recording medium to swell the same whereby to change the reconstruction wavelength of the HOEs created therein.

5. A process for the manufacture of coded arrays of holographic optical elements substantially as hereinbefore described with reference to the accompanying drawings.