HOLOGRAM INCLUDING A COVERT IMAGE

FIELD OF THE INVENTION

This invention relates to methods and apparatus for recording a colour volume reflection hologram including a covert image, and to holograms created using this method, in particular for security applications. In another aspect the invention relates to methods and apparatus for fabricating volume reflection holograms which provide colour mixing effects, and to holograms fabricated using these techniques. In a still further aspect the invention relates to methods and apparatus for producing multicolour, three-dimensional (3D) volume reflection holograms, and to holograms fabricated using these techniques.

BACKGROUND TO THE INVENTION

One of the most successful covert security devices used in the security industry for embossed holography is the ability to reveal a covert code or alphanumeric by application of a pen-light laser to the film in conjunction with a suitably placed viewing screen

There are published patents, for example WO 92/04692, covering methods for surface relief structures for embossed holography which achieve such projected identification images when illuminated by any laser beam. Other companies, for example Dai Nippon, have described the use of e-beam lithographic techniques to achieve a similar effect. Further background prior art can be found in US2005/0179968.

This is a successful technique and provides an image security feature which is difficult to compromise. However, these are embossed holograms of the transmission type where image colour is achieved by utilisation of dispersive line gratings. Thus, whereas the WO'692 masters may be made with a blue laser at say 458nm, as is

commonly used in embossed holography origination, the laser verified image component can be reconstructed with, for example, a typical "key-ring" or "pen-lite" 1-5 mW red laser simply by making an angular adjustment of the incident beam and the viewing screen. Thus it will be seen that the actual colour of the interrogating laser is relatively unimportant, and that the details of the origination method such as precise angle of reference become relatively unimportant and illicit duplication may become somewhat easier to achieve.

In some cases the covert feature is interrogated with a hand held laser and a manually placed screen with the unsatisfactory effect the examiner is often involved in a somewhat haphazard search for the hidden feature. This can have the result that when the hidden feature is finally found, little attention is paid to the image details such as type face which under normal conditions of interrogating a security hologram would be expected to be of fundamental importance in the recognition of the hologram. There is therefore a need for improved techniques.

As the skilled person knows, holography has been frequently and successfully used as a security device for more than 25 years, due to the complexity of the process for the manufacture of mass produced holograms.

Volume holography has important advantages over embossed holography as a security device. Its phase recording mechanism for optical frequencies produces refractive index modulation within the thickness of a layer of transparent recording material and accordingly produces planar fringe structures which are capable of reflective effects with the ability to reproduce full colour (tri-stimulus) three dimensional phase images.

Unlike transmission (embossed) holography, volume reflection holography is able to represent both vertical parallax and stable image colour, even when the hologram, illumination source, or viewing position is varied.

The techniques used in WO1993/024333 and US 5,694,229 to provide security devices for embossed holography, specify a surface relief structure in the form of a

transmission hologram, which reconstructs the Moire interference between displaced overlapping geometric patterns.

To achieve this, two masks, spatially separated in the z-plane, both bearing geometric line patterns, stand in front of a backlit diffuser on the holographic mastering table, in the creation of the Hl master for a rainbow transmission hologram.

The Hl hologram records the animation seen when the viewer moves his line of view to the left or right. This property of a first generation rainbow (Benton) transmission master hologram, to record a whole linear distribution of views with a range of parallax, of a three-dimensional or animated subject, can be easily transferred to a second generation transmission surface relief hologram capable of producing a metal surface relief image for embossing. Typically the first generation Hl master would be recorded as a 'deep' phase transmission hologram in a silver halide recording medium, and the second generation H2 would be reproduced in a photo-resist coating capable only of linear surface relief recording. This surface relief hologram is capable of metallization to produce a nickel embossing shim suitable for a pressing operation to reproduce the holographic image into a thermoplastic material as a mass produced foil at low cost.

Typically the animation of the Moire pattern described resembles the apparent pattern of rushing lines we frequently experience when viewing twin paling fences on bridges over a motorway, for example. However, such a pattern is created by predominantly linear components, whereas the new technique described herein later may involve linear, curved, or bifurcated lines of complex structure.

These Moire techniques have proved a successful security device but are restricted to a single colour in the rainbow format of the embossed Benton hologram. There are also restrictions upon parallax limitation to the horizontal plane. This is based upon the Benton rainbow hologram configuration and specifically covers the situation where the centre of parallax symmetry is in line with the recording slit so that by definition only one rainbow colour is involved. Because of the lack of vertical parallax displayed in embossed holography, the animation effect is limited to the horizontal plane. The viewer moving his eyes vertically will simply see precisely the

same pattern of lines and animation in another colour of the rainbow. It is difficult to 'back-engineer' the holographic image created by this effect as it is not obvious what are the source individual geometric patterns, without the ability to separate them and analyse them individually. Again, however, there is a need for improved techniques.

We now consider the production of multicolour, three-dimensional (3D) volume reflection holograms: It is conventional for holographic stereograms to follow the teachings of Stephen Benton's rainbow hologram theory and eliminate vertical parallax from the image.

Since rainbow holograms are able to prevent the colour smearing experienced by transmission holograms illuminated in white light by effectively substituting their vertical image parallax with colour change — it has become conventional to record slitted (strip) rainbow masters with a lateral series of channels containing individual views of photographic recordings with stereographic properties.

By recording a longitudinally displaced series of such rainbow component masters from colour separations of the stereographic sequence, these can be used to create a full colour holographic image.

These "full colour" images are difficult to define as "true colour" however, because embossed rainbow holograms have the disadvantage that they change colour as the illumination or viewing position is changed in the vertical plane. Whilst the intended colour may well be perceived when the hologram is carefully set up so as to be viewed axially, any attempt to view the image another angle of illumination or view will lead to a considerable change in the gamut of colour experienced.

Thus, say, a human portrait image has the serious disadvantage that as the film is tilted, for example, the lips of the subject will quite suddenly and obviously change to yellow, green and then blue with the effect that the illusion of reality is seriously deteriorated.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is therefore provided a method of recording a colour volume reflection hologram including a covert image, the method comprising: providing a multicolour volume reflection hologram (Hl) master storing an overt multicolour three dimensional image; providing a colour hologram recording medium spaced away from said hologram master and facing said hologram master, a face of said hologram master facing said colour hologram recording medium being a front face of said hologram master; illuminating said front face of said hologram master with multicolour illumination to replay said multicolour three-dimensional image for recording in said recording medium, said multicolour illumination being illumination of at least first and second wavelengths; providing a mask patterned with said covert image on a rear surface of said hologram master; illuminating said rear surface of said hologram master with light of a third wavelength different to wavelengths of said multicolour illumination; and recording in said recording medium said multicolour three-dimensional image and said covert image at said third wavelength to thereby provide a hologram which reproduces said overt multicolour three-dimensional image under white light illumination and which reproduces said covert image, spaced away from said overt image, under substantially monochromatic illumination.

The principle image could, for example, have three colour components and the covert image might be in a fourth colour, and so forth. In embodiments of the method the substantially monochromatic illumination is at the third wavelength - that is the wavelength used to record the covert images is also used to reproduce this image in the recorded volume reflection hologram. However in other embodiments of the method the recording of the 3D image and/or covert image also involves processing the holographic recording material to shift the replay wave-length, for example physically or chemically say by swelling or contracting the recording medium to change the fringe spacing during processing. This can be advantageous, in that the recorded volume reflection hologram can be made harder to copy by moving away from a standard laser wavelength.

In embodiments of the method the reproduced covert image is spaced away from the recorded volume reflection hologram by a distance which is substantially the same as the distance from which the recording medium was spaced away from the hologram master during the recording process. Embodiments of the recording process may employ a so — called "white laser" which comprises multiple sources of substantially monochromatic, coherent light.

The recording medium for the volume reflection hologram may be a film-based medium or a glass plate. The emulsion is preferably on the front surface of the recording medium, that is the side facing the Hl hologram master (to reduce effects from reflections within the carrier for the emulsion). Similarly, preferably the emulsion of the Hl master is on the front face of the master (facing the H2 recording medium). The skilled person will understand the Hl, H2 terminology as referring to first and second generation holograms.

In some preferred embodiments the mask on the rear surface of the hologram master comprises a black, for example lithographic, film patterned in transmission with the covert image. Preferably a diffuser is also provided for the mask, to diffuse light from the covert image over a majority of or substantially all the area of the H2 recording medium. The skilled person will be aware of many suitable diffusing media, for example plastic diffusing media.

In some embodiments of the method the 3D multicolour image is arranged to be substantially adjacent to the H2 volume reflection hologram, for example straddling the film or plate whereas, as previously mentioned, the covert image is spaced asway from the plate. The angle of replay of the covert image (absent any shift due to processing) is substantially the same as the recording angle for the covert image-that is the angle between the replayed covert image and (laser) illumination of the H2 volume reflection hologram is substantially the same as the angle between the multicolour or white laser reference beam used to record the multicolour 3D image and the (laser) light illuminating the mask bearing the covert image.

The invention also provides apparatus for recording a colour volume reflection hologram including a covert image, the apparatus comprising: a holder for a

multicolour volume reflection hologram (Hl) master storing an overt multicolour three dimensional image: a holder for a colour hologram recording medium spaced away from said hologram master and facing said hologram master, a face of said hologram master facing said colour hologram recording medium being a front face of said hologram master; one or more of laser light sources to illuminate said front face of said hologram master with multicolour illumination to replay said multicolour three-dimensional image for recording in said recording medium, said multicolour illumination being illumination of at least first and second wavelengths; wherein a rear surface of said hologram master has a mask patterned with said covert image; a laser for illuminating said rear surface of said hologram master with light of a third wavelength different to wavelengths of said multicolour illumination; and wherein said apparatus is configured for recording in said recording medium said multicolour three-dimensional image and said covert image at said third wavelength to thereby provide a hologram which reproduces said overt multicolour three-dimensional image under white light illumination and which reproduces said covert image, spaced away from said overt image, under substantially monochromatic illumination.

The invention further provides a colour volume reflection hologram including a covert image, which reproduces an overt multicolour three-dimensional image under white light illumination and which reproduces said covert image, spaced away from said overt image, under substantially monochromatic illumination, in particular of a specific, defined wavelength.

The invention still further provides a method of replaying a colour volume reflection hologram storing an overt multicolour three dimensional image and a covert image, the method comprising illuminating said hologram with white light to replay said overt image and illuminating said hologram with substantially monochromatic light to replay said covert image on a screen spaced away from said hologram.

The skilled person will appreciate that, if desired, the H2 volume reflection hologram may be employed to create a subsequent generation of volume reflection hologram(s), for example, and H3 hologram or holograms and so forth.

Colour mixing effects

According to another aspect of the invention there is provided a method of providing a security hologram, the method comprising: recording a plurality of patterns of transmission at different respective wavelengths into a multicolour volume reflection hologram master to record in said hologram master a plurality of projected light patterns at said different wavelengths, said projected light patterns overlapping one another on replay; replaying said recorded volume reflection hologram master using a plurality of substantially monochromatic coherent light sources to create a replayed image comprising said different overlapping projected light patterns; and recording said replayed image in a second volume reflection hologram to provide a volume reflection hologram which replays in white light with a colour mixing effect which changes with viewing angle.

In some embodiments of the method the different mask patterns which produce the different projected light patterns which create the Moire -like colour mixing effects are at different respective distances from the hologram master. However this is not essential because some preferred embodiments of the technique employ a multichannel hologram master in which different views are recorded in different spatial regions, for example, stripes of the hologram master. In this way two adjacent views can produce images which on replay overlap spatially giving a colour mixing effect. Thus colour mixing can appear even when the patterns are substantially the same distance from the hologram master, in effect overlapping laterally. The skilled person will appreciate that with such a multichannel hologram master approach although separate views are encoded in different spatial regions of the hologram master, when this is replayed and copied to the second (H2) hologram these are encoded with substantially the entire (H2) hologram.

In preferred embodiments of the technique the patterns recorded in the hologram master comprise patterns of illumination at different wavelengths projected through different respective masked patterns. It will be appreciated, however, that Moire-like colour mixing effects can be achieved by employing substantially the same pattern recorded with different wavelengths and displaced either laterally or in depth, albeit that more complex patterns can be created with different masked patterns for different

wavelengths. I will also be appreciated that embodiments of the above described method may be employed to generate "black-and-white" patterns — that is patterns which have the appearance of monochrome Moire fringes.

In some embodiments the different wavelengths may be recorded sequentially in the hologram master, for example by using laser light of a first wavelength with a first pattern in a first location with respect to the hologram master, and then a second laser wavelength with a second mask pattern (which may be the same as the first) at a second location with respect to the hologram master. However in other embodiments a mask with different regions having different colours or different wavelength filtering properties may be employed, for example a colour pixellated spatial light modulator such as a colour LCD display. With such an arrangement multiple wavelengths may be recorded simultaneously in the hologram master by displaying a coloured pattern on the SLM. As previously mentioned, in some preferred embodiments a plurality of views of the patterns may be recorded in the hologram master, each representing a view of a pattern at a different respective viewing angle. Such a view may be of a combined or overlapping group of patterns (masks) or of a single pattern (mask). With such an arrangement white light replay of the second volume reflection hologram reproduces a different viewing angle and, more particularly, these appear to change with the viewing angle of an observer of the second volume reflection hologram. This provides a further technique which may be employed to generate a colour mixing effect and hence Moire-like colour mixing patterns. In the hologram master the different channels may be encoded as slots or stripes or pixels or in other ways.

The invention also provides apparatus comprising means for implementing the above described method steps. The invention further provides a volume reflection hologram recorded using an embodiment of a method as described above, more particularly the second volume reflection hologram as described above.

Thus in a further aspect the invention provides a volume reflection hologram which replays under white light illumination a plurality of different light projection patterns at a corresponding plurality of different wavelengths, said patterns being defined by a set of transmission masks, different for said different wavelengths, said masks

overlapping and being displaced from one another in replay space such that by changing a viewing angle said overlap changes create a colour mixing effect from said changing overlap which changes with viewing angle.

The invention still further provides a volume reflection hologram which replays under white light illumination to create moire-like patterns of colour mixing, which colour mixing patterns change with a position of a viewer with respect to said hologram.

It will be appreciated that, broadly speaking, embodiments of the above described methods and holograms employ patterns of projected light rather than projected light rather than patterns of physical obstruction of light which generate Moire fringe patterns. Thus the holograms record these projected light patterns and because these patterns are effectively patterns in three-dimensional space the projected light patterns overlap and therefore also 'mix' in a manner which depends upon the observer, more particularly the location/angle of the observer with the respect to the hologram (the second volume reflection hologram described above). It will also be appreciated that this colour mixing is a property of human vision in that, for example, where red and green colours overlap the eye sees the result as yellow rather than as, say separate reddish green or greenish red images. Thus in embodiments the colour mixing gives the appearance of a colour of a substantially spectrally pure further wavelength where the images overlap. Optionally therefore, a pattern in this third wavelength may also be recorded in the hologram master and then copied to the second volume reflection hologram to still further increase security similar to the approach we have previously described in W02006/077445, providing colours which appear the same to a human observer but which are distinguishable by machine (camera) or by eye only with the aid of special lighting or filters.

Examples of the colours which may be employed to give a colour mixing effect are green (say a frequency -doubled YAG laser at 532nm) and red (for example a Krypton laser at 647 nm or a helium neon laser at 633nm), combining to give yellow. Another example might additionally or alternatively employ a green laser (for example Argon at 514nm and a blue laser (for example Argon at 488nm) to create, on mixing, a cyan colour. The skilled person will readily appreciate that many other combinations of different wavelengths may additionally or alternatively be employed.

The patterns which appear on replay are Moire-like patterns, but are patterns of colour mixing, having a Moire-like appearance in the new colour which results from the mixing, but generally in a pattern in combination with the other colours used to create the mixing effect. Complex patterns may be created in this way.

The skilled person will also appreciate that embodiments of the above described method may readily be extended to create, for example, a third generation (H3) hologram from the second (H2) volume reflection hologram, and so forth.

Multicolour 3D volume reflection holograms

According to a still further aspect of the invention there is provided a method of providing a hologram of a multicolour, three-dimensional (3D) object, the method comprising: inputting three-dimensional data describing said 3D object; generating a plurality of multicolour views of said 3D object from said 3D data, said views of said 3D object comprising views from a grid of different viewing positions ranging over an area, said grid of viewing positions comprising viewing positions varying both horizontally and vertically; displaying each of said multicolour views in turn; recording each said multicolour displayed view in a channel of a multicolour volume reflection hologram master, a said channel of said hologram master being defined by a selected spatial region of said hologram master, said channels being substantially non- overlapping; replaying said recorded multicolour displayed views of said 3D object together; recording said replayed multicolour displayed views simultaneously in a second volume reflection hologram to provide a volume reflection hologram which replays in white light said multicolour 3D object such that a viewer is able to view said object as if from different viewing positions.

Broadly speaking in embodiments of the method we take advantage of the concept that when an observer views a 3D object they are, with either eye, in effect, seeing a 2D view of the object from a particular angle. Thus multiple channels of the volume reflection hologram (Hl master) are employed to encode these different viewing angles and these are all recorded simultaneously in the second generation, H2, volume reflection hologram. More particularly different views are recorded in different

channels of the Hl master to provide a stereographic hologram to recreate the different views of a replayed object seen by the two different eyes of an observer to create binocular vision thus creating a depth effect. Thus the Hl master, and therefore also the H2 hologram encodes a plurality of different views providing this binocular/stereographic depth effect. However embodiments of the technique go further than this in that not only do multiple different channels of the Hl master hologram encode different views from different lateral positions, to provide a stereographic depth effect, multiple channels of the Hl master (and hence also of the H2 second generation hologram) encode (3D) views from different vertical as well as lateral/horizontal positions/angles. This allows for example, a stereographic hologram with a 3D depth effect of, say, the head of a person in which, if the viewer moves their head up and down to view from different vertical angles with respect to the hologram, the viewer can see both the top of the head and under the chin all with full binocular/stereographic depth.

These different channels are encoded in different spatial regions of the Hl hologram master and, on replay and copying the H2 hologram these channels are encoded within substantially the entire H2 hologram. In the Hl master it is conceptually convenient to encode different lateral views corresponding to the same vertical position (a different lateral views giving binocular depth because of the different lateral displacements of an observer's eye) within different regions or pixels of a horizontal stripe across the Hl hologram master. Thus in the Hl master each horizontal stripe may encode a set of different lateral views from a single vertical view. A second horizontal stripe above or below the first may then be employed to encode a further series of lateral views but with a slightly displaced vertical position up or down. In this way a set of horizontal stripes, each comprising a set of pixels, may be recorded on the Hl hologram master, each vertical stripe defining a vertical viewing position of the 3D object of which the hologram is made. Thus, conveniently, the Hl hologram master may be, in effect pixelated with each (x,y) pixel address defining an (x,y) viewing position in space of the 3D object the hologram depicts (taking the (x,y) viewing position as a viewing position in a plane looking towards an object displaced away from the plane in a z-direction, for example perpendicular to the plane).

It will be appreciated from the above that when each channel of the Hl hologram master is recorded a hologram should be recorded of the object from a specified (x,y) viewing position, conveniently corresponding to the (x,y) position of the pixel in the Hl master in which this viewing position is encoded (the other pixels are masked during the recording process into each channel/pixel). To display such a view the hologram may comprise a hologram of an image displayed on a screen such as a computer monitor - although other images, for example photographs, may be employed. Thus if, say, a full 3D colour holographic portrait is being created a plurality of images may be captured of the individual from the different viewing positions and these may then be displayed in turn and sequentially recorded in the pixels of the Hl volume reflection master. This is also convenient as it avoids the need to expose the individual to laser light.

It will be recognised that a hologram may be created of an image displayed on, say, a computer monitor or by displaying the image on say, a spatial light modulator and using this together with a reference beam, for example, a "white" laser beam to form the hologram, more specifically a channel of the Hl master. It will also be appreciated, however, that interesting and novel combinations of imagery and graphics may be created and displayed and employed to generate holograms of computer generated 3D objects. Thus an example might be to employ arial imagery of the earth in relatively high resolution and to combine this with say, graphics such as lines of latitude/longitude and/or text such as labels of continents/countries and the like. The 2D images corresponding to each of the (XY) viewpoints of this imaginary object may then be displayed and captured in separate channels of the Hl hologram master, which is then copied to encode all the channels onto the H2 volume reflection hologram. When replayed this H2 hologram will display the multicolour, 3D object which, in this example, comprises a miniature, high resolution 3D view of the earth as a globe with (or without) super imposed computer graphics. It will be appreciated that a very wide range of images of real or imaginary or difficult to otherwise encode 3D objects may be created in this manner.

The skilled person will readily appreciate that the H2 hologram may be employed to create a third (H3) and optionally subsequent generation hologram by copying the H2 volume reflection hologram.

The invention also provides apparatus configured to implement a method as described above, in particular comprising means to implement the steps of the above described method.

The invention further provides a hologram, more particularly a volume reflection hologram, fabricated, according to a method as described above.

Thus the invention also provides a volume reflection hologram which when viewed under white light illumination, provides a multicolour 3D view of an object, and wherein said 3D view animates with change of viewing position.

Still further, animated 3D objects may be created by encoding the stereographic views of a 3D object in a set of channels of the Hl master and then by creating multiple sets of such channels, each set of channels representing a different 3D " frame" of the animation. Thus, for example, the different frames of the animation may be encoded by vertical (y) position of the viewer (x,y) space. Thus in one example implementation of such a technique a viewer sees a stereographic 3D image with depth (due to the different lateral views presented to the viewers eyes, (presuming the volume reflection hologram is being viewed roughly horizontally), and upon moving the viewing position vertically the animation effects are seen. A simple example given later is that of traffic lights which appear fully 3D with an illusion of depth being created by stereographic/binocular views presented to the observer's eye and which changes from red to green when a viewer displaces their head in a vertical direction. (Here "vertical" can be taken to be perpendicular to a line between the observer's eyes).

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures in which:

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Figure 1 shows recording of a security volume reflection hologram according to an embodiment of the invention, more particularly recording of a first generation master Hl Hologram;

Figure 2 shows the Hl recording process of figure 1 viewed from above;

Figure 3 shows a contact copying process for recording an H2 hologram;

Figure 4 shows replay of an embodiment of a security volume reflection hologram according to an aspect of the invention;

Figure 5 shows an example of a marked viewing screen for use with an embodiment of a security volume reflection hologram according to an aspect of the invention;

Figure 6 shows apparatus and a method for providing a security hologram incorporating a moire colour mixing effect, according to an embodiment of an aspect of the invention;

Figure 7 shows an H2 recording arrangement using an Hl master fabricated with the apparatus of figure 6;

Figure 8 shows, schematically, replay of an embodiment of a security volume reflection hologram according to an embodiment of an aspect of the invention;

Figure 9 illustrates differences between an Hl mastering mode and a Denisyuk technique;

Figure 10 shows an example of slitting an Hl master hologram;

Figure 11 shows an "image plane" concept - any image can be placed at any position with respect to the surface of the final film, such as in front, astride, and behind;

Figure 12 shows vertical parallax according to the prior art - if we substitute the cube for a flat plane of text on the surface neither system will demonstrate vertical parallax;

Figure 13 illustrates a stereogram principle - as the master pixels reduce in size each individual view of the subject through the master loses its individual parallax, but 'eye A' sees pawn obscure bishop more than 'eye B', and eye A sees a view from above horizon, but 'eye B' a lower view revealing the base;

Figure 14 illustrates loss of vertical parallax in a rainbow hologram - since there is no vertical parallax holographers need to produce only one row of photographic angular views; and

Figure 15 illustrates a principle of an embodiment of a volume reflection hologram according to an aspect of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

We will describe means of incorporating covert authentication features into a volume reflection hologram, to provide a covert authentication feature for volume holography.

The technique we describe allows for the incorporation of a viewing jig which will enable more precise reconstruction and describe a method for more precise examination of the hologram.

The method utilises volume reflection holograms to enable the covert encoding of images of a specific colour, which is later matched by a specially supplied laser pen or monochromatic LED device in the verification process and is therefore highly selective. A prototype sample has been made with a "covert" feature imaged with a blue laser in a red/green hologram and thus is able to reconstruct a blue image upon a suitably placed screen when the H3 is illuminated with a blue laser of wavelength 491nm.

In the future, a multi-colour projected image may be considered commensurate with the availability of cheap pen lasers of wavelengths other than 660nm. To achieve further advantage, the use of the aforementioned viewing jig means that the correct

7 laser or LED wavelengths can be delivered at a precise angle of incidence so as to reconstruct the covert image in a predictable position on an accurately positioned screen. This may optionally have printed marks or even holographic markings which delineate the precise position of the expected reconstructed image so as to further authenticate the genuine hologram. For example, a code comprising the date of manufacture of the hologram could be projected into a screen marked with boxes corresponding to the position of the projected letters in the appropriate colour or plurality of specific colours (R ₅ G ₅ B) ₃ as shown in Figure 5.

Advantageously, since the production of full-colour volume holography masters is a complex business, the present method requires only a small modification to the existing optical table set-up and the master recording technique routinely involved for holograms not containing the device. Thus, a minimal change to the existing optical table configuration is required.

Thus the H2 master hologram has similar properties to the routine masters made previous to the invention and these are not impaired or affected. Additionally, however, in order to facilitate this invention, it contains a separate displaced plane of information which is conveniently positioned at the origination stage, within the surface of the Hl hologram.

This means that as shown in the Figures, the technique maybe realised simply by the addition of an extra laser at a wavelength displaced from those lasers used to create the full colour image. As an example, in the case where the full colour main image is made with a blue laser at 458nm, the green component 532nm, and a red component exposed at 633nm the covert image which is the subject of this invention, can be exposed with a krypton laser of wavelength 647nm or a ruby laser of wavelength 694nm. In the future, the availability of blue "pen-lite" lasers might lead to advantages of the use of HeCd lasers at 442nm, or krypton lasers at 413nm, or dpss lasers at 408nm to incorporate the covert security feature.

In the event that LED's are used as an illumination source in conjunction with some arrangement for collimation or beam shaping, then it is conceivable that any other type of visible or u.v. / i.r. laser source may be useful in this context.

The additional laser is arranged so as to allow its beam to travel co-axial with the other colours. It is divided by a wavelength selective mirror so that one part of the divided beam illuminates the artwork logo attached to a diffuser and attached to the rear of the Hl master. Advantageously, this diffuser may be of a smoked glass type so that its undesirable illumination by the hologram reconstruction beams incident upon the Hl master is restricted. The laser beam is not required in this case to form part of the origination process for the Hl, but goes directly into the H2 phase of the origination process, with its split portion directed towards the rear illumination of the artwork attached to the Hl hologram. Advantageously, the artwork film or plate or image display device is index-matched or coupled to the rear of the Hl master. Normally we would expect it to be an advantage to black the rear of the Hl plate to reduce internal reflection in the glass.

The covert logo artwork which forms the subject matter for the laser projected image of this invention is typically placed in a position to suit the requirement of the proposed interrogation system. Its presence is not deleterious to the ordinary H1/H2 image transfer process.

Referring to Figures 1 and 2, Hl master 10 is replayed by a three colour (eg RGB, red, green, blue) reference beam 12 illuminating the front face of the master. A graphic mask 14 bearing a covert image is provided on the rear of the Hl master; preferably this includes a diffuser 16 at least where the mask transmits, to diffuse light from the (transmitting parts of) the covert image. Backlighting 18, preferably at a wavelength X different to any of the R, G and B wavelengths, also illuminates the Hl master. The replayed image and the covert image are recorded in an H2 recording plate 20. When illuminated with white (RGB) light the overt image is replayed; illumination at wavelength X (or a different monochromatic wavelength dependent on wavelength X - if, say, the H2 recording has been processed to change its replay wavelength by, say, swelling) replays the covert image. The wavelength X illumination may, but need not be, coincident with the RGB illumination to replay the overt image.

Recording the H2 hologram

The H2 hologram is recorded by allowing laser light from the Hl master to interfere with a second reference beam in order to create an image-planed hologram. The H2 may be recorded with simultaneous exposure of a multiplicity of laser beams or alternatively consecutive exposures into a single plate or multiple plates.

In one embodiment, when the Hl plate is chemically processed it is returned to the plate holder after rotating it through 180 degrees so that the recording emulsion is now on the opposite face to its position when Hl plate was recorded. A pseudoscopic real image is produced and the H2 recoding plate is positioned in the plane of the image.

This H2 is an image-planed holograph which after chemical tuning during the processing stage can be used as a contact master for mass production. In this case the plate is again rotated through 180 degrees so that the contact copying process will in general utilise the orthoscopic (mainly virtual) image of the subject or artwork. All of the data can then be transferred in a single simultaneous "white laser" exposure from the H2 into a film copy. Alternatively consecutive exposures may be made. Suitable recording films include high resolution ultra fine panchromatic silver halide film such as that supplied by Colour Holographic Ltd., or a photopolymer recording film as supplied by Bayer Materials Science GmbH. In the case of the photopolymer material a longer exposure is required, or the use of high powered lasers, and advantageously the various spectral exposure is simultaneous.

The diagram of Figure 3 demonstrates the principle of this contact copying process. The film is transported into position and held stationary by for example a holding pressure plate or alternatively a vacuum chuck system. Thus Figure 3 shows a variant contact copying process for recording an H2 hologram. In Figure 3 a multi-colour, for example 3 or 4 colour, laser beam 30 (R,G,B,X) is used to make a contact copy of a master hologram 32 on a support base 34, into a film 36 transported by a film transport mechanism 38. Beams R,G,B and X are incident on the same side of the contact master hologram. An optional spacer is provided between the master hologram 32 and the film 36.

After allowing time for the assembly to settle, a shutter opens to allow the laser beam the expose the film. Light passing through the predominantly transparent recording film, reconstructs the multi-colour holographic image and creates a standing wave of interference between the reference beam and the diffuse image from the hologram contact master. This standing wave is recorded in the recording film. The film is thus transported through the system in as stepping fashion, and after completion of the roll of film exposure, a chemical processing machine is use to create the mass produced holograms, which could be laminated with adhesive and die-cut on the roll, for example prior to application to a product as a security badge. In the case of a photopolymer such as that supplied by Bayer there is no chemical processing stage. There is a instead an operation of flooding the exposed film with ultraviolet after a short pause, and this can conveniently be achieved on-line on the roll prior to any finishing or converting operation.

The grating associated with the covert image may be recorded across the full surface area of the H2 hologram, or it could be confined to a smaller zone of the surface, which could typically be either in the corner of the final mass produced hologram, or perhaps in a more central position disguised by one of the principle image features. We have found that the serial addition of exposures, and more importantly exposure giving rise to diffractive grating structures is incrementally deleterious to the individual components.

Specifically, the addition of third and fourth gratings to a single recording layer is considerably more damaging to the first structure recorded than is the additional of a second component. Our patent application GB0917698.3 of 9 October 2009, entitled "Holographic Film and Recording Methods" (incorporated by reference in its entirety), allows us to reduce deleterious cross-talk between the various gratings in the H2 by producing separate layers of reflective components in our H2 assembly which may have a number of component plates each bearing separate gratings associated with various colours or planes of data in the total holographic image. The present image component may, accordingly, be contained within its own separate layer component or may be included within one of the other component plates.

In more detail, our earlier work describes a holographic film for recording a multicolour volume hologram, the film comprising: a carrier; a first photosensitive recording layer, for example silver halide, sensitive to one or both of red and green laser light; and a second photosensitive recording layer sensitive to blue laser light. In embodiments three photosensitive recording layers may be employed, one sensitive to each of the three primary colours, red, blue and green. The sensitivities of the first and second recording layers may be substantially non-overlapping. Preferably red and green colour components are recorded in the first layer, and blue in the second layer. Preferably the silver halide has a grain size of less than 30 nm, say around 20nm, to reduce Rayleigh scattering in the emulsion. Surprisingly, relatively thin layers of emulsion may be employed, for example less than 7μm, 6 μm, or 5 μm, but greater than 2 μm or 3 μm.

For the reasons described above, it may be advantageous to add the covert grating to only a small part of the surface of the H2. In a single layer H2, this could take the form of a graded zone extending away from one edge of the film, or it could even be defined as an image feature by an accurate masking system attached to the surface of the H2 hologram, or by a more complex shadowing system whereby either or both of the reference or object beams are allowed to fall onto only a small or specific area of the hologram. For example, graphics could be designed to include a border perimeter with a reduced colour content which would be ideal to envelope a covert device. Any of these effects may be most easily achieved with the use of our multi-layer technique mentioned above.

The purpose of the exposure is to provide a graphic feature which when projected onto a suitably placed screen will reveal a code or text which authenticates the hologram. Whereas we have had experience of the counterfeiting of most of the specialist holographic features used in embossed holography, we are not aware of any counterfeit attempt upon features of the equivalent transmission type of projected image feature.

Figure 4 shows the detail of the method of reconstructing the laser projected feature. There is no requirement for coherence of the reconstructing beam. Thus a laser is technically not required. A parallel collimated beam from a monochromatic LED with

a simple lens arrangement may be used to reconstruct the image. Suitable power, approximate collimation and selected monochromaticity are desirable. Embossed holograms are dispersive structures and their features can therefore easily be constructed with any reconstructing wavelength by tilting the hologram or source.

However, the volume hologram of this invention is selective in its colour reflectivity and this a considerable advantage to the security value of the present device, since a specialist authentication device may be supplied, whose selectivity further increase the difficulty of counterfeiting the device. Additionally, the marked viewing screen as shown in Figure 5 may be utilised for further authentication of the hologram. In an advanced embodiment of the method, the covert projected image could manifest itself in full colour, with extreme security advantage since the individual colour component images are of a critical wavelength.

Thus we have described a method to produce a covert security device within a volume reflection hologram which will enable the manufacturer, with only minor change to the existing origination system, to create an authentication code or alphanumeric image, or graphic image of any kind, projected by a purpose-supplied reconstruction source, of a specific defined wavelength, that will be projected onto a suitably placed screen and thus verify authenticity of the hologram, with extreme difficulty being presented to any attempt to duplicate the device by the counterfeiter.

Colour mixing effects

The option we provide to use a range of screens with various patterns is more likely to prevent the counterfeiter from perfecting a duplication method. Furthermore, in commercial terms, a particular customer could take ownership of a specific pair of e.g. line masks for inclusion as a common feature in a series of corporate images.

Advantageously, in this case we can create a negative of the black line pattern to produce a pair of screens which are effectively positives of each line diagram. Now by imaging these in a similar way as previous in two displaced planes, the parallax effect shows itself in the animated mixing of colour where the component lines

superimpose, dependent upon the position of the viewer, illumination, or hologram. Thus source lines of red and green will be seen to create rushing animations of yellow mixing Moire patterns as the view changes.

In volume reflection holography we have full parallax in the vertical and the horizontal plane and thus the ability to have running animation effects occurs when the hologram is tilted either vertically or horizontally. Furthermore there is no restriction upon the use of shadowed or black lines as the sole source of graphic detail; here we can use the addition of light of two colours to cause animated mixing effects as the viewer moves his or her viewing position, or tilts the hologram, or moves the source of illumination, i.e. as the viewer moves, the complex pattern of rushing lines manifests itself as an animated mixing of colours, so that the basic red green and blue guilloche design results in a rushing pattern of white, yellow, cyan and magenta lines as the individual components mix.

Making the second generation hologram master

In each of the explanations a single red laser beam is shown to manufacture the hologram components. It is an important feature of these techniques in advanced reflection holography that multiple colours are introduced into the images and this is achieved in each case by multiple wavelength exposure either simultaneously or consecutively and with changes of artwork where appropriate.

In the diagram of Figure 6 the laser beam is spread through the lens (4) and falls upon a diffuser screen capable of distributing its light over a wide area to include the whole of the surface of the master hologram Hl (5). One advantageous method is to use a holographic diffuser such that the optical device can be designed with the purpose of arranging for the diffuse light to cover precisely the exact master surface with less wasted light.

In Figure 6, two mask plates (2) and (3) are arranged with appropriate spacing between them so that when the viewer's eye (6) moves behind the plate he sees a dynamic pattern of rushing lines. This is precisely the effect which will be recorded in

the hologram Hl, Later, after the recording plate is processed chemically, the viewer will be able to experience the same view when he looks into the hologram despite the fact that the artwork mask plates have now been removed, and the motion of his head will result in the same animated scene.

By rotating the Hl master in the plate holder, the holographer can achieve a real image in front of the glass plate and this can be used to make a second hologram H2 where the graphic image appears close to the film plane - as shown in Figure 7. This H2 hologram can be used as a contact copying master for the puipose of mass production.

In this second stage of the manufacturing process the master hologram Hl is illuminated at the correct reference angle and reconstructs the image of the plurality of line patterns. It is then possible to place another glass recording plate (eg. Colour Holographic Ltd., Trinity Buoy Wharf, London) close to the plane of the reconstructed image, such in the final hologram the image will be close to the surface of the film. The emulsion side of the plate is typically on the rear surface on the reference beam incidence side.

Typically in the case where the layer parallax image component is used as a background fill to a principle image feature, the image might be approximately 2 mm to the reference beam side of the plate surface.

A second hologram is then recorded as is termed H2. This H2 may contain other image information such as a main foreground feature which may be introduced from a second Hl master in register with the parallax component. In that case there may optionally be a "blockout" mask in place when the original "moire" Hl is created such that the final hologram view will not allow sight of the animated background through the foreground logo or design. In embodiments the second generation hologram is in effect a contact master.

Additionally, there is available to us a hybrid method which involves a similar principle of interaction between simple geometric line patterns, but which does not

rely upon the parallax effects from simply blocking the view of the illuminated screen in the form of a Moire shadowing device.

In the diagram of Figure 8, the illuminating laser beam is incident upon the diffuser screen (1) as before. In this embodiment however, it is spatial light modulator (2) such as that supplied by Leisegang, Germany, which encodes the hologram data into the Hl recording. Alternatively a digital DMD (digital micro-mirror device) could be used to project the image data, such as that supplied by Texas Instruments Inc.

Since the L.C.D. panel is addressed by computer its displayed image can be easily and quickly changed. Thus multiple channels of information can conveniently be incorporated.

The Hl plate (5) is divided into individual zones which are individually unmasked for exposure. This masking may be achieved manually or automatically with an electrically controlled masking system. Masks are held on both sides of the plate to protect the covered zones from exposure to light whilst an individual zone is exposed to the reference and object laser beams from both sides. The holographer can examine the image changes which will be recorded in the hologram from position (6) prior to installation of the recording plate and its masks.

Thus we can create a negative of the black line pattern to produce a pair of screens which are effectively positives of each line diagram and by imaging these in, say, two displaced planes, the parallax effect shows itself in the animated mixing of colour: The component lines superimpose, dependent upon the position of the viewer, illumination, and/or hologram.

In a stereographic mode (different views, e.g. to provide a depth effect by imitating binocular vision) we have available not only the viewer's position controlling the geometric detail of the image seen but also the important ability to use the artificial multiple image channels to input both subtle and radical changes to the view seen, by actually physically adjusting the displacement of the component films between the exposures in a multi-channel hologram.

This method allows us to produce for example a predicted sequence of radical changes to geometry over and above the more subtle angular variations which occur when the viewer, the illumination or the hologram moves.

Using interference between projected light patterns embodiments of our stereographic system are capable of producing virtual line diagrams in colours which will effectively produce equivalent animations. These stereographic versions can demonstrate the parallax animation in the horizontal plane image vertical strips of graphics with no vertical parallax, as well as a stereogram system with multiple rows of pixels in the Hl representing both vertical and horizontal image. Subtle covert features such as a hologram with vertical parallax above the viewing horizon but zero parallax below it, also become possible and provide a feature easily recognisable to the human eye, but sufficiently subtle to avoid the attention of a counterfeiter.

It is feasible that complete changes of graphic style can be organised within the channels of the hologram so that for example the view of the hologram from the left shows horizontal animation of lines and a sudden change of image occurs such that as the viewer moves to the right, a vertical motion suddenly begins. This type of change in the pattern of animation is not limited to just two channels of course but could apply to a multiplicity of changes of motion. The surface of the "Hl" recording master could be divided into an unlimited number of zones, each of which gives rise to a separate view of an animated line pattern. The difficulty of "back-engineering" a hologram containing such features means that the security value is high, since the counterfeiter is faced with a well nigh impossible task. Advantageously, such an effect can be used as a background feature which provides high security and yet does not detract at all from the primary subject matter of a hologram, which may typically be a company logo or three-dimensional feature constructed close to the surface of the film layer.

An animated file in Illustrator has been created to demonstrate an "animated guilloche" type appearance, eminently compatible with security imagery, which this technique produces. This was then used to create a range of film separations of various line patterns which can be used to produce an innumerable range of Moire- like effects. As described above, an individual customer could realistically take

permanent possession of a pair of screens such that they permanently own a particular animated feature for inclusion only in their own series of holograms.

With computer graphics stereographic methods it is feasible that interference patterns in the form of text or other symbolic graphics can be made to appear.

Multicolour 3D volume reflection holograms

We will describe techniques for using the "full parallax" capability of volume reflection holography to provide stereographic images, optionally animated stereographic images, for example to create highly secure holograms.

The volume reflection holograms we describe, however, have the advantage that their colour remains predominantly consistent throughout any small changes in the viewing and lightning conditions employed. This is because the diffractive fringe structure is a planar matrix of index modulation, which is effectively a dielectric reflector capable of wavelength selectivity. Tilting this structure with respect to the angle of incidence and reflection therefore results in only a relatively minor colour change. Indeed, the degree of change itself can be controlled to some extent by control of the configuration of the volume grating. Essentially, a thicker grating with more reflective fringe planes, frequently accompanied by a reduction in the associated index modulation, will reflect a narrower band of frequencies, with a consequent slight reduction in the range of viewing angles through which the image may be viewed.

When the reflection (volume) hologram recording technique is applied to a three- dimensional model as the subject of the image however, the resulting recording has the advantageous quality that both vertical and horizontal parallax is recorded. This phenomenon sets reflection holography apart from its embossed equivalent, but additionally, it is exceptionally attractive to the viewer in terms of its realistic appearance.

Viewing a volume reflection hologram is essentially similar to the experience of viewing a real scene through a window, whereas the viewing of a rainbow hologram

has been described as similar to the view through a letter box — there is no vertical parallax information recorded within the image, and it is thus impossible to look over the top of a item in the foreground to see what lies behind it, whereas lateral movement of the viewing position will reveal ordinary parallax effect as experienced in a real scene.

Human perception tends not to notice this paradox, and in his invention of the "rainbow hologram", Steven Benton utilised the lack of need for vertical parallax to enable the master recording to be slitted down to a narrow lateral band of view. Since there was then no change in the vertical aspect of the subject recorded, when it was smeared into a range of colours by the dispersive action of the transmission grating illuminated by white light, there was no visual confusion to the viewer, who saw precisely the same image cycle, with visual clarity, through a spectrum of colour as he moved up and down.

The more perceptive viewer was aware of the lack of vertical parallax and the frustration that when a foreground object obscured a background detail, it was impossible to look over or underneath it, in the same way as was possible in the horizontal plane.

Embossed hologram producers have made a forceful argument that the trade off between lost parallax and attractive colour switching is worthwhile, but the techniques we describe allow the present holograms to benefit from both advantageous properties.

There is therefore an advantage in extending the full parallax viewing qualities described above with reference to reflection holograms of three-dimensional subjects; to the holograms based upon stereographic image design principles, so that we can supply images of the style expected by the security industry, but bearing the advantages offered by reflection volume format.

Whereas human perception is principally based upon the binocular configuration of laterally displaced eyes detecting left/right parallax effects, with little attention paid to the existence of up/down parallax in the normal viewing experience, there is

frequently a need for a viewer to improve upon the perception of a scene by raising or lowering the head to explore the resulting parallax change.

Holography should aim in principle to target all of the vision cues which accompany the viewing of a real three dimensional image in life, and the means we have chosen of recording a "redundant", displaced Hl master is capable of doing so, as it can record multiple image channels with animation, full colour, and both horizontal and vertical parallax.

One feature of this repertoire is that it has the ability to exploit a stereographic method of creating three dimensional images with respect to horizontal parallax associated with binocular vision, whilst providing the opportunity, absent from embossed holography, to provide vertical parallax and a range of viewing angle in the vertical plane which is not accompanied by changing colour.

When one views an open aperture Hl master hologram, it is relatively easy to see that since the surface of the Hl phase hologram recording plate contains a continuous spectrum of views of the subject matter as seen from a solid angular range, so that if that planar master surface is divided into "pixels" of insufficient size to allow the observer to move his eye sufficiently to experience any parallax capability within the individual pixel, then it makes little difference whether the subject matter for each individual pixel is in fact a real object, or whether it is a flat graphic representation, such as a photographic transparency, of that object viewed from the appropriate direction with consequential perspective changes.

Substituting two dimensional graphics into the manufacturing procedure in place of expensive, custom made sculpted objects, offers many advantages to the holographer. These include the ability to easily incorporate covert features, accommodate flexibility of image size, and importantly to achieve by very simple means, accurate control of image colour, including the ability to incoiporate intricate colour and graphic detail.

However, the provision of a whole series of 'stereographic' views in a two dimensional array, with genuine parallax effects in all directions, involves a huge

graphic data store for each hologram image. Such a complex sequence of graphics can realistically only be incorporated into the hologram Hl master by means of an automated origination system involving servo driven masks for the hologram master and an image projection system so as to eliminate all of the problems of positioning numerous artwork films etc.

Furthermore, for the graphic artist in the design studio, the difficulty of designing suitable full artificial parallax image graphics, as described above, involves the use of specialist computer design software modified for the purpose, and is also exceedingly time consuming and requiring extensive computer hardware facilities for the rendering operation.

Therefore one embodiment of the present technique allows for the preparation of holographic images which have a distinct advantage in terms of security value and defence against counterfeiting but do not necessarily require the calculation or creation of numerous complex vertical rows of views of the subject matter as described above.

One of the most advantageous ways of realising artwork for the purpose of holographic stereogram creation is the use of cinematic camera film equipment to record a scene and this method has been used by companies such as Applied Holographies to produce embossed holograms such as the album cover for the singer Stevie Nicks " The Other Side of the Mirror" and the security hologram for APACS for plastic card protection known as the "Bard Card" and featuring a portrait of an actor resembling William Shakespeare. This type of portraiture has a very high value in the security print arena.

Such holograms were made with a specially constructed camera system comprising a computer controlled precision "dolly rail" capable of tracking the cine camera smoothly over a distance of several metres whilst an electronically controlled shear lens moved in front of the camera body in order to re-centre the image upon the film. Such a technique enabled the creation of a film recording of a configuration precisely compatible with the holographic mastering process.

In general, the stereogram images which we routinely create contain two specific functions, which are inter-related but which are to a small extent deleterious to one another. These separate functions are:

(a) A continuous incremental range of binocular stereographic image pairs which give rise to three dimensional effects, and

(b) Subtle animation effects which enable minor movements and changes to the scene as the viewing position moves.

Since the human eyes and brain achieve three dimensional impressions by continual comparison of pairs of related images of a subject comprising laterally displaced views — it is expected that these views represent the left and right displacements of the same object. However, when we introduce animation such as the subject of a human portrait smiling or waving, then the stereographic continuity itself is actually damaged to some extent because the left and right views observed by the viewer of the hologram are not, in effect, views of exactly the same subject. As a result, three dimensional interpretation of the image is damaged or prohibited.

In practical terms, this means that if the subject of a hologram portrait moves too much during the filming sequence the 3D impression is lost. For example, in a human portrait, if the waving of a hand involves too much movement of the arm then the arm and hand suffer from the very disconcerting defect that they appear quite flat in the image almost in defiance of their apparent image parallax relative to other stationary three dimensional image features behind them.

Such effects are sometimes known as "Time smear". This photographic term tends to refer in holography to the effects mentioned above, wherein a sequence of individual frames of motion are recorded in such a way that excessive movement occurs far too quickly when a viewer changes his position laterally when examining the hologram. Thus individual views of the subject change far too radically to allow the viewer the recognise stereographic image pairs when viewing the hologram so that in the first instance, perception of three-dimensional depth and parallax is lost, and in the case of a more extreme degree of animation, spurious effects may occur such as an arm

appearing to bend backwards. These unsatisfactory defects are addressed by the techniques we describe.

By separating these functions, the present technique enables us to produce a hologram which has unique qualities. Such a separation is readily available to us because unlike in the rainbow hologram configuration, where the Hl master is slitted down to a number of channels or rows representing the range of dispersive colours which will reconstruct the hologram, the reflection hologram technology utilises an "open- aperture" master, which allows the viewer of the final H2 or H3 hologram to look at the image through a virtual window which appears in the space directly in front of the image which is typically astride or close to the surface of the film bearing the final image.

Thus as the viewer moves his eyes to the left, he or she will see a view of the subject corresponding to the left of that subject and if he moves to the right, he will see a right view of the subject. Whether the three-dimensionality of the subject is achieved by stereographic means or by using an actual direct hologram recording of a three dimensional model is to some extent irrelevant to the viewer.

However, to the holographer, there is considerable advantage in the use of the stereographic route since the colours of the image and the size of the image can be easily controlled or adjusted without the time consuming process of multiple model making.

Whereas animation is typically incorporated in the graphic series of stereographic images in embossed holography at the cost of deletion of three-dimensional effect, as explained above, in reflection holography we have the ability to incorporate the data for animation on the master hologram in the longitudinal direction and thus achieve a quite different distribution of the animation, such that each stage of an animated series has complete undamaged three-dimensional perception.

As a very simple example, we can for instance produce a stereographic sequence of a set of traffic signals, wherein the movement of the hologram or the viewer or the light source in the vertical plane only was responsible for the activation of the animated

series. Movement of the hologram or the viewer or the reconstruction light source in the horizontal is unnecessary, since the viewer will always be presented with a correctly spatially distributed pair of views suitable to construct a 3D image. Turning the hologram film to the left or right will simply enable a realistic change of view corresponding to a view of a real object from either flank, since the viewer's eyes coincide throughout the window of view, with a suitable pair of stereo graphic views from either a left, central, or right biased position.

Tilting the film in the vertical plane results in the viewer experiencing a sequence of views of the same subject, or alternatively a similar or a completely different subject. This sequence also bears a perfectly solid three-dimensional appearance, but with no deletion due to animation effects.

In the above example of a traffic signal image, in the simplest case this animation effect might be due solely to a change of colour of the lights themselves. So for example the traffic signal might show a red light at the top view, which changes through red and amber to green at the centre and reverses to amber and finally red again at the lowest view. In all cases, the 3D qualities (parallax) of the scene are perfectly represented, and the viewer can look to the left and right of the subject by tilting the hologram in the horizontal plane. However, by tilting vertically, the viewer sees an outstandingly clear colour animation whilst retaining full undiminished three dimensional quality

This example illustrates very well the advantage that reflection volume holography has over existing embossed hologram technology, where the action of tilting the film vertically would clearly result in a complete change of colour of the whole scene with a resulting loss of the appreciation of the reality of the viewing experience.

In the event that we wish to demonstrate a higher level of image complexity and hence security value, it is then possible to create a graphic series representing elevated and declined views so that not only does the viewer see the changing light sequence when tilting the film vertically but also from that elevated or declined view, he witnesses an appropriately angled view of the traffic signal itself, with perhaps a view including more detail of the top of the mast and its lamp housings.

One way to achieve such an effect in a fashion suitable to record human portraits would be to have say ten or more high resolution digital cameras (or one or more linearly moveable cameras) on an elevating track or boom. In this way as the boom rises, incremental sequences of stereographic recordings can be made whilst the subject moves more freely than is possible in previous techniques with solely horizontal recording, wherein the aforementioned deleterious effect upon the stereographic fidelity results from excessive animation of the subject. Such a sequence of graphic imagery (suitable graphics files) may be produced using a computer system but the simpler alternative of a separation of "colour animation" of the type we describe may be preferable: Our image projection system combined with an automated masking system provides a convenient and efficient method to create a multi-channel master hologram capable of producing an H2 image with unhindered full parallax and extensive animation properties.

We have thus described a method and equipment to utilise the full parallax capabilities of volume reflection holography via a mastering technique which is able to conveniently provide animation effects which are not deleterious to the three dimensional qualities of the primary image and provide an increase in the difficulty of illicit duplication of the hologram for the benefit of its use as a security device.

Figure 9 shows the hologram recording configurations typically used in the origination of holographic images. The angle of view of the final hologram is dictated by a viewing "window" which is effectively defined by the master hologram. The "Hl" master is recorded in a suitable recording material, which could for example be a glass or film coating of a suitable ultra fine grain silver halide emulsion. Providing the final generation hologram is properly lit with a suitable point light source the viewer of the final hologram is able to view the subject matter as if he or she is looking into a displaced, out of plane window equivalent to the Hl master hologram. As illustrated, in the Hl mode the object is enhanced by additional lighting whereas in a Denisyuk mode single beam illumination is employed.

In the case of the "Denisyuk mode" hologram on the right of figure 9, subject matter is close to the master and as such is able to be viewed through a wide range of angles

both horizontally and vertically as the viewer moves his head. Although highly realistic views of the subject matter can be seen by the viewer over a wide viewing angle a disadvantage of this method is that since all of the light used to illuminate the subject matter has passed through the recording plate before being incident upon the subject, the ratio of object: reference light in the interference recording which gives rise to the hologram is strictly limited. This has a serious detrimental effect upon the maximum possible brightness of the hologram.

By contrast, in the "redundant Hl configuration on the left of Figure 9 we are able to light the object independently and thus control the beam ratio, with the result that the reconstructed image can be very bright. This brightness can be maintained through successive hologram generations. Such improved brightness of image is commercially invaluable in security applications of holography.

The 'full parallax' capability inherent in reflection holograms is seen in both diagrams of Figure 9.

In embossed holography, there is reliance upon the Benton rainbow principle to allow a surface relief hologram to be created. Figure 10 shows the slitting of the Hl master to a narrow band of exposed emulsion before or after the recording of the Hl image. Clearly the horizontal slit provides only a very narrow viewing window for the image when the master Hl is illuminated with the original laser. Thus the substantial elimination of vertical parallax means that the viewer looking through the slit experiences a view (as if looking through a letter box into an entrance hall) which provides no information about the top and bottom of the subject; only the data relating to the "head-on" view are recorded in the reconstructed image.

A second generation hologram made from this image thus contains only horizontal parallax. When the natural colour dispersion of a transmission hologram occurs (caused by the effect of a linear relief grating on white light) the viewer sees an image which changes colour when the viewing position moves vertically but which does not blur, or smear into a range of coloured images with displaced perspective. This is the basis for successful embossed rainbow holography.

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Figure 11 shows how the image associated with the Hl master plate can be placed in a range of different positions with respect to the recording plate for the H2 hologram. Thus the viewer will experience a white light viewable hologram whose image plane could be in front of, planar with, or behind the recording film. This is an important concept in holographic applications, and especially in the field of security devices. This is because it is the case that features within the depth of the image which fall precisely on the film plane of the final hologram will appear sharp even in diffuse lighting — often a vital requirement for a security feature.

Figure 12 shows the viewing characteristics of a transmission rainbow hologram (on the left of the figure) and a reflection hologram (on the right of the figure). For top, T, middle, M, and bottom, B, viewing positions respectively, when viewing a rainbow hologram, RAINBOW, the viewer sees the front of a red cube, the front of a green cube, and the front of a blue cube. For top, T, middle, M, and bottom, B, viewing positions respectively, when viewing a reflection hologram, REFLECTION, the viewer sees the top of a green cube, the front of a green cube, and the bottom of a green cube.

On the left a range of movement by the viewer's eye results in a transition of colour through the visible spectrum as the thin linear grating disperses white light incident upon the hologram surface. Conversely on the right, the viewer witnesses a monochromatic reconstruction of the image. He or she can see the top and bottom of the subject within the limits of the viewing angle described above, with little change in the colour of the image.

Figure 13 shows the principle of multi-channel hologram recording. If the master Hl hologram is divided into pixels as shown, the viewer will see, through each of the pixels or zones in turn, a different view of the subject matter. As we reduce the size of the individual pixel, the viewer will experience a reduction in the parallax or image perspective which is experienced regarding the view of the subject through that pixel. As the area of the pixel reduces toward zero, there is no parallax about the individual image view. At this point, the three-dimensional object of the hologram recording process could effectively be replaced by a high quality photograph of the subject without such a change being apparent to the viewer. In this way the principle of a

stereographic hologram arises; the need for a solid subject for the hologram is replaced by the use of a photographic series.

Figure 14 shows a range of viewing positions through which a viewer's perspective varies without any taking account of the vertical plane. Thus for typical embossed stereograms, a sequence of photography or computer generated imagery is used which has the corresponding perspective properties in accordance with the range of possible views through the slitted master. The more individual frames, the higher the stereographic resolution of the image.

Figure 15 shows how an equivalent Hl master for a reflection hologram presents a full aperture which can be divided in two dimensional pixels in the same way as the slitted rainbow master is divided into sections in only one dimension. The traffic signal image shown on the left could provide a stereographic three-dimensional image which appears as a solid object with perspective when the viewing position is moved left to right. However, in the horizontal rows of the master shown, the 2D images used for the recording of say the top row of pixels may for instance include a colour change such that this top row records a three-dimensional series of images where the actual traffic signal sequence is in the red position. Lower in the grid of pixels, the image rows may correspond to the sequence of colour changes expected from a traffic signal i.e. the illuminated lamp changes to say red and amber in one row, amber in the next and green in the lowest for example.

Thus in one embodiment an individual tile encodes an image with depth (a 3D view), columns of tiles encode a stereographic sequence of changes to the replayed image, and rows of tiles are used to encode a further dimension of change to the image, in the illustrated example a change in colour but alternatively, say, a change shape or, more generally a change in content of the replayed image. This may be employed to produce, for example, a hologram which replays a 3D "solid" view of an object with animation, as in the traffic lights example. Broadly speaking, each tile encodes a view a camera would see of the replayed 3D image when located at an (X, Y) position in a viewing plane.

The human portrait shown on the right could contain photographic sequences where a range of camera declination results in views ranging from the top of the head of the subject to a view from below the chin. Equally animation such as smiling may be encoded in the vertical plane (as with the traffic lights described above), in embodiments so as to ensure that the binocular properties of the horizontal information sequences are not disrupted by moment as explained above.

Thus in embodiments each 3D view may be a 3D view of an image, for example a head, from an (X, Y) viewing position. The 3D views are arranged so that they change with (X, Y) position. This may be achieved, for example, by capturing images from different (X ₅ Y) positions and/or by employing computer generated imagery, or by a combination of both these approaches (captured "real-life" images in combination with computer graphics/image manipulation). Thus computer graphics from an artificial 3D view may be incorporated into, or substituted for/combined with real/captured images, in a 3D (full colour) hologram.

Thus we have described a method of providing a hologram of a multicolour, three- dimensional (3D) object. In one embodiment this method comprises inputting three- dimensional data describing the 3D object and generating a plurality of multicolour views of the 3D, these views comprising views from a grid of different viewing positions ranging over an area or region both horizontally and vertically. In one embodiment each of said multicolour views is displayed in turn and recorded into a channel of a multicolour volume reflection hologram master, the channel defined by a selected spatial region of the master, for example a tile or pixel. Preferably the channels are substantially non-overlapping. Then the recorded multicolour displayed views of the 3D object are replayed together and recorded, preferably simultaneously, into a second (generation) volume reflection hologram. This technique is thus able to provide a volume reflection hologram which replays the multicolour 3D object in white light, in such a way that a viewer is able to view the object in 3D (and multicolour), as if looking at the object from different viewing positions. In preferred implementations three colours are employed to provide a substantially full colour replayed image.

No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.