WO2019180432A1

WO2019180432A1 - Methods of manufacturing diffractive optical devices

Info

Publication number: WO2019180432A1
Application number: PCT/GB2019/050783
Authority: WO
Inventors: Brian William Holmes
Original assignee: De La Rue International Limited
Priority date: 2018-03-23
Filing date: 2019-03-20
Publication date: 2019-09-26
Also published as: GB201804689D0; GB2572219A

Abstract

Non-interferometric (fig 1) and interferometric (Fig 9) methods of manufacturing a diffractive optical device. The Non-interferometric method, comprising providing a photo-modifiable layer (2), controlling a spatial light modulation (11) device to display a first fringe pattern corresponding to a first diffractive effect, and using an optical system (12) to illuminate the SLM device with a first pulse of light from a light source (13) and to project an image of the first fringe pattern onto a first region (Rl) of the photo-modifiable layer. The first region of the photo-modifiable layer is modified in accordance with the first fringe pattern and thereby reproduces, under illumination, the first diffractive effect. The interferometric method recording interferences between a reference beam and an object beam from an object displayed on the SLM.

Description

METHODS OF MANUFACTURING DIFFRACTIVE OPTICAL DEVICES

This invention relates to methods of manufacturing diffractive optical devices, both interferometric and non-interferometric. The methods can be used to form optical devices such as holograms, diffraction gratings and the like. Such optical devices have many applications, including decorative uses, though are particularly well suited for use as security devices.

Diffractive devices such as holograms can be manufactured using various different techniques. Typically, a master hologram defining the holographic information, for example in the form of a surface relief, is originated and then replicated a large number of times to form the individual devices. For instance, the replication may involve embossing the generated surface relief profile into an appropriate carrier material and optionally applying a reflection enhancing layer conforming to the profile in order to improve the visibility of the holographic playback.

Origination of the master hologram (or other diffractive device) can also be achieved in a number of different ways. Traditionally, a layer of photosensitive material is exposed to an interference pattern formed by the meeting of two coherent light beams: an object beam which has been modified by interaction (e.g. reflection off) the object of which a holographic image is to be formed), and a reference beam which has not been so modified. If the two beams impinge on the photosensitive layer from the same side, a transmission hologram is formed which can be viewed by transmitting light through the device, and if the two beams impinge on the photosensitive layer from opposite sides, a reflection hologram is formed which will playback in reflected light. The interference pattern (which typically takes the form of a series of light/dark fringes in one or two dimensions) is recorded in the photosensitive medium which can then be processed to preserve the pattern, for instance as a surface relief or in the form of local refractive index variation. The precise manner in which the information is preserved will depend on the nature of the photosensitive material. It should be noted that whilst in some cases the object could be a complex, three-dimensional object (thereby resulting in a holographic image of the object with a three-dimensional appearance), this is not essential and the object could for instance be a flat mirror in which case the diffractive device formed will effectively be a diffraction grating and the playback is of two-dimensional appearance with a varying colour.

Directly originating a master hologram from an object in this way is complex and costly. As such, alternative techniques are available in which the necessary interference pattern required to generate a holographic image of a certain object is calculated, for instance by computer modelling, and then written directly into a photosensitive layer. This can be achieved, for example, using an optical fringe writer or electron beam lithography. In each case, a collimated beam of either laser light or electrons is focussed onto the photosensitive layer and is controlled to move along a path in accordance with the calculated interference pattern so that the corresponding portions of the layer are exposed to the beam, thereby recording the pattern in the layer. Due to the high precision required and complex pattern to be drawn by the beam, point by point, this can be a slow process.

As a result of the laborious nature of available origination processes, in practical terms the usefulness of holograms and other diffractive devices has been limited to high-volume runs of identical devices, all deriving from the same origination. With current technology, it is not time or cost effective to produce unique or personalised diffractive devices, in which the diffractive information itself varies from device to device since this would require a new origination for each one.

Faster and more flexible methods of manufacturing diffractive devices would therefore be highly desirable.

In accordance with a first aspect of the invention, a non-interferometric method of manufacturing a diffractive optical device is provided, comprising

a) providing a photo-modifiable layer;

b) controlling a spatial light modulation (SLM) device to display a first fringe pattern corresponding to a first diffractive effect;

c) using an optical system to illuminate the SLM device with a first pulse of light from a light source and to project an image of the first fringe pattern onto a first region of the photo-modifiable layer, whereby the first region of the photo- modifiable layer is modified in accordance with the first fringe pattern and thereby reproduces the first diffractive effect.

The first aspect of the invention also provides an apparatus for non- interferometric manufacture of a diffractive optical device, comprising:

a support for a photo-modifiable layer;

a spatial light modulation (SLM) device;

a light source;

an optical system adapted to direct light from the light source to the SLM device and to project an image thereof towards the support; and

a control system configured to control the SLM device to display a first fringe pattern corresponding to a first diffractive effect and the optical system to illuminate the SLM device with a first pulse of light from the light source and project an image of the first fringe pattern onto a first region of a photo- modifiable layer arranged in use on the support, to enable the first region of the photo-modifiable layer to be modified in accordance with the first fringe pattern and thereby reproduce the first diffractive effect

A spatial light modulation (SLM) device is a device having a surface comprising an array of pixels each of which can be individually controlled to change how it modulates incoming light. For example, the SLM device could comprise an array of liquid crystal pixels each of which can be switched on and off by addressing electrodes so as to change the transmissivity or reflectivity of the pixel. Reflective LCOS (liquid crystal on silicon) SLM devices are a preferred example. Alternatively, the SLM device could comprise an array of micro-mirror pixels, each one being individually orientated such that incident light will be redirected differently in different parts of the SLM surface. Whatever its implementation, the SLM device can be controlled by a suitable controller (such as a computer or other digital controller) to display a certain image by arranging selected ones of its pixels to modulate incoming light in accordance with that desired image. Typically, SLM devices can be controlled to change the displayed image near-instantaneously, e.g. within a fraction of a second. By using a SLM device to display a fringe pattern corresponding to a diffractive effect and projecting that pattern on to a photo-modifiable layer, a diffractive optical device can be manufactured far more quickly than previously possible. The fringe pattern, which will typically be all or part of an interference pattern calculated digitally (e.g. by a computer), can be near-instantaneously displayed on the SLM device and, if desired, can then be quickly changed to a different fringe pattern as discussed further below. In addition, since the technique is non-interferometric (i.e. not involving interference between light beams), there is no minimum duration imposed on the light pulse as no coherence between beams needs to be achieved. Moreover, since the whole fringe pattern displayed on the SLM at any one time is recorded into the photo-modifiable layer simultaneously, the process is much faster than previous direct-writing origination techniques. Together, these features allow for substantially real-time, digital origination of diffractive optical devices configured to exhibit any desired diffractive effect. Further, due to the speed of the process, the origination can be performed directly on a photo-modifiable layer which ultimately forms part of the end product, such as a security article or even a security document. That is, the steps of forming a master hologram (or other diffractive device) and then replicating it can be avoided, and the device originated directly“in situ”. This is of course optional (though preferred) and the disclosed method can if desired be used to originate a master hologram (or other device) which is then replicated in conventional manner.

The photo-modifiable layer can take various forms, as discussed below, and could be a self-supporting layer or could be provided on a carrier. Either or both of the photo-modifiable layer and the carrier could ultimately form part of the end product. Depending on the nature of the diffractive device and the photo-modifiable layer, the diffractive effect may or may not be visible from the photo-modifiable layer alone: further processing steps may be required to render the effect visible, such as application of a reflection enhancing layer. Nonetheless, it is the photo-modifiable layer which reproduces, i.e. carries, the necessary information for display of the diffractive effect.

The above-described method could form the entire diffractive optical device. However, in preferred embodiments, each exposure forms only a part of the finished device, which is built up by exposing multiple regions of the photo-modifiable layer to respective fringe patterns displayed on the SLM device. Thus, preferably, the method further comprises repeating steps (b) and (c), optionally a plurality of times, whereby second and optionally subsequent regions of the photo-modifiable layer are modified in accordance with respective second and optionally subsequent fringe patterns displayed sequentially by the SLM device and illuminated by second and optionally subsequent pulses of light from the light source so as to sequentially project images thereof onto the respective regions of the photo-modifiable layer, the respective regions of the photo-modifiable layer being laterally offset from one another, the second and optionally subsequent regions of the photo-modifiable layer thereby reproducing respective diffractive effects corresponding to the second and optionally subsequent fringe patterns. Likewise, in the case of the apparatus, the control system is preferably further configured to control the SLM device and the optical system to achieve this. It will be appreciated that, in this context, the term “laterally offset” means that the regions are beside one another, preferably non overlapping. The regions could be spaced from one another but preferably abut one another so that together they form a contiguous area exhibiting diffractive effects. Any number of regions may be so-exposed: this will depend on the size of the overall device to be formed.

Preferably, although not essentially, the first and second fringe patterns, and the corresponding first and second diffractive effects, are different from one another. The use of a SLM device lends itself well to this since its pixels can be controlled to display different images very quickly. Likewise, any third and subsequent fringe patterns may also be different from the other fringe patterns. However, this will depend on the overall effect to be formed and it may be that two or more of the regions of the photo-modifiable layer are exposed to the same fringe pattern. In advantageous embodiments, the first, second and optionally subsequent diffractive effects reproduced by the respective regions of the photo- modifiable layer collectively form an aggregate diffractive image. This is a diffractive effect which appears contiguous and/or continuous to the observer. By“image” we mean any item of information such as alphanumeric character(s) or text, a symbol, logo, portrait or other indicia. The image can be two- dimensional or three-dimensional in appearance. In some preferred embodiments, the first fringe pattern represents a diffraction grating. In this case, the first region of the photo-modifiable layer reproduces a flat, colour-changing effect (i.e. an area which exhibits a different colour depending on the viewing position). Advantageously, each of the first, second and optionally subsequent fringe patterns represents a respective diffraction grating. This can be used to build up an aggregate diffractive image of which different parts (formed by the different regions) have different appearances (e.g. colours) at any one viewing angle, thereby defining the desired information. For instance, the resulting aggregate diffractive image could be a Kinegram™ or a pixelgram. Effects such as these can be formed by configuring the various fringe patterns to represent diffraction gratings with different parameters, such as the grating spacing (pitch) and/or orientation.

In other preferred embodiments, the first fringe pattern represents all or a part of an interference pattern formed by light from an object and coherent reference light, whereby the first diffractive effect is a holographic image of all or a part of the object. As explained above, the fringe pattern will be determined beforehand, e.g. by computer modelling of the said object and its resulting interference pattern. The object could be a two-dimensional or three- dimensional object. Preferably, each of the first, second and optionally subsequent fringe patterns represents a respective part of an interference pattern formed by light from an object and coherent reference light, whereby the first, second and optionally subsequent diffractive effects are holographic images of respective parts of the object. In this case, the calculated interference pattern will be divided into parts corresponding to the layout of the regions of the layer to be exposed, and the SLM device will be controlled to display each of the parts sequentially. Most preferably, the aggregate diffractive image is a holographic image of the object, as can be achieved by repeating the method a sufficient number of times that the entire calculated interference pattern is recorded across the various regions of the layer (in aggregate) and positioning the regions to substantially abut one another to create a continuous appearance. It should be noted that the holographic image could be a transmissive, reflective or volume hologram. In order for the so-produced diffractive device to exhibit a strong diffractive effect under visible light illumination, the pattern recorded in the photo-modifiable layer preferably has a pitch of 2 microns or less and hence a fringe width of 1 micron or less. If the SLM device can display the fringe pattern at this level of resolution then the image projected onto the photo-modifiable layer may be a real-size image of the SLM device. However, more typically the pixels of a SLM are larger than this, for instance of the order of 5 to 10 microns in each direction. Therefore, preferably, the optical system further comprises a module configured to demagnify the fringe pattern such that the image of the fringe pattern projected onto the photo-modifiable layer is smaller than the fringe pattern displayed on the SLM device, preferably by a factor of at least 10, more preferably by a factor of at least 20, still preferably by a factor of 25. In this way any constraints on the resolution of the SLM device are relaxed and preferably removed entirely.

As already discussed, in preferred implementations the method is repeated a number of times so as to record respective fringe patterns into different regions of the photo-modifiable layer. Hence steps need to be taken to arrange for the image of the SLM to be projected onto different regions of the layer. This can be achieved in various ways. In some preferred embodiments, after each repetition of steps (b) and (c), the photo-modifiable layer and the optical system are moved relative to one another such that the next fringe pattern will be projected onto a new region of the photo-modifiable layer. In the apparatus, this can be achieved by providing a transport system configured to move a photo-modifiable layer arranged on the support in use and the optical system relative to one another to enable the apparatus to project the image onto different regions of the photo- modifiable layer. The transport system can be controlled to advance the photo- modifiable layer (or the optical system) after each exposure. The movement may be in one dimension or two.

Alternatively or in addition the optical system may preferably comprise a positioning module controllable to change the position of the projected image of the fringe pattern relative to the optical system and, after each repetition of steps (b) and (c), the positioning module is controlled so as to move the position of the projected image such that the next fringe pattern will be projected onto a new region of the photo-modifiable layer. The position module could comprise, for example, one or more movable mirrors and/or prisms which redirect the light to a different lateral position depending on their location.

In accordance with a second aspect of the invention, an interferometric method of manufacturing a diffractive optical device is provided, comprising:

a) providing a photo-modifiable layer having first and second surfaces; b) controlling a spatial light modulation (SLM) device to display a first object;

c) using an optical system to split a first pulse of light from a light source into an object beam and a reference beam, the object beam and the reference beam being coherent, to reflect the object beam off the SLM device and then towards the first surface of the photo-modifiable layer and to direct the reference beam towards the first or second surface of the photo-modifiable layer, the object beam and the reference beam meeting at a predetermined (zero or non- zero) interference angle and generating a first interference pattern in the vicinity of the photo-modifiable layer, whereby a first region of the photo-modifiable layer is modified in accordance with the first interference pattern and thereby exhibits a diffractive image of the first object.

The second aspect of the invention also provides an apparatus for interferometric manufacture of a diffractive optical device, comprising:

a support for a photo-modifiable layer having first and second surfaces; a spatial light modulation (SLM) device;

a light source;

an optical system adapted to split light from the light source into an object beam and a reference beam, the object beam and the reference beam being coherent, to reflect the object beam off the SLM device and then towards the first surface of a photo-modifiable layer arranged in use on the support and to direct the reference beam towards the first or second surface of the photo-modifiable layer, the object beam and the reference beam meeting at a predetermined (zero or non-zero) interference angle to thereby generate an interference pattern in the vicinity of the photo-modifiable layer; and

a control system configured to control the SLM device to display a first object and the optical system to generate a first interference pattern from a first pulse of light from the light source, to enable a first region of the photo- modifiable layer to be modified in accordance with the first interference pattern to thereby exhibit a diffractive image of the first object.

The SLM device and the photo-modifiable layer are each as already defined in relation to the first aspect of the invention. Similarly, the diffractive image may not be directly visible from the photo-modifiable layer but further steps may be required to render it visible such as the application of a reflection enhancing layer, in the same manner as the first aspect.

By using a SLM device to display the object of which a diffractive image is formed, different diffractive images can be generated by controlling the SLM device to display different objects. The object beam is reflected off the SLM and thereby modulated by the SLM before meeting the reference beam to produce the interference pattern. The reference beam circumvents any SLM and consequently is not modified by an SLM. As described above, the pixels of the SLM device can be controlled to change from one display configuration (here providing the object) to another near-instantaneously. Since the whole diffractive image of the object is then formed in a single exposure, this allows for fast, sequential formation of different diffractive images. Of course, the object (and hence the diffractive image) will be substantially two-dimensional, but still a vast array of different objects can be utilised, including any alphanumeric character(s) or text, symbols, logos, portraits or any other indicia that can be displayed on the SLM device. Indeed, this approach is particularly well suited to the formation of unique or personalised diffractive images, e.g. containing a person’s name or portrait, or a serial number.

Hence whilst the method could be performed just once, in preferred embodiments the method further comprises repeating steps (b) and (c), optionally a plurality of times, whereby second and optionally subsequent regions of the photo-modifiable layer or of other photo-modifiable layer(s) are modified in accordance with respective second and optionally subsequent interference patterns corresponding to respective second and subsequent objects displayed sequentially by the SLM device and generated by the optical system from second and optionally subsequent pulses of light from the light source, the respective regions of the photo-modifiable layer(s) being laterally offset from one another, the second and optionally subsequent regions of the photo-modifiable layer(s) thereby exhibiting respective diffractive images of the second and optionally subsequent objects. In the case of the apparatus, the control system is preferably further adapted to achieve this. As before,“laterally offset” means that the regions are side-by-side and preferably non-overlapping. In this second aspect of the invention, since each exposure preferably forms a standalone optical device, preferably the regions are spaced from one another and in especially preferred implementations are each on a different photo- modifiable layer (it should be noted in this case the location of each region within its photo-modifiable layer may well be the same in each case).

Most advantageously, the first and second objects, and preferably any optional subsequent objects, are different from one another. However, since the SLM can be controlled to display any object at will, it is not essential that every single one of the objects be different from one another. In preferred examples, each of the first and any second or subsequent objects displayed by the SLM device comprises an indicia, such as any of: a number, a letter, alphanumerical text, a symbol, an image, a logo, a portrait, a photograph, a barcode, a 2D barcode or a biodata pattern. In especially preferred implementations, each of the first and any second or subsequent objects displayed by the SLM device comprises an item of information which uniquely identifies the object and/or is personalisation information, preferably identifying a person.

As noted above, it is preferred that each of the regions are located on different, discrete photo-modifiable layers, preferably forming part of different, discrete documents (either at the time of exposure or subsequently). This is especially the case where each diffractive effect contains unique identification or personalisation information as mentioned above. Alternatively, each of the regions could be located on one photo-modifiable layer, optionally in the form of a web, with the regions spaced from one another, preferably by a distance at least as great as the width of one of the regions. In this case the method preferably further comprises cutting the photo-modifiable layer into separate parts, each part carrying one of the regions.

In preferred implementations of the second aspect of the invention, just as in the first, where more than one diffractive image is to be formed, it is necessary to expose a different region of the photo-modifiable layer(s), and this can be achieved in corresponding ways as before. In a first preferred embodiment, after each repetition of steps (b) and (c), the photo-modifiable layer(s) and the optical system are moved relative to one another such that the next interference pattern will modify a new region of the photo-modifiable layer(s). In the apparatus this can be achieved for instance by providing a transport module as previously described. Alternatively or in addition the optical system may comprise a positioning module controllable to change the position of the generated interference pattern relative to the optical system and, after each repetition of steps (b) and (c), the positioning module is controlled so as to move the position of the generated interference pattern such that the next interference pattern will modify a new region of the photo-modifiable layer(s).

As mentioned in connection with the first aspect of the invention, in interferometric methods it is necessary to ensure coherence between the two light beams and hence, preferably, any optical path difference between the object beam and the reference beam is less than c.At, where c is the speed of light and At is the duration of the or each pulse of light. Desirably, the optical path difference (if any) is kept small so as to enable the use of short pulse durations. For instance, in particularly preferred embodiments, the optical path difference (if any) is 1.8 metres or less, preferably substantially less, which enables the use of pulse durations (At) of around 6 nanoseconds or correspondingly less (although does not prohibit the use of longer pulse durations). Preferably, the optical system comprises a partial mirror configured to split the light from the light source into the object beam and the reference beam.

Advantageously, the optical system further comprises an interference angle module controllable to change the interference angle at which the object beam and the reference beam meet. This, in turn, provides control over at what viewing / illumination angles the diffractive image will playback. In preferred implementations, the interference angle module comprises at least one optical element arranged in the reference beam path and/or in the object beam bath which is configured to redirect the respective beam(s) towards the photo- modifiable layer. The optical element(s) could for instance be moved (e.g. tilted) to change the angle at which the two beams meet. For example, the optical elements could comprise one or more reflective elements, such as mirrors or prisms. In particularly preferred examples, the at least one optical element comprises a diffractive element arranged in the reference beam path. It would (alternatively or additionally) be possible to insert such a diffractive element in the object beam path, but in this case the object beam (carrying the object information) would be distorted by the diffractive element and this would need to be compensated for. In especially preferred embodiments, the diffractive element is a diffraction grating having a pitch which varies with position along the grating, and the interference angle module further comprises one or more optical elements controllable to direct the reference beam (or the object beam) onto different portions of the diffraction grating to thereby change the angle by which the reference beam is redirected towards the photo-modifiable layer.

The following preferred features are common to both the first and second aspects of the invention (except where indicated otherwise):

As already discussed, the spatial light modulation (SLM) device can take various different forms. In particularly preferred implementations, the SLM device is a reflective SLM device, preferably a liquid crystal on silicon (LCOS) device or a digital light projection (DLP) micro-mirror device. Preferably, the light source comprises a laser source, most preferably a pulsed laser source. As explained below, if the photo-modifiable layer is of a suitable material, high energy light sources can be used to ablate the material and thereby record the desired pattern extremely quickly. In particularly preferred implementations, the duration of the or each light pulse is 100 nanoseconds or less, preferably 20 nanoseconds or less, still preferably between 3 and 12 nanoseconds. For instance, as mentioned above, light pulses of around 6 nanoseconds may be utilised.

Advantageously, the optical system further comprises a shutter module controllable to selectively block illumination of the SLM device and/or projection of the image onto the photo-modifiable layer, the shutter module preferably comprising an acousto-optical modulator disposed between the light source and the SLM device. This can be used to assist control of multiple exposures, ensuring that the photo-modifiable layer is not exposed to an image of the SLM device for the instant while its display changes. Thus, preferably control of the SLM device and of the shutter module are synchronised such that when the fringe pattern (or the object, in the second aspect of the invention) displayed by the SLM changes, the shutter module is blocking illumination of the SLM device and/or projection of the image onto the photo-modifiable layer.

It is also advantageous if the optical system further comprises an isolator module configured to prevent reflected light returning to the light source. This is a preventative measure for avoiding inadvertent damage to the light source.

The photo-modifiable layer could comprise any material which is responsive in some manner to exposure to light from the light source. In preferred embodiments, the layer may comprise one of: a metal layer, an ink layer, a photo-sensitive material, a photo-curable material and a (preferably polymeric) material comprising a photo-absorbent additive. The nature of the modification to the material caused by the exposure will depend on both the material and on the light source. In preferred examples, the modification of the photo-modifiable layer is ablation, change in refractive index, photopolymerization or photodissociation thereof.

For example, in particularly preferred embodiments, the photo-modifiable layer may be a substantially opaque layer such as metal or ink, and the light source may be configured to have a sufficiently high energy delivered in each pulse so as to ablate the layer - for instance a laser source may be used. That is, material is removed from the layer (e.g. is vaporised), either through its entire thickness or just in a surface region thereof. This technique is particularly fast and also requires little or no post-processing and so is especially advantageous. Examples of laser ablation recording used in otherwise conventional holography methods, including suitable photo-modifiable materials and laser sources, are disclosed in “Printable Surface Holograms via Laser Ablation” by F.C. Vasconcellos et al, ACS Photonics, 2014, 1 (6), pp 489-495.

Similar effects can be achieved through the use of materials (such as polymers) for the photo-modifiable layer which contain a laser absorbent additive. Such materials may otherwise be transparent and hence may require the subsequent application of a reflection enhancing layer to render the surface structure visible.

In other preferred implementations, the photo-modifiable layer could be of a sort which undergoes a change in refractive index upon exposure to the light from the light source. Examples of suitable materials are disclosed in EP-A-0324482.

In still further preferred embodiments, the photo-modifiable layer could comprise a material such as a positive or negative resist, which exhibits either photopolymerization or photodissociation upon exposure to light from the light source. For instance, the material could form crosslinks between polymer chains upon exposure or such crosslinks could be broken. Photo-modifiable materials of these sorts may require further processing steps such as washing to remove portions of the material which are more soluble than other portions after exposure, resulting in either a surface relief or isolated islands of material if the full thickness of the layer is removed. A reflection enhancing layer such as metal, a metallic ink or a high-refractive index material may be applied so as to conform to the resulting structure to improve visibility of the playback.

Examples of suitable resist materials, which could be used as the photo- modifiable layer in embodiments of the present invention, include Diazonaphthoquinone-based resists (“DNQ”), also known as ortho quinine diazides (“OQDs”), such as 1 , 2 - Naphthoquinone Diazide.

In other cases the photo-modifiable layer may be further provided with a reflection enhancing layer thereon (prior to exposure), preferably comprising a high refractive index material. In this case, the reflection enhancing material may be transparent to the radiation from the light source or it too may be modified (e.g. ablated) by the radiation.

As discussed at the outset, optical devices produced by the above methods and apparatus (according to the first and second aspects of the invention) have many applications and could, for instance, have a purely decorative function. However, in preferred embodiments, the diffractive optical device is a security device and the photo-modifiable layer forms preferably part of a security article or security document. It should be noted that the photo-modifiable layer might already be incorporated into or carried on the security article or security document at the time of exposure. Alternatively, it may be incorporated into or applied onto the security article or security document after exposure (and, optionally after any necessary post-processing steps). By“security device” we mean a feature which is used to prove authenticity of an item such as a security document. Key to the success of security devices is that it is not possible to reproduce the appearance of the device accurately by taking a visible light copy, e.g. through the use of standardly available photocopying or scanning equipment. Diffractive devices such as those made by the presently disclosed technique achieve this aim.

The first and second aspects of the invention therefore further provide diffractive optical devices made in accordance with the respective methods described above, wherein the diffractive optical device is preferably a security device. Also provided are security articles each comprising such a diffractive optical device, wherein the security article is preferably a security thread, strip, patch, insert or foil. Further provided are security documents each comprising a diffractive optical device or a security article of the sorts described above, wherein the security document is preferably a banknote, a polymer banknote, a passport, an identification card, a bank card, a licence, a visa, a cheque or a certificate. Personalised security documents (such as passports, identification cards, bank cards, driving licences and the like) are particularly suitable applications for the presently disclosed techniques given their adaptation to producing unique or personalised diffractive devices.

Examples of methods of manufacturing diffractive optical devices, apparatus therefor, and devices made according to such methods will now be described with reference to the accompanying drawings, in which:

Figure 1 schematically illustrates a first embodiment of apparatus for manufacturing diffractive optical devices;

Figure 2(a) shows an example of a fringe pattern displayed on a SLM device in an exemplary implementation of the first embodiment, Figure 2(b) showing an enlarged detail thereof, and Figure 2(c) showing a demagnified image of the fringe pattern of Figure 2(a);

Figure 3 is a flow chart showing steps of a first embodiment of a method for manufacturing diffractive optical devices;

Figure 4 illustrates part of an exemplary optical system as may be used in a variant of the first embodiment of apparatus for manufacturing diffractive optical devices;

Figure 5 schematically shows a first example of an optical device made using the first embodiment method and apparatus, in plan view;

Figure 6 schematically shows a second example of an optical device made using the first embodiment method and apparatus, in plan view; Figures 7(a) to (d) illustrate exemplary photo-modifiable layers as may be used in all embodiments of the invention, (i) before exposure and (ii) after exposure and optional post-processing;

Figure 8 illustrates an exemplary photo-modifiable layer as may be used in all embodiments of the invention, after exposure and post-processing;

Figure 9 schematically illustrates a second embodiment of apparatus for manufacturing diffractive optical devices;

Figure 10 is a flow chart showing steps of a second embodiment of a method for manufacturing diffractive optical devices;

Figures 1 1 (a), (b) and (c) show three examples of objects displayed on an SLM device in respective exemplary implementations of the second embodiment; Figure 12 illustrates an exemplary security document carrying a security device made using the second embodiment method and apparatus;

Figure 13 schematically illustrates a variant of the second embodiment of apparatus for manufacturing diffractive optical devices;

Figures 14, 15 and 16 show three exemplary security documents carrying optical devices made in accordance with embodiments of the present invention (a) in plan view, and (b) in cross-section; and

Figure 17 illustrates a further embodiment of a security document carrying an optical device in accordance with the present invention, (a) in front view, (b) in back view and (c) in cross-section.

Examples of apparatus and methods for non-interferometric manufacture of diffractive optical devices will be presented first with respect to the first embodiment and variants thereof, followed by examples of apparatus and method for interferometric manufacture of diffractive optical devices with respect to the second embodiment of the invention and variants thereof.

Referring first to Figure 1 , an apparatus 10 according to a first embodiment of the invention is schematically illustrated. The apparatus 10 is configured to record a diffractive fringe pattern into a photo-modifiable layer 2 such that ultimately the photo-modifiable layer 2 will reproduce a diffractive effect corresponding to the pattern recorded therein. In this example, the photo- modifiable layer 2 is supported on a carrier 3 which could be for example the substrate of a document such as a security document, or any other item. Alternatively, the carrier 3 may be a disposable support layer from which the photo-modifiable layer 2 is ultimately removed and applied to a document or other item. In other cases, the photo-modifiable layer 2 could be a self- supporting layer and no carrier 3 need be present. Examples of suitable photo- modifiable layers 2 will be discussed further below.

The apparatus 10 comprises a spatial light modulation (SLM) device 11 , an optical system 12 and a controller 15. As explained above, the SLM device 1 1 has a display surface 11 a comprising a two-dimensional array of pixels, each of which can be individually controlled in order to display a two-dimensional pattern on the surface 1 1a. The individual pixels can be controlled to modulate light incident on them in various different ways, such as by increasing or decreasing each pixel’s reflectivity or adjusting the direction in which each one redirects incoming light. Preferred examples of suitable SLM devices include liquid crystal based devices such as liquid crystal on silicon (LCOS) devices in which an array of liquid crystal pixels are controlled by suitable electrodes and their opacity can be individually adjusted by adjusting the signals applied to those electrodes. Other preferred examples include devices based on an array of micro mirrors, each of which can be individually orientated. The display 1 1A is controlled by controller 15 which comprises a computer or other digital processor enabled to output suitable control data to the SLM device 1 1 to display the desired pattern thereon.

In use, the controller 15 is configured to control the SLM device 1 1 to display a fringe pattern thereon, of which examples will be provided below. The fringe pattern is effectively a representation of an interference pattern (or a part of an interference pattern) and comprises for example a sequence of light and dark fringes in one or two dimensions. The displayed fringe pattern corresponds to a first diffractive effect which that particular fringe pattern will give rise to under suitable illumination conditions. The configuration of the fringe pattern is calculated either on-the-fly or in advance by controller 15 or another computer (the resulting data being transferred to controller 15), based on the desired diffractive effect. The optical system 12 is configured to project an image of what is displayed on the SLM device display 1 1 a onto a first region Ri of the photo-modifiable layer 2 to thereby reproduce that diffractive effect. As discussed below, it may be necessary to perform one or more post-processing steps on the photo-modifiable layer 2 before the diffractive effect can in fact be viewed. Nonetheless, its information will be carried by the photo-modifiable layer 2 after exposure thereof.

The optical system 12 comprises a light source 13 which is arranged to illuminate the display 1 1 a of SLM device 1 1 , the reflected light being directed towards the photo-modifiable layer 2 via suitable optics 18 to project an image of the displayed fringe pattern onto the first region Ri. In the exemplary optical system 12 shown, this is achieved by directing light from the light source 13 onto a partial mirror 17 which redirects the incident light onto the SLM device 1 1. The light, modified by the fringe pattern displayed on the SLM device 1 1 , is reflected back through partial mirror 17 and through optics 18 which focus an image of the fringe pattern onto photo-modifiable layer 2. The first region Ri of the photo- modifiable layer 2 is thus exposed to an image of the fringe pattern displayed on SLM device 1 1. As a result, the photo-modifiable layer 2 is modified in accordance with the fringe pattern, with different portions of the photo-modifiable material receiving different intensities of light as a result of the patterning and so leading to different levels of modification reproduction of the fringe pattern.

Figure 2(a) shows an exemplary fringe pattern FPi as may be displayed on the SLM device 1 1. Figure 2(b) shows an enlarged detail thereof illustrating the formation of the fringe pattern from pixels 1 T of the device 1 1. Depending on the size of the pixels, it may be necessary to de-magnify the image of the fringe pattern 1 displayed on the SLM device 1 1 so that the image of the fringe pattern recorded in the photo-modifiable layer 2 is of the same magnitude as the wavelength of visible light, so as to produce a strong diffractive effect. Figure 2(c) shows an exemplary image of the fringe pattern, IFPi, which has been de- magnified relative to the pattern FPi displayed on SLM device 1 1 , here by a factor of about 4 in each direction. In practice, the pixels of an SLM device 1 1 are typically in the range of 5 to 10 microns in width, whereas fringe pitches of around two microns are required on the photo-modifiable layer in practice. Therefore, de-magnification factors of at least 10 or more are typically preferred, still preferably 20 or more, e.g. around 25. The necessary components for de- magnification are here represented as part of optics 18 but it should be noted that this functional aspect of optics 18 is optional. Also, the functions of focussing and demagnification could be performed respectively by separate optics.

Figure 1 also shows various other optional features of optical system 12 including an isolator module 14a and a shutter module 14b together with a beam expander 16. The isolator 14a is configured to protect the light source 13 from any light reflected from components of the optical system. For example, the isolator 14a could be a Faraday Isolator, which comprises two crossed polarisers which sit either side of a combination 45 degree Faraday rotator or a 45 degree quartz rotator. Light travelling away from the light source 13 gets polarised by the first polariser and then rotated by 90 degrees so it mostly passes through the second polariser. Any light reflected back towards the light source 13 from the optical system would be polarised at 90 by the second polariser and then the quartz and Faraday rotations cancel out so there is no net additional rotation and the reflected light is therefore stopped by the first polariser. The shutter module 14b may be provided to assist in preventing exposure of the photo-modifiable layer 2 to any unintended image of the SLM device 1 1 such as its appearance while changing from one displayed fringe pattern to another. In preferred embodiments, the shutter 14b may be controlled by the controller 15 and synchronised with its control of SLM device 1 1 to ensure that the SLM device 1 1 is only illuminated at the desired instances. A preferred example of a suitable shutter module 14b is an acoustic optic modulator.

The light source 13 can take various forms but in preferred embodiments comprises a laser source and most preferably a pulsed laser source. The use of short pulses of light to record the desired information in the photo-modifiable layer 2 is preferable due to the fast speed at which the entire exposure process can therefore operate. Examples of suitable laser sources include those described in “Printable Surface Holograms via Laser Ablation” by F.C. Vasconcellos et al, ACS Photonics, 2014, 1 (6), pp 489-495. Two categories of laser sources are particularly preferred (depending on the photo-modifiable material selected). These are:

1. Nano-pulsed lasers (e.g. 5 to 10 ns pulse duration) which are based on Nd:YAG laser technologies with exemplary relevant wavelengths at 532nm (suitable for patterning certain photopolymers for example) and 355nm (suitable for patterning certain photo-resists); and

2. Femto-pulsed lasers (e.g. less than 200 fs or less than 10 ps pulse duration) which are based on Ti:Sapphire laser technology with output wavelengths in the range 700-1080nm.

Turning to Figure 3, a first embodiment of a method for manufacturing diffractive optical devices will now be described. The method can be carried out using the apparatus shown in Figure 1 and is non-interferometric. With the photo- modifiable layer 2 in place as shown in Figure 1 , in the first step S101 , a first fringe pattern is displayed on the SLM device 11. As described above, the first fringe pattern is calculated by controller 15 or another processor to give rise to a corresponding first diffractive effect under appropriate playback conditions. In step S102, the optical system 12 is controlled by controller 15 to illuminate the SLM device 1 1 with a first pulse of light from light source 13, thereby projecting an image of the first fringe pattern onto the first region Ri of the photo-modifiable layer 2. Examples of the first fringe pattern and the corresponding image thereof are as already discussed with respect to Figure 2 above.

Depending on the desired nature of the diffractive optical device to be formed, the performance of steps S101 and S102 may complete the method, with the complete optical device having been formed by a single exposure of just one region of the photo-modifiable layer 2. Thus, all further steps shown in Figure 3 and discussed hereinafter are optional, as illustrated by their being depicted in dashed lines. However, in more preferred embodiments, the method is repeated a plurality of times in order to produce a larger diffractive optical device which covers a greater area of the photo-modifiable layer 2 and also to increase its complexity. Thus, once step S102 has been completed, the apparatus is adjusted in step 103 so that a second region of the photo-modifiable layer 2 can be exposed to another fringe pattern. As described hereinafter in more detail, this can either be achieved by moving the optical system 12 and the photo- modifiable layer 2 relative to one another, or the optical system 12 may include a positioning module which is configurable to change the position at which the image is output from the optical system 12.

In the next step S104, a second fringe pattern is displayed on SLM device 1 1 , which may be the same as or different from the first pattern displayed. This will depend on the nature of the overall optical device to be formed. Again, the second fringe pattern is calculated by the controller 15 or another processor for display on the SLM device 1 1. The second fringe pattern corresponds to a second diffractive effect. Next, in step S105, the optical system 12 is controlled to illuminate the SLM device 1 1 with another pulse of light to thereby project an image of the second fringe pattern onto the second region of the photo- modifiable layer. In this way the second fringe pattern will be recorded into the second region of the photo-modifiable layer 2, which will then playback the second diffractive effect upon appropriate playback conditions.

As illustrated by step S106, the process will be repeated until the desired number of regions of the photo-modifiable layer 2 have been exposed to respective fringe patterns. Once all of the regions have been exposed, the optical device may be complete and this may be the end of the method. However, depending on the nature of the photo-modifiable material 2, it may be necessary to perform one or more post-processing steps as indicated by step S107, in order for the diffractive effect(s) to be rendered visible. Examples of such post-processing steps will be discussed below.

As noted above in connection with step S103, in preferred embodiments, it is necessary that different regions of the photo-modifiable layer 2 be exposed to the various respective fringe patterns displayed on SLM device 11. In some implementations, this will be achieved by mounting either the optical system 12 or the photo-modifiable layer 2 on a suitable transport module (not shown, but a suitable example will be described in relation to the second embodiment). This can be operated between exposure steps to physically change the portion of the photo-modifiable layer 2 which is presented to the optical system 12. Alternatively or additionally, the optical system 12 can include a positioning module 20 of which an example is shown schematically in Figure 4. Here, the positioning system 20 comprises two mirrors 20 and 21 , each of which is individually rotatable about respective perpendicular axes. The reflected light from SLM device 1 1 is incident on first mirror 21 which is rotatable about an axis which lies in the X-Z plane. Light is reflected off first mirror 21 onto second mirror 22 which is rotatable about an axis parallel to the Y axis, and then redirected through optics 18 which, as described above, achieves focussing of the image onto photo-modifiable layer 2 and optionally de-magnification thereof to achieve the desired dimensions. Through control of mirrors 21 and 22, the image of the fringe pattern displayed on SLM device 1 1 can be moved from one region of the photo-modifiable layer 2 to another. Three exemplary regions Ri, R₂ and R₃ are illustrated in Figure 4, and rectangle V indicates the periphery of the area within which a region can be selected though movement of the mirrors 20 and 21. It will be appreciated that the positioning module 20 could be implemented in various different ways, including the use of a single gimbal- mounted mirror and/or equivalent optical components such as prisms.

The respective first, second and any subsequent regions (such as Ri, R₂ and R₃) forming the ultimate optical device could have any location relative to one another, including some or all of the regions being spaced from one another such that non-exposed portions of the photo-modifiable layer 2 remain between them. However, in this first embodiment of the invention it is typically preferred that some or all of the regions abut one another, so as to form a contiguous area of the photo-modifiable layer which is exposed to the fringe patterns and thereby exhibits diffractive effects. The diffractive effects exhibited by the respective exposed regions of the photo- modifiable layer 2 could be independent of one another, for instance randomly generated, but in particularly preferred implementations, the respective diffractive effects exhibited by the regions collectively form an aggregate diffractive image. That is, the regions in combination exhibit a cohesive, continuous diffractive image such as one or more items of information. For instance, the diffractive optical device as a whole could take the form of a kinegram™ or pixelgram -type device, or the regions could collectively exhibit a holographic image of all or part of an object.

Figure 5 schematically shows an example of an optical device D made according to the above-described method and apparatus, which exhibits an aggregate diffractive image of the first sort described immediately above. Thus, in this case the optical device D comprises a total of 49 regions, selected ones of which are labelled Ri, R₂, R₃, R₄ and R₅, arranged in a two-dimensional array abutting one another so as to form a rectangle or square shaped optical device D, all of which exhibits diffractive effects. It will be appreciated that the periphery of the area formed by the regions could take any shape and could itself denote an item of information, such as an alphanumerical character or similar if desired. Each region R₁, R₂ etc. has been exposed to a projected image of a respective fringe pattern displayed on the SLM device 11 in the manner described above. In this example, each of the fringe patterns corresponds to a diffraction grating. Thus, each of the exposed regions R₁, R₂ etc. exhibits (under suitable playback conditions) a respective uniform colour which changes with viewing angle. The particular colour which is seen at any one viewing angle, and the speed at which is varies upon changing the viewing angle, depends on the pitch and the orientation of the diffraction grating. This can be varied at will from one region to another simply by appropriate choice of the fringe pattern displayed on the SLM device 1 1 for the corresponding exposure.

In the example shown in Figure 5, only two different diffraction gratings with different parameters are utilised. All of the regions shaded with horizontal lines carry a first diffraction grating, including regions R₁, R₂, R₃ and R₄, whereas those shaded with diagonal lines carry a second diffraction grating with different grating parameters (pitch and/or orientation). Region R₅ is an example of this second diffraction grating. The regions are configured so that, between them, a diffractive image Hi is formed which preferably conveys at least one item of information, here the letter “M”. Thus, upon appropriate illumination for playback, the letter“M” will appear differently coloured from the background surrounding it and may undergo a different variation in colour as the viewing angle changes. It will be appreciated that much more complex effects can be achieved through the use of more types of different diffraction gratings and/or more complex arrangements of the regions.

Figure 6 shows a second example of an optical device D formed in accordance with the above-described method and apparatus which here exhibits a holographic image H₂ of an object, in this case a scroll. Again, the optical device D comprises 49 regions arrayed in two dimensions to form a rectangle or square of which five are labelled Ri, R₂, R₃, R₄ and R₅ as before. However, in this case for clarity the fringes recorded into each region are not represented since they will be of a highly complex nature. Instead, what is shown is the holographic image H₂ played back by the fringe pattern recorded into the regions collectively upon appropriate illumination. To form the device D, the controller 15 or another processor will use computer modelling to determine the interference pattern which would be formed by an object light beam modified by the selected object and intercepted by a coherent reference light beam. The so-generated interference pattern will then be digitally laid out across the intended set of regions and divided up into sections corresponding to the size of each region. The respective sections of the calculated interference pattern will then be displayed sequentially on the SLM device 11 and projected onto respective regions R₁, R₂ etc. of the photo-modifiable layer 2 so that, once complete, the optical device D as a whole will exhibit a holographic image H₂ of the entire object. Of course, it is not essential to exhibit the entire holographic image and if desired, only parts of the calculated interference pattern may be recorded into the photo-modifiable layer 2, for example the left half of the device shown in Figure 6 such that only the left half of the scroll will be displayed. It will be appreciated that the holographic image H₂ could be of any object, two- dimensional or three-dimensional, although three-dimensional effects are preferred. As in the previous example, the periphery of the optical device D could take any shape and may itself define an item of information if desired.

Typically, depending on the size of each projected fringe pattern image, a typical optical device D may comprise many hundred exposed regions Ri, R₂ etc. As such, it is desirable that each exposure be performed as quickly as possible in order to keep the overall duration of the process short. Non-interferometric methods such as the one described above lend themselves well to this, since there is no minimum threshold placed on the duration of the light pulses, as may be the case in interferometric methods (as described below in relation to the second embodiment). Rather, the only constraint on the light pulses will be the nature of the photo-modifiable material 2 and the duration for which it must be exposed to the radiation from the light source 13 in order to properly record the image. For the same reasons, high powered light sources such as lasers, most preferably pulsed lasers, are highly preferred in order to deliver the required dose of energy to the photo-modifiable layer 2 quickly.

The manner in which the photo-modifiable layer 2 is modified by the light and thereby records the information will depend on the nature of the photo-modifiable layer 2 and on the light source 13. Figures 7(a) to (d) show some preferred examples, in each case: (i) before exposure, and (ii) after exposure and optional post-processing steps. A particularly preferred implementation is to use a laser light source to ablate a suitable light-modifiable material 2 such as a layer of ink, a layer of metal (or alloy), or any other typically opaque material. The laser power and duration is set such that those portions of the laser-modifiable layer 2 exposed to the light source in accordance with the fringe pattern are ablated or entirely vaporised, leaving either a surface relief or gaps through the laser- modifiable layer 2, as represented by the pattern P shown in Figure 7(a)(ii).

In other examples, the laser-modifiable material may be of a sort which undergoes either polymerisation or dissociation upon exposure to the light from the light source 13. This is illustrated in Figure 7(b). For instance, the photo- modifiable layer 2 may be a positive or negative resist material which either forms cross-links or breaks cross-links when exposed to the light. Thus, the portions of the photo-modifiable layer 2 exposed to the light from the light source 13 in accordance with the pattern will become either more soluble or less soluble, relative to the unexposed portions of the material, resulting in a surface relief pattern P, as shown in Figure 7(b)(ii). To reveal the pattern P, it may be necessary to perform one or more post-processing steps on the light-modifiable layer 2 after exposure, such as washing the layer in a suitable solvent to remove those portions which are more soluble than the others.

Examples of suitable resist materials, used in embodiments of the present invention, include Diazonaphthoquinone-based resists (“DNQ”), also known as ortho quinine diazides (“OQDs”), such as 1 , 2 - Naphthoquinone Diazide. For instance, five exemplary compositions of suitable resist materials which can be utilised in embodiments of the invention are as follows (“g” = gram):

1 ) 1 g V215 by Varichem Co. Ltd. or 1 , 2 - Naphthoquinone-2- Diazide-5- sulfonyl chloride; 10g Cyclopentanone; 1 g MEK; and 0.03g Surfynol 61 (from Air Products).

2) 1 g V215 by Varichem Co. Ltd. or 1 , 2 - Naphthoquinone-2- Diazide-5- sulfonyl chloride; 10g Cyclopentanone; 1g MEK; and 0.01 g Byk-055 (from Byk Chemie).

3) 1 g V215 by Varichem Co. Ltd. or 1 , 2 - Naphthoquinone-2- Diazide-5- sulfonyl chloride; 10g Cyclopentanone; 1g MEK; and 0.01 g Byk-022 (from Byk Chemie).

4) 1 g V215 by Varichem Co. Ltd. or 1 , 2 - Naphthoquinone-2- Diazide-5- sulfonyl chloride; 10g Cyclopentanone; 1 g MEK; and 0.2g Isopropyl alcohol.

5) 0.95g V215 by Varichem Co. Ltd. or 1 , 2 - Naphthoquinone-2- Diazide-5- sulfonyl chloride; 0.05 Novolak resin; 10g Cyclopentanone; 1g MEK; and 0.05g Surfynol 61 (from Air Products). Figure 7(c) illustrates a further preferred type of photo-modifiable layer 2 which is a material, such as a polymer, containing a laser-absorbent additive. For instance, the material 2 may be otherwise transparent, including visually transparent. Upon exposure to the light source 13, those portions of the layer 2 exposed to the light in accordance with the fringe pattern absorb the radiation and as such undergo modifications such as blackening, charring, foaming or indeed ablation. The ablation may be through the whole thickness of the layer 2 or only a part of it, the latter of which is illustrated in Figure 7(c)(ii). The result is once again a surface relief pattern P in accordance with the fringe pattern. Figure 7(d) shows a variant of the last example in which the photo-modifiable layer 2 is provided prior to exposure with a reflection enhancing layer 2a. For instance, this may be formed from a high refractive index (HRI) material with a refractive index greater than that of the layer 2, e.g. by at least 0.3. For instance, the reflection enhancing layer 2a could be a vapour-deposited layer of zinc sulphide. If the reflection enhancing layer 2a is transparent to the radiation from the light source, it may not be modified by the exposure. Alternatively, the layer 2a could be locally vaporised by the irradiation. Such a reflection enhancing layer 2a could be pre-applied to any of the various types of photo- modifiable layer 2 herein disclosed.

Depending on how the diffractive optical device is to be viewed, whichever of the above types of photo-modifiable material 2 is used, additional steps may be necessary in order to visual the diffractive effect. In particular, it may be necessary to apply a reflection enhancing layer over any surface relief profile generated (i.e. after the formation of that surface relief). An example of such a reflection enhancing layer 2b is shown in Figure 8 and here, whilst the photo- modifiable layer 2 is illustrated as taking the same form as in Figure 7(c), it should be appreciated that there is no such limitation and any type of light- modifiable layer 2 may be combined with a reflection enhancing layer 2b as now described. The reflection enhancing layer 2b preferably conforms on at least one and advantageously both of its sides to the surface relief profile formed in the photo-modifiable layer 2. The reflection enhancing layer 2b could be formed of one or more metals or alloys, a high refractive index (HRI) material as described above, or a reflective ink such as a metallic or colour shifting ink. The reflection enhancing layer 2b can be applied for example by vapour deposition techniques or by printing techniques.

In still further examples, rather than produce a surface relief profile, the photo- modifiable material may undergo other responses to radiation from the light source 13, such as a change in its reflective index. In such cases, the resulting device will be viewable from the material itself, e.g. by transmission of light therethrough, and as such no reflection enhancing layer is typically necessary. Such devices are typically referred to as amplitude-difference devices (rather than phase-difference devices), or volume holograms. Examples of suitable materials for this purpose are disclosed in EP-A-0324482.

The photo-modifiable layer 2 in which the optical device D is formed could then go on to be incorporated into or applied onto an item, such as a security article (e.g. a security thread, stripe, patch or foil) or a security document (such a banknote or passport for example). However, preferably the photo-modifiable 2 already forms part of such an item when exposure is performed. This enables the optical device D to be manufactured in situ, which lends itself well to the production of individualised or personalised optical devices D, in which the diffractive effect is specific to the device in question. For example, for a given set of the so-produced optical devices, each one could have a unique diffractive effect overall. The diffractive effect may preferably incorporate information such as personalisation information relating to the holder of a security document into which the optical device D has been or will be incorporated. For example, the diffractive effect could include one or more items of information such as alphanumerical character(s) or text (such as the example shown in Figure 5), other symbols, logos or even portraits of the holder. Likewise, security devices formed in accordance with the example shown in Figure 6 could have any form of holographic image which again may contain personalised or unique information. As noted at the outset, the optical devices herein disclosed can have many applications including purely decorative functions but are particularly well suited to use as security devices and hence in preferred embodiments are incorporated into security articles or security documents. Exemplary techniques for incorporating security devices such as these into such articles and documents will be described below following discussion of the second embodiment but are equally applicable to optical devices produced using the first embodiment.

A second embodiment of the invention will now be discussed with reference to Figures 9 to 13. Here, the described method and apparatus for manufacturing a diffractive optical device differs from the first embodiment in that it is an interferometric approach. That is, it actively interferes two coherent light beams to produce an interference pattern to which the photo-modifiable layer 2 is exposed during performance of the method. An example of a suitable apparatus 10’ in accordance with the second embodiment of the invention is shown schematically in Figure 9. Some of the components of the apparatus 10’ are common to those of the apparatus 10 described above in connection with the first embodiment of the invention and are labelled using like reference numerals. These will not be described here again since they can be implemented in the same manner as already described in connection with the first embodiment.

Thus, as in the first embodiment, the second embodiment of apparatus 10’ includes an SLM device 1 1 which can be an LCOS device, a micro mirror device or the like as already described. Once again, the SLM device 1 1 is controlled by a controller 15. However, rather than display a fringe pattern corresponding to a diffractive effect on the surface 1 1 a of the SLM device 1 1 , in the second embodiment the controller 15 controls the SLM device 1 1 to display thereon an object, here indicated schematically as Oi . Examples of objects will be given below but in essence the object could comprise any two-dimensional graphic which can be displayed through control of the pixels forming the SLM device 1 1 , such as one or more items of information. The optical system 12’ is configured to divide a light beam emanating from light source 13 into two coherent light beams (note that the light from light source 13 is polarised in the direction transverse to the plane of the page in Figure 9 - this makes it non-angularly dependent in that plane and in this way the brightness of the two beams can be maximised in the plane of interference with the photo- modifiable layer 2). In the embodiment depicted, this division is achieved by partial mirror 17. The light from light source 13 strikes partial mirror 17 and part of it is transmitted in a straight line therethough towards the SLM device 1 1. This is referred to hereinafter the object beam OB. The object beam is reflected by the surface 11 a of SLM device 11 , having been modified by the displayed object Oi. When the reflected and modified object beam OB strikes the partial mirror 17 for a second time, a portion of it is reflected towards the photo- modifiable layer 2. Meanwhile, the portion of light from light source 13 which is not transmitted by partial mirror 17 becomes the reference beam RB. The reference beam circumvents the SLM and consequently is not modified by a SLM. The optical system 12 is configured to return the reference beam back towards the photo-modifiable layer 2 so that it intersects the object beam adjacent to the photo-modifiable layer 2 at an interference angle Q (which may be zero or non-zero). In this embodiment, the optical system 12’ comprises two mirrors 19a and 19b arranged to reflect the reference beam RB through 180 degrees and a diffraction grating 19c which redirects the reference beam RB towards first region Ri of the photo-modifiable layer 2, upon which the object beam OB is also incident. As a result, at the point of intersection, the object beam and reference beam interfere with one another to form an interference pattern IPi to which the region Ri of photo-modifiable layer 2 is exposed. It should be noted that, whilst in the arrangement depicted the object beam and the reference beam strike the photo-modifiable layer 2 from the same side thereof, this is not essential and in other cases the beams could be arranged to meet from opposite directions through the layer 2, to form a reflection-type device rather than a transmission-type device.

The interference pattern IPi is thus recorded in the first region Ri of the photo- modifiable layer 2 through appropriate modification of the material and thereinafter the region Ri carries a diffractive image of the object Oi displayed on SLM device 1 1. As in the case of the first embodiment, however, it should be appreciated that depending on the nature of the photo-modifiable material 2, post-processing steps may be necessary in order to visualise the diffractive image. Nonetheless, it is the photo-modifiable layer 2 which carries the necessary information for replay of the image once exposure is complete.

Other components of the optical system 12’ including light source 13, isolator 14a, shutter 14b and beam expander 16 are all of the sorts already described in relation to the first embodiment.

Figure 10 is a flow diagram setting out steps of an exemplary method in accordance with the second embodiment as may be implemented using the apparatus shown in Figure 9. With the photo-modifiable layer 2 in place as shown, a first object Oi is displayed on the SLM device 1 1 in step S201. Next, the optical system 12’ is controlled in step S202 to illuminate the SLM device 1 1 with the object light beam OB, in order to generate a first interference pattern IPi in the manner already described by meeting the object beam with the reference beam RB and exposing the first region Ri of the photo-modifiable layer 2 to the resulting interference pattern. In this way, a diffractive optical device D is formed which carries a diffractive image of the first object Oi and this may be the end of the method with all further steps described below being optional as illustrated by the dashed lines in Figure 10. However, in other cases the process may be repeated either to form a plurality of discrete diffractive optical devices D or to produce a larger diffractive optical device through exposure of multiple adjacent regions in the manner described with reference to the first embodiment.

Thus, in step S203, the apparatus is adjusted such that a different region of the (or another) photo-modifiable layer 2 will be exposed to the next interference pattern generated by the apparatus. This can be achieved either by moving the optical system 12’ and the photo-modifiable layer 2 relative to one another or, as described in more detail below, the optical system 12’ may include a positioning module which is configurable to change the location at which the interference pattern will be formed.

Next, in step S204, a second object is displayed on SLM device 1 1 through control by controller 15. The second object may be the same or different from the first object. Then, in step S205, the optical system 12’ is controlled to illuminate the SLM device 11 once again with the object light beam to generate a second interference pattern by meeting the object beam OB with the reference beam RB in the manner described and to expose a second region of the photo- modifiable layer to the resulting pattern. In this way, a second diffractive image, this time of the second object, will be formed. As represented by step S206, the process may be repeated any number of times until all of the desired diffractive images of the respective objects have been formed. Finally, if required, the so- exposed photo-modifiable layer(s) may undergo post-processing as necessary to render to the diffractive effect visible. Any of the types of photo-modifiable layer 2 already discussed with reference to the first embodiment can be used in the second embodiment, including those shown in Figure 7 and the same post- processing options are available, including washing steps and/or the application of a reflection enhancing layer 2b as necessary. It should further be appreciated that it is not essential to wait until all the repetitions of the exposure method have been performed before performing post-processing step S207. In particular, if a single exposure takes place on each discrete photo-modifiable layer 2, post- processing of that photo-modifiable layer 2 may take place before the next photo-modifiable layer 2 is exposed, or entirely independently of any further exposure steps.

Some examples of objects which may be displayed on the SLM device 1 1 in the second embodiment are shown in Figures 1 1 (a), (b) and (c). In general, the objects can comprise any graphic which the pixels on surface 1 1a of SLM device 1 1 can be controlled to display, such as one or more alphanumeric characters or text, symbols, logos and the like. In particularly preferred embodiments, the objects may comprise at least one item of information, most preferably personalisation information or information uniquely identifying the diffractive optical device relative to other like devices. For example, Figure 11 (a) shows an example of a first object Oi which here comprises a denomination identifier “$50”. In Figure 1 1 (b), a second object 0₂ is shown which here is the text “NAME” but it will be appreciated that this can replaced by any set of letters or numbers and can be personalised to display the actual name of a document holder. For example, when the diffractive optical device D is to be formed on a particular security document such as a passport or driving licence, the name of the holder could be either input into controller 15 or retrieved from controller 15 by a database and an appropriate data file output to SLM device 11 in order to display the name, or part thereof, on the surface 1 1 a thereof. Figure 1 1 (c) shows an example of a third object 0₃ which here is a 2D barcode. The 2D barcode could contain, for example, bibliographic information relating to a holder of the document on which the device is to be formed or could carry a unique identifier or a link to a webpage. Again, the 2D barcode may be generated by controller 15 or may be retrieved from a database. Other objects which may usefully be employed include biometric information such as fingerprints, eye scans or facial profiles. In all cases, the resulting diffractive optical device D will exhibit a diffractive version of the displayed object such as objects Oi, 0₂ or 0₃.

Figure 12 schematically shows an example of a security document 100, here a card-form driving licence, which carries a security device D formed using the method and apparatus of the second embodiment. The security device D may have been formed directly in situ on the card 3 forming the security document 100 or may have been formed on another support substrate and then transferred thereto. In the former case, the card forming the security document may initially have been provided with an area (denoted by the dashed line 2) across which the photo-modifiable material 2 may originally have been present and, after exposure in the manner described, the depicted diffractive image D of an object, here a 2D barcode as in Figure 1 1 (c), is visible. In this example, the device D may contain information relating to the holder of the security document 100 who is identified on the document elsewhere through his photo 102 and/or other bibliographic information such as his name, date of birth and address indicated at 103. These personalised items of information may be applied to the security document, for instance by printing or laser marking, during a process performed substantially at the same time as applying the diffractive optical device D. Meanwhile, the security document 100 may also contain other features such as text 101 (reading “driving licence” in this case), which is common to all documents of the same sort and is typically applied to all such documents at some central location during the initial manufacturing process, prior to personalisation which may be carried out at local distribution centres.

It should be noted that whilst the depicted security device D on document 100 has been described as being formed using the method of the second embodiment of the invention, the same type of security device D could be formed using the first embodiment of the invention if the controller 15 uses an object such as a 2D barcode to calculate the interference pattern therefrom and record it into the photo-modifiable material 2 in the manner described with respect to Figures 1 and 3, using as many regions of the photo-modifiable layer 2 as necessary. However, the second embodiment has the advantage that the whole optical device D will be manufactured in a single exposure of the photo- modifiable layer 2. In this way, the second embodiment lends itself well to fast, on-the-fly recording of diffractive optical devices containing different information.

On the other hand, the use of an interferometric process does impose some constraints on the system, since the object beam OB and reference beam RB must be coherent to one another at the point of intersection. For a short pulse of light from light source 13, this imposes a maximum path length difference between the object beam and the reference beam. In particular, any such path difference between the two beams must be less than (o.DT), where c is the speed of light and DT is the pulse duration. Thus, for a pulse duration of 6 nanoseconds for example, a path difference of up to 1.8 metres is allowable. Nonetheless, it is preferred that any optical difference between the two beams should be substantially less than this in order to ensure coherence between the two beams. As noted above, between exposure steps the apparatus 10’ needs to be adjusted so that a different region of the photo-modifiable layer 2 (or another photo-modifiable layer 2) will be exposed next. This can be achieved by providing a suitable transport system so that the photo-modifiable layer 2 will be moved relative to the optical system 12’ or vice versa. Alternatively, this could be achieved instead (or in addition) by providing the optical system 12’ with a positioning module, such as positioning module 20 already described in relation to the first embodiment and depicted in Figure 4. This could be inserted for instance between the partial mirror 17 and the photo-modifiable layer 2 in apparatus 10’.

Another aspect which is it desirable to be able to control in the second embodiment is the interference angle Q between the reference beam RB and the object beam OB when they meet to form the interference pattern. The interference angle Q will determine at what angle the diffractive image of the object plays back at from the exposed photo-modifiable layer 2. Thus, Figure 13 shows a variant of the apparatus 10” used in the second embodiment which enables control over the interference angle Q. All other aspects are the same as in the Figure 9 apparatus 10’, although in Figure 13, we also illustrate an exemplary transport module 50 for moving the photo-modifiable layer 2 relative to optical system 12’.

To enable control over the interference angle Q, in the apparatus of Figure 13, mirror 19b is movable in the direction indicated by the arrow, and diffraction grating 19c is of varying pitch (or orientation) along its length in the same direction as the arrow. By adjusting the position of the mirror 19b, optionally under the control of controller 15, the reference beam can be caused to impinge on different positions along the length of the diffraction grating 19c. Due to the varying pitch (or orientation) along the grating, this will also change the angle by which light passing through the diffraction grating is redirected and as a result this will change the angle at which it meets the object beam, thereby controlling the interference angle Q. It should be noted that control of the interference angle Q could be implemented in various different ways. For instance, components 19a, b and c could be inserted into the object beam instead of the reference beam. However, this would cause distortion of the object information which would require compensation. Alternatively, other optical elements could be used to redirect the angle of one or both of the beams, such as one or more reflective elements, e.g. mirrors and/or prisms.

The transport module 50 in this example is controlled so that the photo- modifiable layer 2 can be moved while keeping the optical system 12’ stationary. Here, the transport system comprises a web 53 which is supported by two transport rollers 51 and 52 and a support plate 54 to move in a machine direction MD under control of the rollers. In this example, three discrete photo-modifiable layers 2a, 2b and 2c are depicted, each one arranged separately from the others on the web 53. It should be noted that the photo-modifiable layers 2a, 2b and 2c could be affixed to the transport web 53 (which could be some form of polymeric backing layer to be removed later) or could simply sit thereon. Moreover, each photo-modifiable layer 2a, 2b and 2c could already be carried on separate documents such as the driving licence 100 shown in Figure 12 and simply conveyed on web 53 (in the form of a belt) for this process. In this example, the first region Ri of the photo-modifiable layer 2 is located on first discrete portion of the material 2a whilst second region R₂ is located on second discrete portion 2b and likewise the third region R₃ is located on a third photo-modifiable layer 2c. In still further examples, the web 53 could be provided with a continuous photo-modifiable layer 2 (not shown), spaced regions of which are sequentially exposed in the manner described and later the web and the photo-modifiable layer thereon are cut into individual articles.

A transport module 50 such as that illustrated in Figure 13 can also be utilised in the first embodiment to move the photo-modifiable layer 2 relative to the optical system 12. In both embodiments, it may be desirable to provide a transport module such as this to achieve relative movement in the machine direction MD and a positioning module such as module 20 shown in Figure 4 to achieve selection of different regions in the orthogonal direction.

Diffractive optical devices D of the sorts described above, in the form of security devices, can be incorporated into or applied to any article for which an authenticity check is desirable. In particular, such devices may be applied to or incorporated into security documents such as banknotes, passports, driving licences, cheques, identification cards etc.

The security device or article can be arranged either wholly on the surface of the base substrate of the security document, as in the case of a stripe or patch, or can be visible only partly on the surface of the document substrate, e.g. in the form of a windowed security thread. Security threads are now present in many of the world's currencies as weli as vouchers, passports, travelers' cheques and other documents. In many cases the thread is provided in a partially embedded or windowed fashion where the thread appears to weave in and out of the paper and is visible in windows in one or both surfaces of the base substrate. One method for producing paper with so-called windowed threads can be found in EP-A-0059056. EP-A-0860298 and WO-A-03095188 describe different approaches for the embedding of wider partially exposed threads into a paper substrate. Wide threads, typically having a width of 2 to 8mm, are particularly useful as the additional exposed thread surface area allows for better use of optically variable devices, such as that presently disclosed.

The security device or article may be subsequently incorporated into a paper or polymer base substrate, either on a non-transparent surface there of so that it is viewable from just one side, or in such a way that that it is viewable from both sides of the finished security substrate. Methods of incorporating security elements in such a manner are described in EP-A-1 141480 and WO-A- 03054297. In the method described in EP-A-1 141480, one side of the security element is wholly exposed at one surface of the substrate in which it is partially embedded, and partially exposed in windows at the other surface of the substrate. Base substrates suitable for making security substrates for security documents may be formed from any conventional materials, including paper and polymer. Techniques are known in the art for forming substantially transparent regions in each of these types of substrate. For example, WO-A-8300859 describes a polymer banknote formed from a transparent substrate comprising an opacifying coating on both sides of the substrate. The opacifying coating is omitted in localised regions on both sides of the substrate to form a transparent region. In this case the transparent substrate can be an integral part of the security device or a separate security device can be applied to the transparent substrate of the document. WO-A-Q039391 describes a method of making a transparent region in a paper substrate. Other methods for forming transparent regions in paper substrates are described in EP-A-723501 , EP-A-724519, WO-A-03054297 and EP-A-1398174.

The security device may also be applied to one side of a paper substrate so that portions are located in an aperture formed in the paper substrate. An example of a method of producing such an aperture can be found in WO-A-03054297. An alternative method of incorporating a security element which is visible in apertures in one side of a paper substrate and wholly exposed on the other side of the paper substrate can be found in WO-A-2000/39391.

Examples of such documents of value and techniques for incorporating a security device will now be described with reference to Figures 14 to 17.

Figure 14 depicts an exemplary document of value 100, here In the form of a banknote. Figure 14a shows the banknote in plan view whilst Figure 14b shows the same banknote in cross-section along the line Q-Q'. In this case, the banknote is a polymer (or hybrid polymer/paper) banknote, having a transparent substrate 102. Two opacifying layers 103a and 103b are applied to either side of the transparent substrate 102, which may take the form of opacifying coatings such as white ink, or could be paper layers laminated to the substrate 102. The opacifying layers 103a and 103b are omitted across an area 101 which forms a window within which the security device 1 is located. As shown best in the cross-section of Figure 14b, a light modifiable layer 2 having an optical device 2 formed therein is provided on one side of the transparent substrate 102,. The optical device D is as described above with respect to any of the disclosed embodiments, such that the device D displays one or more diffractive effects (an image of the letter“A” is depicted here as an example). It should be noted that in modifications of this embodiment the window 101 could be a half- window with the opacifying layer 103b continuing across ail or part of the window over the security device 1. In this case, the window will not be transparent but may (or may not) still appear relatively translucent compared to its surroundings. The banknote may also comprise a series of windows or half-windows in this case the security device(s) could be configured to display different diffractive effects in different ones of the windows.

Figure 15 shows such an example, although here the banknote 100 is a conventional paper-based banknote provided with a security article 105 in the form of a security thread, which is inserted during paper-making such that it is partially embedded into the paper so that portions of the paper 104 lie on either side of the thread. This can be done using the techniques described in EP0059056 where paper is not formed in the window regions during the paper making process thus exposing the security thread in is incorporated between layers of the paper. The security thread 105 is exposed in window regions 101 of the banknote. Alternatively the window regions 101 which may for example be formed by abrading the surface of the paper in these regions after insertion of the thread. The security device(s) D are formed on the thread 105, which comprises a transparent substrate with a photo-modifiable layer 2 provided on one side in this example, the security device D is configured to playback in reflected illumination if desired, several different security devices D could be arranged along the thread, with different or identical diffractive effects displayed by each. !n Figure 18, the banknote 100 is again a conventional paper-based banknote, provided with a strip element or insert 108. The strip 108 is based on a transparent substrate and is inserted between two plies of paper 109a and 109b. The security device D is formed in a photo-modifiable layer 2 disposed on the strip substrate. The paper plies 109a and 109b are apertured across region 101 to reveal the security device D, which in this case may be present across the whole of the strip 108 or could be localised within the aperture region 101.

A further embodiment is shown in Figure 17 where Figures 17(a) and (b) show the front and rear sides of the document 100 respectively, and Figure 17(c) is a cross section along line Q-Q\ Security article 1 10 is a strip or band comprising a security device according to any of the embodiments described above. The security article 1 10 is formed into a security document 100 comprising a fibrous substrate 102, using a method described in EP-A-1 141480. The strip is incorporated into the security document such that it is fully exposed on one side of the document (Figure 17(a)) and exposed in one or more windows 101 on the opposite side of the document (Figure 17(b)). Again, the security device is formed on the strip 1 10, which comprises a transparent substrate with a photo- modifiable layer 2 on one surface carrying optical device D.

In Figure 17, the document of value 100 is again a conventional paper-based banknote and again includes a strip element 110. In this case there is a single ply of paper. Alternatively a similar construction can be achieved by providing paper 102 with an aperture 101 and adhering the strip element 1 10 on to one side of the paper 102 across the aperture 101. The aperture may be formed during papermaking or after papermaking for example by die-cutting or laser cutting. Again, the security device is formed on the strip 1 10, which comprises a transparent substrate with a light redirecting layer 10 formed on one surface and colour layer 20 formed on the other.

The security device of the current invention can be made machine readable by the introduction of detectable materials in any of the layers or by the introduction of separate machine-readable layers. Detectable materials that react to an external stimulus include but are not limited to fluorescent, phosphorescent, infrared absorbing, thermochromic, photochromic, magnetic, electrochromic, conductive and piezochromic materials. For instance, the photo-modifyable layer 2 could itself be magnetic and/or electrically conductive.

Suitable magnetic materials include iron oxide pigments (Fe₂0₃ or Fe₃0₄), barium or strontium ferrites, iron, nickel, cobalt and alloys of these. In this context the term “alloy” includes materials such as Nickel:Cobalt, lron:Aluminium:Nickel:Cobalt and the like. Flake Nickel materials can be used; in addition Iron flake materials are suitable. Typical nickel flakes have lateral dimensions in the range 5-50 microns and a thickness less than 2 microns. Typical iron flakes have lateral dimensions in the range 10-30 microns and a thickness less than 2 microns.

Claims

1. A non-interferometric method of manufacturing a diffractive optical device, comprising

a) providing a photo-modifiable layer;

2. A method according to claim 1 , further comprising repeating steps (b) and (c), optionally a plurality of times, whereby second and optionally subsequent regions of the photo-modifiable layer are modified in accordance with respective second and optionally subsequent fringe patterns displayed sequentially by the SLM device and illuminated by second and optionally subsequent pulses of light from the light source so as to sequentially project images thereof onto the respective regions of the photo-modifiable layer, the respective regions of the photo-modifiable layer being laterally offset from one another, the second and optionally subsequent regions of the photo-modifiable layer thereby reproducing respective diffractive effects corresponding to the second and optionally subsequent fringe patterns.

3. A method according to claim 2, wherein the first and second fringe patterns, and the corresponding first and second diffractive effects, are different from one another.

4. A method according to claim 2 or claim 3, wherein the first, second and optionally subsequent diffractive effects reproduced by the respective regions of the photo-modifiable layer collectively form an aggregate diffractive image.

5. A method according to any of the preceding claims, wherein the first fringe pattern represents a diffraction grating.

6. A method according to claim 5 when dependent on any of claims 2 to 4, wherein each of the first, second and optionally subsequent fringe patterns represents a respective diffraction grating.

7. A method according to any of claims 1 to 4, wherein the first fringe pattern represents all or a part of an interference pattern formed by light from an object and coherent reference light, whereby the first diffractive effect is a holographic image of all or a part of the object.

8. A method according to claim 7 when dependent on any of claims 2 to 4, wherein each of the first, second and optionally subsequent fringe patterns represents a respective part of an interference pattern formed by light from an object and coherent reference light, whereby the first, second and optionally subsequent diffractive effects are holographic images of respective parts of the object.

9. A method according to claim 8 when dependent on claim 4, wherein the aggregate diffractive image is a holographic image of the object.

10. A method according to any of the preceding claims, wherein the optical system further comprises a module configured to demagnify the fringe pattern such that the image of the fringe pattern projected onto the photo-modifiable layer is smaller than the fringe pattern displayed on the SLM device, preferably by a factor of at least 10, more preferably by a factor of at least 20, still preferably by a factor of 25.

1 1. A method according to any of the preceding claims when dependent on claim 2, wherein after each repetition of steps (b) and (c), the photo-modifiable layer and the optical system are moved relative to one another such that the next fringe pattern will be projected onto a new region of the photo-modifiable layer.

12. A method according to any of the preceding claims when dependent on claim 2, wherein the optical system comprises a positioning module controllable to change the position of the projected image of the fringe pattern relative to the optical system and, after each repetition of steps (b) and (c), the positioning module is controlled so as to move the position of the projected image such that the next fringe pattern will be projected onto a new region of the photo- modifiable layer.

13. A method according to any of the preceding claims, wherein the spatial light modulation (SLM) device is a reflective SLM device, preferably a liquid crystal on silicon (LCOS) device or a digital light projection (DLP) micro-mirror device.

14. A method according to any of the preceding claims, wherein the light source comprises a laser source, preferably a pulsed laser source.

15. A method according to any of the preceding claims, wherein the duration of the or each light pulse is 100 nanoseconds or less, preferably 20 nanoseconds or less, still preferably between 3 and 12 nanoseconds.

16. A method according to any of the preceding claims, wherein the optical system further comprises a shutter module controllable to selectively block illumination of the SLM device and/or projection of the image onto the photo- modifiable layer, the shutter module preferably comprising an acousto-optical modulator disposed between the light source and the SLM device.

17. A method according to claim 16, wherein control of the SLM device and of the shutter module are synchronised such that when the fringe pattern displayed by the SLM changes, the shutter module is blocking illumination of the SLM device and/or projection of the image onto the photo-modifiable layer.

18. A method according to any of the preceding claims, wherein the optical system further comprises an isolator module configured to prevent reflected light returning to the light source.

19. A method according to any of the preceding claims, wherein the photo- modifiable layer comprises one of: a metal layer, an ink layer, a photo-sensitive material, a photo-curable material and a material comprising a photo-absorbent additive.

20. A method according to any of the preceding claims, wherein the modification of the photo-modifiable layer is ablation, change in refractive index, photopolymerization or photodissociation thereof.

21. A method according to any of the preceding claims, wherein the photo- modifiable layer is further provided with a reflection enhancing layer thereon, preferably comprising a high refractive index material.

22. A method according to any of the preceding claims, wherein the diffractive optical device is a security device and the photo-modifiable layer forms preferably part of a security article or security document.

23. A diffractive optical device made in accordance with the method of any of claims 1 to 22, wherein the diffractive optical device is preferably a security device.

24. A security article comprising a diffractive optical device in accordance with claim 23, wherein the security article is preferably a security thread, strip, patch, insert or foil.

25. A security document comprising a diffractive optical device in accordance with claim 23 or a security article in accordance with claim 24, wherein the security document is preferably a banknote, a polymer banknote, a passport, an identification card, a bank card, a licence, a visa, a cheque or a certificate.

26. An apparatus for non-interferometric manufacture of a diffractive optical device, comprising:

a support for a photo-modifiable layer;

a spatial light modulation (SLM) device;

a light source;

a control system configured to control the SLM device to display a first fringe pattern corresponding to a first diffractive effect and the optical system to illuminate the SLM device with a first pulse of light from the light source and project an image of the first fringe pattern onto a first region of a photo- modifiable layer arranged in use on the support, to enable the first region of the photo-modifiable layer to be modified in accordance with the first fringe pattern and thereby reproduce the first diffractive effect.

27. An apparatus according to claim 26, wherein the control system is further configured to control the SLM device to sequentially display a plurality of fringe patterns corresponding to respective diffractive effects and the optical system to illuminate the SLM device with respective pulses of light from the light source to sequentially project images of the fringe patterns onto respective regions of the photo-modifiable layer, the respective regions of the photo-modifiable layer being laterally offset from one another, to enable the regions of the photo- modifiable layer to be modified in accordance with the respective fringe patterns and thereby reproduce the respective diffractive effects.

28. An apparatus according to claim 27, wherein the at least two of the fringe patterns, and the corresponding diffractive effects, are different from one another.

29. An apparatus according to 27 or 28, wherein the diffractive effects reproduced by the respective regions of the photo-modifiable layer collectively form an aggregate diffractive image.

30. An apparatus according to any of claims 26 to 29, wherein the optical system further comprises a module configured to demagnify the fringe pattern such that the image of the fringe pattern projected onto a photo-modifiable layer arranged in use on the support is smaller than the fringe pattern displayed on the SLM device, preferably by a factor of at least 10, more preferably by a factor of at least 20, still preferably by a factor of 25.

31. An apparatus according to any of claims 26 to 30, further comprising a transport system configured to move a photo-modifiable layer arranged on the support in use and the optical system relative to one another to enable the apparatus to project the image onto different regions of the photo-modifiable layer.

32. An apparatus according to any of claims 26 to 31 , wherein the optical system comprises a positioning module controllable to change the position of the projected image relative to the optical system to enable the apparatus to project the image onto different regions of a photo-modifiable layer arranged on the support in use.

33. An apparatus according to any of claims 26 to 32, wherein the spatial light modulation (SLM) device is a reflective SLM device, preferably a liquid crystal on silicon (LCOS) device or a digital light projection (DLP) micro-mirror device.

34. An apparatus according to any of claims 26 to 33, wherein the light source comprises a pulsed laser source.

35. An apparatus according to any of claims 26 to 34, wherein the optical system further comprises a shutter module controllable to selectively block illumination of the SLM device and/or projection of the image towards the support, the shutter module preferably comprising an acousto-optical modulator disposed between the light source and the SLM device.

36. An apparatus according to claim 35, wherein the controller is further configured to synchronise control of the SLM device and of the shutter module such that when the fringe pattern displayed by the SLM changes, the shutter module is blocking illumination of the SLM device and/or projection of the image towards the support.

37. An apparatus according to any of claims 26 to 36, wherein the optical system further comprises an isolator module configured to prevent reflected light returning to the light source.

38. An interferometric method of manufacturing a diffractive optical device, comprising:

c) using an optical system to split a first pulse of light from a light source into an object beam and a reference beam, the object beam and the reference beam being coherent, to reflect the object beam off the SLM device and then towards the first surface of the photo-modifiable layer and to direct the reference beam towards the first or second surface of the photo-modifiable layer, the object beam and the reference beam meeting at a predetermined interference angle and generating a first interference pattern in the vicinity of the photo- modifiable layer, whereby a first region of the photo-modifiable layer is modified in accordance with the first interference pattern and thereby exhibits a diffractive image of the first object.

39. A method according to claim 38, further comprising repeating steps (b) and (c), optionally a plurality of times, whereby second and optionally subsequent regions of the photo-modifiable layer or of other photo-modifiable layer(s) are modified in accordance with respective second and optionally subsequent interference patterns corresponding to respective second and subsequent objects displayed sequentially by the SLM device and generated by the optical system from second and optionally subsequent pulses of light from the light source, the respective regions of the photo-modifiable layer(s) being laterally offset from one another, the second and optionally subsequent regions of the photo-modifiable layer(s) thereby exhibiting respective diffractive images of the second and optionally subsequent objects.

40. A method according to claim 39, wherein the first and second objects, and preferably any optional subsequent objects, are different from one another.

41. A method according to any of claims 38 to 40, wherein each of the first and any second or subsequent objects displayed by the SLM device comprises an indicia, such as any of: a number, a letter, alphanumerical text, a symbol, an image, a logo, a portrait, a photograph, a barcode, a 2D barcode or a biodata pattern.

42. A method according to any of claims 38 to 41 , wherein each of the first and any second or subsequent objects displayed by the SLM device comprises an item of information which uniquely identifies the object and/or is personalisation information, preferably identifying a person.

43. A method according to claim 39 or any of claims 40 to 42 when dependent on claim 39, wherein each of the regions are located on different, discrete photo-modifiable layers, preferably forming part of different, discrete documents.

44. A method according to claim 39 or any of claims 40 to 42 when dependent on claim 39, wherein each of the regions are located on one photo- modifiable layer, optionally in the form of a web, and the regions are spaced from one another, preferably by a distance at least as great as the width of one of the regions.

45. A method according to claim 44, further comprising cutting the photo- modifiable layer into separate parts, each part carrying one of the regions.

46. A method according to claim 39 or any of claims 40 to 45 when dependent on claim 39, wherein after each repetition of steps (b) and (c), the photo-modifiable layer(s) and the optical system are moved relative to one another such that the next interference pattern will modify a new region of the photo-modifiable layer(s).

47. A method according to claim 39 or any of claims 40 to 46 when dependent on claim 39, wherein the optical system comprises a positioning module controllable to change the position of the generated interference pattern relative to the optical system and, after each repetition of steps (b) and (c), the positioning module is controlled so as to move the position of the generated interference pattern such that the next interference pattern will modify a new region of the photo-modifiable layer(s).

48. A method according to any of claims 38 to 47, wherein any optical path difference between the object beam and the reference beam is less than c.At, where c is the speed of light and At is the duration of the or each pulse of light.

49. A method according to any of claims 38 to 48, wherein the optical system comprises a partial mirror configured to split the light from the light source into the object beam and the reference beam.

50. A method according to any of claims 38 to 49, wherein the optical system further comprises an interference angle module controllable to change the interference angle at which the object beam and the reference beam meet.

51. A method according to claim 50, wherein the interference angle module comprises at least one optical element arranged in the reference beam path and/or in the object beam bath which is configured to redirect the respective beam(s) towards the photo-modifiable layer.

52. A method according to claim 51 , wherein the at least one optical element comprises a diffractive element arranged in the reference beam path which is configured to redirect the reference beam towards the photo-modifiable layer; wherein preferably the diffractive element is a diffraction grating having a pitch which varies with position along the grating, and the interference angle module further comprises one or more optical elements controllable to direct the reference beam onto different portions of the diffraction grating to thereby change the angle by which the reference beam is redirected towards the photo- modifiable layer.

53. A method according to any of claims 38 to 52, wherein the spatial light modulation (SLM) device is a reflective SLM device, preferably a liquid crystal on silicon (LCOS) device or a digital light projection (DLP) micro-mirror device.

54. A method according to any of claims 38 to 53, wherein the light source comprises a pulsed laser source.

55. A method according to any of claims 38 to 54, wherein the duration of the or each light pulse is 100 nanoseconds or less, preferably 20 nanoseconds or less, still preferably between 3 and 12 nanoseconds.

56. A method according to any of claims 38 to 55, wherein the optical system further comprises a shutter module controllable to selectively block illumination of the SLM device and/or generation of the interference pattern, the shutter module preferably comprising an acousto-optical modulator disposed between the light source and the SLM device.

57. A method according to claim 56, wherein control of the SLM device and of the shutter module are synchronised such that when the object displayed by the SLM changes, the shutter module is blocking illumination of the SLM device and/or generation of the interference pattern.

58. A method according to any of claims 38 to 57, wherein the optical system further comprises an isolator module configured to prevent reflected light returning to the light source.

59. A method according to any of claims 38 to 58, wherein the photo- modifiable layer comprises one of: a metal layer, an ink layer, a photo-sensitive material, a photo-curable material and a material comprising a photo-absorbent additive.

60. A method according to any of claims 38 to 59, wherein the modification of the photo-modifiable layer is ablation, change in refractive index, photopolymerization or photodissociation thereof.

61. A method according to any of claims 38 to 60, wherein the photo- modifiable layer is further provided with a reflection enhancing layer thereon, preferably comprising a high refractive index material.

62. A method according to any of claims 38 to 61 , wherein the diffractive optical device is a security device and the photo-modifiable layer preferably forms part of a security article or security document.

63. A diffractive optical device made in accordance with the method of any of claims 38 to 62, wherein the diffractive optical device is preferably a security device.

64. A security article comprising a diffractive optical device in accordance with claim 63, wherein the security article is preferably a security thread, strip, patch, insert or foil.

65. A security document comprising a diffractive optical device in accordance with claim 63 or a security article in accordance with claim 64, wherein the security document is preferably a banknote, a polymer banknote, a passport, an identification card, a bank card, a licence, a visa, a cheque or a certificate.

66. An apparatus for interferometric manufacture of a diffractive optical device, comprising:

a light source;

an optical system adapted to split light from the light source into an object beam and a reference beam, the object beam and the reference beam being coherent, to reflect the object beam off the SLM device and then towards the first surface of a photo-modifiable layer arranged in use on the support and to direct the reference beam towards the first or second surface of the photo-modifiable layer, the object beam and the reference beam meeting at a predetermined interference angle to thereby generate an interference pattern in the vicinity of the photo-modifiable layer; and

67. An apparatus according to claim 66, wherein the control system is further configured to control the SLM device to sequentially display a plurality of objects and the optical system to generate a respective interference patterns from a respective pulses of light from the light source, to enable respective laterally offset regions of the photo-modifiable layer or of other photo-modifiable layers to be modified in accordance with the respective interference patterns to thereby exhibit respective diffractive images of the sequentially displayed objects.

68. An apparatus according to claim 67, wherein the plurality of objects are different from one another.

69. An apparatus according to any of claims 66 to 68, wherein the or each object displayed by the SLM device comprises an indicia, such as any of: a number, a letter, alphanumerical text, a symbol, an image, a logo, a portrait, a photograph, a barcode, a 2D barcode or a biodata pattern.

70. An apparatus according to any of claims 66 to 69, wherein the or each object displayed by the SLM device comprises an item of information which uniquely identifies the object and/or is personalisation information, preferably identifying a person.

71. An apparatus according to any of claims 66 to 70, further comprising a transport system configured to move a photo-modifiable layer arranged on the support in use and the optical system relative to one another to enable the apparatus to modify different regions of the photo-modifiable layer.

72. An apparatus according to any of claims 66 to 71 , wherein the optical system comprises a positioning module controllable to controllable to change the position of the generated interference pattern relative to the optical system to enable the apparatus to modify different regions of a photo-modifiable layer arranged on the support in use.

73. An apparatus according to any of claims 66 to 72, wherein the optical system is configured such that any optical path difference between the object beam and the reference beam is less than c.At, where c is the speed of light and At is the duration of the or each pulse of light.

74. An apparatus according to any of claims 66 to 74, wherein the optical system comprises a partial mirror configured to split the light from the light source into the object beam and the reference beam.

75. An apparatus according to any of claims 66 to 74, wherein the optical system further comprises an interference angle module controllable to change the interference angle at which the object beam and the reference beam meet.

76. An apparatus according to claim 75, wherein the interference angle module comprises a diffractive element arranged in the reference beam path which is configured to redirect the reference beam towards the photo-modifiable layer.

77. An apparatus according to claim 76, wherein the diffractive element is a diffraction grating having a pitch which varies with position along the grating, and the interference angle module further comprises one or more optical elements controllable to direct the reference beam onto different portions of the diffraction grating to thereby change the angle by which the reference beam is redirected towards the photo-modifiable layer.

78. An apparatus according to any of claims 66 to 77, wherein the spatial light modulation (SLM) device is a reflective SLM device, preferably a liquid crystal on silicon (LCOS) device or a digital light projection (DLP) micro-mirror device.

79. An apparatus according to any of claims 66 to 78, wherein the light source comprises a pulsed laser source.

80. An apparatus according to any of claims 66 to 79, wherein the optical system further comprises a shutter module controllable to selectively block illumination of the SLM device and/or generation of the interference pattern, the shutter module preferably comprising an acousto-optical modulator disposed between the light source and the SLM device.

81. An apparatus according to claim 80, wherein the controller is further configured to synchronise control of the SLM device and of the shutter module such that when the object displayed by the SLM changes, the shutter module is blocking illumination of the SLM device and/or generation of the interference pattern.

82. An apparatus according to any of claims 66 to 81 , wherein the optical system further comprises an isolator module configured to prevent reflected light returning to the light source.