It relates particularly to a diffractive optically variable device (OVD) which, under appropriate illumination and observation conditions, generates one or more images which appear to be three-dimensional.

OVDs are useful in a number of different applications, particularly as anti- forgery security devices on banknotes, credit cards, cheques, share certificates and other similar documents. In general, good OVDs are easily viewed and verified by the colour and pattern variation of images which they generate.

Early OVDs used for anti-counterfeiting included the hologram OVD's used on VISA and MasterCard credit cards first issued in 1984. These hologram devices are not ideally suited for application to the surface of flexible documents such as banknotes, as their image characteristics deteriorate when they are not kept flat. Their image characteristics are best observed under illumination from a point light source such as a single incandescent globe, whereas they become blurred and indistinct under extended light sources such as fluorescent light tubes, particularly in office and supermarket environments where multiple-source fluorescent lighting is typically used.

To address problems such as these, and to increase security against copying, new technologies have been developed. These include dot matrix hologram technology (EP 0 467 601 A2), KINEGRAM technology (EP 105099, EP 330 738, EP 375 833) first used on the Saudia Arabia passport in 1987 and later on the Austrian 5000 schilling banknote in 1990, CATPIX grating technology (PCT AU89/00542) used on the Australian plastic ten dollar banknote issued in 1988 and the Singapore plastic 50 dollar banknote in 1990, PIXELGRAM diffraction technology (PCT AU90/00395, U. S. Patent 5,428,479) and EXELGRAM diffraction technology (PCT AU94/00441) which appeared first on the Australian opal stamps and Vietnam bank cheques issued in 1995 and which appeared on Amex travellers cheques and on Hungarian banknotes in 1997.

The contents of International Patent Applications PCT AU90/00395 and PCT AU94/00441, both in the name of the present applicant, are hereby incorporated herein by reference.

The OVD technologies listed above are able to generate a range of optical effects including moving guilloche and graphic effects. PIXELGRAM'and EXELGRAMTM technologies also have the ability to display high resolution portraiture effects that change from positive tone to negative tone images as the angle of view is changed. Printed high resolution portraiture has long been used on banknotes as a security feature because of the particular ability of the human eye to notice errors or defects in an image of the human face. This was one of the reasons that the PIXELGRAM and EXELGRAM technologies were developed to include portraiture OVD effects.

The technologies listed above, except the hologram devices of the type appearing on VISA cards, are based on a two-dimensional imaging process, and images generated by the effect of illumination on these devices are two dimensional. The hologram OVDs on VISATM cards generate images which appear to be three dimensional, but as stated above they blur under normal light conditions.

An object of the present invention is to provide an OVD which generates a three-dimensional image which can be clear and easily viewed on illumination by a normal fluorescent tube, a lamp or daylight.

Summary of the Invention According to the present invention there is provided a diffractive device having a surface relief structure which, when illuminated by a light source, generates two or more diffraction images which, when observed by an observer from an appropriate range of viewing angles, appear as one or more three- dimensional images.

A person with normal eyesight perceives an object or scene as three- dimensional by capturing two simultaneous but slightly different images of the object or scene through the person's left eye and right eye, and processing the two images with the person's brain to derive a feeling of depth and perspective based on the differences in the two images. The invention preferably works by

generating a left-eye image and a right-eye image, which possess slight differences simulating the differences between the left-eye and right-eye images which are observed in viewing an actual three-dimensional object or scene.

In addition to separate left-eye and right-eye images, the diffractive device may generate a centre image which is visible to both eyes.

Separate left-eye and right-eye images may be generated as a result of any suitable characteristics of the surface relief structure. In one arrangement, separate images may be generated as a result of differing orientation of surface regions of the diffractive structure, with a left-eye diffraction image being generated by light diffracted from a plurality of surface regions each of which has a diffractive structure with a first orientation, and a right-eye diffraction image being generated by light diffracted from a plurality of surface regions each of which has a second orientation which differs in angle from the first orientation, so that the left-eye image and the right-eye image are observable from locations which are separated by a distance equivalent to the distance between a person's eyes. In another arrangement, a left-eye image of a particular colour (such as green) may be generated by light diffracted from a plurality of surface regions each of which has a diffractive structure with a first spatial frequency, and a right- eye image of a different colour (such as red) may be generated by light diffracted from surface regions with a second spatial frequency, and the observer may use optical filters of matching frequencies (such as glasses with a green left lens and a red right lens) to ensure that the observer's left eye sees the left-eye image and the right eye sees the right-eye image.

As an optional feature, the diffractive device may generate one or more full colour three-dimensional images. This can be done by allocating surface regions to generate three or more left-eye primary colour images (such as a red image, a green image and a blue image) and three or more right-eye primary colour images, which together create a full colour three dimensional image. Because the colour sensitivity of human eye is relatively imprecise, in some arrangements it is possible to convey the impression of a full colour three-dimensional image without the necessity of three colour images for each eye.

The invention will hereafter be described in greater detail with reference to the attached drawings which show example forms of the invention. It is to be understood that the particularity of those drawings does not supersede the generality of the preceding description of the invention.

Detailed Description Figure 1 is a schematic illustration of an object and the location of an observer's eyes.

Figure 2 is a representation of some regions of a diffractive surface structure according to the invention, divided into left-eye and right-eye"channels".

Figure 3 is a schematic illustration of the mechanics of illuminating and viewing a diffractive device according to the invention.

Figure 4 is a representation of a region of a diffractive structure according to the invention showing curved diffractive structures with left-eye and right-eye "channels".

Figure 5 is a representation of a similar region with structures which are short and straight rather than curved.

Figure 6 is a representation of some regions of a diffractive surface structure according to the invention, divided into left-eye, right-eye and centre "channels".

Figure 7 is a representation of some regions of a full-colour diffractive surface structure according to the invention, with red, green and blue left-eye and red, green and blue right-eye"channels".

Figures 8 and 9 show two different images which were encoded into a diffractive device according to the invention.

Figure 10 shows a combination of the two images of Figures 8 and 9, which appears to be three-dimensional when the diffractive device is viewed.

As best shown in Figure 3, a diffractive device 1 according to the invention has a surface relief structure 2 which, when illuminated by a light source 3, generates two or more diffraction images which, when observed by an observer 4 from an appropriate range of viewing angles, appear as one or more three- dimensional images. The invention preferably works by generating a left-eye image, perceived by the observer at left-eye observation point 5, and a right-eye

image, perceived by the observer at right-eye observation point 6. The two images possess slight differences simulating the differences between the left-eye and right-eye images which are observed in viewing an actual three-dimensional object or scene.

In practice optically variable devices (OVDs) (of which diffractive devices according to the invention form a subset) are typically formed by stamping a surface relief structure on a flat foi; so that the base of an OVD is two dimensional. It is thus relatively straightforward to generate two-dimensional diffractive images. Most OVDs fall into this two-dimensional category. The present invention is based on the application to diffractive devices of techniques for synthesising three-dimensional images from a two-dimensional substrate. For example, three-dimensional movies are displayed on a two-dimensional screen, and three-dimensional photo pictures are a pair of flat images viewed in a three- dimensional viewer. These photo pictures create a convincing illusion of a three- dimensional scene.

The difference between two-dimensional and three-dimensional images is that the two-dimensional image is constructed from the same image for both human eyes; but in the three-dimensional real world a normal person's two eyes give two slightly different images. This is because they are in two different positions in space, separated horizontally by an amount of about 6.5 cm for an average adult human. The brain accepts the small horizontal separation between those two images, and in return gives a single image with accurate depth perception. This ability is known as stereoscopy. Because of stereoscopy, humans experience a real sense of three-dimensional depth or location of objects.

This is illustrated in Figure 1, which shows a left eye 5 and a right eye 6, together with objects 7,8 and 9. The images of those objects perceived by left eye 5 clearly differs from that perceived by right eye 6. For example, in the image perceived by left eye 5, object 9 obscures object 7 apart from a small part of object 7 to the left of object 9; whereas in the image perceived by right eye 6, object 9 obscures object 7 apart from a small part of object 7 to the right of object 9. The observer's brain processes this information so that the observer becomes aware that object 9 is closer than object 7 and slightly smaller.

Stereo pair of images A preferred process of producing a three-dimensional OVD according to the invention is quite similar to a standard process for creating a three-dimensional photo picture. The first step is to create two normal images for left and right eyes respectively, i. e., a stereo pair. This may be achieved simply by taking two photos, or observing images, in the human left eye and right eye positions respectively. As indicated above, for a normal adult human the separation is about 6.5 cm. This can also be done by means of a commercial three-dimensional camera.

Alternatively, a suitable pair of images can be calculated and generated by computer software. It is also possible to mix images taken by a camera with images created by computer. In this process the base (distances of about 6.5 cm) may be increased or decreased relative to the scale of the scene in order to have an appropriate significant stereo effect.

After the images are created, the next step is to design regional surface relief structures, each of which has the function of generating (under appropriate illumination conditions) one or more components of the stereo pair of images. Two channel"palettes", each consisting of a discrete number of different regional surface relief structures, can be employed for the two images. Some care is required in choosing the channel palettes, and consideration has to be given to how a stereo pair of images is to be reconstructed by the individual regional surface relief structures. If appropriate regional surface relief structures are not selected, there is a significant risk of cross-talk between the two channels which can reduce or even cancel the stereo three-dimensional effect.

An example of diffractive regional surface relief structures is shown in Figure 2. In the case illustrated, the structures consist of grooves or gratings. It will be appreciated that diffractive structures may consist of dots or any other structure with appropriate periodicity, and suitable structures are not limited to line gratings.

Information for the left-eye and right-eye images is coded into two channel grating structures. In Figure 2, the regional surface relief structures which contribute to the left-eye image are marked L, and the structures which contribute to the right-eye image are marked R.

In the particular configuration illustrated in Figure 2, the image information is concentrated in the broader grating regions 10, whereas the inter-grating regions 11 can be used to enhance the contrast of the images generated.

An arrangement for observing the three-dimensional diffractive device 1 is shown schematically in Figure 3. The device is illuminated by light source 3.

Clearest results are obtained if the light source approximates a point source, but good results are still available under diffuse illumination such as fluorescent tube lighting because images generated by diffractive devices of the present type are typically significantly sharper than the images generated by commonly known credit card holograms.

A first order diffraction image for the left eye is generated at location 5, and a first order diffraction image for the right eye is generated at location 6. The brain of the observer processes these two images and perceives the overall image as a three-dimensional one. If the left-eye and right-eye positions of the observer are reversed, the observer may perceive an"inverted depth"image, in which closer objects appear to be further away, and further objects appear to be closer.

The best stereo three-dimensional effect is generated if the information for the left eye and right eye is deflected into left eye and right eye respectively without cross-talk between the two. Preferably, each eye sees only its own image which is created by the first order diffraction of incident illumination, observed at the observation distance (typically 25 cm to 50 cm). A three-dimensional image produced according to the method described above is able to be viewed ("free- view"), in most cases, without the requirement of a special viewer. The embodiment described above is therefore unlike three-dimensional movies which have to be viewed with polarisation glasses. This has been achieved by having a particular orientation of regional surface relief structures for all left-eye image information, and a slightly different orientation for all right-eye image information.

As a result, light which is incident on left-eye surface relief structure regions is diffracted into the observer's left eye and light which is incident on right-eye surface relief structure regions is diffracted into the observer's right eye without cross-talk. The example regional surface relief structures shown in Figure 2 are governed by the equation:

R = 2 D sin 2 0 sin a (1.1) where R is the"base" (eye separation of about 6.5 cm), 6 is the angle of the grating's orientation and a is diffraction angle. 6 can thus be obtained: More general and complex structures can be fabricated. Figure 4 is a representation of a surface relief structure region which has curved structures, the left half of which are designated"left", and the right half being designated"right".

The curvature of the structures causes the diffractive effect generated by the structures to be observable from a broader range of viewing angles than is the case for straight-line structures. The"left"structures contribute to the"left- channel"image which is viewed by the observer's left eye, and the"right" structures contribute to the"right-channel"image which is viewed by the observer's right eye.

Figure 5 is a representation of a region similar to that of Figure 4, but with each"left"and"right"structure being divided into three substantially straight structures. The overall effect is similar to that provided by the curved structures of Figure 4.

Stereo Triple Image Diffractive Device The foregoing embodiment relates to a diffractive device which generates a three-dimensional image from a stereo pair of images. A three-dimensional image also can be coded into a stereo triple of images. The stereo triple of images can be obtained by means of a 3-lens stereo camera, three separate photographs, computer processing of one or more two-dimensional or three-dimensional computer images, or any one of various other known techniques. A three- dimensional image is then reconstructed from these three images which include a stereo pair of images as described with reference to the preceding embodiment

(diffracted with a surface relief structure relative orientation angle of +S, and each observed by one eye only) and the third image which is viewed by both eyes (diffracted with a surface relief structure relative orientation angle of zero). An example of suitable surface relief structure regions for this"three-channel" embodiment is shown in Figure 6. A three-dimensional image created by a stereo triple of images can be finer in three-dimensional depth and image resolution than an image created by a stereo pair.

In more general terms, a three-dimensional image can be reconstructed <BR> <BR> <BR> from a stereo 2N or 3N (oo2N21) channel diffractive device. Each the the left,<BR> <BR> <BR> <BR> <BR> right and middle channel may be divided into N (oo 2 N 21) sub-channels. Thus a total of 2N or 3N channels of images may be coded into the diffractive devices and form a three-dimensional image for an observer. N may be the number of three- dimensional depth levels or the orientation of objects. In preferred embodiments N images are coded into the diffractive device in construction of a three-dimensional image.

A diffractive device which generates a full colour three-dimensional image as discussed below is a special case of N =3.

Anaglyphic Three-Dimensional Image Instead of coding left-eye and right-eye channels by use of angular surface relief structure orientation differences as discussed above, the left-eye and right- eye channels can be coded into two colours. Monochrome (grey) three- dimensional images can be observed with a pair of glasses made from two different colour filters, for example, red and blue. Thus, for example, the periods of the surface relief structure regions for the left and right channels could be selected as: d1=1250 nm, and d3=900 nm respectively.

In this embodiment, the precise arrangement of angles over which the right- eye and left-eye images are observable is not important, because the colour filters ensure that only the appropriate coloured information reaches each of the observer's eyes.

Full colour three-dimensional OVD Optically Variable Devices which incorporate full colour effects are described in more detail in International Patent Application PCT/AU97/00800 entitled"Colour Image Diffractive Device", which is also in the name of the present applicant. The contents of that patent specification are hereby incorporated herein by reference.

A full range of colours can be reconstructed with three primary colours.

Red, Green and Blue can be chosen as primary colours, although it is known that other combinations are equally suitable. Any full colour image can be decomposed into three colour images in the three primary colours. Each pixel of a full colour image has a particular hue and brightness which can be expressed as the sum of a brightness level for each of the three primary colours. Although brightness level is a continuous quantity, a reasonable approximation may be made by adopting a discrete number of different possible levels. If, for example, 16 levels of intensity are selected for each colour, there are total of 16x16x16=4096 different colours which can be produced, and each pixel colour in a full-colour image may be mapped to the nearest available combination.

A diffractive device according to this embodiment of the invention creates a full colour image in a similar way to the way in which a monitor displays a portrait or image on a screen. The image is composed of an array of pixels. Each of the pixels contains three sub-pixels which provide intensity levels for the three primary colours respectively as the effect of the first order diffraction of incident illumination by surface relief regions which are designed for that purpose. This mechanism therefore allows for the production of a full range of true colours with various hue and brightness values.

When a diffractive optical element with diffractive structure period d is illuminated by a collimated white light beam in normal incidence, the light with different wavelengths (or colours) k is diffracted through different angles, a, which are governed by the equation: <BR> <BR> <BR> sin a =- (2.1)<BR> <BR> <BR> <BR> <BR> <BR> d

Here only the first order of diffraction is considered because most of the light energy is diffracted into the first order. If the diffractive surface relief structure uses only a single spatial frequency, the OVD produces mono-colour images.

As illustrated in Figure 3, in a typical viewing of an image on an OVD, a light source illuminates the OVD with a particular incidence angle and the observer views the image from a particular observation angle. The viewing angle a is thereby determined. It is possible to determine from equation (2.1) the appropriate periodicity or spatial frequency of a surface relief structure region in order to generate diffracted light of a given wavelength at the viewing angle. We are interested in three primary colours Red, Green and Blue in three wavelengths <BR> <BR> <BR> <BR> jazz 2 and 23 respectively. They are diffracted by surface relief structure regions<BR> <BR> <BR> <BR> <BR> <BR> with three different periods d,, d2and d3 to give appropriate intensity levels for three colour sub-pixels respectively, which are perceived by the eye as creating a single pixel of a given hue and intensity. <BR> <BR> <BR> <BR> <P> 2 23<BR> <BR> <BR> (2. 2)<BR> dl d2 d3<BR> <BR> <BR> <BR> <BR> <BR> <BR> For example, for the three primary colours with wavelengths AI=600 nm, 22=500 nm and 23=450 nm, and the diffraction angle a=30 degrees, the three surface relief structure periods are d1=1200 nm, d2=1000 nm and d3=900 nm in each sub-pixel of each pixel.

There are many ways to control brightness of the diffractive light from a diffractive relief structure region (a sub-pixel) on a diffractive device, such as by varying grating depth, grating profile, area of gratings and grating curvatures.

A full colour three-dimensional OVD according to the embodiment described above is a special case of the three-dimensional OVD of stereo 2N or 3N images for N=3 in which the three sub-channels are coded for the primary colour images, Red, Green and Blue. An example of the diffractive surface relief structure of six regions which together make up a pixel of a full colour three- dimensional image is shown in Figure 7. Intensity characteristics associated with each surface relief structure region may be adjusted as required by varying grating depth, grating profile, area of gratings or grating curvatures.

Typical steps in a process of manufacturing diffractive devices which generate a full colour three-dimensional image may include: Art work with images 1. Take a set of colour stereo 2N or 3N images ; 2. Split each image into three separate primary colour images such as red, green and blue (this step may be done on a computer); 3. Convert each of the primary colour images into a grey scale image (this step may be done on a computer); 4. Code the converted images into a digital file containing discrete pixel intensity information; Palette development 5. Develop six palettes, one for each primary colour in left-eye and right-eye channels, each palette containing a discrete number of different regional surface relief structures, wherein each structure in a given palette is designed to generate the same colour at a different intensity in the image; 6. Apply and test the surface relief structures selected from the palettes with test three-dimensional images; Manufacturing diffractive devices 7. Write a master plate using a computer-controlled electron beam or other suitable output device, substituting a regional surface relief structure selected from one of the six palettes for each item of pixel intensity information in the coded file produced in step 4 above; 8. Reproduce from the master plate a plurality of copies of the diffractive device.

Transparent Multi-layer OVD Figures 8 to 10 illustrate a transparent multi-layer OVD. Figure 8 shows a first example of artwork, consisting of the words"CSIRO 3D". Figure 9 shows a map of Australia, with different areas appearing in different colours.

Using a stereo pair of images, it is possible to make one of the images appear to stand in front of the other image. A diffractive device was made using the images of Figures 8 and 9. Figure 10 shows a two-dimensional composite image; however, because it is not possible to reproduce the stereoscopic effect in a single printed representation, Figure 10 does not show the effect which is

observed by viewing the diffractive device. The observed effect is that the words "CSIRO 3D"are semi-transparent and appear to be floating in space about 2cm in front of the map of Australia. The effect is observed only from a limited range of viewing angles; from another range of angles, only the words"CSIRO 3D"are observable, and from another range of angles only the map of Australia is observable.

Similar techniques can be used to create multi-layer images, in which several images appear to be located at different"depths"in the image.

Security Issues A three-dimensional diffractive device according to the invention is capable of providing an increased level of security. There are at least two channels of information for a three-dimensional image. The degree of security of an optically variable device (OVD) is proportional to the ratio of spatial variation to the dimension. When a surface structure varies with a variation AS (period or orientation of formations in the surface relief structure) in a distance l; the degree of security is then defined by OS se The higher the D. value of an OVD, the more difficult it is to copy the OVD by holographic techniques. A three-dimensional image can have 2N or 3N (N is an integer) channels within 30 micron. It is clear that the three-dimensional diffractive device has a greater degree of surface variation, which results in an increased degree of security. Furthermore, an additional security feature is present because the channels are not independent, and the information in them is combined to form one image.