WO1998023979A1

WO1998023979A1 - Colour image diffractive device

Info

Publication number: WO1998023979A1
Application number: PCT/AU1997/000800
Authority: WO
Inventors: Robert Arthur Lee; Xiaoping Yang
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 1996-11-26
Filing date: 1997-11-26
Publication date: 1998-06-04
Also published as: AUPO384796A0

Abstract

A colour image diffractive device has a surface relief structure which, when illuminated by a light source, generates a diffraction image observable at a range of viewing angles around the device. The observed diffraction image is composed of pixels of numerous different colours, with the hue and intensity of most pixels being similar to those of immediately adjacent pixels, so that the image has a substantially continuous smooth colour tone. Each combination of hue and intensity is generated by the combined diffractive effect of the physical characteristics of one or more regions of the surface relief structure. Brightness is modulated by phase cancellation method (shifting adjacent gratings) or by changing the effective area of gratings within a sub-pixel.

Description

Colour Image Diffractive Device

This invention relates to a colour image diffractive device. It relates particularly but not exclusively to a diffractive device which, when illuminated, generates an image which has a substantially continuous, smooth, substantially full, colour tone.

The invention represents improvements and variations on the inventions described in U.S. Patent 5,428,479 and International Patent Applications PCT AU/94/00441 and PCT/AU94/00279, the contents of which are hereby incorporated herein by reference. Background

Diffractive Optically Variable Devices (OVDs) have been used as anti- counterfeiting devices to protect valuable documents for some time now. These devices have the advantage that because the observed image varies with angle of view, the image characteristics cannot be copied by photographic means and hence documents carrying these devices are well protected from counterfeiting by normal printing and photocopying techniques.

The first examples of diffractive optically variable devices used as anti- counterfeiting devices were the hologram OVDs used on VISA™ and MasterCard™ credit cards in 1984. These hologram devices are not ideally suited to application to flexible surfaces such as banknotes. Their image characteristics also become blurred and indistinct under extended light sources. To overcome these problems, new technologies were developed. These included 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™ technology (U.S. Patent 5,428,479), and EXELGRAM™ technology (PCT AU/94/00441) first used on the Australian opal stamps and Vietnam bank cheques issued in 1995.

The OVD technologies listed above are able to generate a range of optical effects including moving guilloche and graphic effects. PIXELGRAM™ and EXELGRAM™ 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 ability of the human eye readily to perceive errors or defects in an image of the human face. It was for this reason that the PIXELGRAM™ and EXELGRAM™ technologies were developed to include portraiture OVD effects. However these portraiture effects were limited to near monochromatic images consisting of a fixed number of brightness levels (usually 16) or "greyness" values. Optically variable portraiture would be much more valuable as a security feature if it could be extended to include full colour imaging with variable brightness and colour. International patent application PCT/AU94/00279 describes a pixellated diffractive device known by the trademark VECTORGRAM in which each pixel is divided into multiple sub-pixels to generate multi-component image elements. Claims 5 and 18 of that patent application indicate that the multiple sub-pixel structure of the device provides a possible mechanism for producing full colour portraiture type imaging. However there is no detailed specification in PCT/AU94/00279 as to how the relative spatial frequencies of the diffractive elements or the amplitudes of the image elements may be modulated to generate the correct optical fields for full colour imaging within a systematic and predictive framework.

European patent applications EP 0 240 261 A2 and EP 0 240 262 A2 describe methods for producing diffractive devices generating full colour imaging effects. The first of these European applications describes a method for making full-colour diffraction images by creating colour separation masks for a colour image and then using the masks sequentially to create interference patterns on a photosensitive media, resulting in diffraction gratings which reproduce the original image when illuminated and observed from particular angles. The second European application describes a method of making full colour diffraction images by dividing a "white grating canvas" into many small areas and dividing each small area into three component diffractive areas of three different spatial frequencies. The intensity values of light diffracted from each component area is then established by erasing parts of each component area in an inverse manner to the intensity value required from each component area. For both methods it is necessary to have very precise alignment of masks, and the type of diffractive structure produced is limited to structures which can be produced using interference patterns. Moreover, the two applications do not reveal any method for incorporating two or more diffraction images into the same surface area of a diffractive device.

European patent EP 375833 and European patent application EP 0 423 680 A2 describe methods for producing diffractive images where the intensity of the image at a particular image point can be controlled by varying the area of the pixel diffraction grating at the respective image point. 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 a diffraction image observable at a range of viewing angles around the device, wherein the observed diffraction image is composed of pixels of numerous different colours, with the hue and intensity of most pixels being similar to those of immediately adjacent pixels so that the image has a substantially continuous smooth colour tone, wherein each combination of hue and intensity is generated by the combined diffractive effect of the physical characteristics of one or more regions of the surface relief structure.

In a preferred embodiment of the invention, each pixel in the observed diffraction image has hue and intensity characteristics selected from a predetermined palette of available hues and intensities, and each combination of hue and intensity in the palette is generated by the combined diffractive effect of the physical characteristics of one or more regions of the surface relief structure, and the physical characteristics of the regions of the surface relief structure are selected from a predetermined palette of available physical characteristics which have known diffractive effects. The physical characteristics of the regions may be any suitable characteristics. In a preferred arrangement the regions include diffracting arrays of polygonal formations (being either protrusions or indentations) having either sharp or rounded edges and corners. It is preferred that the maximum dimension of such formations be less than 60 microns, and further preferred that the maximum dimension be less than 30 micron.

The intensity of individual pixels and the intensity of each colour component of individual pixels may be provided in any suitable manner. It is preferred that the intensity characteristics of pixels or component colours of pixels be varied by varying one or more of the widths, lengths, depths, angular characteristics, and spatial frequency of diffracting formations on regions of the surface relief structure.

Alternatively, the intensity characteristics of pixels or of component colours of pixels may be varied by means of phase cancellation in diffracted light attributable to physical characteristics of diffracting formations on regions of the surface relief structure which cause simultaneous diffraction of light with differing phase characteristics.

The image generated by the diffractive device may have any suitable characteristics. It is preferred that the characteristics of the image be recognisable diffraction characteristics. One preferred characteristic is the ability of the image to change from a positive tone image to a negative tone image as the angle of view is changed. Another preferred characteristic is the ability to make the image appear to move as the angle of view is changed. Another preferred characteristic is the ability to make the image change smoothly from one image to another (morphing) as the angle of view is changed.

As a preferred feature, the diffractive device of the present invention may, when illuminated by a light source, generate two or more diffraction images which are observable at different ranges of viewing angles around the device.

In an especially preferred embodiment, the surface relief structure is comprised of groups of regions, and most groups include three regions for each diffraction image generated by the device, with each of the three regions generating a different primary colour of a particular intensity, and the three regions together generating a colour pixel in the image to which those three regions contribute. According to another aspect of the invention there is provided a method of forming a diffractive device which under appropriate illumination conditions generates N diffraction images, including the steps of:

(a) decomposing each of N full colour images into three component primary colour images; (b) dividing each of the 3N primary colour images into a multiplicity of

M pixels;

(c) determining an intensity value for each of the pixels;

(d) determining for each pixel a regional surface relief structure for the diffractive device which under appropriate illumination conditions generates diffracted light of the applicable primary colour and intensity value.

(e) grouping each pixel with a pixel of corresponding location from each of the other 3N-1 primary colour images;

(f) for each such grouping forming a composite surface relief structure consisting of the regional surface relief structures placed adjacent to each other; and

(g) applying the composite surface relief structures to the surface of the diffractive device.

The invention will hereinafter be described in greater detail by reference to the attached drawings which show an example form 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.

Figure 1 is a schematic diagram of a pixel of an optically variable diffractive device with three primary colour sub-pixels.

Figure 2 is a schematic diagram of the manner in which a diffractive surface relief structure diffracts incident light.

Figure 3 is a schematic diagram of the mechanism of a full colour diffractive device. Figure 4 is a schematic diagram of a region of surface relief structure which generates a primary colour sub-pixel at maximum brightness.

Figure 5a is a schematic diagram of the region of surface relief structure shown in Figure 4, but incorporating structural modifications resulting in a phase shift, which causes phase cancellation and therefore reduction in brightness in the observed sub-pixel. Figure 5b is an enlarged view of a portion of Figure 5a.

Figure 6 is a schematic diagram of the region of surface relief structure shown in Figure 4, but with a 90° phase shift resulting in maximum phase cancellation or minimum brightness in the observed sub-pixel.

Figure 7 is a graph showing brightness as a function of the palette element index of colour in one sub-pixel, with a=0.3.

Figure 8 shows the same function as Figure 7, but with a=0.5.

Figure 9 shows the same function as Figure 7, but with a=1. Figure 10 shows the same function as Figure 7, but with a=2.

Figure 11 shows a region of the surface relief structure which generates a primary colour sub-pixel.

Figure 12 shows a PIXELGRAM™ type region of diffractive pixels of a monochrome image device. The four parts of Figure 12, which increase in magnification from top to bottom, demonstrate the effect of varying size of gratings on control of brightness.

Figure 13 shows a graph of brightness as a function of the number of palette element colour index for a=1.

Figure 14 shows the same graph as Figure 13, but with a=0.5. Figure 15 shows the same graph as Figure 13, but with a=2.

Figures 16a and 16b show two regions of the surface relief structure, with geometrical shapes forming the diffractive pattern being oriented for highest brightness (shown in Figure 16a) and lowest brightness (shown in Figure 16b) respectively. Figure 17 is a plot of diffraction brightness as a function of surface relief structure depth. Figure 18 is a schematic diagram of a group of regions on the surface relief structure where the surface relief structure generates three separate images observable from three different ranges of viewing angles around the device, with three of the regions contributing to the first image, three regions contributing to the second image, and three regions contributing to the third image.

Figure 19 is a similar schematic diagram for a diffractive device which generates four different diffractive images.

Figure 20 is an alternative EXELGRAM™ type arrangement for a diffractive device which generates four different diffractive images.

Figure 21 is a schematic diagram showing full colour image multiplexing for a diffractive optically variable device.

Figure 22a, Figure 22b, Figure 22c and Figure 22d are images showing a full colour picture and respective red, green and blue colour separations for that full colour picture.

Figures 23a and 23b are two images taken under a microscope at different magnifications of a full colour diffractive device according to the present invention with an EXELGRAM™ type structure.

Figure 24 is another image of the type shown in Figures 23a and 23b, but with the separate red, green and blue channels labelled. Detailed Description of the Preferred Embodiments

It is well known that the full range of colours can be reconstructed with three primary colours; for example Red, Green and Blue are usually chosen as primary colours. Any colour image can be decomposed into three colour images in the three primary colours. Each of the colours may have many intensity levels of brightness. If we choose 16 levels of intensity in each of the three primary colour images, there are 4,096 different colours which may be produced. An OVD device creates a portrait in the same way as a monitor displays a portrait or image on a screen which decomposes the image into many pixels. Each of the pixels contains three sub-pixels which provide the three primary colours respectively with different brightness values. This mechanism therefore allows for the production of a wide range of true colours with various hue and brightness values. If these optical elements are produced with colours of various brightness, full colour images can be created easily.

When a diffractive optical element with grating period (or spatial frequency) d is illuminated by a collimated white light beam in normal incidence, the light with different wavelength, λ (or colour) is diffracted into different angles, α which are governed by the equation:

Here only the first order of diffraction is considered because most of the light energy is diffracted into the first order. If only single spatial frequency gratings are fabricated within a pixel of an OVD, the OVD produces mono- colour images.

When a person observes an image on an OVD, there is usually a fixed incidence angle (the direction of the light source) and a fixed angle of observation (if the observer does not move around too much). Therefore the viewing angle α is fixed (See Figure 2). From equation (1.1), the outgoing light in various wavelengths (colours) can be achieved by varying d, the periods of the gratings. We are interested in three primary colours Red, Green and Blue in three wavelengths λ, , λ₂ and λ₃ respectively. They are diffracted by the gratings with three different periods d, , <i₂ and d₃ in the three colour sub-pixels respectively. When the gratings are illuminated by normal white light from the sun, a fluorescent tube or another light source,

in the same viewing angle α . For example, the three primary colours with wavelengths 11=600 nm, λ2=500 nm and λ3=450 nm, and the diffraction angle α=30 degrees, correspond with the grating periods d7=1200 nm , c/2=1000 nm and 3=900 nm in each sub-pixel of each grating pixel. Figure 3 illustrates how the coloured light is diffracted from these sub-pixels. Three primary colours are created by the gratings in the three spatial frequencies. In order to manipulate the hue to achieve full colour, the next step is to control the brightness value of each primary colour from the sub-pixels.

There are many ways to control brightness of the diffractive light from regions on the surface of an OVD, such as varying grating depth, grating profile and grating curvatures. Several types of elements are discussed below as examples of the types which are the most easily fabricated in practice.

Figures 4 to 6 illustrate a method of controlling the brightness of each primary colour sub-pixel by use of phase cancellation inherent in the structure of regions of surface relief structure. In this method, each grating comprising each (Red, Green, or Blue) sub-pixel is divided into 16 notional columns in a perpendicular direction. Every second notional column in the grating is moved down in order to modulate the brightness value of the sub-pixel while retaining the same colour characteristic. This causes a phase difference in the output light field between even and odd columns of gratings. The phase difference, θ, of the outgoing fields from the two parts of gratings with the same period is,

θ =π - , for δ < - (2.1) d 2 where d , is period of the grating and δ is shift between the gratings (see Figure

5b). The output diffractive field interferes due to the phase difference. The intensity of the diffracted beam is,

/ = I₀ cos² θ = I₀ cos² (— ) (2.2) a where l₀ is the diffracted intensity without the phase shift of gratings. Thus the brightness of the pixel can be varied from maximum with zero phase shift (see

Figure 4 for δ=0) to a minimum with a 90 degree phase shift (See Figure 6 for δ=d/2). Therefore, the colour of the pixel depends on the number of grooves in a pixel (spatial frequency); and the brightness value of the pixel depends on the shift between even and odd columns of gratings.

Except for the difference of brightness value, the characteristics of OVD devices with phase gratings are almost the same as for fully flat gratings ( δ=0 ). An image is created by the device with a narrow viewing angle and sharp colour variance.

Using the phase-cancellation method of brightness control, palettes with various brightness value scales can be developed; for example linear scale and Log scale palettes. Generally, for the number-n palette element of a set of N- level brightness values (n<N, and N is total number of palette elements within the palette), the shift of the number-n palette element is a function of n: δ „ = fin) (2.3) and the outgoing intensity of the number n-palette element is

By choosing a different function f(x), we can get various intensity scale palettes.

(a) The gratings phase shift is a power function of the palette element number.

( /i - l ) δ = -^d α > 0 , and n ≤ N (2.5).

" 2 (N - l )

Figures 7 to 10 are plots of the intensity as a function of n (the palette element index) for varying powers ; a— 0.3, 0.5, 1 , and 2 respectively. N = 16 has been chosen, (b) In general, the gratings phase shift is a function f(x) of palette-number

and 0<= jx; <=1.

The brightness value of the colours can also be varied by changing the effective area of gratings within a sub-pixel (See Figure 11). In the sub-pixel, the diffracted outgoing light brightness is proportional to the diffracting area of the sub-pixel (that is, the area of the sub-pixel which has a grating or diffracting structure on it, as opposed to any flat area of the sub-pixel):

/= /„ S=hw, and S₀ = h₀w_Q (2.7). The advantage of this method is that the brightness scale is exactly linear. However a non-linear scale can be created as well by changing h or w as a function of the number n non-linearly. In one sub-pixel (red, green or blue), the nth-level of brightness is created by varying the area as a function of index number n.

where 1 > f(x) > 0 (2.8)

Figure 11 shows a PIXELGRAM™ type sub-pixel element for h<h₀. There is a gap between the sub-pixel and the pixel next on top or at the bottom. For fixed length of a sub-pixel h=h₀ and varying the width w only, the OVD is an EXELGRAM™ type device. The diffractive elements join at the boundary between pixels. Figure 12 shows pictures of a PIXELGRAM™ type diffractive pixel of a monochrome image device under varying magnification. Examples of these have been fabricated by electron beam lithography. The brightness of each pixel depends on the size of the gratings cells. As examples of a brightness scale, we choose f(x) = x^a , _a > 0 . It is clear that the brightness is a linear function when a = 1 . The brightness as a function of the index number n is plotted in Figure 13, Figure 14 and Figure 15 for a = 1 , 0.5, and 2 respectively.

The two methods described above provide a great advantage in that we can control the brightness scale exactly as we want. OVDs with non-linear scales are also able to be observed by human eyes, but it is very difficult to copy the OVD. This turns out to have some high security features and advantages.

The methods discussed above usually produce only positive portrait images. Negative images can be useful as an additional security feature. This can be done by using a set of palettes in which grating structures are orientated in variable directions for various brightnesses. Two palette elements of an example palette for a sub-pixel are plotted in Figures 16a and 16b. They correspond to the highest (16a) and lowest (16b) brightness value respectively within the palette set. The negative images appear in 0 degrees and positive images are able to be viewed within a small angle. Diffractive efficiency of gratings depends also on the depth of the grating grooves. Figure 17 shows the brightness as a function of the depth to period ratio h/d for a sinusoidal grating with refractive index n₀=1.66. The brightness keeps increasing when the depth of grating increases for 2.5>h/d>0. The scale of brightness is almost linear. Therefore the brightness of the sub-pixel is able to be controlled by varying the depth of gratings within the sub-pixel.

We have described the methods for producing single channel full colour images (where a "single channel" image is the only diffractive image produced by the surface relief structure). Multi-channel OVD's (that is, diffractive devices which generate more than one diffraction image, the different images being observable from different ranges of viewing angles) are also desired for many applications. Spatially divided space may be the easiest way to achieve these multi-channel devices. A notional pixel region on the device can be divided into several parts according to the number of channels. Each channel contains three colour sub-pixels for red, green and blue. As an example, a PIXELGRAM™ type pixel with non-uniform size in each channel on a three- channel OVD is shown in Figure 18. Pixels of uniform size in each channel for four channel PIXELGRAM™ type and EXELGRAM™ type devices are illustrated in Figure 19 and Figure 20 respectively. Many effects can be produced by multi-channel OVD structures. For example:

(a) Kinetic full-colour portraiture where the portrait is perceived to move as the angle of view is changed;

(b) Two or three channel full colour portraiture where each portrait is observed at a different angle.

(c) Morphing portraiture where the image changes its form as the angle of view is changed.

(d) Transparent portraiture where the foreground and background vary in intensity as the angle of view is changed. (e) A vectorized image generation mechanism where each point of each observed image is formed by a singular mapping of diffraction fields from several component areas for each channel within each pixel of the OVD. This vector generation mechanism is the basic mechanism underlying full colour OVD portraiture.

(f) Movement effects with palettes of changing grating orientation. An attractive feature of this method is that movement effects can be produced by a method associated with multi-channel techniques. For example we may want to create a four-stage movement. In this case we use a four channel palette. There is a constant increment of rotated angles of the gratings between adjacent channels. Therefore a four-channel OVD is produced. The four images are observed in sequence 1-2-3-4 (in four different positions), when the device is rotated. Thus the image looks like a movement from position 1 to position 4.

As discussed in earlier, in order to produce a true colour OVD, we create three sub-pixel primary colour palettes, usually corresponding to the colours red, green and blue for each channel of a multi-channel OVD. Once the palettes are designed, the palette elements can be used for the various different full colour OVD effects.

A full colour OVD may be produced in the following steps:

(a) Split each true colour image (art work) into three images for red, green, and blue, and save as three separate grey scale images respectively. Figure 21 shows an example of a four-channel full colour OVD process.

(b) Convert the grey scale images of each channel into a palette of different colour intensities. Figures 22a to 22b show how a full colour image (Figure 22a) is separated into a monochrome red component image (Figure 22b), a monochrome green component image (Figure 22c), and a monochrome blue component image (Figure 22d).

(c) Convert the grey scale intensity palette into a corresponding grating palette.

(d) Transfer the data to the electron beam lithography machine and fabricate the device. (e) Develop the plate.

(f) Electroform a master shim.

(g) Emboss the master shim OVD onto foil. Pictures of surface structures (viewed under a microscope) for a single channel full-colour OVD are shown in Figures 23a, 23b and 24. The OVD has been developed using the method of changing the effective area of gratings within the sub-pixels. One can see in the pictures the width of each track is varying from pixel to pixel. The width is proportional to the brightness of colour created by the sub-pixel.

In general, the degree of security of an OVD is proportional to the ratio of spatial variation to the dimension. For example the period of the gratings can vary from d_] to d₂ in a distance / ; the degree of security is then defined by

Physically the higher the D_χ value of an OVD, the more difficult it is to copy the OVD by holographic techniques. However, the device may also become hard to view. In reality, we have to optimise the two parameters of an OVD: degree of security and visual effectiveness of the device. A full colour OVD has much higher security than a monochrome OVD with the same pixel size, because a pixel of a colour OVD according to the preferred embodiments of this invention is divided into three parts and there are three complete discrete spatial frequencies within each pixel. Normally a copier can only focus onto one of the spatial frequencies. Thus the images on counterfeited devices become monochromatic.

It is to be understood that various alterations, additions and modifications may be made to the parts previously described without departing from the spirit of the invention.

Claims

CLAIMS:

1. A diffractive device having a surface relief structure which when illuminated by a light source generates a diffraction image observable at a range of viewing angles around the device, wherein the observed diffraction image is composed of pixels of numerous different colours, with the hue and intensity of most pixels being similar to those of immediately adjacent pixels so that the image has a substantially continuous smooth colour tone, wherein each combination of hue and intensity is generated by the combined diffractive effect of the physical characteristics of one or more regions of the surface relief structure.

2. A diffractive device according to claim 1 wherein each pixel in the observed diffraction image has hue and intensity characteristics selected from a predetermined palette of available hues and intensities, and each combination of hue and intensity in the palette is generated by the combined diffractive effect of the physical characteristics of one or more regions of the surface relief structure, and wherein the physical characteristics of the regions of the surface relief structure are selected from a predetermined palette of available physical characteristics which have known diffractive effects.

3. A diffractive device according to claim 1 or claim 2 wherein the regions of the surface relief structure include diffracting arrays of polygonal formations (being either protrusions or indentations) having either sharp or rounded edges and corners.

4. A diffractive device according to claim 3 in which the intensity characteristics of pixels or of component colours are varied by varying one or more of the width, length, depth, angular characteristics and spatial frequency of the diffracting formations on the regions of the surface relief structure.

5. A diffractive device according to claim 3 wherein the intensity characteristics of pixels or of component colours of pixels are varied by means of phase cancellation in diffracted light attributable to physical characteristics of WO 98/23979 . _fi PCT/AU97/00800 the diffracting formations on the regions of the surface relief structure which cause simultaneous diffraction of light with differing phase characteristics.

6. A diffractive device according to claim 3 wherein the maximum dimension of the polygonal formations is less than 60 microns.

7. A diffractive device according to claim 3 wherein the maximum dimension of the polygonal formations is less than 30 microns.

8. A diffractive device according to claim 1 wherein the diffraction image is a full colour portrait of a face or object or scene.

9. A diffractive device according to claim 8 wherein the image changes from a positive tone image to a negative tone image as the angle of view is changed.

10. A diffractive device according to claim 8 wherein the image appears to move as the angle of view is changed.

11. A diffractive device according to claim 8 wherein the image changes smoothly from one image to another as the angle of view is changed.

12. A diffractive device according to claim 1 wherein the surface relief structure, when illuminated by a light source, generates two or more diffraction images which are observable at different ranges of viewing angles around the device.

13. A diffractive device according to claim 1 or claim 12 wherein the surface relief structure is comprised of groups of regions, and most groups include three regions for each diffraction image generated by the device with each of the three regions generating a different primary colour of a particular intensity and the three regions together generating a colour pixel in the image to which those three regions contribute.

14. A diffractive device according to claim 13 wherein the primary colours are red, green and blue.

15. A diffractive device according to claim 1 wherein the attributes of each pixel are related directly to the vector components of a point on a chromaticity diagram.

16. A diffractive device according to claim 1 wherein the diffraction image includes an abstract graphic pattern of non-primary colours formed by superposition of primary colour components of different intensities diffracted from the surface structure regions and wherein the observed colour pattern changes to a different colour pattern of non-primary colours as the angle of view is changed.

17. A method of forming a diffractive device which under appropriate illumination conditions generates N diffraction images, including the steps of: (a) decomposing each of N full colour images into three component primary colour images;

(b) dividing each of the 3N primary colour images into a multiplicity of M pixels;

(c) determining an intensity value for each of the pixels; (d) determining for each pixel a regional surface relief structure for the diffractive device which under appropriate illumination conditions generates diffracted light of the applicable primary colour and intensity value.

(e) grouping each pixel with a pixel of corresponding location from each of the other 3N-1 primary colour images; (f) for each such grouping forming a composite surface relief structure consisting of the regional surface relief structures placed adjacent to each other; and

18. A method according to claim 17 wherein N equals 1.