WO1999012742A1

WO1999012742A1 - Improved security printing method for printing secure documents

Info

Publication number: WO1999012742A1
Application number: PCT/GB1998/002324
Authority: WO
Inventors: Richard Ashwell; Paul Singleton
Original assignee: The House Of Questa Ltd.
Priority date: 1997-09-08
Filing date: 1998-08-03
Publication date: 1999-03-18
Also published as: AU8637098A; GB9718958D0

Abstract

A method of printing security documents is disclosed wherein images on the security document are comprised of patterns of dots of different colours, the dots being of a diameter of less than 15 νm. This reduced dot size allows for a number of security features to be incorporated into an image printed on a security document. For example, dots of this diameter printed on a document at a certain tint weight cannot be reproduced by currently available photocopying equipment, and therefore what appears in the original document to be a complete image is incomplete after photocopying, particular portions of the image 'dropping out'. Alternatively, the reduced dot size allows for the offset lithographic and gravure printing of complex images with multiple colours depending on the concentrations, interspersion and/or overlap of dots of different colours. This facilitates the creation of images whose colour varies imperceptibly in two dimensions across a substrate, an effect never before achieved in security document printing. Also, micro-fine text can be printed which is substantially indecipherable to the human eye and to a photocopier but which can be easily read on magnification of the original document. A further security feature available is the printing of an image constituted entirely of miniaturised replicas of itself, the miniaturised replicas being indecipherable both to the human eye and to a photocopier. One or more of these security features can be incorporated in a security document to prevent compromise thereof.

Description

Improved Security Printing Method for Printing Secure Documents

The invention relates to security printing, and more specifically to security printing processes used in the production of bank notes, bonds, vouchers, cheques, stamps and like items which have value associated therewith, known hereinafter as security documents, where there is a requirement that the security document be incapable of being copied without being instantly recognisable, or recognisable after the performance of a simple test, such as a photocopy.

Many security features currently provided on security documents are so provided with the aim of defeating the most powerful currently available colour photocopiers, such as a particular paper type commercially unavailable, the existence of watermarks in the paper or the security document, and the printing of micro-fine text within linework on the security document which is recognised by the photocopier simply as a line. Another security feature directed to this aim is the provision patterned and angled linework on the security document which embodies an image visible to the naked eye, but invisible to a photocopier, as disclosed in EP0384897. The angles and patterns of the linework are such that as a result of the unidirectional sweep of the powerfully illuminated head of the copier across the security document, such linework is not transposed onto the copier paper and voids appear on the copied document which are immediately visible to the naked eye. Furthermore, additional linework may be angled such that as a result of the sweep of the copier head, such linework is attributed with a bolder shade than appears on the security document, and when these two types of linework are interspersed on a security document, the photocopied document has a mottled effect, and is thus instantly recognisable as counterfeit. Currently, offset lithography is in widespread use for the printing of all types of documents, and a specific type of offset lithographic printing is often used for security documents. The general process is as follows:

Ink in a duct is applied to an inking roller in contact with a transfer roller, which is in turn in contact with a number of different rollers, usually arranged approximately pyramidally within the printing press. This configuration of rollers ensures that a uniform thickness of ink is applied to a plate roller positioned beneath the pyramidally arranged rollers, and on which the lithographic plate, usually of aluminium, containing the image is mounted. The ink thus adheres only to the desired areas on the plate cylinder and is then transferred to a rubber blanket cylinder which transfers the ink to the paper under pressure provided by the reaction against an impression cylinder, the paper passing between impression cylinder and rubber blanket cylinder.

Colour security documents, such as bank notes, are often provided with linework which gradually changes colour across the width or length of the document. This effect is known as a vignette and may incorporate a number of different distinct ink colours, usually no more than six, such colours varying imperceptibly over the document in one direction or other. The printing of vignetted linework on a document is effected by splitting the ink duct at the top of the printing press into a number of discreet sections and filling each section with a different colour ink. The merging of the colours is effected by a roller which oscillates laterally of the press along its axis of rotation, said roller being provided within the pyramidal roller configuration. There is a physical limitation to the number of colours which a vignette applied in this manner can contain, as each section of the ink duct containing a different colour must be at least of a width equal to the maximum lateral displacement of the oscillating roller. If the criterion is not met, more than two colours may be mixed on the oscillating roller, and the resulting colour is muddy and lacks the desired aesthetic quality. It will be appreciated immediately by the skilled addressee that such a method of applying coloured ink to paper can only ever result in a gradual colour change in one direction, and in particular axially of the laterally oscillating roller.

Almost exclusively, the originating artwork for current security documents is currently produced on computer. Many powerful, commercially available software products allow for the creation of extremely intricate images and artwork, and current offset lithographic and other printing techniques are all disadvantaged in that it is practically impossible to transfer the complex artwork created on a computer to a printing plate exactly. The high resolution of a computer cannot be transposed to the physical plate effectively, and the loss in resolution during the transposition period results in blurring. Alternatively, this drawback prohibits computer artists from using resolutions on their artwork above a certain level. Maintaining resolution is important in the security printing industry, as very high resolution printed documents are not effectively copied.

Lithographic printing may be effected by one of two methods. In the first method, a mask of the entire artwork to be printed by a single plate is produced, and developed photographically onto a lithographic plate subsequently mounted on said plate cylinder. Thus an entire image is transferred to the paper by the rubber blanket cylinder, whether in a single colour or with a colour vignette as described above. This method is currently extensively used in the security printing of documents. The second method is applicable to the construction of a lithographic printing plate from computer originated artwork, and involves the generation of so-called "bit-map" images from the computer artwork. The computer generated image is processed line by line (the thickness of the lines depending on the desired resultant screen resolution of the lithographic plate) by a Raster Image Processor (RIP) which sends the bit-map information to an image setter. From the original computer image, four bit-mapped masks comprising a great many dots will typically be produced from the image setter, one relating to each of the colours Cyan, Yellow, and Magenta, these colours being inverse to the logical primary colours Red, Green, and Blue, and from which any desired colour may be produced by mixing different quantities of each, and one for Black. As in the first method, these masks are used in the creation of lithographic plates mounted on the plate cylinders, but in this method, only one colour is applied to each plate, the printing press having at least four consecutively arranged press units each containing a plate cylinder through which the paper passes. Thus, for example, the first plate will transfer a cyan dot pattern to the paper, a second plate will apply a different dot pattern in magenta to the paper, and a third plate will transfer a further different dot pattern in yellow, the different dot patterns applied to the paper in precise register with one another such that a completed image identical to that of the computer originated image results. (A fourth plate applies a black pattern or image to the paper as desired). The different colours used in the computer generated image are reproduced on the paper as a result of the juxtaposition and overprinting of the dots of different inks which visually creates the desired colour. Obviously, where any of the colours cyan, magenta, or yellow are explicitly required in the image, dots of different colours will not overlap. Although this method is very flexible for the production of colour images, as an image with a great number of colours may theoretically be printed, this method is flawed in that the current dot size (approximately 22 μm in diameter) is too large to make this method a viable proposition for the printing of security documents. In fact, this method of lithographic printing is in such widespread use that security documents would be vulnerable if produced by this method as counterfeits would be simple to print. Hence, the first method of lithographic printing is currently primarily used for the printing of security documents.

The applicability of the second method to computers is seen when it is understood that computer files containing computer images, however complex and intricate, are comprised exclusively of binary information, and thus from said images may be extracted the cyan, magenta, and yellow components, each of these components further comprising a great many dots which may be either filled in (corresponding to the binary "on" or " 1" state), or not filled in (corresponding to the binary "off" or "0" state). The currently available size of the dots («22μm) and thus the number of "dots per inch" or "d.p.i." («350, known as 350 screen), and the thickness of each line of dots which can effectively be printed onto the paper are not suitable for the security printing market.

A further printing process used commercially for the printing of non-secure documents is the gravure method, whereby the printing plate is chemically etched using gelatin as a resist. In general, the ink is applied to the print cylinder and adheres only to those areas to which the gelatin had been previously applied. As with lithography, this process is not ideally suited to the printing of secure documents, on account of its widespread availability and the limit on screen resolution (essentially a measure of the output quality of an image) which can be achieved.

It is an object of this invention to permit the printing of security documents using offset lithographic techniques or gravure techniques by printing dot patterns on the paper in different colours.

It is a further object of this invention to facilitate the printing of features on security documents which are incapable of being effectively copied.

It is a yet further object of this invention to increase the resolution achievable using the offset lithographic and gravure printing processes thus improving the security of currently available security features and enabling the printing of previously unavailable security features.

According to the invention there is provided a method of printing security documents wherein an image printed on said document is comprised of one or more patterns of dots each of a particular colour, said dot patterns being applied to the document as it passes through consecutive printing units all being part of a press, said dot patterns being in precise register with one another, a single dot pattern of a single colour being applied by each unit, characterised in that the diameter of the dot is less than 15μm.

Preferably the registration of the dot patterns allows for the interspersion and/or overlap of dots of different colours such that in that portion of the image where said interpsersion or overlap of dots occurs, a colour is perceived by the naked eye which corresponds to the proportions of the coloured dots from each dot pattern applied in that portion.

Preferably, the image on the security document is created from three distinct dot patterns, one of which is printed in cyan, one of which is printed in magenta, and one of which is printed in yellow, and further preferably, a fourth dot pattern printed in black is also provided.

Arising from the reduced dot size of the invention, there is accordingly provided in a first aspect of the invention a security feature comprising an image printed on a coloured or clear substrate, which may be translucent, said image portion having at least one colour different from the colour of said substrate and being comprised exclusively of dots, said colour gradually fading and thus merging with the colour of the substrate, or gradually merging with another colour of the image portion, characterised in that the fade or merge is smooth when perceived by the human eye and in that the fade or merge is effected in two dimensions on the substrate.

By this is meant that the fade or merge renders the individual colours indistinguishable from one another in the region of the fade or merge, and that such a fade or merge may be effected in any direction on the paper substrate.

Also arising from the reduction in dot size achievable, a second aspect of the invention accordingly provides a security feature for printing on paper of a particular colour, said security feature embodying an image portion comprised of a pattern of dots, characterised in that the dot size is such that on photocopying said security feature, said image portion is not reproduced.

Preferably, the image portion is superimposed on a background portion which also comprises a pattern of dots whose colour is identical to that of the dots forming the image portion, the size of the dots being such that the background portion is reproduced on photocopying the security feature whereas the image portion is not. Preferably the colour of the paper on which the security feature is printed is white, and the colour of the dots is black. Alternatively, the security feature may be printed on clear or coloured translucent material, the dots being printed thereon in black or other colour.

Further preferably, the respective dot sizes and separation of the dots within the background and image portions is such that the image portion is practically indistinguishable from the background portion to the naked eye when within the background portion, the perceived colour (or so called "tint weight") of both portions being substantially identical. As a result of the hidden nature of the image portion described above, such image portions are often referred to as latent images.

Also according to the second aspect of the invention, there is also provided a latent image decoding device comprising a translucent screen used to reveal a latent image printed on a security document with a predetermined tint weight, said image being comprised of dots in a particular pattern and substantially concealed within a background portion of similar tint weight, said screen having printed thereon a second pattern of dots which is similar to the dot pattern of the latent image, characterised in that superposition of the screen on the latent image results in interference between the respective dot patterns thereof when the screen is accordingly aligned such that the latent image is revealed to the naked eye.

It is preferable that the screen is provided with at least one straight edge for alignment with a straight edge of either any printed image on the security document or with a straight edge of the security document itself. Alternatively, both screen and security document may be positioned in an aligning device which automatically aligns the dot pattern of screen and latent image. It is further preferable that a plurality of different dot patterns are printed on the screen in different areas thereof, each said different dot pattern being similar to the dot pattern of the latent image such that superposition thereof results in interference, in at least one of the areas of the screen, between the dot pattern of the latent image and the dot pattern of that particular area of the screen.

Henceforth, imprecise alignment of the screen may still result in revelation of a portion of the latent image to the user of the decoding device.

Again arising from the reduction in dot size achievable, a third aspect of the invention accordingly provides a security feature embodied within an image or which constitutes an image printed on a security document, the feature comprising text printed along a line of a particular shape and orientation characterised in that the text is comprised of exclusively of dots, and in that on photocopying the security document only the line is reproduced and the text becomes indecipherable in the copied document.

Preferably, the size of the dots is less than 15μm.

It is preferable that the line of text is not immediately apparent to the naked eye, and only visible thereto after magnification. Further preferably, the text is indecipherable in the photocopied image and appears only as a line of colour.

It is further preferable that the security feature comprises a guilloche pattern, at least some of the lines of which comprise text.

It is further preferable that the text of the security feature gradually changes colour along the line. This vignette effect can be achieved as the text simply forms part of the image which is printed as a number of dot patterns, each printed in a different colour. Thus the constituent dot patterns of the overall image may be varied to allow any particular colour vignette along the line of text and in any direction. Such a security feature may be of great advantage to the industry and cannot currently be achieved.

Finally, according to a fourth aspect of the invention which again results from reduced dot size and concomitant increased resolution, there is provided a security feature printed on a security document comprising an image, characterised in that the image is made up exclusively of miniaturised images identical to the image itself, each miniaturised image and thus the entire image being constituted of a pattern of dots.

It is preferable that the size of the dots is less than 15μm.

It is preferable that the miniaturised images are neither immediately apparent to the naked eye without magnification, and that on photocopying the security feature, the miniaturised images are not reproduced.

In all aspects of the invention, the size of the dot is preferably l l μm.

In all aspects of the invention, the shape of the dot may be selected as desired, and may be circular, elliptical, square, or otherwise shaped.

Preferably two or more of the security features described above are incorporated a single security document.

Certain of the high resolution photocopiers currently available on the market may be selectively programmed to detect a particular security feature within a document. However this "focussing" of the CCD within the photocopier to detect a particular security feature reduces the ability of that CCD to simultaneously detect other security features within the same security document being photocopied. The combination of two or more security features in the same document practically prevents any compromise of the document by photocopying.

Previously, gravure printing processes could only achieve 300 screen resolution, but by laser etching the printing plate it is now possible to achieve resolutions of up to 500 screen, and thus achieve the dot size required in the performance of the invention.

It will be immediately appreciated that the printing of security documents according to all aspects of the inventions represents a radical departure from current practice, as the printing of images constructed entirely of interspersed and overlapped patterns of coloured dots has not previously been considered for the security market on account of the size of dot currently available, which we believe is approximately 22 μm, the concomitant lack of definition and imprecise resolution in the resulting image, and the ease with which such images could have been counterfeited by other printers.

It should be mentioned that the resolution of the resulting image which is achievable depends on the quality of the paper substrate and the surface finish thereof, because it is essential, especially when printing dots of a size according to the invention, that bleeding, dispersion, and absorption of ink by the substrate are minimised. However the particular quality of paper substrate selected is not a feature of the invention, as printing according to the invention may be effected on a number of currently commercially available paper substrates. Although it was previously possible to apply different dot patterns in different colours to a sheet of paper, it was impossible to create a uniform colour change along a curved line using such a process on account of the dot size. The individual dots could be seen by the naked eye, the two-dimensional vignette effect was never achieved, and colour photocopying of the image produced an acceptable reproduction of the feature. None of these qualities was acceptable for the security market.

The process according to the first aspect of the invention allows for a gradual, imperceptible colour change in any direction on the surface of the paper which cannot be effectively photocopied.

In the other aspects of the invention, the reduced dot size allows for the printing of specific security features of such definition and resolution that the most precise monochrome and colour photocopiers commercially available cannot reproduce such features effectively. Furthermore, such an increase in resolution has been achieved that continued development of such photocopiers may not result in precise reproduction of such security features for some time, especially as such copiers are currently extremely expensive (∞£70,000-£80,000) and their output is currently generally acceptable for all documents except those of a secure variety.

Specific embodiments of the invention are now given by way of example only with reference to the accompanying drawings, wherein:

Figure 1 shows a schematic perspective view of the application of a single dot pattern to a sheet of paper;

Figure 2 shows a schematic representation of the second aspect of the invention, and Figures 3a, b show enlarged views of portions of Figure 2

Referring firstly to Figure 1, a printing unit indicated generally at 10 and of which there may be many (2-6, or even more) in a single printing press, is provided with a plate cylinder 12 which contacts a rubber blanket roller 14. The unit is also provided with an impression cylinder 16 which acts against the blanket cylinder 14 such that pressure is applied by both said cylinders to a substrate sheet 18 passing therebetween. The directions of motion of the cylinders are indicated by arrows 12a, 14a, 16a, and the direction of travel of the substrate 10 through the unit is indicated by arrows 18a, 18b.

The plate cylinder 12 has mounted thereon a lithographic plate 20 onto which a dot pattern 22 has been photographically or photochemically developed such that ink particles adhere to the dots and not elsewhere on the plate as in conventional lithography. Ink is transferred from to the plate cylinder 12 through a series of rollers (not shown) usually arranged above said plate cylinder. The inked dot pattern is then transferred to the rubber blanket cylinder 14 during which there may be some infinitesimal size gain in the dot as it spreads onto the surface of the rubber blanket. The ink pattern on said blanket is then transferred to the substrate 18 under pressure exerted by the reaction of the impression cylinder 16 thereon as the substrate passes between said cylinders.

The substrate is typically of a desired length to allow a number of consecutive dot patterns to be printed thereon as shown at 24, 26. The substrate is then transferred to a second and subsequent printing units (not shown) similarly constructed to that unit shown in the Figure which apply different dot patterns in different colours. According to the invention, the size of the dots are of the order of l lμm, which allows the printing of documents at a resolution of 850 lines per inch, known in the trade as 850 screen.

The printed image which results after the substrate has passed through all the printing units of the press, each of which applies a dot pattern of a different colour to the substrate, is of high quality, and individual dots cannot be seen by the naked eye. Even under 25x magnification, the colour dots are only just visible.

The distinct advantage of this method of printing security documents over prior art methods is the capability of achieving much higher resolutions of printed material than previously, and combining the printing equipment with high-end computer image processing equipment to create intricate colour images comprised entirely of dots, and in which colour vignettes can be applied in two dimensions, and thus in any desired direction, across the surface of the substrate.

Referring now to Figure 2, a security feature 30 is printed on a substrate 32 by one of the printing units shown in Figure 1. In this instance, it is to be assumed that the substrate 32 is white paper, although any colour material, translucent or opaque may be used and the hatching shown in the Figure in the security feature 30 is intended to be representative of shading.

The security feature 30 consists of a background portion 34 and an image portion 36, both of which consist of a pattern of black dots applied to the substrate to give the effect of a shade of grey. Where the colour of the substrate is different, the application of a pattern of coloured dots to the paper may result in a shade of an alternative intermediate colour. For instance if a cyan substrate is used, and a magenta dot pattern is printed thereon, a blue shade will result. The dots within the pattern which forms the background of the security feature are ideally of a size and separation to be distinguishable by current photocopiers which thus authentically reproduce the said background portion. Therefore, photocopying this portion of the security feature results in a shade of grey being reproduced on the photocopy. However, the image portion 36 of the security feature is ideally comprised of dots of a size ideally of l lμm. The spacing of such dots in the dot pattern which forms the image portion is such that the image portion is not discernible from the background portion of the security feature by the naked eye, the two portions appearing to be the same shade of grey, or other particular colour. The dot size in the image portion is also not resolvable by current photocopiers, the reason being shown clearly with reference to Figures 3a, 3b, and the following description.

When the brightly illuminated head of a photocopier scans over the background portion, shown magnified in Figure 3a, the Charge Coupled Device (CCD) therein is capable of resolving the large dot size, and reproducing the particular shade of grey of said background portion. However, although the shade of grey in the image portion is essentially identical to the naked eye, a much larger area of the white substrate 32 is seen by the CCD of the photocopier, and as the brightly illuminated photocopier head passes thereover, the CCD is "blinded" by the amount of reflected white light, and thus cannot resolve any of the dot pattern of the image portion. Furthermore, even if the CCD could resolve any of said dot pattern, the particles of toner which are electrostatically charged to the paper on which the photocopy is printed are of a size greater than the dots within the image portion, and thus significant distortion of the image portion would result in the photocopy. In either event, a currently impregnable security feature results. As stated above, a further application of the invention is to the provision of microtext as a security feature within a security document. The resolution and definition which can be achieved using smaller dot sizes allows minute lines of text, optionally either curved and/or provided with a colour change along the length of the line, to be included within the original computer generated image. Commercially available packages allow for easy implementation of such features, but it was previously impossible to achieve such definition in the printed document as small text simply disintegrated and became illegible negating its use as a security feature. Text of a height of 500μm and less can now be achieved easily, whether provided with a vignette, on a curved line, or within a guilloche or otherwise. Such a security feature can thus be easily hidden within a much larger security document, and furthermore, encrypted codes may be included within a repeating line of text to further ensure the security of the document. The size of this text is simply too small to be resolved by the most powerful photocopiers, and appears in the resulting photocopy simply as a black (or other colour) line.

Finally, the application of the invention to the provision of a security feature comprising an image which is constituted exclusively of a miniaturised images identical to the overall image again defeats currently available photocopiers on account of the size of the miniaturised images.

The complete image (for example that of the Queen's head) is loaded or scanned into a computer running image processing software and divided into a large number of portions of the same shape as the image itself. These portions are typically of a size of 1- 2mm square, or smaller, and have associated with them a percentage of the portion which is inked. This average "tint weight" in each portion is calculated by the computer and is used to adjust the brightness or darkness of a miniaturised copy of the image which is then transposed into each portion accordingly. Thus the overall image retains is shape, features, and shading, and to the naked eye appears as if no transformation has been effected, whilst being constituted exclusively of miniaturised images which may be view by the naked eye under magnification (x25 is sufficient).

This aspect of the invention is primarily intended for use in providing security features which are monochromatic, but may apply equally to coloured images, the percentage of each different colour within the divided portions of the overall image being calculated by computer, which then subsequently adjusts the tint weights of all these particular colours in the miniaturised image thus maintaining the overall colour balance in the resulting transposed image.

Monochromatic or multicolour images which have been subjected to the process described above are incapable of being photocopied effectively as the photocopier is capable of resolving only the entire image and not the miniaturised images from which it is composed.

Claims

CLAIMS;

1. A method of printing security documents wherein an image is printed on a substrate passing through a press in a particular direction, at least a portion of said image being comprised of at least one pattern of dots of a particular colour different from the colour of the substrate, characterised in that the diameter of the dots in the said dot pattern is less than 15╬╝m.

2. A method according to claim 1 wherein at least a portion of the image the image is comprised of at least two patterns of dots each being of a different colour applied to the substrate consecutively, and characterised in that the interspersion and/or overlap of the differently coloured dot patterns provides an imperceptible colour change across that image portion in two dimensions when viewed by the human eye.

3. A method according to claim 1 wherein at least a portion of the image is comprised of a pattern of dots, characterised in that the tint weight of the dot pattern in the image portion is such that on photocopying, said image portion is not reproduced.

4. A method according to claim 3 characterised in that at least a portion of the image comprises first and second juxtaposed dot patterns the tint weights of which are substantially equal, the dots of said first pattern being of a size greater than 15╬╝m and the dots of said second dot pattern being of a size less than 15╬╝m such that on photocopying, only said first dot pattern is reproduced.

5. A method according to claim 4 characterised in that the respective dot sizes and separation of the dots within the first and second dot patterns is such that the image portion is substantially indistinguishable from the background portion to the naked eye.

6. A method according to claim 4 or 5 characterised in that said second dot pattern is in the shape of lettering which is instantly discernible from the first dot pattern in a photocopy of the original image.

7. A deciphering device comprising a translucent screen printed according to the method of claim 1 and used to reveal a portion of an image printed according to the method of claim 4, characterised in that the pattern of dots printed on said screen is marginally different from the second dot pattern in the image portion such that superposition of the screen on the image results in interference between the dot pattern of the screen and the second dot pattern of the image portion when the screen is accordingly aligned, said interference making visible to the naked eye the shape of the second dot pattern.

8. A device according to claim 7 characterised in that the screen is provided with at least one straight edge for alignment with a straight edge of either the image on the security document or with a straight edge of the security document itself.

9. A device according to claim 7 or 8 characterised in that a plurality of different dot patterns are printed on the screen in different areas thereof, each said different dot pattern being similar to the second dot pattern of the image portion such that superposition of the screen thereon results in interference, in at least one of the different areas of the screen, between the second dot pattern of the image portion and the dot pattern of that particular area of the screen.

10. A method according to claim 1 wherein the image portion comprises text printed along a line of a particular shape and orientation, characterised in that said text is comprised of exclusively of dots and is of a size which is not reproduced on photocopying.

11. A method according to claim 11 characterised in that the text is illegible without magnification.

12. A method according to either claims 10 or 11 characterised in that text is printed along at least some of the lines of a guilloche pattern.

13. A method according to both claims 2 and 10 wherein a line of text, at least a portion of which is at an angle to the direction of travel of the substrate through the press, said line being comprised of dots of at least two colours such that colour of said line appears to change along its length.

14. A method according to claim 1 wherein in that at least a portion of the image is comprised of a sub-image which is recognisable to the human eye, characterised in that said sub-image is comprised of miniaturised replicas of the sub-image itself, each miniaturised replica in turn being comprised of dots.

15. A method according to claim 14 characterised in that the miniaturised replicas are neither immediately decipherable by the naked eye without magnification, and that on photocopying the security feature, the miniaturised replicas are not reproduced but the sub-image as a whole is reproduced.

16. A method according to any of the preceding claims characterised in that the size of the dot is l l╬╝m.

17. A method according to any of the preceding claims characterised in that the shape of the dot is circular.

18. A method according to any of the preceding claims characterised in that the shape of the dot is square.

19. A method according to any of the preceding claims characterised in that the shape of the dot is elliptical.

20. A method according to any of the preceding claims characterised in that the image on the security document is created from three distinct dot patterns, one of which is printed in cyan, one of which is printed in magenta, and one of which is printed in yellow.

21. A method according to any of the preceding claims characterised in that at least one dot pattern is printed in black.

22. A method according to any of the preceding claims characterised in that the substrate is white and opaque.

23. A method according to any of claims 1-21 characterised in that the substrate is translucent.