TRANSITORY IMAGE STRUCTURE

Abstract: A transitory image structure comprises linear markings (1-10) on a substrate. The linear markings define image (31, 32) and background (30) portions respectively, the markings cooperating together such that an image generated by the image portion is not visible from all viewing angles. At least some of the lines of the markings of at least one of the portions have a physical characteristic, such as thickness, which varies in a substantially random manner along their lengths.

Description: TRANSITORY IMAGE STRUCTURE The invention relates to a transitory image structure and a method for generating a transitory image. Transitory images are images which can only be seen from certain angles of viewing. Commonly, these comprise latent images which cannot be seen when the structure is viewed normally but can be seen at certain off-normal angles, and transient images which can be seen when the structure is viewed normally but not at certain off -normal angles. Transitory images, particularly latent images, are particularly useful as security features on documents of value and security documents and can also be used as anti- counterfeiting devices. US-A-4033059 discloses a variety of latent image structures formed by intaglio printing and intended for this purpose. Typically, transitory image structures comprise linear markings on a substrate, the linear markings defining image and background portions respectively, the markings cooperating together such that an image generated by the image portion is not visible from all viewing angles. A drawback of conventional transitory image structures is that the presence of such structures on articles can often be detected even when the structure is viewed at an angle at which the transitory image should not be visible. Attempts have been made in the past to disguise the presence of such a structure by incorporating other patterns which are visible at all angles but these have not been completely successful . In accordance with one aspect of the present invention, a transitory image structure comprises linear markings on a substrate, the linear markings defining image and background portions respectively, the markings cooperating together such that an image generated by the image portion is not visible from all viewing angles and is characterised in that at least some of the lines of the markings of at least one of the portions have a physical characteristic which varies in a substantially random manner along their lengths . We have found that utilising linear markings for one and preferably both of the image and background portions, in which a physical characteristic, for example width or depth, of at least some of (and preferably all) the linear markings varies in a substantially random manner, produces a very flat or uniform appearance when the structure is viewed at angles at which the transitory image should not be visible, considerably improving upon the problems outlined above. Typically, the structure presents a speckled appearance without any indication of the presence of the transitory image. Although the invention can be used to generate a transient image, it is particularly suitable for generating latent images and the following description ' will concentrate on that aspect . In the preferred embodiment, changes in the physical characteristic of the linear markings of the at least one portion occur at substantially random intervals and the physical characteristic of the linear markings of the at least one portion has a dimension which varies in a substantially random manner. This results in maximising concealment of the transitory image. However, in some cases, only one of these changes could be implemented. Typically, the linear markings will be substantially parallel and preferably are rectilinear. As in conventional transitory image structures, preferably the lines of the image portion are not parallel to the lines of the background portion. However, in some cases, both sets of lines could be parallel. We also envisage linear markings forming substantially concentric polygons such as octagons. A single set of concentric polygons could form the first and second sets of linear markings, or the two sets of linear markings could be defined by different groups of concentric polygons. Usually, the markings will be defined by relief elements such as may be produced by intaglio printing or embossing, including blind embossing. However, in some cases, the markings may be surface printed on the substrate, for example by offset or lithographic printing. The surface of the substrate which carries the structure may be colour coated with one or a number of colours or be specularly metallised. The structure may be incorporated into a variety of authenticatable items such as banknotes, printed monetary tokens, travellers cheques, bonds, visas, certificates, vouchers, passes, tickets, passport pages, printed paper or film seals, and tamper evident labels or tags. It is particularly aimed as a visually detectable feature for use against counterfeiting. The structure is particularly suitable for incorporation into documents such as banknotes during the intaglio printing process. The intaglio plate having the mould for imparting the three dimensional effect may carry ink or it may be ink free. In practice, however, it is expected that the structure will be formed on an area of paper such as security printable document grade paper during an intaglio printing process. If the structure is to be printed by the application of intaglio ink then this will generally be to an absorbent surface comprising cellulosic paper fibres or the like. The area onto which the structure is to be formed may previously have been printed by lithography or the like to form an evenly coloured or lustrous background. Alternatively, the intaglio impression may be made in the so-called "blind" embossing style in which no ink is applied to the recesses in the plate corresponding to the device indentations. In such instances, the surface to be embossed will preferably be glossy or specularly reflecting such as may be found by the sheen exhibited by a metallic effect printing ink, or the mirror reflectivity exhibited by a metallised film, or indeed the optical variability induced by the diffraction of light such as in white light viewable holograms, multiple layer light interference coatings and the like. The area of the substrate to be embossed with the structure should be substantially planar and capable of being deformed in accordance with the shape of the mould. A particular advantage of the transitory image structure according to the invention is that it presents the appearance of a random screen even at close viewing distances while at greater distances it will appear as a continuously coloured patch. This allows the structure to be incorporated into other designs on the substrate in such a way that even the presence of the structure is difficult to discern. It should be understood that although the linear markings will normally be continuous, they could be discontinuous and formed, for example, by discrete dots. In another embodiment, the parallel line sets may slightly diverge and optionally the maximum permitted width of the random widening may be caused to vary along the length of the lines. The transitory image structure can be generated in a variety of conventional ways, typically by generating a suitable screen of linear markings having the random variation required and then superimposing that screen on a conventional transitory image pattern. This can conveniently be done using conventional computer aided design systems, intaglio plates or the like then being created from the resultant combined pattern. The intaglio plate will be therefore provided with a fine, uneven textured pattern which will appear to be essentially random. In part of the area will be the parallel line pattern which causes the transitory image effect when the device is tilted. The parallel line set will generally form a simple image such as a number, a letter or a symbol which will be revealed at certain angles of viewing. The parallel line set image may be substantially even, in that the depth of the lines does not change. Alternatively, the lines may themselves exhibit a small degree of speckle with a periodicity similar to that of the background lines. In addition to the line sets being parallel but randomised in the chosen physical characteristic, the line sets could be formed as a matrix of miniature parallel line sets of random thickness, the sets having different orientations of the parallel line sets. In a particularly preferred approach, and in accordance with a second aspect of the present invention, a method of generating a transitory image comprises generating digital data defining linear markings at least some of which have a physical characteristic which varies in a substantially random manner along their lengths; and combining the linear markings with image and background data to generate data defining image and background portions of the transitory image. In this aspect of the invention, the linear markings are generated digitally, typically using a computer programme, which allows considerable freedom in choosing the manner in which the screen is defined. Although the generating and combining steps are set out separately, in some cases, these may be carried out simultaneously. In the preferred method, the form of the linear markings with a varying physical characteristic is determined by: a) i) determining for each segment of the line whether or not it should have the same value of the physical characteristic as the preceding segment; and, if not, ii) generating a new width value for the segment, and, b) generating data in accordance with the determined physical characteristic value, wherein one or both of steps a) i) and ii) are carried out in a substantially random manner. This results in a relatively simple method for generating the random screen. Typically, the physical characteristic variation (usually width or depth) will be constrained within maximum and minimum limits. In addition, typically, the same algorithm will be used to generate the linear markings for the image portion and for the background portion. Although the transitory image will usually be discernable to the naked eye, it could also be machine readable or indeed only machine readable. An example of a latent image structure according to the invention will now be described with reference to the accompanying drawings, in which: - Figure 1 illustrates in enlarged form an example of part of a set of linear markings; Figure 2 is a flow diagram of the algorithm for generating the linear markings shown in Figure 1; Figure 3 illustrates in enlarged form the appearance of a latent image structure utilizing the linear markings shown in Figure 1; and, Figure 4 illustrates the appearance of an example of a latent image structure according to the invention when viewed normally. The invention is primarily concerned with the generation of linear markings having a width which varies in a substantially random manner along its length. However, a variation in depth or some other physical characteristic is also envisaged. Figure 1 illustrates a small portion of a set of linear markings suitable for constructing a transitory image structure, in considerably enlarged form. As can be seen in Figure 1, the linear markings 1-10 have centre lines which are substantially parallel but each exhibits changes in width which occur at substantially random intervals and between substantially random width values . In order to generate a linear marking of this type, a computer is suitably programmed to generate digital data defining lines composed of successive short elements typically having a minimum length of 0.075mm, the width of each element being defined by the program parameters. Figure 2 illustrates a flow diagram of an example of a computer programme for achieving this. Initially, in a step 20, the line spacing between the centre lines of adjacent linear markings is set. A typical example is in the range 0.2-0.5mm centre-to-centre, commonly 0.25mm. In a subsequent step 21, the operator enters the minimum and maximum widths which each line may have, typically, in the range 50-200 microns. He also enters a decision threshold T in the range 0.1-0.9. The computer then commences the generation of the line starting from a predetermined initial width value (step 22) . Initially (step 23) the computer determines the value of a first random number RND1. This random number varies between 0 and 1.0 and in step 24 it is determined whether or not RND1 > T (typically T = 0.5) . If it is greater than T, this is used to initiate a change of width in the line. In order to determine what value that width should be, a second random number RND2 is inspected (step 25) . This also varies between 0 and 1.0 and the computer determines in a step 26 the value of the new width by using the value of RND2 to define a fraction of the range between the previously set Max and Min widths. (In an alternative implementation, the new width could be determined from RND2 in some other way, for example by using RND2 as an address into a previously defined look-up table of widths.) Following selection, the next element is generated (step 27) with a width corresponding to the selected new width. If in step 24 it was found that RND1 was not greater than T then the next element would be generated with a width value corresponding to the previously selected width, in other words the same width as the previous element . In a step 28, the computer determines whether or not the end of the line has been reached and if it has not then steps 23-27 are repeated. Although this algorithm has been described for generating widths which vary on an element by element basis, that variation could be changed by allowing the operator to vary the length of the element . The digital data defining each line is stored and is then combined in a conventional manner with a transitory image, for example to form a bit map. The resultant data (bit map) is then used to construct an intaglio plate, the appearance of that plate and hence the transitory image structure being illustrated in an enlarged form in Figure 3. As can be seen, the transitory image structure defined by the plate comprises a background portion 30 defined by vertical lines and image portions 31,32 defined by horizontal lines, together forming a "head and shoulders" image. It will be noted that the lines forming both the background and image portions 30-32 each have a similar random width variation and line spacing although this is not essential. Furthermore, the background portion 30 could be defined by horizontal lines and the image portions 31,32 by vertical lines. The intaglio plate can then be used to print or blind emboss the structure onto a substrate such as a banknote or the like. The resultant appearance of the structure when viewed normally is shown in Figure 4 where it will be seen that the transitory image located at 40 is completely obscured and indeed even the presence of the structure is not readily apparent since it blends into the surrounding banknote design. Although not shown, the structure could also include a pattern which is visible from substantially all viewing angles. This could be produced by controlling the background linear markings to have a width of particular value in the predetermined pattern.

Claims: CLAIMS 1. A transitory image structure comprising linear markings on a substrate, the linear markings defining image and background portions respectively, the markings cooperating together such that an image generated by the image portion is not visible from all viewing angles characterised in that at least some of the lines of the markings of at least one of the portions have a physical characteristic which varies in a substantially random manner along their lengths. 2. A structure according to claim 1, wherein at least some of the linear markings of both the image and background portions have a physical characteristic which varies in a substantially random manner along their lengths. 3. A structure according to claim 1 or claim 2, wherein the lines of the markings of the at least one of the portions are substantially parallel. 4. A structure according to any of the preceding claims, wherein the lines of the markings of the at least one of the portions are rectilinear. 5. A structure according to any of the preceding claims, wherein the lines of the image portion are not parallel to the lines of the background portion. 6. A structure according to claim 5, wherein the lines of the image portion are at substantially 90° to the lines of the background portion. 7. A structure according to any of the preceding claims, wherein the markings defining the background portion further define a pattern visible from substantially all angles . 8. A structure according to claim 7, wherein the lines of the background portion in the region of the pattern have substantially the same physical characteristics. 9. A structure according to any of the preceding claims, wherein the markings are defined by relief elements. 10. A structure according to claim 9, wherein the markings are intaglio printed. 11. A structure according to any of claims 1 to 8, wherein the markings are surface printed on the substrate. 12. A structure according to any of the preceding claims, wherein the transitory image is a latent image. 13. A structure according to any of the preceding claims, wherein the structure is incorporated into part of a larger design on the substrate. 14. A structure according to any of the preceding claims, wherein changes in the physical characteristic of the linear markings of the at least one portion occur at substantially random intervals. 15. A structure according to any of the preceding claims, wherein the physical characteristic of the linear markings of the at least one portion has a dimension which varies in a substantially random manner. 16. A structure according to any of the preceding claims, wherein the physical characteristic comprises one of the width and depth of the linear markings. 17. A document of value or security document including a transitory image structure according to any of the preceding claims. 18. A document of value or security document according to claim 17, the document being selected from the group comprising banknotes, printed monetary tokens, travellers cheques, bonds, visas, certificates, vouchers, passes, tickets, passport pages, printed paper or film seals, and tamper evident labels or tags. 19. A method of generating a transitory image, the method comprising generating digital data defining linear markings at least some of which have a physical characteristic which varies in a substantially random manner along their lengths; and combining the linear markings with image and background data to generate data defining image and background portions of the transitory image. 20. A method according to claim 19, wherein the form of the linear markings with a varying physical characteristic is determined by: a) i) determining for each segment of the line whether or not it should have the same value of the physical characteristic as the preceding segment; and, if not, ii) generating a new width value for the segment, and, b) generating data in accordance with the determined physical characteristic value, wherein one or both of steps a) i) and ii) are carried out in a substantially random manner. 21. A method according to claim 19 or claim 20, wherein the physical characteristic comprises one of the width and depth of the linear markings. 22. A method according to claim 20 or claim 21, further comprising constraining the physical characteristic variation within maximum and minimum limits. -23. A method according to any of claims 20 to 22, wherein the lines are substantially parallel. 24. A method according to any of claims 20 to 23, wherein the image and background data are combined with respective portions of the linear markings which are not parallel . 25. A method according to claim 24, wherein the linear markings of the image and background portions are at substantially 90° to one another. 26. A method according to any of claims 20 to 25, wherein the transitory image is a latent image. 27. A method according to any of claims 20 to 26, further comprising controlling an output device in accordance with the generated digital data to generate a visual representation of the transitory image. 28. A method according to claim 27, wherein the output device comprises a printer.

Inventors: RAISHBROOK MARTIN, SHENTON COLIN NORMAN, TRINDER ROSEMARY, RAISHBROOK, MARTIN, SHENTON, COLIN, NORMAN, TRINDER, ROSEMARY
Applicants: RAISHBROOK MARTIN, RUE DE INT LTD, SHENTON COLIN NORMAN, TRINDER ROSEMARY, DE LA RUE INTERNATIONAL LIMITED, RAISHBROOK, MARTIN, SHENTON, COLIN, NORMAN, TRINDER, ROSEMARY
Application year: 1998
Main classification: B41M3
IPC codes: B41M3, B42D15