FOR USE IN THE METHOD

This invention relates to a method of forming a composite image arrangement, comprising a pattern and a further pattern, substantially parallel to the first pattern, each comprising a plurality of visually contrasting regions, the patterns being in fixed positions relative one to the other on either side of a light transmissive lamina, the visually contrasting regions of at least one pattern comprising relatively more and relatively less light transmissive regions, the patterns being aligned one to the other such that if viewed together in ttaiuπiission through the said at least one pattern in a given direction relative to a normal to the plane of the patterns an image is produced which appears using noπnal vision to be non-random, the pattern and the further pattern being such that if they are viewed in isolation using noπnal vision each appears substantially random, The invention also relates to a lamina for use in the method. Composite images find particular, though not exclusive, application in the field of security elements for documents of value (such as bank notes, for example) to deter counterfeiting. Simple composite images are employed in bank notes of certain currencies. The nature of the patterns used (for example a segmented D in the case of German bank notes) enables the result of their ∞rnbination to be predicted. The segments arc large in scale. The composite image does not change or disappear with the angle of view.

Such images have only a limited degree of deterrence to counterfeiting. This is because modem double-side photocopying techniques can provide images having a sufficiently good front to back registration to deceive a significant proportion of the public. However, if images are used with very fine scak areas (made up firoin, for example, dots havi g a diameter of approximately 10 mir-xometers), or images which can change with angle of view, the registration of the front and back patterns becomes critical and difficult to achieve. The registration becomes even more difficult if the two patters are on a pair of thin flexible substrates. Such composite images are disclosed for example in WO 9509731.

Unfortr ately, although such structures can make counterfeiting extremely difficult there is an unexpected disadvantage. The properties which make the composite images difficult to counterfeit iresultm a cc∞cmitant increase m fø of πianufacturing the composite images in the first place. The registration and handling of thin flexible films is particularly difficult. This can make the use of such composite uneconomic.

RECTIFIED SHEET RUL£ 91

Solutions to similar problems in the past have concerned making machines capable of feeding a plurality of sheets for processing in good registration, as described for example in US patent number 5,335,572. Such machines are expensive and difficult to maintain because of the number of moving parts. It is an object of the present invention to provide a method of making a composite image as described in WO 9509731.

According to a first aspect of the invention there is provided a method of forming a composite image as defined in the first paragraph above, characterised in that the composite image is foπned using a lamina having a first layer sensitive to Ught in a first wave band, a further layer sensitive to light in a different wave band, and an at least partly transparent layer provided therebetween, the method comprising a) iUuminating the lamina with light from a master image, the master image consisting of at least two elemental areas each having a respective colour, the colours being such that the first layer of the lamina is sensitive to light from at least one elemental area and the further layer of the lamina is sensitive to light from, at least one clher elemental area, thereby forming a first latent pattern in the first layer and a further latent pattern in the further layer, and b) developing the latent patterns thereby foπning a first pattern and a further pattern which together constitute a composite image arrangement.

The two patterns can thus be generated almost simultaneously in registration on either side of a light transmissive lamina in a single process which is suitable for large scale manufacturing, The first and further patterns appear substantially random when viewed in isolation . This can give the advantage of making the composite image more difficult to counterfeit

Preferably, the size of the elemental areas and the thickness of the at least partly transparent layer are such that the composite image is visible only for a range of angles to a given direction relative to the normal to the plane of the patterns.

According to a second aspect of the invention there is provided a larninafor iecording composite images as defined in the first paragraph above, comprising a first layer sensitive to light in a first wave band, a further layer sensitive to light in a different wave band, and an at least partly transparent layer provided therebetween.

The at least partly transparent layer is preferably constituted by a sheet which carries the first and further light sen^v layers on opposite n ajor smiaces. This can make a further supporting substrate unnecessary.

A lamina of this general type for recording X-ray images is disclosed in patent number US 4,585,729. In this case the lamina is a direct X-ray film which is designed to be placed between two sheets of a scintillator which emit visible light when struck by an X-ray photon, and the photosensitive emulsion layers on either side of the supporting substrate are designed to respond to a broad band of visible wavelengths to incxeaβe the film speed. Thus the layers on either side of the support are sensitive to light having the same colour. Such a lamina would not form a composite image as described above if used in the present invention.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which:- Figure 1 shows an image pattern.

Figure 2 shows a pattern appearing random using normal vision. Figure 3 shows a farther pattern appearing random using noπnal vision. Figure 4 shows a simulation of the image obtained by aligning fig. 2 and fig. 3 and viewing together in transmission.

Figure 5 shows a cross-section of a lamina,

Figure 6 shows a flow diagram of the method used to make the lamina of fig. 5.

Figures 1-5 are taken from WO 9509731, and therefore correspond to known composite image arrangements.

In figure 5, a composite image arrangement has a pattern comprising visually contrasting regions having a plurality of relatively more and less light transmissive regions numbered 3 and 2 respectively, and a further pattern having visually contrasting regions comprising a plurality of relatively more and less light transmissive regions numbered 4 and 5 respectively, arranged in fixed positions on either side of a light transmissive lamina 1. The patterns both appear random when viewed in isolation usώg normal vision. The patters are aligned one to the other to give a non-random image (not shown in this Figure) when viewed from position 7 by light from a source 6 passing through the light transmissive lamina 1. Examples of a pattern and a corresponding fiirtter pattern are shown in figure, 2 and figure 3 respectively. Figure 2 comprises a random array of rectangular regions or pixels which have been printed onto a surf-U-β of a white paper sheet. The regions are either black or colourless expending upon whether ink has been printed in the region or not Figure 3 also comprises a random array of pixels of the same size (i.e. 50 per square inch) as those in figure 2. Although both patterns are raαlom information has been encoded into Figure 3 such that when the patients are carefully aligned and viewed in transmission, and image similar to that shown ύ Figure 4 (viz. CRL) is seen. Figure 4 is a

computer simulation of the combination of the patterns in Figures 2 and 3 when aligned and viewed together in transmission.

The patterns are made using a known method comprising the following steps, (a) providing a pattern comprising relatively more and less light transmissive regions, the pattern appearing random using normal vision (i.e. Figure 2), (b) providing an image pattern comprising relatively more and less tight transmissive regions or visually contrasting regions, the image pattern appearing non-random using normal vision (i.e. Figure 1), (c) combining the pattern and the image pattern using a combining function forming a further pattern comprising relatively more and less light transmissive regions, the further pattern appearing random using normal vision (i.e. Figure 3), and (d) aligning the first-mentioned pattern and the further pattern to give an approximation of the image pattern (i.e. Figure 4) when viewed together in transmission.

The combining function used to generate, for example, Figure 3 from Figure 1 and Figure 2 is the "exclusive or" function (also known as a modulo two sum). Other more complex functions may be used but are more difficult to generate (for example by programming a computer). By combining Figure 3 with the inverse of Figure 2 (i.e. in which each dark pixel is changed to light transmissive and each light transmissive pixel is changed to dark) a negative image is produced in which the CRL lettering appears dark on a random grey background. The pixel pitch of the two patterns is preferably equal, and is preferably approximately equal to the separation between the two patterns. As an example, with a thickness of 0.004" , the pixels are printed 300 to the inch. When such patterns are carried on either side of a transparent sheet and aligned one to the other to within 0.001", the transmitted image is visible if viewed normal to the surface of the sheet. If however, the patterns are viewed at an angle of 30° to the noπnal the two patterns are apparently displaced by one half pixel. If the first pattern has been appropriately chosen (for example to be random) the transmitted image will effectively disappear. To reproduce this angle dependency, the printing on opposite sides of the bank note must be aligned to better than approximately 0.001 ". This is beyond the capability of existing photocopy technology and thus makes counterfeiting difficult. Although in the above examples the pattern and further pattern have approximately equal numbers of light transmissive pixels and dark pixels, arranging one image to have more dark pixels will make viewing the transmitted pattern easier from the side with the lighter pattern. This will likewise make viewing the image more difficult from the other (darker) side of the lamina. For bank note printing, the patterns of the composite image arrangement preferably printed on either side of the sheet of paper simultaneously using equipment which has been very carefully aligned.

Although in the above examples paper has been used as the light tram missive lamina, other materials such as acetate film or other transparent or translucent materials such as more and less exposed photographic film may be used as alternatives. It is particularly convenient for both patterns to be pixelated - i.e. made up from a plurality of small picture elements which can each have a respective tone or colour value.

Although in the above examples the pixel pitch is preferably equal for the two patterns the arrangement will work even when the pitches are unequal, provided corresponding parts of the two patterns appear in alignment from the position of an observer. In W 9509731 , it is suggested that the patterns are foπned by malting first and further patterns separately, and then aligning the patterns on either side of a light transmissive sheet. The present invention describes a way of producing the pair of patterns simultaneously or in rapid succession on either side of a lamina using a photographic process which enables separate registration to be dispensed with. In figure 6, the block numbered 100 denotes the first step of the method according to the present invention, and the box numbered 101 denotes the second step of the method. An example of a method according to the vention will now be described.

In the first step, a plurality of latent patterns is foπned by illuminating a lamina (as shown in Figure 7) with light from a master image (201), the lamina (200) having a first layer (202) sensitive to light of a first colour (in the present example a photographic emulsion layer 1 micron thick sensitive to blue light only), a further layer (203) sensitive to light of a different colour (in the present example a photographic emulsion layer 1 micron thick sensitive to red light only) and an at least partly transparent layer (in the present case a polyester sheet (204) 50 microns thick which carries the two light sensitive layers on opposite major surfaces thereof) provided therebetween. The master image (201) comprises hght-transmissive elemental areas (220, 221, 222) having different respective colours, plus opaque areas (223). Colliroated light (230) from an optical system (not shown) is made to shine through the master image. This light subsequently passes through the lamina (200). As a result, two different latent patterns (205, 206) are formed almost simultaneously on either side of the light transmissive layer (204)

(depending, of course, on the relative speed of the two emulsion layers and the time taken for the light to pass through the lamina).

Tlie master image may be a single image having a plurality of elemental areas each having a respective colour (in the present example blue elemental areas and red elemental areas and transparent elemental areas), the colours in the roaster image being such that the first layer carried by the lamina is sensitive to light from at least one elemental area and the further layer is sensitive to light from at least one other elemental area, thereby

forming a first latent pattern in the first layer and a further latent pattern in the further layer One such pattern may be that as shown in Figure 4, in which the apparently black areas shown in the Figure in practice have an apparently random pattern of red and blue coloured pixels, and the areas making up the CRL logo in the Figure have an apparently random mixture of red and blue together with transparent pixels which appear white in the Figure. In the embodiment shown in Figure 7 the elemental areas are either red (220), blue (221), transparent (222) or black (223). Of course, instead of having light being transmitted by the master image, the master image may comprise, for example, red, blue and white and black pixels which scatter light from an incident white light source on the same side of the image as that of a camera holding the lamina 200.

As an alternative, the first pattern may comprise black and white areas as shown in Figures 2 but be photographed through a red filter, then the pattern shown in Figure 3 may be photographed through a blue filter by way of a multiple exposure - without moving the photosensitive lamina between exposures and carefully aligning each pattern prior to each exposure.

In the second step of the method, denoted by box 101 in Figure 6, the latent patterns are developed (in the present case by immersing the lamina in a tank of standard photographic developer in the absence of ambient light to which the light sensitive layers are sensitive) to form a first pattern and a further pattern which together constitute a composite image.

The patterns are then fixed in the same way as any other photographic image, rinsed and dried in filtered air.

In the above example the photographic emulsion layers may be positive or negative working. The sensitivity of the layers and the colours of the master image are arranged such that areas in the blue light sensitive layer corresponding to blue elemental areas of the master image appear dark after processing and all other areas in that layer appear transparent. Similarly, areas in the red light sensitive layer corresponding to red elemental areas in the master image appear dark after exposure and all other areas in the red light sensitive layer appear transparent after processing. It will be apparent to people skilled in the manufacture of photographic film materials that the thickness of the photosensitive layers may be varied over ranges extending from a tenth of a micron to several tens of microns if required. The speed of the photosensitive layers will depend more upon the amount of photosensitive material present in the layers than by their thickness. Although in the above example visible light of different colours is used, the wave bands of the light used need not be in the visible pan of the electromagnetic spectrum. For example, ultra-violet light or infra-red light may be used as an alternative or in

addition to visible light. Although in the above example a light transmissive master image is used, the master image need not be light transmissive. For example, it may reflect light of different colours which subsequently is made to impinge upon the lamina alternatively or in addition to being light transmissive. In the above example, the elemental areas of the master image are square dots 30 microns on a side. The elemental areas can, of course, be of any shape and of any size. However, dots between 2 microns across and 300 microns across are preferred. The only real limitation on the smallness of the elemental areas is the resolution of the optical system used to illuminate the master image and lamina. Although the at least partly transparent layer (204) described above was 50 microns thick, many different thicknesses may be employed as alternatives. Layers with thicknesses in the range 3 - 300 microns are preferred for document protection. The inventors have specifically made embodiments with sheets 100 microns thick. Very thin layers would need to be supported to make handling easier. The size of the elemental areas needs to be approximately equal to the thickness of the at least partly transparent layer (to within say a factor of 3 or 4) if a composite image with an appearance which changes with the angle of view is desired. If a non- variable appearance is wanted then the elemental areas may be correspondingly larger than this.

The lamina described in the above description may take the form of a roll of double-sided photographic film, or a sheet of material. Preferably the composite images are produced having lateral dimensions of a few millimetres. Preferably the images are subsequently attached to a document of value (such as a bank note, for example) by a lamination or bonding process. Each image may be unique if desired - for example each having a different serial number.

Although polyester sheet has been specifically described above, other materials such as glass or perspex, or thin flexible sheets of plastics materials may be used as an alternative. The use of such images on window panes may form eye-catching displays suitable for advertising purposes. Such displays will have a thicker light transmissive sheet constituted by the window pane, and the elemental areas in the two patterns must consequently be larger.