Structure density compensation in synthetic image devices

Abstract

A synthetic image device comprises an array of line focusing elements and an image layer (20) having linearly extending cells The image layer is positioned at a distance relative to the array of line focusing elements such that a limited part of the image layer becomes visible when the image layer is viewed through the array of line focusing elements. Each cell comprises at least two sub-cells, provided side-by-side in a transverse direction (T). The sub-cells of the image layer have individual visual-contrast creating items in patterns (21) forming sub-images. Composed images, each associated with a set composed by one sub-image from each cell, are visible from different viewing angles in the transverse direction. Within overlap areas (42E-G), an average pattern density of the visual-contrast creating items in at least two sub-cells giving rise to the overlapping composed images is less than a complete coverage.

Description

[0001] STRUCTURE DENSITY COMPENSATION IN SYNTHETIC

[0002] IMAGE DEVICES

[0003] TECHNICAL FIELD

[0004] The present technology relates in general to synthetic image devices and manufacturing methods thereof and in particular to synthetic image devices based on arrays of line focusing elements.

[0005] BACKGROUND

[0006] Synthetic image devices have for a long time been used e.g. as security items on valuable items, such as bank notes, or as eye-catching features, e.g. in labels. Common for these applications is that the synthetic image devices are difficult to copy but may still provide easily detectable features used to verify the authenticity of the item to which it is adhered.

[0007] If synthetic image devices are to be incorporated or attached to thin articles, such as a bank note, the thickness of the synthetic image device has to be small. In synthetic image devices based on arrays of microlenses, the size of the microlenses and the image features of the image layers that the microlenses depict has to be very small. Smaller sizes in turn put higher requests on registration between image features of the image layers and microlenses. The small size also put some restrictions on what manufacturing methods that can be used.

[0008] Many kinds of optical effects have been used, comprising e.g. switching between different images, “unlogical” movement of images or simple animations. Colour shifts have also been used for enhancing different effects.

[0009] In the published European patent EP 2 707 228 Bl, a lenticular lens device is presented in which the image strips present a cyclically repeating sequence from an image of a first form into an image of a second form and then ending up again at the first form again, upon changing the viewing angle. The combined image sequences present a contiguous variation in the image form. The cyclic change can for instance be an expansion / contraction or a shape change.

[0010] When designing lenticular lens devices presenting switching between different images or simple animation image sequences, the appearance for the viewer depends on how perfect the focusing elements operate. There are manufacturing tolerances that may give un-ideal imaging and even perfectly shaped focusing elements have different kinds of aberrations. A result of such imperfect imaging may be the appearance of “ghost” images when turning the synthetic image device from one viewing angle to another.

[0011] In the published international patent application WO 2022 / 162380 Al, similar effects are noticed. A way to at least partly compensate for this was presented. Image devices are proposed where active zones are placed together with static zones. The static zones have a tone of the same colour as the image in the active zone, which makes it easier to get the image to “disappear” or “appear” without ghost images. Different colours of different image parts are used for giving “combined” colours of different kinds.

[0012] However, there are still requests for improving the appearance for a viewer of optical effects of a synthetic image device.

[0013] SUMMARY

[0014] A general object is to enhance the appearance of optical effects in thin synthetic image devices.

[0015] The above object is achieved by methods and devices according to the independent claims. Preferred embodiments are defined in dependent claims. In general words, in a first aspect, a synthetic image device comprises an array of line focusing elements and an image layer. The line focusing elements is one of lenticular lenses and linear rows of non-lenticular lenses. The line focusing elements extend in a longitudinal direction. The image layer has linearly extending cells extending in the longitudinal direction and having a width in a transverse direction, perpendicular to the longitudinal direction, corresponding to a pitch of the array of line focusing elements in the transverse direction. The image layer is positioned in parallel to the array of line focusing elements and at a distance relative to the array of line focusing elements such that a limited part of the image layer becomes visible when the image layer is viewed through the array of line focusing elements. Each cell comprises at least two sub-cells. The at least two sub-cells are displaced relative to each other in the transverse direction. The at least two sub-cells of the image layer have individual visual-contrast creating items in patterns, preferably patterns of micro geometrical structures, forming sub-images. At least two composed images, each associated with a set composed by one subimage from each cell, are visible from different viewing angles in the transverse direction. The at least two composed images together present an image switch or an animation upon changing the viewing angles. Overlap areas are defined as areas of the image layer forming, when viewed through the array of line focusing elements, at least two composed images seen at a same, overlapping, area at the synthetic image device but in different viewing angles. Within the overlap areas, an average pattern density of the visual-contrast creating items in at least two sub-cells giving rise to the overlapping composed images is less than a complete coverage by the visual-contrast creating items.

[0016] One advantage with the proposed technology is that the appearance of ghost images is counteracted. Other advantages will be appreciated when reading the detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which:

[0018] FIGS. 1A- 1J illustrate the operation of synthetic image devices based on an array of line focusing elements;

[0019] FIG 2A illustrates schematically an embodiment of a change in composed images upon tilting of a synthetic image device;

[0020] FIG 2B illustrates schematically overlap areas of a synthetic image device;

[0021] FIGS 3A-3B illustrate parts of different overlap areas of an embodiment of an image layer of a synthetic image device having three sub-cells per cell;

[0022] FIGS 4A-4C illustrate parts of different overlap areas of an embodiment of an image layer of a synthetic image device having four sub-cells per cell; and

[0023] FIGS. 5A-D illustrate schematically different shapes and distributions of part-sub-cell areas free from visual-contrast creating items in a synthetic image device.

[0024] DETAILED DESCRIPTION

[0025] Throughout the drawings, the same reference numbers are used for similar or corresponding elements.

[0026] For a better understanding of the proposed technology, it may be useful to begin with a brief overview of the basic features of a synthetic image device based on an array of line focusing elements.

[0027] Figure 1A illustrates a view of a part of an image layer 20 of a synthetic image device 1. The image layer 20 has linearly extending cells 22 extending in a longitudinal direction L. The cells 22, each has a width W in a transverse direction T, perpendicular to the longitudinal direction L. Each cell 22 comprises in this embodiment three sub-cells 24A, 24B, 24C, provided displaced relative to each other in the transverse direction. In a typical case, each sub-cell is a rectangular sub-cell, whereby the rectangular sub-cells are provided side-by-side in the transverse direction, Such sub-cells will, due to its simplicity, be used in this disclosure during the explanation of the present ideas. It is also presently considered as a preferred embodiment.

[0028] However, there are also other plausible designs of sub-cells. A sub-cell may e.g. be composed by e.g. a number of hexagonal, or otherwise shaped, parts provided along a line parallel to the longitudinal direction. The individual parts of the different sub-cells may be intervening in the transverse direction, but the average position of the parts in the different sub-cells is displaced relative to each other in the transverse direction.

[0029] In Figure 1A, three sub-cells are illustrated. In other embodiments, two subcells or more than three sub-cells can be used. The sub-cells 24A, 24B, 24C of the image layer 20 have individual visual-contrast creating patterns, preferably patterns of micro geometrical structures 21 , forming sub-images, in the figure illustrated as hatchings. Different hatchings correspond to subimages giving rise to different composed images (see further below.) The micro geometrical structures 21 may cover the entire sub-cell 24 or a part thereof. Note that part illustrated in the figure is only a very small portion of the entire synthetic image device and furthermore that the size of the features in the figure is enormously exaggerated compared to typical synthetic image devices, where the width W of the cells 22 typically may be less than 200 pm, and preferably much less than that. These figures are provided for illustrating the basic operation.

[0030] Figure IB illustrates a part of an array 10 of line focusing elements 11 of a synthetic image device 1. In this embodiment, the line focusing elements 11 are lenticular lenses 12. As will be discussed further below, the line focusing elements 11 may alternatively be linear rows of non-lenticular lenses. The line focusing elements extend in the longitudinal direction L and a pitch of the array of line focusing elements 11 in the transverse direction T is essentially equal to the width W of the cells in the image layer 20, c.f. figure 1A.

[0031] Note also that, although not illustrated in Figure IB, the width W of the array 10 of line focusing elements 11 can be narrower than the pitch, leaving a small gap between the lenses.

[0032] In alternative embodiments, there may also be a top coat covering the array 10 of line focusing elements 11.

[0033] As illustrated in a cross-sectional view in Figure 1C, in the synthetic image device 1, the image layer 20 is positioned in parallel to the array 10 of line focusing elements 11 and at a distance D relative to the array 10 of line focusing elements 11 such that a limited part of the image layer 20 becomes visible when the image layer 20 being viewed through the array 10 of line focusing elements 11. The dotted lines in the figure illustrates a viewing direction from the left side, which selects the sub-cells 24C to be viewed through the array 10 of line focusing elements 11.

[0034] The result of such a viewing angle is illustrated in Figure ID. The figure illustrates the appearance of the synthetic image device 1 as seen from the side of the array 10 of line focusing elements 11. The respective sub-cells of the different cells of the image layer are viewed to occupy the entire width of the associated focusing element 11, forming a slightly enlarged sub-image 24C*. A composed image 23A is thus created associated with a set composed by one sub-image 24C* from each cell of the image layer. The composed image 23A in this figure is thus visible when the image layer is viewed through the array 10 of line focusing elements 11 in the angle in the transverse direction T illustrated in Figure 1C.

[0035] Figure IE illustrates the synthetic image device 1, indicated to be viewed in another angle in the transverse direction T. In this viewing angle, the array 10 of line focusing elements 11 here depicts the sub-cells 24B. The result is illustrated in Figure IF, where a new composed image 23B is seen based on enlarged sub-image 24B*.

[0036] Figure 1G illustrates the synthetic image device 1, indicated to be viewed in yet another angle in the transverse direction T. In this viewing angle, the array 10 of line focusing elements 11 here depicts the sub-cells 24A. The result is illustrated in Figure 1H, where a new composed image 23C is seen based on enlarged sub-image 24A*.

[0037] As a summary, the image layer 20 is positioned in parallel to the array 10 of line focusing elements 11. The distance D relative to the array 10 of line focusing elements 11 is such that a limited part of the image layer 20 becomes visible when the image layer 20 is viewed through the array 10 of line focusing elements 11. Composed images 23A-C are seen in different viewing directions. In this embodiment, there are three composed images 23A-C, where each composed image 23A-C is associated with a set composed by one sub-image 24A-C from each cell 22. The composed images 23A-C are thus visible from different viewing angles in the transverse direction T. In other embodiments, where there are only two sub-cells in each cell, or more than three sub-cells in each cell, two composed images and more than three composed images, respectively, are provided.

[0038] As mentioned above, the line focusing elements 11 can be constituted by linear rows of non-lenticular lenses 13, as illustrated in Figure II. The non-lenticular lenses 13 will, in addition to the focusing effect in the transverse direction T, also provide a focusing effect in the longitudinal direction. However, if the subimages 24A-C have a variation in the longitudinal direction L that has typical sizes that are larger than the size of the non-lenticular lenses 13, this will not influence the composed image considerably. Thus, for composed images with large structures non-lenticular lenses 13 will give essentially the same result as line focusing elements 11 constituted by lenticular lenses. As mentioned above, the shape of the sub-cells does not need to be rectangular. As exemplified in Figure 1J, a sub-cell 24A-F may therefore be composed of a number of non-rectangular parts 25, provided along the longitudinal direction L. The average position in the transverse direction T is shifted between the sub-cells 24A-F. This may also be used to overcome some limitations due to the resolution of the technology used to define the patterns. The parts 25 of the different sub-cells 24A-F may also overlap in the transverse direction T. This may be beneficial, especially if a higher number of composed images is desired. In Figure 1J, six sub-cells 24A-F are provided within the same width W that without overlap only should have had place for four subcells. However, for illustrational simplicity, in the rest of this disclosure, subcells comprising one rectangular sub-cell each, which abuts with neighbouring sub-cells are used as examples of sub-cells.

[0039] The composed images 23A-C can in a general application be any type of composed images 23A-C. If the composed images 23A-C are considerably different, the viewer will experience a relatively abrupt shift or switch between the different composed images 23A-C when tilting the synthetic image device 1 in the transverse direction T. However, in other applications, the viewer will experience a smoother transition between the different composed images 23. This can be achieved by having composed images that are related to each other, changing in position, direction, size and / or shape in small steps between neighbouring sub-cells. In the present technology, the composed images, at least three, together present an image switch or an animation upon changing said viewing angles.

[0040] Note that the hatchings in Figures 1A- 1I are illustrated differently for the different images, intended for an increased understanding of the basic functionality. In a typical case, the different micro geometrical structures 21 in the sub-images may be of a same constitution.

[0041] The micro geometrical structures 21 can be produced by different methods. For example, they can be formed on a polymer-based film as voids in a radiation cured resist, and subsequently filled or coated using a pigmented coloring material. They can also be produced by means of intaglio-type methods such as described in the published international patent applications WO 2014 / 070079 Al and WO 2011 / 102800 Al. Any other known methods giving rise to a visual contrast can of course also be used. The visual contrast can be due for example to diffraction, differences in refractive indexes, difference in reflection (for example by means of a metal coating), etc.

[0042] In one embodiment, the patterns of micro geometrical structures 21 are provided as printed features, and / or recesses that are coated and / or filled.

[0043] The typical application of this type of synthetic image device benefits from a very thin device. It is therefore preferred to produce the synthetic device so that it has a thickness smaller or equal to 200 pm, or even more preferably smaller or equal to 50 pm.

[0044] The width, in the transverse direction, of the cells corresponds to a pitch of the array of line focusing elements. Each line focusing element creates a subimage extending essentially over its entire surface, for the different viewing directions. In order to keep the device thickness small, the width of the cells cannot be too large. Due to limitations in refractive index of plausible lens material, it will be impossible to get a sufficiently small focus length for a wide cell. A practical limit for the cell width using most common lens materials is at the most 100 pm. For particular selections of materials, the limit may be even smaller.

[0045] In one embodiment, the width of the cells is smaller or equal to 100 pm, preferably smaller or equal to 20 pm, and most preferably smaller or equal to 10 pm.

[0046] Figure 2A illustrates a synthetic image device 1 as seen in three different viewing angles. In a first angle, as illustrated at the top of the figure, a first composed image 30A in the shape of a rectangle is seen. In a second angle, as illustrated in the middle of the figure, a second composed image 30B in the shape of a triangle is seen. In a third angle, as illustrated at the bottom of the figure, a third composed image 30C in the shape of a circle is seen. The bent arrows illustrate a change of viewing angle.

[0047] In Figure 2B, the synthetic image device 1 is schematically illustrated, where contours of the first, second and third composed images 30A-C are indicated. In area 41, no composed image is seen at any viewing angle. In areas 40A, 40B and 40C, only one of the composed images 30A-C may be seen, at different respective viewing angles. However, in overlap area 42A, both the first composed image 30A and the second composed image 30B may be seen, at different respective viewing angles. Also, in overlap area 42B, both the first composed image 30A and the third composed image 30C may be seen, at different respective viewing angles. Also, in overlap area 42C, both the second composed image 30B and the third composed image 30C may be seen, at different respective viewing angles. Finally, in overlap area 42D, all composed image 30A-C may be seen, at different respective viewing angles.

[0048] In other words, the image layer, when viewed through the array of line focusing elements, may form composed images 30A-C seen at a same, overlapping, area 42A-D at the synthetic image device but in different viewing angles. The corresponding areas in the image layer are defined as overlap areas. With reference to Figure 2B, the image layer within overlap area 42A comprises visual-contrast creating patterns in two different sub-cells, associated with the composed images 30A and 30B but no visual-contrast creating patterns in the sub-cell associated with the composed image 30C. The image layer within overlap area 42B comprises visual-contrast creating patterns in two different sub-cells, associated with the composed images 30A and 30C, but no visual-contrast creating patterns in the sub-cell associated with the composed image 30B. The image layer within overlap area 42C comprises visual-contrast creating patterns in two different sub-cells, associated with the composed images 30B and 30C, but no visual-contrast creating patterns in the sub-cell associated with the composed image 30A. The image layer within overlap area 42 D comprises visual-contrast creating patterns in three different sub-cells, associated with the composed images 30A, 30B and 30C.

[0049] As described in the background, “ghost” images may reduce the quality of a composed image. Ghost images can be described as composed images which are slightly visible at viewing angles where they are not intended to be visible. For example, if image 23 A or 23 C are slightly visible at the viewing angle shown in Figure IE, they are denoted as ghost images.

[0050] A general explanation is that the micro geometrical structures 21 present in the sub-cells NOT giving rise to the composed image intended to be seen at a specific angle influence the appearance of this composed image. As mentioned in the background, imperfections in synthetic imaging devices explain the contributions from the neighboring sub-cells to the displayed composed image.

[0051] Considering e.g. Figure 2B, a well-known consequence of ghost images is for example that areas 40B, 40C and 42C have another appearance than area 41 at the viewing angle where image 30A is displayed. This is maybe the most obvious consequence for an observer, as the contrast between image 30A and its background gets reduced at these locations.

[0052] Another maybe less noticeable consequence is that areas 42A, 42B and 42D present a darker appearance compared to 40A at the viewing angle where image 30A is displayed.

[0053] It has now been found that this latter kind of ghost effects can be avoided or at least reduced if a variation of the pattern density at the image layer can be kept limited in these areas. With reference to Figure 2B, the pattern density within the overlap area 40D is 100%, i.e. a complete coverage of the visualcontrast creating patterns over all three sub-cells, whereas the pattern density in overlap area 42A is about 67%, where a complete coverage of the visualcontrast creating patterns is present in two out of three sub-cells. In the single composed image area 40A, the pattern density is about 33%, where a complete coverage of the visual-contrast creating patterns is present only in one in subcell. Since the composed images rely on visual-contrast creating patterns in the different sub-cells, it is not possible to remove an entire sub-cell to lower the density. However, it has been found that it at the contrary is possible to reduce the pattern density within each sub-cell instead.

[0054] In other words, according to the present technology, within the overlap areas, an average pattern density of the visual-contrast creating items in at least two sub-cells giving rise to overlapping composed images is less than a complete coverage by the visual-contrast creating items.

[0055] In a synthetic image device based on the functionality of an array of lenticular lenses, there is a small magnification effect in the transverse direction in order to magnify the content of one sub-cell to cover the entire lenticular lens width. However, there is no magnification at all in the longitudinal direction. When visual-contrast items are placed very close to each other, the human eye cannot resolve the separate visual-contrast items and the human brain will interpret the image as a continuous item. If parts of an originally continuous visual-contrast item are removed that are smaller than the human eye resolution, the interpretation will just be a reduction in intensity. Since there is no magnification in the longitudinal direction, any removed visual-contrast items smaller than the human eye resolution will give the same effect.

[0056] If linear rows of non-lenticular lenses, e.g. spherical lenses, are used, there is a small magnification also in the longitudinal direction. However, this is typically limited to a magnification in the same order of magnitude as is provided in the transverse direction. An absence of visual-contrast items in the longitudinal direction will then be magnified, but if the original width of this absence is small enough, also the magnified image will be smaller than the human eye resolution. Presently, it is considered that absence of visualcontrast items being less than 20 pm will be useful for both lenticular lenses as well as for linear rows of non-lenticular lenses and still be interpreted as just a reduction in intensity.

[0057] Figure 3A illustrates schematically an enlarged portion of an embodiment of an image layer 20. This image layer 20 comprises cells 22 having three subcells 24A, 24B, 24C each. The hatching represents areas with micro geometrical structures 21 or other visual-contrast creating items. In the left part of the figure, the image layer 20 is illustrated as it would look like if a prior art provision of micro geometrical structures 21 within the sub-cells 24A-C is made. The sub-cells 24A and 24B comprises micro geometrical structures 21 while sub-cell 24C is free from micro geometrical structures 21. This means that the illustrated part is a part of an overlap area 42, where two overlapping composed images are produced in different viewing angles.

[0058] In the right part of the figure, it is illustrated how a corresponding image layer 20 according to the present technology could look like. The sub-cells giving rise to the overlapping composed images, i.e. sub-cells 24A and 24B in Figure 3A, present part-sub-cell areas 26 that are free from visual-contrast creating items. Thereby the average pattern density is reducing below a complete coverage by the visual-contrast creating items within each sub-cell 24A-B in question. In this embodiment, the part-sub-cell areas 26 has the shape of a rectangle 27. Preferably, the part-sub-cell areas 26 each have an extension in the longitudinal direction L of less than 20 pm. The angle between a direction parallel to the parallelepipedal shapes and the longitudinal direction L is typically larger than 45 degree, to avoid any undesired moire effect caused by these part-sub-cell areas.

[0059] Both image layers 20 of Figure 3A will, if viewed through appropriate line focusing elements, give rise to images that will appear the same for a viewer, except for the intensity of the image. The intensity of the image produced by the right-hand image layer 20 will be approximately 75% of the intensity of the image produced by the left-hand image layer 20. The interruptions in the coverage by the visual-contrast creating items will not be resolved by the human eye.

[0060] Figure 3B illustrates schematically another enlarged portion of an embodiment of an image layer 20. This image layer 20 also comprises cells 22 having three sub-cells 24A, 24B, 24C each. The hatching represents areas with micro geometrical structures 21 or other visual-contrast creating items. In the left part of the figure, the image layer 20 is illustrated as it would look like if a prior art provision of micro geometrical structures 21 within the subcells 24A-C is made. All the sub-cells 24A-C here comprise micro geometrical structures 21. This means that the illustrated part is a part of an overlap area 42, where three overlapping composed images are produced in different viewing angles.

[0061] In the right part of the figure, it is illustrated how a corresponding image layer 20 according to the present technology could look like. The sub-cells giving rise to the overlapping composed images, i.e. all sub-cells 24A-C in the figure, present part-sub-cell areas 26 that are free from visual-contrast creating items. Thereby the average pattern density is reducing below a complete coverage by the visual-contrast creating items within each sub-cell 24A-C in question. In this embodiment, the part-sub-cell areas 26 has a parallelepipedal shape 28. Preferably, the part-sub-cell areas 26 each have an extension in the longitudinal direction L of less than 20 pm.

[0062] Both image layers 20 of Figure 3B will, if viewed through appropriate line focusing elements, give rise to images that will appear the same for a viewer, except for the intensity of the image. The intensity of the image produced by the right-hand image layer 20 will be approximately 66% of the intensity of the image produced by the left-hand image layer 20. The interruptions in the coverage by the visual-contrast creating items will not be resolved by the human eye. In the embodiments of Figures 3A and 3B, each cell 22 comprises more than two sub-cells 24A-C, provided side-by-side in the transverse direction. The more than two sub-cells 24A-C of the image layer 20 have individual visualcontrast creating items 21 in patterns forming sub-images. More than two composed images, each associated with a set composed by one sub-image from each cell 22, are visible from different viewing angles in the transverse direction.

[0063] An overlap area 42 giving rise to only two overlapping composed images is likely to need a lower degree of manipulation of the pattern density than an overlapping area 42 giving rise to three overlapping composed images. In a preferred embodiment, the average pattern density of the visual-contrast creating items in sub-cells 24A-C giving rise to the overlapping composed images depends on the number overlapping composed images within the device overlap area 42.

[0064] In Figure 4A, an enlarged portion of an embodiment of another image layer 20 is schematically illustrated. This image layer 20 comprises cells 22 having four sub-cells 24A, 24B, 24C, 24D each. Sub-cells 24A and 24B give rise to composed images within this overlap area 42. A reduction of the average pattern density similar to the one in Fig. 3A may be performed, as shown in the right part of the figure.

[0065] In Figure 4B, another enlarged portion of an embodiment of an image layer 20 is schematically illustrated. This image layer 20 also comprises cells 22 having four sub-cells 24A, 24B, 24C, 24D each. Sub-cells 24A and 24C give rise to composed images within this overlap area 42. A reduction of the average pattern density, as shown in the right part of the figure, may be performed also here. However, despite the same number of overlapping composed images, there might be a difference in the cross-dependency of the different sub-cells. In Figure 4A, the sub-cells 24A, 24B contributing to the composed images are situated as closest neighbours, whereas in Figure 4B, the sub-cells 24A, 24C contributing to the composed images are separated by empty sub- cells 24B, 24D. A different degree of reduction may therefore be applied in the different cases. In Figure 4B, the size of the part-sub-cell areas 26 are somewhat smaller than in Figure 4A to compensate for the difference in crossdependency of the sub-cells in the respective embodiments.

[0066] In Figure 4C, yet another enlarged portion of an embodiment of an image layer 20 is schematically illustrated. The cells in this embodiment have four subcells. Here, in this portion, four areas of different properties are present. In area 40 only one composed image may be seen, at a specified viewing angle. Therefore, only one sub-cell in each cell presents patterns of micro geometrical structures 21, which is seen as stripes in the longitudinal direction. Here, no reduction of the average pattern density is made, since only one composed image is produced in this area.

[0067] Area 42 E is an overlap area, in which two composed images are produced in different viewing angles by micro geometrical structures 21 in two neighbouring sub-cells. A reduction of the average pattern density in these sub-cells is provided by introducing part-sub-cell areas 26 that are free from visual-contrast creating items. This ends up with a pattern that is two subcells wide in the transversal direction and presenting “empty” part-sub-cell areas 26 interleaved in the longitudinal direction.

[0068] Area 42F is also an overlap area. Also here, two composed images are produced in different viewing angles by micro geometrical structures 21 , but from two non-neighbouring sub-cells. Also here, a reduction of the average pattern density in these sub-cells is provided by introducing part-sub-cell areas 26 that are free from visual-contrast creating items.

[0069] Area 42 G is an overlap area, where three composed images are produced in different viewing angles by micro geometrical structures 21. They are produced by a respective one of three non-neighbouring sub-cells. A reduction of the average pattern density in these sub-cells is provided by introducing part-sub-cell areas 26 that are free from visual-contrast creating items. The degree of reduction is preferably somewhat higher than in the other overlap areas 42E-F, since there are three “contributing” sub-cells.

[0070] One can see that the composed image that will be present in area 40 also is present in the overlap areas 42E-G. Both the composed images of overlap area 42E will also be seen in overlap area 42G, but then also together with a third composed image. Likewise, both the composed images of overlap area 42F will also be seen in overlap area 42G, but then also together with a third composed image.

[0071] In other words, in one embodiment, the average pattern density of the visualcontrast creating items in sub-cells giving rise to the overlapping composed images depends on whether or not there are closest neighbouring sub-cells contributing to the overlapping composed images within the device overlap area.

[0072] In the above presented examples, the part-sub-cell areas are regularly distributed within the image layer overlap areas. This is often a simple way to obtain a requested average pattern density. For synthetic image devices being based on lenticular lenses, this is presently considered as a preferred embodiment.

[0073] However, in other embodiments, the part-sub-cell areas may be irregularly distributed within the image layer overlap areas. This may e.g. be the case when using linear rows of non-lenticular lenses. In such cases, if a regular pattern of part-sub-cell areas has a pitch that is close to the pitch of the linear rows of non-lenticular lenses in the longitudinal direction, different kinds of moire effects may arise. By spreading the part-sub-cell areas irregularly, such risks are eliminated. This kind of part-sub-cell areas can of course also be used with lenticular lenses, even if there is no direct benefit.

[0074] If the part-sub-cell areas without visual-contrast creating items are very small, in the order to less than 1 pm, and repeated with a regular period, they may give rise to different kinds of optical effects. If this is not the intended use for the part-sub-cell areas, in a preferred embodiment, the part-sub-cell areas each have an extension in the longitudinal direction of at least 1 pm and are repeated with a period in the longitudinal direction of at least 2 pm, preferably at least 4pm.

[0075] In the above examples, the part-sub-cell areas have straight edges. This is often easily applicable in different printing techniques and is presently considered as a preferred embodiment. Such part-sub-cell areas may then e.g. have a parallelepipedal shape, a rectangular shape, hexagonal shape etc. However, the shapes of the part-sub-cell areas 26 may also be varied in many other ways. Figure 5A shows circular part- sub-cell areas 26. Since the circular part-sub-cell areas 26 are not extended over the entire width of the sub-cell, more intensity is provided closer to the sub-cell border.

[0076] If the sub-cells contributing to the composed images are situated as closest neighbours, as for example seen in Figure 5B, it is possible to use rectangular shapes, hexagonal shapes etc. seen as apparently being extended over several sub-cells. This can be beneficial when the dimensions of the sub-cells are similar to the technology used to define the micro geometrical structures, which makes it difficult to define shapes within each sub-cell individually.

[0077] Figure 5C illustrates an embodiment, where differently shaped part-sub-cell areas 26 are used in different sub-cells. This can e.g. be used when more than two neighbouring sub-cells have visual-contrast creating items to modulate an intensity in the transverse direction.

[0078] As illustrated in Figure 5D, the part-sub-cell areas 26 may also differ in size and shape within a sub-cell 24. The person skilled in the art realizes that there are uncountable variations that may be utilized. However, the part-sub-cell areas 26 should together give the requested reduction in average pattern density. As indicated above, the magnitude of the reduction of the average pattern density is preferable dependent on the number or percentage of sub-cells having visual-contrast creating items in the overlap area, and possibly also on if these sub-cells are directly neighbouring sub-cells or not. The optimum magnitude will also depend on the quality and properties of the array of line focusing elements that is used. Therefore, an absolute optimum magnitude is not possible to determine in a general case. However, in experiments it has been found that reasonably good results have been achieved for many types of line focusing elements in certain ranges.

[0079] In one embodiment, for a synthetic image device where each cell comprises three sub-cells, the average pattern density within image layer overlap areas associated with two overlapping composed images is preferably in the interval of 50-80% of a complete coverage by the visual-contrast creating items in each sub-cell. Likewise, the average pattern density within image layer overlap areas associated with three overlapping composed images is preferably in the interval of 30-60% of a complete coverage by the visual-contrast creating items in each sub-cell.

[0080] In one embodiment, for a synthetic image device where each cell comprises four sub-cells, the average pattern density within image layer overlap areas associated with two overlapping composed images is preferably in the interval of 50-70% of a complete coverage by the visual-contrast creating items in each sub-cell. The average pattern density within image layer overlap areas associated with three overlapping composed images is preferably in the interval of 40-60% of a complete coverage by the visual-contrast creating items in each sub-cell. The average pattern density within image layer overlap areas associated with four overlapping composed images is preferably in the interval of 30-50% of a complete coverage by said visual-contrast creating items in each sub-cell.

[0081] The reduction of the average pattern density in different sub-cells over the area of the synthetic image device thus gives a reduced variation of the average pattern density over all parts of the synthetic image device that is involved in image composing. In the examples of Fig. 3A and 3B, areas with one sub-cell giving a composed image have an average cell coverage of 33% (not shown), areas as in Fig. 3A have an average cell coverage of 50% and areas as in Fig. 3B have an average cell coverage of 66%. This average cell coverage is calculated by multiplying the ratio of sub-cells contributing to the composed images by the sub-cell coverage. The total variation of the image-contributing areas is thus between 33% and 66%, compared with a variation between 33% and 100% for a prior art device. This gives in general a more attractive behavior of changing images.

[0082] For instance, for a case with 4 sub-cells, an area with one composed image is unmodified, giving an average cell coverage of 25%. An area with two combined images and a reduction of 50-70% as indicated above, gives an average cell coverage of 25-35%. An area with three combined images and a reduction of 40-60% as indicated above, gives an average cell coverage of 30-45%. An area with four combined images and a reduction of 30-50% as indicated above, gives also an average cell coverage of 30-50%.

[0083] For a more general case with a non-specified number of images, it has been found that the average cell coverage of all areas with several combined images preferably should match the average cell coverage of the area with only one composed image. The average cell coverage of all areas with several combined images should preferably not be more than 2 times larger than the average cell coverage of the area with only composed image.

[0084] In other words, preferably, the average cell coverage of all overlap areas, i.e. areas with several combined images, should be within 100%-200% of the average cell coverage of the area with only one composed image.

[0085] It was also found that, in particular when different kinds of intaglio-type printing were used for producing the image layers, the reduced variation in average pattern density also facilitated such printing operations. It is easier to print a high-quality image layer without too large differences in pattern density over the surface.

[0086] In some embodiments, in particular when the cells of the image layer comprises many sub-cells, overlap areas with just two or a few sub-cells that are active in the image composing may appear to have a too low total intensity. In such cases, it may instead be of interest to increase the overall density of visual-contrast creating items. In such cases, small areas of visual-contrast creating items may be distributed over the sub-cells that are not expected to give any composed image. If the areas are small enough, smaller than the resolution of the human eye, no additional image will be created. Instead, the average intensity level will increase somewhat.

[0087] In other words, in one embodiment, an average pattern density, within the image layer overlap areas, of the visual-contrast creating items in a sub-cell not giving rise to the overlapping composed images is larger than zero.

[0088] Such increase of the average pattern density may also be performed in areas where only one composed image is to be seen, c.f. areas 40A-C of Figure 2B. In other words, in one embodiment, within image layer areas where only one sub-cell contributes to a composed image, an average pattern density of the visual-contrast creating items in a sub-cell not giving rise to the any composed images is larger than zero.

[0089] The term “average pattern density” is in the present disclosure used in the meaning of the degree in which a certain area is covered in average by the pattern. This is in agreement with the usual meaning, where the word “density” typically has the meaning of “the degree of compactness”. A “pattern” may have different structural appearances, presenting areas in which the actual pattern is present, separated by areas in which the pattern is missing. A “pattern density” will therefore have the meaning of the degree in which a certain area is covered by the pattern. This is furthermore additionally explained by the word “average”, since the local “pattern density” may differ in different part areas.

[0090] In the present description, this degree in which a certain area is covered by the pattern is exemplified in different embodiments e.g. as the percentage of the area that is covered by a pattern. For instance, in Figure 3A, left part, two of the sub-cells 24A, 24B are completely covered, thus having a 100% coverage, while sub-cell 24C is empty and thus has a 0% coverage. In the right side of Figure 3A, the coverage in the sub-cells is modified. The sub-cells 24A- B now have part areas 26 that are not covered. These part areas cover about 25% of the entire sub-cell area, and the sub-cell 24A therefore has an average pattern density of 75% compared to a fully covered area. In page 13, lines 28 ff, it is explained that this gives an intensity to the viewer of approximately 75% compared to the fully covered sub-cell.

[0091] The non-covered part areas may assume very different shapes. In figures 5A- D, some examples of differently shaped non-covered part areas are presented. The area ratio between covered and non-covered part areas will then be a measure of the average pattern density.

[0092] According to the present invention, it has been found that the appearance for a viewer becomes more attractive if this average pattern density is modified, depending on what coverage degrees are present in adjacent sub-cells. Preferred coverage percentages are presented in the description, depending on how the adjacent sub-cells are constituted.

[0093] In this aspect, the term “average pattern density” seems to be an adequate quantity for achieving the technical effect as well as being clear and concise in its meaning, since it follows the general definition of the words comprised in the phrase.

[0094] The embodiments described above are to be understood as a few illustrative examples of the present invention. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the scope of the present invention. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible. The scope of the present invention is, however, defined by the appended claims.

Claims

CLAIMS1. A synthetic image device (1) comprising:- an array (10) of line focusing elements (11); said line focusing elements (11) being one of lenticular lenses (12) and linear rows of non-lenticular lenses (13); wherein said line focusing elements (11) extend in a longitudinal direction (L);- an image layer (20) having linearly extending cells (22) extending in said longitudinal direction (L) and having a width (W) in a transverse direction (T), perpendicular to said longitudinal direction (L), corresponding to a pitch of said array (10) of line focusing elements (11) in said transverse direction (T); said image layer (20) being positioned in parallel to said array (10) of line focusing elements (11) and at a distance (D) relative to said array (10) of line focusing elements (11) such that a limited part of said image layer (20) becomes visible when said image layer (20) being viewed through said array (10) of line focusing elements (11); wherein each cell (22) comprises at least two sub-cells (24A-D), said at least two sub-cells (24A-D) being displaced relative to each other in said transverse direction (T); wherein said at least two sub-cells (24A-D) of said image layer (20) have individual visual-contrast creating items in patterns, preferably patterns of micro geometrical structures (21), forming sub-images; wherein at least two composed images (30A-C), each associated with a set composed by one sub-image from each cell (22), are visible from different viewing angles in said transverse direction (T); wherein said at least two composed images (30A-C) together present an image switch or an animation upon changing said viewing angles; wherein overlap areas (42; 42A-G) are defined as areas of said image layer (20) forming, when viewed through said array (10) of line focusing elements (11), at least two composed images seen at a same, overlapping, area at said synthetic image device (1) but in different viewing angles;wherein within said overlap areas (42; 42A-G), an average pattern density of said visual-contrast creating items in at least two sub-cells (24A-D) giving rise to said overlapping composed images is less than a complete coverage by said visual-contrast creating items.

2. The synthetic image device according to claim 1, characterized in that each cell (22) comprises more than two sub-cells (24A-D), provided side-by- side in said transverse direction (T), wherein said more than two sub-cells (24A-D) of said image layer have individual visual-contrast creating items in patterns forming sub-images, wherein more than two composed images, each associated with a set composed by one sub-image from each cell (22), are visible from different viewing angles in said transverse direction (T) .

3. The synthetic image device according to claim 2, characterized in that said average pattern density of said visual-contrast creating items in sub-cells (24A-D) giving rise to said overlapping composed images depends on the number overlapping composed images within said device overlap area (42; 42A-G).

4. The synthetic image device according to claim 2 or 3, characterized in that said average pattern density of said visual-contrast creating items in subcells (24A-D) giving rise to said overlapping composed images depends on whether or not there are closest neighbouring sub-cells contributing to the overlapping composed images within said device overlap area (42; 42A-G).

5. The synthetic image device according to any of the claims 1 to 4, characterized in that said sub-cells (24A-D) giving rise to said overlapping composed images present part-sub-cell areas (26) being free from visualcontrast creating items, thereby reducing said average pattern density below said complete coverage by said visual-contrast creating items.

6. The synthetic image device according to claim 5, characterized in that said part-sub-cell areas (26) each have an extension in said longitudinal direction (L) of less than 20 pm.

7. The synthetic image device according to claim 5 or 6, characterized in that said part-sub-cell areas (26) each have an extension in said longitudinal direction (L) of at least 1 pm.

8. The synthetic image device according to any of the claims 5 to 7, characterized in that said part-sub-cell areas (26) are regularly distributed within said image layer overlap areas (42; 42A-G).

9. The synthetic image device according to claim 8, characterized in that said part-sub-cell areas (26) are repeated with a period in the longitudinal direction (L) of at least 2 pm.

10. The synthetic image device according to any of the claims 5 to 7, characterized in that said part-sub-cell areas (26) are irregularly distributed within said image layer overlap areas (42; 42A-G).

11. The synthetic image device according to any of the claims 5 to 10, characterized in that said part-sub-cell areas (26) have straight edges.

12. The synthetic image device according to claim 11, characterized in that said part-sub-cell areas (26) are parallelepipedal (28).

13. The synthetic image device according to claim 12, characterized in that said part-sub-cell areas (26) are rectangular (27).

14. The synthetic image device according to any of the claims 1 to 13, characterized in that each cell (22) comprises three sub-cells (24A-C), wherein said average pattern density within image layer overlap areas (42; 42A-G) in sub-cells (24A-C) associated with two overlapping composed imagesis in the interval of 50-80% of a complete coverage by said visual-contrast creating items, and wherein said average pattern density within image layer overlap areas (42; 42A-G) in sub-cells (24A-C) associated with three overlapping composed images is in the interval of 30-60% of a complete coverage by said visual-contrast creating items.

15. The synthetic image device according to any of the claims 1 to 13, characterized in that each cell (22) comprises four sub-cells (24A-D), wherein said average pattern density within image layer overlap areas (42; 42A-G) in sub-cells (24A-C) associated with two overlapping composed images is in the interval of 50-70% of a complete coverage by said visual-contrast creating items, wherein said average pattern density within image layer overlap areas (42; 42A-G) in sub-cells (24A-C) associated with three overlapping composed images is in the interval of 40-60% of a complete coverage by said visualcontrast creating items, and wherein said average pattern density within image layer overlap areas (42; 42A-G) in sub-cells (24A-C) associated with four overlapping composed images is in the interval of 30-50% of a complete coverage by said visual-contrast creating items.

16. The synthetic image device according to any of the claims 1 to 15, characterized in that an average cell coverage in all overlap areas (42; 42 A- G) should be within 100%-200% of an average cell coverage of an area (40) with only one composed image.

17. The synthetic image device according to any of the claims 1 to 16, characterized in that said patterns of micro geometrical structures (21) are provided as at least one of:- printed features, and- coated and / or filled recesses.

18. The synthetic image device according to any of the claims 1 to 17, characterized in that said synthetic device has a thickness smaller or equal to 200 pm, preferably smaller or equal to 50 pm.

19. The synthetic image device according to any of the claims 1 to 18, characterized in that said width (W) of said cells (22) is smaller or equal to 100 pm, preferably smaller or equal to 20 pm, and most preferably smaller or equal to 10 pm.

20. The synthetic image device according to any of the claims 1 to 19, characterized in that an average pattern density, within said image layer overlap areas (42; 42A-G), of said visual-contrast creating items in a sub-cell (24A-D) not giving rise to said overlapping composed images is larger than zero.

21. The synthetic image device according to any of the claims 1 to 20, characterized in that within image layer (20) areas where only one sub-cell (24A-D) contributes to a composed image, an average pattern density of the visual-contrast creating items in a sub-cell (24A-D) not giving rise to the any composed images is larger than 0%.

Images