SECURITY DEVICES AND METHODS OF AUTHENTICATION THEREOF

Abstract

A security device includes a substrate and a photoluminescent image on or in the substrate. The image includes at least two different visible light emitting photoluminescent quantum dot compositions, each arranged according to different respective photoluminescent sub-images. The at least two different visible light emitting photoluminescent quantum dot compositions have different emission spectra from one another, and the same or different excitation spectra. The at least two different visible light emitting photoluminescent quantum dot compositions emit different respective visible colours from one another when excited. The respective photoluminescent sub-images are configured such that the photoluminescent image formed by the combination of the respective photoluminescent sub-images is multi-coloured, emitting different visible colours in different laterally offset parts thereof upon excitation of the at least two different visible light emitting photoluminescent quantum dot compositions. At least a portion of the photoluminescent image overlaps an at least semi-transparent region of the substrate.

Claims

1-65. (canceled)

66. A security device comprising: a substrate; and a photoluminescent image disposed on or in the substrate, the photoluminescent image comprising at least two different visible light emitting photoluminescent quantum dot compositions, each arranged according to different respective photoluminescent sub-images, the at least two different visible light emitting photoluminescent quantum dot compositions having different emission spectra from one another, and the same or different excitation spectra, the at least two different visible light emitting photoluminescent quantum dot compositions thereby emitting different respective visible colours from one another when excited; wherein the respective photoluminescent sub-images are configured such that the photoluminescent image formed by the combination of the respective photoluminescent sub-images is multi-coloured, emitting different visible colours in different laterally offset parts thereof upon excitation of the at least two different visible light emitting photoluminescent quantum dot compositions; wherein at least a portion of the photoluminescent image overlaps an at least semi-transparent region of the substrate.

67. The security device according to claim 66, wherein the photoluminescent image further comprises a void sub-image in which no visible light emitting photoluminescent quantum dot composition is provided, the void sub-image being defined by and between the at least two different visible light emitting photoluminescent quantum dot compositions.

68. The security device according to claim 67, wherein the at least two different visible light emitting photoluminescent quantum dot compositions include a first photoluminescent quantum dot composition which emits one of red, green or blue light when excited and a second photoluminescent quantum dot composition which emits a different one of red, green or blue light when excited, and the void sub-image corresponds to those parts of the photoluminescent image which require the third one of red, green or blue light, not emitted by either the first or second quantum dot composition.

69. The security device according to claim 68, wherein the first photoluminescent quantum dot composition emits red light when excited, the second photo luminescent quantum dot composition emits green light when excited, and the void sub-image corresponds to parts of the photoluminescent image which require blue light.

70. The security device according to claim 66, further comprising: an optical filter which selectively transmits light of a waveband which excites one or more of the at least two different visible light emitting photoluminescent quantum dot compositions, and wherein the photoluminescent sub-images are arranged such that all of the photoluminescent quantum dot compositions are provided on a first side of the optical filter and at least part of the photoluminescent image overlaps the optical filter.

71. The security device according to claim 70, wherein the visible colour of the waveband of light selectively transmitted by the optical filter is different from each of the visible colours of the at least two different visible light emitting photoluminescent quantum dot compositions when excited.

72. The security device according to claim 69, wherein the visible colour of the wavelength of light selectively transmitted by the optical filter is blue.

73. The security device according to claim 66 further comprising at least one invisible light emitting photoluminescent quantum dot composition.

74. A method of manufacturing a security device, comprising forming a photoluminescent image on or in a substrate, by applying at least two different visible light emitting photoluminescent quantum dot compositions, each arranged according to different respective photoluminescent sub-images, the at least two different visible light emitting photoluminescent quantum dot compositions having different emission spectra from one another, and the same or different excitation spectra, the at least two different visible light emitting photoluminescent quantum dot compositions thereby emitting different respective visible colours from one another when excited; wherein the respective photoluminescent sub-images are configured such that the photoluminescent image formed by the combination of the respective photoluminescent sub-images is multi-coloured, emitting different visible colours in different laterally offset parts thereof upon excitation of the at least two different visible light emitting photoluminescent quantum dot compositions; wherein at least a portion of the photoluminescent image overlaps an at least semi-transparent region of the substrate.

75. The method of manufacturing a security device according to claim 74, further comprising applying at least one invisible light emitting photoluminescent quantum dot composition.

76. The method of authenticating a security device according to claim 66; wherein the at least two visible light emitting photoluminescent quantum dot compositions include: a first photoluminescent quantum dot composition having a first excitation spectra and a first emission spectra; and a second photoluminescent quantum dot composition having a second excitation spectra and a second emission spectra; the method comprising: illuminating the photoluminescent image with light of the first excitation spectra and of the second excitation spectra such that the security device exhibits the photoluminescent sub-images of the first and second photoluminescent quantum dot compositions simultaneously.

77. The method according to claim 76, wherein illuminating the photoluminescent image comprises: positioning the security device between a viewer and a light source emitting light of the first excitation spectra and of the second excitation spectra, such that the portion of the photoluminescent image which overlaps the at least semi-transparent region of the security device is either: illuminated through the at least semi-transparent region; or is visible to the viewer through the at semi-transparent region.

78. The method according to claim 76, wherein the photoluminescent image further comprises a void sub-image in which no visible light emitting photoluminescent quantum dot composition is provided, the void sub-image being defined by and between the at least two different visible light emitting photoluminescent quantum dot compositions, such that when the photoluminescent image is illuminated with light the void sub-image reflects and/or transmits at least one or more wavelengths of the illuminating light.

79. The method according to claim 78, wherein the visible colours emitted by the visible light emitting photoluminescent quantum dot compositions and that of the at least one or more wavelengths of the illuminating light reflected or transmitted by the void sub-image are selected such that when illuminated, the photoluminescent image exhibited by the security device is a full colour image formed by the photoluminescent sub-images and the void sub-image.

80. The method according to claim 79, wherein the void sub-image reflects and/or transmits all visible wavelengths of the illuminating light, the first photoluminescent quantum dot composition emits one of red, green or blue light when excited and the second photoluminescent quantum dot composition emits a different one of red, green or blue light when excited, and the void sub-image corresponds to those parts of the photoluminescent image which require the third one of red, green or blue light, not emitted by either the first or second quantum dot composition, and wherein the illuminating light is the third one of red, green or blue light, not emitted by either the first or second quantum dot composition.

81. The method according to claim 76, wherein the security device further comprises an optical filter which selectively transmits light of a waveband which excites one or more of the at least two photoluminescent quantum dot compositions, the photoluminescent sub-images being arranged such that all of the photoluminescent quantum dot compositions are provided on a first side of the optical filter and at least part of the photoluminescent image overlaps the optical filter, and wherein illuminating the photoluminescent image comprises positioning the security device between the viewer and a light source such that the optical filter is between the photoluminescent quantum dot compositions and the light source.

82. The method according to claim 78, wherein the at least one wavelength of the illuminating light transmitted by the void sub-image corresponds to the waveband transmitted by the optical filter, the first photoluminescent quantum dot composition emits one of red, green or blue light when excited and the second photoluminescent quantum dot composition emits a different one of red, green or blue light when excited, and the void sub-image corresponds to those parts of the photoluminescent image which require the third one of red, green or blue light, not emitted by either the first or second quantum dot composition, and wherein the visible colour of the waveband transmitted by the optical filter is the third one of red, green or blue light, not emitted by either the first or second quantum dot composition.

83. The method according to claim 77, wherein the light source comprises a display screen configured to emit light of the first excitation spectra and of the second excitation spectra across an area corresponding to all or part of the photoluminescent image.

84. A security device comprising: a substrate; and a photoluminescent image disposed on or in the substrate, the photoluminescent image comprising at least two different visible light emitting photoluminescent quantum dot compositions, each arranged according to different respective photoluminescent sub-images, the at least two different visible light emitting photoluminescent quantum dot compositions having different excitation spectra from one another, and the same or different emission spectra; wherein the respective photoluminescent sub-images are each configured to define a different one of a set of image frames which, when excited sequentially, exhibit the photoluminescent image, which is animated.

85. The security device according to claim 84, wherein the at least two different visible light emitting photoluminescent quantum dot compositions have different emission spectra from one another, the at least two different visible light emitting photoluminescent quantum dot compositions thereby emitting different respective visible colours from one another when excited, whereby the animated photoluminescent image is multi-coloured.

86. The security device according to claim 84 further comprising at least one invisible light emitting photoluminescent quantum dot composition.

87. A method of authenticating a security device, the security device comprising: a substrate; and a photoluminescent image disposed on or in the substrate, the photoluminescent image comprising at least two different visible light emitting photoluminescent quantum dot compositions, each arranged according to different respective photoluminescent sub-images, the at least two different visible light emitting photoluminescent quantum dot compositions having the same or different emission spectra from one another, and different excitation spectra; the method comprising the steps of: sequentially illuminating the photoluminescent image with light of different wavelengths, such that the at least two different visible light emitting photoluminescent quantum dot compositions are excited sequentially and the security device exhibits the photoluminescent sub-images sequentially.

88. The method according to claim 87, wherein sequentially illuminating the photoluminescent image comprises: illuminating the photoluminescent image with light at a first wavelength, wherein the first wavelength is within the excitation spectra of a first photoluminescent quantum dot composition of the at least two visible light emitting photoluminescent quantum dot compositions but not within the excitation spectra of a second photoluminescent quantum dot composition of the at least two visible light emitting photoluminescent quantum dot compositions, such that the security device exhibits a first photoluminescent sub-image; and then illuminating the photoluminescent image with light at a second wavelength, wherein the second wavelength is within the excitation spectra of the second photoluminescent quantum dot composition but not within the excitation spectra of the first photoluminescent quantum dot composition, such that the security device exhibits a second photoluminescent sub-image.

89. The method according to claim 88, wherein the steps of illuminating the photoluminescent image with light at the first wavelength and illuminating the photoluminescent image with light at the second wavelength are performed alternately or periodically.

90. The method according to claim 87, wherein the photoluminescent sub-images are each configured to define a different one of a set of image frames which, when excited sequentially, exhibit the photoluminescent image, which is animated.

Description

[0086] Examples of security devices, security articles, security documents and methods for authentication thereof in accordance with the present invention will now be described with reference to the accompanying drawings, in which:

[0087] FIG. 1 depicts a security document according to a first embodiment of the invention, (a) in plan view and (b) in cross-section along the line Q-Q′ shown in FIG. 1(a), and FIG. 1(c) shows exemplary excitation and emission spectra of the QD compositions used;

[0088] FIG. 2 illustrates a security device according to a second embodiment of the invention, (a) in plan view and (b) in cross-section along the line Q-Q′ shown in FIG. 2(a);

[0089] FIG. 3 shows a security device in accordance with a third embodiment of the present invention, (a) in plan view and (b) in cross-section along the line Q-Q′ shown in FIG. 3(a);

[0090] FIG. 4 illustrates a security device in accordance with a fourth embodiment of the present invention, (a) in plan view, (b) in cross-section shown along the line Q-Q′ shown in FIG. 4(a), and FIGS. 4(c)(i), (ii) and (iii) depict three respective sub-images making up the security device shown in FIG. 4(a);

[0091] FIG. 5(a) schematically depicts an example of apparatus suitable for use in a first embodiment of a method for authenticating a security device, and FIG. 5(b) shows the apparatus of FIG. 5(a) in use with an exemplary security document;

[0092] FIG. 6 depicts a security device in accordance with a fifth embodiment of the invention, (a) in plan view, (b) in cross-section along the line Q-Q′, FIG. 6(c) depicting exemplary excitation and emission spectra of the QD compositions used, and FIGS. 6(d)(i), (ii), (iii) and (iv) showing the appearance of the security device in different respective illumination steps;

[0093] FIG. 7 depicts a security device in accordance with a sixth embodiment of the invention, (a) in plan view and (b) in cross-section along the line Q-Q′ shown in FIG. 7(a), FIG. 7(c) depicting exemplary excitation and emission spectra suitable for the QD compositions used, and FIGS. 7(d)(i), (ii) and (iii) show the appearance of the security device in three different illumination steps;

[0094] FIGS. 8(a) and (b) illustrate the exemplary authentication apparatus of FIG. 5(a) used in a second embodiment of a method of authenticating a security device, in two different illumination steps;

[0095] FIG. 9 depicts a seventh embodiment of a security device in accordance with the present invention, (a) in plan view in a first illumination step, (b) in cross-section, (c) in plan view in a second illumination step, FIG. 9(d) illustrating exemplary excitation and emission spectra for the QD compositions used and, in (i) and (ii), six sub-images from which the security device is formed;

[0096] FIGS. 10(a) and (b) illustrate the exemplary authentication apparatus of FIG. 5(a) used to authenticate an exemplary security document carrying the security device of FIG. 9, in two different illumination modes.

[0097] FIGS. 11, 12 and 13 show three exemplary articles carrying optical devices in accordance with embodiments of the present invention (a) in plan view, and (b) in cross-section; and

[0098] FIG. 14 illustrates a further embodiment of an article carrying an optical device in accordance with the present invention, (a) in front view, (b) in back view and (c) in cross-section.

[0099] Throughout the description below, frequent reference will be made to photoluminescent quantum dot compositions or “QD compositions” for short. Quantum dots (“QDs”) are small particles of various semiconductor materials, typically of the order of nanometres in diameter, which emit specific frequencies of light when excited by an incident wavelength to which the particular QD is responsive. The wavelength(s) over which a particular quantum dot will emit (and hence its emitted colour) are defined by its emission spectrum, and the wavelength(s) which will excite it to emit that colour are defined by its excitation spectrum. Both of these spectra can be precisely tuned by changing the size of the quantum dots, their shape and/or their material. Typically, smaller quantum dots (having a diameter between 2 and 3 nanometres, for example) emit colours at the short wavelength end of the visible spectrum (e.g. blue or green) whilst larger quantum dots (having a diameter of between 5 and 6 nanometres, for example) emit longer wavelength colours such as orange or red. Examples of quantum dots suitable for use in embodiments of the present invention are disclosed in US-A-2004/0233465 as well as in EP-A-2025525. For each embodiment described below, quantum dot compositions can be selected from the various types disclosed therein in accordance with the general requirements placed on their emission and/or excitation spectra by each embodiment as explained below.

[0100] All embodiments require the use of at least two different visible light emitting QD compositions, which will emit visible light when excited. Such compositions may or may not also emit light outside the visible spectrum when excited. Throughout the description below, the QD compositions mentioned are of this sort unless explicitly indicated otherwise.

[0101] In all embodiments, it is preferred to select quantum dot compositions with much smaller Stokes shifts than those of conventional fluorescent materials, for example of the order of 50 nanometres rather than around 100 nanometres as is more conventional. This enables the quantum dots to be activated either by visible light or light at the edge of the visible spectrum. This greatly increases the variety of light sources which can be used to activate the quantum dots and for example, a directional torch such as those commonly found on cameras or smartphones could potentially be used as the illuminator. Whilst it is more usual for QDs to have emission spectra at longer wavelengths than their excitation spectra, QDs with “anti-Stokes” shifts are also available, which are excited by wavelengths longer than those they emit.

[0102] The quantum dots are typically contained in an otherwise conventional ink binder composition or similar, which may or may not contain additional substances such as pigments which are visibly coloured under normal ambient lighting conditions. For example, such pigments may be utilised to help conceal the presence of the quantum dots under standard diffuse lighting (e.g. daylight), for example by giving the composition a white or off-white light base colour. In other cases, the compositions may be transparent and preferably colourless under ambient illumination, such that they can be seen through.

[0103] In all embodiments, the various QD compositions can be applied using any convenient technique, such as printing. Conventional security print techniques such as intaglio printing, flexographic printing, lithographic printing and the like can be used, which is particularly desirable where high resolution is the overriding factor. However, due to their small size, quantum dots also lend themselves well to digital printing techniques which do not require the formation of a “master”, such as inkjet printing, diffusion printing and laser printing. Such digital printing techniques are particularly preferred manufacturing techniques for the present invention since this enables the formation of unique and/or personalised security devices, which differ from one instance of the security device to the next, such as passport photos or bibliographic data relating to the holder of the document. Examples will be given below. The various QD compositions forming each security device are preferably applied in sufficiently accurate register with one another such that the different sub-images appear registered to the naked human eye. For instance a registration tolerance of around 100 microns may be acceptable. Techniques for achieving this are well known and available from conventional multi-colour printing techniques.

[0104] FIG. 1 schematically shows a security document 100 having a security device 1 thereon, in accordance with a first embodiment of the invention. The security document 100 is shown in plan view in FIG. 1(a) and in cross-section in FIG. 1(b), taken along the line Q-Q′ depicted in FIG. 1(a). In this example, the security document 100 is a banknote but could be any other document of value such as a passport, identification document, identification card, visa, certificate or the like. The security document may also be provided with additional security features such as security article 90 illustrated, which in this example is a windowed thread emerging on the surface of the security document 100 at spaced windows 95. Security articles such as item 90 can also be used to carry security devices of the sort now disclosed as will be described further below.

[0105] In this embodiment, the security document 100 is provided with a window region 80 which is transparent or translucent relative to the remainder of the document (i.e. it is at least semi-transparent). The construction by which this is achieved in the present embodiment is shown in FIG. 1(b) where it will be seen that the substrate 10 from which the security document 100 is formed has an opacifying layer 12(a), 12(b) on each surface thereof 10a, 10b which is omitted on both sides in the window region 80. The substrate 10 is a transparent or translucent substrate, preferably formed of polymer such as polypropylene (e.g. BOPP), polycarbonate or the like. Most preferably, the substrate is clear and colourless although as discussed below in some embodiments it may incorporate a coloured tint.

[0106] The substrate 10 could be monolithic or may be multi-layered, that is, made up of multiple layers of polymer laminated together. In this embodiment, the security device 1 is wholly applied to a first surface 10a of the substrate 10 but this is not essential and parts thereof may be applied to either the first side 10a or the second side 10b of the substrate 10 as will be discussed further below. Further, if the substrate 10 is a multi-layered substrate, all or part of the security device could be applied to internal layers of the substrate. It should also be noted that while the security device 1 is depicted as being applied directly on to the first surface 10a or the substrate, in practice, one or more intermediate layers may exist between the security device and the substrate 10, such as primer layers to aid adhesion of the security device 1.

[0107] The security device 1 in this example comprises two visible light emitting quantum dot compositions each applied to the substrate 10 in accordance with a respective sub-image 2a, 2b. Together, the sub-images 2a, 2b make up a photoluminescent image 2. In this example, the first sub-image 2a is an upward facing triangle and the second sub-image 2b is a downward facing triangle, with the “peaks” of the two triangular sub-images overlapping one another in the centre such that the resulting photoluminescent image 2 is in the shape of an hourglass. In this example, the security device 1 further includes a working of one or more non-luminescent conventional ink compositions 9 which may be used for example to provide additional detailing and/or colours which are not available from quantum dots (e.g. black).

[0108] The first sub-image 2a is formed of a first quantum dot composition 3a and the second sub-image 2b is formed of a different, second quantum dot composition 3b. Each quantum dot composition 3a, 3b could be applied continuously over the respective sub area 2a, 3b but more typically will be applied in accordance with a pixel array or a screened arrangement as depicted in the cross-section of FIG. 1(b). The sub-images may be half-toned as in conventional colour printing in order to convey variations of shade. Thus, in the portion of sub-image 2a which is not overlapped by sub-image 2b, only pixels of QD composition 3a will be present and in the portion of sub-image 2b which is not overlapped by sub-image 2a, only pixels of QD composition 3b will be present. Meanwhile in the central overlapping portion, pixels of both QD compositions 3a and 3b will be present alongside one another.

[0109] Under standard ambient lighting conditions (e.g. daylight), the photoluminescent image 2 may be invisible or could for example appear as a continuous, single-colour hourglass shape (with a periphery corresponding to the outline of the two overlapping sub-images 2a and 2b). This will depend on whether the QD compositions selected have any visible colour when the QDs are not activated. If the QD compositions 3a and 3b do each have a visible colour, it is preferred that these are selected so as to match one another under normal ambient lighting conditions (e.g. daylight). For example, when the QDs are not activated, both sets of compositions 3a and 3b may appear white or off-white. The concentration of the QDs in the two QD compositions 3a and 3b is preferably selected so that any low level emissions of light from the QDs under normal ambient lighting conditions (e.g. daylight) are concealed or overwhelmed by other light present and hence not noticeable to the naked human eye.

[0110] In this example, the first and second quantum dot compositions 3a and 3b are selected so as to have different emission spectra (λ.sub.em), and excitation spectra (λ.sub.ex) which are the same or at least overlapping. Exemplary emission spectra and excitation spectra for both QD compositions 3a and 3b are shown in FIG. 1(c) which is a plot of intensity (in arbitrary units) against wavelength. It will be seen that the two QD compositions are both excited at wavelengths around 400 to 500 nanometres (i.e. in the deep blue to blue section of the spectrum). However, QD composition 3a when excited will emit at wavelengths in the green portion of the spectrum, whereas QD composition 3b when excited will emit wavelengths in the red portion of the spectrum.

[0111] To view the photoluminescent image 2 and thereby authenticate the security device 1, the security device is preferably illuminated in a transmissive mode as illustrated in FIG. 1(b), in which the security device is placed between the viewer O.sub.1 and an appropriate light source L (which may in practice comprise multiple light sources). In this example, the light source L is configured to emit at least a wavelength λ.sub.1, which as shown in FIG. 1(c) is in the blue part of the spectrum and falls within the overlapping portion of the two excitation spectra of compositions 3a and 3b respectively. Thus, such an illumination activates both of the QD compositions 3a and 3b with the result that the photoluminescent image 2 becomes visible. Now, the portion of first sub-image 2a which is not overlapped by second sub-image 2b will appear green, whilst that of second sub-image 2b which is not overlapped by first sub-image 2a will appear red. The central overlapping portion (2a+2b) will appear yellow, due to the additive mixing of the red and green emitting pixels both present in that area. It should be noted that such simultaneous activation of the first and second QD compositions 3a and 3b could also be achieved by illuminating the security device simultaneously with a wavelength falling inside the excitation spectrum of the first QD composition, and one falling inside that of the second QD composition, e.g. using a broadband light source L.

[0112] It should be noted that the security device could optionally also include one or more invisible light emitting QD compositions, i.e. those which emit only non-visible light (e.g. infrared or ultraviolet) when excited. The excitation spectra of such compositions may be arranged to also overlap the wavelength λ.sub.1 of light source L, or could be configured to be excited at some other wavelength. The emitted light from such compositions will only be detectable by machine and will therefore not compromise the visible image.

[0113] FIG. 2 shows a second embodiment of a security device made according to the same principles and utilising the same first and second QD compositions as described in relation to the FIG. 1 embodiment. Again, the security device 1 comprises a photoluminescent image 2 arranged in a window 80 of a security document 100. In this example, the photoluminescent image 2 includes not only first and second photoluminescent sub-images 2a and 2b, each one formed of a respective quantum dot composition 3a, 3b, but also a void sub-image 4, in which no visible light emitting quantum dot composition is provided. The void sub-image in this case is formed by the absence of any QD composition, but in other examples could be formed by the provision of one or more QD compositions which emit invisible light only. Here, the photoluminescent image is generally circular, formed of two arcuate sections 2a and 2b which together enclose a central circular portion corresponding to the void sub-image 4. The void sub-image 4 can be used to display an additional colour to the viewer upon illumination which in this example will be the colour of the illuminating light itself since there is nothing to modify the visible colour of the incident light in the void sub-region 4. The QD compositions 3a, 3b have the same emission and excitation properties as shown in FIG. 1(c).

[0114] Thus, under ambient lighting conditions such as daylight, as in the case of the FIG. 1 example, the photoluminescent image 2 is either invisible or appears as a single-colour circular area with a hollow centre if the two compositions 3a and 3b have a matching visible colour. When the security device is illuminated with a predominantly blue light having a wavelength λ.sub.1 (corresponding to that shown in FIG. 1(c)), the two QD compositions 3a and 3b will be activated such that the left half of the circle 2a now appears green and the right hand 2b is red. Meanwhile, the centre circle portion defined by void sub-region 4 appears blue.

[0115] It will be appreciated that whilst the FIG. 2 example has been explained using a simplistic graphic for clarity, the presence of all three colours red, green and blue enables the creation of complex full colour images and a further example of this will be shown below. Further, whilst in the example given the QD compositions used have had conventional Stokes shifts (i.e. they are excited by shorter wavelengths than those they emit), and hence the void sub-image 4 has been configured to correspond to the blue portion of the image, this is not essential. In other cases, the two QD compositions could provide any other two of the RGB channels (e.g. blue and green), and the void sub-image could provide the other (e.g. red).

[0116] Another way to create a full colour RGB image is to provide a security device with a third quantum dot composition of appropriate emitting colour, in a corresponding third luminescent sub-image. An example of this is shown in FIG. 3 which depicts a security device according to a third embodiment of the invention. In this example, the photoluminescent image 2 comprises three circular areas partially overlapping one another, corresponding to three sub-images 2a, 2b and 2c. First sub-image 2a is formed of first QD composition 3a, second sub-image 2b is formed of second QD composition 3b and third sub-image 2c is formed of third QD composition 3b. The first and second QD compositions 3a and 3b can be of the same types as already discussed in relation to FIGS. 1 and 2, the properties of which are illustrated in FIG. 1(c). The third QD composition 3c preferably has an excitation spectra in the region around 400 to 500 nanometres such that it at least partially overlaps those of both the first and second QD compositions 3a and 3b, and an emission spectrum which is also in the blue portion of the spectrum, such that upon excitation the third sub-region 2c appears blue.

[0117] On illumination with an appropriate light source L which excites all three quantum dot compositions 3a, 3b and 3c simultaneously, the complete photoluminescent image 2 becomes visible and exhibits the full range of RGB colours. Where the first and second sub-regions 2a and 2b overlap (only) the image will emit yellow light due to additive colour mixing, where the second and third sub regions 2b and 2c overlap (only), the image will emit magenta light due to additive colour mixing, and where the third and first sub-images 2c and 2a overlap (only) the image will emit cyan light, due to additive colour mixing. In the central portion of the device where all three sub-images overlap, the additive colour mixing will result in white light. Again, complex full colour images can now be formed, with any black portions thereof being provided either by regions of the image in which all three QD compositions are absent and/or by the provision of one or more conventional non-luminescent inks such as item 9 shown in FIG. 1.

[0118] An example of a more complex photoluminescent image 2 will now be illustrated with reference to FIG. 4, which depicts a security device 1 in accordance with a fourth embodiment of the invention. FIG. 4(a) shows in plan view the appearance of the photoluminescent image 2 when all of the QD compositions forming it are simultaneously activated. The result is a full colour portrait, preferably of photographic detail. For example, the portrait could be a passport photo showing the holder of the security document. In this example, the person's hair is yellow, shirt collar and jacket blue and cravat green. His face is depicted in various skin tones to convey contour and shading. The complete photoluminescent image 2 is formed in this example of two photoluminescent sub-images 2a and 2b and one void sub-image 4. The arrangement of pixels resulting from these overlapping sub-images is depicted purely schematically in the cross-section of FIG. 4(b). FIGS. 4(c)(i), (ii) and (iii) show the separate sub-images in plan view. The first sub-image 2a is shown in FIG. 4(c)(ii) and corresponds to the green channel of the image. Thus, the first QD composition 3a is laid down in pixels according to the sub-image 2a depicted (it should be noted that in the Figure, light portions of the sub-image 2a indicate a high intensity of the corresponding QD composition and dark portions thereof are low intensity). The second sub-image 2b, corresponding to the red channel of the image, is shown in Figure (c)(i). The second QD composition 3b will be applied in accordance with this sub-image 2b in a typical pixelated manner.

[0119] Finally, the third sub-image shown in FIG. 4(c)(iii) corresponds to the blue channel of the image and here is provided in the form of a void sub-image 4. That is, this sub-image is not itself printed or otherwise applied to the substrate 10, since it is defined by the absence rather than the presence of material. In practice, its area is defined between the bounds of the other sub-images 2a, 2b applied and, if necessary, a conventional non-luminescent ink may be applied in regions to assist in definition of the void sub-image 4. Thus, the first and second photoluminescent sub-images 2a and 2b are applied to substrate 10 leaving gaps defining void sub-image 4 to complete the photoluminescent image 2. Alternatively, the void sub-image 4 could be printed in an invisible light emitting QD composition. In this example, as in previous cases, all of the sub-images are shown applied to the same surface 10a of the substrate 10, but this is not essential.

[0120] For authentication, the security device could be viewed against a blue light of suitable wavelength which both activates the first and second QD compositions 3a and 3b as well as applies a blue colour to the void sub region 4 such that a full colour RGB plus white image is formed using the same principles as in the FIG. 2 embodiment. However, in a further preferred variant, an optical filter 8 is provided which is arranged in use between the photoluminescent image 2 and the light source L. The optical filter 8 transmits one or more wavelengths which collectively will enable activation of the first and second QD compositions 3a and 3b and give rise to the desired colour of the void sub region 4. Thus, in this example, the optical filter 8 preferably transmits blue light therethrough, including the wavelength λ.sub.1. Now, authentication can be done using for example a white light source L such as a standard torch as may be found on a camera or mobile phone for example. Now, the activated first and second QD compositions 3a, 3b and the transmitted blue light in void sub region 4 will together produce the desired full colour photoluminescent image 2 for observer O.sub.1.

[0121] More generally, it should be noted that an optical filter such as item 8 could be provided in embodiments of the invention with or without a void sub-image 4, in which case the visible colour of the light transmitted by the filter may not be a consideration, or may simply be used to suppress background light so as to render the emitted light from the QD compositions more clearly visible. While the optical filter 8 has been depicted as an additional layer applied to the second surface 10b of substrate 10 in the above embodiment, this is not essential and all that is required of the optical filter is that all of the QD compositions are arranged on the same side of it. For example, the optical filter could be located on the same surface of the substrate as the QD compositions (surface 10a in FIG. 4), between the QD compositions and the substrate. In other cases, the substrate 10 itself could act as the optical filter 8 if it contains a suitable filtering material such as a coloured tint. Alternatively if the substrate 10 is multi-layered, the optical filter 8 could be provided by or on one or more of its internal layers. The optical filter 8 could take the form of a printed or coated layer of suitable material, or could be an additional layer, film or foil applied to the structure.

[0122] As mentioned in connection with the FIG. 2 embodiment, whilst the use of blue illumination or a blue filter 8 is advantageous in that the majority of QD compositions require excitation by a shorter wavelength than they emit, this is not essential. More generally, the two QD compositions can provide any two of the three RGB colours, and the void sub-image can correspond to the third colour channel. For example, if one of the QD compositions has a standard Stokes shift and the other has an anti-Stokes shift, the two photoluminescent sub-images could correspond to the red and blue channels of the image while the void sub image corresponds to the green channel, in which either green illumination or a green optical filter would be used to view the complete image.

[0123] As noted above, the disclosed security devices 1 are suitable for authentication using a wide range of illumination sources L. However, a particularly preferred technique for performing authentication of such security devices will now be described with reference to FIG. 5. The method utilises a display screen as the light source L for performing authentication. Any form of display screen which emits light could be used, such as a backlit liquid crystal display, an LED display screen or a cathode ray tube display screen. Suitable widely available display screens include computer monitors (laptop, desktop or otherwise), TV screens and the screens of mobile devices such as mobile telephones and tablet computers. In FIG. 5, an exemplary mobile device 200 in the form of a now standard smartphone is shown, which has a display screen 201. The device 200 can be controlled by way of a suitable software programme or app configured for carrying out the following authentication procedure. To begin authentication, the user may open the correct app on their device, which may present them with a list of options as to the nature of the security document which is to be authenticated. For example, if the app is configured for authenticating British banknotes, the user could be presented with a menu of the existing denominations to select from. Alternatively, if the app is configured for authenticating passports or other ID documents, some information from the document may be input to the device (such as its serial number) which enables the device 200 to look up the relevant record for that specific document on a database, and optionally to retrieve information as to how the security device 1 on that document should appear.

[0124] The device 200 would then be controlled by the program or app to display a user interface such as that shown in FIG. 5(a) which includes an illumination area 205. The illumination area 205 could occupy the entire screen 201 or, optionally, additional features may be provided outside area 205 as illustrated in this example. Thus, beside area 205 a region 210 may be provided for displaying therein a computer-generated copy 211 of the photoluminescent image 2 which the selected security document 100 should reveal on authentication. The screen may also display an indicator 212 identifying the security document in question. The illumination area 205 could be of any size but is preferably large enough such that the whole of the security device 1 can be comfortably accommodated within it.

[0125] The illumination area 205 is then controlled to display the desired illumination wavelength such as λ.sub.1 which here is blue. The particular wavelength or waveband to be displayed in this region will of course need to be selected in dependence on the nature of the QD compositions carried by the security document 100 in question. Thus, the app may contain or have access to a database of the relevant security documents and corresponding illumination wavelengths that should be used for each one. When the user wishes to carry out authentication, as shown in FIG. 5(b), they place the security document 100 over the display screen 201 of device 200 in such a way that the security device 1 is positioned between the display screen 201 and the viewer, within the region of illumination area 205. The security device 1 is thus illuminated in a transmissive mode by the illumination area 205 and the photoluminescent image 2 is activated. The user is thus able to note the presence of the photoluminescent image 2 and confirm the authenticity of the security document 100. To aid authentication, as noted above, the display screen 201 may optionally show alongside the illumination area 205 a computer generated image 211 of how the photoluminescent image 2 should appear, reproducing not only the outline but also its colours. This enables the user to quickly compare the two and judge whether the security device 1 is authentic.

[0126] In all of the embodiments described so far, the QD compositions utilised have had different emission spectra from one another but substantially the same excitation spectra (although this has not been essential since multiple illumination wavelengths could be used simultaneously in the above embodiments to activate the image 2 if necessary). In other embodiments of the invention, as will now be described, it is the excitation spectra of the various QD compositions which must differ from one another whereas the emission spectra can be the same. An example of this will be described in relation to FIG. 6 which depicts a fifth embodiment of a security device in accordance with the present invention. Once again, FIG. 6(a) shows the security device 1 in plan view, and 6(b) in cross-section along the line Q-Q′. In this case, it will be noted from FIG. 6(b) that the substrate 10′ on which the security device 1 is arranged need not be transparent or translucent, and could for example be opaque. For instance, the substrate 10′ could be an opacified polymer substrate or could be a conventional fibre substrate such as paper or cardboard. Nonetheless, transparent substrates and arrangements such as those shown in the previous Figures can also be used. In the arrangement shown, the security device 1 will be viewed by an observer O.sub.1 under reflected light from a source L.

[0127] In this embodiment, the photoluminescent image 2 is configured as a set of frames which when viewed in sequence reveal an animation effect. Thus, the image 2 is not designed to be viewed with all of its sub-images activated simultaneously, but rather only one or a subset thereof at a time. Of course, it is possible to activate all of the sub-images at once but then the image may appear unintelligible. In the present example, the image 2 is made up of four sub-images 2a, 2b, 2c and 2d, each of which is formed by a corresponding QD composition 3a, 3b, 3c and 3d. Each sub-image 2a, 2b, 2c and 2d takes the form of a chevron and the four chevrons are positioned adjacent to one another along the X direction.

[0128] FIG. 6(c) is a plot illustrating the excitation spectra λ.sub.ex and emission spectra λ.sub.em of the four QD compositions 3a, 3b, 3c and 3d. It will be seen that each of the four QD compositions has a different excitation spectra with peak excitation wavelengths λ.sub.1, λ.sub.2, λ.sub.3 and λ.sub.4 respectively. Meanwhile, all four QD compositions have substantially the same emission spectra, which lie in the red portion of the spectra. It will be noted that a single emission spectrum has been shown in this location, which represents that of each of the four QD compositions. Of course, in practice, there may be some differences between the emission spectra of the four compositions but in this embodiment it is preferred that all emit substantially the same visible colour (here, red—although the specific hue might vary).

[0129] To authenticate the security device, the four QD compositions 3a, 3b, 3c and 3d are activated sequentially by appropriate illumination wavelengths. Thus, as illustrated in FIGS. 6(d)(i), (ii), (iii) and (iv), in a first illumination step the security device 1 is illuminated with a wavelength around λ.sub.1, which activates only the first QD composition 3a and none of the others. Under this illumination condition, as shown in FIG. 6(d)(i), the chevron formed by first sub-image 2a is activated and appears red. In the next illumination step, the security device 1 is illuminated with a second wavelength λ.sub.2. This activates only the second QD composition 3b and none of the others. As shown in FIG. 6(d)(ii), now only the chevron corresponding to second sub-image 2b is activated and appears red. In a third illumination step, a third wavelength λ.sub.3 is used to activate only third sub region 2c, as shown in FIG. 6(d)(iii), and in a fourth illumination step, a fourth wavelength λ.sub.4 is used to activate the fourth sub-image 2d as shown in FIG. 6(d)(iv). Thus, as the sequence of illumination steps progresses, the security device 1 appears to display a red chevron shape which moves in the X direction across the device. Of course, the order of illumination steps could be changed, e.g. if performed in the reverse order, the sub-images would be activated from right to left across the device instead, and the chevron would appear to move backwards, in the −X direction. It would also be possible to perform the illumination steps in any random sequence in which case the chevron may appear to jump from one position to another.

[0130] Also possible is to include intermediate illumination steps between any of the illumination steps already mentioned. In the intermediate illumination steps, two or more of the excitation wavelengths λ.sub.1, λ.sub.2, λ.sub.3 and λ.sub.4 may be used simultaneously to illuminate the device to thereby activate two or more of the sub-images. For example, between the steps of activating first sub-image 2a and then second sub-image 2b, it may be desirable to activate both of them to achieve a smoother animation effect.

[0131] FIG. 7 depicts a sixth embodiment of a security device in accordance with the present invention which also exhibits an animation effect upon sequential illumination. The security device 1 is shown in plan view in FIG. 7(a) and in cross-section in FIG. 7(b) along the line Q-Q′ shown in FIG. 7(a). In this case, the security device 1 is provided on a transparent substrate 10 in the same manner as the first to fourth embodiments, but this is not essential. In this example, the photoluminescent image 2 comprises three luminescent sub-images 2a, 2b and 2c which again are configured as a set of frames which when viewed in sequence will produce an animation effect. The first sub-image 2a is in the shape of a sun symbol, the second sub-image 2b in the shape of a star and the third sub-image 2c in the shape of a crescent moon. As in previous embodiments, the first sub-image 2a defines the area within which pixels of the first QD composition 3a are present, the second sub-image 2b defines the area within which pixels of the second QD composition 3b are present and the third sub-image 2c defines the area within which pixels of the third QD composition 3c are present. In regions where the sub-images overlap, pixels of two or more of the QD compositions will be present.

[0132] FIG. 7(c) is a plot illustrating the excitation and emission spectra of the three QD compositions 3(a), 3(b) and 3(c). In this example, the excitation spectra λ.sub.ex of the three QD compositions are different from one another and the emission spectra λ.sub.em of the three QD compositions are different from one another. Thus, not only will the different sub-images be excited by different illumination wavelengths, but they will also appear with different colours once excited. In this example, the first QD composition 3a has an excitation spectrum in the near UV, the second QD composition 3b has an excitation spectrum in the deep blue and the third QD composition 3c has an excitation spectrum in the blue region of the visible spectrum. Whilst the excitation spectra of the respective QD compositions may overlap to some extent, it is desirable at least a portion of each excitation spectrum is not overlapped by any of the others so that wavelengths can be identified which will each activate only one of the compositions, such as wavelengths λ.sub.1, λ.sub.2 and λ.sub.3 identified in the Figure. The first QD composition 3a has an emission spectrum in the blue part of the spectrum and will therefore appear blue on activation, the second QD composition 3b has an emission spectrum in the green part of the visible spectrum and hence it will appear green on activation and the third QD composition 3c has an emission spectrum in the red portion of the visible spectrum and hence will appear red on activation.

[0133] To authenticate the device, the photoluminescent image 2 is sequentially illuminated, preferably in a transmissive illumination mode, with a series of sequential illumination steps similar to that described with reference to FIG. 6. Thus in a first illumination step, the security device 1 is illuminated at a first wavelength λ.sub.1 which activates the first sub-image 2a, which appears as a blue coloured sun shaped symbol as shown in FIG. 7(d)(i). In the next illumination step the security device 1 is illuminated with a second wavelength λ.sub.2 which now activates only the second sub-image 2b and thus the image 2 appears as shown in FIG. 7(d)(ii) as a green star. In a third illumination step, the security device 1 is illuminated at a third wavelength λ.sub.3 which activates only the third sub-image 2c and thus the security device appears as a crescent moon in the colour red. As the sequence of illumination steps progresses, the security device will therefore appear to show a switching effect, changing between the sun, star and moon shapes illustrated in FIG. 7(d). Of course, if desired, a greater number of frames may be provided, each in a respective different QD composition, and if so configured could be arranged to provide a smoother change from one shape symbol to the next, thus giving rise to a morphing effect. It is also possible to provide many other forms of animation effect such as a zooming or contracting effect or the rotation of a 3D object, through appropriate configuration of each sub-image.

[0134] Again, any suitable illumination means could be used to perform the authentication. However, apparatus such that already discussed with reference to FIG. 5(a) is particularly suitable, and FIG. 8 shows how this can be adapted for use with security devices of the sort described with reference to FIGS. 6 and 7. Thus in FIGS. 8(a) and 8(b), a device 200 is shown which has already been described with reference to FIG. 5 and hence will not be described again. However, in this mode of authentication, which might be selected through selection of the security document in question as indicated by indicator 212, the illumination area 205 no longer displays a static wavelength or colour, but now displays a sequence of different wavelengths one after the other, in order to sequentially activate the respective sub-images as described in relation to FIGS. 6 and 7. In FIG. 8(a), a first illumination step is depicted with the user holding the security document 100 against the display screen 201 of the device 200. The illumination area is emitting a first wavelength λ.sub.1 of light which activates a first sub-image of the security device 1 which in this example is a pound sign (“£”). As before, the device may optionally display a computer generated image 211 of the same image for easy comparison. In the next illumination step, as shown in FIG. 8(b), the illumination region 205 switches to emitting a second wavelength of light λ.sub.2. This causes the security device to stop displaying the pound sign and switch to displaying the second sub-image, which here is the digit “20”. At the same time as switching from the first wavelength λ.sub.1 to the second wavelength λ.sub.2, in the illumination area 205, the computer generated image 211 may also switch to show a copy of the newly expected appearance of the security device. The duration of each illumination step and the point at which the emitted wavelength is switched from λ.sub.1 to λ.sub.2 may be controlled by programming of the device and/or by the user; for example the user may press a button on the device 200 in order to advance to the next illumination step. The device 200 may be controlled to alternately switch between the two illumination steps so that the appearance of the device switches repeatedly between the pound sign and the digit 20. Again, if displayed, the computer generated version of the image 211 should switch in a corresponding manner.

[0135] In the FIG. 8 example, the photoluminescent image illustrated includes a symbol and alphanumeric text which in this case will be common to all of the security documents on banknotes of the same series. However, as mentioned above the present invention lends itself well to providing a unique identifier or personalised information, and so in other examples the photoluminescent image could comprise other alphanumeric data, such as a serial number, the document holder's name or date of birth, etc. More generally the image 2 can comprise any graphic, symbol, alphanumeric text, logo, photo or the like.

[0136] It should be noted that any of the embodiments of FIG. 6, 7 or 8 could include a void sub-image 4 and/or an optical filter 8 and/or additional contributions from non-luminescent ink 9, as described in earlier embodiments.

[0137] A seventh embodiment of the invention will be described with reference to FIG. 9. In this case the security device 1 has the appearance of a full colour, animated image, which here takes the form of a portrait (e.g. a passport photo). As shown in FIG. 9(b), the security device 1 is arranged in a window region 80 on a security document which has a transparent substrate 10 carrying opacifying layers 12(a), 12(b) on either side as already described with reference to FIG. 1. In this case, a first part of the security device 1 is arranged on a first surface 10a of the substrate 10 and a second part of the security device is arranged on the second surface 10b of the substrate 10. FIG. 9(a) shows the appearance of the security device in plan view under a first illumination condition, and FIG. 9(c) shows the same security device also in plan view under a second illumination condition. It should be noted that both of these appearances are viewed from the same side of the security device but under different illumination. Thus it is not the case that one represents the front view and the other the reverse view of the device.

[0138] On the first surface 10a of the substrate 10, three sub-images 2a, 2b and 2c are provided, which collectively form a first frame 5a. Each of the sub-images is provided in a different QD composition 3a, 3b, 3c, the emitted colour of which corresponds to the desired colour of that sub-image. In the first frame 5a, the image formed by the three sub-images 2a, 2b, 2c in combination is that of a person looking to the left. On the second surface 10b of substrate 10, another three sub-images 2d, 2e and 2f are provided which form a second frame 5b. Again, each of the sub-images 2d, 2e and 2f is provided by a corresponding QD composition 3d, 3e and 3f. The second frame 5b is also a full colour portrait of the same subject as that of the first frame 5a but now looking to the right. It should be noted that whilst for convenience all of the sub-images forming first frame 5a have been provided on one surface of substrate 10 and all of the sub-images forming second frame 5b have been provided on the other surface of substrate 10, this is not essential. For instance, the sub-images making up frame 5a could be provided on both the top and bottom surfaces of the substrate and likewise so could those making up the second frame 5b. It is also possible to utilise internal layers within the substrate 10 where this is a multi-layer structure.

[0139] FIG. 9(d) illustrates the excitation and emission spectra λ.sub.ex, λ.sub.em of the six QD compositions 3a to 3f used to form the security device 1. FIG. 9(d)(i) shows the three sub-images 2a, 2b and c which make up first frame 5a. Each of these QD compositions 3a, 3b and 3c have an excitation spectra in the deep blue and can be activated by an illumination wavelength λ.sub.1. Their emission spectra sit in the blue, green and red regions of the visible spectrum respectively and hence when all of the QD compositions 3a, 3b and 3c are illuminated by wavelength λ.sub.1 will combine to exhibit a full colour image as shown in FIG. 9(a).

[0140] FIG. 9(d)(ii) shows the three sub-images 2d, 2e and 2f making up second frame 5b of the image 2, and again each is formed by a corresponding QD composition 3d, 3e and 3f. Each of these three compositions has an excitation spectrum in the blue portion of the visible spectrum and can be activated by illumination wavelength λ.sub.2. Again, the corresponding emission spectra lie in the blue, green and red portions of the visible spectrum and hence when these three compositions are activated by an illumination wavelength λ.sub.2, a full colour image will be exhibited as shown in FIG. 9(c).

[0141] It will be appreciated that in this embodiment it is desirable at least for the compositions 3a, 3b and 3c to be invisible when the quantum dots contained therein are not activated, so as not to obscure the view of the underlying frame 5b. To avoid this problem it is also possible to arrange both frames 5a, 5b to be located on the same surface of substrate 10 (e.g. in an interlaced form) provided a sufficiently high resolution application technique is available.

[0142] Again, the security device shown in FIG. 9 can be authenticated using any appropriate illumination source. However, the device already described with reference to FIG. 5(a) is particularly suitable for this purpose and FIG. 10 shows the use of such in authenticating the FIG. 9 device. Thus, in use the user would select the appropriate security document as indicated at 212 and place the security document 100 against the display screen 201 of the device 200. As shown in FIG. 10, in a first illumination step the illumination area 205 will emit a first illumination wavelength λ.sub.1 which activates all of the sub-images of the first frame 5a and not those of the second frame 5b. Thus, the security device exhibits the full colour portrait looking to the left. In a second illumination step, the illumination wavelength switches to λ.sub.2 and now the sub-images making up second frame 5b are activated whilst those making up first frame 5a are not. As such, the appearance of the security device appears to switch with the full colour image portrait looking to the right. Additional frames could be provided by extending the same principles explained above, to create smoother animation effects if desired.

[0143] It will be appreciated that in the present embodiment it will be desirable for the emitted colours of the six quantum dot compositions 3a to 3f to be closely paired so that, for example, the compositions 3a and 3d emit substantially the same blue hue on activation, the compositions 3b and 3e emit substantially the same green hue on activation and the compositions 3c and 3f emit substantially the same red hue on activation. However, some variation here is acceptable and may be accounted for through the configuration of the respective sub-images.

[0144] Whilst the various authentication methods utilising multiple illumination wavelengths have only been described with reference to the use of two illumination steps, it should be appreciated that any number of illumination steps could be implemented in sequence through appropriate control of the device 200 or other illumination source.

[0145] Security devices of the sorts described above can be incorporated into or applied to any item for which an authenticity check is desirable. In particular, such devices may be applied to or incorporated into documents of value such as banknotes, passports, driving licenses, cheques, identification cards etc.

[0146] The security device or article (Le, an element such as a thread or foil carrying the security device) can be arranged either wholly on the surface of the base substrate of the security document, as in the case of a stripe or patch, or can be visible only partly on the surface of the document substrate, e.g. in the form of a windowed security thread. Security threads are now present in many of the world's currencies as well as vouchers, passports, travellers' cheques and other documents. In many cases the thread is provided in a partially embedded or windowed fashion where the thread appears to weave in and out of the paper and is visible in windows in one or both surfaces of the base substrate. One method for producing paper with so-called windowed threads can be found in EP-A-0059056. EP-A-0860298 and WO-A-03095188 describe different approaches for the embedding of wider partially exposed threads into a paper substrate. Wide threads, typically having a width of 2 to 6 mm, are particularly useful as the additional exposed thread surface area allows for better use of devices such as that presently disclosed.

[0147] The security device or article may be subsequently incorporated into a paper or polymer base substrate so that it is viewable from both sides of the finished security substrate. Methods of incorporating security elements in such a manner are described in EP-A-1141480 and WO-A-03054297. In the method described in EP-A-1141480, one side of the security element is wholly exposed at one surface of the substrate in which it is partially embedded, and partially exposed in windows at the other surface of the substrate.

[0148] Base substrates suitable for making security substrates for security documents may be formed from any conventional materials, including paper and polymer. Techniques are known in the art for forming substantially transparent regions in each of these types of substrate. For example, WO-A-8300659 describes a polymer banknote formed from a transparent substrate comprising an opacifying coating on both sides of the substrate. The opacifying coating is omitted in localised regions on both sides of the substrate to form a transparent region. In this case the transparent substrate can be an integral part of the security device or a separate security device can be applied to the transparent substrate of the document. WO-A-0039391 describes a method of making a transparent region in a paper substrate. Other methods for forming transparent regions in paper substrates are described in EP-A-723501, EP-A-724519, WO-A-03054297 and ER-A-1398174.

[0149] The security device may also be applied to one side of a paper substrate so that portions are located in an aperture formed in the paper substrate. An example of a method of producing such an aperture can be found in WO-A-03054297. An alternative method of incorporating a security element which is visible in apertures in one side of a paper substrate and wholly exposed on the other side of the paper substrate can be found in WO-A-2000/39391.

[0150] Examples of such security document and techniques for incorporating a security device will now be described with reference to FIGS. 11 to 14.

[0151] FIG. 11 depicts an exemplary document of value 100, here in the form of a banknote. FIG. 11a shows the banknote in plan view whilst FIG. 11b shows the same banknote in cross-section along the line Q-Q′. In this case, the banknote is a polymer (or hybrid polymer/paper) banknote, having a transparent substrate 10 (corresponding to that shown in FIG. 1(b)). Two opacifying layers 12a and 12b are applied to either side of the transparent substrate 10, which may take the form of opacifying coatings such as white ink, or could be paper layers laminated to the substrate 10.

[0152] The opacifying layers 12a and 12b are omitted across an area 80 which forms a window within which the security device 1 is located. As shown best in the cross-section of FIG. 11b, the security device 1 is arranged on one surface of the substrate 10 although as mentioned above it could be located partly on one surface and partly on the other. In this example the photoluminescent image is of the letters “ABC” which may be provided in different respective QD compositions. It should be noted that in modifications of this embodiment the window 80 could be a half-window with the opacifying layer 12b continuing across all or part of the window over the security device 1. In this case, the window will not be transparent but will still appear relatively translucent compared to its surroundings. Half-windows are less preferred since the opacifying layer will typically introduce a scattering effect which may reduce the intensity of illumination received by the QD compositions if illuminated in a transmissive mode. However acceptable results may still be achievable depending on the desired design. The banknote may also comprise a series of windows or half-windows. In this case the security device could be configured to display different images in different ones of the windows.

[0153] FIG. 12 shows such an example, although here the banknote 100 is a conventional paper-based banknote provided with a security article 105 in the form of a security thread (similar to item 90 in FIG. 1), which is inserted during paper-making such that it is partially embedded into the paper so that portions of the paper 104 lie on either side of the thread. This can be done using the techniques described in EP0059056 where paper is not formed in the window regions during the paper making process thus exposing the security thread in is incorporated between layers of the paper. The security thread 105 is exposed in window regions 81 of the banknote. Alternatively the window regions 81 which may for example be formed by abrading the surface of the paper in these regions after insertion of the thread. The security device 1 is formed on the thread 105, which comprises a transparent substrate, with the QD compositions located on one or both of its surfaces. In the version shown, the thread 105 only emerges to the surface on one side of the paper such that the window regions 81 are half window regions. However, techniques exist for forming the apertures on both sides of the thread 105 so that the windows 81 are full windows, which is preferred.

[0154] If desired, several different security devices 1 could be arranged along the thread, with different or identical images displayed by each. In one example, a first window could contain a first device, and a second window could contain a second device, each having the same or different combinations of OD compositions. In the example shown, the device collectively exhibits the letters “X, Y, Z”, one in each window, which are preferably each formed of different QD compositions.

[0155] In FIG. 13, the banknote 100 is again a conventional paper-based banknote, provided with a strip element or insert 108. The strip 108 is based on a transparent substrate and is inserted between two plies of paper 109a and 109b. The security device 1 is disposed on one side of the strip substrate, although could be on both sides as previously discussed. The paper plies 109a and 109b are apertured across region 82 to reveal the security device 1, which in this case may be present across the whole of the strip 108 or could be localised within the aperture region 101.

[0156] A further embodiment is shown in FIG. 14 where FIGS. 14(a) and (b) show the front and rear sides of the document 100 respectively, and FIG. 14(c) is a cross section along line Q-Q′. Security article 110 is a strip or band comprising a security device 1 according to any of the embodiments described above. The security article 110 is formed into a security document 100 comprising a fibrous substrate 102, using a method described in EP-A-1141480. The strip is incorporated into the security document such that it is fully exposed on one side of the document (FIG. 14(a)) and exposed in one or more windows 83 on the opposite side of the document (FIG. 14(b)). Again, the security device 1 is formed on the strip 110, which comprises a transparent substrate.

[0157] In FIG. 14, the document of value 100 is again a conventional paper-based banknote and again includes a strip element 110. In this case there is a single ply of paper. Alternatively a similar construction can be achieved by providing paper 102 with an aperture 83 and adhering the strip element 110 on to one side of the paper 102 across the aperture 83. The aperture may be formed during papermaking or after papermaking for example by die-cutting or laser cutting. Again, the security device is formed on the strip 110, which comprises a transparent substrate.

[0158] The security device of the current invention can be made machine readable by the introduction of additional detectable materials in any of the components or by the introduction of separate machine-readable layers. Additional detectable materials that react to an external stimulus include but are not limited to infrared absorbing, thermochromic, photochromic, magnetic, electrochromic, conductive and piezochromic materials.

[0159] When a magnetic material is incorporated into the device the magnetic material can be applied in any design but common examples include the use of magnetic tramlines or the use of magnetic blocks to form a coded structure. Suitable magnetic materials include iron oxide pigments (Fe.sub.2O.sub.3 or Fe.sub.3O.sub.4), barium or strontium ferrites, iron, nickel, cobalt and alloys of these. In this context the term “alloy” includes materials such as Nickel:Cobalt, Iron:Aluminium:Nickel:Cobalt and the like. Flake Nickel materials can be used; in addition Iron flake materials are suitable. Typical nickel flakes have lateral dimensions in the range 5-50 microns and a thickness less than 2 microns. Typical iron flakes have lateral dimensions in the range 10-30 microns and a thickness less than 2 microns.

[0160] In an alternative machine-readable embodiment a transparent magnetic layer can be incorporated at any position within the device structure. Suitable transparent magnetic layers containing a distribution of particles of a magnetic material of a size and distributed in a concentration at which the magnetic layer remains transparent are described in WO03091953 and WO03091952.