Patent Publication Number: US-2021165147-A1

Title: Multilayer optical thin film structure

Description:
FIELD OF THE DISCLOSURE 
     The present disclosure relates to a multilayer optical thin film structure and to an item incorporating the same. 
     BACKGROUND TO THE DISCLOSURE 
     Optical thin film security devices (referred to as OTFSDs) are known devices used for anti-counterfeiting of items such as banks notes and high-value documents. Typically, an OTFSD comprises a layered structure configured such that light incident on the layered structure is reflected from the structure with a particular wavelength or range of wavelengths. Due to the interference effect, the colour perceived from the light reflected off the structure varies as a function of the angle with which the OTFSD is viewed. The colourful fringes observed on soap bubbles or oil patches on water are simple examples of the interference effect. 
     OTFSDs are still popular on major bank notes. For example, compared with other optically variable devices, the optical effect of OTFSDs is more defined and is easier to be identified by a common user. In practice, all one needs to do to verify the authenticity of an item to which is applied an OTFSD is tilt the item and observe whether during the titling the colour of the item (i.e. the colour of the ambient light reflected off the item) varies from one specific colour to another specific colour. Other types of commonly used optically variable devices generally do not exhibit such a well-defined colour changing scheme. Taking for example a holographic security device, while the images produced by such a device can show intense colours and colour variation under certain lighting conditions, it can be difficult, even for an expert, to predict and explain the observed colours and colour variation. Consequently, it would be even more difficult for a layperson to recognize that a holographic device on a banknote may be much different from that on a sticker applied to a cheap consumer good, for example. 
     Furthermore, the lighting conditions suitable for observation of OTFSDs tend to be broader than those for other types of optically variable devices. In particular, the colour variation exhibited by an OTFSD is better observed in an environment of diffused light, such as in a brightly lit shopping mall, paned offices and rooms, or even on a street. In contrast, the ideal lighting conditions for diffraction-based security devices are bright point sources, such as would be found in a theatre or a night bar where surroundings are often painted in dark colours and with dotted lights. It is also more difficult and less economical for a counterfeiter to replicate OTFSDs or mimic their effect. 
     The present disclosure seeks to provide an improved OTFSD. 
     SUMMARY OF THE DISCLOSURE 
     In a first aspect of the disclosure, there is provided a multilayer optical thin film structure comprising: multiple optically absorbing layers; and multiple optically non-absorbing layers. The optically absorbing layers and the optically non-absorbing layers are configured such that light incident on the structure is reflected with a red colour at a first angle, and light incident on the structure is reflected with a yellow colour at a second angle. Accordingly, a new OTFSD is proposed, and in particular one in which the optical thin film structure exhibits a red-to-yellow colour variation. 
     Light having a red colour may be light whose dominant wavelength is about 610 nm to about 740 nm. Light having a yellow colour may be light whose dominant wavelength is about 570 nm to about 590 nm. For example, for some angles of observation the colour yellow may comprise the colour gold which includes a yellow hue. 
     An optically absorbing layer may be a layer configured to absorb one or more wavelengths of visible light (i.e. light in the range of 400 nm-700 nm). 
     An optically non-absorbing layer may be a layer having negligible absorption with respect to visible light (i.e. light in the range of 400 nm-700 nm). 
     The optically absorbing layers may comprise semiconducting layers. 
     The optically absorbing layers may comprise one or more of a metal (such as for example tantalum), a metal alloy, a metalloid (such as for example amorphous silicon or germanium), and a nitride. 
     The optically non-absorbing layers may comprise one or more of silicon oxide, aluminium oxide, magnesium fluoride, an oxide, an oxynitride, and a fluorite. 
     The optical thin film structure may further comprise a bonding layer for bonding the structure to a substrate. 
     The optical thin film structure may comprise at least three optically absorbing layers and at least three optically non-absorbing layers. 
     The optical thin film structure may further comprise a substrate to which are applied the optically absorbing layers and the optically non-absorbing layers. 
     The optically absorbing layers and the optically non-absorbing layers may be arranged in alternating order. In some embodiments, two or more optically non-absorbing layers may be directly adjacent one another. 
     The optical thin film structure may further comprise an optically reflecting layer. The optically reflecting layer may be configured such that visible light incident on the optically reflecting layer is substantially not transmitted through the optically reflecting layer, and may be configured such that reflection of visible light is relatively high. For example, the thickness of the optically reflecting layer may be sufficient as to prevent transmission of incident visible light through the reflecting layer. In some embodiments, the optically reflecting layer (for example a metal of high reflectance) comprises a thickness of at least 20 nm, or for example a thickness in the range 20 nm-50 nm. 
     The optically reflecting layer may comprise a substrate to which is applied the stack of optically non-absorbing and optically absorbing layers. 
     The optically reflecting layer may be a layer configured to reflect at least some wavelengths of visible light (i.e. light in the 400 nm-700 nm range). In some embodiments, the optically reflecting layer may be configured to reflect all wavelengths of visible light. 
     The optical thin film structure may comprise at least three optically non-absorbing layers. In some embodiments the optical thin film structure may comprise at least three optically absorbing layers. 
     The optically absorbing layers and the optically non-absorbing layers may be arranged to form a stack, and the optically reflecting layer may be arranged at an end of the stack. One of the optically absorbing layers may comprise the optically reflecting layer. For example, one of the optically absorbing layers may act as both an absorbing layer and a reflecting layer configured to reflect at least some wavelengths of, or all wavelengths of, visible light. 
     The optically absorbing layers and the optically non-absorbing layers may be positioned on one side of the optically reflecting layer and form a set of layers, and the optical thin film structure may further comprise an identical set of layers positioned on an opposite side of the optically reflecting layer. 
     The optically reflecting layer may comprise one or more of a metal (for example a metal having a suitably high reflectance), a metal alloy, aluminium, silver, gold, chromium, nickel, and tantalum. 
     At least one of the optically absorbing layers may comprises amorphous silicon and at least one of the optically non-absorbing layers comprises silicon dioxide. 
     At least one of the optically absorbing layers may comprise germanium and at least one of the optically non-absorbing layers may comprise silicon dioxide. 
     The optically reflecting layer may comprise aluminium, at least one of the optically absorbing layers may comprise tantalum, and at least one of the optically non-absorbing layers may comprise silicon dioxide. 
     The optically reflecting layer may comprise aluminium, at least one of the optically absorbing layers may comprise a nitride, such as chromium nitride, and at least one of the optically non-absorbing layers may comprise silicon dioxide. 
     The optically reflecting layer may comprise aluminium, at least one of the optically absorbing layers may comprise germanium, and at least one of the optically non-absorbing layers may comprise silicon dioxide. 
     The optically reflecting layer may comprise chromium, at least one of the optically absorbing layers may comprise amorphous silicon, and at least one of the optically non-absorbing layers may comprise silicon dioxide. 
     The first angle may be comprised between about 0 degrees relative to a normal to a plane defined by the structure, and about 28 degrees relative to the normal to the plane. The second angle may be comprised between about 45 degrees relative to the normal to the plane, and about 80 degrees relative to the normal to the plane. 
     The optically absorbing layers and the optically non-absorbing layers may be further configured such that, as an angle of reflection of light incident on the structure varies from the first angle to the second angle, a degree of yellow in the reflected light increases and a degree of red in the reflected light decreases. 
     The optically non-absorbing layers may comprise dielectric layers. 
     The optical thin film structure may be incorporated into an item (such as a bank note) in the form of a patch, stripes, thread, and/or ink flakes. For instance, the optical thin film structure may be formed on a patch which is then incorporated into the item to be protected, or may be incorporated into a thread which is then woven or stitched into the item to be protected. 
     In a further aspect of the disclosure, there is provided a method of securing an item so as to inhibit or prevent counterfeiting of the item. The method comprises applying to the item any of the above-described multilayer optical thin film structures. 
     In a further aspect of the disclosure, there is provided a method of authenticating an item having applied thereto any of the above-described multilayer optical thin film structures. The method comprises observing at a first angle light with a red colour reflected off the item, and observing at a second angle light with a yellow colour reflected off the item. 
     In a further aspect of the disclosure, there is provided an item having applied thereto any of the above-described multilayer optical thin film structures. The item may comprise one or more of a bank note, a document, a passport, an identification card, a bank card, and a valuable good. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Specific embodiments of the disclosure will now be described in conjunction with the accompanying drawings of which: 
         FIG. 1  is a schematic diagram of a multilayer optical thin film structure, according to an embodiment of the disclosure; 
         FIG. 2  is a schematic diagram of a multilayer optical thin film structure, according to an embodiment of the disclosure; 
         FIG. 3  is a schematic diagram of a multilayer optical thin film structure, according to an embodiment of the disclosure; 
         FIG. 4  is a schematic diagram of a multilayer optical thin film structure, according to an embodiment of the disclosure; 
         FIG. 5  is a schematic diagram of a multilayer optical thin film structure, according to an embodiment of the disclosure; 
         FIG. 6  is a CIE chromaticity diagram of approximate colour regions (obtained from http://hyperphysics.phy-astr.gsu.edu/hbase/vision/cie.html); 
         FIG. 7  is a CIE chromaticity diagram showing, as a function of angle of reflection, the colour of light reflected off the multilayer optical thin film structures described herein; and 
         FIGS. 8A and 8B  are plots of reflectance as a function of wavelength. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure seeks to provide an improved multilayer optical thin film structure. While various embodiments of the disclosure are described below, the disclosure is not limited to these embodiments, and variations of these embodiments may well fall within the scope of the disclosure which is to be limited only by the appended claims. 
     Directional terms such as “top”, “bottom”, “upwards”, “downwards”, “vertically” and “laterally” are used in this disclosure for the purpose of providing relative reference only, and are not intended to suggest any limitations on how any article is to be positioned during use, or to be mounted in an assembly or relative to an environment. 
     Additionally, the term “couple” and variants of it such as “coupled”, “couples”, and “coupling” as used in this disclosure are intended to include indirect and direct connections unless otherwise indicated. For example, if a first article is coupled to a second article, that coupling may be through a direct connection or through an indirect connection via one or more other articles. 
     Furthermore, the singular forms “a”, “an”, and “the” as used in this disclosure are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
     Still further, the term “adjacent” is to be considered as encompassing both indirectly adjacent and directly adjacent, unless otherwise stated or implied. When two elements or layers are said to be indirectly adjacent one another, then one or more intervening elements or layers may separate the two elements or layers. On the other hand, when two elements or layers are said to be directly adjacent one another, then the two elements or layers are in direct physical contact with one another. 
     As used herein, a reference to “about” or “approximately” a number or to being “substantially” equal to a number means being within +/−10% of that number. 
     Embodiments of the disclosure are directed at multilayer optical thin film structures or devices. The devices are configured to exhibit a red-to-yellow colour variation when observed at different angles. In order to achieve such a colour variation, the devices comprise a layered structure of optically non-absorbing layers and optically absorbing layers. The specific thicknesses and number of the optically non-absorbing layers and optically absorbing layers, and the specific materials used to form the layers, may be controlled so as to achieve the desired red-to-yellow colour variation. In the following embodiments, the optically non-absorbing layers are dielectric layers, although other optically non-absorbing layers may be used without departing from the scope of the disclosure. 
     Turning to  FIG. 1 , there is shown a first embodiment of a multilayer optical thin film structure or device  10 . Device  10  comprises a number of layers arranged in a stacked formation. In particular, device  10  comprises a first dielectric layer  11  positioned on top of and adjacent a first optically absorbing layer  12 . Optically absorbing layer  12  is positioned on top of and adjacent a second dielectric layer  13 . Second dielectric layer  13  is positioned on top of and adjacent an optically absorbing and reflecting layer  14  (which may simply be referred to as optically reflecting layer  14 ). A bonding layer  15  is used to adhere or bond the stacked arrangement of layers  11 ,  12 ,  13 ,  14  to a substrate  16 . In some embodiments, there is no need for bonding layer  15  and the stacked arrangement of layers  11 ,  12 ,  13 ,  14  may be deposited directly on substrate  16 . 
     While  FIG. 1  shows each of the illustrated layers being directly adjacent one another such that each layer is in physical contact with at least one other layer, in other embodiments additional intermediary layers may be provided to device  10 . Furthermore, device  10  may comprise an additional number of dielectric and/or optically absorbing layers, for example as set out in some of the embodiments described below. The total number of layers, the material used for each layer, and the thickness of each layer may vary in order to achieve a device configured such that light incident on the device is reflected with a red colour at a first angle, and light incident on the structure is reflected with a yellow colour at a second angle. 
     Optically absorbing layer  12  may comprise various materials including, but not limited to, metals, metal alloys, nitrides (such as Cr nitrides and other metal nitrides such as Nb or Ta nitrides), amorphous silicon, germanium, and tantalum. Dielectric layers  11 ,  13  may comprise various materials including, but not limited to, silicon dioxide, aluminium oxide, magnesium fluoride, oxides, and fluorite. Optically reflecting layer  14  may comprise a metal including, but not limited to, aluminium, chromium, and a metal alloy. 
     According to embodiments of the disclosure, the following materials and thicknesses were used in the fabrication of devices according to device  10 . In these examples, the method of deposition of the layers comprised sputtering. 
     It should be noted that, in all of the following examples (Examples 1a-5), the actual thickness of each layer depends on the method of deposition used. The followings thicknesses are therefore purely exemplary in nature, and other layered structures may achieve the same optical colour variation with different thicknesses of layers, assuming for example that the layers were formed using other, different types of deposition techniques. 
     Example 1a 
       
     
       
         
           
               
               
               
               
             
               
                   
                   
               
               
                   
                 Layer 
                 Thickness (nm) 
                 Material 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 14 
                 20 or more 
                 Al 
               
               
                   
                 13 
                 223.7 
                 SiO 2   
               
               
                   
                 12 
                 14.5 
                 Ta 
               
               
                   
                 11 
                 235.8 
                 SiO 2   
               
               
                   
                   
               
            
           
         
       
     
     Example 1b 
       
     
       
         
           
               
               
               
               
             
               
                   
                   
               
               
                   
                 Layer 
                 Thickness (nm) 
                 Material 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 14 
                 20 or more 
                 Al 
               
               
                   
                 13 
                 206.3 
                 SiO 2   
               
               
                   
                 12 
                 20.9 
                 CrN x   
               
               
                   
                 11 
                 240.6 
                 SiO 2   
               
               
                   
                   
               
            
           
         
       
     
     According to another embodiment of the disclosure, the following materials and thicknesses were used in the fabrication of a device according to device  10 . In this example, the method of deposition of the layers comprised sputtering. 
     Example 2 
       
     
       
         
           
               
               
               
               
             
               
                   
                   
               
               
                   
                 Layer 
                 Thickness (nm) 
                 Material 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 14 
                 20 or more 
                 Al 
               
               
                   
                 13 
                 204 
                 SiO 2   
               
               
                   
                 12 
                 7.9 
                 Ge 
               
               
                   
                 11 
                 77.4 
                 SiO 2   
               
               
                   
                   
               
            
           
         
       
     
     Turning to  FIG. 2 , there is shown another embodiment of multilayer optical thin film structure or device  20 . Device  20  is similar to device  10  and like features are numbered using like reference numbers. Unlike device  10 , device  20  includes an additional optically absorbing layer  27  positioned between dielectric layer  23  and an additional dielectric layer  28 . The optically absorbing and reflecting layer  24  is positioned between bonding layer  25  and dielectric layer  28 . 
     According to another embodiment of the disclosure, the following materials and thicknesses were used in the fabrication of a device according to device  20 . In this example, the method of deposition of the layers comprised sputtering. 
     Example 3 
       
     
       
         
           
               
               
               
               
             
               
                   
                   
               
               
                   
                 Layer 
                 Thickness (nm) 
                 Material 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 24 
                 20 or more 
                 Cr 
               
               
                   
                 28 
                 179.8 
                 SiO 2   
               
               
                   
                 27 
                 19.8 
                 a-Si 
               
               
                   
                 23 
                 203.4 
                 SiO 2   
               
               
                   
                 22 
                 6.8 
                 a-Si 
               
               
                   
                 21 
                 90 
                 SiO 2   
               
               
                   
                   
               
            
           
         
       
     
     Light incident on the device  10 ,  20  via dielectric layer  11 ,  21  travels through the stacked arrangement and is reflected at each interface separating adjacent layers. An observer will observe light reflected off the device  10 ,  20 , such light being the complex summation (or interference) of the light having undergone multiple reflections/transmissions at each interface separating adjacent layers. The observed light is therefore a result of the collective effect of each individual layer of the multilayer stack. An observer will observe the colour variation by positioning themselves on the same side of substrate  16 ,  26  as the side from which light is incident on the device  10 ,  20 . 
     Turning to  FIG. 3 , there is shown another embodiment of a multilayer optical thin film structure or device  30 . Device  30  comprises a number of layers arranged in a stacked formation. In particular, device  30  comprises a first dielectric layer  31  positioned on top of and adjacent a first optically absorbing layer  32 . Optically absorbing layer  32  is positioned on top of and adjacent a second dielectric layer  33 . Second dielectric layer  33  is positioned on top of and adjacent a second optically absorbing layer  34 . A bonding layer  35  is used to adhere or bond the stacked arrangement of layers  31 ,  32 ,  33 ,  34  to a transparent substrate  36  (for example a plastic bank note). In some embodiments, there is no need for bonding layer  15  and the stacked arrangement of layers  31 ,  32 ,  33 ,  34  may be deposited directly on transparent substrate  36 . 
     Similarly to the embodiments of  FIGS. 1 and 2 , optically absorbing layers  32 ,  24  may comprise various materials including, but not limited to, metals, metal alloys, nitrides, amorphous silicon, germanium, tantalum, and semiconducting materials. Dielectric layers  31 ,  33  may comprise various materials including, but not limited to, silicon dioxide, aluminium oxide, magnesium fluoride, nitrides, oxides, and fluorite. 
     According to another embodiment of the disclosure, the following materials and thicknesses were used in the fabrication of a device according to device  30 . In this example, the method of deposition of the layers comprised sputtering. 
     Example 4 
       
     
       
         
           
               
               
               
               
             
               
                   
                   
               
               
                   
                 Layer 
                 Thickness (nm) 
                 Material 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 34 
                 26.9 
                 Ge 
               
               
                   
                 33 
                 213.7 
                 SiO 2   
               
               
                   
                 32 
                 7.1 
                 Ge 
               
               
                   
                 34 
                 74.4 
                 SiO 2   
               
               
                   
                   
               
            
           
         
       
     
     Turning to  FIG. 4 , there is shown another embodiment of multilayer optical thin film structure or device  40 . Device  40  is similar to device  30  and like features are numbered using like reference numbers. Unlike device  30 , device  40  includes an additional dielectric layer  47  positioned between optically absorbing layer  44  and an additional optically absorbing layer  48  adhered to a transparent substrate  46  (for example a plastic bank note) via a bonding layer  45 . 
     According to another embodiment of the disclosure, the following materials and thicknesses were used in the fabrication of a device according to device  40 . In this example, the method of deposition of the layers comprised sputtering. 
     Example 5 
       
     
       
         
           
               
               
               
               
             
               
                   
                   
               
               
                   
                 Layer 
                 Thickness (nm) 
                 Material 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 48 
                 47.4 
                 a-Si 
               
               
                   
                 47 
                 184.3 
                 SiO 2   
               
               
                   
                 44 
                 11.4 
                 a-Si 
               
               
                   
                 43 
                 206.2 
                 SiO 2   
               
               
                   
                 42 
                 6.9 
                 a-Si 
               
               
                   
                 41 
                 96.1 
                 SiO 2   
               
               
                   
                   
               
            
           
         
       
     
     Devices according to the embodiments of  FIGS. 3 and 4  are configured to function similarly to those of the embodiments of  FIGS. 1 and 2 . In particular, light incident on the device  30 ,  40  via dielectric layer  31 ,  41  travels through the stacked arrangement and is reflected at each interface separating adjacent layers. An observer will observe light reflected off the device  30 ,  40 , such light being the complex summation (or interference) of the light having undergone multiple reflections/transmissions at each interface separating adjacent layers. The observed light is therefore a result of the collective effect of each individual layer of the multilayer stack. An observer will observe the colour variation by positioning themselves on the same side of substrate  36 ,  46  as the side from which light is incident on the device  10 ,  20 . 
     Turning to  FIG. 5 , there is shown another embodiment of multilayer optical thin film structure or device  50 . Device  50  comprises a number of layers arranged in a stacked formation. In particular, device  50  comprises a first dielectric layer  51  positioned on top of and adjacent a first optically absorbing layer  52 . Optically absorbing layer  52  is positioned on top of and adjacent a second dielectric layer  53 . Second dielectric layer  53  is positioned on top of and adjacent a second optically absorbing layer  54 . Second optically absorbing layer  54  is positioned on top of and adjacent a third dielectric layer  55 . Beneath third dielectric layer  55  is positioned an optically reflecting layer  56 . The stacked arrangement of alternating dielectric and optically absorbing layers is repeated on the other side of the optically reflecting layer  56 . 
     Similarly to the embodiments of  FIGS. 1-4 , optically absorbing layers  52 ,  54  may comprise various materials including, but not limited to, metals, metal alloys, nitrides, amorphous silicon, germanium, and tantalum. Dielectric layers  51 ,  53  may comprise various materials including, but not limited to, silicon dioxide, aluminium oxide, magnesium fluoride, metal oxides and metal fluorite. 
     Optically reflecting layer  56  is sufficiently thick that substantially no light is transmitted through it. Accordingly, the red-to-yellow colour variation may therefore be perceived on either side of device  50 . The embodiment of  FIG. 5  is particularly suited to ink pigment applications. Specifically, the multilayer device  50  may be crushed into small pieces and mixed with an ink carrier for printing. 
     In each of the described embodiments, the layers may be deposited using one or more of a variety of deposition techniques, as known in the art. Such techniques include, but are not limited to, evaporation, vacuum coating, sputtering, a sol-gel process, and printing. The multilayer structure may be deposited on a substrate to be applied to an item to be protected, or alternatively may be deposited directly to objects to be protected, in which case the item to be protected acts as the substrate. In some embodiments, the multilayer structure may be crushed into small flakes to be used as an ink pigment. 
     In each of the described embodiments, the optically absorbing layers and the dielectric layers are arranged in alternating order to form a stack, and (in the cases where one is used) the optically reflecting layer is arranged at an end of the stack. The reflection of light at the interface of two layers is greater the more the materials making up the adjacent layers differ. Accordingly, alternating the layers of the multilayer stack may enable improved reflection of the light. 
     In some embodiments, two or more optically non-absorbing layers may be directly adjacent one another, and/or two or more optically absorbing layers may be directly adjacent one another. Furthermore, the multilayer structure may comprise more optically absorbing layers than non-absorbing layers, or more non-absorbing layers than absorbing layers. 
     Turning to  FIG. 6 , there is shown a CIE 1931 chromaticity diagram. Generally, colours that can be perceived by the human eye are shown in the horseshoe-shaped colour space. Each colour corresponds to a point (x,y) in the diagram. Spectral colors (i.e. colours of single wavelength) are located on the outer boundary of the colour space. Colours of multiple wavelengths are those points inside the colour space. If a point is closer to the outer boundary, its corresponding colour may be considered purer or more saturated. The middle of the horseshoe corresponds to a mixed colour of white, black and grey. It should be noted that the regions shown in the colour space are approximate. 
     By tailoring the thickness and material of each of the dielectric, absorbing, and reflecting layers of the above embodiments, as well as the total number of layers used to form the multilayer device, it is possible to form an optical thin film structure or device that exhibits a specific red-to-yellow colour variation under different viewing angles. In particular,  FIG. 7  shows the desired red-to-yellow colour variation under different viewing angles. As can be seen, at an angle of about 0 degrees relative to a normal to a plane defined by the device (e.g. roughly normal to the layers comprising the device), light reflected off the device has a predominantly red colour. The red colour persists for viewing angles (i.e. angles of reflection) up to about 28 degrees to the normal to the plane, whereupon the colour changes relatively quickly to predominantly yellow, over an approximate 15 degree viewing angle range, from about 28 degrees relative to the normal to the plane to about 45 degrees relative to the normal to the plane. From about 45 degrees relative to the normal to the plane to about 80 degrees relative to the normal to the plane, the reflected light is predominantly of yellow colour. Two adjacent points have a 5 degree difference of angle of observation. 
     Current OTFSDs generally have a relatively narrow range of observation angles in order for each specified colour to be perceived. For example, the current popular three-layer yellow-to-green OTFSD typically has a 10 to 15 degree window for each of the yellow and green colours it reflects. Outside of these 10-15 degree windows, the device exhibits different colours (other than yellow or green). On the other hand, according to embodiments of the present disclosure, the angular ranges for the specified colours of red and yellow are expanded. Thus, even for relatively high angles of observation (e.g. approaching 80 degrees to the normal to the plane of the structure), the hue of the observed colour remains approximately constant. Furthermore, the transition from red to yellow is relatively rapid (i.e. over an approximate 15 degree range). These factors are beneficial for an authentication process, especially by a layperson who may not have the specialist knowledge or skill required for correct observation of the specified colours. 
       FIGS. 8A and 8B  are two graphs showing approximate percentage of light reflectance at different observation angles as a function of wavelength. Structures according to Examples 2, 3, 4 and 5 were shown to exhibit reflectance profiles similar to that of  FIG. 8A , while structures according to Example 1 were shown to exhibit reflectance profiles similar to that of  FIG. 8B . The actual reflectance profile will depend, for example, in practice on the thickness of each individual layer. 
     As would be recognized by the skilled person, the refractive index of each layer will depend on the material of the layer as well as the method of deposition used. Therefore, depending on the deposition technique used to form any given layer, a layer of a different thickness but deposited using a different technique may provide a similar or identical optical effect. As would be also recognized by those skilled in the art, the specific thicknesses, materials, and order of the various absorbing, non-absorbing and reflecting layers may be modified, within the bounds of this disclosure, so as to achieve and maintain the red-to-yellow colour variation described herein. The total number of layers used to form the multilayer device may also be varied, within the bounds of this disclosure. Many different possible layer combinations may, within the bounds of this disclosure, be used in order to achieve the desired red-to-yellow colour variation. 
     While the disclosure has been described in connection with specific embodiments, it is to be understood that the disclosure is not limited to these embodiments, and that alterations, modifications, and variations of these embodiments may be carried out by the skilled person without departing from the scope of the disclosure. It is furthermore contemplated that any part of any aspect or embodiment discussed in this specification can be implemented or combined with any part of any other aspect or embodiment discussed in this specification.