Patent Publication Number: US-11023723-B2

Title: Optical puf and optical reading of a security element

Description:
The present invention relates generally to the optical reading of a security element, and in particular to related methods and systems for determining a unique identifier for such an optically read security element. 
     There is often a need to prove, or disprove, the authenticity of an object or similar. For instance, this might be needed for security purposes, for example to allow or prevent access to certain functionality associated with the object, or simply to allow a user or consumer of the object to be satisfied that they are using an authentic object. It will be appreciated that such tests for authenticity find use in the fields of anti-counterfeiting, security and so on. 
     In order to be able to prove that an object is an authentic object, or in other words to authenticate an object, that object might be provided with a unique identifier in one form or another. “Unique” might not necessarily mean that it is impossible for another object to have the same identifier, but instead that it is statistically highly unlikely for this to be the case, or in other words for the identifier to be accidentally stumbled across by guesswork or simple trial and error. 
     A unique identifier might, for example, take the form of or be derived from a physical (sometimes referred to as physically) unclonable function. This might be in the form of a device or other element, the properties of which depend on small variations in construction or fabrication or similar, but which nevertheless can be used to provide a unique identifier. For instance, in a vast array of memory cells, a certain number of memory cells may be defective, and this number or arrangement of defective cells will be different for different arrays that are produced. Thus, this is a simple example of a unique identifier. Another example might be, for instance, a capacitance or resistance of an electrical component, based on the thickness of layers within that component, or the extent of those layers, and so on. Due to tolerances in manufacturing, each component will likely have a slightly different construction, and so a slightly different, and unique, electrical property. 
     Unique identifiers do not necessarily need to be based on electrical principles. For instance, physical unclonable functions may be probed or otherwise challenged optically in order to determine a unique identifier. For instance, the way in which one or more optical emitters are provided on an object may, as above, yield an overall emission spectrum or map which is unique, again providing a readable unique identifier. 
     Traditionally, the generation of unique identifiers, and/or associated use of physical unclonable functions, have been based on macroscopic effects. More recently though, it has been proposed to incorporate quantum mechanical effects in the generation of unique identifiers. In these more recent examples, for instance, an electrical component exhibiting quantum mechanical confinement (e.g. a resonant tunneling diode) may be used as a quantum mechanical based physical unclonable function. The electrical properties of such a device or structure, and thus the unique identifier, are based on quantum mechanical principles. Similarly, optical based physical unclonable functions may be based on the emissions spectra of quantum dots, or 2-D materials, or similar, located on an object. In both cases, it may be extremely difficult, if not impossible, to be able to physically copy a security element (e.g. being or comprising a physical unclonable function) based on quantum mechanical effects. This is to the extent that the unique identifier provided by such an element may not be circumvented, and certainly not in any practical time frame. 
     Depending on the exact implementation, it might be relatively straightforward to be able to determine (e.g. for the first time, or subsequently, in order to cross-check) a unique identifier based on electrical principles. However, the same cannot necessarily be said for optical-based security elements, for example those security elements that are optically read in order to determine a unique identifier. Whilst it may be relatively easy to optically determine a unique identifier in a laboratory environment, or a highly controlled environment, it will be very difficult, if not impossible, to be able to quickly, or easily, or cheaply do this in a more commercial environment, for example with a typical consumer-like device or a consumer-like environment. Even if it is possible to determine a unique identifier using a more commercial, end-user, device, current approaches or proposals for such use may not offer satisfactory levels of such determination, or related cross-checking for authentication, or related security functionality. 
     It is an example aim or example embodiments of the present invention to at least partially overcome or avoid one or more disadvantages of the prior art, whether identified herein or elsewhere, or to at least provide a viable alternative. 
     According to the present invention there are provided apparatus and methods as set forth in the claims that follow. Other features of the invention will be apparent from the dependent claims, and the description which follows. 
     According to a first aspect of the present invention, there is provided a method of determining a unique identifier for a security element, the method comprising: optically reading the security element via a configurable optical filter system, a readable optical transmission property of the filter system varying with respect to a configuration of the filter system; the reading comprising determining data indicative of an optical property of the security element at a first configuration of the filter system, and determining data indicative of an optical property of the security element at a second, different, configuration of the filter system; and the unique identifier being determined from a map of the variation in determined data indicative of an optical property with respect to the configuration of the filter system, wherein the reading is undertaken for multiple locations across the security element at the or each configuration of the filter system, such that the map is a map of the variation in determined data indicative of an optical property across the security element with respect to the configuration of the filter system, and wherein the reading for multiple locations across the security element is undertaken in a single reading step, using a reader with a two-dimensional sensor. 
     The optical filter system might comprise an optical filter, a readable optical transmission property of the filter varying with respect to an angle of orientation of the filter, such that the optical filter system is configurable with respect to the angle of orientation of the filter. 
     The optical filter system might comprise a plurality of different optical filters, each filter having a different optical transmission property, such that the optical filter system is configurable with respect to which filter is used. 
     The optical filter system might comprise an optical filter, a readable optical transmission property of the filter varying with respect to one or more of a temperature of that filter, an electric field across that filter, or an applied driving frequency when the optical filter is an acousto-optic tuneable filter, such that the optical filter system is configurable with respect to which temperature, electric field, or driving frequency is used. 
     Data indicative of an optical property of the security element might comprise one or more of: an actual optical property; and/or an electromagnetic emission spectrum of at least a part of the security element; and/or a peak, trough, or point of inflection in an electromagnetic emission spectrum of at least a part of the security element; and/or a physical location in relation to the security element of an actual optical property; and/or a physical location of a peak, trough, or point of inflection in an electromagnetic emission spectrum of the security element in relation to the security element. 
     The security element might comprise one or more continuous or discrete components exhibiting quantum mechanical confinement, the or each component being capable of emitting electromagnetic radiation linked to that quantum mechanical confinement, wherein the confinement of the one or more continuous or discrete components optionally confines in one or more of 3D, 2D, or 1D, or 0D. 
     The method may comprise causing the security element to emit electromagnetic radiation, to facilitate the optical reading of the security element, by one or more of: irradiating the security element with electromagnetic radiation; and/or irradiating the security element with electromagnetic radiation such that emission occurs by non-resonant photoluminescence; and/or irradiating the security element with electromagnetic radiation, wherein an irradiation wavelength is different from an emission wavelength of the security element; and/or electrical excitation. 
     One or both of the security element and/or optical filter system may comprise, or be associated with, a designated feature, for use in determining an angle of orientation of the reader with respect to the security element and/or optical filter, wherein the designated feature optionally comprises an alignment marker or a diffraction grating. 
     The optical filter system may comprise one or more of a band pass filter, an edge filter, a notch filter or a tuneable Bragg grating in a fibre. 
     The optical transmission property may be a central transmission wavelength or transmission band. 
     The reader may be a handheld, or wearable, mobile device. 
     The map, or a unique signature or identifier derived from the map, may be stored in an (e.g. secure) location, for use in authentication of the security element on a subsequent reading of that security element. 
     According to a second aspect of the present invention, there is provided a system for determining a unique identifier for a security element, the system comprising: a configurable optical filter system, an optical transmission property of the filter system varying with respect to a configuration of the filter system; an optical reader, for optically reading the security element via the optical filter system; the reading comprising determining data indicative of an optical property of the security element at a first configuration of the filter system, and determining data indicative of an optical property of the security element at a second, different, configuration of the filter system; and the unique identifier being determined from a map of a variation in determined data indicative of an optical property with respect to the configuration of the filter system, wherein the reader is arranged to undertake readings for multiple locations across the security element at the or each configuration of the filter system, such that the map is a map of the variation in determined data indicative of an optical property across the security element with respect to the configuration of the filter system, and wherein the reader comprises a two-dimensional sensor for reading multiple locations across the security element in a single reading step. 
     According to a third aspect of the present invention, there is provided a method of determining a unique identifier for a security element, the method comprising: optically reading the security element via a configurable optical filter system, a readable optical transmission property of the filter system varying with respect to a configuration of the filter system; the reading comprising determining data indicative of an optical property of the security element at a first configuration of the filter system, and determining data indicative of an optical property of the security element at a second, different, configuration of the filter system; and the unique identifier being determined from a map of the variation in determined data indicative of an optical property with respect to the configuration of the filter system. That is, the reading across multiple locations, and/or the single reading with a 2D sensor, as discussed in the first aspect, might not always be required (although highly likely to be the case in a practical implementation). 
     According to a fourth aspect of the present invention, there is provided a system for determining a unique identifier for a security element, the system comprising: a configurable optical filter system, an optical transmission property of the filter system varying with respect to a configuration of the filter system; an optical reader, for optically reading the security element via the optical filter system; the reading comprising determining data indicative of an optical property of the security element at a first configuration of the filter system, and determining data indicative of an optical property of the security element at a second, different, configuration of the filter system; and the unique identifier being determined from a map of a variation in determined data indicative of an optical property with respect to the configuration of the filter system. That is, the reading across multiple locations, and/or the single reading with a 2D sensor, as discussed in the second aspect, might not always be required (although highly likely to be the case in a practical implementation). 
     According to a fifth aspect of the present invention, there is provided a method of authenticating, comprising: optically reading a security element via a configurable optical filter system, a readable optical transmission property of the filter system varying with respect to a configuration of the filter system; the reading comprising determining data indicative of an optical property of the security element at a configuration of the filter system; and the authenticating further comprising comparing the determined data indicative of an optical property with the unique identifier determined using the method of the first or third aspects, or the system of the second or fourth aspects. 
     According to a sixth aspect of the present invention, there is provided a system for authenticating, the system comprising: a configurable optical filter system, a readable optical transmission property of the filter system varying with respect to a configuration of the filter system; an optical reader, for optically reading a security element via the optical filter system; the reading comprising determining data indicative of an optical property of the security element at a configuration of the filter system; and the system being arranged to compare the determined data indicative of an optical property with the unique identifier determined using the method of the first or third aspects, or the system of the second or fourth aspects. 
     According to a seventh aspect of the present invention, there is provided a security element suitable for use in any of the preceding claims, wherein the security element comprises: a first part, capable of emitting electromagnetic radiation; a second part, comprising a configurable optical filter system, a readable optical transmission property of the filter system varying with respect to a configuration of the filter system, the first part being readable via the second part. 
     According to an eighth aspect of the present invention, there is provided a method of determining a unique identifier for a security element, the method comprising: optically reading the security element via a configurable optical filter system, a readable optical transmission property of the filter system varying with respect to a configuration of the filter system; the reading comprising determining data indicative of an optical property of the security element at a first configuration of the filter system; and the unique identifier being determined from a map of the determined data indicative of an optical property with respect to the configuration of the filter system, wherein the reading is undertaken for multiple locations across the security element at the first configuration of the filter system, such that the map is a map of the determined data indicative of an optical property across the security element with respect to the configuration of the filter system, and wherein the reading for multiple locations across the security element is undertaken in a single reading step, using a reader with a two-dimensional sensor. 
     According to a ninth aspect of the present invention, there is provided a system for determining a unique identifier for a security element, the system comprising: a configurable optical filter system, an optical transmission property of the filter system varying with respect to a configuration of the filter system; an optical reader, for optically reading the security element via the optical filter system; the reading comprising determining data indicative of an optical property of the security element at a first configuration of the filter system; and the unique identifier being determined from a map of the determined data indicative of an optical property with respect to the configuration of the filter system, wherein the reader is arranged to undertake readings for multiple locations across the security element at the first configuration of the filter system, such that the map is a map of the determined data indicative of an optical property across the security element with respect to the configuration of the filter system; and wherein the reader comprises a two-dimensional sensor for reading multiple locations across the security element in a single reading step. 
     According to a tenth aspect of the present invention, there is provided a method of authenticating, comprising: optically reading a security element via a configurable optical filter system, a readable optical transmission property of the filter system varying with respect to a configuration of the filter system; the reading comprising determining data indicative of an optical property of the security element at a configuration of the filter system; and the authenticating further comprising comparing the determined data indicative of an optical property with the unique identifier determined using the method of the eighth aspect, or using the system of the ninth aspect. 
     According to an eleventh aspect of the present invention, there is provided a system for authenticating, the system comprising: a configurable optical filter system, a readable optical transmission property of the filter system varying with respect to a configuration of the filter system; an optical reader, for optically reading a security element via the optical filter system; the reading comprising determining data indicative of an optical property of the security element at a configuration of the filter system; and the system being arranged to compare the determined data indicative of an optical property with the unique identifier determined using the method of the eighth aspect, or using the system of the ninth aspect. 
     It will be appreciated that any one or more features described in relation to any one particular aspect of the present invention may be used in place of, or in combination with, any one or more features of another aspect of the present invention, unless such combination or replacement would be understood by the skilled person to be mutually exclusive, based on a reading of this disclosure. In particular, it will be understood that any features described in relation to a method aspect of the present invention can be used in combination with any apparatus aspect of the present invention, and that any features described in relation to an apparatus aspect of the present invention can be used with any method aspect of the present invention. 
    
    
     
       For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic Figures in which: 
         FIG. 1  schematically depicts the optical reading of a security element according to a proposed system and method; 
         FIG. 2  schematically depicts optical reading of a security element via an optical filter system according to an example embodiment; 
         FIG. 3  is a plot schematically depicting a variation in an optical transmission property of the optical filter system of  FIG. 2 , in different configurations of that optical filter system, according to an example embodiment; 
         FIGS. 4 and 5  schematically depict different configurations of an optical filter in terms of their angle of orientation relative to a security element and/or a reader; 
         FIG. 6  schematically depicts a series of optical readings of a security element taken via an optical filter system at different configurations; 
         FIGS. 7 to 9  schematically depict different ways of causing a security element to emit electromagnetic radiation according to example embodiments; 
         FIG. 10  schematically depicts a system and method for determining a unique identifier for a security element according to an example embodiment; 
         FIG. 11  schematically depicts a system and method for determining a unique identifier for a security element according to a different example embodiment; 
         FIG. 12  schematically depicts the creation of a map of a variation in determined data indicative of an electrical property of a security element with respect to configurations of the optical filter system; 
         FIG. 13  schematically depicts principles associated with authenticating of a security element according to example embodiments; 
         FIG. 14  is a flow chart depicting the determination of a unique identifier of a security element, and a subsequent authentication of the security element using that unique identifier; 
         FIG. 15  schematically depicts general methodology associated with example embodiments; 
         FIG. 16  schematically depicts general principles associated with a system according to example embodiments; 
         FIG. 17  schematically depicts different methodology associated with example embodiments; 
         FIG. 18  schematically depicts general principles associated with a system according to example embodiments; and 
         FIG. 19  schematically depicts a security element according to an example embodiment. 
     
    
    
       FIG. 1  schematically depicts a proposed system and method for optically reading a security element. The Figure shows the security element  10 . A reader for reading the security element  10  is shown in the form of a mobile device, in this case a mobile telephone  12 . It has been proposed that the security element  10  may be optically read via the mobile telephone  12  by appropriate irradiation of the security element  10 , for example by way of a flash  14  provided by the mobile telephone  12 , and an appropriate detection of emitted electromagnetic radiation from the security element  10 . In more detail, the security element  10  can be read by an appropriate sensor and related components of the mobile telephone  12 . 
     As with all examples described herein, the security element  10  may comprise one or more components arranged to emit electromagnetic radiation when appropriately stimulated. The components may be, for example, quantum dots, quantum wires, flakes or layers of 2D material. The or each component may emit radiation at a single wavelength, or the or each component may emit radiation with different wavelengths, for example corresponding to a variation in band gap of the or each particular component. 
     The reading of the security element  10  may comprise establishing a two-dimensional (2D) map of how the security element  10  emits radiation across the security element (e.g. across the length and width of the security element). Typically, the one or more components that emit radiation will not be provided within the security element  10  in a controlled, repeatable and consistent manner across different security elements, but will instead be deposited in a random or deliberately uncontrolled manner. It is this deliberate lack of control which will allow the security element  10  to effectively function as a physically unclonable function, and/or to therefore provide a unique identifier (physically unclonable functions are normally employed in specific ways, and unique identification is perhaps a more general feature, function, or description). That is, the emitters will be distributed across the element in a unique manner. 
     A read map may be compared in some way with a stored map, to authenticate the security element  10 . 
     The system and methodology shown in  FIG. 1  may function satisfactorily. Indeed, it can be very difficult to easily replicate the precise layout of the one or more components that emit electromagnetic radiation, to the extent that it is very difficult to copy the unique identifier provided by the security element. However, it is not impossible, and with advances in technology it may be easier to overcome or circumvent the security that the unique identifier provides, since it may be easier to replicate that unique identifier. Also, the proposed methodology and system in  FIG. 1  takes a single reading or snapshot of the emission spectra or spectrum across the security element  10 . Since only a single image or snapshot is used to determine the unique identifier, and likely with a commercial end-user device, only a single unique identifier needs to be in some way copied, or replicated or circumvented in order to overcome the security that is provided. This is not just important in the actual reading phase, but also when information for authentication is transmitted to or from a reader, for example in order to authenticate the security element via a database having a previously determined identifier stored therein, for use in the authentication. 
     It has been realised by the inventors of the present invention that one or more problems associated with the sort of methodology and system as shown in  FIG. 1  can be overcome in a relatively simple manner, but which at the same time provides a vastly improved system and method for determining (e.g. initially, or subsequently for authentication) a unique identifier, and a related system and method for authentication using that unique identifier. 
     In particular, according to the present invention there is provided a method of determining a unique identifier for a security element. The method comprises optically reading the security element by a configurable optical filter system, a readable optical transmission property of the filter system varying with respect to the configuration of the filter system (e.g. the configuration being an inherent configuration of the filter system itself, or of the filter relative to the security element or a reader of the element). The reading comprises determining data indicative of an optical property of the security element at a first configuration of the filter system, and determining data indicative of an optical property of the security element in a second, different configuration of the filter system. The unique identifier is determined from a map (which includes the identifier being the map) of the variation in determined data indicative of an optical property of the security element, with respect to the configuration of the filter system. In most practical implementations, it is most likely that the reading is undertaken for multiple locations across the security element at the or each configuration of the filter system, such that the map is a map of a variation in determined data indicative of an optical property across the security element with respect to the configuration of the filter system. In most practical implementations, it is most likely that the reading for multiple locations across the security element is undertaken in a single reading step, using a reader with a 2D sensor. 
     The main, overall and general concept of the present invention is perhaps best shown in  FIG. 2 .  FIG. 2  shows a security element  20 , and a mobile device  22  for use in reading that security element  20 , for example via the use of a flash  24  of the mobile device  22  for use in irradiating the security element  20 . In these respects, the methodology and system generally depicted in  FIG. 2  is almost identical to that shown in and described in reference to  FIG. 1 . The important difference with  FIG. 2 , and thus the important feature defining the present invention, is the presence of a configurable optical filter system  26 , via which the security element  20  is to be read by the mobile device  22 . 
     The configurable optical filter system  26  allows the optical reading of the security element  20  to be undertaken at a plurality of configurations of the configurable optical filter system  26 . The optical transmission property of the filter system  26 , either inherently or relative to the mobile device  22  or element  20 , are different for each configuration. That is, different optical properties of the element are readable at each configuration. This means that a far richer and more complex, and therefore harder to copy, map of emission wavelengths or energies (or other optical property) from the security element  20  can be very easily obtained, and used in the determination of a unique identifier for the security element  20 . 
     The optical filter system  26  may be configurable in any one of a number of ways in order to ensure that a readable optical transmission property of the filter system varies with respect to the particular configuration. For example, the filter system  26  can be configured to have different transmission properties with respect to the polarisation of emitted radiation. In another example, the filter system  26  may be configurable to vary the intensity of transmitted electromagnetic radiation. However, it is considered that the most effective filter system for the purposed of example embodiments will be one which is configurable with respect to the variation in transmitted electromagnetic wavelengths or energies. This will allow the filter system to be used to selectively discriminate between emitters of different wavelengths forming part of a security element, such that different configurations of the filter system can be used to detect the presence or absence of certain emission wavelengths or energies from one or more components of the security element. 
     The filtering by wavelength is particularly useful when the security element comprises one or more continuous or discrete components exhibiting quantum mechanical confinement, the or each component being capable of emitting electromagnetic radiation linked to the quantum mechanical confinement. Preferably, the confinement of the one or more continuous or discrete components confines in one or more of 3D, 2D, 1D or 0D. The security element will preferably have a mixture of such components and related confinements. In other words, the one or more components forming the security element preferably emits radiation at more than one different wavelength or energy, and/or has or have different emission spectra. For example, components exhibiting 3D quantum mechanical confinement might give a generally flat background emission spectra. 2D confinement might give a broad, sloping background, or perhaps a broad peak. 1D confinement would exhibit narrower peaks in emission energy. 0D confinement would exhibit very sharp peaks in emission spectra. Combinations of one or more degrees of confinement would lead to particularly complex emissions spectra, lending itself to filtration by wavelength, and unique identifiers being based on such filtration. That is, at one configuration an emitter, or its emission, might not be readable, while it can be read at another angle. Different portions of the different emission spectra can be taken advantage of at different configurations. For example, when read with the system used in  FIG. 1 , a single snapshot or reading might not be able to pick up the subtleties in the different natures of emission spectra, and in particular, when these emission spectra are linked to the particular characteristic spectra associated with particular degrees of quantum mechanical confinement. However, and in contrast, the present invention takes advantage of this different spectra in a powerful manner, since changing the configuration of the optical filter system, and undertaking optical readings of the security element at these different configurations, will allow these different emission energies and spectra to be detected or otherwise discriminated, providing a much richer map of emission energies with respect to the different configurations of the optical filter system. This will allow a unique identifier to be determined which is far harder to replicate of circumvent. 
     The optical filter system may be changed from one configuration to another, different, configuration in any appropriate manner. This might, of course, depend on the actual inherent nature or mechanics of the optical filter system, or the nature of emission that is being filtered for. 
     In a basic example, the optical filter system might simply comprise of a plurality of different optical filters, each filter having a different optical transmission property, such that the optical filter system is configurable with respect to which filter of the plurality is used. The different filters could have different polarisations, or transmission bands. This might be a relatively cheap and simple way of implementing a filter system, since there is no absolute requirement for complex control of the filter system, for example using associated electronics or control circuitry. A potential drawback is that a number of optical filters are required, which while relatively easy to achieve in a laboratory, manufacturing or test environment, might be far harder to achieve for an end-user, or consumer, or similar. 
     Other implementations are, of course, possible. For instance, the optical filter system might comprise an optical filter, an optical transmission property (e.g. transmission band) of the filter system varying with respect to one or more of a temperature of that filter, an electric field across that filter, or an applied driving frequency when the optical filter is an acousto-optic tuneable filter, such that the optical filter is configurable with respect to which temperature, electric field, or driving frequency is used. For instance, the temperature of, or electric field across, a filter might affect its dimensions or optical properties (e.g. a configuration of multilayers, or the birefringence), such that different configurations can be achieved. These sorts of optical systems might be more readily tuneable than the use of multiple filters described previously. However, at the same time, such a tuneable filter or associated driving circuitry or similar might add to cost or complexity in comparison with the use of a number of relatively simple, different, optical filters. 
     A perhaps ideal implementation would be a system which has the tuneability described above, but also the simplicity described above. A good example of such a system is one which comprises an optical filter, an optical transmission property of the filter varying with respect to an angle of orientation of the filter, such that the optical filter system is configurable with respect to the angle of orientation of the filter. This approach is very simple, in that only a single optical filter is required either at the manufacturing or test stage, or at the end-user or consumer stage. Also, complex control or driver electronics are not required. There simply needs to be a way of implementing different orientations of the filter relative to the reader or security element to achieve the different configurations. This could amount to a user tilting the filter or reader, or rotating the filter or reader. So, tuneability and simplicity are achieved. The filter need only have transmission properties that vary with respect to the relative orientation, for example by way of a multilayer structure (for tilting) or a property (e.g. thickness or configuration of layers) that varies across the filter (for rotation). 
     On the basis of the above, a perhaps ideal implementation of optical filter system configurable by way of the relative orientation of the filter of that system will now be described in more detail. However, it will be appreciated that general principles will apply to any configurable optical filter system. 
       FIG. 3  schematically depicts a plot  30 , showing transmission properties of an optical filter, in this case a band pass filter. A centre transmission wavelength of the filter is shown as the angle of incidence of the filter with respect to the emission source (or, indeed, angle of reading by a reader) is varied. Similarly, the full width at half maximum (FWHM) bandwidth of the filter is also shown as the angle is varied. 
     It can be seen from  FIG. 3  that the FWHM bandwidth remains substantially constant, irrespective of the angle of orientation. However, and most importantly, it can be seen that the centre transmission wavelength varies quite considerably as the angle of orientation is varied. So, in this example, it can be seen that when the emission wavelengths are incident or read perpendicularly with respect to the filter (zero degrees as shown in the plot) the centre wavelength is 550 nm. As the angle is varied, it can be seen that the centre wavelength decreases to around 485 nm when the angle is 60 degrees, and that there is continual variation in the centre wavelength between these two extremes shown in the plot  30 . It is this variation in the centre wavelength of the filter which allows for the selection and discrimination of different emission wavelengths and energies as alluded to above, which can lead to a relatively simple way of vastly increasing the complexity of the map derived from the security element, and the associated security provided by the unique identifier derived from that map. This is because a map can be created that does not just comprise emission energies and intensities across the security element, but instead emission energies and intensities (or similar, e.g. related data of peak or troughs) across the security element with variation based on orientation. 
     Again, and to reiterate the point, while  FIG. 3  shows a situation of varying the angle of incidence or reading of emitted radiation with respect to a band pass filter, the same general principle shown in  FIG. 3  would apply to the other optical filter system implementations described above, for example the use of different filters with different transmission properties, or the heating of a filter, or the changing of an electric field across that filter, or the changing of the driving frequency of an acousto-optic filter. Again, the advantage is that a map can be created that does not just comprise emission energies and intensities across the security element, but instead emission energies and intensities (or similar, e.g. related data of peak or troughs) across the security element with variation based on configuration of the filter. 
       FIGS. 4 and 5  schematically depict how the angle of orientation of a filter can be changed with respect to emitted electromagnetic radiation. The same principles apply to changing angles at which the radiation is read by a reader. 
       FIG. 4  shows that a band pass filter  40  may be tilted  42  with respect to a general direction of incoming emitted electronic radiation  44 . Optical path lengths within the filter  40  will be altered as a result, changing the optical transmission properties of the filter  42 . 
       FIG. 5  shows how a band pass filter  50  with a wedge shape (or, more generally, a transmission property that varies across the filter  50 ) may be orientated, or re-orientated, by rotating  52  the filter  50  about a longitudinal axis along which emitted electromagnetic radiation  54  generally propagates. Optical path lengths, or paths in general, within the filter  50  will be altered as a result, changing the optical transmission properties of the filter  50 . 
     In one example, an interference filter has alternating layers of different refractive index. In a normal filter these layers would be of fixed width/thickness with respect to distance along or across the filter, and in a wedge-shaped filter such as in  FIG. 5  one or more individual layer thicknesses could also be wedges. That is, there might not be more layers as a function of position—the layers could have a different thickness with respect to position. 
     As might already be appreciated in the plot in  FIG. 3 , the discrimination between different emission energies, or in other words the reading of the security element at different configurations, can reveal quite startlingly different outputs, and ones which are readily detectable by a consumer level mobile device, such as a hand held device or wearable device (e.g. generally portable device) comprising a 2D optical sensor. A typical device might be a mobile telephone, or tablet.  FIG. 6  demonstrates just how varied the readings can be. 
       FIG. 6  schematically depicts a series of optical readings  60  taken of a security element. This series progressively shows the variation in orientation of a band pass filter via which the security element is read with respect to radiation emitted by the security element. The series shows six images, taken in five degree increments away from normal (perpendicular) incidence of the emitted radiation onto and through the filter. In accordance with the plot shown in  FIG. 3 , mapped on to the actual readings shown in  FIG. 6 , it will be appreciated that each image shown in the series  60  reads or otherwise discriminates or filters for emitters or emissions at different wavelengths, ranging from approximately 550 nm at normal incidence (0 degrees) up to around 535 nm at 25 degrees (away from the normal). 
     Using the system of  FIG. 1 , where no deliberate/external filter is used (and certainly no re-configuration of any optical system is used) only one of the readings shown in  FIG. 6  would ever be undertaken, and likely not with a optical band pass or notch or other filter, but simply reading across all visible wavelengths at any one time. Again, while the resulting reading, and underlying distribution of emitters might be quite difficult to copy, it is not impossible. In contrast, the series  60  in  FIG. 6  shows that the readings and associated mapping of emission energies may be vastly more improved, and security provided more complex to overcome, by use of the configurable optical filter system as described above. Quite simply, the use of the optical filter system allows for the discrimination of different emission spectra or different types of emitters, whereas the proposed proposal of  FIG. 1  does not. Again, the advantage is that a map can be created that does not just comprise emission energies and intensities across the security element from a single reading, but instead emission energies and intensities (or similar, e.g. related data of peak or troughs) across the security element with variation based on configuration of the filter. 
     More detailed implementations of the present invention are now discussed and described, and some more general principles associated with systems and methods are also provided. Some of those principles may be interchanged and/or replaced with one another, as will be understood by the skilled person. 
       FIG. 7  shows a security element  70  and a mobile device  72  for use in optically reading that security element  70 . The security element  70  is read via an optical filter system  74 . In this implementation, methodology might comprise causing the security element  70  to emit electromagnetic radiation via a light source, optionally a broadband light source  76  provided by the mobile device  72 , for example a flash or other light source of the mobile device  72 . In response to being irradiated with the light  76  from the mobile device  72 , the security element  70  may emit radiation  78  back towards the mobile device  72 , to be read via the optical filter system  74 . In order to reconfigure the reading via the optical filter system  74 , the optical filter system  74  may be orientated or re-orientated as described above, and/or the device  72  can be orientated or re-orientated as described above. 
       FIG. 8  shows another security element  80  and another mobile device  82 . In this example, the security element  80  can be caused to emit electromagnetic radiation via irradiation by a secondary light source  84 , not forming part of the mobile device  82 . The source  84  might form part of the element  84 , or object to which the element  80  is attached, or be external to the element  80 . Again, the mobile device  82  may read radiation that is emitted  86  by the security element  80  via an optical filter system  88 . In this example, the optical filter system  88  may not be freely located in-between the mobile device  82  and the security element  80 , but may cover or even form part of the security element  80 . The optical filter system  88  may then be reconfigured by appropriate reorientation of the security element  80  on which the filter  88  is located, or of which the security filter  88  forms a part, or via orientation or reorientation of the mobile device  82 . This latter option is still to be understood as a configuration or reconfiguration of the filter system  88  relative to the reader  82 , since the filter is being optically read or inspected at a different angle via the orientation or reorientation, as described above. 
       FIG. 9  shows another security element  90  to be optically read by another mobile device  92 . In this example, the security element  90  can be caused to emit electromagnetic radiation via electrical excitation  94 . Emitted radiation  96  may be read via the mobile device  92  after the radiation  96  has passed through an optical filter system  98 . Much as with the system and methodology shown in  FIG. 8 , the optical filter system can be reconfigured by simply re-orientating the mobile device  92  or the security element  90  such that the filter  98  is orientated at different angles with respect to the reader  92  to achieve different readable configurations. 
     Generally speaking, a security element can be caused to emit electromagnetic radiation in whichever way is suitable, for example, depending on how the security element emits radiation, or the physical mechanism by which one or more components of the security elements may be caused to emit radiation, or depending on the implementation of the element in conjunction with an object to be authenticated (e.g. the availability of an external light source or electrical power supply). Generally speaking, causing the security element to emit radiation might generally involve irradiating the security element with electromagnetic radiation, or via some form of electrical excitation of the security element or components thereof. Irradiating the security element with electromagnetic radiation may be undertaken in such a way that emission occurs by non-resonant photoluminescence, such that it is easier to distinguish the emitted radiation from the radiation used to excite or stimulate the security element. More generally speaking, the irradiation of the security element may be such that an irradiation wavelength is different from an emission wavelength of the security element, again, in order to make it easier to distinguish excitation radiation from emission radiation. 
       FIG. 10  schematically depicts a more detailed exemplary embodiment of the invention for determining a unique identifier for a security element.  FIG. 10  shows that the system comprises a substrate or other object  100  onto which the security element described above has been provided. An electromagnetic radiation source  104  is used to irradiate  106  the security element  102 . An optical reader  108  is used to optically read or inspect the emission from the security element  102  via an optical filter system  110 . An initial reading might be undertaken when the optical reading or inspection is undertaken normally (perpendicularly)  112  with respect to both the filter system  110  and security element  102 . From this reading across multiple locations of the security element  102  (e.g. taken in a single step with a 2D sensor or similar) a map  114  (e.g. a spectrum or similar) of emission energy (wavelength) versus intensity at this first angle  112  is established. 
     Then, the process is repeated when the filter system  110  is orientated at a second angle  116 , in order to establish a second map  118  at this second angle  116 . The method may continue at a third angle  120  to establish a third map  122  at that third angle  120 , and so on. As discussed below, the maps at each angle may be combined into an overall map of how the emission energies vary across the element  102 , and with variation in orientation of the filter system  110 . 
     In a laboratory, manufacturing or testing environment, the angle of orientation of the filter  110  with respect to the reader  108  and/or with respect to the security element  102  can be controlled in a number of different ways. It might be quite straightforward to fix the orientation of the reader  108  and the security element  102 , and to vary the angle of the filter  110  by using a step or servo motor, or some other easily controllable holder or driver. As discussed in more detail below, one or more designated features may be used to determine, at least partially optically, the relative orientation between the elements of the system, in order to establish which angle or angles are involved in the reading phase of the method, and/or to correct for that angle when establishing a map. 
     In a laboratory, manufacturing, or testing environment, there may be a greater degree of control of the components of the system, and/or the associated quality of those components in terms of sensitivities, resolutions, and so on. These may be used to establish a master or high quality map for use in establishing one or more unique identifiers for the security element, for future reference when that same security element is to be used in some form of identification process. The identifier might be the map, or a part thereof, for example the location of one or more peaks, troughs, points of inflection, or so on, at one or more angles, in measured spectra. These could be absolute measured values, or indicative values, for example the 2D location of the peaks or troughs, or the number of peaks or troughs at a given angle, or number of angles. The unique identifier may depend on the nature and level of security that is required. 
       FIG. 11  depicts a slightly different methodology to that shown with reference to  FIG. 10 , where in  FIG. 11  the orientation of a reader is changed with respect to the security element and associated filter system. 
       FIG. 11  shows a substrate or other object  130  onto which is placed or otherwise located a security element  132 . In this Figure, a filter system  134  is located on top of the security element  132 , and may even form a part of that security element  132  in some form of composite or laminate structure or similar. Again, an electromagnetic radiation source  136  is used to irradiate  138  the security element  132 . In this case the irradiation also takes place via the filter  134 . A reader  140  is provided for reading the security element  132 , optionally via a broadband filter  142  which may be used to reduce noise or similar. A reading may initially be undertaken at a first angle  144 , where the reading is undertaken normally (perpendicularly) with respect to the filter  134  and element  132 . 
     As with the system shown in  FIG. 10 , the angle of orientation of the filter  134  with respect to the reader  140  is controllable by one or more appropriate servo or stepper motors or other drivers or controllers. The element-filter  132 ,  134  combination could be oriented as required, and/or the reader  140  could be oriented as required. However, it might be simpler, easier or simply more convenient and more flexible to avoid the need for such control, and to instead actively determine the angle of orientation  144  via analysis of a designated feature  148  associated with one or both of the filter  134  and/or security element  132 . For instance, this might be a dedicated alignment marker, the relative orientation or distortion of which can be used to determine the angle or angles of orientation of the marker  148  and thus the filter  134  and/or security element  132  relative to the reader  140 . A reflective or holographic grating could alternatively or additionally be used, the properties of radiation emitted or otherwise reflected or similar by the grating being used in determining the angles of orientation. A designated feature might be a designated alignment marker that is completely separate to any other component, functional or otherwise, of the security element  132  or overlying filter  134 . Alternatively or additionally, the designated feature might be part of a label provided on the filter  134  or security feature  132  or somehow associated therewith. For instance, the security element  132  might be provided with or be otherwise associated with a brand name or a label, or similar, and one or more features of that brand name or label or logo can be used to determine the orientation of the filter  134  or security element  132  based on determined distortion orientation or similar of that designated feature. Any marker from which angle variation can be determined may be used. The angle calculated might also be used to correct for distortion in the read shape or map of the element. 
     Although finding advantageous use in the present invention, in terms of the freedom that such a feature might provide for use in determining degrees or angles of orientation, further detail on such markers are not provided herein, since such used and related processing is already known in fields such as those detailing or relating to QR codes and similar. 
     The reading, in combination with the determination of the angle, is used to determine a map  150  (e.g. a spectrum) of emission energies versus intensities at that first angle  144 . 
     Further maps  152 ,  154  may then be established at different orientation angles  156 ,  158 , the determination of those different angles being possible via the calculation of the degree of distortion  160  of the designated feature  148  as already described above. 
       FIG. 12  shows how a number of different maps  170  obtained relative to specific orientations of the filter system (e.g. those shown with reference to  FIG. 10 or 11 ), may be combined into an overall map  172  of emission energy (wavelength) versus intensity at different positions or locations across the security element, and at different angles of reading relative to the filter system/different angles of the filter system. 
     The same Figure shows a grid or array  174  which might reflect, be indicative of, or actually equate to an array of pixels in a 2D sensory array—e.g. that used to read the maps  170 . This might also be another way of visualising a map  172 . For a particular pixel, group of pixels, or map point  176 , a plot  180  is shown which represents how the emission energy (wavelength) versus intensity varies at different orientation angles of the filter  182  for the location in the map (and thus location of the security element). It will be appreciated that the width of each energy band at each orientation angle corresponds to the bandwidth or notch or pass wavelength or wavelengths of the filter system at that particular angle of orientation. This principle can be used in a number of different ways, for instance using a very narrow band pass filter at a large number of different orientations to build up a continuous energy spectrum as shown on plot  180 , or to determine the emission spectra at discrete points. A broader or wider transmission band might not resolve the emission spectrum in as much detail, but could nevertheless still be useful. 
     It will be appreciated that the actual measured data could be the unique identifier that is drawn from the map, for example the spectrum at a particular location and at a particular orientation angle of the filter system, or similar. Perhaps more generally, the read or determined data might generally be described as being indicative of an optical property of the security element. It may be indicative in that it might be an actual optical property of the security element, for example a particular wavelength of intensity at that wavelength or energy or similar. Alternatively or additionally, the data indicative of the optical property might be one or more portions of an emitted electromagnetic spectrum of at least a part of the security element. Alternatively or additionally, the data indicative of an optical property might be a peak, trough, or point of inflection in an electromagnetic emission spectrum or at least a part of the security element. Alternatively or additionally, the data may be even more representative, and for example be or equate to a physical location in relation to the security element of an actual optical property, or a physical location of a peak, trough, or point of inflection in an electromagnetic emission spectrum of the security element in relation to the security element. The unique identifier could be the number of peaks at a certain angle, or the number of troughs. Generally, then, the data could be actual data of one or more read features, or representative data, such as a location or count of such features. Features could be read values that do, or do not, exceed a certain value, or which equate to a certain value. That is, the data that is read or obtained or used to provide the unique identifier could be actual absolute measured values, or could be something that is derived from such measured values. Any and all of this can be used to provide a unique identifier, since, as described above, each mapping will be unique to the type and nature of emitters and associated emissions of components forming the security element. 
     The maps  170 ,  172  shown in and described with reference to  FIG. 12  may be stored in an online database, or some other storage medium, so that the database and the maps within can be later accessed for use when subsequently authenticating the security element, for example when the security element is in use and being used to label or identify or otherwise secure an object which the security element is attached or similar. The maps may be stored in a secure location, so that access to the maps is restricted. 
     Once the map of emission energy and intensity as a function of orientation angle of the filter is established (or at least a portion is established), this map can then subsequently be used to authenticate the security element, during a subsequent reading of the security element. Briefly, if a reading of a security element does not reveal a unique identity derived from an already determined map that is present in the database, or match a particular entry in the database, then the security element might not be deemed as authentic. 
     The reading of the security element for authenticating purposes might be much the same as already described above in relation to previous methods and systems. The difference when authenticating is that there will already be a pre-established or determined map to compare newly read data with. 
     When it comes to authenticating a security element, a request or related information from a database (or controlling software) might be sent to a consumer or end-user, or their reader, for undertaking the authentication. This might, of course, be largely transparent to the user. The user might simply be asked to point the camera of the mobile device at a security element at a particular angle, or range of angles, for example taken in the form of a short video, series of photos or similar. The user might simply be asked to point the camera of the mobile device, and the or any reading at the angle of reading then compared with the mapping in the database. That is, the user might not actually be asked to use a particular reading angle. Instead, this might be left to chance or choice of the user. The user might not be aware of the mechanisms underpinning the authentication described herein. 
     In a related example, data from a database representing entries in the map might not need to be sent to the reader of the end-user or consumer. Instead, when authentication is required, the reader of the end-user or consumer might be internally or externally prompted to simply undertake the above methodology for determining optical properties of the security element at a range of angles, or at a particular angle. Once this has been undertaken as described above, this data could then be anonymised and sent back to the database or controlling software for comparison with entries within that database. If these entries match, the database might simply provide identifying location signal or message back to the user or consumer. 
     In a related example, the database might send a unique identifier from a stored map, and this is simply looked for at the reader end. For example, the database might send data indicating that X peaks are expected at angle A, Y peaks are expected at angle B, and Z peaks are expected at angle C, and so on. If one reading matches one of these identifiers, the element may be deemed authentic. Alternatively or additionally, something like a hash function could also be used to abbreviate and/or anonymise the (more complex) map 
     The mapping across different configurations might allow for tiered or more robust security. For example, the unique identifier might be based on features across or within a map at a given angle, and/or across or between angles. In a crude and random example, the unique identifier might be based on there being a peak A at location B and angle C, and there being D peaks at angle E, and there being a change in number of peaks from F to G with a change in angle from H to I. Again, the power is in the mapping being across different angles (configurations), to give a very rich map for possible unique identifier determination. 
     In one example,  FIG. 13  shows how some sort of request may be made or prompted for an end-user to optically read the security element for identification purposes at a subset of angles  200 ,  202 , which will correspond to probing different parts of the (previously mapped) emission spectrum  204 . At one or both of these angles  200 ,  202 , there may be a unique map of optical properties across the security element, and this read by a 2D sensor of the reader as shown by an array or map  206 . Black dots in that array  206  may indicate where, at a particular angle  200 ,  202  or even a combination of those angles  200 ,  202 , a particular optical feature is present. As discussed above, this may be an absolute measured value of the optical feature, or could be simply the location of an emission peak, and emission trough, a plateau, a point of reflection, or similar. 
     The entire measured data set could then be transmitted to a database  210  or controlling software, which stores the mapped optical properties measured previously at the different angles, described above. Alternatively, only particular data may be transmitted, for example the physical location of the particular optical features as discussed above, for example the location of peaks, troughs, points of inflection and so on. This latter example might reduce an amount of data that needs to be generated and/or received. For example, it might be far simpler, easy and quicker to transmit the pixel locations, map locations, or physical locations, (all of which are related to one another) where peaks are detected, than it would be to transmit the entire spectrum across the security element for the different measured angles. Or, the hash (or similar) of this data, could be transmitted, as discussed below. 
       FIG. 14  schematically depicts methodology for optically reading and verifying a unique identifier associated with the security element. Initially, a user chooses or is instructed to choose a random subset of angles for which optical measurement is to take place  220 . 
     Depending on the setup, the user may choose how the measurements are to be undertaken, or be forced by availability of hardware into a particular choice  222 . That is, the user may be able to vary the orientation angle between a reader of a security element, and the filter that is attached to or provided with the security element, or may be able to independently vary the angle of the filter that is moveable separately to one or both of the security element and/or reader, or described above. 
     The optical reading then takes place at the different angles  224 . 
     Once obtained, output data may be anonymised into, for example, a 1D key with an appropriate algorithm (for example a hash function or similar)  226 . 
     This data may then be corrected for environmental noise and other factors, for example using a fuzzy extractor or similar  228 . 
     The user (which includes the reader used by the user) will then announce the angles and possibly band pass filter that were used for the measurement  230 . “Announce” does not necessarily mean that the user has to actually calculate and be aware of the angles and/or band pass filter that has been used. This is likely to all happen in the background, largely transparent to the user. The angles can be determined as discussed above. The type of band pass filter could be known in advance, or optically read via a marker or similar on the band pass filter. It might even be possible to determine the type or nature of band pass filter on-the-fly, e.g. to determine the characteristics of the band pass filter from the measurements that have been taken. It might be possible for the read data to still be useable in some way for comparison with predetermined maps, without knowing exact details of the filter used in the reading. For example, even if intensities are different, there might always be an emission peak at wavelength X, in general location Y, and angle Z, and so on. This sort of coarse data might still be useful in determining a unique identifier. 
     The information is then transmitted to the database which automatically selects the data that is needed for comparison with the measured data, for example based on the announced angles  232 . 
     The calculated data is then put into a similar algorithm (e.g. hash function) as used to minimise the measured data, to create a similar 1D key  234 . 
     The two extracted 1D keys are then compared to one another, and if they agree then the security element is verified as being authentic  236 . 
     Error margins may vary depending on the security requirements, for example with perhaps larger error margins being allowed for less robust security requirements, and being tighter for more secure environments. Hash functions generally don&#39;t allow for margins of error—if input data is changed even slightly then the output should be randomly different. Error correction therefore likely has to be done prior to hashing, by some form of smoothing, or approximating of the data. Another possibility is sending information about features/sections of the security element (e.g. the map data or unique identifier data) hashed separately, and authenticating based on partial success, e.g. one of the hashes passing a comparison test or similar. 
       FIG. 15  depicts perhaps more general methodology for determining an identifier for a security element. 
     The method comprises optically reading the security element via a configurable optical filter system, a readable optical transmission property of the filter system  240  varying with respect of the configuration of the system. 
     The reading comprises determining data indicative of an optical property of the security element at a first configuration of the system, and determining data indicative of an optical property of the security element at a second, different configuration of the system  242 . 
     The unique identifier is determined from a map of the variation in determined data indicative of a property in respect to the configuration of the filter system  244 . 
     In all practical likelihood, the reading will be undertaken for multiple locations across the security element at the or each configuration of the filter system, such that the map is a map of the variation in determined data indicative of the optical property across the security element with respect to the configuration of the filter system  246 . 
     In all practical likelihood, the reading for multiple locations of the security element is likely to be undertaken in a single reading step, using a reader with a 2D sensor  248 . 
       FIG. 16  depicts a general system for determining the unique identifier for a security element  250 . The system comprises a configurable optical filter system  252 , a readable optical transmission property of the filter system  252  varying with respect to configuration of the filter system  252 . The overall system also comprises an optical reader  254 , for optically reading the security element  250  via the optical filter system  254 . The reading comprises determining data indicative of an optical property of the security element  250  at a first configuration of the filter system  252 , and determining data indicative of an optical property of the security element  250  at a second, different configuration of the filter system  252 . The unique identifier is determined from a map of variation in determined data indicative of an optical property with respect to the configuration of the filter system  252 . 
     Although, strictly speaking, not essential, in practice the reader  254  is highly likely to be arranged to undertake readings from multiple locations across the security element  250  at the or each configuration of the filter system  252 , such that the map is a map of the variation in determined data indicative of an optical property across the security element  250  with respect to the configuration of the optical system  252 . 
     Again, although not essential, in practice it is highly likely that the reader  254  will comprise a 2D sensor  256  for reading multiple locations across the security element  250  in a single reading step. 
     In other words, in another embodiment it may not be necessary to read across multiple locations, and/or undertake such readings in a single step. For example, reading at one or more discrete locations might be satisfactory, for example at one or more particular locations of the security element. The readings could be undertaken at one or more, but not all, locations at a time. A one dimensional sensor may be satisfactory, for example reading across or along a line (e.g. row or column or diagonal) of locations of the security element, as opposed to across the element in two dimensions. A sensor that can only read a single location may also be satisfactory, even if used on multiple occasions to take readings across the element. A two-dimensional sensor may simply be more effective and efficient for taking multiple readings across the element, and is the sort of sensor typically found in mobile devices as described herein. That is, specialist equipment is not required. 
       FIG. 17  schematically depicts a general method of authenticating. The method comprises optically reading a security element via a configurable optical filter system, a readable optical transmission property of the optical system varying with respect to a configuration of the filter system  260 . 
     The reading comprises determining data indicative of an optical property of the security element at the configuration of the filter system  262 . 
     The authenticating further comprises comparing  264  determined data indicative of an optical property with the unique identifier determined using the method or system of  FIG. 15 or 16 . The determined data may itself be a unique identifier, such that the unique identifier read at the authentication stage is compared with a previously determined unique identifier. 
     Related to the method of  FIG. 17  is a general authentication or authenticating system, as shown in  FIG. 18 . The system comprises a configurable optical filter system  270 , a readable optical transmission property of the filter system varying with respect to a configuration of that filter system  270 . An optical reader  272  is also provided, for optically reading a security element  274  via the optical filter system  270 . 
     The reading comprises determining data indicative of optical property of the security element at a configuration of the filter system  270 . The overall system is arranged to compare the determined data indicative of an optical property with the unique identifier determined using the method or system of  FIG. 15 or 16 . 
     Again, although not essential, in practice it is highly likely that the reader  274  will comprise a 2D sensor  276  for reading multiple locations across the security element  270  in a single reading step. 
       FIG. 19  schematically depicts a security element for use in accordance with the methods and systems described above. The security element comprises a first part  280  suitable for emitting electromagnetic radiation, as described above. For instance, the first part  280  may comprise one or more continuous or discrete components, particularly in the form of one or more continuous or discrete components capable of emitting electromagnetic radiation as a result of, or associated with, quantum mechanical confinement exhibited by those one or more components. 
     The security element also comprises a second part, in the form of an optical filter system  282 , a readable optical transmission property of the filter system  282  varying in respect to a configuration of the filter system (e.g. inherently, or relative to a reader of the security element). The first part  280  is readable via the second part  282 . The first part  280  may be attached to or form part of an object  284 . The object  284  might be part of the security element, for example a substrate or support for the first part  280 , or might be a separate object, to which the security element is attached for use in authentication. 
     It has been discussed above how the invention is based on optically reading the security element via a configurable optical filter system, a readable optical transmission property of the filter system varying with respect to a configuration of the filter system. This, of course, covers the filter being orientated with respect to the reader, but also includes the reader being oriented with respect to the filter. In either example, a readable optical transmission property of the filter system varies with respect to a configuration of the filter system—i.e. its angle relative to the reader. 
     It might be useful, and easier, to use the same sort of filter when determining the map or identifier for the first time (e.g. in a laboratory, factory, or test environment), as when undertaking the authenticating the security element at some later time. However, this might not be possible or practical. So, the initial determination of the map or identifier might be undertaken for more than one filter, to establish a number of different possible maps, or offsets for a give map for each filter. When one of these filters is then used in an authentication process, it will still be possible to correlate the measurement for authentication with the measurement for the initial determination. Indeed, it might even be possible to not know what filter is being used, and still be able to determine if the security element is authentic from relative changes optical properties at different angles, even if undertaken in a coarse (and perhaps less secure) manner. For example, the presence of peaks in a general location at one angle, and then at a general location at another angle, might be enough for authentication purposed. However, knowing the type or nature of filter will allow for a far more secure process, since more useful information can be obtained and processed. It might well be that the properties of the filter could be obtained from an incidental background reading using ambient light, or a via a deliberate calibration step. 
     The filter system described above is something of a deliberate filter system, where a dedicated filter has been used to obtain the required measurements. That is, the filter system is separate from the device, or added to the device at a later stage. It might be possible to implement the above methodology using filtration principles already existing in a device, for example due to the presence of ultra violet or near infrared filters present on lenses or sensors of existing readers. Such incidental filtering might not have the flexibility of the deliberate, dedicated filters systems described above. 
     Following on from at least the preceding paragraph, it is worth noting that in all embodiments, taking a reading through the configurable filter system is always discussed. It is such a reading, and general approach, which allows the benefits of the invention to be realised, of course. It is therefore clear that even a single reading, at a single given configuration, is still inventive for the reasons already discussed at length above, because the filter system is configurable (even if not always used at different configurations) to facilitate an increase in a richness or depth of a map, and/or configurable to take advantage of a previously obtained map. Also, this is consistent with a single reading at a single configuration being used to compare with (e.g. authenticate via) a map that might well have been built on multiple readings at multiple locations. That is, it is the configurable filter system that is key, even if readings are not undertaken at different configurations in all examples. Whilst multiple readings may be used to establish a rich or deep map, these can clearly (as above) be undertaken with single readings, undertaken at single, different configurations. So, a system and method for undertaking such a single reading at a single configuration, facilitates this. Such use of a configurable filter system has not been contemplated previously. 
     The security element described herein does not necessarily need to be shown, marked or advertised as such. The element could be discretely located on, alongside, on or within an object for which authentication is required. The element could be applied when the object is made, or retrospectively. 
     The security element described herein generally functions as an optically readable physical unclonable function. 
     Determining of a unique identifier, or other information, might comprises defining, obtaining, checking, confirming, or so on, that unique identifier, or other information. 
     Although a few preferred embodiments have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims. 
     Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. 
     All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. 
     Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
     The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.