Optically readable physical unclonable functions on a three-dimensional surface

Abstract

An article comprises a three-dimensional surface covered with a functional layer comprising an optically-readable PUF (physical unclonable function), whereby, in use, reading the PUF from multiple per

Description

The present invention relates, generally, to optically readable physical unclonable functions (PUFs), methods of preparing optically readable PUFs, articles containing optically readable PUFs and methods of using such articles. 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 identity (i.e. identifier) in one form or another. “Unique” might not necessarily mean that it is impossible for another object to have the same identity, but instead that it is statistically highly unlikely for this to be the case or, in other words, for the identity to be accidentally stumbled across by guesswork or simple trial and error. The very same “uniqueness” might be used in other ways, too, for example for highly targeted marketing or data acquisition with respect to the object or a user or consumer of that object. A unique identity might, for example, take the form of or be derived from a physical (sometimes referred to as physically) unclonable function (a PUF). 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 identity. 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 identity. 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 identities do not necessarily need to be based on electrical principles. For instance, physical unclonable functions may be probed or otherwise challenged (i.e. read) optically in order to determine a unique identity. 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 identity. Traditionally, the generation of unique identities, and / or associated use of physical unclonable functions, have been based on macroscopic effects. More recently though, it has been proposed to incorporate micro and even quantum mechanical effects in the generation of unique identities. In these more recent examples, for instance, an electrical component exhibiting quantum mechanical confinement (e.g., a resonant tunnelling 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 identities are based on quantum mechanical principles. Similarly, optical based physical unclonable functions may be based on the emissions spectra of molecules, quantum dots, 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 or exhibiting quantum mechanical effects. This is, to the extent that the unique identity provided by such an element may not be circumvented and certainly not in any practical timeframe. In the provision of optical based physical unclonable functions, it may be difficult or expensive to manufacture them, or difficult to manufacture or apply them in a way that results in a practically useful implementation. It is an example aim of 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 is provided a method 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. The following examples and statements may assist in the understanding of the invention, or related concepts. Again, the scope of the invention is defined by the claims that follow. According to a first example providing useful context for, but not within the scope of, the present invention, there is provided a method of making an optically readable PUF coating composition, the method comprising increasing the entropy of a coating composition by adding a first optically readable material to the coating composition. By “optically readable PUF coating composition” is meant a composition which, when coated onto a substrate, provides an optically readable PUF (i.e., physical unclonable function). The optically readable PUF can be used to provide a unique identity which can be verified by optical means. The properties of the optically readable PUF may depend on small variations in construction or fabrication or similar. By “entropy” is meant the degree of randomness or non-uniformity of distribution of components in the coating composition. Entropy may be measured by the uniformity of an optically readable PUF prepared from the optically readable PUF coating composition, as described in the examples. The method of the first example advantageously provides an optically readable PUF coating composition which comprises a random, non-uniform distribution of an optically readable material. This may provide an optically readable PUF with improved properties, such as uniqueness and readability while maintaining the coatability of the coating composition. The method of the first examplet is surprising as typically, coating compositions with minimum entropy are desirable. The method of the first example may also allow an optically readable PUF to be produced which does not necessarily rely on perhaps more specialised materials or processes, such as those more typically associated with quantum dots, quantum wires, flakes or layers of a substantially two-dimensional material, and so on. Therefore, advantageously, the optically readable PUF coating composition may be more cheaply or easily produced using widely available starting materials. Prior to addition of the optically readable material, the coating composition suitably does not provide an optically readable PUF when coated onto a substrate. For example, the coating composition may be free or substantially free of the first optically readable material. By “substantially free”, is meant that the coating composition comprises no more than trace amounts of the first optically readable material. The coating composition may comprise a solvent. The solvent may comprise a polar solvent and / or a non-polar solvent. Preferably the solvent comprises a polar solvent. The solvent may comprise an organic solvent and / or an inorganic solvent. The solvent may comprise a mixture of an organic solvent and an inorganic solvent, such as water. Preferably, the solvent comprises an organic solvent. The organic solvent suitably comprises a polar organic compound having 6 or fewer carbon atoms. The organic solvent may comprise an alcohol and / or a ketone. Suitable examples of organic solvents include ethanol, propanol, isopropyl alcohol and acetone. Ethanol is preferred. The coating composition suitably comprises a polymer. The polymer may act as a binder to allow the coating composition, and any further components in the composition, to adhere to a substrate and form a coating layer. The polymer may be liquid. Alternatively, in examples where the coating composition comprises a solvent, the polymer may be dissolved in the solvent. The coating composition may be curable (for example when the coating composition comprises a liquid polymer) or air-dryable (for example when the coating composition comprises a polymer dissolved in a solvent). The coating composition may be thermally curable, UV curable, air curable or chemically curable. By “chemically curable”, is meant that the coating composition is cured by mixing a curing agent into the composition. Suitably, the curing agent is mixed into the coating composition just prior to application of the composition to a substrate. Preferably, the coating composition is UV curable. The coating composition is suitably a lacquer. By “lacquer” is meant a coating composition which forms a clear coating. Using a lacquer as the coating composition makes it easier to read the optically readable PUF obtained when the optically readable PUF coating composition provided by the method of the first example is coated onto a substrate. The first optically readable material is, advantageously, more visible in a clear coating. The lacquer may be curable or air-dryable. The lacquer may be thermally curable, UV curable, air curable or chemically curable. Preferably, the lacquer is UV curable. The first optically readable material may comprise any suitable material that can be detected by optical means. The first optically readable material may comprise a quantum dot, a quantum wire, a flake or layer of a substantially two-dimensional material, a metallic nanoparticle, a coloured compound and / or a fluorescent compound. Preferably, the first optically readable material comprises a quantum dot, a quantum wire, a flake or layer of a substantially two-dimensional material, a metallic nanoparticle and / or a fluorescent compound. The first optically readable material may emit electromagnetic radiation at a single wavelength or the first optically readable material may emit electromagnetic radiation with different wavelengths, for example corresponding to a variation in band gap of the first optically readable material. The first optically readable material may, therefore, be an emitter of electromagnetic radiation or, in other words, a material configured or generally able to emit electromagnetic radiation when excited, for example by excitation electromagnetic radiation. In some examples, the first optically readable material comprises a quantum dot, a quantum wire, a flake or layer of substantially two-dimensional material and / or a metallic nanoparticle. The first optically readable material may comprise a plurality of quantum dots, quantum wires, flakes or layers of two-dimensional material and / or metallic nanoparticles. By “substantially two-dimensional material” is meant a material that has a thickness of a few nanometres or less, for example such that motion of electrons into, and out of, a two-dimensional plane is governed by quantum mechanical effects. In some examples, the first optically readable material comprises a coloured compound. By “coloured compound” is meant a compound which absorbs electromagnetic radiation in the visible spectrum (e.g., at a wavelength from 400 to 700 nm). In some examples, the first optically readable material comprises a fluorescent compound. The fluorescent compound is an emitter of electromagnetic radiation or, in other words, a compound able to emit electromagnetic radiation when excited, for example by excitation electromagnetic radiation. The fluorescent compound may comprise a small molecule. The fluorescent compound suitably comprises a polyunsaturated compound, for example a polyaromatic compound. Suitable examples of fluorescent compounds include rhodamine dyes, cyanine dyes, phthalocyanine dyes, porphyrin dyes and quinacridones. Rhodamine dyes and cyanine dyes are preferred. The rhodamine dye may be selected from Rhodamine 6G, Rhodamine 123 and / or Rhodamine B. The cyanine dye may be selected from Cy2, Cy3, Cy3.5, Cy5, C5.5, Cy7 and / or Cy7.5. The first optically readable material may be added to the coating composition in an amount of from 0.1 to 15 mg / mL, such as from 0.5 to 10 mg / mL, such as from 1 to 5 mg / mL based on the total volume of the optically readable PUF coating composition. The first optically readable material may be added to the coating composition in an amount of from 0.01 to 1.5 wt%, such as from 0.05 to 1.0 wt%, such as from 0.08 to 0.4 wt% based on the total weight of the optically readable PUF coating composition. Suitably, the first optically readable material is poorly soluble or insoluble in the coating composition. The first optically readable material may have a solubility of less than 100 mg / mL, such as less than 50 mg / mL or less than 10 mg / mL (e.g., less than 1 mg / mL in the coating composition at a temperature of 20°C and a pressure of 100 kPa). Advantageously, adding the first optically readable material to the coating composition when the first optically readable material is poorly soluble or insoluble in the coating composition may result in the clustering or agglomeration of the first optically readable material. The method of the first example suitably comprises non-uniformly dispersing the first optically readable material in the coating composition. The resulting coating composition suitably comprises a non-uniform distribution of the first optically readable material. For example, the resulting coating composition may comprise clusters or agglomerates of the first optically readable material. The first optically readable material may be mixed into the coating composition but not to an extent that the first optically readable material is uniformly distributed. The method of the first example may comprise adding the first optically readable material to the coating composition without a solvent. In other words, the first optically readable material is not mixed with a solvent before being added to the coating composition. This step helps to ensure that the first optically readable material is non-uniformly distributed in the coating composition. The method of the first example may further comprise mixing a second optically readable material and a solvent to form a mixture and adding the mixture to the coating composition. The second optically readable material may comprise any suitable material that can be detected by optical means. The second optically readable material may comprise a quantum dot, a quantum wire, a flake or layer of a substantially two-dimensional material, a metallic nanoparticle, a coloured compound and / or a fluorescent compound. Preferably, the second optically readable material comprises a quantum dot, a quantum wire, a flake or layer of a substantially two-dimensional material, a metallic nanoparticle and / or a fluorescent compound. The second optically readable material may emit radiation at a single wavelength or the second optically readable material may emit radiation with different wavelengths, for example corresponding to a variation in band gap of the second optically readable material. The second optically readable material may, therefore, be an emitter of electromagnetic radiation or, in other words, a material configured or generally able to emit electromagnetic radiation when excited, for example by excitation electromagnetic radiation. In some examples, the second optically readable material comprises a quantum dot, a quantum wire, a flake or layer of substantially two-dimensional material, and / or a metallic nanoparticle. The second optically readable material may comprise a plurality of quantum dots, quantum wires, flakes or layers of two-dimensional material and / or metallic nanoparticles. In some examples, the second optically readable material comprises a coloured compound. In some examples, the second optically readable material comprises a fluorescent compound. The fluorescent compound is an emitter of electromagnetic radiation or, in other words, a compound able to emit electromagnetic radiation when excited, for example by excitation electromagnetic radiation. The fluorescent compound may comprise a small molecule. The fluorescent compound suitably comprises a polyunsaturated compound, for example a polyaromatic compound. Suitable examples of fluorescent compounds include rhodamine dyes, cyanine dyes, phthalocyanine dyes, porphyrin dyes and quinacridones. Rhodamine dyes and cyanine dyes are preferred. The rhodamine dye may be selected from Rhodamine 6G, Rhodamine 123 and / or Rhodamine B. The cyanine dye may be selected from Cy2, Cy3, Cy3.5, Cy5, C5.5, Cy7 and / or Cy7.5. The first optically readable material and the second optically readable material may be the same or different. Preferably, the first optically readable material is different to the second optically readable material. When the first optically readable material is the same as the second optically readable material, the first optically readable material is suitably optically distinguishable from the second optically readable material in the optically readable PUF coating composition. For example, the difference in the chemical environments of the optically readable materials (such as the medium in which the optically readable materials are dispersed) may result in different emissions. Suitably, the first optically readable material and / or the second optically readable material comprises a quantum dot, a quantum wire, a flake or layer of a substantially two-dimensional material, a metallic nanoparticle and / or a fluorescent compound. Preferably, the first optically readable material and / or the second optically readable material comprises a fluorescent compound. The fluorescent compound may comprise a small molecule. The fluorescent compound suitably comprises a polyunsaturated compound, for example a polyaromatic compound. Suitable examples of fluorescent compounds include rhodamine dyes, cyanine dyes, phthalocyanine dyes, porphyrin dyes and quinacridones. Rhodamine dyes and cyanine dyes are preferred. The rhodamine dye may be selected from Rhodamine 6G, Rhodamine 123 and / or Rhodamine B. The cyanine dye may be selected from Cy2, Cy3, Cy3.5, Cy5, C5.5, Cy7 and / or Cy7.5. Suitably, the second optically readable material is poorly soluble or insoluble in the coating composition. The second optically readable material may have a solubility of less than 100 mg / mL, such as less than 50 mg / mL or less than 10 mg / mL (e.g., less than 1 mg / mL in the coating composition at a temperature of 20°C and a pressure of 100 kPa). Advantageously, adding the second optically readable material a solvent prior to addition to the coating composition allows the second optically readable material to be uniformly distributed in the coating composition The solvent mixed with the second optically readable material may comprise a polar solvent and / or a non-polar solvent. Preferably the solvent comprises a polar solvent. The solvent may comprise an organic solvent and / or an inorganic solvent. The solvent may comprise a mixture of an organic solvent and an inorganic solvent, such as water. Preferably, the solvent comprises an organic solvent. The organic solvent suitably comprises a polar organic compound having 6 or fewer carbon atoms. The organic solvent may comprise an alcohol and / or a ketone. Suitable examples of organic solvents include ethanol, propanol, isopropyl alcohol and acetone. Ethanol is preferred. The second optically readable material may be soluble in the solvent. The second optically readable material may have a solubility of at least 1 mg / mL, such as at least 10 mg / mL, such as at least 50 mg / mL (e.g., at least 100 mg / mL in the solvent at a temperature of 20°C and a pressure of 100 kPa). The mixture of the second optically readable material and the solvent is suitably a solution. The second optically readable material and the solvent may be uniformly mixed. The uniform mixing of the second optically readable material in the solvent may, advantageously, provide the optically readable PUF coating composition with a uniform background emission. The second optically readable material may be added to the solvent in an amount of from 10 to 500 mg / mL, such as from 50 to 200 mg / mL, such as from 80 to 150 mg / mL, based on the total volume of the mixture. The method of the first example may comprise adding a third optically readable material to the mixture of the second optically readable material and the solvent and adding the mixture to the coating composition. The third optically readable material may comprise any suitable material that can be detected by optical means. The third optically readable material may comprise a quantum dot, a quantum wire, a flake or layer of a substantially two-dimensional material, a metallic nanoparticle, a coloured compound and / or a fluorescent compound. Preferably, the third optically readable material comprises a quantum dot, a quantum wire, a flake or layer of a substantially two-dimensional material, a metallic nanoparticle and / or a fluorescent compound. The third optically readable material may emit radiation at a single wavelength or the third optically readable material may emit radiation with different wavelengths, for example corresponding to a variation in band gap of the third optically readable material. The third optically readable material may therefore be an emitter of electromagnetic radiation or, in other words, a material configured or generally able to emit electromagnetic radiation when excited, for example by excitation electromagnetic radiation. In some examples, the third optically readable material comprises a quantum dot, a quantum wire, a flake or layer of substantially two-dimensional material and / or a metallic nanoparticle. The third optically readable material may comprise a plurality of quantum dots, quantum wires, flakes or layers of two-dimensional material and / or metallic nanoparticles. In some examples, the third optically readable material comprises a coloured compound. In some examples, the third optically readable material comprises a fluorescent compound. The fluorescent compound is an emitter of electromagnetic radiation or, in other words, a compound able to emit electromagnetic radiation when excited, for example by excitation electromagnetic radiation. The fluorescent compound may comprise a small molecule. The fluorescent compound suitably comprises a polyunsaturated compound, for example a polyaromatic compound. Suitable examples of fluorescent compounds include rhodamine dyes, cyanine dyes, phthalocyanine dyes, porphyrin dyes, and quinacridones. Rhodamine dyes and cyanine dyes are preferred. The rhodamine dye may be selected from Rhodamine 6G, Rhodamine 123 and / or Rhodamine B. The cyanine dye may be selected from Cy2, Cy3, Cy3.5, Cy5, C5.5, Cy7 and / or Cy7.5. The third optically readable material may be poorly soluble or insoluble in the solvent. The second optically readable material may have a solubility of less than 100 mg / mL, such as less than 50 mg / mL or less than 10 mg / mL (e.g., less than 1 mg / mL in the solvent at a temperature of20°C and a pressure of 100 kPa). The mixture of the second optically readable material, the third optically readable material and the solvent is suitably a suspension of the third optically readable material. The third optically readable material and the solvent may be uniformly mixed. The uniform mixing of the third optically readable material in the solvent may, advantageously, provide the optically readable PUF coating composition with a uniform background emission. The third optically readable material may be added to the solvent in an amount of from 10 to 500 mg / mL, such as from 50 to 200 mg / mL, such as from 80 to 150 mg / mL, based on the total volume of the mixture. Preferably, in examples where a third optically readable material is added, the second optically readable material is soluble in the solvent and the third optically readable material is insoluble or poorly soluble in the solvent. The physical interactions between the second optically readable material and the third optically readable material, advantageously, allow the third optically readable material to be uniformly dispersed in the mixture, for example by allowing the third optically readable material to dissolve in the mixture. The mixture may be added to the coating composition such that the second optically readable material (and third optically readable material, if present) is present in an amount of from 0.1 to 15 mg / mL, such as from 0.5 to 10 mg / mL, such as from 1 to 5 mg / mL, based on the total volume of the optically readable PUF coating composition. The mixture may be added to the coating composition such that the second optically readable material (and third optically readable material, if present) is present in an amount of from 0.01 to 1.5 wt%, such as from 0.05 to 1.0 wt%, such as from 0.08 to 0.4 wt%, based on the total weight of the optically readable PUF coating composition. The method of the first example suitably comprises uniformly mixing the mixture and the coating composition. In the method of the first example, the first optically readable material may be added to the coating composition prior to the mixture containing the second optically readable material, the third optically readable material, if present, and the solvent. Alternatively, the mixture containing the second optically readable material, the third optically readable material, if present, and the solvent may be added to the coating composition prior to the first optically readable material. The method of the first example may comprise increasing the entropy of the coating composition by adding an optically readable material and optionally a solvent to at least two portions of the coating composition and then combining the portions of the coating composition. Preparing at least two separate portions of the coating composition which are then combined advantageously allows the uniformity of the optically readable material in each portion to be tuned independently. For example, one portion of the coating composition may comprise a non-uniform distribution of an optically readable material and another portion of the coating composition may comprise a uniform distribution of an optically readable material. Thus, the non-uniform distribution may advantageously be unaffected by the preparation of the uniform distribution. The optically readable material suitably comprises at least the first optically readable material as defined herein. The optically readable material may comprise two or more optically readable materials. The optically readable material may further comprise the second optically readable material and / or the third optically readable material as defined herein. For example, the optically readable material may comprise the first optically readable material and the second optically readable material; the first optically readable material and the third optically readable material; or the first optically readable material, the second optically readable material, and the third optically readable material. Preferably, each portion of the coating composition is different. Suitably, a different optically readable material or combination of optically readable materials may be added to each portion. For example, the first optically readable material may be added to a first portion of the coating composition and the second optically readable material may be added to a second portion of the coating composition. The third optically readable material may be added to a third portion of the coating composition, or the third optically readable material may be added to the second portion of the coating composition comprising the second optically readable material. A solvent may be added to one or more of the portions of the coating composition. Preferably, the solvent is mixed with an optically readable material before being added to the portion of the coating composition. In some examples, each optically readable material is mixed with a solvent before being added to a portion of the coating composition. In some examples, none of the optically readable materials are mixed with a solvent. In some examples, some of the optically readable materials are mixed with a solvent and some of the optically readable materials are not mixed with a solvent before being added to a portion of the coating composition. For example, the first optically readable material may be mixed with a solvent before being added to a first portion of the coating composition, and the first optically readable material may be added to a second portion of the coating composition without being mixed with a solvent beforehand. Preferably, the first optically readable material is not mixed with a solvent before being added to the first portion of the coating composition. Preferably, the second optically readable material is mixed with a solvent before being added to the second portion of the coating composition. Preferably, the third optically readable material is not mixed with a solvent before being added to the second or third portion of the coating composition. If two or more optically readable materials are added to one portion of the coating composition, the solvent may be mixed with one of the optically readable materials before being added to the portion of the coating composition and the other optically readable material(s) may be added to the portion of the coating composition without being mixed with a solvent beforehand. For example, the second optically readable material may be mixed with a solvent before being added to the second portion of the coating composition and the third optically readable material may be added to the second portion of the coating composition without being mixed with a solvent beforehand. The solvent mixed with an optically readable material before being added to a portion of the coating composition may comprise a polar solvent and / or a non-polar solvent. Preferably the solvent comprises a polar solvent. The solvent may comprise an organic solvent and / or an inorganic solvent. The solvent may comprise a mixture of an organic solvent and an inorganic solvent, such as water. Preferably the solvent comprises an organic solvent. The organic solvent suitably comprises a polar organic compound having 6 or fewer carbon atoms. The organic solvent may comprise an alcohol and / or a ketone. Suitable examples of organic solvents include ethanol, propanol, isopropyl alcohol and acetone. Ethanol is preferred. The portions of the coating composition may be combined with mixing. In some examples, the first optically readable material and the mixture containing the second optically readable material, the third optically readable material, if present, and the solvent may be added to separate portions of the coating composition which are then combined. One advantage of this is that the second optically readable material and third optically readable material, if present, may be uniformly mixed into the coating composition without affecting the non-uniform distribution of the first optically readable material in the coating composition. The method of the first example may comprise: (a) providing a first portion and a second portion of a coating composition; (b) adding a first optically readable material to the first portion of the coating composition; (c) adding a second optically readable material to a solvent to form a mixture; (d) adding the mixture to the second portion of the coating composition; (e) combining the first portion and the second portion of the coating composition. The first portion and the second portion may be combined with mixing. They may be combined at any suitable ratio. The first portion and the second portion may be combined in a ratio of from 1:10 to 10:1, such as from 1:5 to 5:1, such as from 1:3 to 1:1, by volume. According to a second example providing useful context for, but not within the scope of, the present invention, there is provided a method of producing an optically readable PUF, comprising producing an optically readable PUF coating composition in accordance with the method of the first example and applying the optically readable PUF coating composition to a substrate. The method of the second example may further comprise drying or curing the optically readable PUF coating composition. According to a third example providing useful context for, but not within the scope of, the present invention, there is provided an optically readable PUF coating composition produced according to the method of the first example. According to a fourth example providing useful context for, but not within the scope of, the present invention, there is provided an optically readable PUF produced according to the method of the second example. The optically readable PUF may be used as a security element - i.e. an optically readable PUF may be used as, or even defined as an optically readable security element. The optically readable PUF may include an alignment mark, to allow an optical reader to use that mark to correct for scale, perspective and any transforms (e.g., mirror image) that may be applied as part of the reading of the PUF. A ‘unique’ identity, e.g., a serial number, may be printed adjacent to the optically readable PUF to enable further authentication of the PUF. According to a fifth example providing useful context for, but not within the scope of, the present invention, there is provided an article comprising a layer covering (e.g., coating) a three-dimensional surface of the article, the layer providing a function, and an optically readable PUF in the layer, such that the layer provides the function of enabling authentication of the article. Reading, which includes determining (i.e., setting or extracting) an identity, of the optically readable PUF from different perspectives allows the article to be authenticated with different identities from those perspectives of the three-dimensional surface via the reading. The article having a three-dimensional surface means that the article is not merely a flat sticker or label and is substantially non-planar. Typically, the surface is external, but the surface does not have to be external. The surface merely needs to be readable externally, for example by an image capturing device that can view that surface. The article of the fifth example may comprise a functional layer with an integrated optically readable PUF. This integration, advantageously, allows the manufacture of the article to be simplified, since the functional layer and the optically readable PUF may be prepared in a single step, as opposed to requiring the optically readable PUF to be applied to the functional layer in a separate step, for example, as an adhesive label. The surface may comprise a plurality of faces (i.e., sides or other whole or part surfaces that can be provided with an integrated optically readable PUF or a portion of such an integrated optically readable PUF), and the optically readable PUF may be distributed among a set of the plurality of faces. Therefore, an identity may be extracted by reading the set of faces. In other words, extraction of an identity may depend on combining readings of more than one face. Thus, advantageously, the optically readable PUF being distributed among the set of the plurality of faces enhances security. As above, that the surface comprises more than one face does not necessarily mean that the article has edges or corners but means that the article is more complex than a single flat plane, and that different readings identities may be read from different perspectives of the article. For instance, the geometry of the article may be substantially cubic, cuboidal, prismatic, spherical or cylindrical. Another function of the layer may be the improvement of a property of the article. For example, the another function may be the improvement of strength, durability, appearance, surface texture, taste, and / or smell of the article. The function and the another function may be used interchangeably with “first” and “second” function. The article may be any article which would benefit from the authentication function provided by the optically readable PUF. The article may be selected from machine or device parts, clothing, footwear, accessories, bags, food, artwork, electronic components, certificates, ID cards, consumable goods, gemstones, medicine, tax stamps, currency, packaging and labelling. For example, the article may be branded clothing and the optically readable PUF may enable its authentication as a genuine article. As another example, the article may be a medicinal tablet and the PUF may enable the authentication of the tablet as genuine. In one embodiment, the article is food or medicine and the layer is edible. Suitably, the layer is digestible and non-toxic. The optically readable PUF is suitably present in substantially the entire layer. This presence may allow any part of the layer to be used to authenticate the article. Suitably, at least 95%, such as at least 98%, such as at least 99% by surface area of the layer comprises the optically readable PUF. Preferably, 100% by surface area of the layer comprises the optically readable PUF. The layer suitably forms a major surface of the article. The layer may form at least 10%, such as at least 30%, such as at least 50% the surface of the article. The layer may form 100% of the surface (i.e., the layer may completely cover the surface). In one embodiment, the article is food or medicine and the layer forms 100% of the surface. The article may comprise a plurality of optically readable PUFs in the layer. The plurality of optically readable PUFs may be non-uniformly distributed. These plurality of optically readable PUFs could be dedicated areas or regions, physically applied or marked, or they could be different areas or regions of one or more areas or regions (e.g., PUFs) that extend across the surface - i.e., a same, single coating or layer, but read from different perspectives. A (sub) portion of the layer comprising at least a portion of the optically readable PUF may be designated as an optically readable area, for example one or more different areas that are located across the three-dimensional surface, for instance above and below, or front and back, or opposite sides. This may be useful to improve the reliability of readings of the optically readable PUF, as readings taken from different parts of the layer may not be comparable. However, as discussed below, a benefit of the present invention may be that there is no need to actually designate a portion as an optically readable area for reading. The layer may include an alignment mark, to allow an optical reader to use that mark to correct for scale, perspective and any transforms (e.g., mirror image) that may be applied as part of the reading of the PUF. The layer suitably comprises a polymer. The polymer may act as a binder to allow the layer to adhere to the article. The optically readable PUF suitably comprises an optically readable material. The optically readable material may be non-uniformly (e.g., randomly) distributed in the optically readable PUF. The optically readable material may comprise any suitable material that can be detected by optical means. The optically readable material may comprise a quantum dot, a quantum wire, a flake or layer of a substantially two-dimensional material, a metallic nanoparticle, a coloured compound and / or a fluorescent compound. Preferably, the optically readable material comprises a quantum dot, a quantum wire, a flake or layer of a substantially two-dimensional material, a metallic nanoparticle and / or a fluorescent compound. The optically readable material may emit radiation at a single wavelength or the optically readable material may emit radiation with different wavelengths, for example corresponding to a variation in band gap of the optically readable material. The optically readable material may therefore be an emitter of electromagnetic radiation or, in other words, a material configured or generally able to emit electromagnetic radiation when excited, for example by excitation electromagnetic radiation. In some embodiments, the optically readable material comprises a quantum dot, a quantum wire, a flake or layer of substantially two-dimensional material and / or a metallic nanoparticle. The optically readable material may comprise a plurality of quantum dots, quantum wires, flakes or layers of two-dimensional material and / or metallic nanoparticles. In some embodiments, the optically readable material comprises a coloured compound. In some embodiments, the optically readable material comprises a fluorescent compound. The fluorescent compound may comprise a small molecule. The fluorescent compound suitably comprises a polyunsaturated compound, for example a polyaromatic compound. Suitable examples of fluorescent compounds include rhodamine dyes, cyanine dyes, phthalocyanine dyes, porphyrin dyes and quinacridones. Rhodamine dyes and cyanine dyes are preferred. The rhodamine dye may be selected from Rhodamine 6G, Rhodamine 123 and / or Rhodamine B. The cyanine dye may be selected from Cy2, Cy3, Cy3.5, Cy5, C5.5, Cy7 and / or Cy7.5. Suitably, the layer has been applied in a single step. In some embodiments, the optically readable PUF is as defined by the fourth example of the present invention. According to a sixth example providing useful context for, but not within the scope of, the present invention, there is provided a method of making an article according to the fifth example. The method comprises providing a layer covering a three-dimensional surface of the article, wherein the layer provides a function, and wherein the layer comprises an optically readable PUF, such that the layer provides the function of enabling authentication of the article. Reading of the optically readable PUF from different perspectives allows the article to be authenticated with different identities from those different perspectives of the three-dimensional surface via the reading. Providing the layer for the article may comprise applying a coating composition to a substrate. The coating composition may be dried or cured. The coating composition may comprise the optically readable PUF coating composition of the third example. According to the present invention, and in accordance with the claims that follow, there is provided a method of using an article , the article comprising: a coating layer covering a three-dimensional surface of the article, the coating providing a function; and an optically readable PUF in the coating layer, such that the coating layer provides the function of enabling authentication of the article, the method comprising reading of the optically readable PUF from different perspectives, to allow the article to be authenticated with different identities from those different perspectives of the three-dimensional surface via the reading. Advantageously, this may increase security by enabling different (i.e., more than one) identities to be determined. Relatedly, and further advantageously, this also mitigates the effect of degradation: if the coating layer is damaged from one perspective, another perspective of the layer may be relied on. Further still, this is advantageous, because there is no burden on the user to ensure a particular relative orientation of the article and the image capturing device. These advantages analogously apply to the fifth example. This is all in stark contrast to the provision of, for example, a single, flat, square or rectangular label or region, which might provide just a single identity. This is also all in stark contrast to the provision of, for example, a single coating layer that extends across an article, but which provides a single identity from any and all perspectives. The method according to the present invention may further comprise authenticating the optically readable PUF based on the identity. The method according to the present invention may further comprise reading the optically readable PUF a second time using the image capturing device, wherein the image capturing device is at a second perspective relative to the optically readable PUF, the second perspective being different from the first perspective, and determining a different identity of the optically readable PUF. The different identity is based on the second perspective. At least one of the readings may be non-deterministic, advantageously making the reading more difficult to spoof or replicate by a nefarious party. For example, a user may be encouraged or prompted to take random readings (e.g., from different, random perspectives), so that the same part of the article is not always used as the identity. Relatedly, without being prompted, a user might naturally and randomly read the article, since the coating layer extends across a 3-dimensional surface of the article. That is, the user will implicitly be given more freedom to authenticate the article in a natural, less structured way. Brief Description of the Drawings For a better understanding of the invention, and to show how embodiments of (and examples related to) the same may be carried into effect, reference will now be made, by way of example, to the accompanying Figures in which: Figure 1 depicts flash photographs taken of (a) a tag formed from Ink A, and (b) a tag formed from Ink C, wherein Ink C is an optically readable PUF according to the fourth example useful for understanding, but not within the scope of, the present invention; and Figure 2 depicts identities created from the (a) a tag formed from Ink A, and (b) a tag formed from Ink C, wherein Ink C is an optically readable PUF according to the fourth example useful for understanding, but not within the scope of, the present invention. Figure 3 schematically depicts general methodology associated with the method of the first example useful for understanding, but not within the scope of, the present invention. Figure 4 schematically depicts different methodology associated with examples. Figure 5 schematically depicts general methodology associated with the method of the second example useful for understanding, but not within the scope of, the present invention. Figure 6 schematically depicts an article according to the fifth example providing useful context for, but not within the scope of, the present invention. Figure 7 schematically depicts general methodology associated with the method of the sixth example providing useful context for, but not within the scope of, the present invention. Figure 8 schematically depicts general methodology associated with the present invention in accordance with the claims that follow. Examples Definitions Inter-Hamming distance (inter-HD): the number of bits that differ between two identities extracted from two different tags, divided by the total number of bits within an identity. The ideal value is 0.5. Intra-Hamming distance (intra-HD): the number of bits that differ between two identities extracted from the same tag, divided by the total number of bits within an identity. Uniformity: the ratio of O’s to 1 ’s in the identity. The ideal value is 0.5. Degrees of Freedom (DOF): the maximum number of bits needed to completely specify an identity. A high DOF is desirable. Decidability: a measure of the separation of the inter and intra hamming distance distributions. A high decidability is desirable. Example 1 Rhodamine 6G was dispersed in ethanol forming a 100 mg / ml solution. 25 pl of this solution was added to a UV curable lacquer and mixed for 15 minutes using a mixing bit rotating at 400RPM to form Ink A comprising Rhodamine 6G at a concentration of 2.5 mg / ml. Rhodamine B was added directly to a UV curable lacquer and mixed for 5 minutes using a mixing bit rotating at 400RPM to form Ink B comprising Rhodamine B at a concentration of 2.5 mg / ml. Ink B was mixed with Ink Aina volume ratio of 1:3 for 5 minutes using a mixing bit rotating at 400RPM to form Ink C. Example 2 Rhodamine 6G was added directly to a UV curable lacquer and mixed for 5 minutes using a mixing bit rotating at 400RPM to form Ink A comprising Rhodamine 6G at a concentration of 0.1 mg / ml. Rhodamine 6G was dispersed in ethanol forming a 100 mg / ml solution. 50 mg of quinacridone was mixed with 1 ml of the solution to form a mixture comprising 2 parts Rhodamine 6G to 1 part quinacridone by weight. The mixture was added to Ink A and mixed for 15 minutes using a mixing bit rotating at 500RPM to form Ink B. Example 3 Ink A and Ink C from Example 1 were separately coated onto substrates and cured for 60 seconds using a Hg vapour UV lamp to form tags. Emission from the tags was measured by flash photographing the tags using a smartphone camera. The photographs are shown in Figure 1. Background emission from uniformly dispersed Rhodamine 6G is visible over the entirety of the tags formed from Ink A (Figure 1(a)) and Ink C (Figure 1(b). Spots of non-uniformly dispersed Rhodamine B are visible in the tag formed from Ink C. An identity was created using an image of each tag as the source. These identities are shown in Figure 2. Figure 2(a) corresponds to Ink A and Figure 2(b) corresponds to Ink C. The figures of merit for the two identities in figure 2 are shown in the following table: Ink A C Inter-HD (average, standard deviation): 0.476, 0.015 0.488, 0.005 Intra-HD (average, standard deviation) 0.406, 0.028 0.383, 0.025 Uniformity 0.65 0.49 Degrees of Freedom (bits) 1100 1468 Decidability 3.78 5.67 The inter-HD values were produced through the comparison of a number of tags generated for each type (A and C). Ink C generally showed superior figures of merit compared to Ink A. Detailed Description of the Related Examples and Embodiments Figure 3 schematically depicts methodology for making an optically readable PUF coating composition. Initially, a coating composition is provided, (a). A first optically readable material is added to the coating composition, (b). A second optically readable material and a solvent are mixed to form a mixture, (c). The mixture is added to the coating composition, (d). Figure 4 schematically depicts a preferred methodology for making an optically readable PUF coating composition. Initially, a coating composition is provided in a first portion (a1) and a second portion (a2). A first optically readable material is added to the first portion of the coating composition, (b). A second optically readable material is added to a solvent to form a mixture, (c). The mixture is added to the second portion of the coating composition, (d). The first portion and the second portion of the coating composition are combined, (e). Figure 5 schematically depicts methodology for producing an optically readable PUF. Initially, a coating composition is provided, (a). A first optically readable material is added to the coating composition, (b). A second optically readable material and a solvent are mixed to form a mixture, (c). The mixture is added to the coating composition, (d). Following steps (b) and (d), which may be carried out in any order, the coating composition is applied to a substrate, (f). The coating composition may be dried or cured following application to the substrate, (g). Figure 6 schematically depicts an article 10 according to the fifth example providing useful context for, but not within the scope of, the present invention. The article 10 comprises a layer 20 covering a three-dimensional surface (in this case the surface is that of a cylinder, but the surface is not limited to a particular geometry), the layer 20 providing a function and an optically readable PUF such that the layer 20 provides the function of enabling authentication of the article 10.. Reading of the optically readable PUF from different perspectives 22, 24, 26 allows the article 10 to be authenticated with different identities from those different perspectives 22, 24, 26 of the three-dimensional surface via the reading. For example, this might be due to different parts of the article being deliberately provided with different identities. However, as discussed above, a layer comprising one or more optically readable PUFs (e.g., a coating composition) which comprises a random, non-uniform distribution of an optically readable material might inherently provide such different identities across and around the surface, e.g. to be read from different perspectives. A portion of the layer 20 comprising at least a portion of the optically readable PUF may be designated as an optically readable area 30. One such area 30 is shown for simple explanation, but a powerful feature of the invention is that the one or more such areas 30 might extend around or across the layer, to provide or be readable to extract different identities from different perspectives. The designation may be visible to the user. The designation may not be visible to the user. However, a benefit of the present invention may be that there is no need to actually designate a portion for reading. By extending across the surface of the article 10, and being readable from different perspectives 22, 24, 26, reading may be more naturally undertaken by a user, and / or a more robust level of security is provided, by the or an identity effectively being different from or at those different perspectives 22, 24, 26. Figure 7 schematically depicts methodology for making an article. A layer is provided for covering a three-dimensional surface of an article, (h), wherein the layer provides a function, and wherein the layer comprises an optically readable PUF, such that the layer provides the function of enabling authentication of the article. Reading of the optically readable PUF from different perspectives allows the article to be authenticated with different identities from those different perspectives of the three-dimensional surface via the reading. The layer or its component parts will typically be provided as discussed in other parts of this disclosure. Figure 8 schematically depicts a methodology for using an article. The method comprises reading (i) an optically readable PUF a first time using an image capturing device and determining (j) an identity of the optically readable PUF. The image capturing device is at a first perspective (e.g. arrow 22 in Figure 6) relative to the optically readable PUF, and the identity determined is based on the first perspective 22. This determination may be in an initial setting, setup or calibration step, e.g. during or after manufacture of the article. Or, the determination may be later on, to authenticate the article. As discussed above, when performed according to the invention, the method is repeated for different perspectives (e.g., arrows 24 and 26 in Figure 6). Although perhaps subtle, the invention is powerful. As discussed above, the invention brings significant benefits, for example in comparison with the provision of a single, flat, square or rectangular label or region, which might provide just a single identity. Additionally or alternatively, the invention brings significant benefits in comparison to the provision of a single layer that extends across an article, but which provides a single identity from any and all perspectives. 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.

Claims

1. A method of using an article, the article comprising:a coating layer covering a three-dimensional surface of the article, the coating providing a function; andan optically readable PUF in the coating layer, such that the coating layer provides the function of enabling authentication of the article,the method comprising reading of the optically readable PUF from different perspectives, to allow the article to be authenticated with different identities from those different perspectives of the three-dimensional surface via the reading.

2. The method of claim 1, comprising:reading the optically readable PUF a first time using an image capturing device, wherein the image capturing device is at a first perspective relative to the optically readable PUF; anddetermining an identity of the optically readable PUF, wherein the identity determined is based on the first perspective.

3. The method of using an article according to claim 2, further comprising:authenticating the optically readable PUF based on the identity.

4. The method of claim 2 or 3, further comprising:reading the optically readable PUF a second time using the image capturing device, wherein the image capturing device is at a second perspective relative to the optically readable PUF, the second perspective being different from the first perspective; anddetermining a different identity of the optically readable PUF, wherein the different identity is based on the second perspective,5. The method of claim 4, wherein at least one of the readings is non-deterministic.

6. The method of any preceding claim, wherein:the surface comprises a plurality of faces, andthe optically readable PUF is distributed among a set of the plurality of faces.

7. The method of any preceding claim, wherein:the article is selected from machine or device parts, clothing, footwear, accessories, bags, food, artwork, electronic components, certificates, ID cards, consumable goods, gemstones, medicine, tax stamps, currency, packaging and labelling,optionally, the article is food or medicine, andoptionally, the coating layer is edible.

8. The method of any preceding claim, wherein:the optically readable PUF comprises a fluorescent compound, andoptionally, fluorescent compound comprises a rhodamine dye, a cyanine dye, a phthalocyanine dye, a porphyrin dye, and / or a quinacridone.

9. The method of any preceding claim, wherein the optically readable PUF is present in substantially the entire coating layer.

10. The method of any preceding claim, wherein the coating layer completely covers the surface.

11. The method of any preceding claim, comprising a plurality of optically readable PUFs in the coating layer.

12. The method of claim 11, wherein the plurality of optically readable PUFs is non-uniformly distributed in the coating layer.

13. The method of any preceding claim, wherein a portion of the coating layer comprising at least a portion of the optically readable PUF is designated as an optically readable area.

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