Abstract:
A system for tagging articles. A glass is fabricated from ( 1 ) a combination of oxides and ( 2 ) one or more rare earth elements. The glass is divided into particles or fragments, which are attached to the article. When the particles are excited by specific radiation, they emit a characteristic signature, in terms of a collection of frequencies, each frequency having a characteristic amplitude and decay time. However, the particles cannot be counterfeited, or reverse-engineered, because, at present, no systematic data is available which correlates a particle&#39;s characteristic signature with the composition and processing of the particle itself. Thus, at best, a trial-and-error approach must be taken in attempts at counterfeiting, which is considered impossible.

Description:
[0001]     The present invention relates to security markers, which are generally attached to, or embedded in, objects. The security marker contains indicia which can identify its origin and thus the origin of the object. The security marker is difficult to counterfeit under today&#39;s technology.  
       BACKGROUND OF THE INVENTION  
       [0002]     Security markers are used to authenticate items. For example, bank notes typically include security markers such as watermarks, fluorescent inks, security threads, holograms, kinegrams, and such like. However, with advances in copying technology, it is becoming more difficult to provide security markers, which are not only difficult to counterfeit, but also easy to detect, quick to detect in situ, and inexpensive.  
         [0003]     Chemical and biochemical taggants are also used as security markers. However, in many cases such taggants must be removed from the item prior to being analyzed. This is both time-consuming and expensive.  
         [0004]     Optically based approaches, such as fluorescent labels, have also been used. Fluorescent materials emit light when excited by radiation of a particular wavelength. Information can be encoded in fluorescent inks, and can only be retrieved when the mark is illuminated with radiation of the appropriate wavelength.  
         [0005]     An example of a particular type of fluorescent ink is described in U.S. Pat. No. 5,256,193, which is hereby incorporated by reference. The following patents describe various security labeling and printing applications, and are hereby incorporated by reference: JP 8208976; U.S. Pat. No. 4,736,425; U.S. Pat. No. 5,837,042; U.S. Pat. No. 3,473,027; U.S. Pat. No. 5,599,578; GB 2,258,659; U.S. Pat. No. 6,344,261; and U.S. Pat. No. 4,047,033.  
         [0006]     However, known inks and dyes have the disadvantage that they have very broad spectra, which limits the number of inks and dyes that can be used in a particular item. An example will illustrate this limitation.  
         [0007]     Consider the visible spectrum, which ranges from red, through orange, yellow, green, blue, indigo, and violet. One ink may produce a color which spans from red through green. Another may produce a color which spans from green through violet. Thus, if these two inks are used, it is difficult to use a third ink with them, because the first two inks cover the entire visible spectrum.  
         [0008]     For many purposes, it is desirable to use inks having a narrower spectrum, such as an ink which occupies only the red part of the spectrum, or less. In general, fluorescent inks do not offer this property.  
       OBJECTS OF THE INVENTION  
       [0009]     An object of the invention is to provide an improved tag.  
         [0010]     A further object of the invention is to provide a tag which radiates light in response to optical excitation, which radiated light has a narrow spectral width.  
       SUMMARY OF THE INVENTION  
       [0011]     In one form of the invention, a glass composition is fabricated, which produces a unique optical signature in response to exciting radiation, and the glass composition is difficult to copy to form a second composition which produces the same unique optical signature.  
         [0012]     According to a first aspect of the present invention there is provided an optically detectable security marker for emitting light at a pre-selected wavelength, the marker comprising: a rare earth dopant and a carrier incorporating the rare earth dopant, the interaction of the carrier and the dopant being such as to provide a fluorescent fingerprint or response that is different from that of the rare earth dopant.  
         [0013]     The rare earth element which is used as the dopant has an intrinsic set of electronic energy levels. The interaction between the carrier and the dopant is such that these intrinsic energy levels change when the dopant is incorporated into the carrier. For example, when the dopant is incorporated into a glass, new energy levels (from the glass) are made available for transitions, thus altering the electron arrangement and hence the energy levels of absorption and fluorescent emission. These transitions can assist recombinations that were previously prohibited. Altering the rare earth dopant and/or dopant chelate and/or the composition of the carrier changes these energy levels and hence the observed fluorescent fingerprint.  
         [0014]     By virtue of this aspect of the invention an optically detectable security marker is provided that can be tailored to have strong fluorescent light emission at a pre-selected wavelength when illuminated with a particular wavelength of light. This enables a validator to validate the security marker by detecting emission at the pre-selected wavelength in response to radiation at a particular wavelength. Such a security marker is very difficult to replicate by a counterfeiter.  
         [0015]     Preferably, the rare earth dopant is a lanthanide.  
         [0016]     Preferably, the carrier comprises a glass or a plastic. The carrier in which the rare earth dopant is embedded can readily be produced in a variety of formats, e.g. microbeads or fibers suitable for inclusion in products (such as those made from plastic or paper). Alternatively the rare earth dopant may be an integral part of the polymer matrix forming a product.  
         [0017]     Due to the discrete fluorescence wavelength of a carrier doped with a rare earth element, multiple carriers can be used (or a single carrier doped with multiple rare earth elements), each prepared to have a different pre-selected emission wavelength, so that a security profile comprising multiple wavelengths can be provided in a single item without the different wavelengths overlapping each other. This enables a security marker to be provided that has a security profile selected from a large number of permutations, thereby greatly increasing the difficulty in counterfeiting such a security marker.  
         [0018]     The carrier doped with the rare earth ion has a new energy level profile that allows transitions different to those allowed by either the rare earth element or the undoped carrier.  
         [0019]     The new energy profile is particularly advantageous for security purposes because it provides narrow emissions at wavelengths not naturally found in either the rare earth element or the undoped carrier. These narrow emissions can be used as part of a security marker.  
         [0020]     Preferably a plurality of rare earth dopants is used. One or more of these different rare earth dopants may have intrinsic fluorescence emissions that are visible to the unaided human eye and one or more may have intrinsic fluorescence emissions that are invisible to the unaided human eye, for example infra-red or ultra-violet fluorescence emissions.  
         [0021]     Preferably, the combined effect of the carrier and the rare earth dopant is such as to cause the security marker to emit light that is visible by the unaided eye, for example in the range of 390-700 nm.  
         [0022]     Preferably, the security marker can be excited by highly selective, high intensity visible light and the resultant emission can be detected in the visible region.  
         [0023]     It may be desirable to add secondary dopants (such as other rare earth elements) to a carrier including primary dopants (i.e., those dopants that have already been introduced into the carrier to produce fluorescence at the pre-selected wavelength) even though the emissions from these secondary dopants are not conducive to the desired transitions (i.e., the fluorescence at a pre-selected wavelength). This is because the energy levels of these secondary dopants can contribute to otherwise prohibited transitions. Thus, the secondary dopants do not fluoresce at the pre-selected wavelength, but rather they contribute indirectly by strengthening the fluorescence from primary dopants at the pre-selected wavelength.  
         [0024]     Various ratios and concentrations of dopants have been tested. In one example, the doping used was about 3 mol %, based upon the total number of moles of oxides and dopants in the composition. About 1 to 3 mol % is used for single and multi doped beads of glass (i.e., 1 mol % Eu, 1 of Tb 1 of Dy for 1 bead in steps (of each) of 0.5 mol % up to 2 mol % Eu, 3 mol % Tb and 3 mol % Dy). Bead size was about 50 micron (for screen printing). One type of glass used in this example has a soft point of about 740 degrees C., although the exact melting point depends on the specific glass used, and may vary from 700 degrees C. to 1500 degrees C. For some embodiments, efficiency may level off for doping above 3 mol %.  
         [0025]     Different methods of doping glass with rare earth elements are known. The following patents or published applications describe various doping methods, and are hereby incorporated by reference: U.S. Pat. No. 6,153,339; U.S. Pat. No. 5,262,365; and US Published Application 2004/0212302.  
         [0026]     Glass beads have been fabricated and tested (fluorescent spectra has been measured) for beads varying from 5 μm in diameter (which is a suitable size for incorporating in inkjet fluid) to 100 μm in diameter (which is a suitable size for screen printing applications). To obtain beads having a size of approximately 5 μm, the beads can be passed through a micro sieve having 61 μm apertures/reticulations.  
         [0027]     According to a second aspect of the present invention there is provided an item having an optically detectable security feature for emitting light at a pre-selected wavelength, the item comprising: a rare earth dopant and a carrier incorporating the rare earth dopant, the interaction of the carrier and the dopant being such as to provide a fluorescent fingerprint or response that is different from that of the rare earth dopant.  
         [0028]     The item may be validated by irradiating the item and detecting emissions at the pre-selected wavelength.  
         [0029]     The item may be a fluid. Examples of fluids particularly suitable for use with the invention include fuel, paint, ink and such like.  
         [0030]     Preferably, the item is a laminar media item. The laminar media item may be in the form of a web, or in sheet form. Examples of sheet form laminar media items include banknotes and financial instruments such as checks.  
         [0031]     Preferably, the item includes a plurality of security markers, each marker emitting at a different pre-selected wavelength. Alternatively, a marker may includes a plurality of rare earth dopants.  
         [0032]     In a preferred embodiment, the markers have different concentrations of dopant, so that the intensities of the pre-selected wavelength emissions are different.  
         [0033]     By virtue of this feature, the relative emission intensity of different pre-selected wavelengths can be used as a security feature. For example, one pre-selected wavelength intensity may be 100%, another pre-selected wavelength intensity 50%, a third pre-selected intensity 25%, and a fourth pre-selected intensity 50%. More or less than four wavelengths can be used. This provides a large variety of security profiles, where each profile comprises a ratio of intensities of a plurality of wavelengths. This also makes counterfeiting even more difficult, as the quantities of each dopant must be accurately replicated, in addition to the carrier energy difference.  
         [0034]     In one embodiment, the emission from each marker decays over a different time period. By virtue of this feature, the time over which an emission occurs for a particular wavelength can be used as part of a security profile.  
         [0035]     According to a third aspect of the invention there is provided a system for validating an item having an optically detectable security feature emitting light at one of a plurality of pre-selected wavelengths, where the security feature has a carrier incorporating a rare earth dopant, the system comprising: means for illuminating the security feature with one or more wavelengths for producing emissions from the rare earth dopant; means for detecting emission from the security feature at a pre-selected wavelength; means for filtering and comparing the detected emission with a security profile for the item; and means for indicating a successful validation in the event of the emission matching the security profile.  
         [0036]     Preferably, the means for illuminating the item may comprise a pulsed light emitting diode, a laser diode, or a filtered broadband light source, and an illumination filter for ensuring that only a narrow band of wavelengths illuminate the item.  
         [0037]     Preferably, the means for detecting emission comprise a detection filter to filter out all wavelengths except the pre-selected wavelength, and a photodiode to detect the intensity of light passing through the detection filter.  
         [0038]     In a preferred embodiment, the illumination means comprises an array of LEDs, each LED having a different illumination filter, so that the item to be validated is illuminated with multiple wavelengths. In such an embodiment, the detection means comprises an array of photodiodes, each photodiode having a different detection filter, so that the emission at each pre-selected wavelength can be determined.  
         [0039]     According to a fourth aspect of the invention there is provided a method of validating an item having an optically detectable security feature emitting light at one of a plurality of pre-selected wavelengths, the method comprising the steps of: illuminating the security feature with light at one or more wavelengths for producing emissions from the rare earth dopant; detecting emission from the security feature at a pre-selected wavelength; filtering and comparing the detected emission with a security profile for the item; and indicating a successful validation in the event of the emission matching the security profile.  
         [0040]     According to a fifth aspect of the invention, there is provided an optically detectable security marker for emitting light at a pre-selected wavelength, the marker comprising: a rare earth dopant incorporated within a carrier material, the dopant and the carrier material being such as to cause emission of visible light in response to optical stimulation by visible light of a pre-determined wavelength.  
         [0041]     Preferably, the interaction of the carrier and the dopant is such as to provide a fluorescent fingerprint or response that is different from that of the rare earth dopant.  
         [0042]     According to a sixth aspect of the invention, there is provided a security item that includes an optically detectable security marker for emitting light at a pre-selected wavelength, the marker comprising: a rare earth dopant incorporated within a carrier material, the dopant and the carrier material being such as to cause emission of visible light in response to optical excitation by visible light.  
         [0043]     Preferably, the security item is a fluid, for example fuel, paint, ink and such like. Alternatively the security item may be a laminar media item, for example banknotes and financial instruments such as checks.  
         [0044]     Preferably, the item includes a plurality of security markers, each marker emitting at a different pre-selected wavelength.  
         [0045]     Preferably, the interaction of the carrier and the dopant is such as to provide a fluorescent fingerprint that is different from that of the rare earth dopant.  
         [0046]     According to a seventh aspect of the invention, there is provided a security marker comprising a borosilicate based glass, preferably including SiO 2 ; NaO; CaO; MgO; Al 2 O 3  0.29; FeO and/or Fe 2 O 3 ; K 2 O, and B 2 O 3 , and a rare earth dopant, preferably a lanthanide. Preferably the glass has a composition of: SiO 2  51.79 wt %; NaO 9.79 wt %; CaO 7.00 wt %; MgO 2.36 wt %; Al 2 O 3  0.29 wt %; FeO, Fe 2 O 3  0.14V wt %; K 2 O 0.07 wt %, and B 2 O 3  28.56 wt %, not precluding the use of other glass mixes. The glass and the rare earth ion may be formed into a micro-bead.  
         [0047]     The marker comprises a carrier, such as glass or plastic including one or more types of rare earth ion. The interaction of the glass or plastic and the dopant is such that the spectral response of the marker is different from that of the rare earth dopant or the carrier per se. In particular, the interaction between the carrier and the dopant is such that the intrinsic energy levels of the dopant change when it is incorporated into the carrier. For example, when the dopant is incorporated into a glass, new bonds are formed in the doped glass, thus altering the electron arrangement and hence the energy levels of absorption and fluorescent emission. Altering the rare earth dopant and/or dopant chelate and/or the composition of the carrier changes these energy levels and hence the observed fluorescent fingerprint. The preferred dopant is any of the lanthanides except Lanthanum.  
         [0048]     Preferably the rare earth doped glass is formed into micro-beads that can be included in, for example, a fluid such as ink.  
         [0049]     According to an eighth aspect of the invention, there is provided a kit comprising a) a collection of samples derived from a single batch of material, all samples producing a common response signature when illuminated by a set of excitation frequencies, and b) a scanner for illuminating a test sample with the set of excitation frequencies and ascertaining whether the test sample produces the response signature.  
         [0050]     Preferably, the scanner includes data indicating the response signature, and compares a signature obtained from the test sample with the data.  
         [0051]     Preferably, the scanner i) includes one of the samples as a reference, ii) obtains a signature from the reference, iii) obtains a signature from the test sample, and iv) compares the two signatures.  
         [0052]     Several methods for doping standard glass compositions with the selected fluorescent rare earth ions can be employed. As used herein, the word “dopant” refers to (i) additives (for example rare earth elements) introduced to the carrier components before the carrier (for example, glass) is produced, so that when the carrier is produced it contains the additives, which is referred to herein as a “pre-production dopant”; and/or (ii) additives introduced to the carrier after the carrier is produced, so that the carrier is produced without the additives present, which is referred to herein as a “post-production dopant”. Thus, the term dopant covers additives introduced before (pre-production) or after (post-production) the carrier is produced.  
         [0053]     In one method, test samples of doped glass are prepared by the incorporation of the rare earth ions into the batch composition using the appropriate metal salt. The glass is prepared by heating the batch in a platinum crucible to above the melting point of the mixture. In another method, existing standard glass samples are powdered and mixed with solutions of the fluorescent ions. The glass is lifted out of the solvent, washed and then oven dried.  
         [0054]     An example of a glass that could be used as the carrier material for the rare earth dopants is a borosilicate based glass. In particular, a glass that could be used is as follows: SiO 2  51.79 wt %; NaO 9.79 wt %; CaO 7.00 wt %; MgO 2.36 wt %; Al 2 O 3  0.29 wt %; FeO, Fe 2 O 3  0.14 wt %; K 2 O 0.07 wt %, and B 2 O 3 28.56 wt %. This can be made by ball milling soda lime beads (100 μm) for 5 minutes to create a powder to help melting and mixing. Then 5 g of the crushed soda lime beads, 2 g of the B 2 O 3  and 3 mol % of the rare earth dopant, for example Europium, Dysprosium and Terbium but also others, are ball milled together for, for example, 3 minutes. The resulting powder is then put in a furnace and heated up to  550 C. It is left in the furnace at this temperature for about 30 minutes, to ensure that the boric oxide is completely melted. Then the temperature is increased to 1100 C. for 1 hour to produce a homogeneous melt. The temperature is increased again to 1250 C and the molten glass is poured into a brass mould, which is at room temperature, which quenches the glass to form a transparent, bubble free borosilicate glass, doped with rare earth ions.  
         [0055]     The peak emission wavelength for fluorescent emission in the marker depends on the energy levels of the final rare earth doped glass. Altering the weight percentage of the network modifier oxides within the glass matrix will change these levels and hence change the observed peak fingerprint. Hence, to observe the correct wavelength fingerprint, the glass composition has to be known. Likewise, where two or more rare earth dopants are used, varying the ratios, by mole percentage, of these changes the fluorescence intensity in the detected signal. Peak intensities can be used as part of an encoding scheme and so by varying the dopant levels, there is provided an opportunity to provide even more encoding options. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0056]      FIG. 1  illustrates processing which produces one form of the invention.  
         [0057]      FIG. 2  illustrates four excitation wavelengths  10 , and the response produced by each, under one form of the invention.  
         [0058]      FIG. 3  illustrates an excitation frequency F 1  and a response frequency F 2 .  
         [0059]      FIG. 4  illustrates decay over time of response frequency F 2 .  
         [0060]      FIG. 5  illustrates a time delay DEL-T which can exist between the excitation frequency F 1  and the response frequency F 2 .  
         [0061]      FIG. 6  illustrates sequential excitation by four excitation frequencies.  
         [0062]      FIG. 7  illustrates part of a database according to one form of the invention.  
         [0063]      FIG. 8  illustrates a prior-art table of energy levels of various dopants in silicon.  
         [0064]      FIG. 9  illustrates a computer  55  storing database  50 , which is accessible by a remote computer  65 .  
         [0065]      FIG. 10  illustrates one form of the invention, implemented in connection with a photocopier.  
         [0066]      FIG. 11  illustrates a varnish  155  in which are suspended glass particles  150  of the type described herein.  
         [0067]      FIG. 12  illustrates a coating  175  on an article  170 , which coating  175  contains glass particles  150 .  
         [0068]      FIG. 13  illustrates a carrier  210  supporting a glass fragment  215 .  
         [0069]      FIG. 14  illustrates a carrier on which the glass beads can represent data.  
         [0070]      FIG. 15  illustrates a kit according to one form of the invention.  
         [0071]      FIG. 16  illustrates a scanner according to one form of the invention.  
         [0072]      FIG. 17  is a block diagram of a detector arrangement;  
         [0073]      FIG. 18  is a table showing various excitation wavelengths and corresponding emission wavelengths for a Europium dopant in a borosilicate based glass, and  
         [0074]      FIG. 19  is a table similar to that of  FIG. 2  for Europium, but in solution. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0075]     Block  1  in  FIG. 1  illustrates a collection of two types of raw materials: (1) a group of oxides and (2) one or more rare earth elements. The labels W, such as W 1 , indicate that each raw material is present in a specific weight. Thus, the collective labels W 1 -W 10  indicate a specific composition, by weight, of the raw materials.  
         [0076]     The raw materials undergo heat treatment and possibly annealing, as indicated by the arrow labeled PROCESS, to produce a glass billet  2 . The glass billet  2  is then cut into dice, or pulverized, as indicated by the arrow labeled DICE/PULVERIZE/etc.  
         [0077]     Arrow  3  points to a block which represents one of the dice, or a collection of the powder. In the general case, when the block  3  is excited by radiation, indicated by frequencies F 1  through F 5 , the block  3  will re-radiate specific frequencies, indicated by frequencies F 6  through F 10 .  
         [0078]     The specific re-radiated frequencies, and also properties of those re-radiated frequencies, are unique to the specific glass billet  2 . The properties of the re-radiated frequencies are described in detail below, but include (1) intensity of each re-radiated frequency and (2) decay rate of each re-radiated frequency.  
         [0079]     To repeat, in general, if the relative weights W are altered, different re-radiated frequencies, with different properties, will be detected. Also, if the heat treatment, annealing, or both, of the glass billet  2  are changed, then different re-radiated frequencies, with different properties, can also be detected, even if the compositions of two billets  2  are identical.  
         [0080]     Therefore, in the general case, the re-radiated frequencies and their properties, obtained from a given set of excitation frequencies, depend on (1) the composition that is, the relative weights W, (2) the heat treatment, and (3) the annealing (if any) of the glass billet  2 .  
         [0081]      FIG. 2  illustrates a generalized example of the response of a particular glass billet, and is based on  FIG. 18 , described later. Image  10  indicates four excitation wavelengths, at 395, 415, 465, and 535 nanometers, nm. For the glass billet in question, the 535 nm excitation produces wavelength  13 , of relative intensity indicated. The 465 nm excitation produces wavelengths  15  and  17 , of relative intensities indicted.  
         [0082]     The 415 nm excitation produces wavelengths  19  and  21 , of relative intensities indicated. The 295 nm excitation produces wavelengths  23 ,  25 ,  27 , and  29 , of relative intensities indicated.  FIG. 18  sets forth the relative intensities more precisely, in numerical form.  
         [0083]      FIG. 3  illustrates the general principle that an excitation frequency F 1  will produce at least one re-radiated, or output, frequency F 2 . Frequency F 2  will be characterized by an initial intensity, indicated by I 2 . Also, as indicated in  FIG. 4 , output frequency F 2  will be characterized by a decay time, such as T 2 , which is the time required to decay to 50 percent of its initial value.  
         [0084]     In addition to the decay time T 2 , another time interval may be present, such as that shown in  FIG. 5 . The output frequency F 2  can occur after a time interval DEL-T following the excitation frequency F 1 . This delay time DEL-T may also be a property of the output frequency F 2 , and used to identify the glass billet.  
         [0085]     In addition, the delay time DEL-T can be used to solve a particular problem which can arise. The Inventors point out that, in  FIG. 2 , the excitation wavelength of 395 nm produces an output wavelength of 535 nm. However, that output corresponds to an excitation wavelength of the same value. Thus, if the four excitation wavelengths in image  10  were applied simultaneously, a problem could arise in determining whether a detected signal at wavelength 535 nm was caused by the excitation at that wavelength, or by response  23 .  
         [0086]     One solution to this problem is to utilize the time delay DEL-T of  FIG. 5 . The excitation wavelengths are first applied, allowed to decay, and then a detector is activated after DEL-T expires. Then it is known that, if a signal at wavelength 535 nm is detected, it is not due to an excitation at that wavelength.  
         [0087]     In addition, another solution to the problem would be to sequentially apply the excitations, as indicated by the sequence F 1  through F 4  in  FIG. 6 . When each excitation of a specific frequency is applied, a detector looks for a response, either at the same time, or after a delay such as DEL-T in  FIG. 5 .  
         [0088]     The principles just described can be used to construct a database  30  in  FIG. 7 . The column labeled COMPOSITION refers to a specific billet, which contains a specific set of relative percentages of components, and which was subjected to specific heat treatment and annealing, or other specific processing.  
         [0089]     Heat treatment refers to the time-temperature history of the billet in fusing the oxides and the rare earth element(s) together. Annealing refers to the time-temperature history of the billet following heat treatment. Of course, in some cases no annealing may be used, or air quenching can be viewed as the annealing.  
         [0090]     The column labeled EXCITATION refers to the frequency of excitation applied to the billet, or sample of the billet. In the case of COMPOSITION  1 , two excitation frequencies F 1  and F 4  are indicated.  
         [0091]     The column labeled RESPONSE refers to the frequency, decay time, and initial intensity of signals re-radiated in response to the excitation frequency. For example, in the case of COMPOSITION  1 , the excitation frequency F 1  produces re-radiated light of frequency F 2 , initial intensity I 2 , and decay time T 2  and also re-radiated light of frequency F 3 , initial intensity I 3 , and decay time T 3 .  
         [0092]     In addition, Excitation frequency F 4  produces re-radiated light of frequency F 5 , initial intensity I 5 , and decay time T 5 .  
         [0093]     Of course, the specific definitions of intensities, such as I 5 , and decay time, such as T 5 , are here chosen for convenience. Other definitions are possible, and values other than initial intensity and 50-percent-decay-time can be used.  
         [0094]     Also, if a delay time, such as DEL-T in  FIG. 5 , is found significant for a particular billet and excitation frequency, that delay time can be included in the database, and is deemed represented by the times such as T 2 , T 3 , and so on.  
         [0095]     The collection of responses are viewed as a signature which identifies each billet. For example, the responses in dashed box  33  represent one signature of a billet of composition  1 . Of course, a sub-set of the contents of box  33  could be used. For example, the excitation frequency F 1  could be used, and frequency F 4  could be eliminated.  
         [0096]     Several significant features which distinguish the glass dice  3  of  FIG. 1  from taggants in the prior art are the following.  
         [0097]     One is that, at the present time, it is difficult to reverse-engineer the dice. That is, it is difficult for one to excite the glass as indicated in  FIG. 2 , detect the output signature, and then fabricate a glass which produces that output signature. One reason is that a complete database of the type shown in  FIG. 7  is not known to exist. That is, a complete database which covers all possible compositions of glass billets, and their signatures, is not known to be available in the published literature, including printed publications as defined by 35 USC section 102.  
         [0098]     This fact distinguishes the invention from systems which may appear to be similar, but are not. For example, silicon, a crystal, can be doped with different elements. The doped silicon can then be excited, and radiated light of frequency corresponding to the doping element will be detected. Based on the frequency of the re-radiated light, one can consult known tables, and determine the identity of the dopant.  FIG. 8  illustrates such a table. The frequency of re-radiated light will depend on the drop in energy D experienced by an electron, and that drop will depend on the energy level E created by the dopant. One can thus reproduce the silicon-dopant system, based on the table.  
         [0099]     However, to repeat, such tables are not known to exist for the glass systems in question.  
         [0100]     A second feature is that the glass systems in question are not crystalline. Glasses, in general, are supercooled liquids, they are not crystals. Thus, an energy level system corresponding to that of  FIG. 8  is not present or, if present, is different for the different glasses described herein.  
         [0101]     A third feature is that some glasses are classified as refractory materials. Dice, or powders, of such glasses can withstand high temperatures. Such glasses are unaffected by temperatures of 400, 500, 700, 1000 degrees F., and higher. This distinguishes them from most, if not all, fluorescent inks and paints, and the surfaces to which the inks and paints are applied.  
         [0102]     Several applications of the glasses under consideration will be discussed.  
         [0103]     In  FIG. 9 , a database  50  is stored in a computer  55 . The database  50  is generated by a glass foundry (not shown) which fabricated a billet  2  in  FIG. 1  of glass. The glass foundry subjected the billet, or fragments of it, to various excitation frequencies, and measured the signature of the glass. Data concerning the glass, such as the composition, heat treatment, annealing, excitation frequencies and resulting signatures, are stored in the database  50 , and indicated by blocks D 1 -D 8 . The identity of the foundry can be included in the data.  
         [0104]     The glass foundry can repeat the process for another billet of glass, of different composition.  
         [0105]     A user (not shown) would test a sample  60  of the glass billet. For example, the sample may be attached to a specific article (not shown). The user would apply excitation frequencies to the sample  60 , and obtain a signature of the sample  60 .  FIG. 2  illustrates generalized excitation frequencies in image  10 , and the signature which results.  
         [0106]     The signature obtained can be represented as a collection of data, which the user transmits to the computer  55  in  FIG. 9 , over the INTERNET, using the user&#39;s own computer  65 . As database  30  in  FIG. 7  indicates, knowledge of the signature allows one to ascertain the composition of the glass from which the sample  60  in  FIG. 9  originated, or any data associated with the data in the database, such as the identity of the foundry which fabricated the glass.  
         [0107]     In addition, other information can be included in the database  50  in  FIG. 9 . For example, a billet having a given signature can be transferred to a specific party, such as a government. That party can be identified in the database  50 , in connection with the data regarding the billet.  
         [0108]     As a more specific example, fragments of the billet can be pulverized and added to an ink which is used to print currency. If a sample  60  in  FIG. 9  is taken from the currency, and points to the specific billet, then it is known that the currency is associated with the billet delivered to the particular government.  
         [0109]     Thus, in general, a sample  60  in  FIG. 9  of a billet can be used to trace the origin of the sample. Or database  50  in  FIG. 9  can indicate the original owner of the billet from which the sample  60  is derived.  
         [0110]     In another application, the glass can be used to suppress counterfeiting or copying. Block  100  in  FIG. 10  represents a photocopier. Block  105  represents a sheet to be copied, which can take the form of a visual image on a paper carrier. Block  10  represents a fragment of the glass attached to the carrier.  
         [0111]     Block  115  represents a detector, which illuminates the sheet  105  at the copying station, and thereby illuminates block  10 , the fragment of glass. If block  10  produces a particular signature, then the detector  115  blocks copying, so that the photocopier  100  will not copy the sheet  105 .  
         [0112]     Alternately, the system could be designed so that only sheets bearing an authorizing block  110  can be copied. Thus, if the proper signature is detected, copying is allowed, and ordinary sheets lacking a block  110  cannot be copied.  
         [0113]     In another application, fragments  150  of the glass in  FIG. 11  are added to a liquid carrier  155 , such as a varnish. In one embodiment, the fragments take the form of a fine powder, and have no dimension larger than, say, one micron, five microns, ten microns, fifteen microns, or twenty microns. In one embodiment the powder is sufficiently fine that the granules are invisible to the naked eye. In another embodiment, the grains of the powder are approximately the size of the grains of common table salt.  
         [0114]     The liquid carrier is painted onto an article, represented by block  170  in  FIG. 12 , forming a coating  175 . The signature of the particles can be detected in the manner described above, and the database  50  in  FIG. 9  can be used to deduce information about the article  170 .  
         [0115]     Significantly, in some cases, the article  170  can perform a function, the particles do not interfere with that function, and the function can be performed if the particles are not present. For instance, if the article  170  is a handgun, the particles do not interfere with the function of the handgun, and the particles need not be present for that function to exist.  
         [0116]     In another application, it is not necessary to consult a database. A detector, as described herein, can be equipped with data which indicates a signature of fragments from a glass billet. Or the data can indicate multiple signatures, for multiple billets.  
         [0117]     In use, an article  210  in  FIG. 13 , which carries a glass fragment  215 , is submitted to the detector  200  in  FIG. 13 . The detector  200  obtains the signature of the fragment  215  and, if the signature matches a stored signature, the detector thereby deduces information about the article  210 . Such information can relate to authenticity, origin, ownership, information about the article  210 , or any other characteristic which possession of a fragment  215  having a predetermined signature can represent.  
         [0118]     For example, the article  210  can take the form of a document, item of fine art, a label, a registration plate or card for a vehicle or other item commonly registered with a government, a written signature or fingerprint carried on a card, or a storage medium such as a CD or floppy disc. If the fragment  215  displays a specific signature, then that signature indicates that the article  210  may be copied, or is prohibited from being copied, as appropriate.  
         [0119]     As another example, since different billets of glass produce different signatures, those signatures, or the corresponding billets, act as identification numbers. These ID-glasses can be attached to, or embedded in, articles to indicate ownership. This concept is applicable to articles such as items of fine art, precious metals and jewelry, human tissues such as organs, semen, and blood, and certificates.  
         [0120]     As a specific example, an ID-glass can be inserted into a body fluid which is to be tested for illness, or presence of drugs or alcohol. The ID-glass, being inert to most common reagents, will not affect the test results, except perhaps by contaminating an optical test, which would be rare. The ID-glass identifies the owner of the fluid.  
         [0121]     As another example, an ID-glass can identify origin of an article, and thus provide authentication. As a specific example, this can apply to items of fine art, liquors, perfumes, human tissues, admission tickets, entertainment recordings such as video tapes and discs.  
         [0122]     As another example, the ID-number feature of the ID glass can be used to classify articles or substances. As a specific example, ten different ID-glasses, with ten different signatures, can be fabricated. These can be used to distinguish ten ostensibly identical, yet different, articles. For example, contact lenses look identical, but are different. A tiny ID-glass at the edge can identify the contact lens. A similar principle applies to blood type, pharmaceuticals, chemicals, and so on.  
         [0123]     A similar identification can perform a trademark-like function, in identifying authentic goods. Without limitation, this would apply to toner cartridges, fuels, tires, and any fungible articles in which the identity of the manufacturer or supplier is important.  
         [0124]     As another example, the ID-glass can be used to track articles. For example, a fuel tank at a gasoline filling station may acquire a leak. If an ID-glass powder is added to the tank, the powder will migrate to the leak and escape. A detector can be used to elicit the powder to display its signature, to locate the leak.  
         [0125]     This tracking function can be applied to people, animals, weapons, explosives, medical instruments, pollutants, and watercourses. It can also be applied to any article or substance generally which moves, and which motion is to be followed, such as blood in the human circulatory system and food in the human digestive system.  
         [0126]     In the case of treating the article  210  as a human, the tag  215 , if exhibiting the proper signature, can act as an admission permit or key. Thus, tag  215  can grant admission to places or buildings. Or tag  215  can grant permission to use specific equipment.  
         [0127]     In another application, article  210  of  FIG. 13  can represent a person or other living being. A fragment  215  having a predetermined signature can represent a specific characteristic, such as color-blindness.  
         [0128]     In another application, the article  170  in  FIG. 13  bears no visible tags, yet the coating exhibits the signature when excited. Alternately, the coating is applied only in a concealed location on the article  170 .  
         [0129]     In another application, the glass fragments can cooperate with each other to provide information. For example,  FIG. 14  illustrates a card  300 , upon which is superimposed an imaginary grid. Distance D is pre-established by convention. If a glass fragment is positioned within a cell  205  of the grid, that cell is treated as a logical ONE. If a cell  205  is empty, that is, devoid of a glass fragment, then that cell is treated as a logical ZERO.  
         [0130]     A reader (not shown) begins at a pre-established starting point, advances in steps of distance D, and determines whether a ONE or ZERO is present. A binary encoding system is thus established.  
         [0131]     Alternately, glass fragments having two different signatures are used. Now the need to advance in units of D is eliminated, but can still be used if desired. If the two different signatures are A and B, then the sequence AABAABBB can be treated as 11011000, which is another system of binary encoding.  
         [0132]     This principle can be extended. If N types of glass fragment are used, having N different signatures, then an alphabet of N characters is thereby made available.  
         [0133]     In another embodiment, the glass fragments can be used to ascertain abrasion or wear. For example, assume a laminated material, having five layers. Five different glasses are used, having five different signatures. Glass  1  is embedded in layer  1 . Glass  2  is embedded in layer  2 , and so on.  
         [0134]     Prior to any wear occurring, only the glass in the outermost layer can be detected by its signature. After the outermost layer is abraded away, then the glass in the next layer can be detected, and so on.  
         [0135]     In another embodiment, a kit is provided. The kit  400  in  FIG. 15  contains a number of glass beads  405 . A detector  410  is provided, such as that described in connection with  FIG. 16 , and it detects the specific signature of the glass beads  405 . In ordinary practice, the detector  410  will be dormant when contained within the kit  400 . All components of the kit  400  are contained in a common package, such as a thermo-formed blister pack.  
         [0136]     The detector  410  can compare the signature obtained from a sample bead  405  with stored data indicating that signature. Or the detector  410  can be equipped with its own bead, and it compares the signature of that bead with the signature of a sample bead  405 .  
         [0137]     In another embodiment, multiple different ID-glasses are contained in the same article. The composite signature of all ID-glasses can be used for the purposes described herein.  
         [0138]     In one embodiment, one billet of glass is fabricated and its signature is ascertained. This is repeated for numerous billets, to develop a database of glasses and their signatures.  
         [0139]     In one approach, every time a new billet is fabricated, its signature is compared with existing signatures in the database. If the new signature does not deviate sufficiently from an existing signature, the corresponding billets are treated as interchangeable. Since the signatures can be, in effect, treated as numbers, a simple formula can be used to define similarity between signatures. For instance, if one signature has an intensity I, then another signature having an intensity of 0.95I can be defined as similar.  
         [0140]     In one embodiment, no database is used. A glass foundry fabricates a billet of glass, ascertains its signature, divides the billet into fragments or powder, and delivers the fragments/powder to a customer. The foundry may include data indicating the signature. Or the customer may rely on his own testing to deduce the signature. But the foundry does not retain data indicative of the signature, or if it does retain such data, keeps it secret. Or the data is not available in a printed publication as defined in 35 USC section 102.  
         [0141]     Thus, the customer obtains a collection of ID-glass fragments which, as a practical matter, are difficult to replicate. Or at least difficult to replicate by trial and error without undertaking  10 , 000  trials, which is considered an impractical number.  
         [0142]     The number of 10,000 is obtained as follows. Assume that the glass contains eight components. Assume that the final billet can contain 1, 2, 3, . . . 10 grams of each component. Under these assumptions, the final billet can weigh from eight grams (one gram of each component) to 80 grams (ten grams of each component), and any integral number of grams in-between.  
         [0143]     The total number of permutations of components is 10 raised to the eighth. Of course, the permutation 1-1-1-1-1-1-1-1 (one gram of each component) represents the same composition as 2-2-2-2-2-2-2-2 (two grams of each component), because the relative percentages of ingredients are the same. A similar comment applies to multiples: 1-2-1-1-1-1-1-1 has the same relative percentages as 2-4-2-2-2-2-2-2.  
         [0144]     Nevertheless, these identical cases represent a very small fraction of the total number of possibilities. In this example, of the ten-to-the-eighth (or 100 million) there certainly exist at least 10,000 different compositions, having 10,000 different signatures, and at least 10 million different compositions, having 10 million different signatures.  
         [0145]     Thus, when a glass billet is fabricated using eight different components, in effect, a composition is selected from 10,000 possibilities, or 10 million possibilities, depending on how counting is done.  
         [0146]     A given composition, producing a given signature, is difficult to copy to produce an identical composition which produces the same signature, for several reasons. One is that the heat treatment and annealing (if any) affect the signature, and those processing parameters are not apparent from the composition, at least not under current technology.  
         [0147]     A second reason is that the approach would typically be based on trial-and-error. However, as just explained, for eight ingredients, the number of trials required can run into the millions. Further, a trial-and-error approach does not actually amount to copying: the original composition is not copied, but numerous trials are undertaken in pursuit of a composition having a similar property as the original. That is not copying.  
         [0148]      FIG. 16  illustrates a scanner  500 , and a disc  505 , having a central hole  507 , which engages with an axle  508 . The disc  505  which carries a collection of glass beads  510 . The disc  505  contains an indexing hole  515 , which engages with an indexing pin  518 , which allows the scanner  500  to position a desired one of the glass beads at a scanning station indicated by dashed box  520 .  
         [0149]     For example, assuming that a top side of the disc  505  is defined, then the beads  510  can be identified by their rank (first, second, third) in the clockwise direction relative to the indexing hole  515 .  
         [0150]     Of course, the disc  505 , or other carrier, may carry a single bead  510 .  
         [0151]     Scanner  500  may be controlled remotely, as by a computer  550 , which selects a specific bead  510 , or sequence of beads  510 , for scanning. Thus, a sequence of signatures can be arbitrarily generated, to thwart hackers who wish to synthesize the signatures.  
         [0152]      FIG. 17  shows an arrangement for detecting information encoded in accordance with the present invention. This includes a sensor and a platform for supporting an item under test. The sensor has a housing in which are provided an emitter, for example a light emitting diode (LED), at the output of which is provided a narrow band filter. The narrow band filter allows only a very narrow, pre-determined range of wavelengths to be passed. As an example, the filter could be selected to allow a narrow band pass centered on a wavelength of 465 nm to pass through it and toward the sample platform. Adjacent to the emitter is a detector, such as a photodiode. At its input is a narrow band filter that allows only a very narrow, pre-determined range of wavelengths to pass through it. As an example, the filter could be selected to allow light centered on a wavelength of 615 nm to reach the detector.  
         [0153]     In use of this arrangement, light is emitted from the emitter and passed through the first narrow band filter and onto a security item that carries or includes the marker. This light is absorbed by the rare earth dopant, which if it matches the energy levels of the dopant and carrier used causes it to fluoresce. Light emitted from the item is transmitted towards the second filter, and from there, to the detector. Also the emission from each marker decays over a different time period. By virtue of this feature, the time over which an emission occurs for a particular wavelength can be used as part of a security profile. For authentic documents, the light received at the detector should have one or more characteristic features that can be identified. In the event that the detected response has the expected features, the item is identified as being bona fide. In the event that the response is not as expected or is not within an acceptable range of the expected response, the item is identified as being a potential counterfeit.  
         [0154]     The spectral emissions of various marker samples have been investigated. As an example,  FIG. 18  shows a table of the emission wavelengths and intensities for various different excitation wavelengths for a marker comprising of 3 mol % EuCl3 when included in the borosilicate glass described above.  
         [0155]     By way of comparison,  FIG. 18  shows the corresponding results for the EuCl 3 :6H 2 O dopant, but when in solution. From these Figures, it can be seen that in glass the most significant excitation is at 395 nm, which emits at 615 nm and 590.5 nm. The corresponding results for the EuCl 3 :6H 2 O in solution shows that the emission wavelengths here are 592.5 nm, 618.5 nm, 556.5 nm, 536 nm and 526 nm. Hence the spectral response of the marker at 395 nm is significantly different from that of the EuCl 3 :6H 2 O in solution. Also in glass, for excitation at a wavelength of 415 nm, there is an output of 615 nm and 590.5 nm. In contrast for the EuCl 3 :6H 2 O in solution there is effectively no fluorescence at this wavelength. Again, this demonstrates that there is significant and measurable difference caused by the incorporation of the EuCl 3 :6H 2 O in the borosilicate carrier.  
         [0156]     Because rare earth ions have well defined and relatively narrow, non-overlapping spectral bands, this means for many applications it is possible to detect the security marker using a single discrete pre-determined excitation wavelength and likewise a single discrete pre-determined detection wavelength. For example, for the EuCl 3  doped borosilicate glass described above, the emitter filter could be selected to be 465 nm, and the detector filter could be 615 nm. Alternatively, a plurality of stimulating wavelengths could be used. To do this, a number of different suitable emitter filters would be selected, and a plurality of corresponding filters. These would be included in the arrangement of  FIG. 1  to allow the simultaneous measurement of optical response at various different wavelengths.  
         [0157]     A further advantage of the discrete nature of spectral response of rare earth ions is that a number of species can be combined into the one product for improved security for example 3 mole % Eu+3 mole % Tb, not precluding other rare earths at different percentages and more than two. Because the response of the various different dopants is relatively discrete, detection of these is simplified. The narrow emission bands also facilitate the spectral selection of the molecules, making the detection system simpler than those required for systems containing multiple dyes. A further advantage is that many rare earth ions require excitation at wavelengths conducive to existing laser diode technologies. This makes in situ excitation possible because the excitation source is compact, robust and long lived.  
         [0158]     Furthermore, incorporating the rare earth dopants into a suitable carrier, and in particular the glass beads described herein, means that the security marker in which the invention is embodied is extremely stable under adverse chemical, environmental and physical abrasion conditions, thereby ensuring that it has a long lifetime compared to conventional dyes.  
         [0159]     A skilled person will appreciate that variations of the disclosed arrangements are possible without departing from the invention. For example, whilst only a few rare earth ions have been specifically described, it will be appreciated that there is a wide range of fluorescent rare earth ions that could be used. The number of permutations available is therefore greatly enhanced. In addition, whilst some rare earth ions emit in the UV and IR ranges, it is preferred for some applications that both the excitation radiation and the emitted radiation are within the visible range, that is within a wavelength range that is visible to the unaided human eye. Accordingly, the above description of a specific embodiment is made by way of example only and not for the purposes of limitation. It will be clear to the skilled person that minor modifications may be made without significant changes to the operation described.