Abstract:
A device for validating a secure tag comprises: an optical source (such as one or more LEDs); a processor coupled to the optical source for controlling emissions therefrom; and a luminescence detector coupled to the processor. The processor is programmed to control the optical source to create a first and a second pulse sequence having first and second excitation parameters respectfully. The processor is also programmed to control the luminescence detector to measure first and second luminescence in response to the first and second pulse sequences respectively. The processor is programmed to validate the secure tag in the event that the first and second luminescence meet an acceptance criterion. A method of validating a secure tag is also described.

Description:
The present invention relates to secure tag validation. 
   BACKGROUND 
   Secure tags are used for a number of different purposes; a primary purpose being preventing, detecting, and/or deterring counterfeiting of an item to which the secure tag is affixed. 
   One type of secure tag that has recently been developed includes multiple small particles of a host (such as glass) doped with one or more rare earth ions (“RE ions”). This type of secure tag is described in US patent application number 2004/0262547, entitled “Security Labelling,” and US patent application number 2005/0143249, entitled “Security Labels which are Difficult to Counterfeit”, both of which are incorporated herein by reference. 
   These RE particles can be applied to valuable items in different ways. For example, the secure tags can be incorporated in fluids which are applied to valuable items (by printing, spraying, painting, or such like), or incorporated directly into a substrate (paper, metal, rag, plastic, or such like) of the valuable items. 
   In response to suitable excitation, a secure tag comprising RE particles produces a luminescence spectrum having narrow peaks because of the atomic (rather than molecular) transitions involved. The narrow luminescence peaks result primarily from internal (4f to 4f) transitions of the lanthanide ion. Luminescence is a generic term that relates to a substance emitting optical radiation in response to excitation, and includes photoluminescence. 
   Photoluminescence is a generic term that includes fluorescence and phosphorescence, which will now be described with reference to  FIG. 1 , which is a simplified Jablonski energy diagram  10  showing most of the possible transitions in a molecule or atom. In  FIG. 1 , the wavy lines represent dark transitions (transitions that do not emit or absorb light). The solid lines represent transitions that absorb or emit light. 
   The molecule or atom starts out in the ground state (S 0 )  12 . When the atom or molecule absorbs light of the appropriate frequency (illustrated by arrows  14  in  FIG. 1 ), electrons in the molecule or atom are promoted to a first singlet excited state (S 1 )  16  or to a second singlet excited state (S 2 )  18  (each state having multiple vibrational energy levels). The spin on the promoted electrons are preserved during excitation. The electrons are typically excited to a higher vibrational energy level in the first singlet excited state (S 1 )  16  before rapidly relaxing (illustrated by arrows  20  in  FIG. 1 ), to the lowest energy level in the first singlet excited state (S 1 )  16 . This event is termed vibrational relaxation or internal conversion and occurs in about a picosecond or less. The excited state may decay directly back to the ground state by way of fluorescence (illustrated by arrows  22 ), quenching (illustrated by arrow  24 ), or non-radiative relaxation (illustrated by arrows  26 ). The excited state may also transfer energy to the triplet excited state (T 1 )  28 , which is referred to as intersystem crossing, as illustrated by wavy line  30 . The spin on the electron is flipped as it moves from S 1  to T 1 . From the T 1  state the molecule or atom may emit a photon of light (phosphorescence)  32  or lose the energy via non-radiative relaxation  26 . During phosphorescence the spin on the electron is again flipped. The transition from T 1  to S 0  is slow compared to other possible transitions, the timescales are typically between 10 −3  to 10 2  seconds. Thus, in internal conversion the spin is preserved; whereas in intersystem crossing the spin is flipped. 
   Secure tags based on RE ions phosphoresce, which allows a delay to be used between excitation and measuring the stimulated phosphorescence. This ensures that any fluorescence from background material (such as a substrate on which the secure tag is located) has decayed prior to the phosphorescence measurements taking place. 
   To enable quick and accurate validation of a secure tag, a luminescence signature is typically derived from the luminescence measured from that secure tag. This luminescence signature may be based on peak locations, absence of peaks, relative peak intensities, and such like. A luminescence signature is typically derived by converting a large number of data points from a luminescence spectrum into a relatively short code. This short code (the luminescence signature) enables rapid comparison with other, pre-stored luminescence signatures to facilitate validation of the secure tag. 
   It would be desirable to increase the security of secure tags based on RE particles to make them even more difficult to counterfeit, without making validation of the RE particles slower or more expensive. 
   SUMMARY 
   According to a first aspect of the present invention there is provided a secure tag validation method comprising: illuminating the secure tag using a first pulse sequence having first excitation parameters; measuring first luminescence emitted from the secure tag in response to the first pulse sequence; illuminating the secure tag using a second pulse sequence having second excitation parameters; measuring second luminescence emitted from the secure tag in response to the second pulse sequence; validating the secure tag in the event that the first and second luminescence meet an acceptance criterion. 
   The first excitation parameters may be selected to ensure that an intermediate state is saturated, so that the system is stable and the number of electrons entering the intermediate state is approximately equal to the number of electrons leaving the intermediate state. 
   The second excitation parameters may be selected to ensure that an intermediate state is not saturated, so that the number of electrons entering the intermediate state exceeds the number of electrons leaving the intermediate state. 
   The secure tag may include a plurality of rare earth (RE) ions, each RE ion having a different charging time; that is, the time taken to reach saturation for a constant excitation power and frequency. 
   Where two RE ions are used, one pulse sequence may cause both a first and a second RE ion to saturate; another pulse sequence may cause the first RE ion to saturate, but the second RE ion not to saturate. Where three or more RE ions are used, more permutations are possible. 
   By virtue of this aspect of the invention, two different excitation pulse sequences can be used to stimulate luminescence from a secure tag. Where multiple different types of RE ions are used in the secure tag, the RE ions will typically have different charging rates. By selecting at least one set of excitation parameters that does not saturate excited triplet states within all of the RE ions, the first luminescence will differ from the second luminescence. This difference can be used to improve security because a counterfeit secure tag is unlikely to replicate this effect. 
   Those of skill in the art will recognize that to excite a large number of molecules into the T 1  state ( FIG. 1 ), a large number of molecules must remain in the S 1  state for a substantial period of time. The number of molecules in the S 1  excited state is determined by the power and frequency of the illumination. The duration of the pulse determines how much time the molecules have to transition from the S 1  state to the T 1  state. Because a transition from the S 1  state to the S 0  state is much more probable than a transition from the S 1  state to the T 1  state (due to the requirement for the electron spin to flip for the S 1  to T 1  transition), the S 1  state is typically empty within 10 −9  to 10 −7  seconds after the illumination is switched off. 
   When the illumination is in the form of a relatively short pulse (of constant illumination power and frequency), relatively few molecules enter the T 1  state. As the duration of the pulse is increased, the number of molecules entering the T 1  state continues to increase. However, at some point the duration of the pulse reaches a certain value (referred to herein as the “saturation time”) at which the number of molecules entering the T 1  state (through intersystem crossing) equals the number of molecules leaving the T 1  state (through phosphorescence and non-radiative relaxation). This state is referred to as saturation. Any increase in the duration of the illumination beyond the saturation time will not increase the amount of phosphorescence after the illumination is turned off. However, if illumination is used that has a pulse duration shorter than the saturation time, then one RE ion may phosphoresce more strongly than another RE ion because of more efficient filling of the T 1  state. 
   The acceptance criterion may be implemented by deriving a first luminescence signature from the measured first luminescence; ascertaining if the derived first luminescence signature matches a first pre-stored luminescence signature; deriving a second luminescence signature from the measured second luminescence; ascertaining if the derived second luminescence signature matches a second pre-stored luminescence signature; and validating the secure tag in the event that the first luminescence signature matches the first pre-stored luminescence signature and the second luminescence signature matches the second pre-stored luminescence signature. 
   Ascertaining if the derived first luminescence signature matches the first pre-stored luminescence signature may comprise ascertaining whether the derived luminescence signature differs from the first pre-stored luminescence signature by less than a predetermined amount (for example, a five percent difference). In other words, the derived luminescence signature may match the first pre-stored luminescence signature even if there is a relatively small difference between them. Similarly, the second luminescence signature may match the second pre-stored luminescence signature if the difference is less than a predetermined amount. 
   The first and/or second pulse sequence may be a single pulse, or it may be a series of pulses. Where a series of pulses is used, the repetition rate is the pulse to space ratio for that series. 
   The excitation parameters may include: excitation frequency, pulse width (that is, the duration of each pulse), pulse power, repetition rate of each pulse (where a pulse sequence comprises a series of pulses), number of pulses in the sequence, and such like. 
   The step of measuring the first luminescence emitted from the secure tag in response to the first pulse may occur at a time delay (t D ) after the illuminating step has ceased; that is, without any illumination present (no pulse sequence). Alternatively, the step of measuring the first luminescence emitted from the secure tag in response to the first pulse occurs simultaneously with the illuminating step. 
   Preferably, the time delay (t D ) is selected to ensure that background fluorescence has decayed to noise levels prior to the measuring steps taking place. The time delay (t D ) may be between 5 and 2500 microseconds. 
   To achieve a stronger signal, the method may involve taking multiple measurements at the same delay time before a secure tag is validated. For example, a secure tag may be excited, the time delay (t D ) elapses, the luminescence is measured, the secure tag is then immediately excited again, the same time delay (t D ) elapses, the luminescence is measured again, and so on. The multiple measurements (all at the same delay time) are then combined. Although this increases the length of time required to validate a secure tag, it ensures that a short integration time can be used for each measurement. 
   By changing the parameters of the excitation pulse sequences, the resulting phosphorescent spectrum can be altered. The time scale selected for the pulse width parameter can range from nanoseconds to milliseconds. A similar effect can be achieved by changing the power of the excitation pulse, and (where a series of pulses is used) by changing the repetition rate. 
   This aspect of the invention has the advantage of added flexibility and security to the tag validation process, without adding much complexity. 
   According to a second aspect of the invention there is provided a device for validating a secure tag, the device comprising: an optical source; a processor coupled to the optical source; and a luminescence detector coupled to the processor; the processor being operable (i) to control the optical source to create a first and a second pulse sequence having first and second excitation parameters respectfully, (ii) to control the luminescence detector to measure first and second luminescence in response to the first and second pulse sequences respectively, and (iii) to validate the secure tag in the event that the first and second luminescence meet an acceptance criterion. 
   The device may include a network connection to upload or download data. 
   These and other aspects of the present invention will be apparent from the following specific description, given by way of example, with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
       FIG. 1  is a prior art Jablonski energy diagram provided as background information; 
       FIG. 2  is a schematic diagram of a secure tag reader according to one embodiment of the present invention; 
       FIGS. 3A and 3B  are two graphs illustrating differential rates of saturation from an excited state (the S 1  excited state) to an intermediate state (the T 1  state) for two different lanthanide ions; 
       FIG. 4  is a schematic diagram of a banknote incorporating a secure tag for validation by the reader of  FIG. 1 ; and 
       FIG. 5  is a flowchart illustrating steps involved in validating the banknote of  FIG. 4  using the reader of  FIG. 1 . 
   

   DETAILED DESCRIPTION 
   Reference is first made to  FIG. 2 , which is a schematic diagram of a secure tag reader  100  according to one embodiment of the present invention. 
   The reader  100  is a fixed position unit and comprises a housing  102  in which an optical excitation source  104  is mounted. The optical excitation source  104  is in the form of a pair of LEDs  104   a ,  104   b  circumferentially spaced around a collecting lens  108 , diametrically opposite each other. The LEDs  104   a ,  104   b  emit at approximately 395 nm, which is visible to the human eye and corresponds to the deep blue region of the electromagnetic spectrum, and have a power of approximately 8 mW. 
   A Fresnel lens  110  is mounted at a window in the housing  102  to focus radiation (illustrated by arrows  112 ) from the excitation source  104  onto a focus spot (illustrated by broken line  114 ) at which a group of secure tags  116  will be located. 
   Luminescence emitted from the secure tags  116  (illustrated by broken arrows  118 ) is directed by the Fresnel lens  110  onto the collecting lens  108 , which in turn focuses the luminescence onto a luminescence detector  120 , which is an imaging sensor in the form of a CCD sensor. 
   The CCD sensor  120  is coupled to a controller  122 , comprising a processor  124  and non-volatile memory (NVRAM)  126 . 
   The processor  124  receives intensity data from the CCD sensor  120  and processes this data to validate the secure tags  116 , as will be described in more detail below. 
   The NVRAM  126  stores: a processing algorithm  128 , a parameter information file  130 , and pre-stored luminescence signatures  132 . 
   The processing algorithm  128  is used by the processor  124  to derive luminescence signatures from measured luminescence, and to compare the derived luminescence signatures with the pre-stored luminescence signatures  132 . 
   The parameter information file  130  stores (i) excitation parameters used by the processor  124  to control activation and de-activation of the LEDs  104   a ,  104   b , and (ii) detection parameters used by the processor  124  to control activation of the CCD sensor  120 . 
   There are a plurality of sets of excitation parameters (in this embodiment there are three sets of excitation parameters), where each set of excitation parameters is used to create a pulse sequence. There are also a plurality of pre-stored luminescence signatures  132 , with one luminescence signature corresponding to each set of excitation parameters. In this embodiment there are three pre-stored luminescence signatures. 
   The detection parameters indicate when the CCD sensor  120  is to be activated, and how long an integration time is to be used to measure luminescence, so that the sensor  120  detects (or at least transmits to the processor  124 ) luminescence when activated by the controller  122 . The sensor  120  may actually detect luminescence continually but the processor  124  may only receive (or only store) the detected luminescence when the CCD sensor  120  is “activated”. In this embodiment, the detection parameters are the same for each set of excitation parameters. 
   The controller  122  is coupled to a USB port  140  for outputting data, or the results of analysis on the data, and (in some embodiments) for receiving updated parameter information from a remote source via a network  142 . 
   The reader  100  also includes a simple user interface  146  coupled to the controller  122 . The user interface  146  comprises: a trigger  148 , which allows a user to activate the reader  100 ; a red LED  152 , which indicates a failure to validate a secure tag; a green LED  154 , which indicates a successfully validated secure tag; and a loudspeaker  156 , which emits a short beep when a secure tag is successfully validated, and a long beep when a secure tag is not successfully validated. 
   In this embodiment, the reader  100  is intended to read secure tags  116  comprising microbeads of borosilicate glass doped with 3 mol % of Europium and 3 mol % of Dysprosium. The principles of manufacturing borosilicate glass doped with Europium and Dysprosium are described in US patent application publication number 2005/0143249, entitled “Security Labels which are Difficult to Counterfeit”. 
   Reference is now also made to  FIGS. 3A and 3B , which are two graphs illustrating the differential rates of saturation from the S 1  excited state to the T 1  state for Eu and Dy. In each of these Figs., a single pulse of optical excitation is used; however, the pulse width in  FIG. 3A  is shorter than that used in  FIG. 3B . Population of the triplet state in the Eu ion occurs more quickly than for the Dy ion, so the charging curve  180  of the Eu ion in  FIG. 3A  is steeper than the curve  182  of the Dy ion. Since the pulse width  184  in  FIG. 3A  is relatively short, neither the Eu ion nor the Dy ion reaches saturation. A detection window  186  is shown in  FIG. 3A , at which the luminescence intensity from the Eu ion is disproportionately greater than the luminescence intensity from the Dy ion, because the Eu ion is nearer saturation (and therefore has a higher triplet state population). 
   In  FIG. 3B , although the charging curve  190  of the Eu ion is steeper than the curve  192  of the Dy ion, both curves reach saturation because the pulse width  194  is relatively long. A detection window  196  is shown in  FIG. 3B , at which the luminescence intensity from the Eu ion is only slightly greater than the luminescence intensity from the Dy ion, reflecting the steady state condition of saturation. 
   To program the secure tag reader  100  to read secure tags comprising Eu and Dy RE ions, excitation parameters are derived. The excitation frequency is 395 nm and the power is 8 mW, which are the characteristics of the LEDs  104   a ,  104   b  mounted in the reader  100 . The number of pulses, pulse width, and repetition rate can be selected from a number of different variables. 
   First excitation parameters comprise the pulse width, repetition rate, and number of pulses. The first excitation parameters are selected through trial and error to ensure that both the Eu and Dy ions are saturated as a result of a first pulse sequence based on the first excitation parameters. 
   Second excitation parameters comprise a shorter pulse width than for the first excitation parameters, but the same repetition rate, and the same number of pulses. The second excitation parameters are selected through trial and error to ensure that the Eu ion is saturated but the Dy ion is not saturated as a result of a second pulse sequence based on the second excitation parameters. 
   Third excitation parameters comprise a shorter pulse width than for the first or second excitation parameters, but the same repetition rate, and the same number of pulses. The third excitation parameters are selected through trial and error to ensure that neither the Eu ions nor the Dy ions are saturated as a result of a third pulse sequence based on the third excitation parameters. 
   The first, second, and third excitation parameters are loaded into the parameter information file  130  in reader  100 . Detection parameters are also loaded into the parameter information file  130 . In this embodiment, the detection parameters, which are used by the processor  124  to control activation of the CCD sensor  120 , are set to ten microseconds (10 μs) after a pulse sequence has ceased. In this embodiment, the same detection parameter is used for each pulse sequence. 
   Once the parameter information file  130  has been loaded, the reader  100  is ready to validate secure tags, as will now be described with reference to  FIG. 4  and  FIG. 5 .  FIG. 4  is a schematic diagram of a valuable media item  200 , in the form of a banknote, which is printed with ink incorporating secure tags  116  at a tag area  218  on the banknote  200 . The tags  116  comprise small beads (typically having an average diameter of five microns or less) of borosilicate glass doped with 3 mol % of Dysprosium and 3 mol % of Europium. For clarity, in  FIG. 4  the tags  116  are greatly enlarged with respect to the banknote  200 , and only a few tags  116  are shown is a flowchart  250  illustrating steps involved in validating a secure tag.  FIG. 5  is a flowchart illustrating steps involved in validating the banknote of  FIG. 4  using the reader  100 . 
   The first step (step  252 ) is for the user to locate the banknote  200  in the reader  100 . Once the banknote  200  is correctly aligned, the user presses the trigger  148  (step  254 ). The banknote  200  and reader  100  are aligned when the reader&#39;s focus spot  114  is in registration with the tag area  218 . This may be achieved either by moving the banknote  200  or by moving the reader  100 , or both. 
   On receipt of a trigger press, the processor  124  accesses the parameter information file  130  to retrieve the first excitation parameters and detection parameters (step  256 ). Using the retrieved excitation parameters, the processor  124  creates a first pulse sequence, and applies this first pulse sequence to the LEDs  104   a ,  104   b  (step  258 ). The LEDs  104   a ,  104   b  illuminate the secure tags  116  using this first pulse sequence. 
   Once the first pulse sequence has ended, the processor  124  then applies the retrieved detection parameters to activate the CCD sensor  120  and measure luminescence from the secure tags  116  (step  260 ). In this embodiment, the detection parameters define a time delay of a hundred microseconds (100 μs), and an integration time (the length of time over which a measurement is recorded) of five hundred microseconds (500 μs). 
   The processor  124  then derives a luminescence signature from the measured luminescence spectrum of the secure tags  116  using the algorithm  128  (step  262 ). In this embodiment, the algorithm  128  identifies the peaks in the measured luminescence, normalizes the intensities of the identified peaks, compares the ratios of all of the peaks, and creates a unique code based on the peak ratios. This unique code is the luminescence signature for the secure tags  116  in response to the first pulse sequence. 
   The processor  124  then compares the derived luminescence signature with the corresponding luminescence signature  132  pre-stored in the NVRAM  126  (step  264 ) to ascertain if there is a match (step  266 ). If the two signatures do not meet an acceptance criterion, for example, if the two signatures do not match (within a predetermined tolerance) then the secure tag  116  is not validated (step  268 ), and the processor  124  activates the red LED  152  and causes the loudspeaker  156  to emit a long beep. 
   If the two signatures do meet an acceptance criterion, for example, if the two signatures match (within a predetermined tolerance) then the processor  124  ascertains if there are any unused sets of excitation parameters (step  270 ). 
   If there are more unused sets of excitation parameters then the processor  124  increments to the next set of excitation parameters and loops back to step  258  (step  272 ). The processor  124  then uses this next set of excitation parameters to generate a another pulse sequence. This continues until there are no more sets of excitation parameters. 
   If there are no more unused sets of excitation parameters then the secure tags  116  are validated (step  274 ) and the processor  124  activates the green LED  154  and causes the loudspeaker  156  to emit a short beep. 
   This embodiment provides increased security because different pulse sequences are used, each corresponding to a different charge state, so each stimulates a different luminescence signature. It would be extremely difficult to replicate these luminescence signatures using a different substance than the RE ions being used. 
   Various modifications may be made to the above described embodiment within the scope of the present invention, for example, in other embodiments a secure tag based on luminescent particles other than rare earth doped hosts may be used. Where rare earth doped hosts are used, more or fewer than two rare earth ions may be included in each secure tag. The rare earth ion or ions used may be different to Europium and Dysprosium. The rare earth ions may comprise lanthanide ions. In other embodiments, rare earth ions may be incorporated in hosts other than glass. 
   In the above embodiment, the same detection parameters are used for each pulse sequence; in other embodiments, each pulse sequence may have different detection parameters. 
   In the above embodiment, luminescence was measured after excitation ceased; however, in other embodiments, luminescence measurements may be recorded while the secure tag is being excited. 
   The detection parameters may be different to those described above. 
   In the above embodiment, the luminescence signatures were derived from the peaks in the wavelength range; in other embodiments, different parts of a luminescence spectrum may be used, for example, fewer than all of the peaks, areas of the wavelength range that are not peaks, for example, areas of background noise, or areas part-way between a peak and background noise.