Abstract:
A method of coding and authentication includes the steps of irradiating a sample with a harmonically modulated radiation and detecting a component of an emission of the sample in response where the component is out of phase with the radiation. The method further includes modulating the intensity of the radiation and identifying the sample by a phase difference between the radiation and the out of phase component.

Description:
This application claims benefit of provisional application Ser. No. 60/159,171 filed Oct. 13, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to coding and authentication, in particular, by identifying spectral emissions and a response to modulated excitation. 
     BACKGROUND OF THE INVENTION 
     It is well known that valuable items, for example, negotiable instruments, works of art, etc. are susceptible to theft and counterfeiting. 
     With regard to documents, the advancement of color copier technology has made it fairly easy to create a color copy of any document, including currency, using commonly available equipment. 
     One way of protecting valuable items is to utilize the physical characteristics of the item itself as a way to identify the object. As an example, watermarks or signatures are typically produced by taking semantic information of the item to be protected, for example, alphanumeric characters, physical features, etc. or other related information (e.g. ownership information), as an input to a mathematical algorithm that generates a signature or watermark. These signatures or watermarks are typically kept with the item to be authenticated. For example, a digital watermark may be imbedded within digital information to be protected or it may be printed on or within an item that is valuable. In another example, the watermark or signature may be kept separate from the item, but when combined with the item to be authenticated produces proof of authentication. For instance, a smart card could be utilized that when read confirms certain physical characteristics about an item. 
     Objects and Advantages of the Invention 
     It is a first object and advantage of this invention to provide a system for coding and authenticating an item. 
     It is another object and advantage of this invention to code and authenticate an item by using a response to a specific periodic optical excitation. 
     SUMMARY OF THE INVENTION 
     The foregoing and other problems are overcome and the objects of the invention are realized by methods and apparatus in accordance with embodiments of this invention. 
     A method of coding and authentication includes the steps of irradiating a sample with a harmonically modulated radiation and detecting a component of an emission of the sample in response, where the component is out of phase with the radiation. The method further includes modulating the intensity of the radiation and identifying the sample by a phase difference between the radiation and the out of phase component. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above set forth and other features of the invention are made more apparent in the ensuing Detailed Description of the Invention when read in conjunction with the attached Drawings, wherein: 
     FIG. 1 depicts a detection system  10  in accordance with the invention; 
     FIG. 2 shows a block diagram of a detector array; 
     FIG. 3 shows a representation of the phase angle difference between the output of a source and the emissions from a sample. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is directed toward a method and apparatus for authenticating an article or document by its steady state temporal response. Specifically, a shift in phase of an oscillating fluorescence or phosphorescence time signal is manifested when an object is subjected to a periodic excitation. An atomic or molecular fluorescent material system with two or more levels may be optically excited to exhibit a steady state time signature in response to the periodic optical excitation. Through solution of the appropriate rate equations, a particular radiating transition will fluoresce or radiate light with a time signature that is a function of the modulated excitation and the specific delay rates of the system. 
     The same effect can be used with any physical response that has associated with it a well defined relaxation process and/or time including non-linear phenomena. 
     An atomic or molecular fluorescent material system with two or more levels typically exhibits an upper state lifetime T or relaxation rate of γ=T −1 . This response phenomena can be driven by a harmonic forcing function (exciting UV light for example) whose intensity is harmonically modulated. It should be understood that, as employed herein, harmonic modulation means modulating in relationship to the relaxation rates of the materials being driven to an upper state. The forcing function is modulated in a manner that does not preclude detecting emissions from a sample that have characteristics that result from the modulation. The forcing function is given by: 
     
       
         1= I   0  cos ωt  (1) 
       
     
     With this modulation, the electron population of the upper emitting state is given by: 
       N   u   =N   0   BI   0  cos ω t−N   uγ   (2) 
     where it is assumed that there is, on a time scale T, ω −1 , a direct excitation from the ground state N 0 , and that B is the effective Einstein coefficient for the transition, and I 0  is the excitation intensity. 
     Assuming a solution of the form: 
     
       
           N   U ( t )= N   U   A  cos ω t+N   U   B  sin ω t   (3) 
       
     
     we obtain: 
     
       
           N   U ( t )=−ω N   U   A  sin ω t+ωN   U   B  cos ω t   (4) 
       
     
     Insertion of equations (3) and (4) in equation (2) yields: 
     
       
         −ω N   U  sin ω t=−N   U   Bγ sin ω t   (5a) 
       
     
     and 
     
       
         +ω N   U   B  cos ω t=N   0   BI  cos ω t−N   U   A  cos ω t   (5b) 
       
     
     These two equations can be solved to yield:                N   u   A     =       N   U   B                     γ   ω                   and             (6a)                 N   u   B     =       (     ω       ω   2     +     γ   2         )                     N   O                   BI             (6b)                                
     Equation 6a can be used to determine the phase angle of the emitted light, φ, at a modulation frequency, ω:              φ   =       Tan     -   1                         (     ω   γ     )     .               (   7   )                                
     Equation 7 shows that when ω=γ, φ=π/4 or 45 degrees. 
     The out of phase or quadrature component of the fluorescent emission of a material “i” at a wavelength λ is given by:                I   i   λ     =       I   o                   A                   (     ω       ω   2     +     γ   i   2         )               (   8   )                                
     where I 0  is the excitation peak amplitude, A is a factor that depends on the material active density, other rates and optical cross section. γ i  is the relaxation rate of the optically emitting level and includes non radiative relaxation. 
     In the case that a combination of different materials are used, all emitting at or near the same wavelength λ but with different γ i , the quadrature component of the fluorescent light output has an amplitude given by:                ∑     i   =   1     N                     I   i   λ             (   9   )                                
     This enables the encoding of information into substantially one “color”, by discriminating the plurality of relaxation values by the associated phase shifts. 
     Equation (8) shows a resonance response behavior which peaks at ω=γ i  and has a full width at half maximum given by: 
     
       
         Δω i =2{square root over (3)}γ i   (10) 
       
     
     Equation (10) shows that materials with the same λ can all be distinguished from each other by their component or out of phase response if their γ i  are well separated. The critical separation is of the order of:                  γ     i   +   1         γ   i       &gt;       2   +     3         2   -     3         &gt;   14           (   11   )                                
     Using γ i +1/γ i &gt;20, we find that between a range of 10 Hz and 10 MHz, we can uniquely identify four materials with the same emission wavelength using their time response. 
     The temporal response of a material system may be combined with spectral response to obtain a number of unique response signatures. Excitation in the UVA region alone or with other sources can be used to produce fluorescent or phosphorescent emission out to 1000 nm or more. Using the range from 400 nm to 1000 nm (silicon response window) and a typical spectral separation requirement for dyes of 100 nm we can obtain different fluorescent wavelength bins λ 1  . . . λ M , where M is approximately 5. 
     By sweeping the modulation frequency of the excitation source from 1 Hz to 10 MHz and phase detecting the fluorescent light within each wavelength bin, we can obtain up to M N  unique codes. For the case of M=5, and N=4, we have γ N   M =625 available codes. 
     Phase measurement of frequencies well into the MHz range can be effectively implemented using currently available lock in circuits on a single chip. Such chips can determine phase differences with approximately 1% accuracy, which allows for a high precision authentication of the specific fluorescent taggant material based on time response as well as spectral signature. 
     A detection system  10  in accordance with the invention is shown in FIG. 1. A modulated source of radiation  15  excites a sample  20  at a wavelength ω and a periodic rate defined by Equation 8. The source  15  preferably generates UV radiation but may generate any type of radiation that is capable of being harmonically modulated. The sample may be mounted on a positioning device  25  in order to locate the sample  20  for irradiation. The source  15  and detector array  30  may also comprise positioning devices (not shown) for locating these devices for optimum performance. In response to being irradiated by the source  15 , the sample  20  emits a wavelength λ with a time function defined by the modulated wavelength ω and the specific relaxation rate γ of the sample  20 . A detector array  30  with appropriate support circuitry  35  detects the emission from the sample  20 . The detector array is preferably capable of detecting the spectral content of the emission in addition to any phase differences of emissions having the same wavelength. Control circuitry  40  directs the activity of the overall system  10  and in particular controls the source  15 , positioning device  25 , detector array  30  and support circuitry  35 . 
     As shown in FIG. 2, the detector array  30  is preferably comprised of an optical section  45  for focusing the emissions within the detector array  30 , an array of sensors  55  for detecting the emissions, and a filter section  50  for allowing only the frequencies of interest to impinge on the sensors  55 . The sensor array may comprise any array of sensors suitable for detecting the emissions of the sample  55 , for example, a diode array, a CCD array, etc. 
     As a specific example, the sensors  55  may comprise three photodiodes and the filter section  50  may comprise a corresponding number of narrow band filters, one diode filter combination centered on the emission line, and the two others being, for example, +−10 nm relative to the center of the emission line. The relative signal outputs of each diode filter combination serve to authenticate the spectral signature while phase shift measurements authenticate the temporal signature of the specific material. 
     FIG. 3 shows a representation of the phase angle difference between the output  60  of the source  15  and the emissions  65 ,  67  from the sample  20 . 
     It can be appreciated that the techniques and structures described above are useful for authenticating objects based on their materials. It can also be appreciated that by selecting certain materials with the characteristics described above when constructing items, that the techniques and structures disclosed herein are also useful for encoding various types of information into objects, and authenticating those objects, such as valuables, negotiable instruments, works of art, currency, various types of substrates, items that may require sorting, items that are traveling on a conveyor system, etc. 
     Thus, while the invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that changes in form and details may be made therein without departing from the scope and spirit of the invention.