These days, luminescent materials are widely used as anti-counterfeiting measures in security documents. When exposing a banknote to ultraviolet light the illuminants can be recognized and verified by the naked eye, also known as second line verification. Nowadays, however, simple UV luminescent materials and UV lamps are readily available either online or in a shop, which means that these materials are no longer very effective when it comes to anti-counterfeiting and authentication. Security suppliers are now attempting to use the multi-peak or multi-excitation characteristics of a number of luminescent materials in order to improve both the resistance to counterfeiting and the possibilities of machine verification.
In order to achieve improved resistance to counterfeiting, it is necessary to develop specific materials, and to consider ways of using them, for example in complex authentication. Materials which emit light when excited in the infrared wavelength range can, for example, be used in second and third line verification using simple tools and more complex sensors respectively (figure 1). We first attempted to optimize the luminescent material itself.
While maintaining the second level of verification by which luminescence can be excited by ultraviolet radiation, we considered an upgrade to the third level of verification requiring a sensor or machine, while also improving the first level of verification to enable easy authentication by the human eye (figure 2). In order to determine the characteristics of such materials, we focused our research not only on first and second line, but also on the third line of verification. We went on to make a luminescent material with both a fluorescent and a phosphorescent peak in the visible spectrum in a single matrix, and subsequently, attempted to make luminescent materials that could be excited by mechanical energy such as oblique lighting, friction and compression.
Two-peak elastico-luminescent material
In order to improve the ability to authenticate an illuminant and also increase its resistance to counterfeiting, we synthesized an illuminant that enables authentication at all authentication levels: a so-called ‘two-peak elastico-luminescent material’ (figure 3).
Two-peak elastico-luminescent materials have at least two peak wavelengths and for the third line verification we established the illuminant intensity of each peak wavelength. Elastico-luminescent material also shows a phosphorescence peak in its wavelength, so when reading both these different signals the distinction accuracy will be very high. In order to improve verification by the human eye, we applied the elastico-luminescent effect. Generally, a material shows luminescence when exposed to electromagnetic radiation such as UV light or an electron beam. In elastico-luminescence, however, the luminescence is excited not by electromagnetic radiation but by mechanical energy such as friction or compression. This kind of luminescence enables authentication by simply applying finger friction without the need for any tools and shows all the signs to be able to facilitate verification at all three levels. However, facilitating all three levels with a single (however complex) material is not easy; at present we have been most successful at the third level of verification. Our results at the level of elastico-luminescence have not yet been satisfactory.
Elastico-luminescence
The effects of elastico-luminescence were originally applied in the building industry, where it is used to visualize stress distribution in a structure. We chose to apply it as an anti-counterfeiting technique by printing experimental prints using materials which have both optical luminescence and elastico-luminescence. In order to understand how elastico-luminescence works as a method of authentication, we started out making screen prints (figure 4). A faint luminescence is visible where finger friction has been generated. We then made a flexographic print, which uses thinner ink. As the thickness thins, it becomes more difficult to observe the luminescence, which is only faintly visible along the finger friction path. In the dimmer evening light this thinner ink would facilitate the verification of a security document simply by scratching its surface.
Our experimental two-peak elastico-luminescent material has two luminescent peaks at approximately 450nm and 500nm and one phosphorescent peak at nearly 500nm. Although it is possible to authenticate the material using a fluorescent spectrophotometer (figure 5), in order to make authentication simpler with a cheaper device, we chose to use the peak wavelengths of luminescence and long persistence at this 450nm and 500nm.
Machine authentication
We constructed an authentication system to use with a two-peak elastico-luminescent material, consisting of a computer with two measurement devices (figure 6). The two-peak luminescence unit radiates the target print with pulses of UV light [1]. A photo diode [2] detects the luminescence before and after the UV radiation. The detected light is filtered by a band-pass filter [3] and converted to an electrical signal by a photodiode [4]. The electrical signal is then amplified [5] and converted from an analogue to a digital signal by an A/D converter [6], after which the digital signal is sent to the computer [7].
Next, we would like to discuss a method to verify prints which have been given anti-counterfeiting features using a material with both two-peak and elastico-luminescent qualities. This method involves the use of a composite machine reading system with a two-peak luminescent unit as mentioned before, with the addition of an elastico-luminescent measurement unit. The measurements obtained from both the two-peak luminance and elastico-luminescence units.
Figure 7 shows an example of discrimination when reading both the two-peak luminescence and the elastico-luminescence. Reference values and admissible errors are set beforehand using a large amount of measurement data using genuine prints. We defined the voltages acquired from the fluorescence and phosphorescence as respectively V1 and V2 at 450nm, and as V3 and V4 at 500nm. Furthermore, the voltage acquired from the elastico-luminescence is defined as V5. V1 to V5 are first measured, and then every voltage is judged as to whether the acquired value is within the previously determined upper and lower limits of allowable error. A print is accepted as genuine when all detection values are within permissible levels. It is judged to be counterfeit when any one value is outside the permissible range of error.
Multi-peak luminescent material
Multi-peak luminescent materials such as two-peak elastico-luminescent materials have first line verification capabilities and also expanded second line verification qualities. First, they show different luminescent colours according to the different excitation wavelengths. The luminescent phosphorescent colours change only at specific wavelengths of irradiating light. The phosphorescence is intended to be visible, but not necessarily to the human eye. Materials in which phosphorescence persist for a long time have practical applications in, for example, fire exit signs, however when used as a security feature it shouldn’t persist as long because of the risk of detection.
For that reason we have developed a special material which has just the right persistence time and, as mentioned before, different luminescent colours according to the excitation wavelength. Figure 8 shows in what way the fluorescent and phosphorescent colours vary according to the excitation wavelength.
The image on the left of figure 9 shows the luminescent state while the graph on the right side shows the emission spectrum. There are three peak wavelengths in each blue, green, and red spectrum area. We can apply these peaks to machine authentication using the system mentioned before.
Figure 10 shows screen prints using luminescent materials excitable by UV light. The luminescent colour changes at each excitation wavelength and the phosphorescence is visible, although if people are not aware of the latter, they may not actually notice it. This is why such a material would be useful for examining officers and shop clerks who could verify security documents using only simple equipment.
Figure 11 shows the fluorescent spectrum of a multi-peak luminescent material. Prints using such materials can be authenticated by the same system of two-peak elastico-luminescent material. The starting time of the UV exposure is called T0, and the ending time T1. We first measured the intensity of luminescence at each peak and then we verified the six values of each peak. It is now possible to determine authenticity at an even greater level of accuracy by radiating the print with two light sources, for the luminescent spectrum varies with excitation wavelength.
Conclusion
In order to improve counterfeit prevention and authentication, we have been experimenting with ways to exploit the properties of luminescence materials. We have succeeded in developing a new luminescent material with complex photogenic properties in a single matrix. This material has two luminescent peaks and one phosphorescent peak, and applied to machine authentication highly accurate authentications can be achieved. This material also has elastico-luminescent properties, so its authenticity can be verified without using tools such as UV light.
In creating this new field of authentication methods, we have also developed a new variety of luminescent material which has three luminescent peaks and one phosphorescent peak, and which will show different luminescent colours depending on the excitation wavelength. We are confident that multiple luminescent peaks will enable more reliable verification of authenticity, especially in machine authentication.
Although multiple luminescent peaks require UV light, the existence of various features can still be verified at the first level of authentication because phosphorescence is visible with the unaided eye. We have considered a method of machine authentication of security documents printed with materials that use all luminescent properties. We believe that the proposed materials and methods have the potential of becoming an effective new authentication technology.
Tsuyoshi Uematsu graduated from Chiba University in 1984, completing a graduate program with a major in Image Science. He has conducted research on Image Processing Technologies and colour reproduction of computer outputs, and is especially interested in holography. In 1985 he joined the Printing Bureau, Ministry of Finance, Japan, which in 2003 became the National Printing Bureau, an Incorporated Administrative Agency. As a team leader, he developed the holograms for the new Japanese banknotes issued in 2004.
