FR2895124A1 - Procedure and apparatus for making documents secure consists of applying two different substances that produce luminescent radiation in response to excitation - Google Patents

Abstract

The procedure, for use with banknotes or other documents (10), consists of applying a security sign (100) using two different substances with pigments that generate luminscent radiation at different spectral frequencies in response to excitation radiation (210) centred around a wavelength of about 370 nm. The first substance generates luminescent radiation with a broad frequency range, while the second one emits one or more rays outside the spectrum of the first. The emitted radiation (220, 221, 222) can be detected by a machine or the naked eye.

Description

The present invention relates generally to the manufacture and

  Authentication of secure documents.

  More specifically, the invention relates to a method of securing a document, comprising associating the document with a sign of luminescent securance which produces a determined luminous response in response to excitation radiation, the luminous response being effected. at least partially according to a broad spectrum.

  The invention also relates to a secure document obtained by such a method. And the invention also relates to a method for authenticating such a secure document, and an associated authentication machine. It is specified that broad spectrum is understood to mean a spectrum of wavelengths that is different from a single line, covering without interruption a wavelength range that is substantially greater. And a line is defined as a spectrum essentially limited to a very narrow wavelength range i.e. covering a wavelength range of the order of about 10 nm only.

  Correspondingly, low resolution means a resolution greater than 10 nm. And by resolution & levee >> is meant a resolution less than or equal to 10 nm. We already know security procedures of the type mentioned above. It is thus known, for example, to associate with a secure document, such as a banknote, a sign of luminescent securance which produces a response in the visible spectrum in response to excitation radiation which may be in the ultra-violet spectrum. violet (UV). The authentication of the document results from the comparison of the light response of the document exposed to the excitation radiation, with an expected response stored.

  This authentication can be done by a machine. Elie can also be performed in some cases directly by a human observer (especially naked). This type of method thus offers a certain level of security.

  US 6,552,290 describes a structure having one or more regions, each region including a dye in combination with one or more lanthanides selected to produce predetermined wavelengths A, a2, k3. Public authentication can be achieved by subjecting the structure to low level illumination, generated for example by an ultraviolet source and measuring a broadband fluorescent emission generated by the dye. Machine authentication can also be achieved by subjecting the structure to a laser source and measuring a line emission at wavelengths 2 1, 22, 23.

  The object of the invention is to further increase the level of security that can be obtained In order to achieve this object, the invention proposes a method of securing a document, comprising a step of associating a document with a sign security, the safety sign comprising a first medium capable of generating a first light-emitting radiation in response to excitation radiation and a second medium, distinct from the first medium, capable of generating a second light-emitting radiation in response to excitation radiation, the first light-emitting radiation having a broad frequency spectrum and the second light-emitting radiation having a frequency spectrum is one or more lines (s), an unsophisticated observer having at his disposal usual authentication techniques and seeking to characterize the radiation produced by the security sign, will only perceive the first light-emitting radiation generated by the first medium. The minescent mask thus masks the presence of the second luminescent radiation. For this reason, the method of the invention makes it possible to increase the level of security of marking with respect to known methods The method of the invention makes it possible to generate a document that can be authenticated by two possible modes of authentication: a first authentication mode or simplified mode in which the first radiation is analyzed and a second authentication mode or advanced mode in which the second radiation is analyzed.The document can thus be subject to authentication tests by means of conventional authentication or authentication tests by more sophisticated authentication means The second medium thus allows additional security of the document with respect to the first medium alone.In addition, the method of the invention is simple to implement because each luminescent radiation is generated by a distinct medium.

  Each medium can therefore be chosen independently to obtain a desired radiation. Preferred, but not limiting, aspects of the security method according to the invention are the following: the first medium comprises a first pigment capable of generating the first light-emitting radiation and the second medium comprises a second pigment capable of generating the second light-emitting radiation, the second radiation line (s) is (are) located outside the broad spectrum of the first radiation, or the second radiation line (s) is (are) located in a domain wavelength covered by the broad spectrum of the first radiation, - the spectrum of the second radiation comprises firstly one or more line (s) located in a range of wavelengths covered by the broad spectrum of the first radiation and secondly one or more line (s) located outside the broad spectrum of the first radiation, - the excitation radiation has a center frequency spectrum around a wavelength less than or equal to about 370 nm, - the spectator re broad extends in a range of wavelengths included in the visible spectrum, - the broad spectrum extends in a wavelength range between about 500 nm and about 560 nm, - the second medium includes a plurality of second pigments, each second pigment being able to generate one or more line (s), the security sign further comprises a dye, - the dye is capable of filtering wavelengths of a visible spectrum, - the dye constitutes a band-pass filter for wavelengths of the light-emitting radiation emitted by the security sign, - at least one line has a wavelength comprised in a passband of the band-pass filter. The invention also relates to a secure document, such as a banknote, the document comprising a security sign associated, the security sign comprising a first medium capable of generating a first light-emitting radiation in response to a radiation of excitation and a second medium, distinct from the first medium, capable of generating a second light-emitting radiation in response to the excitation radiation, the first light-emitting radiation having a broad frequency spectrum and the second light-emitting radiation having a spectrum constituting one or more line (s). And the invention also relates to a method for authenticating a secure document as defined above, comprising a step of illuminating the document with excitation radiation, the method further comprising, in a first simplified authentication mode steps of: - detecting a first broad frequency spectrum of a first luminescent radiation emitted by the security sign in response to the excitation radiation, and / or in a second advanced authentication mode, steps consisting in to: - detect a spectrum of a second light emitting radiation emitted by the security sign, in response to the excitation radiation, - compare the second detected spectrum with a reference spectrum consisting of one or more line (s), the a method further comprising a step of deriving preceding steps if the secure document is authentic. Preferred, but not limiting, aspects of this authentication method are the following: according to the first simplified authentication mode, the detection step of the first spectrum is carried out by a single observer, first method of authentication simplified, the detection step of the first spectrum is carried out by a machine, - the machine comprises means able to characterize a colorimetry of the first light radiation emitted by the security sign, - according to the second mode of In advance authentication, the detection and comparison steps of the second spectrum are performed by a machine. The invention finally relates to a machine for authenticating a secure document as defined above, comprising means for illuminating the document with excitation radiation, authentication means in simplified mode comprising: means for detecting a first broad frequency spectrum of a first luminescent radiation emitted by the safety sign in response to the excitation radiation, and / or forward mode authentication means comprising: - means for detecting a second spectrum of a second luminous radiation emitted by the security sign, in response to the excitation radiation, means for comparing the second detected spectrum with a second reference spectrum constitutes one or more lines, the machine further comprising means for deducing whether the secure document is authentic. Preferred, but not limiting, aspects of such a machine are the following: the means for detecting the second spectrum comprise a plurality of sensors, each sensor being capable of detecting a line of the second spectrum individually, the means for detecting the second spectrum; spectrum comprise a plurality of sensors, each sensor being able to detect a line of the second spectrum around a wavelength individually associated with the sensor, the set of detected lines covering at least part of a visible spectrum, - each The sensor is capable of detecting a line having a width of the order of 20 nm, the means for illuminating the document are able to emit a center excitation radiation around a wavelength less than or equal to a value of The order of 370 nm. Other aspects, objects and advantages of the invention will become apparent upon reading the following description with reference to the accompanying drawings, in which: FIG. 1 is a schematic representation of a security document such as a bank note, made according to the invention, - Figures 2a and 2b are two schematic representations illustrating two authentication modes of such a security document (Figure 2a corresponding to a simplified authentication mode and Figure 2b corresponding to an advanced authentication mode), - Figure 3 schematically represents means for authenticating such a document, Referring firstly to Figure 1, there is shown schematically a secure document 10 such that a bank note The document 10 comprises at least one region 100 corresponding to a luminescent security sign.

  The region 100 may have been associated with the document by any technique known per se (offset printing, screen printing, letterpress, toner, application or heat transfer of a support strip or a film, integration of a sheet element or a luminescent wire,

  As is known, the region 100 of the document 10 is sensitive to excitation radiation 210. In other words, the region 100 is capable of producing, when exposed to the excitation radiation 210 (as represented in FIGS. 2a and 2b), a response 220 of determined characteristics.

  More specifically, the response comprises a first radiation 221 having a broad frequency spectrum, as is symbolized in the diagram of FIG. 2a (the spectrum of the first radiation 221 covers a wavelength range that is substantially greater than a range of The order of 20 nm).

  It is specified that this diagram (like that of FIG. 2b) represents the frequency spectrum of the radiation, comprising a radiation intensity (in ordinates) as a function of wavelengths (on the abscissa). The characteristics of the first radiation 221 depend on the characteristics of the excitation radiation 210, and the luminescent properties of the security sign constituted by the region 100. In the context of the invention, the response of the security sign of the region 100 to excitation radiation is, however, not limited to the first radiation 221 as represented in FIG. 2a, since, as shown in FIG. 2b, the same security sign of the region 100 is capable of emitting a second radiation 222 in response to the excitation radiation 210. The second radiation 222 is constituted of at least one line (three lines in the case of the representation of FIG 2b) .The superposition of the radiation 221 and 222 forms the response luminescent 220 emitted by the security sign, this response 220 being emitted in response to excitation radiation 210 single.

  Two components of the security sign 100 response can thus be distinguished: a first component (first radiation 221) which has a broadband frequency spectrum; and a second component (second radiation 222) which has a frequency spectrum constituting one or more several lines (s). The superimposition of the components 221 and 222 forms the response to the excitation radiation 210. As indicated above, the second component 222 is distinguished from the first wide-spectrum component 221. The second component 222 is indeed a component whose spectrum is constituted by a light line or a plurality of distinct light lines. These lines may all be outside the wavelength range covered by the broad spectrum of the first component 221.

  These lines may also be in the range of wavelengths covered by said broad spectrum of the first component 221. In this case, the intensity of each line will be substantially greater than the intensity of the first wide spectrum radiation, for window of 20 wavelengths corresponding to the line. It is also possible to predict that the second component 222 comprises a plurality of light lines, distributed both within the wavelength range covered by the first wide-spectrum component 221, and outside this range of lengths. wave. It is this latter variant which is schematically illustrated in FIG. 2b (the graph associated with this figure taking in dotted lines the first component 221 with a wide spectrum). It will be noted that the integral of the response to excitation radiation is substantially equal to the integral of the first component 221 (the contribution of the second component 222 to the integral of the response is negligible, that is, less than 10% of the globate response).

  In the case where the response is issued in the visible domain, an observer at naked eye does not perceive a difference between a global response to the excitation radiation (including the first component 221 and the second component 222) and a response. which would include only the first component 221. In a variant, the observer perceives a response apparently having a broad spectrum, different from that of the first component, but close to a broad spectrum resulting from a security sign which would not contain that a medium.

  However, the derivative of the response to excitation radiation shows one or more "peak (s)" additional (s) relative to the derivative of the first component 221 alone. To obtain such a double response (components 221 and 222), security sign 100 exhibits particular luminescence properties. The sign 100 comprises a first medium capable of generating a first light-emitting radiation in response to the excitation radiation and a second medium, distinct from the first medium, capable of generating a second light-emitting radiation in response to the excitation radiation. The first medium comprises at least one first pigment sensitive to excitation radiation. The first pigment is capable of generating, in response to excitation radiation 210, luminescent radiation having a broad frequency spectrum and constituting the first component 221. The second medium comprises at least one second excitation radiation-sensitive pigment. The second pigment is capable of generating, in response to the excitation radiation 210, one or more light emission lines (s) constituting the second component 222. The sign (100) thus incorporates this (these) pigment (s). (s) by any method known per se. For example, the first and second pigments may be blended with an offset ink which will be deposited on the document to form the region 100. The excitation radiation 210 may have a center spectrum in the UV range.

  The excitation radiation 210 can thus, in one embodiment of the invention, correspond to a spectrum of wavelengths centered around a wavelength of the UV spectrum, this wavelength can for example be less than or equal to a value of the order of 370 nm.

  The first broad spectrum component 221 preferably corresponds to a range of wavelengths within the visible spectrum. For example, it can be predicted that this broad spectrum of the first component 221 will extend in a wavelength range from about 500 nm to about 560 nm. As has been said, each line of the second component 222 is obtained by integrating one or more second specific pigment (s) into the security sign 100. Each second pigment can thus be generating at least one line in response to excitation radiation 210. In addition, the security sign 100 may further comprise a dye.A dye is defined here as means for providing a light filter function, according to a window of desired wavelengths The dye may comprise a substance or powder integrated at the sign of region 100, or any other optical filtering means which only allows radiation to pass through a given wavelength range. In particular, the dye can filter in a wavelength or bandwidth window of the visible spectrum.

  The dye thus constitutes a band-pass filter for the wavelengths of the replies of the security sign 100. This filtering function makes it possible to eliminate any undesirable components of the radiation 221 and / or 222.

  It can be predicted that, when using such a dye, at least one emission line of the second component 222 has a wavelength included in the wavelength window of the dye bandpass filter. In this case, the dye allows the sign 100 to emit the line in question, and this line may be detected by suitable means of authentication. It has been said that the second component 222 is elaborated from at least one line, and that it could consist of several distinct lines. It is also possible to make a coding of the second component 222 from a given combination of different lines. It is also possible to consider decoding this coding for example according to different groups of secure documents. It is thus possible to code: with a first general code the component 222 of a first group of tickets (corresponding for example to a first face value), with a second general code a second group of bills, etc. And within each of these groups, each ticket may optionally be associated with a specific code of the component 222, this specific code corresponding to a particular declination of the general code associated with the group to which the note belongs. Authentication of the security sign is done in the following manner. The document 10 is illuminated with a predetermined excitation radiation 210. According to a first simplified authentication mode, the presence or the absence of a response 220 is detected (by means which will be described later). This first simplified authentication mode is an all or nothing mode in which it is determined whether a response 220 comprises a first expected broadband component 221 emitted by the security sign. According to a second advanced authentication mode, the answer 220 is collected (by means which will be described later), then the response 220 is compared to an expected response, which is stored in the authentication means (for example in a authentication machine, or in a server to which the authentication machine is connected), the stored expected response comprising a second component 222. We will now describe the main steps of the authentication process of the document 10, with particular reference to the This figure schematically represents an authentication device 30 for the document 10.

  The device 30 is fixed, the document 10 defiant facing sensor means of this device in the direction of the arrow F. The device 30 is provided with means for generating an excitation radiation 210. In the representation of FIG. these means comprise a light source 310. In one embodiment, the illumination means 310 of the machine are thus capable of emitting excitation radiation having a center frequency spectrum around a lower wavelength or equal to a value of the order of 370 nm. In all cases, the radiation 210 is directed to a fixed area in front of the title as crossed by the security sign 100 of the document 10 running. It is specified that it is possible alternatively to provide that the document 10 is fixed, and that it is the device 30 which is mobile with respect to the document. When the security sign 100 arrives in an area illuminated by the excitation radiation 210, this security sign produces a response 220 to the radiation 210. If the document is authentic, the response 220 comprises two components 221 and 222. We have seen above that the security document 10 (which may for example be a banknote) has a luminescent security sign 100 to which at least one second pigment is incorporated so as to generate in addition to a first radiation 221 (or first component) a broad frequency spectrum, a second radiation 222 (or second component) whose spectrum is constituted by at least one line, in response to excitation radiation 210.

  It is thus possible to implement the authentication of such a document according to a first authentication mode, said simplifies, and / or according to a second authentication mode, said advance D. In the first authentication mode or mode simplifies, one carries out the following operations: - one illuminates the sign 100 of the document with the excitation radiation 210, - one collects a luminous answer 220 emitted by the security sign 100 in response to the radiation of excitation, with a means broadband sensor of low resolution (less than 10 nm), in order to search only the first component 221 of the response 220 which has a broad spectrum (ie response 220), - if the sensor detects the presence of the first component 221, we conclude the autentity of the sign according to the simplified mode. The wideband sensor means may be a single sensor, or a set of sensors. It is in all cases able to collect the first component 221. In the second authentication mode or advance mode, the following operations are performed: the sign 100 of the document is illuminated with the excitation radiation 210; collects the luminous response 220 of the safety sign 100 to the excitation radiation 210, with a higher resolution sensor means (for example a sensor having 33 channels having a resolution greater than 10 nm) able to detect the presence of isolated light lines , in order to look for the second component 222 of the response 220 which has a spectrum consisting of at least one light emission line, the spectrum of the response 220 collected is compared with a second reference spectrum expected memorized including one or a plurality of lines and, if the two spectra matched satisfy a given correspondence, it is concluded that the sign is autentic in the advance mode. The sensor means capable of detecting the presence of isolated light ray (s) can here again be a unique sensor. It can also be in the form of a plurality of sensors. It can thus notably comprise a sensor for detecting each of the lines of the expected second component 222. And it is possible that the broadband sensor means and the line sensor means are merged into the same sensor means. It will thus be possible for this sensor means to gather a plurality of sensors known as line sensors, individually associated with the detection of the response 220 of the sign 100 according to a given frequency range. In this case, the set of line sensors can cover a spectrum of continuous wavelengths, the wavelength windows associated with the different sensors capable of being adjacent to one another .The spectrum covered by the meeting of the wavelength windows In particular, the components 221 and 222 may cover at least part of the visible spectrum, and these components may in particular cover at least part of the visible spectrum. It is also possible that this spectrum substantially covers the entire visible spectrum.

  The frequency ranges associated with the different line sensors can correspond to wavelength windows of the order of 20 nm wide. The different sensor means are integrated in the authentication machine 30, a schematic illustration of which is given in FIG. 3. In this illustration, the sensor means are thus in the form of a plurality of line sensors 32. The machine also includes means 33 which are connected to the sensor means 32. These means 33 are capable of performing comparison and authenticity diagnostic operations which will be described later in this text. The machine 30 thus comprises the following elements: means 310, 311 for illuminating the security sign 100 of the document 10, means 32 for acquiring the luminous response of the security sign 100, memory means at least one expected component; means for comparing the collected component and a stored expected component. The means 33 comprise the component storage means (s) expected (s) and the comparison means (or may come into contact with a remote memory - for example at a central server). The means 33 are thus capable of detecting the presence or absence of the luminous response of the security sign, These means 33 are furthermore able to compare the response 220 collected by the sensor means with a second reference spectrum 30 memorized, and give an indication of a good comparison (correspondence of the answer collected and of the memorized answer) or of bad comparison.

  As has been said, the acquisition means 32 of the machine may comprise individual sensors making it possible to individually collect a response of the document along one line, in particular to detect the line (s) generated by the second (s) pigment (s) incorporates (s) the security sign of the document, for authentication in advance mode. The authentication in advance mode can be done at the same time as the authentication in simplified mode. It is also possible to implement only authentication in simplified mode, in which the sign 100 is illuminated with the excitation radiation 210 and only the presence or absence of the first component 221 is detected. In this case, the authentication is done without searching in the luminous answer of the security sign for the presence of isolated luminous lines. This possibility may in particular be implemented by simple authentication machines that do not include line sensor means. It will thus be possible to use, for such authentication performed in simplified mode only, existing machines equipped only with low resolution sensors. Such low resolution sensors may, for example, only allow the detection and recognition of a colorimetry of the security sign answer 220 which is defined by a wide spectrum. And the means 33 of the machines used only for authentication in simplified mode thus do not necessarily include means for comparison with a second reference spectrum, or even means for storing a second expected frequency spectrum line (s). Simplified mode authentication can also be performed visually by a human operator.

  Authentication in advance mode is necessarily necessary by machine. In the case where the lines of the second component 222 form a code, the authentication machine 30 is associated not only with means for detecting and authenticating the second component, but also with means for recognizing this code. These means may be included in the means 33. They can also be deported to a central server with which the machine is able to communicate.

  Providing specific means for authentication in advance mode significantly enhances the security level associated with the document. And the combination of components 221 and 222 allows both: recognition of the first component 221 of the security sign with simple authentication machines, provided only with low resolution sensor means, or even by an observer looking at In this case, the document illuminates with excitation radiation 210. The security documents according to the invention can thus be authenticated (like certain existing documents) on the sole basis of the recognition of the first broadband radiation 221. An additional security by the second component 222. It will be noted in particular that a counterfeiter wanting to produce false documents 10 provided with a sign 100, and having detected on an authentic document only the first broadband component 221, will not reproduce in its imitation of the sign 100 that the means to generate the broadband component 221, and will not integrate in this imitation of the sign 100 of second pig (s) (s) to generate the second component (s) 222.

Claims (24)

  A method for securing a document, comprising a step of associating the document with a security sign, the security sign comprising a first medium capable of generating a first light-emitting radiation in response to the excitation radiation and a second medium, distinct from the first medium, capable of generating a second light-emitting radiation in response to excitation radiation, the first light-emitting radiation having a wide frequency spectrum and the second light-emitting radiation having a frequency spectrum constituting one or more lines (s).   2. Method according to claim 1, wherein the first medium comprises a first pigment capable of generating the first luminescent radiation and the second medium comprises a second pigment capable of generating the second luminescent radiation.   3. Method according to one of claims 1 or 2, wherein the or the second radiation line (s) is (are) outside the broad spectrum of the first radiation.   4. Method according to one of claims 1 or 2, wherein the or the line (s) of the second radiation is (are) located in a range of wavelength covered by the broad spectrum of the first radiation. 25   5. Method according to one of claims 1 or 2, wherein the spectrum of the second radiation comprises firstly one or more line (s) located in a range of wavelength covered by the broad spectrum of the first radiation. and on the other hand one or more line (s) 30 (s) outside the broad spectrum of the first radiation.   6. A method according to one of the preceding claims, wherein the excitation radiation has a center frequency spectrum around a wavelength of less than or equal to about 370 nm.   7. Method according to one of the preceding claims, wherein the broad spectrum extends in a range of wavelengths included in the visible spectrum.   8. A method according to one of the preceding claims, wherein the broad spectrum is in a wavelength range from about 500 nm to about 560 nm.   9. Method according to one of the preceding claims, wherein the second medium comprises a plurality of second pigments, each second pigment being capable of generating one or more line (s).   10. A method according to one of the preceding claims, wherein the security sign further comprises a dye. 20   11. The method of claim 10, wherein the dye is capable of filtering wavelengths of a visible spectrum.   12. A method according to one of claims 10 or 11, wherein the dye is a bandpass filter for wavelengths of luminescent radiation emitted by the safety sign.   13. The method of claim 12, wherein at least one line has a wavelength within a passband of the bandpass filter.   14. Secure document, such as a banknote, the document comprising an associated security sign, the security sign comprisinga first medium capable of generating a first light-emitting radiation in response to excitation radiation and a second medium, distinct from the first medium, capable of generating a second light-emitting radiation in response to the excitation radiation, the first light-emitting radiation having a broad frequency spectrum and the second light-emitting radiation having a spectrum is one or more lines (s).   A method of authenticating a secure document according to claim 14, comprising a step of illuminating the document with excitation radiation, the method further comprising, in a first simplified authentication mode, steps comprising detecting a first broad frequency spectrum of a first luminescent radiation emitted by the security sign in response to the excitation radiation, and / or according to a second advanced authentication mode, steps of detecting a second spectrum of a second luminescent radiation emitted by the security sign, in response to the excitation radiation, 20 - comparing the second detected spectrum with a second reference spectrum consisting of one or more line (s), the method further comprising a step of deriving preceding steps if the secure document is authentic. 25   16. The method of claim 15, wherein according to the first simplified authentication mode, the detection step of the first spectrum is performed naked by an observer.   17. The method of claim 15, wherein according to the first simplified authentication mode, the step of detecting the first spectrum is performed by a machine.   18. The method of claim 17, wherein the machine comprises means capable of characterizing a colorimetry of the first light radiation emitted by the security sign.   19. Method according to one of claims 15 to 18, wherein according to the second advanced authentication mode, the detection and comparison steps of the second spectrum are performed by a machine.   20. The authentication machine of a secure document according to claim 14, comprising means for illuminating the document with excitation radiation, means of authentication in simplified mode comprising: means for detecting a first frequency spectrum wide of a first light-emitting radiation emitted by the security sign in response to the excitation radiation, and / or forward-mode authentication means comprising: - means for detecting a spectrum of a second light-emitting radiation emitted by the safety sign, in response to the excitation radiation, means for comparing the second detected spectrum with a second reference spectrum constitutes one or more lines, the machine further comprising means for deduce if the secure document is authentic. 25   21. Machine according to claim 20, wherein the means for detecting the second spectrum comprise a plurality of sensors, each sensor being able to individually detect a line of the second spectrum. 30   22. Machine according to one of claims 20 or 21, wherein the means for detecting the second spectrum comprise a plurality of sensors, each sensor being able to detect a line of the two-speckle around a wavelength individually associated with the sensor, ( set of detected lines covering at least part of a visible spectrum.   23. Machine according to claim 22, wherein each sensor is able to detect a line having a width of the order of 20 nm.   24. Machine according to one of claims 20 to 23, wherein the means for illuminating the document are able to emit a center excitation radiation around a wavelength less than or equal to a value of the order of 370 nm.