EP0946371A1

EP0946371A1 - Photochromic security system for security documents

Info

Publication number: EP0946371A1
Application number: EP97954036A
Authority: EP
Inventors: John Bannard; Dennis Maguire; Shakher Puntambekar
Original assignee: Standard Register Co
Current assignee: Standard Register Co
Priority date: 1996-12-23
Filing date: 1997-12-08
Publication date: 1999-10-06
Also published as: CO4890890A1; ZA9710835B; WO1998028154A1; AU5791598A; ID19169A; CA2276043A1

Abstract

The present invention is directed to a security document which includes a photochromic security system. The photochromic security system comprises a photochrome and a sensitizer which causes the photochrome to colorize at a wavelength of electromagnetic radiation emitted by an image processing device. Desirably, the photochromic system of the present invention will undergo a color change and/or a change in optical density from about 300 nm to about 500 nm.

Description

PHOTOCHROMIC SECURITY SYSTEM FOR SECURITY DOCUMENTS

Field of the Invention

The invention is directed to a photochromic system and an improved security document and, more particularly, to a photochromic system and a security document incorporating a photochromic system which undergoes a color change or change in optical density when exposed to electromagnetic radiation emitted by an image processing device such as a color photocopier, monochrome photocopier or computer scanner.

Background of the Invention

Image processing devices have been used for years to make accurate copies of documents. In many cases, legitimate reasons exist for making such copies. Unfortunately, copies are also made and used for illegal purposes. Specifically, color copiers can be used to reproduce security documents, such as checks, reports, stock certificates, automobile title instruments, birth certificates, college transcripts, prescriptions and other documents of business and personal value, for illegal purposes. This concern has been heightened with the advent of desk top publishing software and hardware, including personal computers and computer scanners. Such desk top publishing systems allow sophisticated image processing and printing not generally available previously.

Many techniques have been developed to prevent improper reproduction of security documents. One of the most successful is the use of a hidden warning message, also known as the Void Pantograph, which is readily apparent on reproduced copies of the document, but which is invisible, or nearly so, on the original document. In this method, relatively large image elements (usually dots) and relatively small image elements

(also usually dots) are printed in a pattern such that a warning word is hidden from view on an original but appears visually on a photocopy due to the inability of photocopier to reproduce the smaller image elements.

With recent improvements in image processing technology, security document protection has become less effective. By manipulating the control settings on such devices, when some of the most commonly used frequencies and size combinations of security elements are used, copies can be made of such documents in which the warning phrase does not appear on the reproductions. For example, by adjusting the settings for sharpness and lightness/darkness, it is still possible with some image processing devices for a skilled individual to produce a copy in which the warning phrase is not visible. Furthermore, desk top publishing systems, which are now available in conjunction with laser printers, offer additional possibilities for unauthorized copying. In an attempt to combat these new image processing methods, treated or coated substrates having light absorption characteristics in the visible spectrum have been used to formulate security documents . With documents printed on this type of substrate, the device "sees" little or no contrast between the substrate background and the images meant to be protected against copying. However, this method is unsatisfactory because very dark colored substrates are required to effectively inhibit copying. Since copiers operate in essentially the same spectral region as the human eye, a document which is "blind" to a copier is also practically blind to the human eye. That is, the substrate background of the original is so dark that it is almost impossible to read information printed on the original.

In another approach, a label, seal or stamped area of specularly reflecting material, such as a metallic foil, a vacuum metallized polymer film or holograms, has been adhered to an original document. Image processing devices cannot reproduce the mirror-like specular reflectance of such materials. However, because this approach does not include words indicating that a copy has been made, a person who does not have access to the original document will not be aware of the reflecting material and, consequently, will be unaware that t e document is a copy.

Recently in the art, it has been proposed that photochromic materials be used to inhibit copying. Photochromism is a characteristic of certain organic and inorganic materials to undergo a color change when exposed to certain wavelengths of electromagnetic radiation. This radiation causes a change in the electron distribution or a change in the bond structure of the photochromic molecule causing a localized rearrangement in interatomic distances and vibrational frequencies. Accompanying this shift in vibrational frequency is a shift in resonant absorption of light energy in the visible spectrum, resulting in a perceived color change and/or change in optical density.

The use of photochromic materials to produce copy resistant documents has been proposed in British Patent No. 1,332,185. This patent teaches creating copy resistant documents by treating paper stock with photochromic materials which change color when exposed to the high levels of illumination intensity of photocopiers. A photochromic material is chosen which does not change color at normal ambient light intensity but does undergo color change at higher intensities, and which will revert to its original color after high intensity exposure is terminated. The color change in the paper stock causes the background and image areas to achieve closely related darkness values, depriving the copier of the necessary contrast to reproduce an image.

A similar method is taught in U. S. Patent No. 3,597,082. In this patent, copy resistant documents are created by impregnating or coating sheets with a composition containing a photochromic material which absorbs incident light within the spectral range of a photocopier light source. When an original bearing the image is copied, the copy is uniformly blank because of loss of contrast between the image and the color- altered background. However, the methods described in GB 1,332,185 and U.S. P. 3,597,082 are ineffective because they teach the use of photochromic security documents in which the photochrome colorizes when exposed to direct sunlight and, in addition, most of the photochromes disclosed in GB 1,332,185 did not colorize fast enough to prevent copying by conventional duotone copiers of the early 1970 's without the use of special adaptations to the copiers. Thus, the photochromes disclosed in GB 1,332,185 would be even less effective against today's high speed duplicating image processing devices. Further, today's image processing devices are constructed to emit less intense light than copiers in the early 1970 ' s and thus the photochromes described in U.S. P. 3,597,082 and GB 1,332,185 may not colorize when exposed to modern image processing devices.

For these reasons, it is necessary to develop a photochromic security document which is activated by the electromagnetic radiation produced by modern image processing devices and which colorizes without the use of specially adapted devices .

Summary of the Invention

The present invention is directed to a photochromic system and a security document incorporating the photochromic system which are useful in indicating that the security document has been copied on an image processing device and in interfering with the legibility of copies of the document made on an image processing device.

One embodiment of the invention concerns a security document comprising a substrate having a top surface for carrying printed indicia; and a security portion carried on the top surface of the substrate, the security portion being formed from a photochromic system including a photochrome and a sensitizer which increases the wavelength at which the photochrome colorizes to a wavelength equal to a wavelength of electromagnetic radiation emitted by an image processing device, whereby, upon copying of the security document by an image processing device, the security portion undergoes a change in optical density. In this embodiment, the security portion may also be placed on the security document so that the resulting copy of the security document is entirely obscured. In another embodiment, the security portion may be a security term.

Another embodiment of this invention concerns a photochromic system useful for making security documents comprising a photochrome; and a sensitizer which causes the photochrome to colorize at a wavelength of electromagnetic radiation emitted by an image processing device.

The most useful photochromes for the photochromic system are spiropyrans, spirothiopyrans, spirooxazines and similar compounds, and, preferably, are benzoindolinospiropyrans, naphthoindolinospiropyrans , benzospiropyrans , naphthospiropyrans and bipyrans . It has been discovered that the photochromes can be sensitized so that their colorization is initiated by electromagnetic radiation emitted by an image processing device. Typically the wavelengths of electromagnetic radiation emitted by an image processing device are greater than 300 nm. Preferably, the photochromic systems of this invention will colorize and/or undergo a change in optical density when exposed to radiation between the wavelengths of about 300nm and about 500nm.

Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims .

Brief Description of the Drawings

Fig. 1 presents a top view of the security document of this invention with a photochromic security portion shown in broken lines. Fig. 2 presents a top view of the security document of Fig. 1 after the photochromic security portion has colorized.

Fig. 3 presents a top view of a security document which bears a security term, shown in broken lines, as the security portion. Fig. 4 presents a top view of the security document of Fig. 3 after the security term has colorized.

Detailed Description of the Invention

Most photochromes are photochromic because they have a C- C, C-O or similar bond which breaks when irradiated and reforms when the radiation ceases and a small amount of thermal energy is available. The critical wave length of radiation to break such a bond is found to be between 250 and 290nm. While the bond is broken, electron-density is fed back into a conjugated system, which is stabilized by resonance, to produce color. If a shift of electron density alone is responsible for coloration, then the process will occur very quickly.

Modern image processing devices do not emit a sufficient quantity of radiation in the 250-290nm range to induce coloration of a photochrome because they are usually fitted with a glass cover which is specifically designed to limit the emission of ultraviolet radiation. Typically, the radiation emitted by a modern photocopier has wavelengths above 300nm.

Certain photochromic materials can be sensitized to have the wavelength at which they change color altered by another molecular entity, so that they are useful to prevent the unauthorized copying of original security documents. To be compatible for use in a security document, a photochrome must have certain characteristics. First, the photochrome must decolorize either thermally or when the source of radiation which causes the colorization is discontinued. Second, the photochrome must have a speed of coloration fast enough to produce a color change during the pass of the lamp of an image processing device. Third, the photochrome must have an extinction coefficient capable of producing a reflectance optical density (OD) such that the photochromic portion of the security document is discernible from the substrate when exposed to an exciting source of radiation. Fourth, the photochromic system must be non-selfscreening. In other words, it must have absorption characteristics such that it has a "window" between the characteristic absorption wavelength of the uncolored form and the characteristic absorption wavelength of the colored form. Fifth, the photochrome must have triplet state excited levels. A photochromic reaction takes place according to the following equilibrium:

Uncolored Form ** Colored Form

wherein K_forward is a function of electromagnetic radiation and ^_backward is a function of temperature and/or electromagnetic radiation. For that reason, some photochromes will not colorize because the speed of the reverse reaction equals or exceeds the speed of the forward reaction. However, it has been discovered that the forward reaction can be sensitized without affecting the reverse reaction. To be properly sensitized, the photochrome must have triplet state excited levels. While not wishing to be bound by a particular theory, it is thought that chemical sensitization can be explained in terms of an energy transfer mechanism involving an energy donor and an energy acceptor. Essentially, the donor is excited at a longer wave length than the acceptor absorbs and a triplet state is formed in the excited donor. Energy is then transferred from the excited triplet state of the donor to a triplet state of the acceptor and the reaction takes place in the triplet state of the acceptor and leads to the formation of products. The only prerequisite for the sensitization reaction are a high yield of the triplet state of the donor, the lowest lying triplet state of the donor molecule must be equal to or marginally higher than that of the acceptor molecule and the donor should have good absorption properties. An additional important characteristic of a photochrome is its extinction coefficient. The greater the extinction coefficient, then the greater will be the optical density for a given concentration.

The lamp of a image processing device, such as a photocopy machine or computer scanner, is focused into the form of a band of light which sweeps the page to be copied. The residence time of the lamp on the page is approximately 500ms. Therefore, to be useful, the photochrome should colorize in a time less than about 500ms so that the image processing device will be affected by the color change of the photochrome.

Preferably, the photochrome will colorize in from about 50ms to about 500ms and, more preferably, in from about 50ms to about

200ms.

As stated above, image processing devices emit electromagnetic radiation at frequencies above about 300nm.

Thus, the photochrome must be sensitized to undergo a change in color and/or optical density at frequency at or above about

300nm. The photochromic systems of the present invention preferably undergo a color change and/or change in optical density from about 300nm to about 500nm.

Based upon the above requirements, it has been determined that photochromes useful to produce security documents can be selected from the group consisting of spiropyrans, spirothiopyrans, spirooxazines and similar compounds. Preferably, the photochromes are benzoindolinospiropyrans, naphthoindolinospiropyrans , benzospiropyrans , naphthospiropyrans and bipyrans . Most preferably, the photochrome is a benzoindolinospiropyran having the following formula (I) :

wherein R_x-R₃ are hydro or alkyl groups and can be the same or different; R-R_ι0 are hydro, halo, nitro, alkyl, alkoxy, phenyl or phenoxy groups and can be the same or different; and R is a hydro or alkyl group. Preferably, the benzoindolinospiropyran is selected from the group consisting of unsubstituted spiro [2H-l-benzpyran-2 , 2-indoline] ; 6 ' , 8 ' -dinitro-1, 3,3- trimethyl-spiro [2H-l-benzpyran-2 , 2-indoline] ; 6 ' , 8 ' -dichloro- spiro [2H-l-benzpyran-2 , 2-indoline] ; 6 ' , 8 ' -dibromo-spiro [2H-1- benzpyran-2 , 2-indoline] ; 6-hydroxy-spiro [2H-l-benzpyran-2 , 2- indoline] ; 6 ' -bromo-8 ' -nitro-spiro [2H-l-benzpyran-2 , 2- indoline] ; 8 ' -bromo-6 ' -nitro-spiro [2H-l-benzpyran-2 , 2- indoline] ; 6-formyl-spiro [2H-l-benzpyran-2 , 2-indoline] ; 6- phenylazo-spiro [2H-l-benzpyran-2 , 2-indoline] ; 6 ' , 8 ' -dinitro-5 ' - methoxy-spiro [2H-l-benzpyran-2 , 2-indoline] ; 5 ' , 6 ' -dichloro-8 ' - nitro-spiro [2H-l-benzpyran-2 , 2-indoline] ; 7-chloro-6 ' -nitro- 1,3,3,3' -tetramethyl-spiro [2H-l-benzpyran-2 , 2-indoline] ; 6 ' - nitro-N-phenyl-spiro [2H-l-benzpyran-2 , 2-indoline] ; and 5- chloro-6 ' -nitro-spiro [2H-l-benzpyran-2 , 2-indoline] .

Sensitizers useful to induce the color change in a photochrome when exposed to the electromagnetic radiation emitted by the lamp of an image processing device will have the following characteristics. First, the sensitizer will absorb radiation within the window of the photochromic system spectrum, i.e., from about 300nm to about 500nm. Second, the sensitizer will be subject to triplet state excitation. Third, it will have a high quantum yield to the triplet state. Fourth, the excited state energy level of the sensitizer will be equal to or slightly greater than the triplet state of the photochrome. The color change process becomes less efficient as the energy levels of the photochrome and the sensitizer approach each other, in that the substrate and sensitizer become indistinguishable in the energy transfer process, i.e. triplet energy from sensitizer to substrate becomes interchangeable. If these criteria are met, the photochrome will be properly sensitized so that it will change color when exposed to the electromagnetic radiation emitted by an image processing device. To properly create a sensitized color change reaction of a photochromic compound, the sensitizer should be present in the photochromic system in a concentration from about 1% to about 50% of the concentration of the photochrome . A sensitizer will be useful provided that orbital overlap occurs between the sensitizer and the photochrome; both the donor and acceptor have excited (triplet) states; and the energy levels of the excited states are compatible. Materials which meet these criteria are aromatic amine, nitro- and ketone compounds in which two centers of different electron affinity linked by a conjugated system exist. Sensitizers useful to increase the wavelength at which spiropyrans, spirothiopyrans, spirooxazines and other similar compounds undergo a change in color and/or optical density are selected from the group consisting of picramide; substituted benzophenones; 1,2- benzanthraquinone; 3-methyl-l-3-diazo-l, 9-benzanthrone; substituted β-naphthothiazolines; 2-methyl-α-nitro-β- naphthothiazolines ; substituted β-naphthothiazolines ; and thioxanthones . Preferably, the sensitizer will be a substituted β-naphthothiazoline or a thioxanthone . Useful substituted β-naphthothiazolines have the following formula (II) :

wherein, R_x and R₂ are hydro, benzoyl, furoyl, substituted benzoyl, and substituted furoyl and can be the same or different .

Useful thioxanthones have the following formula (III) :

wherein R is an alkyl, alkoxy, halo or a C_ι-C₃ haloalkyl group. It has been found that isopropylthioxanthone, chloropropylthioxanthone and chlorothioxanthone are particularly useful because they have absorptions at about 400 nm.

An appropriate binder will not interfere with the colorization of the photochrome either by sterically hindering the photochrome or by quenching the excited state of the sensitizer. The binders typically present in the photochromic system bring the photochrome and the sensitizer into solution and ultimately fix the photochrome and sensitizer to the surface of a substrate. It is thought that, because the rate of colorization of the system is dictated by the efficiency of the sensitizer, the interaction of the sensitizer and binder must be monitored to insure that the above requirements are met. Useful binders are selected from the group consisting of polystyrene-acrylonitrile; polystyrene; poly (vinylchloride) ; polystyrene/butadiene; polyalkylmethacrylates; and poly (vinyl acetate) . The photochrome, sensitizer and binder can be dissolved in a solvent to provide for liquid delivery of the photochromic system. The solvent can be selected from the group consisting of methylethylketone, tetrahydrofuran, benzene, toluene, chloroform, ethylacetate, methanol, acetonitrile, dioxane, methyl ether of ethylene glycol, dimethylformamide, dimethylsulfoxide, ethylene glycol monomethyl ether, morpholine, ethylene glycol, petroleum hydrocarbon oils and vegetable oils, such as peanut oil and soybean oil. The final binder formulation can be a water borne formulation so that it can be applied to the substrate using conventional application methods. One useful system comprises an aqueous emulsion of poly (vinylacetate) stabilized with poly (vinylalcohol) . Typically, the photochrome and sensitizer can be dissolved in dibutylphthalate and added to the emulsion during high speed stirring in an emulsifier. The hydrophobic droplets of photochrome/sensitizer in the dibutylphthalate then migrate to the emulsified particles from the aqueous phase and are stabilized by the surfactant systems. The system may be further modified by mixing the emulsion with an aqueous solution of poly (vinylalcohol) . The primary feature of the photochromic system is contemplated to be useful as a copy indicator. The composition would also be useful for either manual or automatic authentication. The photochromic system containing the sensitized photochrome could be applied to the security document in a manner similar to the "void pantograph." In other words, the photochromic system can be applied to the security document so that, when the security document is exposed to the ultraviolet radiation emitted by an image processing device, a security portion appears on the copy. The original would also show the security portion for a period of time equal to the time required for the photochrome to return to its uncolored state. This would inhibit attempts to make multiple copies of the same document.

The photochromic system can be applied to a security document to produce a security portion on the security document. Indicia can then be applied to the security portion. The security portion remains in an uncolored state, i.e., a state of low optical density, until it is activated by the electromagnetic radiation emitted by an image processing device. After being exposed to the radiation of an image processing device, the security portion undergoes a change in optical density so that a copy of the security portion is rendered illegible. As described below, the security portion can cover a portion of, portions of or the entire security document. The photochromic system is useful to produce security portions on security documents such as bank checks, personal checks, reports, stock certificates, automobile title instruments, birth certificates, college transcripts, prescriptions and other documents of business and personal value. After a period of time, the optical density of the security portion will decrease and the security portion (s) of the original security document will again become legible.

Fig. 1 presents a top view of the security document of this invention with the nonvisible photochromic security portion shown in broken lines. The security document 10 comprises a substrate 12 onto which indicia is printed. The substrate 12 can be any substrate, such as paper or plastic, which is known in the art and onto which security terms can be printed. The top portion 14 of the substrate 12 bears a photochromic security portion 16. The photochromic security portion 16 is formed from the photochromic system which contains the photochrome and the sensitizer. The photochromic security portion 16 is placed onto the substrate 12 by any conventional means. Before being exposed to an image processing device, the security portion 16 remains in an uncolored state 18.

Fig. 2 presents a top view of the same security document after the photochromic security portion has changed color and/or experienced a change in optical density. Once the security document 10 has been exposed to a light source of an image processing device, the photochromic security portion 16 is activated from the uncolored state 18 to a colored state 20 in which it has undergone a change in optical density. As the photochromic security portion becomes colored, the indicia printed in the security portion 16 becomes "invisible" to the image processing device. The security portion 16 colorizes to such a color that the indicia is no longer readable by the image processing device. The indicia on the security portion 16 will be rendered illegible on any copies produced from the security document 10.

The security portion can cover a part of the substrate, parts of the substrate or the entire substrate. If the security portion covers the entire surface of the substrate, then, when the substrate is exposed to the electromagnetic radiation emitted by an image processing device, the entire background colorizes making the background indistinguishable from the printed indicia on the document . With the background being the same color as the document, the entire copy would then be illegible. After a certain period of exposure to visible light, the security portion, i.e., the photochromic system, on the original document decolorizes and the original document again becomes legible. The rate of decoloration is dependent upon the photochrome, binder and temperature used. The security portion may also be a security term. The phrase "security term" is intended to include not only words such as "VOID" as shown in the drawings but also words or phrases which make evident to the observer that the document being observed is not the original . Phrases such as "PHOTOCOPY," "COPY," or "DUPLICATE" may be used for this purpose. Also intended to be included in the definition of "security term" are words or symbols which signify to the individual making the copy that the original document is authentic. The phrase "security term" also encompasses cancellation terms such as "VOID" and "UNAUTHORIZED COPY" which make evident to the observer that the document being inspected is a copy of the original document and that the copy is not authentic.

Fig. 3 presents a top view of a security document which bears a security term, shown in broken lines, as the security portion. In Fig, 3, a security document 30 comprises a substrate 32 onto which indicia is printed. The top portion 34 of the substrate 32 bears a photochromic security portion 36. In this embodiment, the security portion 36 is a security term 38. Before being exposed to an image processing device, the security term 38 remains in an uncolored state 40.

Fig. 4 presents a top view of the security document of Fig. 3 after the security term has become visible. Once the security document 30 has been exposed to the lamp of an image processing device, the security term 38 is activated from the uncolored state 40 to a colored state 42. As the photochromic security portion experiences a change in optical density, the security term 38 becomes "visible" to the image processing device . The image processing device copies the security term 38 in its colored state 42 onto any copies after the security portion 16 has changed color.

The invention described herein can also be used in combination with other security measures to further thwart copying of security documents. Such security measures are described in commonly assigned United States Patent Numbers

5,045,426 entitled "Toner Adhesion-Enhancing Coating for Security Documents"; 5,149,140 entitled "Security Information Document" 5,290,515 entitled "Solvent and/or Pressure Sensitive Ink and Security Document"; 5,250,492 entitled "Coatings for Use with Business Forms, Security Documents or Safety Paper"; and 5,340,159 entitled "Varying Tone Security Document". Other security measures can be found in commonly assigned copending United States Patent Applications 08/185,362 entitled "Security Document"; and 08/611,378 entitled "Security Document". The disclosures of the above patents and applications are hereby incorporated by reference in their entirety.

The security document of the present invention and the photochromic system of the present invention will be effective with any image processing device that produces reproductions or copies of a security document by means of a source of electromagnetic radiation. The security document and photochromic system are particularly effective with image processing devices such as duotone photocopiers, color photocopiers and computer scanners. One skilled in the art will certainly appreciate the myriad of uses of the security document and the photochromic system of this invention. While certain representative embodiments and details have been presented for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes in the compositions and security documents disclosed herein may be made without departing from the scope of the invention, which is defined in the appended claims.

Claims

1. A security document comprising: a substrate having a top surface for carrying printed indicia; and a security portion carried on the top surface of the substrate, the security portion being formed from a photochromic system including a photochrome and a sensitizer which increases the wavelength at which the photochrome colorizes to a wavelength equal to a wavelength of electromagnetic radiation emitted by an image processing device, whereby, upon copying of the security document by an image processing device, the security portion undergoes a change in optical density.

2. A photochromic system useful for making security documents comprising: a photochrome; and a sensitizer which causes the photochrome to colorize at a wavelength equal to a wavelength of electromagnetic radiation emitted by an image processing device.

3. The security document of claim 1 wherein the photochromic system is non-selfscreening.

4. The invention according to claim 1 or claim 2 wherein the photochrome is selected from the group consisting of spiropyrans, spirothiopyrans, spirooxazines and similar compounds.

5. The invention according to claim 4 wherein the sensitizer is selected from the group consisting of picramide; substituted benzophenones; 1, 2-benzanthraquinone; 3-methyl-l-3-diazo-l, 9- benzanthrone; substituted ╬▓-naphthothiazolines ; 2 -methyl -╬▒- nitro-╬▓-naphthothiazolines; substituted ╬▓-naphthothiazolines ; thioxanthone and substituted thioxanthones .

6. The invention according to claim 4 wherein the photochrome is a benzoindolinospiropyran having the following formula (I) :

wherein R╬╣~R₃ are hydro or alkyl groups and can be the same or different; R₄-R₁₀ are hydro, halo, nitro, alkyl, alkoxy, phenyl or phenoxy groups and can be the same or different; and R_n is a hydro or alkyl group.

7. The invention according to claim 5 wherein the sensitizer is a substituted ╬▓-naphthothiazoline having the following formula (II) :

wherein, R_: and R₂ are hydro, benzoyl, furoyl, substituted benzoyl, and substituted furoyl and can be the same or different .

8. The invention according to claim 5 wherein the sensitizer is a thioxanthone having the following formula (III) :

wherein R is a alkyl, alkoxy, halo or a C╬╣_-C₃ haloalkyl group,

9. The invention according to claim 1 or claim 2 wherein the photochromic system further includes a binder which solubilizes the photochrome and the sensitizer and which attaches the sensitizer and the photochrome to the substrate.

10. The invention according to claim 1 or claim 2 wherein the sensitizer is present in a concentration from about 1% to about 50% of the concentration of the photochrome.

11. The security document of claim 1 wherein the security portion covers the top surface of the security document such that, upon copying of the security document, the security portion renders an entire copy illegible.

12. The security document of claim 1 wherein the security portion is a security term.

13. The security document of claim 1 wherein the security portion becomes visually perceptible when the security document is exposed to a light source of a color copier.

14. The invention according to claim 1 or claim 2 wherein the photochrome colorizes in about 50ms to about 500ms.

15. The invention according to claim 14 wherein the photochrome colorizes in about 50ms to about 200ms.

16. The invention according to of claim 1 or claim 2 wherein the photochrome colorizes between the wavelengths from about

300nm to about 500nm.

17. The invention according to claim 9 wherein the binder is selected from the group consisting of styrene-acrylonitrile; polystyrene; poly (vinylchloride) ; polystyrene/butadiene; polyalkylmethacrylates ; and poly (vinyl acetate) .

18. The photochromic system of claim 2 further including a solvent .

19. The photochromic system of claim 18 wherein the solvent is selected from the group consisting of methylethylketone, tetrahydrofuran, benzene, toluene, chloroform, ethylacetate, methanol, acetonitrile, dioxane, methyl ether of ethylene glycol, dimethylformamide, dimethylsulfoxide, ethylene glycol monomethyl ether, morpholine, ethylene glycol, petroleum hydrocarbon oils and vegetable oils, such as peanut oil and soybean oil.