Patent Application
20070238613
Compositions that produce a color change upon exposure to energy in the form of light or heat are of great interest in generating images on a variety of substrates. For example, data storage media provide a convenient way to store large amounts of data in stable and mobile formats. For example, optical discs, such as compact discs (CDs), digital video disks (DVDs), or other discs allow a user to store relatively large amounts of data on a single relatively small medium. Data on such discs often includes entertainment, such as music and/or images, as well as other types of data. In the past, consumer devices were only configured to read the data stored on optical disks, not to store additional data thereon. Consequently, any data placed on the optical disks was frequently placed thereon by way of a large commercial machine that burned the data onto the disc. In order to identify the contents of the disc, commercial labels were frequently printed onto the disc by way of screen printing or other similar methods.
Recent efforts have been directed to providing consumers with the ability to store data on optical disks. Such efforts include the use of drives configured to burn data on recordable compact discs (CD-R), rewritable compact discs (CD-RW), recordable digital video discs (DVD-R), rewritable digital video discs (DVD-RW), and combination drives containing a plurality of different writeable drives, to name a few. These drives provide a convenient way for users to record relatively large amounts of data that may then be easily transferred or used in other devices.
The optical disks used as storage mediums frequently have two sides: a data side configured to receive and store data and a label side. The label side is frequently a background on which the user hand writes information to identify the disc.
An imageable system includes a first thermochromic layer, a second thermochromic layer, and a thermal averaging layer disposed between said first thermochromic layer and said second thermochromic layer.
Additionally, according to one exemplary embodiment, a method for forming an imageable coating includes providing a substrate, dispensing a first thermochromic material on the substrate, dispensing a thermal averaging material on the first thermochromic material, and dispensing a second thermochromic material on the thermal averaging material.
The accompanying drawings illustrate various embodiments of the present system and method and are a part of the specification. The illustrated embodiments are merely examples of the present system and method and do not limit the scope of the disclosure.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.
The present exemplary systems and methods provide for the preparation of an imageable thermochromic system that uses thermal averaging to differentiate between color generation in a first or a second layer. In particular, an imageable thermochromic structure is described herein that can be selectively imaged in a first or a second layer with a single radiation generating device by varying the frequency and/or intensity of the radiation generating device. According to one exemplary embodiment, the present imageable thermochromic structure has two thermochromic coatings, each having different critical marking temperatures, separated by a thermal buffer layer. By placing the thermochromic coating having a higher critical marking temperature on the top of the exemplary structure, a radiation source may be pulsed at a high intensity to selectively radiate the top thermochromic layer. Additionally, the radiation source may provide uninterrupted radiation at a low intensity to selectively mark the lower thermochromic layer. Further details of the present markable structure, as well as exemplary methods for forming the structure on a desired substrate will be described in further detail below.
As used in the present specification, and in the appended claims, the term “imageable discs” is meant to be understood broadly as including, but in no way limited to, audio, video, multi-media, and/or software disks that are machine readable in a CD and/or DVD drive, or the like. Non-limiting examples of imageable disc formats include, writeable, recordable, and rewriteable disks such as DVD, DVD-R, DVD-RW, DVD+R, DVD+RW, DVD-RAM, CD, CD-ROM, CD-R, CD-RW, and the like.
As used in the present specification, and in the appended claims, the term “thermochromic” shall be interpreted broadly as including any material that is configured to change color when exposed to a temperature equal to, or higher than, a critical color changing temperature.
For purposes of the present exemplary systems and methods, the term “color” or “colored” refers to absorbance and reflectance properties that are preferably visible, including properties that result in black, white, or traditional color appearance. In other words, the terms “color” or “colored” includes black, white, and traditional colors, as well as other visual properties, e.g., pearlescence, reflectivity, translucence, transparency, etc.
In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present systems and methods for forming an imageable thermochromic structure configured to differentiate between at least two imageable layers using thermal averaging. It will be apparent, however, to one skilled in the art that the present systems and methods may be practiced without these specific details. Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. The appearance of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment.
Exemplary Structure
As illustrated in
As illustrated in
As introduced, the processor (125) is configured to controllably interact with the radiation generating device (110). While
As mentioned previously, the present media processing system (100) includes a data storage medium in the form of a radiation imageable disc (130) disposed adjacent to the radiation generating device (110). According to one exemplary embodiment, the exemplary imageable disc (130) includes first (140) and second (150) opposing sides. The first side (140) has a data surface formed thereon configured to store data while the second side (150) includes an imageable surface having a plurality of color forming compositions.
With respect to the first side (140) of the imageable disc (130), the radiation generating device (110) may be configured to read existing data stored on the imageable disc (130) and/or to store new data on the imageable disc (130), as is well known in the art. As used herein, the term “data” is meant to be understood broadly as including the non-graphic information digitally or otherwise embedded on an imageable disc. According to the present exemplary embodiment, data can include, but is in no way limited to, audio information, video information, photographic information, software information, and the like. Alternatively, the term “data” may also be used herein to describe information such as instructions a computer or other processor may access to form a graphic display on an imageable surface.
In contrast to the first side of the imageable disc (130), the second side of the imageable disc (140) includes a plurality of imageable coatings including a thermal averaging layer separating the plurality of imageable coatings. According to one exemplary embodiment, discussed in further detail below, the second side of the imageable disc (140) includes two separate thermochromic layers: a bottom thermochromic layer having a relatively low marking temperature, and a top thermochromic layer having a relatively high marking temperature and an optional sensitizing agent in the form of an antenna dye or other radiation absorbing species dispersed in the top thermochromic layer. Further details of the radiation-curable imageable coating including thermal averaging will be provided below.
Exemplary Coating Formulation
As mentioned above, the second side of the imageable disc (140) includes a plurality of layers including a top and a bottom thermochromic layer separated by a thermal averaging layer.
According to one exemplary embodiment, the two thermochromic layers (220, 200) forming the present coating structure include, but are in no way limited to, polymer matrices with acidic activator species dissolved therein and a low-melting eutectic of a leuco dye insoluble or having low solubility at ambient temperature in the matrix, but uniformly distributed therein as a fine dispersion. Additionally, the top thermochromic layer may be sensitized to one or more radiation generating devices by the inclusion of an antenna dye package uniformly distributed/dissolved in at least one and preferably both phase(s) of the top layer (200). According to one exemplary embodiment, the present antenna dye package dispersed in the top thermochromic layer (200) includes a dye having an absorbance maximum corresponding to a wavelength value of a known radiation generating device (110;
As mentioned, the first phase in each of the imageable thermochromic layers (200, 220) includes, but is in no way limited to, a polymer matrix with acidic activator species dissolved therein. According to one exemplary embodiment, the polymer in each of the thermochromic layers (200, 220) may be a lacquer configured to form a continuous phase, referred to herein as a matrix phase, when exposed to heat and/or light. More specifically, according to one exemplary embodiment, top imageable thermochromic layer (200) includes a radiation curable polymer that may include, by way of example, UV-curable matrices such as acrylate derivatives, oligomers, and monomers, with a photo package. A photo package may include a light absorbing species, such as photoinitiators, which initiate reactions for curing of the lacquer, such as, by way of example, benzophenone derivatives. Other examples of photoinitiators for free radical polymerization monomers and pre-polymers include, but are not limited to, thioxanethone derivatives, anthraquinone derivatives, acetophenones, benzoine ethers, and the like. Alternatively, because the bottom thermochromic layer (220) does not receive any significant radiation during the image formation process, the bottom thermochromic layer (200) may include, but is in no way limited to, thermally curable polymers. According to one exemplary embodiment, the thermally curable polymers undergo cure at a temperature substantially below the temperature required for image formation in bottom thermochromic layer (220).
According to one exemplary embodiment, the polymer matrix phases may be chosen such that curing is initiated by a form of radiation or a heat level that does not cause a color change of the color-former present in the coating, according to the present exemplary system and method. For example, the radiation-curable polymer matrix may be chosen such that the above-mentioned photo package initiates reactions for curing of the lacquer when exposed to a light having a different wavelength than that of the leuco dyes. According to one exemplary embodiment, the photo package may initiate reactions at a significantly shorter wavelength then that of the leuco dyes, such as UV or near UV radiation. Matrices based on cationic polymerization resins may require photoinitiators based on aromatic diazonium salts, aromatic halonium salts, aromatic sulfonium salts, and metallocene compounds. Additionally, matrices based on free radical polymerization may further include free radical photoinitiators such as benzophenons, alphahydroxy ketones, isopropylthioxanthones, and the like. Consequently, an example of a suitable lacquer or matrix may include Nor-Cote CLCDG-1250A (a mixture of UV curable acrylate monomers and oligomers) which contains a photoinitiator (hydroxyl ketone) and organic solvent acrylates, such as, methyl methacrylate, hexyl methacrylate, beta-phenoxy ethyl acrylate, and hexamethylene acrylate. Other suitable components for lacquers or matrices may include, but are not limited to, acrylated polyester oligomers, such as CN293 and CN294 as well as CN-292 (low viscosity polyester acrylate oligomer), trimethylolpropane triacrylate commercially known as SR-351, isodecyl acrylate commercially known as SR-395, and 2(2-ethoxyethoxy)ethyl acrylate commercially known as SR-256, all of which are available from Sartomer Co. Similarly, the bottom thermochromic layer (220) may include a thermally curable polymer matrix that is configured to be cured at a temperature other than the temperature desired for initiating color change.
Additionally, a number of acidic developers may be dispersed/dissolved in each of the present polymer matrices. According to one exemplary embodiment, the acidic developers present in the polymer matrices may include a phenolic species that is soluble or partially soluble in the coating matrix while being configured to develop color when reacting with a leuco dye through proton transfer. Suitable developers for use with the present exemplary system and method include, but are in no way limited to, acidic phenolic compounds such as, for example, Bis-Phenol A, p-Hydroxy Benzyl Benzoate, Bisphenol S (4,4-Dihydroxydiphenyl Sulfone), 2,4-Dihydroxydiphenyl Sulfone, Bis(4-hydroxy-3-allylphenyl)sulfone (Trade name—TG-SA), 4-Hydroxyphenyl-4′-isopropoxyphenyl sulfone (Trade name—D8). The acidic developer may be either completely or at least partially dissolved in the polymer matrices.
The second phase of each of the thermochromic layers is a color-former phase including, according to one exemplary embodiment, a leuco dye and/or leuco dye alloy, further referred to herein as a leuco-phase. According to one exemplary embodiment, the leuco-phase is present in the form of small particles dispersed uniformly in each of the thermochromic layers. According to one exemplary embodiment, the leuco-phase includes leuco dye or alloy of leuco dye with a mixing aid configured to form a lower melting eutectic with the leuco dye. Alternatively, according to one embodiment, the second phase of each of the present polymer matrices may include other color forming dyes such as photochromic dyes.
According to one exemplary embodiment, the present thermochromic layers may have any number of leuco dyes including, but in no way limited to, fluorans, phthalides, amino-triarylmethanes, aminoxanthenes, aminothioxanthenes, amino-9,10-dihydro-acridines, aminophenoxazines, aminophenothiazines, aminodihydro-phenazines, aminodiphenylmethanes, aminohydrocinnamic acids (cyanoethanes, leuco methines) and corresponding esters, 2(phydroxyphenyl)-4,5-diphenylimidazoles, indanones, leuco indamines, hydrozines, leuco indigoid dyes, amino-2,3-dihydroanthraquinones, tetrahalop, p′-biphenols, 2(p-hydroxyphenyl)-4,5-diphenylimidazoles, phenethylanilines, and mixtures thereof. According to one particular aspect of the present exemplary system and method, the leuco dye can be a fluoran, phthalide, aminotriarylmethane, or mixture thereof. Several nonlimiting examples of suitable fluoran based leuco dyes include, but are in no way limited to, 3-diethylamino-6-methyl-7-anilinofluorane, 3-(N-ethyl-p-toluidino)-6-methyl-7-anilinofluorane, 3-(N-ethyl-N-isoamylamino)-6-methyl-7-anilinofluorane, 3-diethylamino-6-methyl-7-(o,p-dimethylanilino)fluorane, 3-pyrrolidino-6-methyl-7-anilinofluorane, 3-piperidino-6-methyl-7-anilinofluorane, 3-(N-cyclohexyl-Nmethylamino)-6-methyl-7-anilinofluorane, 3-diethylamino-7-(mtrifluoromethylanilino)fluorane, 3-dibutylamino-6-methyl-7-anilinofluorane, 3-diethylamino-6-chloro-7-anilinofluorane, 3-dibutylamino-7-(o-chloroanilino)fluorane, 3-diethylamino-7-(o-chloroanilino)fluorane, 3-di-n-pentylamino-6-methyl-7-anilinofluoran, 3-di-n-butylamino-6-methyl-7-anilinofluoran, 3-(n-ethyln-isopentylamino)-6-methyl-7-anilinofluoran, 3-pyrrolidino-6-methyl-7-anilinofluoran, 1(3H)-isobenzofuranone,4,5,6,7-tetrachloro-3,3-bis[2-[4-(dimethylamino)phenyl]-2-(4-methoxyphenyl)ethenyl], and mixtures thereof.
Aminotriarylmethane leuco dyes can also be used in the present invention such as tris(N,N-dimethylaminophenyl)methane (LCV); deutero-tris(N,Ndimethylaminophenyl) methane (D-LCV); tris(N,N-diethylaminophenyl)methane (LECV); deutero-tris(4-diethylaminolphenyl)methane (D-LECV); tris (N,N-di-n-propylaminophenyl) methane (LPCV); tris(N,N-dinbutylaminophenyl)methane (LBCV); bis(4-diethylaminophenyl)-(4-diethylamino-2-methyl-phenyl)methane (LV-1); bis(4-diethylamino-2-methylphenyl)-(4-diethylamino-phenyl)methane (LV-2); tris(4-diethylamino-2-methylphenyl)methane (LV-3); deutero-bis(4-diethylaminophenyl)-(4-diethylamino-2-methylphenyl)methane (D-LV-1); deutero-bis(4-diethylamino-2-methylphenyl) (4-diethylaminophenyl)methane (D-LV-2); bis(4-diethylamino-2-methylphenyl) (3,4-dimethoxyphenyl)methane (LB-8); aminotriarylmethane leuco dyes having different alkyl substituents bonded to the amino moieties wherein each alkyl group is independently selected from C1-C4 alkyl; and aminotriaryl methane leuco dyes with any of the preceding named structures that are further substituted with one or more alkyl groups on the aryl rings wherein the latter alkyl groups are independently selected from C1-C3 alkyl.
Additional leuco dyes can also be used in connection with the present exemplary system and method and are known to those skilled in the art. A more detailed discussion of appropriate leuco dyes may be found in U.S. Pat. Nos. 3,658,543 and 6,251,571, each of which are hereby incorporated by reference in their entireties. Additionally examples may be found in Chemistry and Applications of Leuco Dyes, Muthyala, Ramaiha, ed.; Plenum Press, New York, London; ISBN: 0-306-45459-9, incorporated herein by reference.
Additionally, according to one exemplary embodiment, a number of melting aids may be included with the above-mentioned leuco dyes. As used herein, the melting aids may include, but are in no way limited to, crystalline organic solids with melting temperatures in the range of approximately 50° C. to approximately 150° C., and preferably having melting temperature in the range of about 70° C. to about 120° C. In addition to aiding in the dissolution of the leuco dye and/or the antenna dye, the above-mentioned melting aid may also assist in reducing the melting temperature of the leuco dye and stabilize the leuco dye alloy in the amorphous state, or slow down the re-crystallization of the leuco dye alloy into individual components. Suitable melting aids include, but are in no way limited to, aromatic hydrocarbons (or their derivatives) that provide good solvent characteristics for leuco dye and antenna dyes used in the present exemplary systems and methods. By way of example, suitable melting aids for use in the current exemplary systems and methods include, but are not limited to, m-terphenyl, pbenzyl biphenyl, alpha-naphtol benzylether, 1,2-[bis(3,4]dimethylphenyl)ethane. In some embodiments, the percent of leuco dyes or other color-former and melting aid can be adjusted to minimize the melting temperature of the color-former phase without interfering with the development properties of the leuco dye. When used, the melting aid can comprise from approximately 2 wt % to approximately 25 wt % of the color-former phase.
According to one exemplary embodiment of the present exemplary system and method, the above-mentioned leuco-phase is uniformly dispersed/distributed in each of the thermochromic matrix phases as a separate phase. In other words, at ambient temperature, the leuco phase is practically insoluble in matrix phase. Consequently, the leuco dye and the acidic developer component of the matrix phase are contained in the separate phases and can not react with color formation at ambient temperature. However, upon heating with laser radiation, both phases melt and mix. Once mixed together, color is developed due to a reaction between the fluoran leuco dye and the acidic developer. According to one exemplary embodiment, when the leuco dye and the acidic developer react, proton transfer from the developer opens a lactone ring of the leuco dye, resulting in an extension of conjugate double bond system and color formation.
While the above-mentioned color formation is desired, selective color formation of specific portions of each of the thermochromic layers (200, 220) is desired. Consequently, according to one exemplary embodiment, the top thermochromic layer (200) may be sensitized with one or more antenna dyes. According to one exemplary embodiment, the antenna dyes comprise a number of radiation absorbers configured to optimize development of the color forming composition upon exposure to radiation at a predetermined exposure time, energy level, wavelength, etc. More specifically, the radiation absorbing antenna dyes may act as an energy antenna providing energy to surrounding areas of the resulting coating upon interaction with an energy source. However, various radiation absorbing dyes have varying absorption ranges and varying absorbency maximums where the antenna dye will provide energy most efficiently from a radiation source. Generally speaking, a radiation antenna that has a maximum light absorption at or in the vicinity of a desired development wavelength may be suitable for use in the present system and method.
As a predetermined amount of energy can be provided by the radiation generating device (110;
In order to sensitize the top thermochromic layer (200) described above to the radiation of the radiation generating device (110;
According to one exemplary embodiment, the media processing system (100) may include one or more radiation generating devices (110;
As mentioned, a number of dyes having varying absorbance maximums may be used in the above-mentioned coatings to act as radiation absorbing antenna dyes. According to one exemplary embodiment, radiation absorbing antenna dyes having absorbance maximums at approximately 780 nm that may be incorporated into the present antenna dye package include, but are in no way limited to, indocyanine IR-dyes such as IR780 iodide (Aldrich 42,531-1) (1) (3H-Indolium, 2-[2-[2-chloro-3-[(1,3-dihydro-3,3-dimethyl-1-propyl-2H-indol-2-ylidene)ethylidene]-1-cyclohexen-1-yl]ethenyl]-3,3-dimethyl-1-propyl-, iodide (9Cl)), IR783 (Aldrich 54,329-2) (2) (2-[2-[2-Chloro-3-[2-[1,3-dihydro-3,3-dimethyl-1-(4-sulfobutyl)-2Hindol-2-ylidene]-ethylidene]-1-cyclohexen-1-yl]-ethenyl]-3,3-dimethyl-1-(4-sulfobutyl)-3H-indolium hydroxide, inner salt sodium salt). Additionally, phthalocyanine or naphthalocyanine IR dyes such as Silicon 2,3-naphthalocyanine bis(trihexylsiloxide) (CAS No. 92396-88-8) (Lambda max −775 nm) may be used.
Exemplary radiation absorbing antenna dyes having absorbance maximums at approximately 650 nm that may be incorporated into the present antenna dye package include, but are in no way limited to, dye 724 (3H-Indolium, 2-[5-(1,3-dihydro-3,3-dimethyl-1-propyl-2H-indol-2-ylidene)-1,3-pentadienyl]-3,3-dimethyl-1-propyl-, iodide){umlaut over ( )}C(lambda max=642 nm), dye 683 (3H-Indolium, 1-butyl-2-[5-(1-butyl-1,3-dihydro-3,3-dimethyl-2H-indol-2-ylidene)-1,3-pentadienyl]-3,3-dimethyl-, perchlorate {umlaut over ( )}C (lambda max=642 nm), dyes derived from phenoxazine such as Oxazine 1 (Phenoxazin-5-ium, 3,7-bis (diethylamino)-, perchlorate) {umlaut over ( )}C (lambda max=645 nm), both of which are commercially available from “Organica Feinchemie GmbH Wollen.” Appropriate antenna dyes applicable to the present exemplary system and method may also include but are not limited to phthalocyanine dyes with light absorption maximum at/or in the vicinity of 650 nm.
Radiation antennae which can be incorporated into the present antenna dye package for optimization in the blue (˜405 nm) and indigo wavelengths can include, but are not limited to, aluminum quinoline complexes, porphyrins, porphins, and mixtures or derivatives thereof. Non-limiting specific examples of suitable radiation antenna can include 1-(2-chloro-5-sulfophenyl)-3-methyl-4-(4-sulfophenyl)azo-2-pyrazolin-5-one disodium salt (λmax=400 nm); ethyl 7-diethylaminocoumarin-3-carboxylate (λ max=418 nm); 3,3′-diethylthiacyanine ethylsulfate (λmax=424 nm); 3-allyl-5-(3-ethyl-4-methyl-2-thiazolinylidene) rhodanine (λ max=430 nm) (each available from Organica Feinchemie GmbH Wolfen), and mixtures thereof.
Non-limiting specific examples of suitable aluminum quinoline complexes can include tris(8-hydroxyquinolinato)aluminum (CAS 2085-33-8), and derivatives such as tris(5-cholor-8-hydroxyquinolinato)aluminum (CAS 4154-66-1), 2-(4-(1-methyl-ethyl)-phenyl)-6-phenyl-4H-thiopyran-4-ylidene)-propanedinitril-1,1-dioxide (CAS 174493-15-3),4,4′-[1,4-phenylenebis(1,3,4-oxadiazole-5,2-diyl)]bis N,N-diphenyl benzeneamine (CAS 184101-38-0), bis-tetraethylammonium-bis(1,2-dicyano-dithiolto)-zinc(II) (CAS 21312-70-9), 2-(4,5-dihydronaphtho[1,2-d]-1,3-dithiol-2-ylidene)4,5-dihydro-naphtho[1,2-d]1,3-dithiole, all available from Syntec GmbH.
Non-limiting examples of specific porphyrin and porphyrin derivatives can include etioporphyrin 1 (CAS 448-71-5), deuteroporphyrin IX 2,4 bis ethylene glycol (D630-9) available from Frontier Scientific, and octaethyl porphrin (CAS 2683-82-1), azo dyes such as Mordant Orange (CAS 2243-76-7), Merthyl Yellow (CAS 60-11-7), 4-phenylazoaniline (CAS 60-09-3), Alcian Yellow (CAS 61968-76-1), available from Aldrich chemical company, and mixtures thereof.
As mentioned above, the top thermochromic layer (200) and the bottom thermochromic layer (220) are separated by a middle thermal averaging layer (210). According to one exemplary embodiment, the thermal averaging layer (210) is configured to slow the diffusion of heat from the top thermochromic layer (200) to the bottom thermochromic layer (220) during image formation. Consequently, if the top thermochromic layer (220) is being pulsed to form a desired image, the pulses are averaged by the thermal averaging layer (210) to a temperature less than the peak temperature of the pulses. According to one exemplary embodiment, the thermal averaging layer (210) may be formed out of any number of materials configured to average pulsed temperature including, but in no way limited to, an acrylate or any number of polymers having a thickness and thermal conductivity tuned to averaging pulsed thermal energy. According to one exemplary embodiment, the thermal averaging layer (210) may include, but is in no way limited to, an aqueous polymer. Further, the aqueous polymer can contain any number of inorganic particles (sub-micron) or ceramic particles to act as heat sinks. Exemplary methods of forming the above-mentioned structure, as well as methods for forming images on the structure are described in further detail below.
Exemplary Coating Forming Method
With each of the thermochromic coatings prepared for the two imageable thermochromic coatings (200, 220), the coatings may be formed on an optical disc data portion (230), as detailed in
Once the above-mentioned structure is formed on the imageable disk (130;
Continuing with
The temperature v. time graph for the top thermochromic layer (520) illustrates the thermal effect of irradiating the disc with pulsed light energy. As illustrated in
However, as illustrated in the temperature v. time graph for the bottom layer (560), the increase in temperature of the top thermochromic layer (200;
In contrast,
Additionally, according to one exemplary embodiment, the teachings of the present exemplary system may be applied to a full color marking system having three marking layers. Specifically, as illustrated in
In conclusion, the present exemplary systems and methods provide for the preparation and marking of an imageable thermochromic coating using a single radiation generating device, such as a laser. In particular, the present exemplary imageable coating has at least a first and second thermochromic layer, each having different critical marking temperatures. The at least first and second thermochromic layers are separated by a thermal averaging layer. Due to the above-mentioned structure and varying critical marking temperatures, each of the thermochromic layers may be selectively marked by varying the intensity and duration of a single radiation generating device.
The preceding description has been presented only to illustrate and describe the present method and apparatus. It is not intended to be exhaustive or to limit the disclosure to any precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the disclosure be defined by the following claims.
