Abstract:
The current invention provides a persistent phosphor blend, along with techniques for making and using the blend. The persistent phosphor blend is made of at least one persistent phosphor combined with at least one other phosphor, where the excitation spectra of the one or more other phosphors overlap the emission spectra of the one or more persistent phosphors. The choice of the phosphors used allows the decay time and emission colors to be tuned for the specific application. In another embodiment, the invention provides a method for making persistent phosphor blends with tunable colors. In yet another embodiment, applications for such a persistent phosphor blend are provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation in part of U.S. patent application Ser. No. 11/654,191, filed 17 Jan. 2007, now abandoned, which is herein incorporated by reference. 
    
    
     BACKGROUND 
     The current invention relates generally to phosphor blends having long persistence and tunable colors. More specifically, the current invention provides blends of one or more persistent phosphors with one or more other phosphors to create blends with long persistence and tunable colors. 
     A phosphor is a luminescent material that absorbs radiation energy in one portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. One important class of phosphors includes crystalline inorganic compounds of very high chemical purity and of controlled composition, to which small quantities of other elements, called “activators,” have been added for fluorescent emission. With the right combination of activators and inorganic compounds, the color of the emission of these crystalline phosphors can be controlled. Most useful phosphors emit radiation in the visible portion of the electromagnetic spectrum in response to excitation by electromagnetic energy outside the visible range. Well known phosphors have been used in mercury vapor discharge lamps to convert the ultraviolet (UV) radiation emitted by the excited mercury to visible light. Other phosphors are capable of emitting visible light upon being excited by electrons, useful in photomultiplier tubes, or X-rays, such as scintillators used in imaging systems. 
     One important property of phosphors is the decay time, or the time required for the phosphor to stop emitting light after the excitation is removed. Most phosphor compositions have short decay times, with most of the stored energy emitted as light within seconds, or even a small fraction of a second, after excitation ends. Although their decay time is short, these phosphors have many possible color choices and multiple phosphor blends may be made with specific colors for use in lighting applications where continuous excitation is present. 
     New persistent phosphorescent materials have been developed in an attempt to extend the decay time for applications when continuous excitation is not present. However, many of these persistent phosphors have emission spectra that peak in the blue or green spectral regions, which may limit their visibility in certain applications. Accordingly, there is a need for new phosphorescent compositions that have long decay times and emission spectra that may be tuned for the application. 
     BRIEF DESCRIPTION 
     In one embodiment, the current invention provides a persistent phosphor blend, which has at least one persistent phosphor blended with at least one other phosphor, wherein the emission spectrum of the at least one persistent phosphor at least partially overlaps the excitation spectrum of the at least one other phosphor. The at least one persistent phosphor comprises a composition selected from the group consisting of
         a) a composition having a general formula A x-y-z Al 2-m-n-o-p O 4 :Eu y , Dy z , B m , Zn n , CO o , Sc p , where A is Ba, Sr, Ca, or a combination of these elements, x is between about 0.75 and 1.3, y is between about 0.0005 and about 0.1, z is between about 0.0005 and about 0.1, m is between about 0.0005 and about 0.30, n is between about 0.0005 and about 0.10, o is between about 0 and about 0.01 and p is between about 0 and about 0.05, and   b) a composition having general formula A x-y-z Al 2-m-n-o-p O 4 :Eu y , Nd z , B m , Zn n , CO o , Sc p , where A is Ba, Sr, Ca, or a combination of these elements, x is between about 0.75 and about 1.3, y is between about 0.0005 and about 0.1, z is between about 0.0005 and about 0.1, m is between about 0.0005 and about 0.30, n is between about 0.0005 and about 0.10, o is between about 0 and about 0.01, and p is between about 0 and about 0.05.       

     Another embodiment provides a method of producing a persistent phosphor blend with tunable colors. The method comprises blending the at least one persistent phosphor as described above with at least one other phosphor, wherein the emission spectrum of the least one persistent phosphor at least partially overlaps the excitation spectrum of the at least one other phosphor. 
     Another embodiment provides an article of manufacture containing a persistent phosphor blend. The article of manufacture comprises a structure and a phosphor blend that has at least one persistent phosphor as described above and at least one other phosphor, wherein the emission spectrum of the at least one persistent phosphor at least partially overlaps the excitation spectrum of the at least one other phosphor in the composition. 
     Another embodiment provides an article of manufacture coated with layers containing at least one persistent phosphor as described above and at least one other phosphor. The emission spectrum of the at least one persistent phosphor at least partially overlaps the excitation spectrum of the at least one other phosphor. 
     Yet another embodiment provides a coating comprising one or more layers, the coating containing at least one persistent phosphor as described above and at least one other phosphor, wherein the emission spectra of the at least one persistent phosphor partially overlaps the emission spectra of the at least one other phosphor. 
    
    
     
       DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is a diagrammatical representation of a structure impregnated with particles of two different phosphor materials, a persistent phosphor and another phosphor, in accordance with embodiments of the current invention; 
         FIG. 2  is a diagrammatical representation of a layered structure containing different phosphors in the different layers, including a persistent phosphor and another phosphor, in accordance with embodiments of the current invention; 
         FIG. 3  is a graphical representation of exemplary excitation and emission spectra for a persistent phosphor, which may be used in embodiments of the current invention; 
         FIG. 4  is a similar graphical representation of exemplary excitation and emission spectra of a phosphor, (Ca,Sr) 8 (Mg,Zn)(SiO 4 ) 4 Cl 2 :Eu 2+ , Mn 2+  (CaSi), which may be used in embodiments of the current invention; 
         FIG. 5  is a graphical representation of exemplary excitation and emission spectra of another phosphor, Tb 3 A 4.9 O 12 :Ce 3+  (TAG:Ce), which may be used in embodiments of the current invention; 
         FIG. 6  is a graphical representation of exemplary excitation and emission spectra of another phosphor, Sr 4 Al 14 O 25 :Eu 2+  (SAE), which may be used in embodiments of the current invention; 
         FIG. 7  is a graphical representation of exemplary excitation and emission spectra of another phosphor, 3.5MgO*0.5MgF 2 *GeO 2 :Mn 4+  (MFG), which may be used in embodiments of the current invention; 
         FIG. 8  is an elevational view of an exemplary product that may incorporate a phosphor blend in accordance with the invention, in this case a faceplate panel of an automobile radio, with either the faceplate or controls containing a persistent phosphor blend in accordance with embodiments of the present invention; 
         FIG. 9  illustrates another exemplary application, in this case a child&#39;s toy containing a persistent phosphor blend or decorated with a film containing a persistent phosphor blend in accordance with embodiments of the present invention; 
         FIG. 10  illustrates a hard hat either containing a persistent phosphor blend or decorated with a film containing a persistent phosphor blend, in accordance with embodiments of the present invention; 
         FIG. 11  shows an exit sign containing a persistent phosphor blend, in accordance with embodiments of the present invention; 
         FIG. 12  shows an article of clothing either containing a persistent phosphor blend in the material itself, or letters attached to the front of the article of clothing containing a persistent phosphor blend, in accordance with embodiments of the present invention; and 
         FIG. 13  is a door, with an attached “EXIT” sign containing a persistent phosphor blend, in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a matrix  10  containing a blend of two types of phosphors: a persistent phosphor  12 , and another phosphor  14 , in accordance with embodiments of the current invention. In this illustration, the excitation has ended, and the persistent phosphor  12  particles are emitting stored energy as photons. These photons may escape the matrix  10 , as indicated generally by reference numeral  16 , and be visible as a blue or green luminescence. Alternatively, the emitted photons may be absorbed by particles of another phosphor  14 , as represented by reference numeral  18 , which then release the captured energy as longer wavelength photons  20 . The visible combination of the two types of photons  16 ,  20  emitted from the matrix  10  results in a different color for the luminescence from the persistent phosphor blend, as compared to the luminescence from the persistent phosphor by itself. For example, if the longer wavelength photons  20  are yellow and the shorter wavelength photons  16  are blue, the overall structure may appear to have a white luminescence. Those skilled in the art will recognize that the phosphor blend is not limited to one persistent phosphor  12  and one other phosphor  14 . Indeed, the blend may comprise two or more persistent phosphors in combination with two or more other phosphors, depending on the desired persistence time and emission color. In this embodiment of the current invention, the phosphor powders are blended prior to incorporation into a plastic matrix. In other embodiments, each phosphor may be separately incorporated into the matrix to form a final composition containing a phosphor blend. Those skilled in the art will recognize that the matrix is not limited to plastic, but may also encompass other materials such as paint, glass, or other organic or inorganic matrices, including such materials as transparent ceramics. 
     The phosphor blend may be made by any suitable mechanical method. In exemplary embodiments, such methods may include stirring or blending the powders in a high speed blender or a ribbon blender, or combining and pulverizing the powders in a bowl mill, hammer mill, or jet mill. Those skilled in the art will recognize that any number of other techniques may be used to make a well blended mixture of powders. 
       FIG. 2  shows an alternate configuration for a multiple phosphor structure, in accordance with embodiments of the current invention. In  FIG. 2 , a substrate  22  is coated with layers  24  containing a persistent phosphor  12 , and another phosphor  14 . After the excitation is removed, photons emitted from the persistent phosphor  12  may escape, as indicated by reference numeral  16 , and be visible as a blue or green luminescence. Alternatively, the photons emitted from the persistent phosphor  12  may be absorbed by the other phosphor  14 , which then emits the absorbed energy as longer wavelength photons  20 . As discussed with respect to  FIG. 1 , if the short wavelength photons  16  are blue and the longer wavelength photons  20  are yellow, the overall structure will appear to have a white luminescence. Furthermore, those skilled in the art will recognize that this structure may have more than one layer containing a persistent phosphor  12 , and more then one layer containing another phosphor  14 . The ordering of the layers may be controlled to tune the appearance of the emission. 
     In either of the embodiments discussed with respect to  FIGS. 1 and 2 , incorporation of the phosphors into a matrix  10  or layers  23  may be implemented using standard processing techniques for the matrix material chosen. For example, in embodiments of the current invention, the phosphors could be incorporated into a paint composition by mixing a powder blend into the base paint mixture, as if the phosphors were a dry pigment. In other embodiments, the phosphors could be stirred into a solvent to form a slurry prior to incorporation in the base paint mixture. 
     If the matrix is a polymer, incorporation of the phosphors may be done using such techniques as powder blending, compression molding, injection molding, sheet forming, film blowing, fiber forming, or any other plastics processing technique that may incorporate a dry powder blend into a plastic matrix. Those skilled in the art will recognize that the plastic matrix material used in embodiments of the current invention may be any thermoplastic material with sufficient translucency to allow light transfer through thin layers, including, but not limited to, polystyrene, high impact polystyrene (HIPS), styrene-butadiene copolymer, polycarbonate, polyethylene, polyurethane, polyethylene terephthalate (PET), polyethylene terephthalate glycol (PETG), and polypropylene, among others. Furthermore, the plastic matrix may also be a thermo-set material, including, but not limited to, silicone RTV resins, epoxy resins, polyesters, phenol-formaldehyde resins, and melamine, among others. In exemplary embodiments, the phosphors are incorporated into thermo-set resins by mixing the phosphor with one of the two reactant portions. 
     In order for energy to be transferred from the persistent phosphor  12  to the other phosphor  14 , the emission spectrum of the persistent phosphor  12  must have some overlap with the excitation spectrum of the other phosphor  14 . To illustrate this point,  FIGS. 3-7  show the excitation and emission spectra of phosphors that may be used in embodiments of the current invention. For example,  FIG. 3  shows the excitation  26  and emission  28  spectra for an exemplary persistent phosphor, Ca 0.90 Eu 0.005 Nd 0.03 Al 2 O 4 , used in embodiments of the current invention. The emission spectrum  28  has a maximum intensity at about 450 nm, with some intensity in the range of about 400 nm to about 550 nm. This emission intensity can be compared to the excitation, or absorbance, spectra for other exemplary phosphors that may be used in embodiments of the current invention, as represented by the solid lines in the spectra shown in  FIGS. 4-7 . 
       FIGS. 4 and 5  show the excitation and emission spectra for phosphors that have a strong absorbance around 450 nm.  FIG. 4  shows the excitation and emission spectra,  30  and  32 , respectively, of the phosphor (Ca,Sr) 8 (Mg,Zn)(SiO 4 ) 4 Cl 2 :Eu 2+ ,Mn 2+  (CASI), which may be used in exemplary embodiments of the current invention.  FIG. 5  shows the excitation and emission spectra,  34  and  36 , respectively, of the phosphor (Tb,Y,Lu,La,Gd) 3 (Al,Ga) 5 O 12 :Ce 3+  (TAG:Ce), which may be used in exemplary embodiments of the current invention. The high degree of overlap between the excitation spectra  30 ,  34  of these phosphors with the emission spectrum  28  of the exemplary persistent phosphor, discussed with respect to  FIG. 3 , indicates that efficient energy transfer would occur, and a significant portion of the light emitted may come from the CASI or TAG:Ce in blends with the persistent phosphor. 
     In contrast to the strong overlap between the spectra described above,  FIGS. 6 and 7  show the excitation and emission spectra for phosphors that have a weaker absorbance around 450 nm.  FIG. 6  shows the excitation and emission spectra,  38  and  40 , respectively, of the phosphor Sr 4 Al 14 O 25 :Eu 2+  (SAE), which may be used in exemplary embodiments of the current invention.  FIG. 7  shows the excitation and emission spectra,  42  and  44 , respectively, of the phosphor 3.5MgO-0.5MgF 2 —GeO 2 :Mn 4+  (MFG), which may be used in exemplary embodiments of the current invention. The absorbance of these phosphors at 450 nm is lower than that discussed with respect to  FIGS. 4 and 5 , with most of the absorbance at shorter wavelengths, as shown in the excitation spectra  38 ,  42 . This indicates that the efficiency of the energy transfer from the emission  28  of the exemplary persistent phosphor, discussed with respect to  FIG. 3 , may be somewhat lower. However, the existence of some overlap between the emission spectrum  28  and the excitation spectra  38 ,  42  indicates that they may absorb at least a portion of the photons  18  emitted by the persistent phosphor  12  and emit longer wavelength photons  16 , changing the perceived color of the blend. 
     As these examples illustrate, it is not necessary for the emission spectrum of the persistent phosphor to perfectly match the excitation spectrum of the other phosphor. Any energy emitted by the persistent phosphor  12  that is not absorbed by the other phosphor  14  will be emitted from the structure, and become part of the visible light mixture perceived by the viewer. 
     In embodiments of the current invention, the persistent phosphor may have the general formula A x-y-z Al 2 O 4 :Eu y , Nd z , where A may be Ba, Sr, Ca, or a combination of these metals, x is between about 0.75 and 1.3, y is between about 0.0005 and 0.1, and z is between about 0.0005 and 0.1. In other embodiments of the current invention, the persistent phosphor may have the general formula A x-y-z Al 2 O 4 :Eu y , Dy z , where A may be Sr, Ca, Ba, or a combination of these metals, x is between about 0.75 and 1.3, y is between about 0.0005 and 0.1, and z is between about 0.0005 and 0.1. The persistent phosphor compositions shown above are merely examples of phosphors that may be used in embodiments, and are not intended to be limiting. Those skilled in the art will recognize that other persistent phosphor compositions may be used while remaining within the scope of the current invention. 
     In certain embodiments of the present invention, the persistent phosphor  12  comprises a phosphor as described in U.S. patent application Ser. No. 11/954,814, herein incorporated by reference in its entirety. For example, one phosphor suitable for use in the persistent phosphor  12  has the general formula A x-y-z Al 2-m-n-o-p O 4 :Eu y , Dy z , B m , Zn n , Co o , Sc p  where A may be Ba, Sr, Ca, or a combination of these elements, x is between about 0.75 and 1.3, y is between about 0.0005 and 0.1, and z is between about 0.0005 and 0.1. Further m is between about 0.0005 and 0.30, n is between about 0.0005 and 0.10, o is between about 0 and 0.01 and p is between about 0 and 0.05. Phosphors made according to this formulation may have a green luminescence and a longer persistence than other types of phosphors. 
     Alternately, a phosphor suitable for use in the persistent phosphor  12  has the general formula A x-y-z Al 2-m-n-o-p O 4 :Eu y , Nd z , B m , Zn n , Co o , Sc p  where A may be Ba, Sr, Ca, or a combination of these elements, x is between about 0.75 and 1.3, y is between about 0.0005 and 0.1, and z is between about 0.0005 and 0.1. Further m is between about 0.0005 and 0.30, n is between about 0.0005 and 0.10, o is between about 0 and 0.01, and p is between about 0 and 0.05. Phosphors made according to this formulation may have a blue luminescence and a longer persistence than other types of phosphors. 
     For the purposes of describing the compositions above and throughout this description, the term “between” when describing a numerical range shall be interpreted mean a range that is inclusive of the described endpoints. 
     In embodiments of the current invention, the other phosphor  14  may be a blue emitter, a blue-green emitter, a green emitter, a yellow emitter, a yellow-orange emitter, an orange-red emitter, a red emitter, or a blend of phosphors having these emission colors, depending on the final color and persistence properties desired. In such embodiments, the one or more other phosphors chosen may have the general formulas: (Ca,Sr) 8 (Mg,Zn)(SiO 4 ) 4 Cl 2 :Eu 2+ ,Mn 2+  (CASI); (Tb,Y,Lu,La,Gd) 3 (Al,Ga) 5 O 12 :Ce 3+  (TAG:Ce); Sr 4 Al 14 O 25 :Eu 2+  (SAE); 3.5MgO-0.5MgF 2 —GeO 2 :Mn +  (MFG); (Ba,Sr,Ca) 5  (PO 4 ) 3  (Cl,F,OH):Eu 2+ ; (Ba,Sr,Ca)MgAl 10 O 17 :Eu 2+ ; (Ba,Sr,Ca)BPO 5 :Eu 2+ ; Sr 4 Al 14 O 25 :Eu 2+ ; BaAl 8 O 13 :Eu 2+ ; 2SrO.0.84P 2 O 5 .0.16B 2 O 3 :Eu 2+ ; MgWO 4 ; BaTiP 2 O 8 ; (Ba,Sr,Ca)MgAl 10 O 17 :Eu 2+ ,Mn 2+ ; (Ba,Sr,Ca) 5 (PO 4 ) 3 (Cl,F,OH):Sb 3+ ; LaPO 4 :Ce 3+ ,Tb 3+ ; CeMgAl 11 O 19 :Tb 3+ ; GdMgB 5 O 10 Ce 3+ ,Tb 3+ ,Mn 3+ ; GdMgB 5 O 10 :Ce 3+ ,Tb 3+ ; (Ba,Sr,Ca) 5 (PO 4 ) 3 (Cl,F,OH):Eu 2+ ,Mn 2+ ,Sb 3+ ; (Y,Gd,La,Lu,Sc) 2 O 3 :Eu 3+ ; (Y,Gd,La,In,Lu,Sc)BO 3 :Eu 3+ ; (Y,Gd,La)(Al,Ga)O 3 :Eu 3+ ; (Ba,Sr,Ca)(Y,Gd,La,Lu) 2 O 4 :Eu 3+ ; (Y,Gd)Al 3 B 4 O 12 :Eu 3+ ; monoclinic Gd 2 O 3 :Eu 3+ ; (Gd,Y) 4 (Al,Ga) 2 O 9 :Eu 3+ ; (Ca,Sr)(Gd,Y) 3 (Ge,Si)Al 3 O 9 :Eu 3+ ; (Sr,Mg) 3 (PO 4 ) 2 :Sn 2+ ; GdMgB 5 O 10 Ce 3+ ,Mn 2+  Those skilled in the art will recognize that the current invention is not limited to the phosphor compositions disclosed above and that other phosphors may be used, while remaining within the scope of the current invention. 
     The phosphors used in the current invention may be produced by mixing powders of oxygen-containing compounds of the relevant metals, and then firing the mixture under a reducing atmosphere. For example, the persistent phosphor: Ca 0.90 Eu 0.005 Nd 0.03 Al 2 O 4 , used in exemplary embodiments of the current invention, may be produced by mixing powders of oxygen-containing compounds of europium, neodymium, an alkaline-earth metal, and a group 13 metal, and then firing the mixture under a reducing atmosphere. After firing, the phosphor may be ball milled, or otherwise ground, to break up any conglomerates that may have formed during the firing procedure. 
     In exemplary embodiments, the oxygen-containing compounds may be oxides, carbonates, nitrates, sulfates, phosphates, citrates, carboxylates, and combinations of these compounds. In embodiments containing carboxylates, the carboxylates used may have from one to five carbon atoms, such as formates, ethanoates, proprionates, butyrates, and pentanoates. 
     In other embodiments, the mixture of starting materials for producing the phosphor also comprises a flux, such as boric acid, lithium tetraborate, lithium carbonate, hydrogen borate, an alkali hydroborate, or a mixture of these compounds. According to another embodiment of the present invention, the flux may be a halide compound, such as a fluoride, of europium, neodymium, the alkaline-earth metals, or the group 13 metals. The halide compound can comprise up to 10 percent, by weight, of the mixture. The flux may also be an alkali halide, such as lithium fluoride, sodium fluoride, or other alkali halides. In embodiments containing a flux, it may be desirable to wash the product with hot water to remove residual soluble impurities originating from the flux. 
     The oxygen containing compounds may be mixed together by any mechanical method. In exemplary embodiments, such methods may include stirring or blending the powders in a high speed blender or a ribbon blender, or combining and pulverizing the powders in a bowl mill, hammer mill, or jet mill. Those skilled in the art will recognize that any number of other techniques may be used to make a well blended mixture of powders. If the mixture is wet, it may be dried first before being fired. The drying may be carried out at ambient atmosphere or under a vacuum. 
     The mixture of oxide powders is fired in a reducing atmosphere at a temperature in a range from about 900° C. to about 1,700° C. for a time sufficient to convert the mixture to the phosphor. In exemplary embodiments the temperature may be in the range from about 1,000° C. to about 1,400° C. The firing may be conducted in a batch or continuous process, preferably with stirring or mixing to promote good gas-solid contact. The firing time required may range from about one minute to ten hours, depending on the amount of the oxide mixture being fired, the extent of contact between the solid and the gas of the atmosphere, and the degree of mixing while the mixture is fired or heated. The mixture may rapidly be brought to and held at the final temperature, or the mixture may be heated to the final temperature at a lower rate such as from about 10° C./minute to about 200° C./minute. In exemplary embodiments, the temperature is raised to the final temperature at rates of about 10° C./minute to about 100° C./minute. Those skilled in the art will recognize that the precise conditions needed for the synthesis of a particular phosphor composition will depend on the phosphor chosen and are within the ambit of the conditions above. 
     The firing is performed under a reducing atmosphere, which may include such compounds as hydrogen, carbon monoxide, ammonia, hydrazine, or a mixture of these compounds with an inert gas such as nitrogen, helium, argon, krypton, xenon. In one embodiment, a mixture of hydrogen and nitrogen containing hydrogen in an amount from about 0.5 volume percent to about 10 volume percent may be used as a reducing gas. In another embodiment, the reducing gas may be carbon monoxide, generated in situ in the firing chamber by the reaction between residual oxygen and carbon particles placed in the firing chamber. In yet another embodiment, the reducing atmosphere is generated by the decomposition of ammonia or hydrazine. In exemplary embodiments, after firing, the phosphor may be ball milled in a propanol slurry to break up aggregates that may have formed during firing. 
     In addition to the synthesis procedures discussed above, many of the phosphors that may be used in embodiments of the current invention may be commercially available. For example, both of the phosphors Sr 4 Al 14 O 25 :Eu 2+  (SAE) and 3.5MgO-0.5MgF 2 —GeO 2 :Mn 4+  (MFG), used in embodiments of the current invention, are commercially available. 
     Using the techniques discussed with regard to  FIGS. 1 and 2 , the phosphor blends of the current invention may be incorporated into numerous products for use in low light applications, including safety equipment, toys, input devices, signs, emergency exit indicators, instrument panel controls, electrical switches, circuit breaker switches, furniture, communication devices, wristwatch faces, numbers on a wristwatch face, clock faces, numbers on a clock face, kitchen ware, utensils, labels, car dashboard controls, stair treads, clothing, lamps, weapon sights, and displays. For example,  FIG. 8  shows the front faceplate  46  of a car radio with controls  48 . In embodiments of the current invention, a phosphor blend may either be incorporated in the faceplate  46  or in the controls  48 .  FIG. 9  shows a child&#39;s toy  50  with various decorations  52  attached to the outside. A phosphor blend may be incorporated into the structure of the toy  50  or into the decorations  52 , in accordance with embodiments of the current invention. 
     Furthermore, the long persistence and tunable color of the phosphor blends of the current invention make them useful for applications in emergency equipment. For example,  FIG. 10  shows a hard hat  54  with stickers  56  attached to the outside. In embodiments of the current invention, a phosphor blend may be incorporated into the body of the hard hat  54  or into the stickers  56 .  FIG. 11  shows an emergency exit sign  58  with applied lettering  60 . In embodiments of the current invention, a phosphor blend may be incorporated into the sign  58  or into the lettering  60 .  FIG. 12  illustrates an article of clothing  62  with letters  64  attached to the front. A phosphor blend may be incorporated either into the fabric of the article of clothing  62  or into the lettering  64 , in accordance with embodiments of the current invention. In  FIG. 13 , a door  66  has attached letters  68 , spelling the word “EXIT” in this example. In embodiments of the current invention, the lettering  68  may incorporate a phosphor blend. The letters may also be colored, so as to be visible at all times, or clear, so as to be visible only in low light conditions, when the glow from the phosphor blend can be seen. 
     The applications above are but a few examples of embodiments of the present invention and are not intended to limit its application to those uses. Those skilled in the art will recognize that a long lived persistent phosphor can be used in a large variety of applications beyond the ones listed above. 
     While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.