US3525864A - Lighting means excited by ultra-violet radiation - Google Patents

Description

VIOLET RADIATION Filed June 18, 1968 S. L. LEACH Aug. 25, 1970 LIGHTING MEANS EXCITED BY ULTRA- FIG. I

INVENTOR SAM L LEACH FIG. 2

ATTORNEY United States Patent 3,525,864 LIGHTING MEANS EXCITED BY ULTRA-VIOLET RADIATION Sam lL. Leach, Palos Verdes Peninsula, Calif., assignor to F. C. Griblin, Toronto, Ontario, Canada Filed June 18, 1968, Ser. No. 738,018 Int. Cl. H01j 1/54 US. Cl. ZEN-71 12 Claims ABSTRACT OF THE DISCLOSURE luminescent material itself is constructed in a novel fashion in the form of a thin-film sheet or Web in which inorganic solid phosphors are suspended in the medium of an organically activated polymer carrier or film. Adhesive means are coated on one surface of the thin-film luminescent web or sheet such that the web or sheet will adhere to and can be placed on virtually any surface which would be subjected to ultraviolet radiation, thus converting the entire surface into a source of light. Lighting efficiency is accordingly greatly improved over prior art constructions and the versatility of lighting shapes is virtually unlimited, since the construction of the gas discharge tube no longer dictates the dimensions of the light producing area.

This invention generally relates to fluorescent lighting means and particularly concerns the utilization of gas discharge radiation to excite luminescence of inorganic solids suspended in a medium of organically activated polymers, physically displaced from the normal relative position with respect to the excitation wave front radiation.

Normal fluorescent tubes of the the prior art generally consist of a glass-walled gas discharge tube effective to internally generate ultra-violet radiation. The glass discharge tube normally has an energy converting phosphor coating on the inside surface of the glass wall thereof which transforms the impinging ultraviolet radiation into visible light rays which then pass through the transparent glass of the tube. The tube glass, however, is normally not transparent to ultra-violet radiation and thus any excess ultra-violet radiation is blocked. Lighting fixtures and housings of the type commonly in use today normally contain two, three or more of these standard fluorescent tubes and such housings at best comprise light reflectors.

The efliciency of the above-described standard fluorescent lighting fixtures, although much higher than incandescent lighting arrangements, still remains quite poor. With a typical prior art fluorescent tube, for example, fully 78 watts of a 100 watt input excitation is converted into heat at the glass wall of the fluorescent tube while only 22 watts of the 100 watt input are actually converted into visible light. Under conditions of equilibrium in a standard fluorescent tube, about 60% of the 78 watts con verted into heat at the tube wall is removed by conduction type well-known to and convection and approximately 40% of this heat is removed by radiation. Yet, this heat removal ratio is largely dependent on the actual temperature of the fluorescence tube itself and on its placement in a given housing. Under normal conditions, the housing of the lighting fixture acts as a heat sink carrying off the heat produced at the wall of the fluorescent tube. Unless this heat sink is adequate, the temperature of the discharge tube gradually rises producing a number of deleterious effects as will be discussed below.

Standard fluorescent tube utilizes mercury vapor as the gas, and any significant heat accumulation in the tube will shift the mercury resonance line away from the 2537A normal output, such shifting of course, seriously affecting the quantum output efliciency. Heat accumulation in the tube has a further degrading effect on most luminescing phosphors utilized inside the fluorescent tube. Heat accumulation also places an additional burden on the air conditioning ebuipment as well as the ballast circuitry used to drive the tube. In short, gradual heat accumulation such as is easily produced in the standard design fluorescent lamps, is a very considerable hazard which leads to lamp failure, ballast failure, other electrical hazards, and/ or an overloaded air-conditioning systent. The physical configuration of prior art standard fluorescent tubes, and particularly the placement of the phosphor particles on the inside Wall of the tube, leads to still further disadvantages. The phosphor particles, when placed on the inside surface of the discharge tube, are in direct contact with the hot mercury vapors therein, these mercury vapors having a strongly degrading effect on the phosphor particles, resulting in a further loss of efliciency of the lighting unit and a further lessening of the usable life time of the lighting unit.

Prior art fluorescent lighting configuration suffer still another disadvantage in that since the phosphor particles are placed on the inside surface of the discharge tube, the shape and dimensions of the light emitting area is confined to the shape of the tube itself, and, in any event, can be no larger than the area of the inside surface of the discharge tube. Accordingly, if the lighting engineer or other user of standard fluorescent tubes is desirous of providing a complete area of light such as a wall, special light distributing panels must be utilized which, in addition to adding to the expense of a lighting unit, further decrease the lighting efliciency.

Thus, an entirely new concept or approach to fluorescent lighting units is indicated, and it is here that the subject invention comes to the fore. It is a primary object of the subject invention to provide a new concept in fluorescent lighting which eliminates most if not all of the disadvantages discussed above with respect to the prior-art configurations. Further, and more specific, though equally important objects of the subject invention are as follows:

(a) To provide a novel fluorescent lighting means wherein the luminescent phosphor material is physically disposed at a location wherein the degrading effects of the mercury vapor within the gas discharge tube are eliminated;

(b) To provide a novel fluorescent lighting means wherein the generation of harmful heat is reduced to a minimum;

(c) To provide a novel fluorescent lighting means having an extremely high lighting efficiency;

(d) To provide a novel fluorescent lighting means offering an extremely long useful life;

(e) To provide a novel fluorescent lighting means capable of making virtually any surface radiate visible li ht.

The above objects as well as other objects, features, and advantages are implemented by the subject invention wherein gas discharge radiation, such as ultra-violet radiation, is utilized to excite luminescence of inorganic phosphor solids which are suspended in a medium or film of organically activated polymers, the film containing the phosphor particles being physically displaced from the normal relative position with respect to the excitation wave front radiation. In the preferred embodiment of the subject invention, the luminescent material is constructed as a thin-'film web or sheet which is quite pliable. Adhesive bonding means are provided on one surface of the thin film web or sheet such that the web or sheet can be placed upon virtually any surface, the entire surface thus emitting light. For example, and in accordance with the invention, the thin-film luminescent web or sheet is placed on the inside surface or surfaces of a standard lighting fixture of the type now in use that utilize standard fluorescent tubes. In this instance, however, the standard fluorescent tubes would be replaced with a mercury discharge tube having no phosphor coating on the inside walls thereof and further having walls constructed of a glass material which would pass ultra-violet radiation into the housing, the ultra-violet radiation being converted by the thin-film luminescent web or sheet into visible light.

The actual physical surface or configuration upon which the thin-film luminescent web or sheet is placed is by no means critical to the subject invention, as all that is required to carry the subject invention into effect is the provision of any surface upon which the thin-film web or sheet can adhere, and the further provision of any external source of ultra-violet radiation impinging upon the surface.

The subject invention further comprises both a novel construction of the thin-film luminescent sheet utilized as well as a method for making the same.

The subject invention will be better understood from the following detailed description of preferred embodiments thereof, such detailed description making reference to the appended drawings, wherein:

FIG. 1 is a perspective view of a lighting fixture utilizing prior-art fluorescent tubes;

FIG. 2 is a perspective view of a preferred structural embodiment of the invention disclosing the lighting fixture of FIG. 1 with the prior-art fluorescent tubes being replaced by ultra-violet radiation generating mercury vapor discharge tubes, and with a thin film of luminescent material being provided on the inner surface of the actual housing;

FIG. 3 is a perspective view of a further embodiment of the subject invention wherein a thin-film luminescent web or sheet is placed on a typical back-lighted sign in the path of impinging ultra-violet radiation;

FIG. 4 is a perspective view of a roll or web of the thin film luminescent material of the subject invention; and,

FIG. 5 is an exploded view of the structural details of the thin-film luminescent web or sheet of FIG. 4 contained within the circle, portions of the exploded view being shown broken-away and in elevational section for illustrative clarity.

Describing now the drawings, attention is particularly directed to FIG. 1 thereof wherein a typical, prior-art fluorescent lighting unit is disclosed. The lighting unit generally comprises a housing 2 having a top wall 4 and side walls 6 generally constructed in a light-reflecting configuration or shape. A plurality of fluorescent tubes 8 are provided in the housing in well-known fashion. The bottom portion of the housing is provided with a prismatic surface 10, surface 10 either being transparent to visible light or at least translucent thereto.

As shown, each fluorescent tube 8 comprises an elongated glass tube 12 having non-illustrated electrodes at both ends thereof. The elongated tube 12 is hollow and is normally filled with a gas such as mercury. A phosphor coating 14 is provided on the inside surface of the glass tube 12. Now, when the fluorescent tube '8 is started, the electrical discharge products ultra-violet radiation through the medium of the mercury vapor in well-known fashion, the ultra-violet radiation normally being centered about a peak of 2537 angstroms comprising the well-known mercury resonance line. The ultra-violet radiation generated within the tube 12 impinges upon the phosphor coating 14 on the inside surface thereof, and the phosphor coating 14, also in well-known fashion, serves to convert the impinging ultra-violet radiation to visible light, the conversion process being undertaken with certain wellknown efficiency levels. Accordingly, quantums of visible light schematically illustrated by the arrowed lines 16 are emitted through the wall of the glass tube 12 and either pass directly through the prismatic surface 10 of the lighting unit 2 or are variously reflected from the upper surface 4 of the walls '6 of the housing.

As discussed in some detail above, however, this priorart construction suffers a multiplicity of disadvantages. A very large quantity of heat is generated by each of the fluorescent tubes 8 at the walls of the respective glass tubes 12 and therefore, at the location of the phosphor coating 14. This heat has a quite degrading effect both on the quality and life of the phosphor particles of the coating 14 and thus on the quality of light emitted from each tube 8. Further, the light producing area and shape of such a prior-art construction is limited to the area and shape of the inside surface of the glass tube 12, since it is only upon this surface that a phosphor coating is applied. Accordingly, the efficiency of this prior-art construction with respect to light actually generated from a given amount of input power is surprisingly low. Since the phosphor coating 14 placed on the inside surface of the glass tube 12 is in direct contact with the hot mercury vapors of the tube, a certain degradation of the phosphor particles takes place due to mercury absorption, further reducing the efficiency and life of the tube.

Referring now to FIG. 2, a preferred structural embodiment of the subject inventive lighting means is disclosed as applied to a more or less standard light housing. In a similar fashion as the lighting fixture of FIG. 1, the lighting fixture of FIG. 2 also comprises a housing 18 having a top wall 20 and side walls 22 constructed in a light reflecting configuration. A prismatic surface 24 is again provided at the bottom opening of the housing 18, the prismatic surface 24 again being transparent or at least translucent to visible light. A plurality of discharge tubes 26 are also provided in spaced relation Within the lighting housing 18. However, the discharge tubes 26 are not standard fluorescent tubes as was the case in FIG. I, although the outward physical construction of the tubes is very similar thereto. The discharge tubes each contain an elongated hollow glass tube 28, the glass tube 28 again containing mercury vapors. However, no phosphor coating is provided on the inside surface or walls of the glass tubes 28 and further, the glass tube 28 are constructed of a material such as Vycor, or Pyrex or a suitable lime glass such that the ultra-violet radiation produced by the mercury gas discharge therein passes through the wall of the tube. As will be recalled, this structure is contrasted to the fluorescent tube structure of the prior art which utilizes a glass tube which is transparent to visible light but which has very low transmission characteristics for ultra-violet radiation.

A thin-film of luminescent material 27 is placed on the inside surface of the lighting housing 18, both on the inside surface of the upper walls 20 and side walls 22 thereof, as well as upon the inside surface of the prismatic cover or plate 24. The detailed physical construction of the luminescent film 27 will be discussed hereinbelow.

The mechanics of the production of light utilizing the inventive embodiment of FIG. 2 in such that the gas discharge within the tubes 26 produces ultra-violet radiation which passes through the glass walls 28 of each tube 26 as schematically depicted by the lines 30. Each quantum of ultra-violet radiation strikes an area of the luminescent thin-film coating 27 wherein the luminescent coating 27 converts the incoming quantum of ultra-violet energy 30 into light rays designated by the arrowed lines 32.

The configuration of the instant invention as disclosed in FIG. 2 has manifold advantages over that of the prior art as represented by FIG. 1. For one, the thin-film luminescent layer 27 no longer is on the inside surface of the tube 28 and accordingly, is no longer in contact with the hot mercury vapors therein. Thus, with the inventive embodiment, there is no problem whatsoever of absorption of mercury vapors by the phosphor. Additionally because the phosphor particles are removed from the walls 28 of the tubes, the phosphor particles are not subjected to a large amount of heat. Rather, the heat that is generated in the phosphor particles by virtue of the conversion process of the ultra-violet energy into radiant visible light, is generated over a much greater area and thus, each individual phosphor particle is subjected only to a low level of heat. Further, and as should be readily apparent, the actual light producing area of the novel lamp configuration of PEG. 2 is greatly increased over that of the figure of FIG. 1 since the light producing area is not constrained to that of the inside surface of the glass wall of each fluorescent tube, but rather can comprise the entire inside surface of the actual housing 18. Accordingly, the inventive structure of FIG. 2 exhibits a much higher lighting efficiency for the same input power than would be exhibited by the structure of FIG. 1.

It is to be noted that the thin-film luminescent sheet or web 27 is also transparent to light, and it is for this reason that the web 27 can also be placed on the bottom surface, that is on the prismatic plate 24 of the lighting housing 18, as all visible light generated by the other areas within the housing 18 will be transmitted through the thin-film phosphors located on the prismatic plate 24 and then radiated into the room. Accordingly, even greater lighting Cl'fiClEIICY is effected.

As mentioned above, the exact physical configuration of the placement of the thin-film luminescent web or sheet 27 with respect to the source of the ultra-violet radiation 26 forms no critical part of the subject invention, since all that is required is that the luminescent film be placed upon any surface in the path of ultra-violet radiation.

A further example of a structural configuration in ac- The thin-film luminescent web or sheet 27 is adhesively bonded or applied to the back of a transparent or at least translucent panel or plate 36 having a plurality of letters forming a sign generally designed 38 on one surface thereof. A source of ultra-violet radiation again generally designated 26 is provided in the back-lighted sign arrangement and is seen to comprise the elongated hollow glass tube 28 filled with a gas such as mercury and having electrodes 34 at both ends thereof. It is contemplated that the tube 26 be of the hot cathode type and have a relatively low mercury vapor pressure as efliciency of ultra-violet production has been found to decrease as the vapor pressure increases. The wall 28 of the tube 26 is, as described above, constructed of a glass which would be substantially transparent to ultra-violet radiation. Well-known glasses that fit this purpose are quartz, Vycor, Pyrex or the like. Yet, it is generally known that the main resonance line of mercury ultra-violet radiation, i.e., 2537 angstroms, is highly productive of ozone which, of course, is an undesirable attribute in this utilization. Accordingly, the invention contemplates the use of a lime glass instead of quartz, Vycor or Pyrex since such a lime glass would have its major transparency in the range of 3000 to 3200 angstroms, that is, the upper end of the ultra-violet radiation spectrum and at a range which is not highly productive of undesirable ozone.

Accordingly, the ultra-violet radiation lines 30 schematically depicted will be assumed to comprise wave lengths in the range of 3000 to 3200 angstroms and the phosphor particles thus used in the thin-film luminescent sheet or web 27 would accordingly be selected to be particularly responsive to this range of ultra-violet radiation. As the ultra-violet radiation 30 strikes the phosphor particles contained within the thin-film luminescent web or screen 27, the ultra-violet energy is converted into visible light which would then pass through the adhesive coating bonding thin-film 27 to the sign plate 36 to produce a backlighting effect for the sign letters 38. Harmful ultra-violet radiation would not be transmitted through the sign plate 36 due to the construction of the material thereof and also due to the fact that the adhesive contemplated for use, although transparent to visible light, does not have a high transmission efficiency for energy in the ultra-violet range.

As should readily be apparent, the possibilities of different environments and different configurations in which the thin-film luminescent web or sheet 27 could be utilized is almost limitless. Entire walls or ceiling or a room or building could quite easily be lighted in a highly efficient and safe manner merely by placing the thin-film luminescent web or sheet thereon and providing an external ultra-violet source.

FIGS. 4 and 5 of the drawings depict a particular preferred construction of the thin film luminescent Web or sheet of the subject invention. As shown in FIG. 4, the thin-film sheet is contemplated to be quite flexible to facilitate handling and could comprise a roll that could be unwound to any suitable length and cut by the user to any suitable size in accordance with his needs. FIG. 5 is an exploded, partially in section view of the portion of the thin-film web or sheet contained within the circle of FIG. 4.

The preferred construction of the thin-film web or sheet is seen to comprise a plurality of layers, the overall sheet having a thickness of approximately 5 mils. A central layer 40 is provided and preferably comprises a polyvinyl alcohol film utilized as a carrier for phosphor particles 42 embedded therein. The polyvinyl alcohol film is utilized as a carrier material since it is transparent to both visible light as well as ultraviolet radiation and, more importantly, is structurally unaifected by the impingement of ultraviolet radiation. The thickness of the central layer 40 is contemplated to be within the range of 10 to microns. Another suitable film material for the phosphor carrier would be a polyvinyl fluoride such as Tedlar although polyvinyl alcohol is preferred.

The polyvinyl alcohol film carrier for the phosphors, although having the properties of being transparent to visible and ultra-violet radiation as well as being unaffected by ultra-violet radiation, has the characteristic that it is easily dissolvable by water. Accordingly, a weatherproof coating is contemplated to be preferably applied to both sides of the polyvinyl alcohol carrier 40. As shown in FIG. 5, coatings 44 and 46 are placed on either side of the polyvinyl alcohol base and are contemplated to comprise polyvinyl fluoride. The polyvinyl fluoride coatings 44 and 46 serve to protect the polyvinyl alcohol carrier film 40 and specifically, to weather-proof the same. Polyvinyl fluoride is also transparent to both visible light as well as to ultra-violet radiation and is also unaffected by the impinging ultra-violet radiation. The thickness of the polyvinyl fluoride coatings 44 and 46 is contemplated to be within the range of 1 to 3 mils, although this thickness could be varied as desired.

In one construction of the subject inventive thin-film luminescent web or sheet, a further layer designated 48 is provided between the polyvinyl alcohol phosphor carrier 40 and the upper polyvinyl fluoride weather-proofing layer 44. This additional coating 48 is contemplated to comprise a vacuum deposited aluminum layer having a very small thic-kness in the range of 200 to 500 angstrom units. The thin aluminum film is provided, if desired, as an extremely good reflector of ultra-violet radiation impinging thereon. Accordingly, ultra-violet radiation passing through the polyvinyl fluoride coating 46 into the polyvinyl alcohol phosphor carrier would excite some of the phosphor particles 42 and would be reflected by the thin aluminum coating back through the polyvinyl alcohol to excite further phosphor particles 42. Thus, the provision of the aluminum film also serves to increase the efficiency of the light output, although, as stated above, the provision of this additional layer is not critical to the invention. Magnesium oxide and titanium dioxide could also be used instead of aluminum if desired.

The thin-film luminescent Web or sheet is, as stated above, contemplated to be placed upon virtually any surface which would intercept ultra-violet radiation. For this reason, an adhesive coating generally designated 50 is provided on one side of the luminescent web or sheet. The adhesive coating 50 preferably comprises an acrylic adhesive, the acrylic readily passing visible light but having a low transmission efiiciency for ultra-violet radiation. The particular adhesive utilized need only be one that is transparent to visible light if such characteristic is required in a given environment of use and be adherable lucent to visible light, said thin sheet of luminescent mato the polyvinyl fluoride layer 44. In this regard, many silicones and epoxies as well as acrylics could be utilized. One particularly suitable form of adhesive for use as the adhesive coating is that of the micro-encapsulated type which would be released only when the user pressed the 45 luminescent film upon the desired surface.

There are a number of different methods which may be utilized in accordance with the subject invention to produce the thin-film luminescent web or sheet of FIGS.

4 and 5. In this regard, and as one alternative method, the polyvinyl alcohol carrier or film 40 would first be produced by casting polyvinyl alcohol carrying the phosphor particles 42 on a polished drum, for example, in accordance with well-known techniques. After the sheet of polyvinyl alcohol has thus been produced, such sheet could then be laminated to the polyvinyl fluoride films which,

as stated above, are commercially available under the trade name Tedlar, by use of any suitable laminating ad hesive as discussed above. The polyvinyl fluoride layer itself is produced preferably through an extruding process or a casting process.

If desired, the aluminum film or layer 48 could be vacuum-deposited on the polyvinyl fluoride film 44 prior to the lamination of the film 44 to the polyvinyl alcohol carrier 40. The acrylic adhesive layer 50, on the other 5 hand, may be applied to the surface of the polyvinyl fluoride 44 via a roll-coating process although the acrylic adhesive layer could also be sprayed on, if desired.

An alternative method of making the thin-film luminescent Web of the subject invention resides in first providing a polyvinyl fluoride layer 44 and then spraying the polyvinyl alcohol film carrier 40 containing the phosphor particles 42 onto the polyvinyl fluoride film. Alternatively, the polyvinyl alcohol carrier could be roll-coated onto the poyvinyl fluoride layer.

Many different materials can be utilized for the phosphor particles 42 carried in the polyvinyl alcohol base or film 40. Such material can be selected for its color producing properties, and its sensitivity to the impinging ultraviolet radiation, as well as for other characteristics. Table 1 is illustrative of materials found particularly suitable for use with the subject invention, although it is to be expressly understood that the scope of the subject invention is by no means to be limited thereby.

TABLE I Exciting Sensitivity Emittod Emitted range, peak, range, peak,

Phosphor Color angstroms angstroms angstroms angstroms Calcium tungstate Blue 2, 200-3, 000 2, 720 3, 100-7, 000 4, 400 Magnesium tungstate Blue-white 2, 200-3, 200 2, 850 3, 600-7, 200 4, 800 Cadmium silicate- Yellow-pink 2, 200-3, 200 2, 400 4, 800-7, 400 5, 950 Cadmium borate..- Pink 2, 200-3, 600 2, 500 5, 2004, 500 (5,100 Calcium phosphate. Blue (ultra 2, 200-3, 200 2, 500-2, 800 3, 200-4, 500 3, 005 Halophesphate e 1, 800-3, 200 2, 3, 600-7, 500 5, 800

achieved.

What is claimed is:

of said housing means.

As should now be apparent, the objects initially set forth at the outset of this specification have been successfully 1. A lighting apparatus comprising a source of ultraviolet radiation and a thin, pliable sheet of luminescent material comprising an organically activated polymer carrier in which inorganic solid phosphors are suspended, said thin sheet being physically displaced from said source of ultra-violet radiation.

2. A lighting apparatus as defined in claim 1 further including housing means, and wherein said source of ultra-violet radiation comprises a gas discharge tube means disposed in said housing means, said thin sheet of luminescent material being disposed on the inside surface 3. A lighting apparatus as defined in claim 2, wherein said housing means defines a substantially closed compartment having at least one surface that is at least transterial being further disposed on the inside of said housing means upon said at least one surface.

4. A lighting apparatus as defined in claim 1, further including a plate of at least translucent material, and wherein said thin sheet of luminescent material is disposed on said plate in the path of ultra-violet radiation from said source of ultra-violet radiation.

5. A lighting apparatus comprising a source of ultraviolet radiation, a plate of at least translucent material physically displaced from said source of ultra-violet radiation, and a thin sheet of luminescent material disposed on said plate in the path of ultra-violet radiation from said source of ultra-violet radiation, and wherein said thin sheet of luminescent material comprises a layer of polyvinyl alcohol film containing phosphor particles, protecting layers of polyvinyl fluoride disposed on either side of said polyvinyl alcohol film, and an adhesive layer disposed on one of said polyvinyl fluoride layers to bond said thin sheet of luminescent material to said plate.

6. A lighting apparatus as defined in claim 5, wherein said source of ultra-violet radiation comprises a gas dis has a thickness in the range of 1 to 3 mils.

charge tube having glass walls substantially transparent to at least selected frequencies of ultra-violet radiation.

7. A thin sheet of luminescent material comprising a polyvinyl alcohol film having phosphor particles therein, a layer of polyvinyl fluoride on at least one side of said polyvinyl alcohol film, and an adhesive coating on said layer of polyvinyl fluoride.

8. A sheet as defined in claim 7 including layers of polyvinyl fluoride on both sides of said polyvinyl alcohol 9. A sheet as defined in claim 8, wherein said polyvinyl alcohol film has a thickness in the range of 10 to microns, and wherein each said layer of polyvinyl fluoride 9 10 10. A sheet as defined in claim 9 further including a References Cited layer of aluminum having a thickness in the range of 200 UNITED STATES PATENTS to 500 angstroms disposed between said layer of polyvinyl fluoride u on which said adhesive la er is dis osed and 2,430,232 11/1947 Lynch 250*71X said polyviiiyl alcohol film, y P 2,879,614 3/ 1959 Baldanza 250-71 X 11. A sheet as defined in claim 9, wherein said ad- 5 290L647 8/1959 Thomas et a1 313 109 X hesive coating comprises a coating oi micro-encapsulated RALPH G NILSON Primary Examiner acrylic adhesive releasable 1n response to the application f pressure, D, L. WILLIS, Assistant Examiner 12. A film of polyvinyl fluoride having a thickness in 10 the range of 10 to 100 microns and having phosphor particles therein. 250-77, 80; 313-109

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