A Planar Fluorescent and Electroluminescent Lamp Having One Or More Chambers

Europaisches Patentamt European Patent Office 0 Publication number: 0 550 047 A2 Office europeen des brevets

EUROPEAN PATENT APPLICATION

0 Application number: 92122065.3 int. CIA H01J 61/70, H05B 33/12, H05B 33/10 0 Date of filing: 28.12.92

0 Priority: 30.12.91 US 816034 0 Applicant: Winsor, Mark D. 311 Capital Way North 0 Date of publication of application: Olympia, Washington 98501 (US) 07.07.93 Bulletin 93/27 0 Inventor: Winsor, Mark D. 0 Designated Contracting States: 311 Capital Way North BE DE FR GB IT NL Olympia, Washington 98501 (US)

0 Representative: Patentanwalte Grunecker, Kinkeldey, Stockmair & Partner Maximilianstrasse 58 W-8000 Munchen 22 (DE)

0 A planar fluorescent and electroluminescent lamp having one or more chambers.

0 A planar fluorescent and electroluminescent lamp having two pairs of electrodes. Planar electrodes on an outer surface of the lamp create a plasma arc by capacitive coupling. The planar electrodes also cause embedded phosphor to emit light on the electroluminescent phenomena. In one embodiment, a second chamber is on top of the first chamber and light passes from a primary chamber through the second chamber, and is emitted by the lamp.

CM < FIGURE 1

Rank Xerox (UK) Business Services (3. 10/3.6/3.3. 1) 1 EP 0 550 047 A2 2

Field of the Invention weight and size of the light-directing components, when added to the bulb volume, result in a bulky This invention relates to planar fluorescent package usually exceeding one inch in thickness. lamps, and more particularly, to a planar fluores- Furthermore, miniature fluorescent tubes are inher- cent lamp having two pairs of electrodes and which 5 ently very fragile and more costly to produce than emits light by both fluorescent and electrolumines- the large-sized commercial counterparts. Despite cent phenomena. the significant drawbacks of fluorescent tubes, they are often chosen to provide the backlighting re- Background of the Invention quired in today's LCD displays or aircraft cockpits. io Planar fluorescent lamps are well known in the Thin, planar, and relatively large area light art. Envelopes are formed by sealing molded glass sources are needed in many applications. Bac- pieces together along their edges. Some prior art klights must often be provided for LCDs to make planar lamps include labyrinthine discharge chan- them readable in all environments. Thin backlights nels. See, for example, U.S. Patent Nos. 3,508,103; for LCDs are desired to preserve as much as w 3,646,383; and 3,047,763. Because of the complex possible the LCDs' traditional strengths of thin pro- glass molding and stamped metal housings, the file, low cost, and sunlight readability while permit- prior art fluorescent flat panels are difficult to man- ting readability at numerous angles and in low light ufacture and expensive. These lamps had non- conditions. Lamps for use in the avionics environ- uniform light intensity ouput across the lamp and ment, such as airplane cockpits, are preferably as 20 were often too thick and too inefficient for portable lightweight, thin, and low power as possible. computer screens using batteries. Many demanding challenges exist for engineer- One flat fluorescent lamp, as shown in U.S. ing a thin, planar source of uniform light. If in- Patent No. 4,851,734 ('734), utilizes transparent candescent lamps or LEDs are used as the light electrodes on planar glass plates. Unfortunately, source, the optics for dispersing and diffusing light 25 the narrow gap between the plates constricts the from the multiple point sources to the planar view- length of the positive column, resulting in low ul- ing surface must be provided to avoid local bright traviolet radiation and low illumination. Further, in or dim spots. Additionally, provision must be made the embodiment with the electrodes on the outside, to dissipate the heat generated by the incandes- the usable power is reduced because the glass cent or LEDs, or alternatively, to utilize only high- 30 must be sufficiently thick to withstand normal at- temperature materials for LCDs. mospheric implosion when the chamber is vacuum- Recent developments in large LED arrays have evacuated. In the embodiment shown in Figure 4 of made them appear suitable for use in flat panel the '734 patent, the electrodes are directly exposed displays. However, arrayed LEDs still consume rel- to each other, with no insulating layer in-between, atively high amounts of power and require careful 35 severely limiting their practical use. Further, the attention to avoid the thermal effects from the unprotected transparent thin-film electrodes sputter LEDs. Furthermore, the problems of diffusing the away very quickly from ionic bombardment within a light emitted by the LED arrays must still be over- fluorescent tube. come as well as the spectral limitations inherent in A flat fluorescent lamp designed for LCD bac- an LED. 40 klighting is disclosed in U.S. Patent No. 4,767,965. The introduction, some thirty years ago of elec- Two parallel glass plates are supported by a troluminescent lamps, is a possible choice for a framepiece, including two cold cathode electrodes planar lamp. Unfortunately, electroluminescent placed opposite each other. A plasma discharge at lamps suffer from a short life at high frequencies the optimum mercury vapor pressure ranges con- and have low ultimate brightness at about one 45 ducts current as an arc and not in a planar fashion. lumen per watt. Nevertheless, the electrolumines- This results in a discharge which is nonuniform in cent lamp is sometimes selected as a solution to the planar chamber and brightness variations as low light display outputs, despite its spectral limita- great as 60% across the face of the lamp. In tions and intrinsic problems with life expectancy. addition, these parallel glass plates must be thick Another choice for generating light for a display 50 to avoid atmospheric implosion when a vacuum is is fluorescent technology. Fluorescent lamps have drawn in the envelope. the advantage of being relatively efficient and ca- Some problems of the prior art have been pable of generating sufficiently bright light. Min- attempted to be overcome by using a combination iature fluorescent lights made for backlights are of a hot cathode surrounded by a cold cathode in typically tubular structures having selected diam- 55 tubular fluorescent lamps as taught in U.S. Patent eters and lengths. Backlighting schemes using tu- Nos. 4,117,374 and 3,883,764. These lamps are bular fluorescent lamps generally require a reflector designed for large currents and are opaque to and a diffuser to distribute the light. The additional visible light, thus exhibiting nonuniform dark areas

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at the lamp envelope ends. the upper chamber. In one embodiment, the upper A need remains for a planar lamp that is thin in chamber or chambers are vacuumevacuated and cross-section and uniformly bright across the entire phosphor-lined to emit light when ultraviolet radi- face thereof. ation from the primary chamber impinges thereon. 5 The top glass of the lower chamber is thin to Summary of the Invention permit the ultraviolet mercury radiation to pass through it and enter the upper chamber as well. According to principles of the present inven- Alternatively, the upper chamber is filled with a tion, a planar fluorescent lamp includes a pair of cooling liquid to maintain the overall temperature of planar electrodes on an inside surface of the vacu- io the lamp at a selected value. Still alternatively, the um chamber. At least one of the planar electrodes upper chamber is open to the atmosphere, and air- is transparent to visible light. A thin dielectric layer filled to permit cooling air to pass therethrough and completely covers each of the electrodes within the evenly disburse the light prior to output from the chamber. The chamber is evacuated and refilled lamp. with an inert gas to a selected pressure. Mercury is The phosphor layer of the emission chamber is vapor is placed therein to permit fluorescent illu- preferably very thin and is crystallized into the mination from the phosphor layers. The dielectric glass dielectric layer itself. The glass dielectric layers capacitively couple the high frequency pow- layer is preferably lead-free so as to not degrade er source across the low-pressure chamber for the phosphor. A glass is selected which has a creating a plasma which emits ultraviolet radiation. 20 reflow temperature of approximately 600 °C and In one embodiment, two pairs of electrodes are preferably well below the 700 ° C at which phosphor provided: one pair of planar electrodes and one begins to degrade. pair of internal cathodes. Each pair of electrodes is The phosphor is applied to the glass in a slurry individually driven by a different power source. The and the combination heated until the glass be- power sources are preferably at different frequen- 25 comes somewhat sticky and wet, as would occur at cies. Alternatively, the power sources are at the approximately the reflow temperature of the glass. same frequency, but out of phase with each other The glass with the phosphor coating in place is by exactly 90° to ensure the electrical separation then cooled to form phosphor crystals embedded of each power source. into the glass layer itself. In the final product, In one embodiment, the chamber has walls 30 portions of phosphor crystals are embedded in and therein to provide a serpentine, elongated dis- surrounded by glass and portions of the phosphor charge column. It is generally known that the crystals are exposed to the mercury chamber itself. length of the discharge path is one of the factors in Light efficiently passes directly from the phosphor determining the light output, and the longer the crystals into the glass for emission while minimiz- discharge path, the greater the output and the 35 ing the reflectance of the light from the phosphor luminous efficiency, according to Pascend's law. It glass interface. In addition, the light is also emitted is also known that in low-pressure, positive-column based on electroluminescence directly from the lamps with phosphors excited by mercury radi- phosphors and into the glass. ation, it is possible to obtain improved efficiency and greater output when the discharge column is 40 Brief Description of the Drawings constructed out of round. Accordingly, the serpentine, thin-film cavity is segregated by planar wall Figure 1 is a cross-sectional view of a planar members with an electrode at each end of the lamp according to one embodiment of the inven- serpentine chamber. tion. In one embodiment, the phosphor includes a 45 Figure 2 is a top cross-sectional view of an combination of fluorescent phosphors and elec- alternative embodiment having a serpentine dis- troluminescent phosphors. The electroluminescent charge chamber. phosphors emit light directly into the glass plates Figure 3 is a side cross-sectional view taken when an electric field is applied. The light emitted along lines 3-3 of Figure 2. by the electroluminescent phosphors is generally 50 Figure 4 is an isometric view of the alternative uniform across the lamp. The light emitted by the embodiment of Figure 2. fluorescent phosphors provides the desired high Figure 5 is a side cross-sectional view of an brightness. alternative embodiment of the invention having two The planar lamp may include a total of two, chambers and ground electrodes. three, or more chambers, if desired, according to 55 Figure 6 is an enlarged view of a region of the one embodiment. The upper chamber is positioned cross-sectional view of Figure 5. on top of the lower chamber such that any light Figure 7 is a cross-sectional view of an alter- exiting from the lower chamber must pass through native embodiment of a serpentine, multi-chamber

3 5 EP 0 550 047 A2 6 lamp. layers 30 and 32 to emit viewable, white light Figure 8 is a top plan view of the lamp of according to known fluorescent lamp phenomena. Figure 7. The phosphor layers 30 and 32 on the inside of Figure 9 is a top plan view of a combined hot the chamber 12 convert the ultraviolet light created and cold cathode. 5 by these two power sources into longer, visible Figure 10 is an isometric view of the combined light at high efficiencies. Light is emitted from both cathode of Figure 9. the top and bottom of the lamp 10 when all planar Figure 11 is an isometric view of an alternative electrodes are transparent. Alternatively, light is embodiment of a cold cathode. reflected from the bottom and is emitted only from io the top, as shown in other embodiments herein. Detailed Description of the Invention A pair of internal cathodes 34 and 36 are also positioned within the chamber 12. The internal Figure 1 illustrates a lamp 10 having a cham- cathodes may be of the extended bar type shown ber 12. The chamber 12 is formed by the sealed in U.S. Patent No. 4,767,965, or the shorter type as enclosure of a pair of planar plates top plate 14 15 shown in U.S. Patent No. 3,508,103, both incor- and bottom plate 16 and a sidewall structure 17 porated herein by reference. Preferably, the inter- having a pair of sidewalls 18 and 20. A pair of nal cathodes 34 and 36 are of the flat sheet type planar electrodes 22 and 24 are on an inner sur- shown in Figures 9-11 and explained later herein. face of the planar plates 14 and 16, respectively. At The internal cathodes 34 and 36 can be of the hot least one of the planar electrodes 22 and 24 is 20 cathode type, the cold cathode type, or the com- transparent to permit light to exit from the chamber bination hot and cold cathode type, as explained in 12. Conductive wire mesh or other known conduc- more detail herein. Thus, the term "vertical elective transparent conductor can be used, such as trode" refers to the type of electrode, one that is those taught in U.S. Patent Nos. 4,266,167 ('167) or within the chamber 12 to create an electron flow 4,851 ,734 ('734), both incorporated herein by refer- 25 within the gas, and not to a particular shape of ence. The planar electrodes 22 and 24 extend over electrode. the majority of the inner surface of the plates 14 An electric ground shield 37 having electrodes and 16 of the chamber 12. 38 and 40 may also be provided, if it is desired, to Dielectric glass layers 26 and 28 overlie planar block any electric fields which the lamp 10 may electrodes 22 and 24, respectively. At least one, 30 generate outside of itself. The ground shield 37 and usually both, of the dielectric layers 26 and 28 may be omitted if desired. are transparent. In one embodiment, the dielectric Power is applied simultaneously to planar elec- layers are soda-lime, lead-free ceramic glass hav- trode pairs 22, 24 and vertical electrode pairs 34, ing the desired temperature characteristics as de- 36 to cause the lamp 10 to emit light. An AC power scribed herein. Overlying the dielectric layers 26 35 supply 42 powers planar electrodes 22 and 24. A and 28 are phosphor layers 30 and 32, respec- separate AC power supply 44 powers internal cath- tively. Known phosphors are suitable for layers 30 odes 34 and 36. The AC power supplies 42 and 44 and 32, as explained in the patents incorporated by may be of the high-frequency type as disclosed in reference; alternatively, the phosphors may be spe- the '167 patent or of a low-frequency type as used cifically formulated and applied as explained more 40 in standard fluorescent lamps today. The two pow- fully herein. er supplies 42 and 44 preferably are at different The chamber 12 is filled with an ionizable frequencies to ensure that the electrodes do not atmosphere that produces ultraviolet radiation when short to an electrode not in their pair. Typically, electrically excited. A gas or mixture of inert gases drive frequencies are in the range of 400-2000 Hz from group O of the periodic table, for instance, 45 at 700 volts. In one embodiment, the drive fre- argon at a low pressure, and mercury vapor having quency is at 25 kHz at 700 volts, but only 13 watts a partial pressure in the range of 1-10 microns, of power is required, thus resulting in very low form the atmosphere within chamber 12. Generally current. If power supplies 42 and 44 are the same droplets of mercury in the liquid state are within the frequency, they are set 90° out of phase to mini- chamber 12 and a portion of the chamber 12 is 50 mize the interference between them. In one em- held in the temperature range of 40°C-50°C to bodiment, the power supplies 42 and 44 are pro- produce mercury resonance radiation in the range vided from a single power source but circuitry of 2537 A. As is known, the mercury vapor pres- electrically separates it into two power supplies and sure is determined by the coolest portion of the offsets their phase by 90 ° . (For example, a DC chamber 12 and it is not necessary for the entire 55 power supply can be converted to AC power at the chamber 12 to be at this temperature, so long as a selected frequency for power supplies 42 and 44.) part of it is. Ultraviolet radiation emitted by the Having the AC power sources 42 and 44 90° out plasma within the chamber 12 causes the phosphor of phase with each other ensures that the planar

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electrodes 22 and 24 act as one pair and the The horizontal electrodes also act to reduce internal cathodes 34 and 36 act as a separate pair. the space charge effect, thus causing a corre- Driving each pair of electrodes with a separate sponding reduction in voltage drop which yields a power source and 90° out of phase ensures that higher phosphor life and overall efficacy. the respective pairs operate independent of each 5 A highly uniform light output across a thin, other. planar, rectangular lamp permits it to be used in a The planar electrodes 22, 24 create an electric wide variety of applications. This lamp can be used field by capacitive coupling, causing excitation of as backlighting for LCD screens on computers, the plasma in primary chamber 12. The electrodes avionic displays, signs, or the like. In addition, 22 and 24 are plates of a capacitor and the dielec- io many lamps can be coupled together, edge-to- tric layer of the capacitor is the combination of the edge, to form a large area, uniform light output dielectric layers 26 and 28 and the atmosphere source for large signs or other uses. within the chamber 12. In one embodiment, only 3 Figures 2-4 illustrate an alterative embodiment single electrode, either 22 or 24 is covered with a of the planar lamp 10 having interior walls 48. dielectric and the other electrode is not so coated. is As shown in Figure 2, internal cathodes 34 and Having a single electrode coated is suitable, though 36 are positioned at each end of a discharge having both uniformly coated with exactly the same column 46. The discharge column 46 extends as a thickness of dielectric layers 26 and 28 is pre- single, narrow column from electrode 34 to elec- ferred. The excitation of a mercury plasma by trode 36 in serpentine fashion, that is, bending capacitive coupling produces a stable and uniform 20 back and forth. Walls 48 within the chamber 12 are plasma and a uniform source of ultraviolet light, a sealed at the top plate 14 and the bottom plate 16. condition conducive to uniform light generation. Each of the walls 48 is sealed to the sidewall On the other hand, the internal cathodes 34 structure 17, for example, to either sidewall 50 or and 36 create an electric discharge when the volt- sidewall 52 and extends towards the other sidewall age across the internal cathodes 34 and 36 rises 25 for most of the width of the chamber. Each wall 48 above a threshold value, called the breakdown volt- terminates prior to reaching the opposing wall to age, creating a positive column. The discharge arc provide a single connected discharge path as illus- is sustained by a flow of electrons emitted by the trated in Figure 2. Generally, the length of the cathode and collected by the anode. In AC opera- discharge path 46 is a principal factor in determin- tion, the electrodes at both ends are identical and 30 ing the light output and the luminous efficiency of a operate alternatively as the cathode and anode. lamp, the longer the discharge path, the greater the The phenomenon, known as space charge effect, light output and efficiency according to Pascend's produces a voltage drop across the lamp causing law. The serpentine discharge path 46 provides a the atmosphere in the chamber 12 to conduct, longer discharge path between electrodes 34 and which accelerates electrons, thus changing the 35 36, further increasing the efficiency of the lamp. In electrical energy into kinetic energy. Mercury addition, the discharge chamber 46 is constructed atoms emit high amounts of ultraviolet light in this out of round, either rectangular or square. Im- plasma. proved efficiency of operation and greater output The two pairs of electrodes operate simulta- per wattage is generally achievable when the dis- neously to produce a bright and highly uniform 40 charge is constructed out of round. The lamp 10 light source. Each phenomenon compliments the may be in the range of 0.2 - >1 2 inches across and other to overcome the respective weak points. For in the range of 0.2 - .75 inch in thickness. The example, an arc discharge is known to produce serpentine walls 48 also permit a larger, thin lamp high light output with great efficiency, that is, many because they provide support to plates 14 and 16 lumens per watt. However, plasma discharge at 45 so they can be thinner without danger of implosion. optimum mercury pressure conducts as an arc and In the embodiment of Figures 2-4, cathodes 34 is often not a uniform discharge over a large sur- and 36 are combined cathodes of hot cathode and face area. As a result, nonuniformities of brightness cold cathode. Electrode 34 includes a hot cathode exist across the face of prior art lamps between the 54 and a cold cathode 56. Similarly, electrode 36 internal cathodes. Planar capacitive electrodes 22 50 includes a hot cathode 58 and a cold cathode 60. and 24 act to create very uniform plasma across Planar electrodes 22, 24 are capacitive coupling the entire chamber 12. This complements the high electrodes. Conductors 90-96 permit coupling the light output capability of the internal cathodes 34 electrodes to an outside power supply. As shown in and 36. The shape of the plasma between internal Figure 4, electrodes 92 and 93 are coupled to the cathodes 34 and 36 is altered to be more uniform 55 cold cathodes 56 and 60. Electrodes 94 and 96 are by the horizontal electrodes to create a highly coupled to the hot cathodes 54 and 58. Electrodes uniform, high light output arc across the entire 90 and 91 are coupled to the planar electrodes 22 chamber. and 24, respectively. The vertical cold cathodes 56

5 9 EP 0 550 047 A2 10 and 60 preferably have vertical, opposing metal off by the arc current of the plasma heating alone, strips and are open on the top and bottom to supplemental heating of the hot cathode is not permit light to be emitted, as shown in Figures 2-4. required. However, at lower lamp temperatures, the DC power supply 74 provides the heating pow- hot spot may become too cold and as the hot er for the hot cathodes 54. DC to AC inverters 51 , 5 cathode begins to operate in a cold cathode man- 53, and 55 (sometimes called an electronic ballast) ner; however, the material and structure of the hot convert DC voltage from a DC power supply (not cathode is unsuited for cold cathode operation. shown) to the desired AC frequency, generally one Therefore, when the arc current does not supply inverter for each pair of electrodes. For example, sufficient heat to the hot spot for proper hot cath- an inverter 51 converts the power for the hot cath- io ode operation, it is necessary to supply supple- odes 54 and 58, an inverter 53 converts the power mentary heating to the filament, such as by resis- for the cold cathodes 56 and 60, and an inverter 55 tive heating from DC power supply 74. converts the power for the planar electrodes 22 A hot cathode is generally more efficient be- and 24. In one embodiment, the same inverter is cause the power lost at the cathode is minimized. used for each electrode pair and the phases are is The most efficient operation is provided when sup- offset by other circuitry. Alternatively, a separate plemental heating is not required to maintain the and direct AC power supply is provided for each cathode at the desired operating temperature. A electrode pair or DC power supply 74 is used for hot cathode also has the advantage of a higher the power. As will be appreciated, the power sup- light output for a given amount of power. plies to the electrodes can be configured a variety 20 Cold cathodes do not use a high temperature of suitable ways to provide the power. filament, but rather have a large emitting surface A discussion of the cathode-fall zone as altered area, typically coated with an emissive coating. by either a hot cathode or a cold cathode may be From the cold cathode, electrons enter the plasma useful. Current flows at the transitional region just by field emission, also called secondary electron in front of a cathode producing a cathode-fall, or 25 emission. The temperature of the cold cathode is voltage drop, which pulls electrons away from the generally in the range of 150°C and there is a cathode. The work function of the cathode material cathode-fall of usually greater than 80 volts. Cold at the temperature of the cathode as well as the cathodes generally have a lower ultimate bright- ionization characteristics of the current carrying gas ness than hot cathodes, usually less than a thou- determine the magnitude of the cathode-fall. As 30 sand footlamberts in miniature fluorescent tubes. cathode-fall increases, a greater number of heavy At very low currents, the cold cathode is more mercury ions impact into the cathodes, slowly sput- efficient than the hot cathode because the filament tering the cathode away and turning energy into of the hot cathode requires supplementary heating heat. The cathode-fall causes a power loss in the to maintain incandescence at the filament when at region immediately adjacent the cathode. In a flu- 35 low currents. Cold cathodes can thus easily be orescent discharge chamber this results in a small dimmed down without complicated drive circuitry. dark region adjacent the cathode. The large area of the cold cathode also gives Hot cathodes are filaments which glow, similar longer life because, with low current flow, few cath- to the glow given off by incandescent globes, but ode electrons are required to sustain the arc and not as bright. The hot cathodes utilize a thermionic 40 the electrons of the large cathode are not depleted emission in which the electrons are essentially so quickly. The disadvantage of the cold cathode is boiled into the arc stream from the hot coiled that the higher cathode-fall voltage, usually greater filaments which must have a temperature in the than 80 volts and sometimes as high as 200 volts range of 1000°C. Electrons stream from a hot spot results in greater losses and less efficiency. on the filament, which results in a total cathode-fall 45 Use of the hot cathode has the advantage of of only 12-15 volts. A brightness of several thou- providing a much brighter light for a given lamp. If sand footlamberts is achievable. The hot cathode an extremely high light output is desired, a hotter lamp can thus be brighter than a cold cathode cathode may be used. For high temperature opera- lamp. In addition, the cathode-fall region is gen- tion, all surfaces including the upper plate are erally very short so that the dark space near the 50 constructed of a high temperature ceramic or hard end of a lamp is correspondingly short and the glass. The sidewall structure 17, interior walls 48, light is more uniform through the discharge cham- and lower plate 16 may all be an opaque, IR- ber. absorbing ceramic. A hot spot on the hot cathode must be held in The actual wattage expended at the electrodes the temperature range of approximately 1000°C for 55 is a product of the voltage drop at the electrodes the cathode to remain a hot cathode. When the hot times the plasma arc current. Because of the high spot on the filament of the cathode can reach an voltage drop of the cold cathode and the lesser operating temperature from the temperature given equivalent voltage drop for a hot cathode, there is

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greater wattage dissipation, consequently, more traviolet light back into the phosphor film 32, in- heat generated at the terminals of cold cathode creasing the lamp's overall efficacy. The U.V. re- lamp than at the terminals of a hot cathode lamp. flective film could also be composed of other ma- Because of the wattage loss at the electrodes, a terials, such as ZnO, AI2O3, Zirconia, or the like. A hot cathode lamp will always be more efficient in 5 grounding shield of electrodes 38 and 40 can be overall lumens per watt, because the expenditure provided if desired. A dielectric layer 39 is pro- of watts into the arc stream for both the hot and a vided below the grounding electrode 40 to isolate it cold cathode will be the same. For the reasons from the surrounding environment. explained above, hot cathodes have generally The upper plate 65 of chamber 12 is an implo- found use as backlights for LCD screens despite 10 sion resistance plate that is transparent to white their efficiencies because of the numerous light. Light emitted by the phosphor layers 30 and drawbacks. 32 shines out of the chamber 12 by passing For more detailed information on hot and cold through the transparent plate 65. cathodes, see "Fluorescent Backlights For LCDs," The secondary chamber 62 is defined by by Mercer and Schake, Information Display, pp. 8- 15 planar face plate 68, upper plate 65 (the upper 13, November 1989. plate 65 is actually the lower plate of the secondary In the embodiment of Figures 2-4, the cold and chamber 62) and sidewalls 70 and 72. In the em- hot cathodes can be operated simultaneously. Al- bodiment of Figures 5 and 6, the upper chamber ternatively, the cold cathodes may operate alone 62 is at atmospheric pressure and is open to the when low light levels are desired; the hot cathode 20 air. Cooling air, or alternatively, cooling fluid, flows may operate alone. As a further alternative, and through the secondary chamber 62 to cool the usually preferred, the hot and cold cathodes will lamp as needed. Overlying face plate 68 is a operate simultaneously with the planar electrodes diffuser coating 74 and a grounding electrode 38 22 and 24. on the inside surface of the secondary chamber 62. An alternative embodiment is shown in Figures 25 Overlying the top surface of the face plate 68 is a 5 and 6, in which the lamp 10 includes a primary dichroic mirror 69. The dichroic mirror is construct- chamber 12 and a secondary chamber 62. Light 64 ed from a dichroic film of a known material that is is emitted only out of the top of lamp 10. (The transparent to white light but reflects heat, such as details of the electrodes are not shown for simplic- infrared radiation, back into the lamp. ity in illustration. The electrodes and drive circuitry 30 As illustrated in the simplified view of Figure 5, could be any combination of those in the prior art the lamp 10, in one embodiment, includes a plural- or those previously discussed with respect to other ity of chambers. (Figure 5 shows features of a two- embodiments of this invention.) chamber lamp and does not show other features The primary chamber 12 is defined by an present in the lamp for similarity of illustration.) A upper plate 65, a lower plate 66, and sidewall 35 secondary chamber 62 is formed on top of primary structure 17 having sidewalls 18 and 20. The pri- chamber 12. The primary chamber 12 is generally mary chamber 12 contains an inert gas and mer- the chamber at the lowest pressure and usually cury vapor at a selected pressure as described includes the mercury vapor which emits high with respect to Figure 1. Planar, horizontal elec- amounts of ultraviolet light. The secondary cham- trodes 22 and 24 overlay the respective plates 65 40 ber 62 includes a planar face plate 68 and a and 66. Lead-free glass layers 26 and 28, respec- sidewall structure 67. The top plate 14 of the tively, overlays each of the horizontal electrodes 22 primary chamber 12 forms the bottom plate of the and 24. The lead-free glass layers 26 and 28 are secondary chamber 62. The secondary chamber dielectrics which insulate the respective electrodes has many uses and configurations, examples of 22 and 24 from the interior of the chamber 12. A 45 which will now be described. soda lime glass, or other lead-free glass, is accept- The secondary chamber 62 permits thinner able for use as the dielectric layer 28. Overlaying plates to be used in the lamp 10 without danger of each of the glass layers 26 and 28 are respective imploding. It is known that mercury vapor should phosphor layers 30 and 32. be held in the pressure range of 3-8 microns for a The lower plate 66 is constructed of a black 50 maximum light output or overall efficiency. In addi- ceramic glass which acts as an infrared heat ab- tion, the chamber 12 must be evacuated of air and sorber to draw heat away from the front of the lamp refilled with a very low-pressure inert gas, such as and towards the back. As an alternative to using a argon, to a selected pressure. If the gas pressure black glass for the lower plate 66, a black ceramic within the chamber 12 becomes too low, the lamp film coating may be applied which provides the 55 will implode, with the planar plates 14 and 16 same function of absorbing heat in the form of collapsing into the chamber 12, destroying the infrared light. A titanium-doped ceramic film may lamp 10. In the past, this danger of imploding has be applied on top of the plate 66 to reflect ul- been guarded against by making the plates 14 and

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16 sufficiently thick to withstand the pressure dif- Figures 5-8 illustrate examples of multiple ference between the low pressure inside of the chamber lamps for specific applications. Figures 5 chamber 12 and atmospheric pressure. The thicker and 6 show a double chamber lamp for use in plates to prevent implosion have the disadvantage avionics, such as the backlighting of an aircraft of causing greater light losses because the visible 5 display panel. Figures 7 and 8 are miniaturized, light emitted by the phosphors must travel through serpentine primary chamber overlaid by a light- the planar plates from the inside of the chamber emitting secondary chamber for use in LCD dis- 12. plays where dimming is desired, or other uses. The According to one embodiment, the pressure in phosphor layers 30 and 32 are specially formed to the secondary chamber 62 is at an intermediate io provide improved performance. An example of the pressure, between atmospheric pressure and the formation of phosphor layer 32 on dielectric layer low pressure of the chamber 12. This places less 28 will now be described in detail for illustrative stress on the planar plate 14 between the two purposes. chambers. The planar plate 14 may therefore be The phosphor 32 is applied to the glass layer made significantly thinner without danger of implo- is 28 while both are cool, prior to the lamp 10 being sion. The lower planar plate 16 can remain as thick assembled. The phosphor layer 32 is applied by as desired because the light is emitted through the any acceptable technique, including screen print- upper planar plate 14. The face plate 68 is the ing, thick film printing, spraying, dipping, brushing, thickness required to prevent implosion caused by or other acceptable techniques. The phosphor 32 is the pressure difference between secondary cham- 20 usually mixed into a slurry prior to applying it to ber 62 and atmospheric pressure. The face plate the glass layer 28, the techniques of making a 68 can therefore be quite thin because the pres- phosphor slurry being well known in the art. The sure in chamber 62 may be only slightly less than glass used for the dielectric layer 28 has a se- atmospheric pressure and will generally be higher lected reflow temperature, that is, the temperature than the pressure in chamber 12. 25 at which the glass becomes sticky and begins to In alternative embodiments, the secondary melt. Preferably, the glass reflow temperature is chamber 62 is open to the ambient air. In this approximately 600 ° C. embodiment, the secondary chamber 62 may per- The glass layer 28 is then heated to approxi- mit ambient cooling or forced air cooling. Alter- mately its reflow temperature, with the phosphor natively, the secondary chamber 62 is filled at 30 layer on top of it. At the reflow temperature, the selected locations with thermal fluids which vapor- glass becomes quite sticky and begins to melt ize to locally cool the primary chamber 12 at slightly at the surface. A reflow temperature below selected locations from maintaining at least some the temperature at which phosphor degrades is portion of the mercury vapor pressure in the tem- selected. The dielectric layers should have the perature range of 40°C-50°C while permitting the 35 proper thickness to create a uniform elective field hot cathodes to achieve temperatures in the range within the chamber 12. For most glass materials, a of 1000°C. A cooling fluid may be forcibly cir- thickness greater than 5 microns is used to ensure culated within the second chamber to maintain the a uniform covering without pin holes in the dielec- lamp below a selected temperature. Alternatively, tric layer. Preferably, the thickness is less than 25 the secondary chamber 62 forms a thermal vacu- 40 microns to provide good light transmission prop- um to block heat from being emitted out of the erties. Thus a dielectric layer for 26 and 28 in the front of the lamp 10. The secondary chamber may range of 5-25 microns is acceptable; though other also be a light diffuser, providing uniform light out thicknesses may be used in some environments. of the lamp from a nonuniform light source in For those, phosphors that degrade at or below chamber 12. The ultimate use of the secondary 45 700 °C, the reflow temperature of the glass 28 is chamber is dependent upon the specific applica- selected to be below this temperature, preferably in tion. the range of 600 ° C - 650 ° C. A soft glass which is As will be appreciated, features in Figures 5 free of heavy metals is selected so that the phos- and 6 are not to scale. For example, the conductive phor will not be degraded in the glass. A non- films that form electrodes 22 and 24 are in the 50 vitrifying glass having the proper reflow tempera- range of less than a micron in thickness while the ture is acceptable. glass plate 66 is in the range of 1/8 of an inch in Hard glass is generally more transparent to thickness. The phosphor crystals have an average U.V. light than soft glass and is usually preferred diameter in the range of 3-4 microns and the for upper plate 65 and face plate 68. In some dielectric layer 28 has a thickness sufficient to 55 environments, a hard glass for the dielectric glass provide a pin hole free surface, usually greater than layers 26 and 28 could be used. For example, an 5 microns and likely in the range of 10-30 microns. alumina-silicate, boro-silicate, quartz, pyrex, or the like, which are considered hard glasses and gen-

8 15 EP 0 550 047 A2 16

erally having a reflow temperature of 650 ° C, could be exposed to the atmosphere over a large surface be used for dielectric layers 26 and 28. Hard glass area and partially surrounded by other crystals. generally has a higher reflow temperature than soft The phosphor layer 32 is shown in stepped fashion glass, and a glass having an even higher reflow to illustrate that some crystals 71 are embedded temperature, in the range of 700 °C - 1000°C, 5 completely within the glass layer 28 and some could be used; but if such a choice is made, crystals 73 are completely outside of the glass preferably phosphors are selected which will not layer 28, on a surface region thereof. The embed- significantly degrade when heated to the reflow ded crystals 71 are in solid form, completely em- temperature of glass selected for dielectric layer bedded within the glass layer 28 and are not ex- 28. If hard glass is used for the dielectric layers, io posed to the atmosphere within chamber 12. On the upper and lower plates are also a hard glass to the other hand, the crystals 72 are exposed to the ensure that they have a similar coefficient of ther- atmosphere of the chamber 12. mal expansion. Preferrably, the reflow temperature Reference has been made to dielectric glass of the dielectric layer of glass is not higher than the layer 28 and the phosphor layer 32 to illustrate one reflow temperature of the plates, to ensure that the is technique for applying the phosphor layer to glass. plates do not melt when the temperature is raised The dielectric layer 26, as well as any dielectric to embed the phosphors in the dielectric layer. and phosphor layers of the various embodiments of As the glass layer 28 becomes somewhat the invention, can be similarly constructed if de- sticky at the surface region, the phosphor layer sired. For example, the lamp of Figures 1-3 which becomes embedded into the glass at a very slight 20 have only a single chamber can be constructed depth. The glass 28 is not heated to its liquid state with an embedded/exposed phosphor, as de- melting point; it is only heated sufficiently that the scribed. surface becomes sticky and the surface region As can be appreciated, the phosphor layer 32 melts slightly. The glass layer and 28 with the is embedded into the glass layer 28 so that it can- phosphor on its surface is then cooled to trap the 25 be used in a lamp 10 of which embodiments are phosphor crystals embedded within the glass layer. show in Figures 1-8. Prior to applying the phosphor The rate and manner of cooling is not particularly layer 32 to the glass layer 28, the glass layer 28 is critical and can be carried out naturally by letting overlaid on an electrode 24 which is affixed to a the glass cool toward room temperature by turning planar plate 16, or alternatively an opaque plate 66. off the kiln and venting it to ambient air to permit it 30 The plate 16 which forms a part of the chamber is to cool over time. A cooling rate in the range of a glass having a higher reflow temperature than the 1 ° C-25 ° C per minute has been found acceptable. temperature of the glass layer 28. The plate 16 During mass production, forced cooling using cir- may be, for example, an alumina-silicate glass, a culating fluid, such as air, may be necessary if a borasilicate glass or other hard glass having a large mass of glass plates are together; however, 35 comparable reflow temperature above 650 ° C. After cooling techniques of glass in the construction of the plate 16 has been prepared by applying the fluorescent lamps is generally known in the art and electrode 24 and the glass layer 28, the phosphor any suitable technique which maintains the integrity layer 32 is applied and the entire assembly is of the glass to keep it free of cracks is acceptable. heated, and then cooled in the manner described. After the glass 28 cools, the loose phosphor is 40 The glass plate does not melt because it has a wiped off the glass. The phosphor layer 32 which higher reflow temperature than that of the glass remains is adhered to the glass or to phosphor layer 28. crystals that adhere to the glass. The phosphor After the upper plate 14 and lower plate 16 layer 32 is therefore quite thin, usually 3 to 5 layers have been prepared as has been described, the of crystals. In one embodiment, only a very thin 45 plates are assembled into a completed lamp 10 layer of phosphor is originally applied to the glass similar to that shown in Figures 1-8. Assembling layer 28, and wiping off of excess phosphor is the lamp may be performed by positioning the omitted because it is not required. glass plates and sidewall structures which will form The phosphor layer 32 is shown with a portion the lamp adjacent to each other and bonding them of it extending into the glass layer 28, a portion of 50 together with an appropriate adhesive. Appropriate it at the surface, and a portion of it extending out of adhesives include glasses or ceramics having a the glass layer 28. As the glass cools with the selected reflow temperature to bond to each of the phosphor layer 32 thereon, some of the crystals glasses, a U.V. epoxy resin, a silicon adhesive will be completely embedded within the glass layer (such as the type used in aquariums), or other 28, some of the crystals will be partially embedded 55 suitable adhesive for permanently bonding the and completely surrounded by other crystals, other glass structures of the lamp to each other. crystals will partially embedded and partially ex- In one embodiment, the phosphor layers are posed to the atmosphere, while other crystals will applied to the dielectric layers and the lamp is

9 17 EP 0 550 047 A2 18 assembled prior to an additional heating step. Dur- light when subjected to an intense alternating cur- ing final assembly of the lamp, the entire lamp is rent electric field. The ceramic may be the dielec- heated to bond the members together, as may tric of a capacitor, for example, electroluminescent occur if a glass having a low reflow temperature is lamp in which a ceramic layer 7 having particles of used in the bonding. During the heating up of the 5 phosphor embedded therein is exposed to an elec- entire lamp during the bonding process, the glass tric field to cause the solid ceramic block to emit layers 26 and 28 may also slightly melt, causing light is shown in U.S. Patent No. 2,900,545. After the phosphor to become partially embedded within finding that electroluminescent phosphors emit light the layers which they overlay. according to the electroluminescent phenomenon, Having the phosphor layer 32 crystalized within io it became desirable to construct a structure that the glass layer 28 partially embedded and partially would simultaneously operate on electrolumines- exposed provides significant advantages that en- cent and fluorescent phenomena. hance the emission of light and lamp brightness The phosphor layer 32 preferably includes both controllability, as will now be explained. fluorescent and electroluminescent phosphors. In Having some of the crystals of the phosphor is one embodiment, zinc sulfide, a known elec- layer 32 embedded within the glass layer 28 in- troluminescent phosphor, doped with a suitable ele- creases the efficiency of the light transmission. ment , such as copper, silver, manganese, chlorine, Light generated by the phosphor layer 32 by the or the like, is used. Also mixed in the same phos- fluorescent phenomenon passes directly through phor slurry are fluorescent phosphors. Many flu- the crystal structure of the phosphor layer 32 and 20 orescent phosphors are known and, preferably, a into the glass layer 28 with high efficiencies. Addi- mixture of three triband rare earth phosphors, one tionally, light generated by phosphor crystals within red, one green and one blue, are mixed in the the dielectric layer 28 by both the fluorescent and slurry. The selected phosphors are combined in electroluminescent phenomenon passes directly various proportions to give the desired spectral and from the crystal embedded in the glass into the 25 brightness output. (Fluorescent phosphors are glass itself with high transmission efficiencies. This known in the art; a person of ordinary skill would is distinguished from the prior art in which the select the particular rare earth phosphors and phosphor layer is merely dusted onto the glass and spectral proportions desired for each application is not embedded within the glass itself. In the prior following well-known techniques published in the art, some of the light emitted by the phosphor is 30 literature, see, for example, previously cited article reflected by the phosphor/gas/glass interfaces, de- by Mercer, or Waymouth, John F., "Electric Dis- creasing the transmission efficiency of light from charge Lamps," MIT Press, IBSN 0262-23058-8.) the phosphor to the exterior of the lamp. The In one embodiment, the phosphor slurry in- embedded phosphor layer decreases reflections of cludes 90% fluorescent phosphors by weight and white light from the phosphor/glass interface. 35 10% electroluminescent phosphors by weight. For The phosphor layer 32, formed as described, example, the slurry may include 10% zinc sulfide emits light under a vacuum fluorescence phenom- by weight and 90% rare earth phosphors. Other enon and also under electroluminescence phenom- proportions, such as 20% and 80%, 45% and 55%, enon. For background purposes, an explanation of or 55% and 45% can be used. the vacuum fluorescence phenomenon and the 40 If desired, an additional thin film of magnesium electroluminescence phenomenon may be useful. oxide may be overlaid directly on top of the ce- The vacuum fluorescence phenomenon is the ramic dielectric film approximately to a thickness in emission of visible light from ultraviolet light strik- the range of 250 angstroms to 5 microns prior to ing the phosphors, the ultraviolet light being pro- applying the phosphor layer 32. As is known in the vided by mercury vapors within the chamber 12. 45 art, the additional layer of magnesium oxide will When power is applied to the electrodes of the lower the on/off threshold for light emission by the discharge chamber 12, ultraviolet electromagnetic phosphor. Other materials which alter the on/off radiation at approximately 2537 angstroms is emit- threshold for secondary emission can be used, if ted. The ultraviolet electromagnetic radiation im- desired, such as Y203, Al203, Ti02, Zn02, BNG, pinges on the phosphor coating 32 and excites the 50 Si02, or BaTi02, etc. phosphor to cause it to emit. The visible light is In the embodiment of Figures 5 and 6, the then emitted by the lamp 10. Fluorescence is thus lamp 10 simultaneously outputs fluorescent light the excitation of visible light photons when ul- and electroluminescent light. The light from both traviolet light strikes the phosphor. phenomena is combined as the light output. Electroluminescence, on the other hand, is a 55 The fluorescent phenomenon is created by ver- solid-state, electric field phenomenon. Some solid tical cathodes 34 and 36 creating a plasma arc, or materials, such as a ceramic having a zinc sulfide positive column within the fluorescent chamber 12 powder embedded therein, has been shown to emit to convert the electrical energy into ultraviolet radi-

10 19 EP 0 550 047 A2 20

ation that the phosphor layer 32 converts into visi- bined fluorescent and electroluminescent phenom- ble light. Planar electrodes 22 and 24 also aid in enon. creating a more uniform plasma arc within the Figures 7 and 8 illustrate a lamp 10 having a atmosphere of the chamber 12 to provide uniform, sealed secondary chamber 62. As previously de- bright light based on the fluorescent phenomenon. 5 scribed with respect to the other figures, the lamp The horizontal electrodes 22 and 24 also impose 10 of Figure 7 includes a primary chamber 12 an electric field on the solid dielectric layer 28 having serpentine walls 48 therein. The serpentine which includes phosphor crystals from phosphor walls support the upper plate 65, permitting it to be layer 32 embedded therein. The solid dielectric made somewhat thinner than would otherwise be material emits visible light directly, based on the io possible without the intermediate walls. The vertical electroluminescence phenomenon when exposed cathodes 34 and 36 include respective hot cath- to this electric field. The internal cathodes 34 and odes 58 and 54 and cold cathodes 56 and 60 as 36 and the horizontal electrodes 22 and 24 tend to have been previously described. A power supply be individually controlled to selectively control the 55 provides power to planar electrodes 22, 24, 76, percentage of light output based on the fluorescent is and 77. Electrodes 24 and 76 are coupled to one phenomenon or the electroluminescent phenom- side of power supply 55 and electrodes 22 and 77 enon. Generally, the light emitted will be a com- are coupled to the other side. Power supplies 51 bination of fluorescent light and electroluminescent and 53 provide power to internal cathodes 34 and light, both phenomena operating simultaneously 36. DC power supply 74 provides additional power within the single lamp. 20 to heat the hot cathodes 58 and 54 as necessary. Electroluminescent materials have the advan- The inner surfaces of upper chamber 62 in- tage of emitting uniform light while operating at cludes phosphor layer 78 on the upper surface. In relatively low temperatures. However, elec- one embodiment, a pair of planar electrodes 76 troluminescent lamps generally have a low ultimate and 77 are overlaid by respective dielectric layers brightness, about 1 lumen per watt. The low bright- 25 82 and 83 and an electric field is applied on the ness of electroluminescent is generally considered secondary chamber 62. The second pair of planar a disadvantage. However, in the present invention, electrodes 76 and 77 is powered from the same a lamp having the low-level light output of elec- power supply 55 as the first pair of planar electroluminescence is advantageously used in com- trodes 22 and 24. Alternatively, a separate power bination with the high-level light output of fluores- 30 supply is provided for each pair of planar elec- cent lamps to provide a useful lamp. In some trodes. environments, it is desirable to vary the light out of The interior of secondary chamber 62 is filled the lamp over a wide range. As previously ex- with the appropriate atmosphere, such as an inert plained, while hot cathodes emit great amounts of gas, at a suitable pressure. The secondary cham- light and are very efficient at full power, it is 35 ber 62 does not include mercury vapor in one extremely difficult to dim a fluorescent lamp having embodiment, but does include mercury vapor in an hot cathodes because the cathodes do not maintain alternative embodiment. Similarly, in one embodi- the required operating temperature. ment, there are no internal cathodes within the According to principles of the present inven- secondary chamber 62. However, in an alternative tion, a hot cathode fluorescent phenomenon is 40 embodiment, vertical and planar electrodes are used in conjunction with the cold cathode and the both provided. electroluminescent phenomenon from the phos- The pressure of secondary chamber 62 is in- phors. When it is desirable to dim the lamp, the hot termediate between atmospheric pressure and the cathodes may be shut off completely, so that they very low pressure of the primary chamber 12. A draw no power; the desired level of dim light is 45 pressure in the range of 8-25 mm of mercury is provided by the combined cold cathode and elec- acceptable for the secondary chamber 62, the pri- troluminescent phenomenon of the very same mary chamber 12 being in the range of 2-6 mm of lamp. For even more low light control, the cold mercury. A relatively thick, implosion-resistant low- cathodes are turned off and only the planar elec- er plate 66 prevents implosion due to the dif- trodes 22 and 24 remain on. The planar electrodes 50 ference between atmospheric pressure surrounding create uniform electroluminescent light of low the plate 66 and the low interior pressure of the brightness, as may be desired in some applica- primary chamber 12. On the other hand, the upper tions. The planar electrodes can also create flu- plate 65 is a significantly thinner plate and is not orescent light by capacitive coupling, depending on necessarily sufficiently strong by itself to prevent the applied voltage. The voltage is adjustable to 55 implosion based on the pressure difference be- provide the desired light output. The low-light level tween atmospheric pressure and the low pressure illumination range and adjustability of the lamp is of primary chamber 12. However, the upper plate therefore significantly increased using the com- 65 is not subjected to atmospheric pressure. Rath-

11 21 EP 0 550 047 A2 22 er, it is only subjected to the pressure difference mits the light to be uniformly bright across the between the secondary chambers 62 and the pri- entire face of the lamp, even to the very edges. mary chamber 12. The upper plate 65 can there- The lamps can then be placed edge-to-edge to fore be made extremely thin and thus more trans- form an array of many lamps to cover a large area parent to white visible light and ultraviolet light. 5 and emit light uniformly, even though many lamps Some ultraviolet light passes from primary cham- are used. ber 12 completely through upper plate 65 and into The cathodes of Figures 9-11 can be fixed secondary chamber 62. This ultraviolet light im- directly to the walls they are adjacent, if desired, pinges upon phosphor layer 78 within the upper but preferably are spaced from the walls by a small chamber 62, causing this upper layer 78 to emit io distance, in the range of 10-1000 microns. fluorescent light. Therefore, even if electrodes 76 The invention has been described and illus- and 77 are not present, the phosphor layer 78 trated with respect to various alternative embodi- emits light based on the ultraviolet radiation escap- ments. It will be understood by those of ordinary ing from primary chamber 12. This secondary skill in the art that numerous inventive features source of white light emissions provides more uni- is described in one embodiment may be used in form, brighter light because a greater percentage combination with inventive features described in of the ultraviolet radiation is being used. other embodiments. Various embodiments of lamp In an alternative embodiment, power is sup- 10 have been described. Specific features are illus- plied to upper planar electrodes 76 and 77, creat- trated in the various embodiments. The features of ing a plasma arc within the upper chamber 62 for 20 one embodiment can be combined with the fea- local generation of ultraviolet radiation that im- tures of other embodiments if desired. For exam- pinges upon phosphor layer 78, causing it to emit ple, phosphor layers formed by standard prior art white light. The secondary chamber 62 may in- techniques as shown in Figure 1 can be used for clude more phosphors and may operate at a dif- the layers in the lamps of Figures 2-8 rather than ferent pressure, generally a significantly higher 25 the embedded layers. Similarly, the single open pressure than the primary chamber 12. This per- chamber configuration of Figure 1 could have walls mits thinner, larger area glass plates to be used for 48 therein to form a serpentine chamber. Alter- top faceplate 68 and upper plate 65 without the natively, the lamps of Figures 2-8 could be all open danger of implosion. In this embodiment, the sec- area chambers. The planar electrodes of Figure 1 ondary chamber 62 emits light based on the elec- 30 are not required in the two-chamber embodiments troluminescent phenomenon locally generated and of Figures 5-8, such lamps being operable with the fluorescent phenomenon caused by ultraviolet only internal cathodes in the chamber itself if de- light escaping from primary chamber 12. sired. All other features of the various embodi- Figures 9-11 illustrate possible shapes for in- ments could also be combined, as desired, without ternal cold cathodes 56 and 60. In the embodiment 35 using all the features in one lamp and such lamp of Figures 2-8, the cold cathodes 56 and 60 are would still fall within the scope of this invention. formed of flat conductive strips bent at two loca- Additionally, equivalent structure may be substi- tions. The metal strips are open on the top and tuted for the structure described herein to perform bottom so they do not block light that may be the same function in substantially the same way emitted out of the top or bottom. Preferably, the 40 and fall within the scope of the present invention, two sides of the cold cathode are adjacent the wall the invention being described the claims appended 20 and internal 48, and the back is adjacent the hereto and not restricted to the embodiments wall 52, as shown in Figure 2. The AC power shown herein. supply is electrically connected to both the cold cathode and the hot cathode in one embodiment, 45 Claims as shown in Figures 9 and 10. The DC power supply is coupled only to the hot cathode to pro- 1. A planar lamp having at least one transparent vide supplemental heating as necessary. electrode, comprising: As shown in Figure 11, the cold cathode may a pair of facing, planar plates forming a top be a generally flat, thin strip for use in the open 50 and bottom wall of a chamber, each of said chamber lamp of Figure 1 . The strip is flat so as to plates having an inner surface facing an inside not block U.V. from striking the phosphors at the of said chamber and an outer surface facing an edges of the lamp or white light that may be outside of said chamber, at least one of said emitted. The ends may be bent and extend for a electrodes being transparent to light; short distance along either side of the lamp, though 55 a sidewall structure coupled to said sup- this is not required and in one embodiment, the port plates at a peripheral region and retaining cathode is a planar, flat metal strip for its entire said plates a fixed distance from each other, length. Using a planar strip for the cathodes per- said sidewall structure having a height sub-

12 23 EP 0 550 047 A2 24 stantially less than the inside surface area of electrodes, giving off light according to an said plates, said sidewall structure and said electroluminescent phenomena simultaneously plates forming an enclosure for a chamber; with the fluorescent material emitting light ac- a gas of a selected composition and at a cording to the fluorescent phenomena. selected pressure within said chamber; 5 a pair of planar electrodes coupled to re- 6. The lamp according to claim 1, further includ- spective inside surfaces of said plates, at least ing: one of said electrodes being transparent to a third planar plate positioned above a light, said electrodes having a large surface transparent plate within the said first pair of area, approximately equal to the inside surface 10 plates, the third planar support plate forming a area of said plates; wall of a second chamber above the first a thin dielectric layer coupled to at least chamber; and one inside surface of one of said plates and a second pair of sidewalls positioned at a completely covering the electrode, coupled to peripheral region between the third planar that plate, said dielectric layer being transpar- is plate and the transparent support plate, the ent to light; second pair of sidewalls, the third planar plate an electric power source coupled to the and transparent plate defining the second pair of electrodes to selectively apply an elec- chamber above the first chamber with the sec- tric voltage to said pair of electrodes for ond chamber positioned in the light emission powering said lamp; and 20 path of the first chamber such that light emit- a fluorescent material on an inside surface ted by the first chamber passes through the of said dielectric layer and exposed to said gas second chamber. within said chamber so that when an electric voltage is applied to said electrodes, light is 7. The lamp according to claim 6 wherein the emitted from said lamp according to a fluores- 25 second chamber is hermetically sealed by the cent phenomena. second pair of sidewalls being hermetically sealed to the third planar support plate and to The planar lamp according to claim 1, further the transparent plate to a form an hermetically including: sealed second chamber; a second pair of electrodes extending into 30 a gas at a selected pressure within the said chamber and surrounded by the gas with- second chamber; and in said chamber for creating a plasma arc a fluorescent material on an inside surface within the chamber; and of the second chamber, the fluorescent ma- a second electric power source coupled to terial emitting light as ultraviolet light passes the second pair of electrodes to selectively 35 through the transparent plate, into the second apply an electric voltage to said second pair of chamber, and strikes the fluorescent material, electrodes for providing a current path through the light being emitted by the lamp being a the gas within said chamber from one elec- combination of the light from the first chamber trode in said second pair to the other electrode and the second chamber. in said second pair. 40 8. The lamp according to claim 7 wherein the The lamp according to claim 2 wherein said pressure of the gas in the first chamber is first power source and said second power lower than the pressure of the gas in the source are both alternating current power sour- second chamber for permitting the transparent ces at the same frequency and are driven out 45 electrode to be exposed to less of a pressure of phase with respect to each other. difference between its outside and inside surfaces and thus permitting said transparent The lamp according to claim 2 wherein said plate to be thinner than if it were exposed to first power source and said second power atmospheric pressure on an outside surface source are both alternating current power sour- so and the lower pressure on the inside surface. ces at different frequencies. 9. The lamp according to claim 6, further includ- The lamp according to claim 1, further including: ing: a second pair of planar electrodes within an electroluminescent material fixed to 55 the second chamber, each of the second pair said dielectric layer within said chamber, said of electrodes being transparent, one of the electroluminescent material emitting light when second pair of electrodes being positioned on a voltage is applied to the pair of planar of an outside surface of the transparent electrode

13 25 EP 0 550 047 A2 26

of the first pair and on an inside surface of the 15. A lamp that emits light based on the fluores- third planar electrode; cent phenomena and the electroluminescence a thin dielectric layer completely covering phenomena simultaneously, comprising: each of said second pair of planar electrodes, a sealed chamber having a gas at a se- respectively, said dielectric layers being trans- 5 lected pressure therein, said gas including parent to light; mercury vapor for emitting ultraviolet light an electric power source coupled to the when subjected to an electrical field; second pair of planar electrodes to selectively a pair of electrodes adjacent said sealed apply an electric voltage to said second pair of chamber and positioned to apply an electric electrodes; and io field to said gas within said sealed chamber, a fluorescent material on said dielectric each electrode of said pair being separated layers covering the second pair of planar elec- from said chamber by a dielectric layer; trodes and exposed to said gas within said a light transparent member forming a wall second chamber so that when an electric volt- of said chamber; age is applied to said second pair of planar is a fluorescent phosphor adjacent said light electrodes, light, is emitted from said second transparent member within said chamber and chamber simultaneously with light being emit- positioned to be exposed to ultraviolet radiation ted from the first chamber. emitted by said gas when an electrical field is applied by said electrodes; and 10. The lamp according to claim 2, further includ- 20 an electroluminescent phosphor at least ing: partially embedded within said light transparent a plurality of interior walls extending from member, said electroluminescent phosphor the sidewall structure into the first chamber emitting visible light when an electric field is and terminating within the first chamber, the applied by said electrodes. interior walls extending from the top plate to 25 the bottom plate to define an extended length, 16. The apparatus according to claim 15 wherein bending channel region for the plasma arc, one said fluorescent phosphor is attached to an of the internal cathodes being located at one inside surface of said light transparent mem- end of the bending channel region and the ber. other of the internal cathodes being located at 30 the other end of the bending channel region. 17. The lamp according to claim 15 wherein said electroluminescent phosphor is at least par- 11. The lamp according to claim 2, further includ- tially embedded with said light transparent ing: member. a third pair of electrodes extending into 35 said chamber and surrounded by the gas with- 18. A fluorescent lamp having two pairs of elec- in said chamber for creating a plasma arc trodes, comprising: within the chamber, said third pair of elec- a sealed chamber having a gas at a se- trodes being of the hot cathode type and said lected pressure therein, said gas including second pair of electrodes being of the cold 40 mercury vapor for emitting ultraviolet light cathode type, said third pair of electrodes be- when subjected to an electrical field; ing powered by said second electric power a first pair of electrodes adjacent said seal- source. ed chamber and positioned to apply an electric field to said gas within said sealed chamber, 12. The lamp according to claim 1, further includ- 45 each of said electrodes being separated from ing: said gas by a dielectric layer; a layer of magnesium oxide between said a second pair of electrodes within said fluorescent material and said dielectric layer, sealed chamber, surrounded by said gas, and said fluorescent material being overlaid on top positioned to conduct current through said gas of said magnesium oxide to lower the on/off 50 within said sealed chamber; and threshold of said fluorescent material. electric power supply means for providing power to said first and second pair of elec- 13. The lamp according to claim 1 wherein each of trodes. said electrodes is covered with a thin dielectric layer of generally uniform thickness. 55 19. The apparatus according to claim 18 wherein said electric power supply means includes two 14. The lamp according to claim 12 wherein said independent power sources for providing elec- dielectric layers are a heavy metal free glass. tric power to each of said pair of electrodes

14 27 EP 0 550 047 A2 28

independent of each other. 24. A method of applying a phosphor to a surface for use in a lamp, comprising: 20. The apparatus according to claim 18 wherein mixing a phosphor with a liquid to form a said electric power supply means includes a slurry; single DC power supply and invertor circuitry 5 placing said slurry in contact with a sur- to provide AC power to each of said pair of face region of the transparent glass layer; electrodes at different frequencies. heating said transparent glass layer with the slurry in contact therewith to the approxi- 21. A fluorescent lamp having two sealed cham- mately the reflow temperature of said glass bers, one positioned on top of the other, com- io layer until the glass layer begins to partially prising: melt at the surface region with which the slurry a first sealed chamber having a gas at a is in contact to cause said phosphor layer to at selective pressure therein, said gas including least partially enter the surface of said glass mercury vapor for emitting ultraviolet light layer; and when said gas is subjected to an electric field; is cooling said glass layer with said phosphor a first pair of electrodes positioned to ap- slurry partially embedded therein to obtain a ply an electric field to said gas within said first glass member having phosphor crystals em- sealed chamber for causing said gas to emit bedded therein and phosphor crystals projec- ultraviolet light; ting from a surface region thereof. a light transparent member forming a wall 20 of said first sealed chamber, said light being 25. The method according to claim 24, further emitted out of said light transparent member; including the step of: a fluorescent phosphor adjacent said light wiping off excess phosphor from said transparent member and positioned within said glass member after said cooling step to re- chamber to be exposed to ultraviolet radiation 25 move phosphor which is not adhered to said emitted by said gas when an electric field is glass member. applied by said electrodes to cause light to be emitted out of said light transparent member; 26. The method according to claim 24, further and including the step of: a second chamber attached to said first 30 applying a layer of magnesium oxide to sealed chamber and positioned above said said surface region of said transparent glass light transparent member such that light emit- member prior to placing said slurry on said ted from said first sealed chamber passes surface region. through said second chamber prior to being emitted by said fluorescent lamp, said second 35 27. The method according to claim 24, further chamber including at least one light transpar- including the steps of: ent member. placing said glass layer on a glass member prior to placing the phosphor slurry on the 22. The fluorescent lamp according to claim 21 glass layer, said glass member having a higher wherein said second chamber is a sealed 40 reflow temperature than said glass layer; chamber having a gas at a selected pressure coupling a plurality of said glass members therein; and to wall structures after said cooling step of the a fluorescent phosphor within said second glass layer to form a lamp using said glass sealed chamber and positioned to be exposed members. to ultraviolet radiation emitted by said gas with- 45 in said first chamber such that said fluorescent phosphor of said second chamber emits light when exposed to said ultraviolet radiation from said first chamber. 50 23. The fluorescent lamp according to claim 21 wherein said second chamber is a sealed chamber having a cooling fluid therein, said cooling fluid absorbing heat emitted by said first sealed chamber so that said lamp is effec- 55 tively cooled.

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