Compact fluorescent lamp

Abstract: A substantially flat compact arc-discharge fluorescent lamp (1) comprises a flat chamber filled with a discharge gas. The chamber has a top wall (3) opaque to fluorescent light and a bottom wall (2) transparent to fluorescent light. An inner wall structure (12) sealingly connects the top and bottom walls (3,2) to form a continuous discharge path (4) extending in a substantially flat spiral pattern within the chamber. The length of the discharge path (4) and the height of the lamp (1) are selected to provide a wide range of lumens per unit volume while improving the lamp life at low cost. A phosphor coating (23) is formed on the wall structure (12) and on the inner surfaces of the top and bottom walls (3,2). A pair of electrodes (8,10) are disposed on the opposite ends of the discharge path (4). A reflective coating (21) formed adjacent to the phosphor coating (23) is useful to direct light in a desired direction, while obviating the need for a separate reflector in a lamp fixture. The lamp (1) may comprise a ballast (17) and an incandescent-type base (31) to allow the lamp (1) to be detachably attached to a socket of a regular incandescent lamp.

Description: TECHNICAL FIELD The present invention relates to a substantially flat compact, arc discharge, fluorescent lamp. BACKGROUND ART Fluorescent lamps can provide a higher lumen output and longer lamp life than regular incandescent lamps. Typically, the efficiency of an incandescent lamp is 16 lumens per watt (lpw) of electrical energy input; while a fluorescent lamp provides, typically, 50-60 lpw. The incandescent lamps last only about 750 hours; while the fluorescent lamps last anywhere from six to ten thousand hours. In commercial establishments where one has to pay labor to change light bulbs, the less often a bulb needs to be changed the more (measurably) economical the installation becomes. Thus, substantial energy savings and cost savings would be obtained by replacing the incandescent lamps with the fluorescent lamps. For such a replacement, it is desired to make the fluorescent lamps more compact to fit them into a smaller space because the general environment around incandescent bulbs is substantially smaller than the environment that existing fluorescent bulbs need. Known compact fluorescent lamps are typically formed by bending a glass tube in a U-shaped or circular form to minimize its volume. Extensions of this basic design are Double-U, Triple-U and Quadruple-U forms which are simply the joining of the basic-U. However, these compact fluorescent lamps tend to be long to maintain an arc length required for a desired light output. Further, an external reflector (fixture) is needed if these are to be employed for preferential directional lighting. In addition, the compact fluorescent lamps involves a tradeoff among many parameters. In other words, it is often impossible to optimize all the parameters simultaneously. For example, the smaller size, the less the efficiency and often the higher cost. Furthermore, as the size is reduced, heat removal becomes a problem. Heat has to be removed so as to maintain an optimum mercury vapour pressure for optimum efficiency. Similarly, as size is reduced the phosphor loading (current density per surface area of phosphor or power per surface area of phosphor) increases leading to faster phosphor light output deterioration. Again, as size is reduced (due to the difficulty of heat removal) electrical components in a base of the lamp tend to get very hot which leads to shorter life for the lamp. Attempts to solve these heat problems include coating the phosphor with alumina and using amalgams, perforated outer jackets, heat transfer conduits, etc. Another application where substantially flat compact, fluorescent lamps are used is related to the expanded use of lap-top personal computers. With the advent of Liquid Crystal Displays (LCDs) for the lap-top computers, the demand of the flat fluorescent lamps as Area-lamps increased. This increase is due to LCDs' requirement for some form of backlighting. This application requires: (i) the lamp needs to be flat with either a rectangular geometry or a square geometry, (ii) the brightness over the lamp surface has to be uniform, (iii) a dimming ratio should be > 1000:1. In a lap-top computer, it is preferred that a thickness of the flat fluorescent lamp in millimeters or less. In an overhead lighting fixture for general lighting purposes, it is preferred that the thickness of the flat fluorescent lamp is in centimeters or more. For example, U.S. Patent No. 3,226,590, to Christy, discloses a large area substantially flat fluorescent lamp called "panel-lamp" that consists of two corrugated glass plates laid over each other and sealed to withstand the atmospheric pressure. By virtue of its corrugation and seal pattern, an electrical discharge path in the shape of a serpentine or labyrinthine is generated. A similar corrugated structure is adopted for a round, substantially flat lamp which is disclosed in U.S. Patent No. 3, 243,630, to W.C. Martyny. However, these lamps are not practical for retrofitting into incandescent lamp applications. Other substantially flat arc- discharge fluorescent lamps are disclosed in US- A-2406146, wherein a sealed flat chamber is filled with a discharge gas, said chamber having a top or bottom wall opaque to fluorescent light, a bottom or top wall transparent to fluorescent light and an inner wall structure sealingly interconnecting said top and bottom walls to form a continuous, substantially flat spiral discharge path, a phosphor coating formed on said wall structure and on inner surfaces of said top and bottom walls, and a pair of electrodes disposed at opposite ends of said discharge path for establishing and maintaining an arc or said discharge gas, and in US- A- 2405518, wherein the inner wall structure includes an outer wall and a plurality of inner walls, the inner walls being formed of a continuos strip. For improving the above problems, the present invention as disclosed in the independent claims is directed to a substantially flat compact, arc discharge, fluorescent lamp having a unique discharge path or pattern. The lamp comprises a flat chamber filled with a discharge gas. The chamber has a top wall opaque to fluorescent light and a bottom wall transparent to fluorescent light. An inner wall structure sealingly connects between the top and bottom walls to form a continuous discharge path extending in a substantially flat spiral pattern within the chamber. A phosphor coating is formed on the wall structure and inner surfaces of the top and bottom walls. A pair of electrodes are disposed at the opposite ends of the discharge path to establish and maintain an arc of the discharge gas between the electrodes. In the present invention, a length of the discharge path is substantially determined by a width of the discharge path. That is, as the discharge path width is reduced, at the same lamp dimension, the discharge path length can be increased. Therefore, the present fluorescent lamp can provide a wide range of lumens per unit volume of the chamber without changing the lamp dimension. Additionally, flat lamps have to be a structure capable of withstanding the atmospheric pressure. In the present fluorescent lamp, since the flat chamber is uniformly reinforced with the inner wall structure, any cracking due to the pressure on the flat chamber can be effectively prevented. In particular, this structural feature would be favorable as an area of the flat surface of the chamber is increased. Therefore, a primary object of the present invention is to provide a substantially flat compact, arc discharge, fluorescent lamp with a spiral discharge path for supplying a requested lumen output. In a preferred embodiment of the present invention, the discharge path is of a rectangular or rounded cross section. In a further preferred embodiment of the present invention, a reflective coating is partially formed on one side of the phosphor coating such that light striking the reflective coating is directed back towards the opposite side of the phosphor coating. Many conventional compact fluorescent lamps use separate reflectors. However, in the present invention, since the reflective coating is formed within the chamber, it will results in saving on a lighting fixture. Therefore, the present fluorescent lamp can provide preferential directional lighting such as downlighting without using any external and expensive lighting fixture. In a still further preferred embodiment of the present invention, a thickness of the wall structure, the discharge path width and a vertical height of the lamp are determined so as to provide for a range of lumens per chamber volume of about 4 to about 17 lm/cm 3 (about 70 to about 280 lumen per cubic inch). In another preferred embodiment of the present invention, the fluorescent lamp comprises a ballast and incandescent-type base to allow the fluorescent lamp to detachably attach to a socket of regular incandescent lamps. Therefore, it is possible to replace the incandescent lamps with the present fluorescent lamp by the use of the same physical setting as the incandescent lamps. The fluorescent lamp explained above can be made in accordance with the following process of the present invention. An outer glass ring, a plurality of inner glass rings with different diameters, top and bottom glass plates having the same diameter as the outer ring and a plurality of back discharge suppresser plates are prepared. Each of the inner rings has a slot. A reflective coating is formed on the inner rings and a flat surface of the top plate. The outer and inner rings are sealingly attached in a concentric pattern to the bottom plate to define an inner channel between adjacent rings. The suppresser plates are sealingly attached to the bottom plate and between the adjacent rings such that each of the inner channels is joined with adjacent inner channel through the slot to form a continuous, flat spiral discharge path on the bottom plate. Subsequently, a phosphor coating is formed on the reflective coating and a flat surface of the bottom plate on which the discharge path extends. A pair of electrodes are sealingly attached to the top plate at positions corresponding to the opposite ends of the discharge path. It is preferred that one of the electrodes has a mercury dispensing getter. Then, the top plate is sealingly attached to free edges of the inner and outer rings on the bottom plate to form a lamp vessel. The lamp vessel is filled with mercury and at least one rare gas to thereby obtain the fluorescent lamp of the present invention. To stabilize the lamp characteristics, an aging treatment would be performed to the lamp. In general, a bending step of a glass tube, which inherently is a slow process, is required to form serpentine or labyrinthine discharge paths of current fluorescent lamps. However, in the present process, it is possible to efficiently make the fluorescent lamp with the spiral discharge path without carrying out such a bending step. In addition, a distance between the adjacent rings, that is, the discharge path width, can be determined by changing the number of the inner rings. Therefore, it is convenient to design the spiral discharge path for providing a requested lumen output. Other objects, features and advantages will be apparent from the following detailed description of preferred embodiments referring to the attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a flat rounded, arc discharge, fluorescent lamp of a first embodiment of the present invention, in which a viewing glass plate is not shown; FIG. 2 is a side view of a unidirectional lamp having the fluorescent lamp and a ballast; FIG. 3 is a cross-sectional view of the fluorescent lamp for illustrating an arrangement of phosphor/reflective coatings; FIG. 4 is a cross-sectional view of the fluorescent lamp for illustrating another arrangement of phosphor/reflective coatings; FIG. 5 is an exploded perspective view helpful understanding a process of making the fluorescent lamp of the present invention; FIG. 6 is a perspective view of a flat rectangular, arc discharge, fluorescent lamp of a second embodiment of the present invention, in which a viewing glass plate is not shown. DETAIL DESCRIPTIONS OF THE PREFERRED EMBODIMENTS First Embodiment A substantially flat, rounded, arc discharge, fluorescent lamp 1 of the first embodiment is explained in detail. In FIG.1, a viewing glass plate 2 (shown in FIG. 6) transparent to fluorescent light is removed from the lamp 1 , so that a continuous, flat spiral discharge path 4 is viewable. The discharge path 4 has a rectangular cross section. There are concentric rings 6 that extend vertically from a cathode glass plate 3 . The concentric rings 6 consist of an outer ring 11 and three inner rings 12 having different diameters. There are slotted opening 9 in each of the inner rings 12 and back discharge suppresser plates 13 joining each ring with adjacent ring to obtain the spiral discharge path 4 . First and second electrodes 8 and 10 are disposed at or near the opposite ends of the discharge path 4 . The first electrode 8 has a mercury getter ring 14. Numeral 5 designates an exhaust tube connecting with a through-hole in the cathode plate 3 , as described later. The lamp 1 is filled with mercury and a rare gas as a discharge gas. If necessary, it is possible to use any kind of rare gas or rare gas mix at any pressure plus a mercury drop of any weight or a mercury amalgam of any composition. Numeral 7 designates a pair of electrical leads for the first electrode 8 which make electrical connections. The electrode 10 also has similar electrical leads. In this embodiment, the diameter of the lamp 1 is 76 mm (3"), and the discharge path length is about 800 mm (about 32"). In this lamp, a reflective coating 21 such as alumina (Al 2 O 3 ) or silicon oxide (SiO 2 ), which is capable of reflecting UV and visible light, is formed on both of inside and outside surfaces of the inner rings 12 and an inside surface of the cathode plate 3 . The reflective coating 21 is also formed on an inside surface of the outer ring 11 to prevent a visible radiation through the outer ring. A phosphor coating 23 is formed on the entire surface of the reflective coating 21 and an inside surface of the viewing plate 2 , as shown in FIG. 3. It is possible to use any kind of phosphor or phosphor mix for the phosphor coating. Therefore, the lamp 1 of this embodiment is adequate for a unidirectional lighting such as downlighting. Of course, It is possible to adopt another arrangement of the reflective coating 21 in the lamp 1 . For example, as shown in FIG. 4, since the reflective coating 21 is not formed on the inside surface of the outer ring 11 , visible light is radiated through the outer ring as well as the viewing plate 2 . This kind of flexibility is not found in a bent tube approach of the prior art's fluorescent lamps. By the way, the rectangular cross section of the discharge path 4 gives a larger surface area than a circular one. This is important because the larger the surface area of the phosphor coating for the same power loading, the better the lumen maintenance and the phosphor coating suffers less deterioration. A rate at which the phosphor coating loses luminosity determines the life of the lamp. Therefore, the flat fluorescent lamp 1 having the discharge path 4 of the rectangular cross section is likely to have better lumen maintenance or show a longer lamp life when compared with a tubular or bent tube fluorescent lamp of equal lumen output. Alternatively, of course, when the power loading in the fluorescent lamp 1 is increased so as to be equal to the power loading in the bent tube approach, the lamp 1 results in the same lumen maintenance and lamp life but higher lumens per unit volume. This leads to a fundamentally more compact fluorescent lamp. This is one of the advantages of the flat lamp approach. If necessary, the fluorescent lamp 1 of the present invention may be formed into a package of equal size and shape to an incandescent lamp. That is, as shown in FIG. 2, a ballast 17 is disposed at the cathode plate side of the lamp 1 to obtain a unidirectional lamp 30 . The lamp 30 has a screw-type electrical power fitting 31 that matches a typical incandescent bulb receptacle (not shown). Thus, the regular incandescent lamp can be replaced with the present unidirectional lamp. The fluorescent lamp 1 of FIG. 1 can be made in accordance with the following process of the present invention, as shown in FIG. 5. Four glass rings, R1 , R2 , R3 and R4 are cut out of soda-lime glass tubes with inner diameters of 72 mm, 49 mm, 32.5 mm and 15 mm, respectively. All of the rings has a height of 25.4 mm. Except for the ring R1 , the rings R2-R4 have the slotted openings 9 of widths 10 mm ( R2 ), 7 mm ( R3 ) and 6 mm ( R4 ), respectively. After the rings are pre-cleaned, inner and outer surfaces of the rings R2-R4 and an inner surface of the ring R1 are coated with alumina (Al 2 O 3 ) as the reflective coating 21 . A frit glass paste is applied to edges of the rings R1-R4 , and then the rings are attached to a circular glass plate with a diameter D1 of 76.2 mm as the viewing plate 2 in a concentric pattern to define an inner channel between adjacent rings. The back discharge suppresser plates 13 are sealingly attached to the viewing plate 2 and between the adjacent rings such that each of the inner channels is joined with adjacent inner channel through the slotted opening 9 to form the continuous spiral discharge path 4 , to thereby obtain an intermediate product. The intermediate product is sealed at a temperature of 450 °C. An outer channel defined between the rings R1 and R2 has a channel width of 10.5 mm. Two inner channels are defined between the rings R2 and R3 , and between the rings R3 and R4 , respectively. A channel width of each of the inner channels is of 7.5 mm. The internal surfaces of the intermediate product are coated with a phosphor material to form the phosphor coating 23 thereon. The phosphor coating 23 is baked for organic burn-out. Free edges of the rings R1-R4 are cleaned free of the phosphor material. The frit glass paste is applied to the free edges of the rings and dried. A preparation of the cathode plate 3 is completed in the following sequence : (1) A circular plate 40 of the soda-lime glass with a diameter of 76.2 mm and thickness of 3 mm is cut. (2) Two pairs of holes 41 of diameter 2.3 mm are drilled at predetermined locations on the circular plate 40 . A fifth hole 42 for attaching the exhaust tube 5 is drilled in the circular plate 40 so as to be arranged away from the pairs of the holes 41 . (3) Two pairs of the electrical leads 7 of Ni-Fe-Cr alloy are heat-treated and frit sealed through the two pairs of the holes 41 . (4) A flat surface of the circular plate 40 is coated with alumina (Al 2 O 3 ). The alumina coating is baked for organic burn-out. (5) The phosphor coating 23 is formed on the alumina coating and baked for burn-out. (6) Each of triple carbonate coated filaments as the electrodes 8 and 10 is welded to the pair of the electrical leads 7 . In FIG. 5, the mercury dispensing getter 14 is attached to the electrical lead 7 of the electrode 8 . The intermediate product and the cathode plate 3 are placed in alignment to obtain a lamp assembly. After the frit glass paste is applied to an edge of the exhaust tube 5 , the exhaust tube is laid over the fifth hole 42 of the cathode plate 3 . A sealing pressure is applied over the entire lamp assembly and frit-sealed in an oven at a temperature of 450 °C to obtain a sealed unit. The sealed unit is exhausted through a special exhaust procedure and then filled with argon gas and mercury to obtain the fluorescent lamp 1 . The lamp is subjected to an aging procedure for stabilization. In a process comprising a bending step of glass tubes for making fluorescent lamps, a thickness of the glass tube has to be large enough to withstand the atmospheric pressure. However, since the inner rings R2-R4 are disposed within the lamp 1 and are not exposed to the atmospheric pressure, the inner rings with a thinner thickness can be used in the present process. This feature saves space and is less expensive. In addition, due to the thin thickness of the inner rings, visible light absorption decreases, so that a more efficient light source would be obtained. The performance characteristics of the fluorescent lamp 1 manufactured by the above process are as follows. When the lamp is at ambient temperature of 25 °C, a typical lamp voltage is about 75V, at 200 mA for a lamp power of 13.6 W. The lamp exhibits a normal fluorescent volt-ampere characteristic. Second Embodiment As shown in FIG. 6, a substantially flat, rectangular, arc discharge, fluorescent lamp 1A of the second embodiment is explained below. A viewing glass plate transparent to fluorescent light is not shown in FIG. 6, so that a continuous, rectangular spiral discharge path 4A is viewable. In this embodiment, the lamp 1A comprises a wall structure 6A . The wall structure 6A consists of an outside wall 11A and a plurality of inside walls 12A. The inside walls 12A are formed with one continuous strip that is bent into a square structure forming the spiral discharge path 4A . The discharge path 4A leads from one electrode 8A to the other electrode 10A . Except for the above features, the fluorescent lamp 1 A of the second embodiment is a substantially same as the lamp 1 of the first embodiment. Therefore, no duplicate explanation to common parts and operation is deemed necessary. Like parts are designated by like numerals with a suffixed letter of " A ". The lamp 1A of the second embodiment can be manufactured in accordance with the process of the first embodiment. In these embodiments, many variations from the above particular designs and process are available to those of ordinary skill in the art. The number of the inner channels can be increased or decreased without substantially changing outer dimensions of the lamp. This would change lumen output. Similarly, the vertical height of the lamp can be altered depending on the needs. As the height is reduced, at the same current, an amount of power that can be coupled is increased to lead a higher lumen output.

Claims: A substantially flat arc-discharge fluorescent lamp (1) comprising a sealed flat chamber filled with a discharge gas, said chamber having a top wall (3) opaque to fluorescent light, a bottom wall (2) transparent to fluorescent light, and an inner wall structure sealingly interconnecting said top and bottom walls to form a continuous, substantially flat spiral discharge path (4), a phosphor coating (23) formed on inner walls of said chamber, and a pair of electrodes (8, 10) disposed at opposite ends of said discharge path (4) for establishing and maintaining an arc of said discharge gas, characterised in that said inner wall structure includes a plurality of concentric rings (11, 12) of different diameters, wherein each ring (12) except the outermost (11) has a slot (9) and the annular space between each two adjacent rings is interrupted by a back discharge suppresser plate (13). A substantially flat arc-discharge fluorescent lamp (1A) comprising a sealed flat chamber filled with a discharge gas, said chamber having a top wall (3) opaque to fluorescent light, a bottom wall (2) transparent to fluorescent light, and an inner wall structure sealingly interconnecting said top and bottom walls to form a continuous, substantially flat spiral discharge path (4A), a phosphor coating (23) formed on inner walls of said chamber, and a pair of electrodes (8A, 10A) disposed at opposite ends of said discharge path (4A) for establishing and maintaining an arc of said discharge gas, characterised in that said inner wall structure includes an outer wall (11A) and a plurality of inner walls (12A), the inner walls being formed of a continuous strip bent into a rectangular structure. The lamp of claim 1 or 2, wherein said wall structure (12; 12A) extends perpendicularly to said top and bottom walls (3, 2). The lamp of any preceding claim, wherein the thickness of said wall structure (12; 12A), the width of said discharge path (4; 4A) and the vertical height of said lamp (1; 1A) are determined to provide a light flux per chamber volume in the range of about 4 to about 17 ℓm/cm 3 . The lamp of any preceding claim, wherein said discharge path has a rounded or rectangular cross section. The lamp of any preceding claim, comprising a reflective coating (21) partially formed on one side of said phosphor coating (23) such that light striking said reflective coating is directed back towards the opposite side of said phosphor coating. The lamp of any preceding claim, comprising a ballast (17) and a single base (31) adapted to be inserted in a power supply socket. A process for making a substantially flat arc-discharge fluorescent lamp (1), comprising the steps of: preparing a plurality of glass rings (R1-R4) of different diameters, top and bottom glass plates (3, 2) having the same diameter as the largest one (R1) of said rings, and a plurality of back discharge suppresser plates (13), each of said rings except the largest one having a slot (9), coating said rings except the largest one and a flat surface of said top plate with a light reflective material to obtain a reflecting film (21) thereon, sealingly attaching said rings in a concentric pattern to said bottom plate to define an inner channel between adjacent rings, sealingly attaching said suppresser plates to said bottom plate between adjacent rings such that each of said inner channels is interrupted and joined with an adjacent inner channel through said slot to form a continuous, flat spiral discharge path (4) on said bottom plate, coating a phosphor on said reflecting film and a flat surface of said bottom plate along which said discharge path extends to form a phosphor film (23) thereon, sealingly attaching a pair of electrodes (8, 10) to said top plate at positions corresponding to opposite ends of said discharge path, sealingly attaching said top plate to free edges of said rings on said bottom plate to form a lamp vessel, and filling said lamp vessel with mercury and at least one rare gas to obtain said fluorescent lamp. The process of claim 8, comprising the step of coating an inside surface of said largest ring (R1) with said reflective material. The process of claim 8 or 9, comprising the step of providing in said top plate (3) two pairs of holes (41) for sealingly attaching said electrodes (8, 10) and an exhaust hole (42) for attaching an exhaust tube (5). The process of any of claims 8 to 10, comprising the step of mounting a mercury dispensing getter (14) on one of said electrodes (8 and 10). The process of any of claims 8 to 11, comprising the step of ageing said fluorescent lamp (1) to stabilise the lamp characteristics.

Inventors: ANANDAN MUNISAMY, MAYA JAKOB, ANANDAN, MUNISAMY, MAYA, JAKOB
Applicants: MATSUSHITA ELECTRIC WORKS LTD, MATSUSHITA ELECTRIC WORKS, LTD.
Application year: 1995
Main classification: H01J61
IPC codes: H01J61, H01J9