LOW PRESSURE ARC DISCHARGE LIGHT SOURCE UNIT

Abstract: In a low pressure arc discharge light source unit for unipolar or bipolar operation comprising a vacuum-­ tight glass envelope (1) translucent at at least one side thereof, a rare fill gas and a quantity of mercury therein, a coating of fluorescent phosphor on the inner side of the envelope, and at least two electrodes opposed to each other within the envelope and connected to lead-in wires or the like, the envelope (1) is flat and essentially two-dimensional comprising two planar areas essentially parallel to each other at a small distance. The unit provides for better light output, better radiation efficacy and simpler and less costly manufacture, at the same time being more compact. At least 50 % of the radiation of the planar area of the front of the envelope (1) is effective. Means are provided allowing a single cathode to cooperate with a multiplicity of anodes in e.g. a pixel.

Description: The invention relates to a low pressure arc discharge light source unit comprising a vacuum-tight glass envelope having two planar areas essentially parallel to each other at a distance corresponding to the width of a rim or spacer between them, said envelope being translucent at at least one side thereof, a rare fill gas and a quantity of mercury therein, a coating of fluorescent phosphor on the inner side of the envelope, and electrodes opposed to each other within the envelope and connected to lead-in wires or the like, at least one wall being provided within the envelope between the planar areas for forming a discharge space of multiple arc length, said wall extending essentially perpendicular to said planar areas. A unit of this kind is known from US-A-3 508 103 and is suitable for use as a dome light in a car and is flat as necessary for the said purpose. The distance between the two planar areas in the embodiment mentioned is a half inch, i.e. 12,7 mm. Length and width are five times and four times that distance, resp. Further, the unit in accordance with the said prior art has only one discharge space of multiple length. Only one color can be provided and only two electrodes are present. From JP-A-57-180067 (see English abstract) is known a low pressure discharge lamp, the rectangular cylindrical glass envelope of which comprises a phosphor layer on its inner surface, 3 sets of electrodes and 2 separating walls. The 3 discharges do not interfere and the luminous output is increased. From DE-A-2 952 535 a discharge lamp is known, the envelope of which comprises 2 trough-shaped glass bodies having planar areas and rims, both bodies being bonded together along their rims. From FR-A-2 549 640 is known a low pressure arc discharge light source unit comprising a translucent and vacuum-tight glass envelope, a rare fill gas and a quantity of mercury therein, a coating of fluorescent phosphor within the envelope, and electrodes opposed to each other within the envelope and connected to lead-in wires or the like. Same comprises a U-shaped envelope made from glass tube material and having two parallel legs ending in sockets providing for connecting pins. The known unit can be used for optical presentation of information, i.e. presentation of alphanumeric signs, graphics and pictures displayed on a screen or display, respectively. Such a display consists of a matrix of picture elements, each picture element consisting of a monochrome light signal source in case of a monochrome display. In case of color presentation of information one picture element is composed of three single light source units of the primary colors red, green and blue forming a so-called pixel. The desired or required, respectively, color impression is then created physiologically by additive mixture of the three primary colors within the human eye/brain system. Presenting information to a large audience in the open air means looking on a correspondingly large area display which is distinctly visible not only at night but also during day light and with sufficient optical resolution from a greater viewing distance. In case of presentation of rapidly moving pictures, like in television, the picture information changes up to 100 times per second (and up to 120 times per second in the U.S.). At the same time the temperature of the outside environment can fluctuate over a wide range, e.g. -20° to +50° C. While the known light source unit mentioned above in the second place is able to fulfill the demands stipulated above, there are some drawbacks present: 1. The known light source unit is presenting towards the audience the curved portion of the U-shaped envelope only so that no more than approximately 20% of the radiation is effective. The rest is dissipating, especially through the parallel legs of the U-shaped envelope which are arranged substantially normal or perpendicular, respectively, to the plane of fixation of a unit, said plane being also substantially normal to the viewing direction of the spectators. 2. The production cost are relatively high in view of special workmanship and single manufacture of each unit being necessary, including application of the sockets by hand. The objective underlying the invention is to be seen in the provision of a low pressure arc discharge light source unit as mentioned initially above rendering better light output or intensity or brightness, respectively, enhancing overall radiation efficacy and presenting a more compact unit which can be manufactured simpler and used for otical presentation of information, e.g. in a matrix. This objective is met with a low pressure arc discharge light source unit mentioned initially above in which the width of the rim between the planar areas is small so that the envelope is almost two-dimensional, wherein two or more discharge spaces are provided, at least one of these discharge spaces having a multiple arc length and at least one of them having a normal arc length or a different arc length, by means of at least two walls, at least one of the walls extends up to and joins with the rim with only one end whereas the other end(s) keep(s) a distance from the rim, wherein two or more fluorescent phosphors of different spectral power distribution are provided within different discharge spaces, and wherein three or more electrodes are arranged within the envelope. The invention provides for a number of advantages: 1. At least 50 % of radiation of the planar area representing the front of the envelope is effective. It is to be understood that this percentage can be further increased by providing a reflective layer on the back, whether within or outside the envelope. 2. The light source unit in accordance with the invention can be manufactured at decreased cost and in a continous manner, e.g. by use of a belt feed furnace for combining the two planar envelope areas essentially parallel to each other at a small distance, whereby the lead-in wires can be melted in. There is no need for the bending of a glass tube and for fixing sockets to the ends thereof. The envelope of the light source unit in accordance with the invention may comprise two planar bodies of glass being fixed to each other by spacers and glass solder. In a further embodiment the envelope comprises a planar glass body and a trough-shaped glass body having a planar area and a rim, both bodies bonded to each other along the rim of the trough-shaped body by glass solder. A still further embodiment is characterized in that the envelope comprises two trough-shaped glass bodies having planar areas and rims, both bodies bonded together along their rims by glass solder. It is preferred that there is a distance of approximately 3 mm to 10 mm between planar bodies or a planar body and the planar area of a trough-shaped body or the planar areas of two trough-shaped bodies. The trough-shaped bodies can be cover glasses whereas the planar bodies can be made from float glass. In a preferred embodiment on at least one of the inner sides of the planar area there is a fluorescent phosphor coating whereas on the inner or outer side of the opposite planar area there is a reflective coating so that the light output, as mentioned above, can be reinforced for unidirectional viewing. Without any reflective layer viewing will be bidirectional, of course. It is to be understood that the general shape of the envelope can be rectangular, square, circular or even polygonal as desired. In/forming the envelope the use of float glass plates is proper with spacers or a spacer frame. Also one glass plate and a flat cover glass or sinter glass as a trough-shaped body could be used, even two cover glasses. For effecting the joints glass solder of a low-melting point of similar thermal expansion coefficient is preferred. The leads or electrode connectors, respectively, can be made of wire or ribbon or can be formed by thin or thick film layers and fed either laterally or from the rear through the envelope, preferably single-ended. In accordance with a rectangular embodiment or the invention one or more separating walls are provided within the envelope between the planar bodies or the planar areas, respectively, said wall or walls extending essentially perpendicular thereto and in parallel relationship to each other. Each discharge space being provided with electrodes, different and independent control is possible. In a preferred embodiment different colors can be produced in the manner described above dependent from the phosphors and the energy input used, and this can be accomplished in a very advantageous manner by one single light source unit comprising only one flat and essentially two-dimensional envelope in accordance with the invention. At least one separating wall within the envelope extends up to and joins with the spacer or the rim or the rims of the envelope with only one end thereof whereas the other end keeps a distance from the spacer or the rim or the rims. By doing so discharge spaces having multiple arc length and/or discharge spaces allowing a common electrode are provided. So the arc can turn around the wall at the end thereof keeping a distance from the rim so that double arc length is provided within a single light source unit if one separating wall is present in a single-ended configuration, the wall separating the electrodes from each other at the end therof joining with the rim of the envelope at the opposite end of the separating wall. Two separating walls and more can be provided in an opposed arrangement for multiple arc length, more specifically, the separating walls within the envelope in parallel relationsship to each other can force the arc into a zig-zag-configuration. Two separating walls of this kind mean triple the arc length with a double-ended light source unit. Various configurations are possible within one and the same envelope, allowing different arc length for different brightness and different colors within one and the same unit, all of the discharge spaces being independently controllable. Longer discharge spaces provide for efficiacy, of course. For unipolar operation a recessed space for the cathode can be provided so that the effective discharge length or the positive column of discharge, respectively, will be 100 % within the area of the phosphor layer whereas the dark space present near the cathode is outside of the effective area of the light source, hence a better yield of the visible area of the light source unit in accordance with the invention is obtainable and advantageous especially when the unit is used for display purposes. An aspect of the invention of special importance is the possibility to save material and labour in connection with using a common cathode, at the same time providing for better performance of the light source unit. As an example, if two separating walls are provided within a light source unit, the two separating walls providing for three discharge spaces and separating three anodes from each other, however, not extending up to opposite sides but keeping a distance from the opposite spacer or rim, one common cathode will be sufficient to provide for independent operation of each discharge space. Continuous heating of the cathode makes it possible to ignite each discharge arc independently from the other two discharge arcs, whether there will be different phosphors in different discharge spaces or not. It is readily apparent that in the presence of a continuously heated cathode the time of response of each discharge within the light source unit to the ignition pulse will be shorter as compared with a situation in which individual cathodes are present for each discharge space and should be heated not before ignition thereof is intended. In addition thereto, only two passages for the lead-ins are necessary instead of six in the case of three cathodes being present. Further, only one exhaust tube and only one exhaust procedure are necessary instead of three. Unipolar operation is preferred, notwithstanding that there is no denying the fact that also bipolar operation is possible, using electrodes at both ends of the arc length instead of cathode and anode. In case of unipolar operation anodes being plates or conductive coatings inside the envelope are preferred. The invention and its preferred embodiments are described in more detail in view of the accompanying drawings wherein Fig. 1 to 11 do not fall within the terms of the scope of claim 1 but help to understand the invention. Figure 1 is a plan view of a first embodiment of the construction of the light source unit; Figure 2 is an elevational view of the first embodiment; Figure 3 is a plan view of a second embodiment; Figure 4 is an elevational view of the second embodiment; Figure 5 is an elevational view of a third embodiment; Figure 6 is a plan view of a fourth embodiment; Figure 7 is an elevational view of the fourth embodiment; Figure 8 is a plan view of a fifth embodiment; Figure 9 is an elevational view of the fifth embodiment; Figure 10 is a plan view of a sixth embodiment; Figure 11 is an elevational view of the sixth embodiment; Figure 12 is a plan view of a first embodiment of the invention. Figure 13 is a plan view of a further embodiment of the invention. It is to be understood that the embodiments of Fig. 12 and 13 can incorporate any of the embodiments of Fig. 1 to 11, as regards the construction of the light source unit. Figures 1 and 2 show a low pressure arc discharge light source unit for bipolar operation comprising a vacuum-tight glass envelope 1 which is in accordance with the invention flat and essentially two-dimensional and comprises two planar areas essentially parallel to each other at a small distance a. In this embodiment the envelope 1 comprises two planar bodies 2 and 3 of glass being fixed to each other by spacers 4, 5, 6 and 7 and glass solder (not shown). As to be seen from Figure 1, the spacers 4, 5, 6 and 7 form a rectangular spacer frame to which the glass bodies 2, 3 are soldered. The rectangular spacer frame could be formed also unitary or as a one-piece-configuration, respectively. The low-temperature glass solder or frit, respectively, produces a vacuum-tight fit. An exhaust tube 8 (Figure 2) allows the unit to be exhausted and filled with an inert fill gas and a quantity or drop, respectively, of mercury. Coatings 9 and 10 of fluorescent phosphor are provided on the inner side of the envelope or on the inner sides of the two planar glass bodies 2, 3,respectively. It is to be understood that when using the unit for display purposes it is advisable to provide for a reflective layer on the glass body adjacent to the display panel (not shown) which body is preferably body 3 to which lead-in wires 11 and 12 extend from the exterior towards electrodes 13 and 14. The lead-in wires 11 and 12, of course, pass through glass body 3 in a sealed- or melted-in manner. A reflective layer (not shown) could be disposed between the inner surface 13' of glass body 3 and coating 10, however, application of a reflective layer on the outside is possible also. As shown in Figures 1 and 2 the electrodes are provided for bipolar operation. The same kind of electrodes 13 and 14 is used with the embodiment according to Figures 3 and 4 and so the same numerals are used also for the lead-in wires 11 and 12 and for the planar glass body 3. The only difference of this embodiment as compared with the embodiment of Figures 1 and 2 resides in the facts that instead of the second planar glass body a trough-shaped glass body 14', a cover glass, is used, disposing of the frame forming spacers which are necessary with the preceding embodiment. In the embodiment of Figures 3 and 4 the trough-shaped glass body 14' comprises not only a planar area 15 and a rim 16, the latter being sufficient for forming a bond or joint with the planar glass body 3 by means of glass solder, but also a flange 17 for providing a broader surface of contact between bodies 14' and 3. The embodiment according to Figure 5 is a light source unit for unipolar operation using a cathode 18 and an anode 19. The rest of the construction is like the embodiment of Figures 1 and 2, except for a recessed space 20 for cathode 18 extending outwardly from the bottom of planar glass body 3'. The advantages of this configuration have been elucidated above already. It is to be understood that instead of planar glass body 2 a cover glass corresponding to the embodiment in accordance with Figures 3 and 4 can be used. The embodiment of Figures 6 and 7 corresponds to the embodiment of Figures 1 and 2 except for the fact that it is destined for unipolar operation and, therefore, comprises a cathode 21 and an anode 22 instead of identical electrodes. Further, thin ribbons 11' and 12' are used as electrical conductors instead of wires. The embodiment of Figures 8 and 9 corresponds to the embodiment of Figures 6 and 7 except for the realization of the electrical conductors and spacers 5 and 7. The electrical conductors are layers 23 and 24 in thick or thin film technique out of metal or graphite, not only for the leads for the cathode 21 but also for the leads of the anode 22, the latter forming a film layer inside of spacer 5'. In the embodiment of Figures 8 and 9, the planar glass bodies 2 and 3 extend beyond the two spacers 5' and 7' of spacer frame 4, 5', 6, 7' so that there will be areas of the electrical conductor layers 23 and 24 being exposed to the outside of unit 1 so that contact can be made as desired, planar glass body 3 forming at its end a substrate for such connecting ends of the layers. It is preferred to provide for anodes of large areas. The electrodes and/or the cathodes can be oxide-coated tungsten filaments. The embodiment of Figures 10 and 11 corresponds to the embodiment of Figures 1 and 2 except for the unipolar configuration, i.e. it has a cathode 25 and an anode 26 instead of identical electrodes, anode 26 corresponding essentially to anode 22 of the embodiment of Figures 6 and 7, except for the electrical connectors which are not ribbon-like but lead-in wires or pins as it is the case with the embodiment of Figures 1 and 2. Again to the same parts the same reference numerals have been assigned. Figures 12 and 13 present two embodiments of the inventive principle offered by means of example. It is to be understood that the variations possible are virtually infinite. The variation includes not only the method of electrical operation (unipolar and bipolar) but also the number of emitted colors and the length of the arc or arcs within one unit. With the shown embodiments the electrical connectors are situated in accordance with the embodiments of Figures 6, 7 and 8, 9, respectively, i.e. the connectors are ribbons 11', 12' or layers 23, 24. In all these further embodiments reference numerals used in previous embodiments are used also here for identical parts if not otherwise indicated. For display purposes the devices or units, respectively, are generally single-ended at the rear as it is the case with the embodiments of Figures 1, 2 and 3, 4 and 10, 11. The arcs and their lengths are shown by dotted lines. In view of partially separating walls 28 in the embodiment according to Fig. 13 a common cathode 29 can be provided. It is to be understood that in spite of the common cathode 29 the discharge spaces can be ignited and controlled independently from each other so that the unit can switch over from one color to the other notwithstanding the fact that in doing so not only different discharge spaces but also different phosphors are involved. By the way, this embodiment can provide, of course, for the impression of different colors at the spectator by using one or more of the discharge spaces. Further, also a monochrome configuration is possible providing for different brightness of one and the same color depending from whether only one discharge space is used or more. Additionally, switching operation is accomplished at a shorter time in view of the fact that cathode 29 serves three (or possibly more) anodes 32. Figure 12 is a unipolar three color unit having the two outer discharge spaces at double arc length and the middle discharge space at normal, i.e. simple arc length. Same applies to the embodiment in accordance with Figure 13. In Figure 12, of course, the middle discharge space having only half the length of the outer discharge spaces is fenced in by separating walls 27 extending at both ends up to and sealingly joining with the rim of the unit leaving no distance. In Fig. 12 and in Fig. 13 the color green having the highest electro-optical efficiency will be chosen for the middle discharge space in order to compensate for the different brightness caused by different arc lengths. It is to be understood that in every modification chosen for a special display application the longest possible arc length or path in order to achieve maximum efficiency should be chosen. In separating the discharge spaces always glass can be used, however, in case of multicolor devices dark glass is to be preferred in order to avoid color mixing. Attention has been drawn already to the fact that color mixing or the formation of different colors not being plainly red, green and blue, respectively, is to be carried out by the human eye/brain system. Application of the inventive light source unit for display purposes requires extensive brightness control. This can be achieved by unipolar operation of the D.C. configuration and by controlling the current and/or pulse width in a pulse modulation system. To obtain maximum electro-optical conversion efficacy it is important to use an optimum temperature to produce an optimum mercury vapour pressure. With the unit in accordance with the invention this can be achieved by thermostatic control of a thermal conductive metallic flange to be arranged at the backside of the unit, i.e. the side at which the unit will be affixed to a display panel or the like. Good thermal conductivity may be obtained by the use of e.g. alumina filled adhesive or silicone grease. In order to keep power losses due to cathode and anode fall as low as possible, it is essential to maintain the gas discharge at as high an arc voltage as possible. Also this can be achieved by means of long arc paths which can be obtained by the separating or partitioning, respectively, walls 28 as described in more detail above. Special attention is drawn to the advantages single-ended units obtained by this technique have. As pointed out above the general shape of the light source units in accordance with the invention has not necessarily to be rectangular or square, the shape could also be circular or polygonal. Accordingly, also the separating walls are not necessarily to be planar, e.g. in case of a circular shape of the unit the wall may take the form of an archimedic spiral. It should be understood that reflective coatings are advantageous with uni-directional displays; bi-directional displays do not need reflective layers, of course. If reflective material is used, this can be metals if the reflective coating is deposited on the outer surface of the envelope, e.g. Ag, Al and Cr, or white pigments, e.g. alumina, barium sulphate or magnesia if used inside the envelope. As pointed out previously also the reflective layer can be coated with a fluorescent phosphor if arranged inside the envelope. Emphasis is given to the advantage residing with the invention with regard to the fact that the forming of the envelope including the separating walls and the electrodes or cathodes and anodes, including the appertaining electrical conductors, can be accomplished by one and the same manufacturing step in the furnace, preferably feed belt furnace. As elucidated above, the common cathode 29 (Fig. 13) which in operation will be heated constantly will provide for an easy and quick response to ignition and instantaneous fluorescence of the three colors in common. In addition thereto it is possible to ignite each discharge arc independently from the other two discharge arcs, whether there will be different phosphors in different discharge spaces or not. Only one exhaust tube and only one exhaust procedure are necessary with the embodiment in accordance to the said Figures, notwithstanding the fact that an exhaust tube is not shown therein. It is to be understood that the invention is not limited to pixels or units having only three discharge spaces, respectively. It was pointed out above that the number of possible embodiments is virtually indefinite and that e.g. four, five, six or more discharge spaces can be incorporated within one and the same envelope enclosing the necessary number of separating walls and electrodes. The concept of only one cathode opposing a multiplicity of anodes is emphasized again.

Claims: Low pressure arc discharge·light source unit comprising a vacuum-tight flat glass envelope (1) having two planar areas essentially parallel to each other at a distance corresponding to the width of a rim or spacer between them, said envelope being translucent at at least one side thereof, a rare fill gas and a quantity of mercury therein, a coating of fluorescent phosphor on the inner side of the envelope, and electrodes opposed to each other within the envelope and connected to lead-in wires or the like, at least one wall being provided within the envelope (1) between the planar areas (15) for forming a dischage space of multiple arc length, said wall extending essentially perpendicular to said planar areas, wherein the width (a) of a rim between the planar areas is small so that the envelope (1) is almost two-dimensional, wherein two or more discharge spaces are provided, at least one of these discharge spaces having a multiple arc length and at least one of them having a normal arc length or a different multiple arc length, by means of at least two walls (27, 28), at least one (28) of the walls extends up to and joins with the rim (16) with only one end whereas the other end(s) keep(s) a distance from the rim, wherein two or more fluorescent phosphors of different spectral power distribution are provided, within different discharge spaces, and wherein three or more electrodes are arranged within the envelope (1). Light source unit according to claim 1, characterized in that the envelope (1) comprises a planar glass body (3) and a trough-shaped glass body (14') having a planar area (15) and a rim (16), both bodies bonded to each other along the rim of the trough-shaped body by glass solder. Light source unit according to claim 1, characterized in that the envelope (1) comprises two trough-shaped glass bodies having planar areas (15) and rims (16), both bodies bonded together along their rims (16) by glass solder. Light source unit according to claim 1, 2 of 3, characterized in that there is a distance (a) of approximately 3 mm to 10 mm between planar bodies (2, 3) or a planar body (3) and the planar area (15) of a trough-shaped body (14') or the planar areas of two trough-shaped bodies. Light source unit according to claim 1 or 2, characterized in that on at least one of the inner sides (13') of the planar bodies (3) and/or planar areas (15) defining a discharge space a fluorescent phosphor coating (9, 10) is provided whereas on the inner or outer side of the opposite planar area there is a reflective coating. Light source unit according to claim 1, characterized in that at least one wall (27) within the envelope (19) extends at both ends up to and sealingly joins with the spacer (4, 5, 6, 7) or the rim (16) of the envelope (1) to form different discharge spaces. Light source unit according to claim 1, characterized in that a plurality of discharge spaces having a multiple arc length is provided, at least one of said discharge spaces having a multiple arc length being different from the multiple arc lengths of the other discharge spaces. Light source unit according to claim 1 or 7, characterized in that all the discharge spaces are formed by walls extending up to and joining with the rim with only one end whereas the other end keeps a distance from the rim, and that one common cathode is arranged within said distance serving all the discharge spaces present. Light source unit according to any of the preceding claims, characterized in that its shape is circular or polygonal. Light source unit according to claim 1, 7, or 8, characterized in that a thermally conductive metallic flange is arranged at the backside thereof at which the unit is affixed. Light source unit according to claim 1 for unipolar operation, characterized in that a recessed space (20) for the cathode (18) is provided. Light source unit according to claim 1, characterized in that for unipolar operation the anode of each discharge space is a plate or a conductive coating inside the envelope (1). Use of the light source unit in accordance with claim 8 wherein the common cathode is heated constantly. Use according to claim 13, wherein two or more discharge spaces are ignited in common. Use of the light source unit in accordance with claim 1 unipolarly for display purposes, wherein current and/or pulse width are controlled in a pulse modulation system to provide extensive brightness control. Use of the light source unit in accordance with claim 10, wherein the metallic flange is thermostatically controlled. Method of manufacturing the light source unit according to any of the preceding claims, wherein the forming of the envelope including the walls, the electrodes or cathodes and anodes and the appertaining electrical conductors is accomplished by one and the same step in a furnace, preferably feed belt furnace.

Inventors: LOY, HELMUT M.
Applicants: GTE LICHT GMBH
Application year: 1985
Main classification: H01J61
IPC codes: H01J61