LOW-PRESSURE DISCHARGE LAMP

Abstract: The low-pressure discharge lamp has a lamp vessel (1) into which hollow cylindrical electrodes (3) enter, between which a discharge path extends. A hollow body (5) lies in the extended direction of at least one of the electrodes (3) at a distance from an end (4A, 4B) thereof. The hollow body is connected to the electrode by electrically conductive means (7) and is coated with electron emitter (6). The electron emitter comprises at least one mixed oxide of at least one of the elements Ba and Sr with at least one metal from the group comprising Ta, Ti, Zr, Sc, Y, La and the lanthanids, wherein electron emitters of the composition BAxSr1-xY2O4, x being in the range of 0 to 1, are excluded. Lamps according to the invention provide a relatively high luminous flux.

Description: Low-pressure discharge lamp. The invention relates to a low-pressure discharge lamp, comprising a tubular glass lamp vessel which is closed in a vacuumtight manner and which has end portions; an ionizable filling comprising a rare gas in the lamp vessel; hollow cylindrical electrodes which enter the lamp vessel each at a respective end portion and which each have an internal end inside and an external end outside the lamp vessel. Such a low-pressure discharge lamp is known from EP-A 0 562 679 (PHN 14.189). The known lamp is of a simple construction which is easy to realise. The hollow cylindrical electrodes therein have a multiple function: they act as electrodes inside the lamp vessel, as current supply conductors and current lead-throughs inside the lamp vessel and in the lamp vessel wall, and also as tubes through which the lamp vessel can be cleaned and be provided with its filling. The lamp vessel may be closed in a vacuumtight manner in that a glass tube is fused to each of the electrodes outside the lamp vessel and is closed at its free end, for example by fusion. The construction of the known lamp renders it easy to realise lamps of a comparatively small internal diameter, for example 1.5 to 7 mm, and of a comparatively great length of, for example, 1 m or more. The ionizable filling may comprise a rare gas or a mixture of rare gases, or in addition a component capable of evaporation such as, for example, mercury. The lamp vessel wall may be provided with a fluorescent material. The lamp may be used for lighting purposes, or as a signal lamp, for example with a neon filling as a tail lamp or stop lamp in vehicles. In the latter application the lamp has the advantage over an incandescent lamp that it emits its full light after 10 ms already, instead of 300 ms after being energized. The high cathode fall (* 180 volts) and high work function of axially configured, emitterless and hollow electrodes typically used in the known lamp limit their use to lamp currents of less than 10 to 15 mA. Lower current results in a low light output ( < 900 lm/m) and the high cathode fall reduces the lamp efficacy. High current narrow diameter (ND) fluorescent and neon lamps are highly desirable yet are non-existent. No electrodes are presently available for ND fluorescent lamps with a current between 20 and 50 mA. The requirement for such lamps, among others, is a low cathode fall of, for example, less than 80 volts. There is therefore a need in the art for high current and high efficacy ND lamps. Such higher current ND fluorescent lamps may be used in automobile interior lighting or as backlights in laptop computers. The cathode fall of an electrode in a lamp can be reduced by promoting electron emission. In traditional larger diameter and high current ( > 200 mA) fluorescent lamps, a tungsten coil coated with triple carbonates (for example a mixture of barium, strontium and calcium carbonates) is used as the electrode. Consequently, these lamps have four terminals, two for each electrode on either side. During lamp manufacturing, in an extra process step, the carbonates are thermally converted into oxides in the lamp by passing a current through the tungsten coil. In the lamp, these oxides [(Ba,Sr,Ca)O] promote electron emission via thermionic emission when the electrode is heated to 1000-1300°C, either by passing a heating current through the tungsten coil or by ion-bombardment. It would be desirable to have novel electrodes which do not require the extra thermal in-lamp processing step during manufacture, particularly since this step requires expensive processing time. An ND lamp requires single-lead electrodes because of geometrical constraints and therefore ion-bombardment is the only source of cathode heating. Due to the absence of a coil the use of carbonates in single-lead ND lamps would require external RF heating to convert them to oxides during manufacturing. This adds an additional, even more costly step to the manufacturing process. The non-prepublished Application IB 95/00951 (PHN 15023) describes a narrow diameter lamp according to the above mentioned kind in which a tube is arranged in front of the electrodes. The tube may be covered with Ba x Sr,. x Y 2 O 4 as an emitter, wherein x is, for example, 0.75. It is an object of the present invention to provide a low-pressure discharge lamp of the kind described in the opening paragraph which is capable of providing an increased luminous flux. According to the invention, this object is realised in that a hollow body lies in the extended direction of at least one of the electrodes at a distance from an end thereof, which hollow body is coated with an electron emitter on at least one of its surfaces and is connected to the electrode by electrically conducting means forming a thermal isolator, said electron emitter comprising at least one mixed oxide of at least one of the elements Ba and Sr with at least one metal from the group comprising Ta, Ti, Zr, Sc, Y, La and the lanthanids, wherein electron emitters of the composition Ba x Sr 1 . x Y 2 O 4 , x being in the range of 0 to 1, are excluded. The lamp according to the invention was found to provide an increased luminous flux against the same consumed power. The discharge arc is found to apply itself mainly to the inside of the electrode during starting of the lamp. The arc also hits the hollow body and raises its temperature. After some time the arc applies itself mainly to the hollow body and remains there. The hollow body assumes a comparatively high temperature during lamp operation. This results in a good electron emission of the emitter materials applied at the hollow body. The electrically conducting means provide the hollow body with a thermal insulation, so that the electrode itself remains comparatively cool, cooler than the electrode of the known lamp. This manifests itself in the temperature of the electrode at the area where it makes contact with the lamp vessel, and outside the lamp vessel. The lamp vessel and the electrode outside the lamp vessel as a result may be in contact with or in connection with materials which have a comparatively low resistance to heat during operation. The electron emitters used need not be activated and show no moisture uptake even after prolonged exposure to air. Preferably the electron emitter comprises one or more mixed oxides selected from the group consisting of Ba Ta 2 O 9 , Ba 5 Ta 4 O,j, BaY 2 O 4 , BaCeO 3 , Ba 2 TiO 4 , BaZrO 3 , Ba » Sr,. x TiO 3 , and B& x Sτ l ..ZτQ i , wherein x ranges from a value of 0 to 1. Most preferably the electron emitter comprises one or more mixed oxides selected from the group consisting of Ba 4 Ta 2 O 9 , BaCeO 3 , Ba 2 TiO 4 , BaZrO 3 , Ba 5 Sr 5 TiO 3 and BajSr } ZrO 3 The lamp which has only one electrode provided with a hollow body is highly suitable for DC operation. The electrode with the hollow body is the cathode then. It is favourable, however, for example for AC operation, when both electrodes are fitted with such a hollow body. The electrically conducting means may be formed by a metal wire which is welded to the electrode and to the hollow body, for example with resistance welds or laser welds. Alternatively, however, said means may comprise two or more wires. This embodiment may be preferable in lamps which are subjected to accelerations during operation, for example owing to shocks or vibrations. In a favourable embodiment, the hollow body is integral with the electrode. In that case material has been removed from the shell of a cylinder from which the electrode and the hollow body were formed over a longitudinal portion thereof, for example by sawing, grinding, drilling, burning, or etching. One or several connections between the hollow body and the electrode may have been maintained then so as to serve as electrically conducting means. Three such connections distributed over the circumference provide a mechanically strong construction. The wall of the hollow body is formed, for example, from a solid material, for example the same material as the electrode, for example, the hollow body is integral with the electrode. It is favourable when the hollow body is internally coated with emitter. Alternatively, however, the hollow body may be coated externally, or both internally and externally. The discharge arc preferentially applies itself to the inside of the hollow body in the case of internal coating. Any material detached from the hollow body then remains substantially inside the hollow body instead of depositing itself on the lamp vessel wall. A hollow body allows itself to be coated particularly easily both internally and externally when it is immersed in a suspension of emitter material. The external emitter may then act as a spare reservoir if the internal emitter stock should become exhausted towards the end of lamp life. The thermal insulation of the hollow body may be chosen through the choice of the distance between the hollow body and the electrode, the number of connections between the hollow body and the electrode, and the average cross-section thereof. If the hollow body and the electrode are an assembled unit, the insulation is also adjustable through the choice of the material of said means, in particular the heat conductivity thereof. Those skilled in the art may readily make this choice in a small test series for each lamp type. The electrode, and thus possibly the hollow body, may be made of a metal which has a coefficient of expansion which corresponds to that of the glass of the lamp vessel, for example a CrNiFe alloy in the case of lime glass, for example Cr 6% by weight, Ni 42% by weight, and the rest Fe. For a hard-glass lamp vessel, for example of borosilicate glass, an electrode may be used, for example made of Ni/Fe or NiCoFe, for example Ni 29% by weight, Co 17% by weight, the rest Fe, for example with a diameter of 1.5 mm and a wall thickness of 0.12 mm. Alternatively, the hollow body in an assembled unit of electrode and hollow body may consist of, for example, CrNiFe with 18% Cr by weight, 10% Ni by weight, and the rest Fe, or of Ni. The electrically conducting means may then be, for example, NiCr, for example Ni80Cr20 (weight/weight), for example in the form of wire of 0.125 or 0.250 mm diameter. In an embodiment of the lamp according to the invention, the hollow body is open at both ends and is positioned inside the lamp vessel. Practically all radiation generated by the discharge arc is utilized in this embodiment, which is particularly attractive for a comparatively short lamp vessel. The lamp vessel may be closed in that a glass tube was fused to one or both electrodes outside the lamp vessel and closed. It is alternatively possible, however, that a seal has been made in the electrode tube itself outside the lamp vessel. For this purpose, the hollow body may have been closed by fusion, for example with a laser, or pinched, or pinched and fused. In another embodiment of the lamp according to the invention, the hollow body is positioned outside the lamp vessel in front of the electrode. This has the advantage that material detached from the hollow body during operation will end up substantially outside the lamp vessel, so that the lamp vessel itself remains clear. The lumen output accordingly remains high during lamp life. This embodiment is of particular importance for lamps whose filling comprises a component capable of evaporation. Since the discharge arc applies itself mainly to the hollow body during normal operation, the space outside the lamp vessel, where the hollow body is accommodated, assumes a comparatively high temperature. The evaporation component can thus have a comparatively high vapour pressure. The opposite side facing away from the electrode may be open, as is the side facing the electrode, or it may alternatively be closed, for example in that it has been pinched. In the drawing: Figure 1 shows a first embodiment of the low-pressure discharge lamp according to the invention partly in side elevation, partly broken away. Figure 2 shows in more detail an end-portion of the lamp of Figure 1. Figure 3 shows an end-portion of a lamp according to a second embodiment of the invention. Figure 4 shows a third embodiment of the invention. Figure 5 shows a detail of this embodiment. Figure 6 shows an end-portion of a lamp according to the prior art. The low-pressure discharge lamp in Figure 1 has a tubular glass lamp vessel 1 which is closed in a vacuumtight manner and has end portions 2. It has an ionizable filling comprising rare gas, in the drawing a filling of argon and mercury. A mixture of phosphors 8 covers the inner surface of the lamp vessel for the major part. Hollow cylindrical electrodes 3 enter the lamp vessel each at a respective end portion 2 and have ends 4A, 4B inside and outside the lamp vessel. The lamp vessel 1 is closed by glass tubes 9 which are fixed to the ends 4B of the electrodes 4, which tubes 9 are closed at their free ends. A hollow body 5, shown in more detail in Fig.2, has been laser or resistance welded onto the electrode 3 with electrically conducting means 7, forming a thermal isolator, for example, a Ni or Ni-Cr wire. The hollow body 5 is coated with an electron emitter 6 on at least one of its surfaces, and preferably on an internal surface. The electron emitter 6 comprises a mixed oxide of Ba, Sr and mixtures thereof with one or more of the metals from the series comprising Ta, Ti, Zr, Sc, Y, La and the lanthanids. A second embodiment of the lamp of the invention is shown in Figure 3. Parts in this figure corresponding to those in Figure 1 and 2, have a reference number that is 10 higher. In this embodiment the hollow body 15 is cup shaped and has an open side 15A facing away from the electrode 13. Figures 4 and 5 show a third embodiment. Parts corresponding therein to parts in Figures 1 and 2 have a reference numeral that is 20 higher. In the third embodiment, the electrode 23 and the hollow body 25 are integral. The electrically conducting means 27 forming a thermal isolator was obtained by forming an incision having a width w 1 , w2 of about 1 mm. The hollow body 25 has a length 1 of 2 mm. Lamps according to the first embodiment of the invention were subjected to a life test. The cathode fall was measured at several points of time during this life test. The results are shown in Table 1. TABLE 1 Electron emitter Lifetime [hr] 0 500 1000 2000 3000 4000 Ba. 5 Sr.jTiO 3 (+ Y 2 0 3 ) 30 60 70 65 63 96 Ba 5 Sr s TiO 3 64 84 67 BaY 2 O 4 31 68 75 BaZrOj 27 30 40 45 75 45 BaTiO 4 34 43 43 45 45 40 BaCeO 3 30 45 48 45 44 45 Ba 4 Ta 2 O, 30 45 42 45 44 45 The lamps of the invention have a relatively low cathode fall, enabling an increased luminous flux with the same power consumption. Lamps of the third embodiment of the invention, as shown in Fig. 4, 5, were operated at a DC lamp current of 10 mA. At seven positions at the wall of the lamp vessel the temperature was measured after 10 minutes of operation. The positions at which the temperature was measured extend from 'a' near the anode to 'g' near the cathode. Subsequently the polarity of the DC-current, and therewith also the position of cathode and anode and the positions a-g were reversed. Again after 10 minutes of operation, the temperatures were measured at positions a-g. The above mentioned temperature measurements were performed for four identical lamps of each kind, so that eight values were obtained for each of the positions a-g. The mean of these eight values is listed in Table 2 for lamps invl and inv2 according to the invention. In the lamps of type invl and inv2 the hollow body is covered with the emitter material BaZrO 3 and Ba^a^, respectively. For comparison also temperatures were measured at lamps refl and ref2 not according to the invention. In these lamps both of the end portions are of the construction as shown in Figure 6, wherein a hollow body in front of the electrode is not present. In the lamp refl , the electrode is free from electron emitter. In the lamp ref2, the electrode is covered with Ba 4 Ta 2 O,. TABLE 2 Life Temperature (C) at time, 1 hr. glass position No. LAMP 1 2 3 4 5 6 7 refl 60 60 60 50 177 177 230 ref2 66 70 77 54 120 130 150 invl 52 72 80 48 95 83 59 inv2 44 46 46 46 100 104 102 After a life test of 400 hours the above described measurements were repeated for lamps ref2, invl and inv2. The results thereof are shown in Table 3. TABLE 3 Life Temperature (C) at time, 400 hr. glass position No. LAMP 1 2 3 4 5 6 7 refl ref2 66 71 75 54 121 142 160 invl 43 49 57 52 70 77 67 inv2 60 66 72 54 103 111 115 The measure of the invention results in significant lower temperatures at the surface of the lamp vessel. This makes it possible to use relatively cheap materials for luminaires. Lamps of the first and the third embodiment having a filling of neon were cyclically switched on and off in a further life test. In the lamp according to the first embodiment the hollow body was covered with BaZrO 3 as an electron emitter. The lamp is still in operation after 3500 h and 565,500 cycles. In the lamp according to the third embodiment the hollow body is covered with Ba,Ta 2 O 9 . This lamp is still in operation after 3500 h and 580,000 cycles.

Claims: CLAIMS: 1. A low-pressure discharge lamp, comprising a tubular glass lamp vessel (1) which is closed in a vacuumtight manner and which has end portions (2); an ionizable filling comprising a rare gas in the lamp vessel; hollow cylindrical electrodes (3) which enter the lamp vessel each at a respective end portion (2) and which each have an internal end (4A) inside and an external end (4B) outside the lamp vessel; characterized in that, a hollow body (5) lies in the extended direction of at least one of the electrodes (3) at a distance from an end (4A, 4B) thereof, which hollow body (5) is coated with an electron emitter (6) on at least one of its surfaces and is connected to the electrode (3) by electrically conducting means (7) forming a thermal isolator, said electron emitter comprising at least one mixed oxide of at least one of the elements Ba and Sr with at least one metal from the group comprising Ta, Ti, Zr, Sc, Y, La and the lanthanids, wherein electron emitters of the composition Ba x Sr 1 . x Y 2 O 4 , x being in the range of 0 to 1, are excluded. 2. A lamp as claimed in Claim 1 , wherein the electron emitter comprises one or more mixed oxides selected from the group consisting of Ba 4 Ta 2 O 9 , Ba 5 Ta 4 O, 5 , BaY 2 O 4 , BaCeO 3 , Ba 2 TiO 4 , BaZrO 3) Ba x Sr,. x TiO 3 , and Ba x Sr,. x ZrO 3 , wherein x ranges from a value of 0 to 1. 3. A lamp as claimed in Claim 2, wherein the emitter material comprises one or more mixed oxides selected from the group consisting of BajTajO,, BaCeO 3 , Ba 2 TiO 4 , BaZrO 3 , Ba 5 Sr 5 TiO 3 and Ba s Sr 5 ZrO 3 4. A low-pressure discharge lamp as claimed in Claim 1 , 2 or 3, characterized in that the hollow body (5) is open at both sides and is positioned inside the lamp vessel (1) in front of the electrode (3). 5. A low-pressure discharge lamp as claimed in Claim 1 , 2 or 3, characterized in that the hollow body (15) is positioned inside the lamp vessel (11) in front of the electrode (13) and in that it is cup-shaped, having an open side facing away from the electrode. 6. A low-pressure discharge lamp as claimed in Claim 1 , 2 or 3, characterized in that the hollow body is positioned outside the lamp vessel in front of the electrode. 7. A low-pressure discharge lamp as claimed in one of the previous Claims, characterized in that the hollow body (5) is internally coated with emitter (6). 8. A low-pressure discharge lamp as claimed in Claim 7, characterized in that the hollow body (5) is coated with emitter (6) internally and externally. 9. A low-pressure discharge lamp as claimed in one of the Claims 1 to 4 or 6 to 8, characterized in that the electrode (23) and the hollow body (25) are integral. 10. A low-pressure discharge lamp as claimed in one of the preceding Claims, characterized in that both electrodes (3) have a hollow body as defined in that Claim.

Inventors: LANGEVOORT JEROEN, MCGEE SUSAN, MCGEE THOMAS F, MEHROTRA VIVEK, YOUNG EDWARD, LANGEVOORT, JEROEN, MCGEE, SUSAN, MCGEE, THOMAS, F., MEHROTRA, VIVEK, YOUNG, EDWARD
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1996
Main classification: H01J61
IPC codes: H01J61