Title of Invention	"LOW POWER HIGH-PRESSURE SODIUM LAMP"
Abstract	Low power high-pressure sodium lamp with a lamp power being less than or equal to 100 W and having a discharge vessel which contains at least sodium and xenon and optionally contain mercury, pNaB being the operating filling pressure of the sodium and pxeK being the cold filling pressure of the xenon, characterized in that pNaB= 20 to 100mb, pxeK= 1 to 5 bars, and

Full Text

Technical field The invention proceeds from a high-pressure sodium dis¬charge lamp in accordance with the preamble of Claim 1. At issue here, in particular, are high-pressure sodium discharge lamps with a power of at most 100 W and very high xenon pressure. Normally, such lamps have a circu¬larly cylindrical discharge vessel made from aluminium oxide, which is accommodated in a transparent outer bulb. Prior art The basic lineaments of the design of high-pressure sodium discharge lamps have already been known a long time. Likewise, it had already been known for a long time to use xenon with a relatively high pressure to increase the light efficiency in these lamps. For example, it is stated in the relevant monograph "The High-Pressure Sodium Lamp" by DE GROOT/VAN VLIET (Philips Technical Library, Deventer, 1986) on pages 299 and 300 that it is possible to increase the light efficiency by 10 to 15% if - in so-called SUPER lamps -a xenon cold filling pressure of 20 to 40 kPa (200 to 400 mb) is used instead of the customary standard filling pressure of approximately 30 mb. At the same time, it is pointed out there on page 299 that the light efficiency in high-pressure sodium dis¬charge lamps decreases sharply with falling lamp power. Even in the case of increased xenon pressure, it is at most 85 Im/W for a 50 W lamp power, while a light effi¬ciency of approximately 138 Im/W can be reached for a lamp power of approximately 400 W. - 2 -DE-C 26 00 351 describes an Hg-free high-pressure sodium lamp- specifically suitable for so-called self-stabilizing operation, which has a sodium operating pressure of pNaB = 4 to 93 mb, a xenon operating pressure of pxe(hoti - 80° ™& anc^ a pressure ratio of PwaB/Pxethot) - 1/20. Taking account of the usual factor 8 (see DE-C 28 14 882, column 2, middle) for converting between the xenon operating pressure and xenon cold filling pressure of pX6K' this therefore yields a pres¬sure ratio of pXeic/PNaB ^2.5. In the case of self-stabi¬lizing operation, the aim is to operate a high-pressure sodium lamp without a ballast. This mode of operation requires a long decay time of the plasma formed from the filling gas. In order to achieve this long decay time, use is made in a manner known per se of a rela¬tively high xenon pressure as well as a relatively large inside diameter of the discharge vessel (see also the abovementioned relevant monograph by DE GROOT/VAN VLIET on pages 126 and 154) . According to DE GROOT/VAN VLIET page 155, the self-stabilizing operation of high-pressure sodium lamps has not found any practical application, because of problems in starting and in sudden variations in the system voltage. The high-pressure sodium discharge lamp described by way of example in DE-C 26 00 351 has a high power of 400 W and a very large inside diameter of 7.6 mm. The xenon cold filling pressure is 260 mb and the pressure ratio of pXeK/PNaB is approximately 3.5. As a result, for the high power of 400 W, a rather moderate light effi¬ciency of only 110 mm/W is achieved. This reference neither aims for nor achieves a particularly high light efficiency by comparison with other high-pressure sodium lamps. According to Figure 10.18 of DE GROOT/VAN VLIET (page 299), it is possible, however, to achieve light efficiencies of up to 138 Im/W for a power of 400 W. This principle that the light efficiency depends on the lamp power is represented here once again as Figure 3 for comparative purposes (see below). A Hg-free high-pressure sodium lamp without self-stabilization is described in DE-B 28 14 882. In this case, a value of between 1.25 •is again recommended for the xenon cold filling pres¬sure pxeK, referred to the sodium operating pressure (PN&B = sodium operating pressure) . This value for the pressure ratio of pxeK/pNau is, however, in good agree¬ment with that described in DE-C 26 00 351. However, DE-B 28 14 882 (column 3, line 41f) advises against further increasing the xenon pressure over this upper limit, since there is the disadvantage that starting is rendered more difficult "without there being an opposing increase in light efficiency". In the exemplary embodiments with a low lamp power of 70 and 100 W, pNaB = 230 mb, and the xenon cold filling pres¬sure is approximately 500 mb. This corresponds to a pressure ratio of pxeK/PwaB of approximately 2 to 2.5. A light efficiency of 97 or 105 Im/W, respectively, for a power of 70 W or 100 W, respectively, is thereby achieved. These values are plotted for comparative pur¬poses as crosses in Figure 3 (see below). Representation of the invention Accordingly, there is provided a low power high-pressure sodium lamp with a lamp power being less than or equal to 100 W and having a discharge vessel which contains at least sodium and xenon and further may contain mercury, pNaB being the operating filling pressure of the sodium and pXeK being the cold filling pressure of the xenon, characterized in that pNaB= 20 to 100mb, pxeK= 1 to 5 bars, and PxeK /PNaB= 10 tO 30. The low-power high-pressure sodium discharge lamp according to the invention has a discharge vessel which contains at least sodium and xenon. In this case, low power is understood, in particular, to be a lamp power of less than or equal to 100 W. In this case pNaB is the operating filling pressure of the sodium, and pXeK is the cold filling pressure of the xenon. Surprisingly, in the case of low powers it is possible by contrast with the previous teaching to achieve a further increase in the light efficiency by typically 20% if pNaB = 20 to 100 mb and pXeK = 1 to 5 bars are selected and if, furthermore, the condition PxeK/PwaB - 10 is simultaneously satisfied. The pressure ratio PxeK/PnaB is advantageously between 10 and 30. Mercury can be added to the lamp filling to increase the operating voltage. The xenon pressure exceeds by a factor of 3 to 10 the values customary in the case of previously known high-pressure sodium discharge lamps with a high xenon pressure (for example the NAV SUPER lamps from OSRAM)x The result in this case is a light efficiency which is increased by typically 20% by comparison with these NAV-SUPER lamps. The already mentioned previously known increase in the light efficiency of high-pressure sodium lamps in the case of an increase in the xenon pressure (see QE GROOT/VAN VLIET, page 153 and pages 299-300-f is used commercially in so-called NAV SUPER lamps. However, the increase in light efficiency achieved by the present invention in the case of a further increase in the xenon pressure by comparison with the values in NAV-SUPER lamps is unexpectedly high and has not been known to this extent. Thus, for example, a light effi¬ciency increased by 10 to 15% by comparison with the so-called standard lamps (with 30 mb xenon cold filling pressure) in conjunction with an increase in the xenon filling pressure (cold) to 200 to 400 mb is described in QE GROOT/VAN VLIET page 300. • A further pressure increase is- excluded there because of the more diffi¬cult starting. The surprising behaviour of the lamps according to the invention is based on specific utilization of the state of affairs known but so far not taken into account by the experts. Certainly, it is known that the light efficiency of high-pressure sodium lamps decreases clearly towards low lamp powers (DE GROOT/VAN VLIET, page 299; see Figure 3, below). The explanation given there, that this law is due to the circumstance that for a low lamp power the efficiency of the radiation is lower and the electrode losses are higher than in the case of a higher lamp power is, however, not correct. The basic reason is, rather, that the relative component of the heat loss in the discharge arc as part of the lamp power becomes larger with decreasing lamp power. This heat loss can, however, be reduced by the low thermal conductivity of xenon when it is used as buffer gas at a sufficiently high pressure. This effect acts on the light efficiency more favourably the smaller the lamp power. In this case, it is the pressure ratio between xenon and sodium which is of decisive importance, because by contrast with xenon sodium has a high thermal conductivity. The higher the xenon pressure with respect to the sodium pressure, the better the heat losses can be stemmed. The final effect of this is the observed additional increase in light efficiency for small powers. The very high xenon pressure of at least one bar (cold) has yet further advantages in addition to increasing the light efficiency: 1. A low wall temperature of the discharge vessel can be achieved by the smaller heat losses. This can, for example, be utilized to lengthen the service life. Alternatively, the discharge vessel can be- 6 - reduced, so that the wall temperature originally present is achieved again. In this case, the light efficiency is yet further increased by the higher power density. 2. A high xenon pressure prevents diffusion. This reduces the evaporation of electrode components during the starting operation and reduces the blackening, resulting, therefrom, of the wall of the discharge vessel in the region of the elec¬ trodes. This effect is known qualitatively from NAV SUPER lamps. In the case of a very high xenon pressure, it is even more strongly pronounced, as a result of which the service life is further lengthened. 3. In the case of the lamps according to the inven¬ tion, because of its very high pressure xenon makes a substantial contribution to the operating voltage. This contribution is independent of the temperature of the discharge vessel, since, by contrast with sodium, the xenon is also present in gaseous form at room temperature. This has a stabilizing effect with respect to fluctuations in the system voltage or to manufacturing tolerances. By contrast with this, in the case of all previously known lamps (for example, in accordance with s DE-B 28 14 882)"; the contribution of the xenon atoms to the operating voltage is insubstan¬ tial. The operating voltage is determined there virtually solely by the number of the sodium atoms, which is strongly influenced by the tem¬ perature of the coldest spot, and thus by fluctua¬ tions in the system voltage or manufacturing tolerances. In the case of a mercury additive, the latter also acts in setting the operating voltage. 4. The very high xenon pressure produces a particu¬ larly low restarting peak in operating the lamp.- 7 - This lengthens the service life because of the lighter loading of the electrodes, and provides higher security against extinction in the case of sudden fluctuations in system voltage. 5. In the sodium spectrum, xenon effects a widening of the peak spacing in the spectra profile of the sodium resonance line v(D-line) , which is widened by pressure and is self-absorbing at its centre. This effect is known in principle (see DE GROOT/VAN VLIET, in particular page 16a, plate Ic) . As a result, the sodium pressure can be reduced in conjunction with the same colour tem¬perature and colour rendition. This effect operates drastically in the case of a very high xenon pressure of at least one bar (cold) . In the case of the present invention, the sodium pressure is especially preferably set so low in relation to the xenon pressure that a peak spacing of the two wings of the resonance line of typically 10 nm, at most 12 nm, results. An essential precondition in this case is to select the ratio pxeK/pMas ^ 10 and pNaB = 20 to 100 mb. It has emerged that optimum light efficiencies are produced under these conditions. By contrast, in the case of the conditions specified in..DE-B 28 14 882, there is a peak spacing of the two wings of the sodium D line of at least 15 to 20 nm, since pN3B is relatively high there (see above). This can be estimated with the aid of the equation (3.28) specified in (DE GROOT/VAN VLIET, page 87 . Items 3 and 5 provide an additional justification for selecting the low operating pressure, typical of the present invention, of the sodium vapour of 20 to 100 mb. This low sodium pressure has, for its part, a plurality of advantages: 1. In the case of a sodium vapour pressure of 20 to 100 tub-, the temperature of the discharge vessel at the coldest spot is only 840 to 950 K. This coldest spot is always in the vicinity of the seal. Consequently, the seal is now typically colder by 150 K than in the case of previously known lamps (see DE-B 28 14 882), from which there follows a reduction in the failure of lamps due to leaks in the region of. the seal. 2. The corrosion, caused by sodium, of the wall of the discharge vessel, which preferably occurs in the centre of the vessel, is reduced because of the low sodium partial pressure. The result is an additional improvement in the service life. The disadvantage, mentioned in DE-B 28 14 882, of impairing the ability to start, can be effectively countered precisely in the case of low lamp powers ( (above 5 bars). The xenon pressure is advantageously limited to values of up to 3 bars. These improved parts are already being used in commercially available metal halide lamps from OSRAM (for example: HQI-E 100 W/NDL and WDL) . By contrast, it is not possible in the case of lamps according to the invention to achieve starting by using conventional starting devices for low-power NAV lamps. By contrast with DE-C 26 00 351,""the lamps described here are neither intended nor suitable for self-stabi¬lizing operation. In this case, the xenon operating pressure achieved in accordance with the invention is also, at 8 to 24 bars, higher than the typical value of 1.8 bars specified there.The heating, described in DE-C 26 00 351, of the dis- charge vessel, which is necessary there for starting (alternatively, a conventional ballast can be used) is not required in the case of the discharge vessel 5 according to the invention. The discharge vessel according to the invention preferably has an appendix (a niobium tube open at the beginning) by means of which it is possible, in a way known per se, to add xenon as a filling at high pressure, and which is 10 sealed after the filling operation. As well as sodium and xenon, the lamps according to the invention can, in particular, additionally contain mer¬cury in the filling. The increase in the light effi-15 ciency is of similar magnitude in lamps with or without a mercury additive. A typical lamp filling with a mer¬cury additive uses an amalgam with 18% by weight of Na. ' Preferably, the inside diameter of the discharge vessel 20 is between 2.5 and 5 mm, in particular at most 4 mm. Self -stabilization is excluded from the very first given these dimensions. By way of comparison, the inside diameters specified .in DE-C 26 00 351 "are larger .by a whole power of 10. Although the discharge vessel 25 is generally a circular cylinder, it can also have a different geometry, for example have a bulge in the middle . The high-pressure sodium discharge lamps advantageously 30 additionally have a capacitive starting aid, for example a wire along .the discharge vessel. By contrast with (DE-C 26 00 351, the lamps according to the invention do not, however, require preheating. 35 These lamps frequently have a niobium tube appendix, as described, for example, in DE GROOT/VAN VLIET on page 251, Figure 8.30.Such lamps can be operated using a conventional, or frequently also an electronic ballast. The discharge vessels described here are preferably used in circularly cylindrical or elliptical outer bulbs. The invention is to be explained below with the aid of a plurality of exemplary embodiments. In the drawing: Figure 1 shows a high-pressure sodium discharge lamp, Figure 2 shows a comparison of the light efficiencies of different high-pressure sodium lamps (each with a power of 50 W) having a different xenon pressure (with and without Hg) , and Figure 3 shows a comparison of the light efficiency of different high-pressure sodium lamps for different lamp powers and a different xenon pressure. Description of the drawings The high-pressure sodium discharge lamp shown in Figure 1 and with a power of 50 W, has a discharge vessel 1 made from aluminium oxide. Said discharge vessel is arranged in a cylindrical outer bulb 2 made from hard glass, which is sealed at its first end by a screw cap 3 and at its second end by a dome 9. The outer bulb 2 is evacuated. Situated opposite one another in the discharge vessel 1, having an inside diameter of 3.3 mm, are two elec¬trodes 4 which have an electrode spacing EA of 30 mm. The first electrode 4, remote from the cap, is con¬nected via a tubular niobium bushing 5 with an appendix 6 to a lead wire 7 which is connected to a solid outer feeder 8 which leads along the discharge vessel to a contact in -the screw cap 3. The second electrode 4 is likewise connected via a nio-5 bium bushing 5 (without appendix, however) to a metal wire 15. The latter is connected via a further conduc¬tor 16 to a second contact in the cap 3. The discharge vessel is. fitted with a capacitive 10 starting aid which is formed by a starting wire 17 along the discharge vessel. The starting wire 17 is connected in an electrically conducting fashion to the second electrode 4. 15 The lamp is, for example, connected via starter circuit in the lamp cap to a 220 V AC voltage system. The starting voltage is 4 kV. The discharge vessel 2 contains a filling which com-20 prises only sodium and xenon. The cold filling pressure of the xenon (pXeK) is 3 bars and the operating filling pressure of the sodium (pNaB) is 100 mb, with the result that pXeK/pNaB = 30. 25 This lamp reaches a light flux of 5100 1m and a light efficiency of 102 Im/W (see Figure 2, solid triangular measuring point #1 at 3000 mb xenon cold filling pres¬sure) . By comparison with this, previous 50 W lamps with a xenon cold filling pressure of 300 mb (SUPER 30 type) have only achieved a light flux of 4200 1m corresponding to a light efficiency of 81 Im/W (see Figure 2, open triangular measuring point). Also speci¬fied in Figure 2 is the light efficiency for further lamps with the usual low xenon pressure of at most 35 100 mb (standard type) . It amounts to approximately 70 Im/W at 30 mb (see Figure 2, open triangular measuring point).Figure 3 represents the dependence of the light efficiency -on the lamp power schematically along the lines of DE GROOT/VAN VLIET. The value achieved with the above exemplary embodiment (102 Im/W for a lamp power of 50 W) is plotted as a rhomboidal measuring point. It is clearly above the prior art. In a SECOND exemplary embodiment, a lamp of identical design is operated only with one bar of xenon pressure and 50 mb of sodium pressure. The ratio here is pXeK/PNaB 20. At 95 Im/W, see Figure 2, solid triangular measuring point #2 at 1000 mb xenon cold filling pres¬sure) , the light efficiency is still substantially higher than in the case of the previously known lamps. Because of the lower xenon pressure, starting is ren¬dered easier by comparison with the first exemplary embodiment. The starting voltage is at 3 kV. These two lamps are particularly suitable for new installations having a stronger starting device. In a THIRD exemplary embodiment, the identically designed 50 W lamp is additionally filled with mercury. Use is made for this purpose of an amalgam having 18% by weight of sodium, the rest being mercury. This lamp exhibits a light efficiency of 105 Im/W (solid circular measuring point #3 in Figure 2) in conjunction with the xenon cold filling pressure of 2 bars, a sodium operating pressure of 80 mb and a pressure ratio of PxeK/PNaB = 25.0. Correspondingly, a FOURTH exemplary embodiment (50 W) with a xenon cold filling pressure of 1 bar in conjunc¬tion with an identical Na/Hg ratio exhibits a light efficiency of 93 Im/W (solid circular measuring point #4 in Figure 2). The corresponding light efficiencies of sodium lamps containing mercury and having a lower xenon cold filling pressure (SUPER and standard types) are like¬wise specified (open circular measuring points for 30 to 300 mb in Figure 2), for the purpose of comparison. In a FIFTH exemplary embodiment, an essentially similar lamp is operated with a power of 63 W. The filling con¬tains 1 bar of xenon and 50 mb of sodium, but no mer¬cury. The pressure ratio is PxeK/PwaB = 20. The light efficiency is 98 Im/W. This lamp is conceived as a direct substitute for high-pressure mercury lamps with a power of 125 W, which have the same light flux. It has a power reduction circuit (phase-gating control) and a starting circuit in the lamp cap. In a SIXTH exemplary embodiment of a 35 W lamp, a dis¬charge vessel with an inside diameter of 3.3 mm and an electrode spacing of 23 mm is filled only with sodium and xenon. The xenon cold filling pressure is pX6K = 2 bars, and the sodium operating pressure is pNaB = 90 mb. The pressure ratio is therefore pXeK/PNaB = 22.2. The light efficiency is 98 Im/W (see Figure 3, rhom-boidal measuring point #6) and is thus substantially higher than was to be expected to date [lacuna] lamps of this power. In a SEVENTH exemplary embodiment of a 70 W lamp, a discharge vessel with an inside diameter of 3.3 mm and an electrode spacing of 36 mm is filled with sodium/mercury amalgam (see above) and xenon. The xenon cold filling pressure is pXeK = 2 bars, and the sodium operating pressure is pNaB = 75 mb. The pressure ratio is consequently PxeK/PwaB = 26.7. The light efficiency is 115 Im/W (see Figure 3, rhomboidal measuring point #7) and is thus likewise substantially higher than was to be expected to date in the case of lamps of this power. In an EIGHTH exemplary embodiment of a 70 W lamp, a discharge vessel with an inside diameter of 3.7 mm and an electrode spacing of 37 mm is filled with sodium/mercury and xenon. The xenon cold filling pres¬sure is Pxeic = 1-5 bars, and the sodium operating pres¬sure is pNaB =85 mb. The pressure ratio is consequently = 17.6. The light efficiency is 108 Im/W. WE CLAIM: 1. Low power high-pressure sodium lamp with a lamp power being less than or equal to 100 W and having a discharge vessel which contains at least sodium and xenon and optionally contain mercury, PNBB being the operating filling pressure of the sodium and pxeK being the cold filling pressure of the xenon, characterized in that pNaB= 20 to 100mb, pxeK= 1 to 5 bars, and pxeK /pNaB= 10 to 30. 2. High-pressure sodium lamp as claimed in claim 1 wherein pxeK__£ 3 bars. 3. High-pressure sodium lamp as claimed in claim 1 wherein the filling is free of mercury. 4. High-pressure sodium lamp as claimed in claim 1 wherein the discharge vessel is a circular cylinder. 5. High-pressure sodium lamp as claimed in claim 4 wherein the inside diameter of the discharge vessel is between 2.5 and 5 mm. 6. High-pressure sodium lamp substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

Documents:

2766-del-1997-abstract.pdf

2766-del-1997-claims.pdf

2766-del-1997-correspondence po.pdf

2766-del-1997-correspondence-others.pdf

2766-del-1997-description (complete).pdf

2766-del-1997-drawings.pdf

2766-del-1997-form-1.pdf

2766-del-1997-form-13.pdf

2766-del-1997-form-19.pdf

2766-del-1997-form-2.pdf

2766-del-1997-form-3.pdf

2766-del-1997-form-4.pdf

2766-del-1997-form-6.pdf

2766-del-1997-gpa.pdf

2766-del-1997-petition-137.pdf

2766-del-1997-petition-138.pdf

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