High-pressure sodium lamp

Abstract: A high-pressure sodium lamp of the invention radiates white light with a colour temperature higher than 2250 K. The lamp is provided with a discharge vessel (1) with a ceramic wall (2). In the discharge vessel (1), electrodes (10a, 10b) are arranged which are connected to current supply conductors (20a, 20b) which are passed through the wall (2) in a gastight manner. The electrodes (10a, 10b) each have a rod (11a) ending in a tip (12a) and with a length l and diameter d. The rod (11a) of the electrodes (10a, 10b) is provided with a single winding (13a) of a wire with a diameter of at most d/3, which winding (13a) is remote from the tip (12a) of the rod (11a) over a distance of at most 2d. This construction enables a quick ignition of the lamp.

Description: The invention relates to a high-pressure sodium lamp which in the nominal operating condition radiates white light with a colour temperature higher than 2250 K, provided with a discharge vessel surrounding a discharge path and having a ceramic wall, in which vessel at least two electrodes are arranged, each electrode comprising a rod which has a diameter d, a length l, a tip, and is provided with a winding which is removed from the tip of the rod by a distance of at most 2d,the discharge path extending between said tips, while each electrode is connected to a current supply conductor which is passed in a gastight manner through the wall of the discharge vessel. Such a high-pressure sodium lamp is known from GB 2 083 692 A. The term "ceramic wall" is understood to mean in the present description and Claims a wall of a refractory material such as monocrystalline metal oxide, for example sapphire, polycrystalline metal oxide, for example densely sintered aluminium oxide or yttrium oxide, or crystalline non-oxidic material such as aluminium nitride. The filling of the discharge vessel may comprise, for example, mercury in addition to sodium and one or several rare gases. The winding consists, as does the rod of the electrode, of refractory material such as, for example, tungsten or, for example, an alloy of tungsten and rhenium. The area in the colour triangle within which the light of a high-pressure sodium lamp is called "white" is bounded by straight lines through points having coordinates (x, y): (0,400; 0,430), (0,510; 0,430), (0,485; 0,390) and (0,400; 0,360). The colour temperature may reach values of up to approximately 4000 K in that case. The known lamp consumes a power of 400 W during nominal operation. The electrode rods have a diameter of 1.2 mm and are provided with a double winding of wire with a diameter of 0.6 mm. The electrodes are free from alkaline earth metals. Preferably, the electrodes are entirely devoid of emitter. The absence of emitter material has the advantage that chemical reactions with components of the filling are avoided and that the properties of the lamp during its life cannot change as a result of the gradual disappearance of emitter material from the electrode. The absence of emitter material in the electrode construction of the known lamp may result in a comparatively long ignition time and a wide spread in ignition times among lamps. The term "ignition time" is understood to mean the time required by the lamp for achieving a stable arc discharge via a glow discharge condition after the moment the ignition voltage is applied. A comparatively long ignition time is disadvantageous because the electrodes are highly loaded during lamp ignition, which in general adversely affects lamp life. In an ambience in which several lamps are arranged, a wide spread in ignition times has the disadvantage that the ambience is unevenly illuminated for some time after switching on of the lighting. The invention has for its object inter alia to provide a measure for avoiding the said disadvantages. According to the invention, a lamp of the hand mentioned in the opening paragraph is for this purpose characterized in that each of the windings is a single winding of wire having a diameter of at most d/3. Because of the absence of a second winding, the electrodes are easy to manufacture. The measure achieves a short ignition time, also in the absence of emitter material, as well as a small spread in the ignition times among lamps, especially for lamps whose discharge vessels contain Xe with a filling pressure of at least 20 kPa. There is evidence to suggest that the favourable effects of the measure according to the invention are caused by the fact that the winding around the rod has a favourable influence on the heat balance of the electrode both during lamp ignition and during nominal operation. It is suspected that the favourable ignition behaviour of the lamp according to the invention is realized in that the comparatively thin wire conducts only little heat in its longitudinal direction and thus quickly assumes a temperature required for nominal lamp operation upon being struck by the glow discharge arc, as a result of which the transition from the glow discharge to the arc discharge takes place. The heat transfer from the winding to the rod is still small at comparatively low temperatures because the substantially cylindrical wire is in contact with the rod over a comparatively small surface area. During nominal operation, on the other hand, when the rod and the winding assume temperatures of the order of 2400 to 2700 K, there is a good thermal coupling between the rod and the winding as a result of radiation. The invention is particularly suitable for use in lamps with a power rating of up to 100 W. In the lamp according to the invention, the winding preferably extends up to substantially the tip of the rod, the interspacing remaining between the tip and the winding being of the order of at most 100 µm. The result of the measure is found to be greatest when the winding is provided around the electrode rod with minimal pitch. Since the single winding of the comparatively thin wire is removed from the rod tip by at most 2d, it is found that the discharge still strikes the winding during nominal operation. The electrode of the lamp according to the invention has an effective thickness d e which is equal to the rod diameter d plus twice the thickness of the wire forming the winding. In the case of a greater interspacing, it is found that the discharge no longer strikes the winding in the nominal operating condition. The winding is preferably removed from the location where the current supply conductor is passed through the discharge vessel wall by at least 1 mm. It is prevented by this that the winding absorbs amalgam between the turns as a result of capillary action. In an embodiment which is advantageous on account of a favourable manufacturing technology, the winding extends over the full length of the rod. This renders it possible to manufacture the electrode in a simple manner in that it is sawn off as a segment from a rod with wire coiled around it which is comparatively long compared with the electrode. The operation of providing the winding for each individual electrode is avoided in this way. A ratio I/d e 3/2 above 2 and below 5 is clearly preferable for the effective thickness of the electrodes. In this expression, I is the lamp current in A in the nominal operating condition and d e is the effective thickness of the electrode in mm. This and other aspects of the high-pressure sodium lamp according to the invention will be explained in more detail with reference to the drawing in which: Fig. 1 is an elevation of an embodiment of a high-pressure sodium lamp according to the invention, Fig. 2 shows an end of the discharge vessel of the lamp of Fig. 1 provided with an electrode according to a first embodiment, partly in elevation and partly in longitudinal section, and Fig. 3 shows an end of the discharge vessel of the lamp of Fig. 1 provided with an electrode according to a second embodiment, partly in elevation and partly in longitudinal section. It is noted that the lamp and the components are not drawn true to scale in the drawing. Fig. 1 shows a high-pressure sodium lamp which in the nominal operating condition radiates white light with a colour temperature of at least 2250 K. The lamp is provided with a discharge vessel 1 with a wall 2 of densely sintered aluminium oxide as the ceramic material, enclosing a discharge path. Two electrodes 10a, 10b are arranged in the discharge vessel 1. The electrodes 10a, 10b are each connected to a current supply conductor 20a, 20b which is passed in a gastight manner through the wall 2 of the discharge vessel 1. The discharge vessel 1 is provided with a filling consisting of 10 mg of an amalgam of sodium and mercury with a weight ratio of 15/40 and xenon with a filling pressure of 53 kPa. The discharge vessel 1 is accommodated in an outer bulb 30 which is provided with a lamp cap 40. The outer bulb 30 may be filled with an inert gas, for example with nitrogen. Fig. 2 shows an embodiment of an electrode 10a of the lamp of Fig. 1. The other electrode 10b has a similar construction. The electrode 10a is fixed on a niobium lead-through element which serves as the current supply conductor 20a. The current supply conductor is passed through the wall 2 of the discharge vessel 1 in a gastight manner via a seal 5a of melting ceramic through a tube 4a sintered into an end 3a of the discharge vessel 1. The tube 4a, the seal 5a of melting ceramic and the discharge vessel are depicted in longitudinal section here. The electrode 10a has a rod 11a with a diameter d and a length l (see Fig. 2), which has a tip 12a and is provided with a winding 13a. The rod 11a of the electrode 10a is provided with a single winding 13a of wire with a diameter of at most d/3. The winding 13a is removed from the tip 12a of the rod 11a by at most 2d. In a practical embodiment, the length l of the rod is 7.2 mm and its thickness d is 500 µm. In this practical embodiment, the winding is removed from the tip 12a by less than 0.1 mm and extends from there alongside the rod 11a over a distance of 4 mm. The winding accordingly is approximately 3 mm removed from the location where the current supply conductor is passed through the wall of the discharge vessel. The spacing between the tips of the electrodes 10a, 10b is 9.6 mm. The winding 13a is made of tungsten wire of 100 µm thickness and has the smallest possible pitch, i.e. 100 µm. The effective thickness of the electrode is 700 µm as a result. The lamp current is 2.5 A during nominal operation. The ratio I/d e 3/2 , therefore, is 4.27 and lies within the limits 2 and 5 recommended for this ratio. In Fig. 3, which shows a second embodiment of an electrode of the lamp of Fig. 1, parts corresponding to parts of Fig. 2 have reference numerals which are 100 higher. In this embodiment of the lamp, the winding 113a extends over the entire length of the rod 111a, and the current supply conductor 120a extends 1 mm into the discharge vessel. The winding, accordingly, is 1 mm removed from the location where the current supply conductor is passed through the wall of the discharge vessel. The ignition behaviour of lamps according to the invention was investigated. The supply circuit of the lamps consisted of a ballast with an electronic starter and an electronic control unit. The supply circuit yielded a rated maximum effective voltage of 108 V and, during ignition, pulses with a duration of 410 ns and an amplitude of approximately 3 kV. Out of 80 tested lamps according to the invention, all had ignited within one minute. The average ignition time and the standard deviation of the ignition times of these lamps, of which 95% even had an ignition time shorter than 5 seconds, were 7.8 s and 12.0 s, respectively. Lamps according to the invention were subjected to an endurance test. At the start and after 100, 500 and 1000 hours photometric characteristics such as luminous flux and colour temperature, and electrical characteristics such as lamp voltage and power, were measured. Average values of the test results are given in Table 1 for lamps provided with electrodes according to the first embodiment of the invention (A) and for lamps provided with electrodes according to the second embodiment (B). In the Table, t is the time in hours (h), V la is the lamp voltage in V, P la is the power consumed by the lamp in W,  is the luminous flux in lm, η is the luminous efficacy in lm/W, and T c is the colour temperature in K. Up to approximately 500 hours of operation, the lamp according to the second embodiment has a comparatively high luminous efficacy compared with the lamp according to the first embodiment.

Claims: A high-pressure sodium lamp which in the nominal operating condition radiates white light with a colour temperature higher than 2250 K, provided with a discharge vessel surrounding a discharge path and having a ceramic wall, in which vessel at least two electrodes are arranged, each electrode comprising a rod which has a diameter d, a length l, a tip, and is provided with a winding which is removed from the tip of the rod by a distance of at most 2d, the discharge path extending between said tips, while each electrode is connected to a current supply conductor which is passed in a gastight manner through the wall of the discharge vessel, characterized in that each of the windings is a single winding of wire having a diameter of at most d/3. A high-pressure sodium lamp as claimed in Claim 1, characterized in that the winding extends up to substantially the tip. A high-pressure sodium lamp as claimed in Claim 1 or 2, characterized in that the winding is at least 1 mm removed from the location where the current supply conductor is passed through the wall of the discharge vessel. A high-pressure sodium lamp as claimed in Claim 1, 2 or 3, characterized in that the winding extends over the entire rod.

Inventors: DIELIS HARALD RENE, JACOBS CORNELIS ADRIANUS JOHAN, MEEUWSSEN MARINA MARIA JOSEPHA, DIELIS, HARALD RENE, JACOBS, CORNELIS ADRIANUS JOHANNES, MEEUWSSEN, MARINA MARIA JOSEPHA, Dielis, Harald René
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1992
Main classification: H01J61
IPC codes: H01J61