WO2005029534A2

WO2005029534A2 - Metal halide lamp

Info

Publication number: WO2005029534A2
Application number: PCT/IB2004/051668
Authority: WO
Inventors: Hendrik A. Van Esveld; Gregory J. Nelson; Catharina J. M. Daemen
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-09-22
Filing date: 2004-09-02
Publication date: 2005-03-31
Also published as: WO2005029534A3; TW200523983A

Abstract

A metal halide lamp comprising a discharge vessel with a ceramic wall enclosing a discharge space containing a rare gas and an ionizable filling, wherein in said discharge space two electrodes are arranged whose tips have a mutual interspacing EA so as to define a discharge path between them, with the special feature that the quotient of the outer wall area and the inner wall area is larger than 2.

Description

Metal halide lamp

The present invention relates to a metal halide lamp comprising a discharge vessel with a ceramic wall enclosing a discharge space containing a rare gas and an ionizable filling, wherein in said discharge space two electrodes are arranged whose tips have a mutual interspacing EA so as to define a discharge path between them. The invention also refers to a method for manufacturing such a lamp.

Such a lamp is known from international (PCT-) patent publication no. WO 99/28946 in the name of the same Applicant. This known electric discharge lamp has a tubular, light-transmissive ceramic lamp vessel, for example of polycrystalline aluminum oxide, and a first and a second current conductor which enter the lamp opposite to each other and each support an electrode in the lamp vessel, for example a tungsten electrode which is welded to the current conductors. The lamp vessel has an ionizable filling comprising a rare gas, such as Xenon or Argon etcetera, and/or mercury and metal halides. A disadvantage of this known lamp is that light-technical properties deteriorate the lifespan due to corrosion of the innerwall of the discharge vessel and/or tungsten deposition on the inner wall in the optical path.

It is an object of the invention to obviate these disadvantages of the prior art, that is, to propose a metal halide lamp wherein said light-depreciation is minimized or avoided. In order to accomplish that objective a metal halide lamp of the type referred to in the preamble according to the invention is characterized in that the quotient of the outer wall-area and the inner wall-area is larger than 2, whereby the inner wall area is determined by the inner wall-load and the outer wall area is determined by the outer wall load. The outer wall- load is defined as the quotient of the operating power of the lamp and the outer surface of the discharge vessel excluding the extended plugs that contain the feedthrough for the electric current. The inner wall load is then defined as the input power of the lamp divided by the total inner area of the discharge vessel. For metal halide lamps an outer wall load of approximately 15 to 20 W/cm2 is found to give a satisfactory light output and life for general lighting. Taking into account that the operating power of the lamp of a certain type is more or less constant over its entire lifetime, the above indicated quotient is increased compared to prior art lamps, on the one hand, by maintaining the outer wall-load (that is by not changing the outer shape of the metal halide lamp) and, on the other hand, by increasing the inner wall- load (that is by decreasing said inner surface). Decreasing said inner surface is realized by increasing the wall thickness of the discharge vessel perpendicular to the discharge arc, wherein the endpieces are not changed. Increasing the wall thickness of the discharge vessel has the following advantageous consequences: a. An increase in the wall thickness of the discharge vessel (on the inner surface thereof) urges the discharge arc between the tips of the electrodes in a more inward direction (that is in a position more or less in the middle between said electrodes), so that the average temperature of the inner surface of the wall of the discharge vessel will be raised. As the wall of the discharge vessel is made of a ceramic material (i.e. polycrystalline aluminum oxide) with a rather high thermal conductivity compared to quartz, for example, said thermal conductivity will play an important role in levelling the internal temperature differences within the discharge vessel of the present lamp. Therefore, a uniform ("flatter") temperature distribution within the discharge vessel will be achieved. b. As a result of the uniform temperature distribution in view of said high thermal conductivity, the average temperature within the discharge vessel may be higher than with prior art lamps, although the highest temperature inside the discharge vessel will be lower than with prior art lamps. After all, the lowest temperature is higher than with prior art lamps. c. Due to a higher average temperature inside the discharge vessel tungsten- particles that are deposited on the inner surface of the discharge vessel form larger crystals instead of a grey tungsten film consisting of very small particles in the prior art, resulting in a less obscured optical path even with the same amount of tungsten on the inner wall and therefore better maintenance (less light depreciation over life) of the lamp. In a preferred embodiment of a metal halide lamp according to the invention the inner wall-load is larger than or equal to 30 watt/cm2, wherein the outer wall-load is larger than or equal to 15 watt/cm2. Particularly, the inner wall-load ranges between 30 watt/cm2 and 45 watt/cm2, wherein the outer wall- load ranges between 15 watt/cm2 and 20 watt/cm2. The invention also refers to a method for manufacturing a metal halide lamp in accordance with the invention, wherein a discharge space enclosed by a discharge vessel with a ceramic wall of the lamp is filled with a rare gas and an ionizable filling and wherein two electrodes are arranged in said discharge space in such a manner that tips of said two electrodes have a mutual interspacing EA so as to define a discharge path between them, characterized in that the quotient of the outer wall area and the inner wall area is larger than 2, wherein the inner wall area is determined by the inner wall-load and the outer wall area is determined by the outer wall-load, as previously defined.

The above and further aspects of the amp in accordance with the invention will be explained hereinafter with reference to a drawing (not true in scale), in which Fig. 1 schematically shows a lamp in accordance with the invention, and Fig. 2 is a detailed representation of the discharge vessel of the lamp in accordance with Fig 1.

Fig. 1 shows a metal-halide lamp provided with a discharge vessel 3 having a ceramic wall which encloses a discharge space 11 containing an ionizable filling including at least Hg, an alkali halide and CeJ3. However, the filling does not necessarily have to contain Hg or Ce-Halide. Two electrodes whose tips are a mutual distance EA apart are arranged in the discharge space, and the discharge vessel has an internal diameter Di at least over de distance EA. The discharge vessel is closed at one side by means of a ceramic projecting plug 34, 35 which encloses a current lead-through conductor (Fig. 2: 40, 41, 50, 51) to an electrode 4,5 positioned in the discharge vessel with a narrow intervening space and is connected to this conductor in a gastight manner by means of a melting-ceramic joint (Fig. 2: 10) near to an end remote from the discharge space. The discharge vessel is surrounded by an outer bulb 1 which is provided with a lamp cap 2 at one end. A discharge will extend between the electrodes 4, 5 when the lamp is operating. The electrode 4 is connected to a first electrical contact forming part of the lamp cap 2 via a current conductor 8. The electrode 5 is connected to a second electrical contact forming part of the lamp cap 2 via a current conductor 9. The discharge vessel, shown in more detail in Fig. 2 (not true to scale), has a ceramic wall and is formed by a cylindrical part with an internal diameter Di which is bounded at either end by a respective end wall portion 32a, 32b, each end wall portion 32a, 32b forming an end surface 33a, 33b of the discharge space. The end wall portions each have an opening in which a ceramic projecting (extended) plug 34, 35 is fastened in a gastight manner in the end wall portion 32a, 32b by means of a sintered joint S. The ceramic projecting plugs 34, 35 each narrowly enclose a current lead-through conductor 40, 41, 50, 51 of a relevant electrode 4, 5 having a tip 4b, 5b. The current lead-through conduct is connected to the ceramic projecting plug 34, 35 in a gastight manner by means of a melting-ceramic joint 10 at the side remote from the discharge space. The electrode tips 4b, 5b are arranged a mutual distance EA apart. The current lead-through conductors each comprise a highly halide-resistant portion 41, 51 for example in the form of a Mo-A1203 cermet H; and a portion 40, 50 which is fastened to a respective end plug 34, 35 in a gastight manner by means of the melting-ceramic joint 10. The melting- ceramic joint extends over some distance, for example approximately 1 mm, over the Mo cermet 41 , 51. It is possible for the parts 41 , 51 to be formed from a material other than Mo- A1203 cermet. Other possible constructions are known, for example, from EP 0 587 238 (US-A 5,424,609). A particularly suitable construction was found to be, inter alia, a highly halide-resistant coil applied around a pin of the same material. Mo is very suitable for use as a highly halide-resistant material. The parts 40, 50 are made from a metal whose coefficient of expansion corresponds well to that of the end plugs. Nb, for example, is a highly suitable material. The parts 40,50 are connected to the current conductors 8, 9, respectively, in a manner not shown in any detail. The lead-through construction described renders it possible to operate the lamp in any desired burning position. Each of the electrodes 4, 5 comprises an electrode rod 4a, 5a which is provided with a winding 4c, 5c near the tip 4b, 5b. The projecting ceramic plugs are fastened in the end wall portions 32a and 32b in a gastight manner by means of a sintered joint S. The electrode tips then lie between the end surfaces 33a, 33b formed by the end wall portions. The quotient of the outer wall area and the inner wall area is larger than 2, so that a more uniform ("flatter") temperature distribution within the discharge vessel 1 is obtained. Said large quotient is realized by decreasing the inner surface of the discharge vessel and by not changing the outer shape of the lamp. Due to a higher average temperature inside the discharge vessel tungsten-particles that are deposited on the inner surface of the discharge vessel form larger crystals instead of a grey tungsten film consisting of very small particles in the prior art, resulting in a less obscured optical path even with the same amount of tungsten on the inner wall and therefore better maintenance (less light depreciation over life) of the lamp. PHNL031137 PCT/IB2004/051668

A comparison between a standard 70 Watt metal halide lamp and a metal halide lamp according to an embodiment of the invention reveals the following:

Prior Art Invention

Inner length (IL) 7.0 mm 9.0 mm

Inner diameter (ID) 6.85 mm 5.2 mm

Wall thickness (WD) 0.8 mm 1.2 mm

Electrode distance (EA) 7 mm 7 mm

Outer diameter (OD) 8.45 mm 7.6 mm

Inner wall-load 33 W/cm2 40 W/cr

T upper: 1425 K 1390 K

T under: 1325 K 1340 K

Tmax-Tsalt 100K 90 K

The temperatures are measured in a horizontal operating position of the lamp.

Claims

CLAIMS:

1. A metal halide lamp comprising a discharge vessel with a ceramic wall enclosing a discharge space containing a rare gas and an ionizable filling, wherein in said discharge space two electrodes are arranged whose tips have a mutual interspacing EA so as to define a discharge path between them, characterized in that the quotient of the outer wall area and the inner wall area is larger than 2, whereby the inner wall area is determined by the inner wall-load and the outer wall area is determined by the outer wall-load.

2. A metal halide lamp according to claim 1, wherein the inner wall- load is larger than or equal to 30 watt/cm2 and wherein the outer wall-load is larger than or equal to 15 watt/cm2.

3. A metal halide lamp according to claim 2, wherein the inner wall-load ranges between 30 watt/cm2 and 45 watt/cm2 and wherein the outer wall- load ranges between 15 watt/cm2 and 20 watt/cm2.

4. A method for manufacturing a metal halide lamp according to any of the preceding claims 1 through 3, wherein a discharge space enclosed by a discharge vessel with a ceramic wall of the lamp is filled with a rare gas and an ionizable filling and wherein two electrodes are arranged in said discharge space in such a manner that tips of said two electrodes have a mutual interspacing EA so as to define a discharge path between them, characterized in that the quotient of the outer wall area and the inner wall area is determined by the inner wall-load and the outer wall area is determined by the outer wall-load.