METAL HALIDE ARC LAMP

Abstract: A metal halide arc lamp includes a quartz arc tube (1) having a pair of electrode shafts (3, 27; 5, 29). The quartz arc tube (1) which encloses a fill including mercury and metal halide materials comprises a hollow illuminating portion (25) and a pair of squeezed portions (21, 23) each of which have an individual elongate space (31, 33), extending about the electrode shaft (27, 29). The minimum value of the width, in the direction perpendicu­ lar to the electrode shaft, of the elongate spaces (31, 33) is controlled within a specific range of values, related to the diameter of the electrode shaft, to reduce a probability of crack occurrence in the squeezed portion (21, 23) by a pressure produced when the fill is vapored rapidly in the elongated space (31, 33) of the squeezed portion (21, 23).

Description: The present invention relates to metal halide arc lamp as described in the first part of claim 1. In particular, the invention relates to a metal halide arc lamp containing a fill including mercury and metal halide materials such as NaI, ScI₃, etc.. Such lamps are well known in the art. In general, metal halide arc lamps have a quartz arc tube enclosing a pair of electrode shafts therein. The electrode shafts face each other, one shaft in each side, inside the tube, connecting with an individual external lead through a metalfoil. The metalfoil is made of high-melt point metals such as molybdenum. Each end of tube is squeezed to form a flat surface. The arc tube is filled with a starting rare gas such as argon, mercury, and a metal halide material such as NaI, ScI₃. To enclose a pair of main electrodes consisting of the electrode shafts and the metalfoils in an arc tube, the squeezed parts are formed by using the following procedure. The main electrodes face each other along the elongated axis of the arc tube, one in each end of the arc tube. Each end is softened by heating and opposite sides are squeezed with a pair of pinchers. By using this method, a gap is created around the electrode shaft because the diameter of electrode shaft is large. The gap is created lengthwise along the electrode shaft. The width of gap extends to the breadth direction of the squeezed part, or in the direction perpendicular to the elongated axis of the arc tube. This gap is required to absorb a difference in the thermal expansion coefficient between the metal of shaft and the squeezed glass. In a metal halide lamp with the above-described constructions, the metal halide enclosed in the arc tube enters into the gap. When the lamp is energized, the temperature of the electrode shaft rises and the halide evaporates quickly. Evaporated halide provides high internal pressures in the narrow gap. When the compression strength of squeezed glass part is lower than an internal pressure, a shelly crack is created in the squeezed part. For some arc tubes, this crack caused filler in the arc tube to leak, or the arc tube to be damaged. In addition, since the thermal expansion coefficient of the electrode shaft is different from that of the squeezed part of tube, when an arc lamp was turned on and off, a crack was created in the glass of the squeezed part. The result of observation and testing conducted by the inventors shows that there is a trend towards a smaller gap of a squeezed part, with increasing leak and defect in arc tubes. The other result shows that larger gap lowers the initial pressure strength of the squeezed part of arc tube. The present invention seeks to provide a metal halide arc lamp with a long life. The present invention provides a metal halide arc lamp comprising a quartz arc tube having a hollow portion with squeezed portions formed at opposite ends of the hollow portion; a fill including mercury and metal halide material in the arc tube; two electrodes located on the longitudinal axis of the arc tube at respective opposite ends of the hollow portion of the arc tube and each having a shaft which is disposed within the adjacent squeezed portion; characterised in that the shaft of one of the electrodes extends along an elongated space which leads from the hollow portion into the squeezed portion and the minimum value (L MIN ) of the width of the space in the direction perpendicular to the length of the shaft satisfies the equation: 0.1 mm ≦ L MIN ≦ 0.3 mm when the diameter (D) of the shaft is 0.4 mm and less and satisfies the equation: 1/6 D + 1/30 mm ≦ LMIN ≦ 1/6 D + 7/30 mm when the diameter (D) of the shaft exceeds 0.4 mm. A preferred embodiment of the present invention will be described with reference to the accompanying drawings, wherein like reference numerals throughout the various figures denote like structure elements and wherein:- Figure 1 shows a vertical longitudinal sectional view of an embodiment of the present invention; Figure 2 shows an enlarged sectional view taken in the direction of the arrows along the line A'A' of Figure 1; and Figure 3 shows a graph illustrating the characteristic of an arc tube obtained by varying the relationship between the diameter of the shaft of an electrode and the width of a space created in a squeezed portion of the arc tube. Referring to the accompanying drawings, an embodiment of this invention will be described. FIGURE 1 shows a vertical section of an arc tube of a metal halide arc lamp with a 100 W rating. An arc tube 1 has a quartz envelope containing a fill of a proper amount of starting rare gas, such as argon, mercury and metal halide materials, e.g. NaI and ScI₃. NaI and ScI₃ are able to improve characteristics of visible spectrum emitted from mercury. Na and Sc, however, quickly react on quartz. To prevent these metals from being reacted, Na and Sc are individually combined with iodine to be halogenated before they are enclosed in arc tube 1. In addition to NaI and ScI₃, rare earth metals such as Dy (Dysprosium) and Tm (Thulium) can be used as a filler. These materials are individually used, or used together with one another. In this case, the rare earth metals are halogenated and filled in the arc tube as described above. A pair of main electrodes 3 and 5, made of metal such as tungsten, faces each other, one in each end of arc tube 1. An auxiliary electrode 7 is arranged close to main electrode 3. Main electrodes 3 and 5, and auxiliary electrode 7 are connected to external leads 15, 17 and 19 through metal foils 9, 14 and 13 respectively. Metal foils 9, 11 and 13 are made of a metal with a high melt point, such as molybdenum Both ends of arc tube 1 are heated and compressed to form squeezed parts 21 and 23 respectively. As the result, arc tube 1 has a hollow portion 25 between squeezed parts 21 and 23. Main electrodes 3 and 5 have electrode shafts 27 and 29, respectively, connected to metal foils 9 and 11 respectively. Main electrodes 3 and 5 are arranged opposite to one another in portion 25. Electrode shafts 27 and 29 are arranged in squeezed parts 21 and 23 respectively. When squeezed part 21 is formed, a gap 31 is created between electrode shaft 27 and glass material 21a of squeezed part 21. Gap 31 extends along electrode shaft 27, and expands breadthwise to electrode shaft 27. In the same way, a gap 33 is created between electrode shaft 29 and glass material 23a of squeezed part 23, and a gap 35 between a base portion 7a of auxiliary electrode 7 and glass material 21a of squeezed part 21. Refering to FIGURE 2, the configuration of gaps 31, 33 and 35 will be described. Because gaps 31, 33 and 35 have the same configuration respectively, FIGURE 2 shows the section crossing along line A-A' in FIGURE 1, which illustrates squeezed part 23 of arc tube 1. The width (L) of gap 33 extends breadthwise to squeezed part 23 or to the direction perpendicular to the compressed direction of squeezed part 23. As can be seen in FIGURE 1, the width (L) of gap 33 defined by electrode shaft 29 and glass material 23a of squeezed part 23 is formed such that it becomes gradually wider from the middle portion of the gap towards both ends. In this embodiment, the diameter of electrode shaft 29 is set to 0.4 mm, and the minimum value (Lmin) of the width of gap 33 in the squeezed part 23 is set to 0.2 mm. Normally, an arc tube is enclosed in an external tube (not illustrated in Figures) to be formed as a lamp. When a lamp with the construction described above is installed in a vertical position with squeezed part 23 faced downward, metal halide and mercury filled in arc tube 1 accumulate in luminous area 25 of arc tube 1. Then metal halide and mercury enter into gap 33 between electrode shaft 29 and squeezed part glass material 23a. When the lamp is energized, quick temperature rise of electrode shaft 29 causes the metal halide and mercury to be evaporated rapidly. Therefore, the pressure caused by evaporation of the metal halide and mercury is applied to squeezed part glass 23a defining gap 33. Since the minimum value (Lmin) of width (L) of gap 33 is set to as wide as 0.2 mm, the applied pressure is moderated. This effect prevents squeezed glass 23a from being cracked. In addition, gap 33 which has a sufficient width absorbes a difference in thermal expansion coefficient between electrode shaft 29 and squeezed glass 23a. This provides advantage that occurrence of cracks in the glass of squeezed part 23 caused by temperature changes occurring when the lamp is turned on and off is prevented. The width (L) of gaps 31, 33, and 35 can be limited to a specified value by changing the shape of pincher or the rate of application of pressure used for manufacturing a lamp. When a lamp is turned on in the vertical position as above-described embodiment, no filler enters into gaps 31 and 35 produced in squeezed part 21 located at the upper part of arc tube 1, or only a small amount of filler enters. For that reason, the minimum value (Lmin) of the width (L) of gap 31 does not have to be considered. However, if the squeezed part to be located in the lower position cannot be predicted, the width (L) of gaps 31 and 33 produced in squeezed parts 21 and 23 has to be limited to the minimum value (Lmin) as described above. For gap 35 created in the squeezed part 21, such consideration is not required even if auxiliary electrode 7 is located in the lower position. The reasons; 1: A large current does not flow in an auxiliary electrode 7. 2: Current flows in auxiliary electrode 7 for an extremely short duration until arcing starts between main electrodes 3 and 5. For that reason, temperature of auxiliary electrode 7 rises extremely slowly unlike temperatures of main electrodes 3 and 5. Therefore, since any filler in gap 35 evaporates slowly, squeezed glass 21a is not damaged. The following table shows the comparison between the minimum values (Lmin) of width (L) of gap 33 produced between electrode shaft 29 located in the lower position and squeezed glass 23a and a number of lamps cracking. Lamps of the same type as that in the embodiment described above are used as the sample. A total amount of the sample is 20. This result shows that the number of lamps cracking is extremely small (the probability of lamp cracking is 1/20) when the minimum value (Lmin) is 0.1 mm. Cracking of this level is not associated with leakage or failure of arc tube 1. In contrast, when the minimum value (Lmin) is smaller, specifically 0.07 or 0.05 mm, more lamps are cracked, and some of these cracks are large enough to cause arc tubes to be cracked or damaged. Evaluations of these data indicate that the minimum value (Lmin) should be at least 0.1 mm. In contrast to the number of lamps cracking, the initial pressure resistance of a lamp can be improved with reducing the minimum value (Lmin) of gap 33. Assuming that the population of initial pressure resistance is normally distributed, if the limited value is represented as the average value - 3σ, the lower limit is 11,1 · 10⁵ Pa (11 atmospheres) when the minimum value (Lmin) is 0.4 mm. The internal pressure of arc tube 1 is about 10,1 · 10⁵ Pa (10 atmospheres) when a 100 W rating metal halide arc lamp is activated, and there is some fluctuation of this internal pressure during manufacturing. In consideration of these facts, it can be concluded that the minimum value (Lmin) is set to less than 0.3 mm rather than 0.4 mm. This indicates that it is desirable that the minimum value (Lmin) is from 0.1 mm to 0.3 mm to meet both the crack and the initial pressure resistance characteristics. FIGURE 3 shows the results of the tests carried out on lamps with various different main electrode diameters D and lamp inputs characteristics, in a similar way to the tests described above. The hatched region A in FIGURE 3 is the region where the probability of crack occurence is low during the life of the arc lamp, and where a squeezed part of an arc tube with initial pressure resistance enough for practical use can be obtained. The region B represents the area where the probability of crack occuring during the service life is high. The region C represents the area where the initial pressure resistance is low. FIGURE 3 shows the following correlation. When the diameter (D) of an electrode shaft is 0.4 mm or less, the minimum value (Lmin) of width of the gap should be; 0.1 mm < Lmin < 0.3 mm It is undesirable that the minimum value (Lmin) is 0.1 mm or less, because the probability of a lamp crack being created is high. It is undesirable that the minimum value (Lmin) exceeds 0.3 mm, because the initial pressure resistance is low. When the diameter (D) of an electrode shaft is more than 0.4 mm, the minimum value (Lmin) should be; 1 / 6 D + 1 30 < Lmin < 1 6 D + 7 30 If Lmin < 1 6 D + 1 30 , the probability of a lamp crack being created is high. If Lmin > 1 6 D + 7 30 , the initial pressure resistance is low. In summary, it will be seen that the embodiment of the present invention overcomes the disadvantage of the prior art and provides an improved metal halide arc lamp in which the probability of a lamp crack being created during the service life thereof is low, and a squeezed part thereof has initial pressure resistance enough for practical use to be obtained.

Claims: A metal halide arc lamp comprising a quartz arc tube (1) having a hollow portion (25) with squeezed portions (21, 23) formed at opposite ends of the hollow portion; a fill including mercury add metal halide material in the arc tube; two electrodes (3, 5) located on the longitudinal axis of the arc tube at respective opposite ends of the hollow portion (25) of the arc tube and each having a shaft (27, 29) which is disposed within the adjacent squeezed portion (21, 23); characterised in that the shaft of one of the electrodes extends along an elongated space (31, 33) which leads from the hollow portion (25) into the squeezed portion and the minimum value (L MIN ) of the width of the space in the direction perpendicular to the length of the shaft satisfies the equation: 0.1 mm ≦ L MIN ≦ 0.3 mm when the diameter (D) of the shaft is 0.4 mm and less and satisfies the equation: 1/6 D + 1/30 mm ≦ L MIN ≦ 1/6 D + 7/30 mm when the diameter (D) of the shaft exceeds 0.4 mm. A metal halide arc lamp as claimed in claim 1, characterized in that the shaft of the other electrode extends along an elongated space (31, 33) which leads from the hollow portion (25) into the squeezed portion and the minimum value (L MIN ) of the width of the space in the direction perpendicular to the length of the shaft satisfies the equation: 0.1 mm ≦ L MIN ≦ 0.3 mm when the diameter (D) of the shaft is 0.4 mm and less and satisfies the equation: 1/6 D + 1/30 mm ≦ LMIN ≦ 1/6 D + 7/30 mm when the diameter (D) of the shaft exceeds 0.4 mm. A metal halide arc lamp as claimed in claim 1 or 2, characterised in that the shaft (27, 29) of each electrode (3, 5) is connected electrically to an electrically conductive foil (9, 11) disposed within the respective squeezed portion (21, 23) of the arc tube. A metal halide arc lamp as claimed in claim 1, 2, or 3, characterised in that the width of the or each elongated space increases from the middle portion of the space towards the opposite ends of the space.

Inventors: HONDA, KAZUO PATENT DIVISION, INUKAI, SHINJI PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1986
Main classification: H01J61
IPC codes: H01J61