The present invention relates to a discharge lamp arc tube having a closed glass bulb containing electrodes arranged to be opposite to each other and luminous materials, etc., sealed therein, especially relates to an arc tube having a closed glass bulb without any tip-off portion, and a method of producing the same.
FIG. 6 shows a conventional discharge lamp device. The discharge lamp device has a structure in which front and rear end portions of an arc tube 5 are supported by a pair of lead supports 3 and 4 projecting forward from an electrically insulating base 2. The reference character G designates an ultraviolet screening globe for cutting off an ultraviolet component in a wavelength region harmful to human bodies from light emitted from the arc tube 5.
The arc tube 5 has a structure in which a closed glass bulb 5a is formed between a pair of front and rear sides pinch seal portions 5b, 5b such that a pair of electrode rods 6, 6 are disposed so as to be opposite to each other in the glass bulb 5a by the pinch seal portions 5b, 5b respectively and luminous materials are sealed in the glass bulb 5a. A piece of molybdenum foil 7 which connects the electrode rod 6 projected into the inside of the closed glass bulb 5a and a lead wire 8 led out from the pinch seal portion 5b to each other is sealed in the pinch sealed portion 5b, so that the airtightness in each of the pinch seal portions 5b is secured.
That is, tungsten rods excellent in durability are most suitably used as the electrode rods 6 but tungsten is largely different in linear expansion coefficient from glass and inferior in airtightness because tungsten is hardly fitted to glass. Accordingly, molybdenum foil 7 having a linear expansion coefficient close to that of glass and relatively well fitted to glass, is connected to each of the tungsten electrode rods 6 and sealed by each of the pinch seal portions 5b so that airtightness in each of the pinch seal portions 5b is secured.
A method for producing the arc tube 5 is disclosed, for example, in Japanese Patent Application Laid-open No. Hei. 6-231729. As shown in FIG. 7(a), first, an electrode assembly A including an electrode rod 6, a piece of molybdenum foil 7 and a lead wire 8 to which the rod 6 and the foil 7 are integrally connected is inserted into a cylindrical glass tube W from one opening end side of the glass tube W. The glass tube W has a spherically swollen portion w.sub.2 formed in the middle of the glass tube W, that is, between linear extension portions w.sub.1. A position P.sub.1 near the spherically swollen portion w.sub.2 is primarily pinch-sealed. Then, as shown in FIG. 7(b), luminous materials P, etc., are introduced into the spherically swollen portion w.sub.2 from the other opening end side of the glass tube W. Then, as shown in FIG. 7(c), after another electrode assembly A is inserted, a position P.sub.2 near the spherically swollen portion w.sub.2 is heated and secondarily pinch-sealed while the spherically swollen portion w.sub.2 is cooled by liquid nitrogen so that the luminous materials, etc., are not vaporized. In this manner, the spherically swollen portion w.sub.2 is sealed hermetically, so that an arc tube 5 having a tipless closed glass bulb 5a is finished.
Incidentally, in the primary pinch sealing step shown in FIG. 7(b), pinch-sealing is performed while an inert gas (generally, inexpensive argon gas) is supplied, as a forming gas, into a glass tube W so that the electrode assemblies A are not oxidized. Further, in the secondary pinch sealing step shown in FIG. 7(c), pinch-sealing is performed in a nearly vacuum state because the glass tube W with its opening ends closed is cooled by liquid nitrogen so that luminous materials, etc., are not vaporized.
In the conventional arc tube, however, the linear expansion coefficient of the molybdenum foil 7 sealed by the pinch seal portions 5b is not quite equal to that of glass even though the molybdenum foil 7 is well fitted to glass. Further, the temperature difference of the lamp is large between at the time of switching on and at the time of switching off, so that heat stress due to the temperature change is generated in the interface between the molybdenum foil 7 and glass. Further, the vibration of an engine and vibration due to the running of a car are transmitted to the arc tube. Accordingly, there arises a problem that a gap is formed between the molybdenum foil 7 and a glass material in long-term use, that is, foil floating occurs to cause the leakage of materials sealed in the closed glass bulb.
Therefore, the present inventor conducted experiments and made considerations on the aforementioned problems. As a result, the inventor confirmed that foil floating was reduced if molybdenum foil having its surface oxidized was sealed in the pinch-seal portion. Thus, the inventor has achieved the present invention.
That is, the present invention is based on the aforementioned problems and the inventor's findings and its object is to provide a discharge lamp arc tube free from foil floating in pinch seal portions and a method of producing the same.