1. Field of the Invention
The present invention relates to electric discharge lamps and, more particularly, to an improved sealing structure for use in such discharge lamps and a method for sealing discharge lamps.
2. Description of the Prior Art
Discharge lamps such as extra-high pressure mercury lamps and metal halide lamps are widely used in optical instruments such as liquid crystal projectors, OHPs and motion picture projectors and in general lightings. Such discharge lamps are highly advantageous in that their energy efficiency is three to five times higher than that of incandescent lamps such as halogen lamps, which emit light by heating filament, and their life time is five to ten times longer than that of such incandescent lamps.
A conventional method of making discharge lamps is briefly described with reference to FIGS. 2(A) and 2(B) illustrating such conventional method.
First, quartz glass is shaped into a glass tube (21) having a hollow light-emitting portion (22) and sealing tubes (23) formed at opposite ends of the light-emitting portion (22). Then, a hole (24) is formed at the light-emitting portion (22) of the glass tube (25) is connected to the hole (24).
Subsequently, a mount (26) having an electrode is inserted through an end of each sealing tube (23) into the glass tube (21) with use of a clamp, while nitrogen gas is introduced into the glass tube (21). The opposite sealing tubes (23) are then sealed by heating and pressing. Thus, the mount (26) is embedded in each sealing tube (23) so that the electrode is located in the light-emitting portion (22).
In turn, a vacuum is provided in the light-emitting portion (22) through the tip tube (25), followed by required treatments such as washing. Subsequently, filer substances such as mercury or a metal halide and a rare gas are filled into the light-emitting portion (22). The connecting portion of the tip tube (25) is then melt and cut to seal the light-emitting portion (22), thereby providing a discharge lamp.
With such method, however, a seal-cut trace (27) of the tip tube (25) remains on the light-emitting portion (22). The seal-cut trace hinders certain light path to cause a shadow, resulting a loss of 10 to 20% in lighting efficiency. Accordingly, when the discharge lamp is used as the light source of a projector or the like, unevenness in the screen brightness results.
Further, since the internal gas pressure if exerted on the light-emitting portion of the discharge lamp, the presence of the seal-cut trace (27) in the light-emitting portion is not preferable in terms of the strength. With the construction having such seal-cut trace, the filling gas pressure is limited, which leads to a lamp having a shorter life time.
To solve the problems associated with the seal-cut trace of the tip tube, so-called xe2x80x9ctiplessxe2x80x9d discharge lamps which can be made without using any tip tube have been developed.
A conventional method of making such tipless discharge lamp is described below with reference to FIGS. 3(A) to 3(C).
First, a first mount (36a) is inserted through an outer end of one sealing tube (33a) into a glass tube (31) as shown in FIG. 3(A). The sealing tube (33a) containing the first mount (36a) therein is then sealed by heating and pressing.
Then, in a vacuum chamber capable of providing a highly reduced pressure, filler substances such as mercury and a rare gas are introduced through an outer end (37) of another sealing tube (33b) which is not yet sealed. Thereafter, a second mount (36b) having a bent lead (35) is inserted through the outer end (37) of the sealing tube (33b). The outer end (37) is then sealed by the use of a laser or a plasma burner, thus closing the glass tube (31) as shown in FIG. 3(B).
In this state, since the lead (35) of the second mount (36b) is bent, it abuts against the inner wall of the sealing tube (33b). Accordingly, although the second mount (36b) is located within a hollow portion of the sealing tube (33b), it can be retained without the aid of any additional retaining means.
The glass tube is then taken out of the vacuum chamber and the unsealed portion (i.e., the peripheral wall embracing the second mount) of the sealing tube (33b) is sealed by heating and pressing. The end portion (37) is then cut away as shown in FIG. 3(C), thus completing the tipless discharge lamp.
In the tipless discharge lamp thus fabricated, the seal-cut trace is not formed and, hence, the light path is not hindered. Thus the unevenness in brightness is less likely to occur. Further, since the lamp thus provided has an enhanced strength, the pressure within the light-emitting portion (32) can be greatly increased.
With this method, however, it is impossible to clean the interior of the glass tube through a tip tube which would otherwise be used after the sealing tube is sealed. Accordingly, impurities generated during such operations as encapsulating the rare gas and finally sealing the glass tube remain within the light-emitting portion (32), which may lead to any failure or a shorter life time.
Further, the cost of equipment such as the vacuum chamber and the like requires is high and, hence, the products are inevitably expensive. This hinders the widespread use of apparatus such as projectors using tipless discharge lamps.
Additionally, since the mount is retained within the glass tube by mere contact engagement between the lead (35) and the inner wall of the sealing tube (33b), the precise positioning of the electrode (36) is difficult, which leads to a poor yield.
Therefore, need exists for means for providing discharge lamps which is less likely to cause unevenness in brightness, which enables easier positioning of an electrode, and which ensure a longer life time at a low cost.
In accordance with a first aspect of the present invention, there is provided an electric discharge lamp comprising: a lamp envelop including a first and a second sealing tubes both of which are sealed and a light-emitting portion located between the sealing tubes; a pair of electrodes disposed in the light-emitting portion; and a filler substance encapsulated within the lamp envelop, one of the sealing tubes having a seal-cut trace of a tip tube that has been used for introducing the filler substance into the lamp envelop therethrough.
With this construction, the seal-cut trace of the tip tube is not located on the light-emitting portion, light emanates without interference by the seal-cut trace. Accordingly, the discharge lamp is less likely to suffer from a loss in the luminance and an unevenness in the brightness. Further, the light-emitting portion can be strengthened and hence, the pressure within the light-emitting portion can be increased thereby ensuring the lamp enjoying a longer life time.
In accordance with a second aspect of the present invention, there is provided a method of making a discharge lamp comprising the steps of: providing a lamp envelop including a first and a second sealing tubes, a light-emitting portion located between the sealing tubes, and a tip tube connected to the second sealing tube for communication therbetween; inserting a first mount having a first electrode into the first sealing tube through an open end thereof so that the first electrode is located in the light-emitting portion and then entirely sealing the first sealing tube; inserting a second mount having a second electrode into the second sealing tube through an open end thereof so that the second electrode is located in the light-emitting portion and then sealing a portion of the second sealing tube so as to maintain the communication between the tip tube and the second sealing tube; cleaning the inside of the light-emitting portion an then introducing a filler substance and a rare gas through the tip tube into the light-emitting portion; removing the tip tube from the second sealing tube by sealing and cutting; and sealing the rest of the second sealing tube in which a seal-cut trace of the tip tube is present.
With this method, the lamp envelop including the second sealing tube connected with the tip tube for communication therebetween is used, and accordingly, a discharge lamp is fabricated which has the seal-cut trace of the tip tube on the second sealing tube but not on the light-emitting portion. Further, since the mounts can be held firmly by clamp, the position of each electrode can be easily adjusted, which leads to a higher production yield.
Furthermore, this method allows the operations of reducing the pressure in the lamp envelop and introducing the filler substance such as mercury into the lamp envelop to be easily performed through the tip tube. Hence, a large vacuum chamber is not needed. Moreover, impurities within the lamp envelop can be discharged through the tip tube and, therefore, it is possible to provide discharge lamps which avoid any failure or any shortened life time associated with such impurities. In addition, the presence of the seal-cut trace of the tip tube on the second sealing tube but not on the light-emitting portion enables the filling gas pressure to increase rather than the conventional level, which leads to lamps enjoying a longer life time.
These and other objects, features and attendant advantages will become apparent from the following detailed description when read in conjunction with the attached drawing, in which;