Electric lamp and discharge devices: systems – Current and/or voltage regulation
Reexamination Certificate
2001-07-18
2004-03-30
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Current and/or voltage regulation
C313S639000
Reexamination Certificate
active
06713972
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to ultra-high pressure discharge lamps and, more particularly, to novel methods of lighting such ultra-high pressure discharge lamps, ultra-high pressure discharge lamps to which the methods of the invention are applicable, and ballasts for use in such ultra-high pressure discharge lamps.
2. Description of the Related Art
In these years, a liquid crystal projector market has grown rapidly, and a market for consumer-oriented data projectors and rear projection television sets adapted for digital broadcasting is expected to grow explosively in near future. In meeting the demand of such a growing market, the life of a light source and countermeasures against flicker are critical factors.
Heretofore, brightness has been the only one standard for estimating the performance of such a light source and, hence, there has been a keen competition for a higher brightness. Ultra-high pressure discharge lamps developed as a result of such a keen competition generally exhibit a high luminance attained by shortening the arc length to the limit and increasing the pressure of mercury in the discharge bulb to 150 atm or higher so as to raise the arc temperature.
Though the luminance of such a conventional ultra-high pressure discharge lamp has been improved in the manner described above, tradeoffs have occurred such that an electrode surface adjacent the location at which an arc is generated is consumed too rapidly and that flicker due to such rapid consumption of the electrodes is likely to occur. At present, these tradeoffs cannot be controlled. Conventional ultra-high pressure discharge lamps cause steady flicker to occur after 400 hours' use (refer to Table 1), and the illuminance of a screen illuminated by a projector incorporating such a conventional ultra-high pressure discharge lamp lowers by about 50% after 1,000 hours' use of the lamp. Thus, conventional ultra-high pressure discharge lamps also have a problem in keeping the screen illuminance.
Though there has been developed an ultra-high pressure discharge lamp assuring a certain illuminance kept for 1,000 hours or longer, an electrode surface adjacent the arc generating location in such a lamp becomes roughened and, hence, the discharge initiating point frequently moves about, which is observed as flicker on the screen. It is, therefore, needless to say that light sources to be incorporated in consumer-oriented products have to be flickerless while keeping a satisfactory screen illuminance for a longer time.
An invention related to an ultra-high pressure discharge lamp is disclosed in Japanese Patent Gazette No. 2,829,339. The Gazette mentions use of a typical halogen cycle for preventing blackening due to evaporation of tungsten and describes that if the amount of a halogen used is large, the electrodes are consumed heavily and, hence, the life of the lamp becomes as short as several hundreds of hours. The invention of this Gazette consists in reducing the amount of the halogen to the limit in order to prevent such a rapid consumption of electrodes due to the halogen. However, such an extremely reduced amount of the halogen is critical and hence may fail to ensure a desired halogen cycle. As a result, the discharge bulb is sometimes blackened due to evaporation of tungsten.
The Patent Gazette teaches the fact that the life of 5,000 hours or longer was attained by reducing the amount of the halogen to the limit. Even an ultra-high pressure discharge lamp that escaped being blackened at its discharge bulb, however, was observed to have its electrodes consumed or roughened at their surfaces adjacent the arc generating location. In an extreme case plural projections were formed on the electrodes. Due to such roughness at the surfaces of the electrodes, the discharge initiating point frequently moved about on the electrode surface roughened and such a movement was reflected as flicker on a screen. Thus, the light source according to the Patent Gazette was not a practically acceptable one as a light source having a service life rated at 5,000 hours which assures a service life of 5,000 hours presently demanded.
As described above, the Patent Gazette mentions the amount of a halogen as the cause of the occurrence of electrode consumption. Specifically, the Patent Gazette concludes that encapsulation of a halogen in an amount of 1×10
−4
&mgr;mol/mm
3
or more is the only factor dominating the electrode consumption. In the experiment conducted by the inventors of the present invention, however, it was observed that an ultra-high pressure discharge lamp encapsulating a halogen in an amount less than 1×10
−4
&mgr;mol/mm
3
was blackened due to an incomplete halogen cycle while at the same time an electrode surface adjacent the arc generating location was deformed (roughened or formed with projections) after lapse of a few hundreds of hours from the starting of the experiment, that the discharge initiating point began moving about after lapse of 500 hours, and that flicker appeared on a screen after lapse of 1,000 hours. From this experiment it is deduced that other factors than the amount of a halogen which dominate the consumption of electrodes and the roughening of an electrode surface exist.
Japanese Patent Gazette No. 2,980,882 discloses an invention that intends to prevent blackening or blurring of a discharge bulb and consumption of electrodes by encapsulating an increased amount of a halogen as large as 2×10
−4
to 7×10
−3
&mgr;mol/mm
3
in order to absorb and reduce short wavelength ultraviolet ray. According to this Gazette, the mechanism of the occurrences of such inconveniences is that short wavelength ultraviolet ray generated during lighting of the lamp cuts off the silicon (Si)-oxygen (O) bond of quartz glass forming the discharge bulb of the lamp thereby to cause SiO to evaporate and such evaporated SiO in turn causes blurring of the discharge bulb and consumption of the electrodes. Stated otherwise, the Patent Gazette concludes that short wavelength ultraviolet ray is the factor dominating the blackening or blurring of a discharge bulb and consumption of electrodes.
The inventors of the present invention actually constructed an ultra-high pressure discharge lamp according to the scope claimed by the latter Patent Gazette and verified the effect of this lamp. As a result, it was found that consumption of the electrodes proceeded unusually rapidly and this fact was far worse than stated in the Gazette. More specifically, it was observed in the ultra-high pressure discharge lamp in which a large quantity of a halogen was encapsulated in order to inhibit the generation of short wavelength ultraviolet ray that the electrodes were consumed or roughened at their surfaces adjacent the arc generating location after lapse of 100 hours from the starting of the experiment, while at the same time the discharge initiating point moved around frequently, and that flicker appeared on a screen after lapse of 400 hours from the starting of the experiment. It is concluded from these results that the life of the ultra-high pressure discharge lamp according to the Patent Gazette in question cannot reach even 2,000 hours, which is the shortest life presently required of projector light sources and that it is absolutely impossible for the lamp to have a life of 10,000 hours, which is required of applications in television. This experiment also proved that the life of an ultra-high pressure discharge lamp could not be prolonged by absorbing and reducing short wavelength ultraviolet ray with an increased amount of an encapsulated halogen.
In view of the foregoing, the inventors of the present invention conducted experiments on the assumption that factors dominating the consumption or roughening of electrodes and formation of projections on the electrodes in an ultra-high pressure discharge lamp operated with alternating or direct current include a thermal factor, an oxygen factor and a silicon factor
Hujii Toshitaka
Hurukawa Hisao
Ikeda Tomihiko
Nakagawa Atsuji
Shirai Tetsuya
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Philogene Haissa
Phoenix Electric Co. Ltd.
Tran Chuc
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