Low pressure rare gas discharge lamp

Abstract: In a low-pressure rare gas discharge lamp wherein a rare gas as a light emitting gas is sealed in a bulb (1) and light emitted from the gas by electric discharge is utiliz­ ed, an isolation film (2) for isolation from a discharge space is proviced at least on the inner surface portion of the bulb which portion surrounds a positive column.

Description: The present invention relates to a low-pressure rare gas discharge lamp wherein a rare gas as a light emitting gas is sealed in a bulb. Particularly, the present invention is concerned with a low-pressure rare gas discharging fluorescent lamp for use in office automatic (OA)-related machinery and apparatus such as facsimiles and copying machines. In order to avoid white discoloration a coating of titanium dioxide on the inner surface of the bulb of a low-pressure rare gas discharge lamp containing a small quantity of mercury has been proposed in US-A-3 912 828. As further prior art, for example on pages 1079 - 1082 of "Toshiba Review", Vol. 40, No. 12 (1985) there is described a low-pressure rare gas discharge lamp with several ten Torr to several hundred Torr of xenon sealed therein in place of mercury used in ordinary fluorescent lamps. More particularly, since mercury vapor is used in ordinary fluorescent lamps, this vapor pressure changes with change of the ambient temperature, and light output also varies, while the use of xenon is advantageous in that the light output does not vary over a wide temperature range because mercury is not used. This advantage is utilized to attain the extension of use as a light source for OA-related machinery and apparatus. On the other hand, for example, as reported by Mr. Okuno of Matsushita Electric Industrial Co., Ltd. at the 1975 national meeting of the illumination society, it is known that in a xenon-sealed low-pressure rare gas discharge lamp, the best light emitting efficiency is realized by making the sealed gas pressure extremely low, not higher than 0.1 Torr. However, as also pointed out by the same report, there has been the problem that in such a low pressure region, xenon is extinguished by a clean-up phenomenon during discharge and the service life of the lamp expires in a short time. Thus, in a low-pressure gas discharge lamp, if the sealed gas pressure is set low, there will be an increase of luminance and improvement of efficiency, but the life of the lamp will expire in an extremely short time due to a clean-up phenomenon. Therefore, in order to ensure the service life of the lamp it has inevitably been required to increase the gas pressure under the sacrifice of luminance and efficiency. The present invention has been accomplished for overcoming the above-mentioned problems. According to our finding, the clean-up phenomenon of a low-pressure rare gas discharge lamp is closely related to the relation between the residue in the glass tube and rare gas ion, and this reaction is suppressed by isolating the rare gas ion and the residue in the glass tube from each other. The present invention is based on this finding and it is the object thereof to provide a low-pressure rare gas discharge lamp capable of preventing the cleanup phenomenon even at an extremely low pressure of a rare gas sealed in the lamp and having high luminance and efficiency and a prolonged service life in a low gas pressure region. This object is accomplished by claim 1. In drawings: Figure 1 is a partially sectional view showing an embodiment of the present invention; Figure 2 is a life characteristic diagram of a low-pressure rare gas discharge lamp using a titanium oxide film. Preferred embodiments of the present invention will be described with reference to the drawings. In Figure 1, which is a partially sectional view of a low-pressure rare gas discharge lamp according to an embodiment of the present invention, a reference numeral 1 denotes a gas bulb having a tube diameter of 15.5 mm. The glass bulb 1 is formed by soda glass which is very common and in which there are contained as residues about 0.004 wt% of fluorine and 0.031% of chlorine. A numeral 2 denotes a titanium dioxide film formed as an isolation film on the inner surface of the glass bulb 1. The titanium dioxide film 2 is formed by applying tetrabutyl titanate to the bulb inner surface, then drying and baking it for decomposition. A numeral 3 denotes a fluorescent substance layer formed on a face of the titanium dioxide film 2, using GP₁G₁ green fluorescent substance (a product of Kasei Optonix, Ltd.). Numerals 4, 5 and 6 denote a reflective film, an aperture, and a filament, respectively. Though not shown in the figure, an electron emitting substance is applied to the filament 6, and xenon 100% gas is sealed in the interior of the glass bulb 1. In the glass bulb 1, moreover, there is provided a sufficient amount of barium getter for the purpose of adsorbing impure gases throughout the service life of the lamp. As to lighting conditions, a sinusoidal high frequency of 30 KHz was used as a power source, and the lamp current was set constant at 100 mA. Figure 2 shows life characteristics in varied gas pressures in the lamp constructed as above, in which the amount of titanium oxide deposited on the inner surface of the glass bulb is used as a parameter. The life is shown in terms of a relative value, assuming that the life of the lamp having a sealed xenon pressure of 100 Torr is 100%. Reference to the figure shows that as the amount of titanium dioxide deposited increases, the life of the lamp is prolonged to a remarkable extent. When the filaments of lamps whose lives had expired were observed, there scarcely remained an electron emitting substance in lamps in which the amount of titanium oxide deposited exceeded 0.05 mg/cm². This state was close to that of the filaments of lamps each having a sealed gas pressure of 50 Torr or higher and with titanium dioxide not deposited. According to another similar experiment, krypton proved to make the lamp life shorter than in the use of xenon. Generally, rare gases are called inert gases which are extremely small in reactivity, and it is said that this tendency is enhanced with reduction in size of atoms. However, according to experiments made by the present inventors, smaller atoms were more apt to react in plasma. This is presumed to be because krypton is higher in the ionization level than xenon so the electron energy of krypton is higher than that of xenon during discharge and hence the reaction is accelerated. Likewise, when xenon 100% gas and xenon 10% plus neon 90% gas were sealed respectively in discharge lamps at the same pressure, the latter was higher in both electronic energy and luminance, but was shorter in the service life. Table 1 below shows several experimental examples. In Table 1, as the aluminum oxide and silicon oxide there were used Aluminum Oxide C (a product of DEGUSSA AG), etc., but these materials used not only were ineffective but also showed a tendency to somewhat shortening the service life of the lamps. This is presumed to be because not only the function as an isolation film, namely glass shield, is imperfect but also fine particles of the materials scratches the inner surface of the glass bulb, causing impurities (residues) in the glass to be exposed. It is known to form a coating of titanium dioxide on the inner surface of a glass bulb, as shown, for example, in Japanese Examined Patent Publication No. 7240/1961 and Japanese Unexamined Patent Publication No. 35967/1975. However, the coatings disclosed therein are for suppressing the reaction between an electroconductive film formed on the inner surface of the glass bulb and mercury. On the other hand, in Japanese Unexamined Patent Publication No. 93184/1977 there is disclosed a titanium oxide coating for suppressing the deposition of sodium in glass to prevent the reaction of sodium with mercury. Thus, all of the above conventional titanium oxide coatings are for suppressing the reaction with mercury to improve luminous flux. It is not suggested thereby at all that in a low pressure region of a rare gas discharge lamp not containing mercury, a titanium oxide film suppresses the reaction between the residues in glass and the rare gas ions to greatly improve the life characteristic. In the present invention, as set forth above, since an isolation film is formed on the inner surface of a glass bulb which surrounds a positive column, it is possible to suppress the reaction between a light emitting gas and the residues in the glass bulb which causes the clean-up phenomenon, whereby the life of the lamp can be prolonged. Consequently, the luminance and efficiency can be greatly improved without impairing the life of the lamp.

Claims: Low-pressure rare gas discharge lamp wherein a rare gas as a light emitting gas is sealed in a bulb (1) and light emitted from the gas by electric discharge is utilized, wherein an isolation film (2) consisting of titanium dioxide for isolation from a discharge space is provided at least on the inner surface portion of the bulb (1), no mercury being contained in said lamp. A low-pressure rare gas discharge lamp according to claim 1, characterized in that said titanium dioxide film (TiO₂) is formed by thermal decomposition of tetrabutyl titanate, as a thin film having a light transmitting property.

Inventors: KAMANO YUJIRO MITSUBISHI DENKI, KOBAYASHI TOSHIHIKO MITSUBISHI, MITSUHASHI SEISHIRO MITSUBISHI, MURAKAMI KATSUO MITSUBISHI DEN, OSAWA TAKASHI MITSUBISHI DENKI, KAMANO, YUJIRO, MITSUBISHI DENKI K.K., KOBAYASHI, TOSHIHIKO, MITSUBISHI DENKI K.K., MITSUHASHI, SEISHIRO, MITSUBISHI DENKI K.K., MURAKAMI, KATSUO, MITSUBISHI DENKI K.K., OSAWA, TAKASHI, MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1990
Main classification: H01J1
IPC codes: H01J61