Rare gas discharge fluorescent lamp device

Abstract: The invention provides a rare gas discharge fluorescent lamp device which is long in life and high in brightness and efficiency. The lamp device comprises a rare gas discharge fluorescent lamp including a bulb (1') having rare gas such as xenon, argon or krypton gas enclosed therein, a fluorescent layer (2a) formed on an inner face of the bulb (1'), a reflecting film (2b), and a pair of electrodes (3a', 3b') located at the opposite ends of the bulb (1'). The lamp device further comprises a power source (8) for applying a voltage across the electrodes (3a', 3b'), and pulse voltage forming means (9, 10) connected between the electrodes (3a', 3b') and the power source (8) for forming a dc pulse voltage from a voltage supplied from the power source (8). The dc pulse voltage thus formed is applied across the electrodes (3a', 3b') to cause the lamp to be lit. The pulse frequency of the pulse voltage and the enclosed gas pressure are determined depending upon the rare gas employed.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a rare gas discharge fluorescent lamp for use with an information device such as a facsimile. a copying machine or an image reader wherein fluorescent substance is excited to emit light by ultraviolet rays generated by rare gas discharge. Description of the Prior Art In recent years, the performances of information terminal devices such as a facsimile, a copying machine and an image reader have been improved together with advancement of the information-oriented society. and the market of such information devices is rapidly expanding. In developing information devices of a higher performance. a light source unit for use with such information devices is required to have a higher performance as a key device thereof. Conventionally. halogen lamps and fluorescent lamps have been employed frequently as lamps for use with such light source units. However, since halogen lamps are comparatively low in efficiency, fluorescent lamps which are higher in efficiency are used principally in recent years. However, while a fluorescent lamp is high in efficiency, it has a problem that characteristics thereof such as an optical output characteristic vary in accordance with temperature since discharge from vapor of mercury is utilized for emission of light. Therefore. when a fluorescent substance is used, either the temperature range in use is limited. or a heater is provided on a wall of a tube of the lamp in order to control the temperature of the lamp. However. development of fluorescent lamps having stabilized characteristics are demanded eagerly for diversification of locations for use and for improvement in performance of devices. From such background. development of a rare gas discharge fluorescent lamp which makes use of emission of light based on rare gas discharge and is free from a change in temperature characteristic is being proceeded as a light source for an information device. FIGS. 7 and 8 show an exemplary one of conventional rare gas discharge fluorescent lamp devices which is disclosed, for example, in Japanese Patent Laid-Open Publication No. 63-58752 (and DE-A-37 18 216), and wherein FIG. 7 is a diagrammatic representation showing a longitudinal section of a rare gas discharge fluorescent lamp and an entire construction of the device, and FIG. 8 is a cross sectional view of the lamp. Referring to FIGS. 7 and 8, the rare gas discharge fluorescent lamp of the device shown includes a bulb 1 in the form of an elongated hollow rod or tube which may be made of quartz or hard or soft glass. A fluorescent coating 2 is formed on an inner face of the bulb 1, and rare gas consisting of at least one-of xenon, krypton, argon. neon and helium gas is enclosed in the bulb 1. A pair of inner electrodes 3a and 3b having the opposite polarities to each other are located at the opposite longitudinal end portions within the bulb 1. The inner electrodes 3a and 3b are individually connected to a pair of lead wires 4 which extend in an airtight condition through the opposite end walls of the bulb 1. An outer electrode 5 in the form of a belt is provided on an outer face of a side wall of the bulb 1 and extends in parallel to the axis of the bulb 1. The inner electrodes 3a and 3b are connected by way of the lead wires 4 to a high frequency invertor 6 serving as a high frequency power generating device, and the high frequency invertor 6 is connected to a dc power source 7. The outer electrode 5 is connected to the high frequency invertor 6 such that it may have the same polarity as the inner electrode 3a. Operation of the rare gas discharge fluorescent lamp device is described subsequently. With the rare gas discharge fluorescent lamp device having such a construction as described above, if high frequency power is applied across the inner electrodes 3a and 3b by way of the high frequency invertor 6, then glow discharge will take place between the inner electrodes 3a and 3b. The glow discharge will excite the rare gas within the bulb 1 so that the rare gas will emit peculiar ultraviolet rays therefrom. The ultraviolet rays will excite the fluorescent coating 2 formed on the inner face of the bulb 1. Consequently, visible rays of light are emitted from the fluorescent coating 2 and emitted to the outside of the bulb 1. Another rare gas discharge fluorescent lamp is disclosed, for example, in Japanese Patent Laid-Open Publication No. 63-248050. The lamp employs such a hot cathode electrode as disclosed, for example, in Japanese Patent Publication No. 63-29931 in order to eliminate the drawback of a cold cathode rare gas discharge lamp that the starting voltage is comparatively high. The rare gas discharge fluorescent lamp can provide a comparatively high output power because its power load can be increased. However, it can attain only a considerably low efficiency and optical output as compared with a fluorescent lamp based on mercury vapor. In summary. conventional rare gas discharge fluorescent lamps cannot attain a sufficiently high brightness or efficiency as compared with fluorescent lamps employing mercury vapor because fluorescent substance is excited to emit light by ultraviolet rays generated by rare gas discharge. A rare gas discharge lamp device from which the preamble of claim 1 starts out is disclosed in EP-A-285 396. This document focuses on the lamp itself and proposes a pressure of the rare gas enclosed in the lamp in a range between 2.7 and 27 kPa, but it pays no attention to means for driving the lamp. Different electric circuits for driving fluorescent lamps are disclosed in US-4 388 563 and Xerox Disclosure Journal, vol. 7, No. 5, Sept/Oct 1982, pages 307 and 308. SUMMARY OF THE INVENTION The present invention has been made to eliminate such problems as described above, and it is an object of the present invention to provide a rare gas discharge fluorescent lamp device offering a high brightness and a high efficiency. This object is solved by the device set forth in claim 1. The subclaims are directed to preferred embodiments of the invention. In a rare gas discharge fluorescent lamp device according to an implementation of the present invention, a pulse-like voltage is applied across a glass bulb so that molecules of a gas, which is enclosed in the bulb and contributes to emission of light, may be excited at such an energy level that a great amount of ultraviolet rays of the gas are produced by resonance, in order to increase the emission of light, improve the efficiency, and restrain wear of electrodes. To this end, pulse-like or intermittent discharge which involves die periods of lamp current is caused in the lamp by a half-wave rectified voltage supply from a lighting device having a simple construction wherein a current limiting element and a diode are added to a conventional high frequency power source, and a voltage is supplied across the lamp at a suitable frequency depending upona balance between an energization period and the die period of the pulse-like discharge. A lighting device where a half-wave rectified voltage is utilized as described above is constituted from a series circuit of a high frequency power source and a current limiting element, and a diode connected in parallel to the series circuit, and either a half-wave rectified voltage having a frequency higher than 4 KHz but lower than 200 KHz is supplied across the lamp in which xenon gas is enclosed at a pressure higher than 1300 Pa (10 Torr) but lower than 27 kPa (200 Torr) in order to cause the lamp to be lit, or a half-wave rectified voltage having a frequency higher than 5 KHz but lower than 200 KHz is supplied across the lamp in which krypton gas is enclosed at a pressure higher than 1300 Pa (10 Torr) but lower than 13 kPa (100 Torr) in order to cause the lamp to be lit. Under the construction conditions described above, pulse-like discharge which involves die periods of lamp current takes place in the lamp, and a voltage is applied across the lamp at a suitable frequency depending upon the energization period, and besides xenon gas or krypton gas is enclosed in the lamp at such a pressure that it may be excited in a high efficiency by pulse-like lighting. Accordingly, xenon gas or krypton gas is excited in a high efficiency, and radiation of ultraviolet rays is increased and the lamp efficiency is improved. Other objects and features of the invention will be more fully understood from the following detailed description and appended claims when taken with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic representation of an entire construction of a rare gas discharge fluorescent lamp device showing an embodiment of the present invention wherein a half-wave rectified voltage is utilized; FIG. 2 is a diagram showing a relationship between an enclosed gas pressure and a lamp efficiency when xenon gas is used with the device shown in FIG. 1; FIG. 3 is a diagram showing a relationship between a lighting frequency and a lamp efficiency when xenon gas is used with the device shown in FIG. 1; FIG. 4 is a diagram showing a relationship between an enclosed gas pressure and a lamp efficiency when krypton is used with the device shown in FIG. 1; FIG. 5 is a diagram showing a relationship between a lighting frequency and a lamp efficiency when krypton is used with the device shown in FIG. 1; FIG. 6 is a diagrammatic representation of an entire construction of a rare gas discharge fluorescent lamp device showing another embodiment of the present invention wherein a half-wave rectified voltage is utilized; FIG. 7 is a diagrammatic representation showing an entire construction of a conventional rare gas discharge fluorescent lamp device which makes use of a high frequency current; and FIG. 8 is a cross sectional view of a lamp of the device shown in FIG. 7. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following, several embodiments of the present invention are described with reference to the accompanying drawings. Referring to FIG. 1, there is shown as an embodiment of the present invention an entire construction of a rare gas discharge fluorescent lamp device which makes use of a half-wave rectified voltage. The lamp device shown includes a rare gas discharge fluorescent lamp which includes a bulb 1' made of glass. a fluorescent layer 2a and a reflecting film 2b both formed on an inner face of the bulb 1'. The fluorescent layer 2a and the reflecting film 2b are not formed at a slit portion 2c on the inner face of the bulb 1'. The lamp further includes a pair of electrodes 3a' and 3b' each formed from a filament coil to which an electron emitting substance is applied. The lamp device includes, in addition to the lamp, a high frequency power source 8. a capacitor 9 connected in series to the high frequency power source 8 and acting as a current limiting element, a diode 10 connected in parallel with the series circuit of the high frequency power source 8 and the capacitor 9, and a power source 11 for heating the electrode 3b'. Operation of the device is now described. With the rare gas discharge fluorescent lamp device shown in FIG. 1, when a positive voltage is applied to the electrode 3a', the voltage is applied across the bulb 1' so that a lamp current flows through the lamp. When a negative voltage is applied to the electrode 3a', however. the lamp is short-circuited by the diode 10. and consequently, no voltage is applied across the bulb 1' and no current flows through the lamp. Accordingly, with the rare gas discharge fluorescent lamp device of the construction described above, a high frequency half-wave rectified voltage is applied across the lamp so that pulse-like discharge wherein the lamp current presents die periods takes place in the bulb 1', which is different from ordinary high frequency lighting. Here, the capacitor 9 functions as a current limiting element for allowing only an appropriate electric current to flow through the bulb 1' when a high frequency voltage is applied. FIG. 2 shows a relationship between a pressure of enclosed gas and an efficiency of the lamp when xenon gas is enclosed in the rare gas discharge fluorescent lamp shown in FIG. 1. Here, the bulb 1' of the lamp has an outer diameter of 15.5 mm and an overall length of 300 mm, and the lamp power is constant at 7 W and the frequency is 20 kHz. In FIG. 2, a solid line curve indicates the relationship when the lamp device of the construction shown in FIG. 1 is lit in a pulse-like fashion while a broken line curve indicates the relationship in the case of high frequency lighting by an ordinary ac sine wave. It can be seen from FIG. 2 that the lamp device of the embodiment of the present invention shown in FIG. 1 presents an effect of improvement in lamp efficiency and such effect of improvement in lamp efficiency depends upon a pressure of the enclosed xenon gas. Also it can be seen from FIG. 2 that a maximum efficiency is obtained where the enclosed xenon gas pressure is within a region of several kPa and that the significant effect of improvement in efficiency by the present invention as compared with that in ordinary high frequency lighting can- be obtained within a range of the enclosed xenon gas pressure between 1300 Pa to 27 kPa. Such improvement in lamp efficiency arises from the fact that pulse-like discharge wherein an energization period and a die period alternately appear modulates electron energy of a positive column to a high degree to increase the energy to excite the xenon gas so as to increase the ultraviolet rays generated from the xenon gas, and also from emission of after-glow light during such die periods. For example, the value of 1300 Pa at which the lamp efficiency presents significant improvement corresponds to a pressure at which emission of after-glow light during die periods. which hardly appears at several 100 Pa, appears significantly. By the way, the improvement in efficiency is comparatively low at a high pressure, but this phenomenon arises from the fact that, if the pressure is excessively high, then the electron energy is restrained by frequent collisions of electrons with xenon gas, and consequently, the electron energy is not modulated readily by pulses. FIG. 3 shows a relationship between a lighting frequency and a lamp efficiency. In FIG. 3, a solid line curve indicates the relationship when the lamp device of the construction shown in FIG. 1 is lit by pulses, while a broken line curve indicates the relationship in the case of ordinary high frequency lighting. Here, the rare gas discharge fluorescent lamp encloses xenon gas at 4 kPa therein, and the lamp power is constant at 7 W. From FIG. 3, it can be seen that a high efficiency is obtained at a frequency higher than 4 kHz with the rare gas discharge fluorescent lamp device of the embodiment of the present invention shown in FIG. 1 as compared with that in ordinary high frequency lighting. It can also be seen that, if the frequency rises to about 200 kHz, the efficiencies in the two cases present substantially same levels. Accordingly, the frequency should be higher than 4 kHz but lower than 200 kHz. It is to be noted that the reason why the efficiency drops at the high frequency and becomes substantially equal to that in the case of ordinary high frequency lighting is that a plasma parameter of a positive column cannot follow such high frequency and gradually approaches a fixed condition similar to a dc current. In this manner, with the rare gas discharge fluorescent lamp device having such a construction as shown in FIG. 1, the lamp efficiency can be improved significantly and the lighting device is so simplified in construction that it can be realized readily at a reduced cost. Further, since a capacitor is employed as the current limiting element, the power loss of the lighting device is low. Besides, since a voltage equal to twice as much as that of the power source is generated by the combination of the diode and the capacitor, a high voltage required for starting of discharge can be obtained readily. In addition, since the discharge current can have a waveform which has a moderate rising feature in the form of a half-wave rectified sine wave, there is an effect that higher harmonic wave components are reduced and electromagnetic noise which causes a problem in pulse discharge is also reduced. It is to be noted that, while the lamp in the embodiment described above has an outer diameter of 15.5 mm as an example, an examination which was conducted with lamps having outer diameters ranging from 8 mm to 15.5 mm revealed that such improvement in efficiency as described above is obtained with the construction shown in FIG. 1 irrespective of the outer diameters of the lamps. Further, while one of the filament coils in the embodiment described above is of the hot cathode type, since the improvement in efficiency arises from the improvement in efficiency of a positive column, it may otherwise be, for example, of the cold cathode type without depending upon the electrode structure. However, where a filament coil electrode is employed as in the embodiment described above, it is effective for reduction of a starting voltage and increase in life of an electrode to heat the cathode as seen in FIG. 1. Further, since xenon gas is lowest in ionization potential and excitation potential among rare gases, even if some other rare gas or gases are mixed with xenon as the enclosed gas, emission of light by xenon can be obtained similarly. Further, while a capacitor is employed as the current limiting element in the embodiment described above, the current limiting element may otherwise be constituted by an inductor as shown in FIG. 6 in which another embodiment of the present invention is shown. Also with the rare gas discharge fluorescent lamp device shown in FIG. 6, a lighting device is obtained which is low in power loss and inexpensive. Also with the rare gas discharge fluorescent lamp device where the current limiting element is constituted by an inductor in this manner, characteristics similar to such lamp efficiency characteristics with respect to an enclosed gas pressure or a frequency as shown in FIGS. 2 and 3 which were obtained from the rare gas discharge fluorescent lamp device of the construction shown in FIG. 1 are obtained. Subsequently, efficiency characteristics where krypton gas is enclosed in the bulb 1' of the rare gas discharge fluorescent lamp device which makes use of a half-wave rectified voltage will be described. Referring to FIG. 4, there is shown a relationship between an enclosed gas pressure and a lamp efficiency where krypton gas is enclosed in the bulb 1' of the rare gas discharge fluorescent lamp device having such a construction as shown in FIG. 1. It is to be noted that the lamp used has an outer diameter of 15.5 mm and an axial length of 300 mm, and the lamp power is constant at 7 W and frequency is 20 kHz. In FIG. 4. a solid line curve indicates the relationship when the lamp is lit based on pulse-like discharge with the construction shown in FIG. 1 while a broken line curve indicates the relationship in the case of high frequency lighting based on an ordinary ac sine wave. From FIG. 4, it can be seen that the rare gas discharge fluorescent lamp device of the present embodiment has an effect of improvement in lamp efficiency, and the effect of improvement in lamp efficiency depends upon an enclosed gas pressure of krypton gas. It can be seen also from FIG. 4 that the maximum efficiency is obtained where the enclosed krypton gas pressure is within the range of several kPa . and a significant effect of improvement in efficiency of the embodiment with respect to that in ordinary high frequency lighting can be obtained within the range from 1300 Pa to 13 kPa. Such improvement in lamp efficiency relies upon a similar action of krypton gas to that of xenon gas described above. FIG. 5 shows a relationship between a lighting frequency and a lamp efficiency of the rare gas discharge fluorescent lamp device which employs krypton gas as the enclosed gas. Referring to FIG. 5, a solid line curve indicates the relationship when the lamp is lit based on pulse-like discharge while a broken line curve indicates the relationship in the case of ordinary high frequency lighting. It is to be noted that the lamp of the rare gas discharge fluorescent lamp device encloses krypton gas therein at 400 Pa, and the lamp power is constant at 7 W. From FIG. 5, the rare gas discharge fluorescent lamp device wherein krypton gas is enclosed in the lamp presents a high efficiency in a frequency range higher than 5 kHz as compared with that in ordinary high frequency lighting. Further, the maximum efficiency is exhibited at a frequency of about 20 kHz. and the efficiency drops at a higher frequency such that it is so low at a frequency of about 200 kHz that it is near to the efficiency in the case of ordinary high frequency lighting. It is to be noted that such drop of the efficiency in a high frequency region arises from a similar action of krypton gas to that in the case of xenon gas described above. In this manner, the lamp efficiency can be improved significantly also with the rare gas discharge fluorescent lamp device wherein krypton gas is enclosed in the lamp. and the lighting device can be simplified significantly in construction and can be realized readily at a reduced cost. Further, since a capacitor is used as the current limiting element, the power loss of the lighting device is low. The current limiting element may otherwise be constituted by an inductor as shown in FIG. 6 and as described hereinabove. Also where the current limiting element is constituted by an inductor, characteristics similar to such lamp efficiency characteristics with respect to an enclosed gas pressure or a frequency as shown in FIGS. 4 and 5 are obtained. It is to be noted that while the lamp has an outer diameter of 15.5 mm as an example in the embodiment described above wherein krypton gas is enclosed in the lamp, an examination which was conducted with such lamps that have outer diameters ranging from 8 mm to 15.5 mm revealed that similar improvement in efficiency was obtained irrespective of the diameters of the lamp bulbs. Further, while the filament coil is of the hot cathode type, since the improvement in efficiency depends upon improvement in efficiency of a positive column. the filament coil may otherwise be, for example. of the cold cathode type without depending upon the electrode structure. However, where a filament coil electrode is employed, it is effective for reduction of the starting voltage and increase in life of an electrode to heat the cathode as seen in FIG. 1. Further, even if argon, neon or helium which have a higher ionization potential than krypton is mixed with krypton for the enclosed gas, emission of light can be obtained similarly to that only by krypton gas itself. In summary, according to the present invention. in case a half-wave rectified voltage is used. where xenon gas is enclosed in a bulb of a lamp of a rare gas discharge fluorescent lamp device, the enclosed gas pressure is set to a value higher than 1.3 kPa but lower than 27 kPa, and a half-wave rectified voltage having a frequency higher than 4 kHz but lower than 200 kHz is supplied to the bulb to cause the bulb to be lit, but where krypton gas is enclosed, the enclosed gas pressure is set to a value higher than 1.3 kPa but lower than 13 kPa, and a half-wave rectified voltage having a frequency higher than 5 kHz but lower than 200 kHz is supplied to the bulb to cause the bulb to be lit. Accordingly, there are effects that the rare gas discharge fluorescent lamp device is simplified in construction and can be produced at a reduced cost and that a high lamp efficiency can be obtained. Accordingly, there is an effect that a rare gas discharge fluorescent lamp device of a high brightness and a high efficiency can be obtained without deteriorating the life as compared with that in conventional dc lighting or in ordinary high frequency ac lighting.

Claims: A rare gas discharge fluorescent lamp device, comprising: a rare gas discharge fluorescent lamp including a bulb (1') having rare gas enclosed therein, a fluorescent layer (2a) formed on an inner face of said bulb, and a pair of electrodes (3a', 3b') located at the opposite ends of said bulb; characterised by: a pulse voltage forming means (8, 9, 10; 8, 9', 10) which applies half-wave rectified pulses across said electrodes to cause said lamp to be lit, and which comprises a series circuit of a high frequency power source (8) and a current limiting element (9, 9'), and a diode (10) connected in parallel with said series circuit, to form said half-wave rectified pulses, the frequency of said pulses being higher than 4 kHz but lower than 200 kHz; said rare gas being enclosed in said bulb (1') at a pressure higher than 1.3 kPa but lower than 27 kPa. A rare gas discharge fluorescent lamp device as claimed in claim 1, wherein said rare gas is xenon gas. A rare gas discharge fluorescent lamp device as claimed in claim 1, wherein said rare gas is krypton gas enclosed in said bulb at a pressure lower than 13 kPa and the frequency of said pulses is higher than 5 kHz. A rare gas discharge fluorescent lamp device as claimed in any one of claims 1 to 3, wherein a power source (11) for heating is provided for either one of said electrodes. A rare gas discharge fluorescent lamp device as claimed in any one of claims 1 to 4, wherein said current limiting element is a capacitor (9). A rare gas discharge fluorescent lamp device as claimed in any one of claims 1 to 4, wherein said current limiting element is an inductor (9').

Inventors: ANZAI YOSHINORI, MITSUHASHI SEISHIRO, MURAKAMI KATSUO, OHSAWA TAKASHI, SAIKATSU TAKEO, SAKURAI TAKEHIKO, YAMAZAKI HIROYOSHI, ANZAI, YOSHINORI, MITSUHASHI, SEISHIRO, MURAKAMI, KATSUO, OHSAWA, TAKASHI, SAIKATSU, TAKEO, SAKURAI, TAKEHIKO, YAMAZAKI, HIROYOSHI, ANZAI, YOSHINORI, C/O MITSUBISHI DENKI K.K., MITSUHASHI, SEISHIRO, C/O MITSUBISHI DENKI K.K., MURAKAMI, KATSUO, C/O MITSUBISHI DENKI K.K., OHSAWA, TAKASHI, C/O MITSUBISHI DENKI K.K., SAIKATSU, TAKEO, C/O MITSUBISHI DENKI K.K., SAKURAI, TAKEHIKO, C/O MITSUBISHI DENKI K.K., YAMAZAKI, HIROYOSHI, C/O MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1989
Main classification: H01J61
IPC codes: H01J61, H05B41, G03G15