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 fluores­ cent 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, and particularly where dc rectangular pulses are used, the duty ratio is also deter­ mined 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. 13 and 14 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. 13 is a diagrammatic representation showing a longitudinal section of a rare gas discharge fluorescent lamp and an entire construction of the device, and FIG. 14 is a cross sectional view of the lamp. Referring to FIGS. 13 and 14, 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 Laid-Open 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 further fluorescent lamp device is disclosed in US-A-4 128 788. This device includes a discharge lamp filled with a rare gas of 10⁻⁴ to 10 kPa. The lamp is driven by a pulse voltage of 0.06 to 20 kHz and 0.1 to 10% duty cycle to improve its light producing efficiency. The pulse voltage is generated by a circuit including a vacuum tube which is connected in series with the discharge lamp and is periodically switched on and off by means of a pulse voltage applied to its gate. 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 simple rare gas discharge fluorescent lamp device offering a high brightness and a high efficiency. This object is solved by the device of claim 1. In a rare gas discharge fluorescent lamp device according to 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 is 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 power source of simple construction which provides dc rectangular pulses to be applied to the lamp. 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 dc rectangular pulse 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 starting voltage characteristic with respect to an enclosed gas pressure when xenon gas is used with the device shown in FIG. 1; FIG. 4 is a diagram showing a lamp efficiency with respect to an energization time of a pulse commonly when xenon gas, argon gas or krypton gas is used with the device shown in FIG. 1; FIG. 5 is a diagram showing a lamp efficiency with respect to a pulse duty ratio when xenon gas is used with the device shown in FIG. 1; FIG. 6 is a diagram showing a life characteristic with respect to a pulse duty ratio commonly when xenon gas, argon gas or krypton gas is used with the device shown in FIG. 1; FIG. 7 is a diagram showing a characteristic of a relationship between an enclosed gas pressure and a lamp efficiency when argon gas is used with the device shown in FIG. 1; FIG. 8 is a diagram showing a starting voltage characteristic with respect to an enclosed gas pressure when argon gas is used with the device shown in FIG. 1; FIG. 9 is a diagram showing a lamp efficiency characteristic with respect to a pulse duty ratio when argon gas is used with the device shown in FIG. 1; FIG. 10 is a diagram showing a characteristic of a relationship between an enclosed gas pressure and a lamp efficiency when krypton gas is used with the device shown in FIG. 1; FIG. 11 is a diagram showing a starting voltage characteristic with respect to an enclosed gas pressure when krypton gas is used with the device shown in FIG. 1; FIG. 12 is a diagram showing a lamp efficiency characteristic with respect to a pulse duty ratio when krypton gas is used with the device shown in FIG. 1; FIG. 13 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. 14 is a cross sectional view of a lamp of the device shown in FIG. 13. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following, several embodiments of the present invention are described with reference to the accompanying drawings. Referring now to FIG. 1, there is shown a rare gas discharge fluorescent lamp device wherein dc rectangular pulses are utilized. The lamp device shown includes a bulb 1″ made of glass and having a straight cylindrical configuration having a diameter of 15.5 mm and an axial length of 300 mm. The bulb 1″ has a film of a fluorescent substance formed on an entire inner peripheral surface thereof. A pair of electrodes 3a″ and 3b″ are located at the axial opposite ends in the bulb 1″. Though not particularly shown, an aluminum plate having a width of 3 mm is secured to and extends along an outer surface of the bulb 1″ and serves as an auxiliary starting conductor. The lamp device further includes a dc power source 7′ connected to the electrodes 3a″ and 3b″ of the rare gas discharge fluorescent lamp for supplying a dc voltage across the electrodes 3a″ and 3b″. A switching element 12 such as an FET (Field Effect Transistor) is connected in parallel with the rare gas discharge fluorescent lamp and acts to connect or disconnect a dc voltage to be applied to the lamp. The lamp device further includes a pulse signal source 13 connected to the switching element 12. The switching element 12 thus receives pulses from the pulse signal source 13 and performs switching on and off in accordance with a period and a pulse width of the pulses received to change a voltage to be applied to the bulb 1″ into dc rectangular pulses. The lamp is thus lit intermittently by the pulse voltage. The lamp device further includes a resistor 14 serving as a current limiting element. An examination of measuring a brightness and an efficiency of the rare gas discharge fluorescent lamp device described above with xenon gas, argon gas and krypton gas enclosed individually in the glass bulb 1″ was conducted individually changing the pressure of enclosed gas in the lamp, the ratio of an energization time within a period (hereinafter referred to as duty ratio), the energization time and so forth in intermitting lighting of the lamp. FIG. 2 shows a relationship between a pressure of enclosed xenon gas and a lamp efficiency. It is to be noted that the lamp efficiency is determined from a value obtained by dividing a brightness by an electric power. In FIG. 2, a solid line curve A indicates the relationship when the rare gas discharge fluorescent lamp is lit by rectangular wave dc pulses having a duty ratio of 60 % while a broken line curve B indicates the relationship in the case of ordinary high frequency ac lighting (sine wave), and in both cases, the frequency is 20 kHz and the power consumption is the same. It can be seen from FIG. 2 that, at an enclosed gas pressure lower than 1300 Pa, there is no significant difference in efficiency between pulse lighting and ac lighting, but at an enclosed gas pressure higher than 1300 Pa, the efficiency in pulse lighting is higher than the efficiency in ac lighting. However, if the enclosed gas pressure exceeds about 9300 Pa, then the efficiency of the lamp in ac lighting still rises but the efficiency of the lamp in pulse lighting begins to drop, and then at 27 to 40 kPa , the efficiency of the lamp in pulse lighting approaches the value of the efficiency in ac lighting again. On the other hand, FIG. 3 shows a relationship between an enclosed gas pressure and a starting voltage. It can be seen from FIG. 3 that, as the enclosed gas pressure increases, a progressively high voltage becomes necessary for starting. Since such rise of the starting voltage is remarkable particularly at an enclosed gas pressure higher than 27 kPa, preferably the enclosed gas pressure is lower than 27 kPa. Accordingly, from FIGS. 2 and 3, the optimum enclosed gas pressure at which the efficiency is higher than that in high frequency lighting and pulse lighting wherein a practical starting voltage can be attained is higher than 1300 Pa but lower than 27 kPa. On the other hand, several lamps having diameters ranging from 8 mm to 15.5 mm and a length of 300 mm with xenon gas enclosed therein at a pressure of 4 kPa were produced, and characteristics of the lamps were measured changing the dc pulse lighting conditions variously. Results of such measurement are shown in FIGS. 4 and 5. FIG. 4 shows a relationship between an energization time within a period of a dc pulse and a lamp efficiency while the de-energization time is held fixed to 100 µsec. From FIG. 4, it can be seen that the shorter the pulse energization time, the higher the efficiency, and the effect is particularly remarkable where the pulse energization time is shorter than 150 µsec. FIG. 5 shows relationships between a lamp efficiency and a pulse duty ratio in the case of pulse lighting at frequencies of 5kHz, 20 kHz and 80 kHz (curves C, D and E). Further, efficiency values in high frequency ac lighting (sine wave) at frequencies of 5 kHz, 20 kHz and 80 kHz which are used commonly are shown for comparison in FIG. 5 (lines F, G and H). From FIG. 5, it can be seen that the efficiency is raised by decreasing the duty ratio of pulses as compared with that in dc lighting (duty ratio = 100 %), and even compared with that in ac lighting at the same frequency, the efficiency is much higher if the pulse duty ratio is made lower than 70 %. Further, several lamps having diameters ranging from 8 mm to 15.5 mm with xenon gas enclosed therein at pressures of 1300 Pa to 27 kPa were produced, and a life test of the lamps was conducted changing the pulse duty ratio while keeping the lamp power fixed. Results are shown in FIG. 6. Here, the terminology "relative life" signifies a ratio of an average life time when the lamp is lit at a varying duty ratio to an average life time when the lamp is lit at a predetermined fixed duty ratio (for example, 40 %). From FIG. 6, it can be seen that the relationship between a pulse duty ratio and a relative life presents such a variation that, if the pulse duty ratio is reduced until it comes downs to 5 %, the relative life exhibits a little decreasing tendency, and after the pulse duty ratio is reduced beyond 5 %, the life drops suddenly. It is presumed that, where the duty ratio is lower than 5 %, the pulse peak current of the lamp increases so significantly that wear of the electrodes progresses suddenly. Accordingly, the pulse duty ratio is preferably higher than 5 % when the life is taken into consideration. While the results of the examination wherein xenon gas was used are described above, a similar examination was conducted for characteristics of the lamps wherein argon gas and krypton gas were used. Results of the examination were obtained in a similar manner as described above. In particular, FIG. 7 shows a relationship between a pressure of enclosed argon gas and a lamp efficiency. Referring to FIG. 7, a curve A′ indicates the relationship in the case of lighting by rectangular wave dc pulses having a duty ratio of 60 % while another curve B′ indicates the relationship in the case of ordinary high frequency ac lighting (sine wave) when the frequency is 20 kHz and the electric power is the same. It can be seen from FIG. 7 that there is no significant difference in efficiency between pulse lighting and ac lighting at an enclosed gas pressure lower than 1300 Pa, but at an enclosed gas pressure higher than 1300 Pa, the efficiency in pulse lighting is higher than that in ac lighting. On the other hand, FIG. 8 shows a relationship between an enclosed gas pressure and a starting voltage, and from FIG. 8, it can be seen that, as the enclosed gas pressure rises, a progressively high voltage is required for starting. Since such rise of the starting voltage is remarkable particularly where the enclosed gas pressure is higher than 13 kPa, the enclosed gas pressure is preferably lower than 13 kPa. Accordingly, from FIGS. 7 and 8, the optimum enclosed argon gas pressure at which the efficiency is higher than that in high frequency lighting and pulse lighting wherein the starting voltage is practical can be attained is higher than 1300 Pa but lower than 13 kPa. On the other hand, several lamps having diameters ranging from 8 mm to 15.5 mm and a length of 300 mm with argon gas enclosed therein at a pressure of 4 kPa were produced, and characteristics of the lamps were measured changing the dc pulse lighting conditions variously. Results of such measurement are shown in FIGS. 4 and 9. In particular, from FIG. 4, it can be seen, similarly as in the case wherein xenon gas is enclosed as described above, that the lamp efficiency is remarkable particularly where the pulse energization time is shorter than 150 µsec. On the other hand, FIG. 9 shows relationships between a lamp efficiency and a pulse duty ratio in the case of pulse lighting at frequencies of 20 kHz and 80 kHz (curves D′ and E′). Further, efficiency values in high frequency ac lighting (sine wave) at frequencies 20 kHz and 80 kHz which are used commonly are shown for comparison in FIG. 9 (lines G′ and H′). From FIG. 9, it can be seen that the efficiency is raised by decreasing the duty ratio of pulses as compared with that in dc lighting (duty ratio = 100 %), and even compared with that in ac lighting at the same frequency, the efficiency is much higher if the pulse duty ratio is made lower than 80 %. Further, several lamps having diameters ranging from 8 mm to 15.5 mm with argon gas enclosed therein at pressures of 1300 Pa to 27 kPa were produced, and a life test of the lamps was conducted changing the pulse duty ratio while keeping the lamp power fixed. Results are the same as those shown in FIG. 6 in which the results where xenon gas was enclosed in the lamp as described above are shown. Accordingly, it can be seen that preferably the pulse duty ratio is also higher than 5% when the life is taken into consideration. Further, a relationship between an enclosed gas pressure and a lamp efficiency where krypton gas was used is shown in FIG. 10. Referring to FIG. 10, a solid line curve A″ indicates the relationship in the case of lighting by rectangular wave dc pulses having a duty ratio of 60 % while the curve B″ indicates the relationship in the case of ordinary high frequency ac lighting (sine wave) when the frequency is 20 kHz and the electric power is the same. It can be seen from FIG. 10 that there is no significant difference in efficiency between pulse lighting and ac lighting at an enclosed gas pressure lower than 1300 Pa, but at an enclosed gas pressure higher than 1300 Pa, the efficiency in pulse lighting is higher than that in ac lighting. On the other hand. FIG. 11 shows a relationship between an enclosed gas pressure and a starting voltage, and from FIG. 11, it can be seen that, as the enclosed gas pressure of krypton gas rises, a progressively high voltage is required for starting. Since such rise of the starting voltage is remarkable particularly where the enclosed gas pressure is higher than 13 kPa, the enclosed gas pressure is preferably lower than 13 kPa. Accordingly, from FIGS. 10 and 11, the optimum enclosed gas pressure of krypton gas at which the efficiency is higher than that in high frequency lighting and pulse lighting wherein the starting voltage is practical can be attained is higher than 1300 Pa but lower than 13 kPa. Further, several lamps were produced with a pressure of enclosed krypton gas of 4 kPa under the same conditions as those where argon gas was used, and characteristics of the lamps were measured changing the dc pulse lighting conditions variously. Results of such measurement are shown in FIGS. 4 and 12. As described hereinabove, from FIG. 4, it can be seen that the lamp efficiency is remarkable particularly where the pulse energization time is shorter than 150 µsec, similarly as in the case where xenon gas or argon gas is enclosed as described hereinabove. Also as described hereinabove, from FIG. 12, the lamp efficiencies in pulse lighting with frequencies of 20 kHz and 80 kHz (D″ and E″) are much higher if the pulse duty ratio is made lower than 70 %, when compared with efficiency values (G″ and H″) in high frequency ac lighting (sine wave) of the same frequencies which are used commonly. Further, a life test of such lamps with krypton gas enclosed therein was conducted, and results of such life test proved that the pulse duty ratio is preferably higher than 5 % as seen in FIG. 6. In summary, according to the described embodiments of the present invention, in case a dc rectangular pulse voltage is used, where xenon gas is enclosed in the bulb, the enclosed gas pressure is set to a value higher than 1.3 kPa but lower than 27 kPa, and the pulse energization time is set to 150 µsec while the duty ratio is set to a value higher than 5 % but lower than 70 %; where argon gas is enclosed, the enclosed gas pressure is set to a value higher than 1.3 kPa but lower than 13 kPa, and the pulse energization time is set to 150 µsec while the duty radio is set to a value higher than 5 % but lower than 80 %; and 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 the pulse energization time is set to 150 µsec while the duty ratio is set to a value higher than 5 % but lower than 70 %, and the lamp is caused to be intermittently lit in such conditions as described above. 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 formed on an inner face of said bulb, and a pair of electrodes (3a″, 3b″) located at the opposite ends of said bulb; and a power source (7′, 12, 13, 14) which applies a dc pulse voltage across said electrodes to cause said lamp to be lit, said power source comprising a series circuit of a dc power source (7′) and a current limiting element (14), and a switching element (12) connected in parallel with said lamp and with said series circuit and connected to a pulse generator (13) so as to be switched in response to an output of said pulse generator, to form dc rectangular pulses, the frequency of said pulse voltage being higher than 4 kHz but lower than 200 kHz, and the duty ratio of the dc rectangular pulses being higher than 5 % but lower than 80 %; 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 and the duty ratio of the dc rectangular pulses is lower than 70 %. A rare gas discharge fluorescent lamp device as claimed in claim 1, wherein said rare gas is argon gas enclosed in said bulb at a pressure lower than 13 kPa. 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 duty ratio of the dc rectangular pulses is lower than 70 %. A rare gas discharge fluorescent lamp device as claimed in any of claims 1 to 4, wherein a reflecting film is formed on an inner surface of said fluorescent layer.

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