Metal halide lamp and temperature control system therefor

Abstract: In a metal halide lamp which includes a discharge tube (2) retaining a fill of mercury and at least one metal halide added as a luminous material, an energy density of the arc discharge portion (3) represented by a product E × j is in the range of 70.0 ≤ E × j ≤ 150.0 (VA/mm 3 ) where E=V/d , j=I/S , assuming that I is a lamp current in amperes with a lamp voltage of V volts applied between the paired discharge electrodes in a stable lighting condition of the lamp and that each of the electrodes has a tip face (1a, 1a') of which a cut area in section is S mm 2 and the gap distance is d in millimeters, and thus a high luminous flux retention rate and high luminance of an arc discharge portion can be accomplished with a longer life of the lamp, suppressing a lamp voltage varying rate, avoiding a change in color temperature, which remarkably improves additional merits when in utilization as a light source in various display apparatuses such as optical projection systems.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention generally relates to a low-power, high-pressure discharge lamp, and in particular to a metal halide lamp having a discharge envelope vessel retaining a metal halide fill in a mercury atmosphere, and to a temperature control system for a stable lighting condition of the lamp, maintaining a high luminous flux retention rate of the lamp. Description of the Prior Art Conventionally, a metal halide lamp has been fabricated under consideration of various quantitative restrictions such as restriction on lamp power consumption required for sufficient luminous energy or quantity of light in view of provision of a lighting circuit, and in particular, when a lamp is used as a light source in an optical projector system, there have been required further restrictions such as a gap distance or arc length between a pair of discharge electrodes. The electrodes, which are made of tungsten and the like material, are fabricated in a specific shape and size for increasing a luminance or brightness of an arc discharge portion to be produced between the electrodes in view of an optical requirement and an upper limit in quantity of a mercury fill restricted for ensuring a pressure-proof property of an arc discharge tube. Moreover, in recent years, there has been increasing a strong demand for developing a metal halide lamp for use as a light source having characteristics of high luminance and high luminous flux retention rate in a essential part of an optical display incorporated in e.g. a projection optical system. In particular, it is essentially important to optimize a contour of the discharge electrodes per se having a specific shape and dimension in fabricating a metal halide lamp because the design thereof exerts a great influence on the characteristics of the lamp such as a luminous flux retention rate, luminance of the arc discharge portion and lamp voltage varying rate. However, in the conventional manufacturing method of the lamp, there has not been yet taught or established a guiding principle for providing a suitable design of electrodes to have optimum lamp characteristics, i.e., high luminous flux retention rate, high luminance of the arc discharge portion and small lamp voltage varying rate, under consideration of the restrictions of the lamp power, gap distance between electrodes, and upper limit of the fill of mercury. Therefore, the fabrication of an optimum metal halide lamp has been mainly carried out by experiences. In this conventional metal halide lamp, there have been drawbacks that, the discharge tube wall of quartz glass is easily reactive with a metal halide at a high temperature of about 1100 °C or higher, and if the quantity of the metal halide sealed inside the tube is reduced by the reaction with the glass tube wall, the luminous flux retention rate are undesirably reduced to deteriorate the life property of the lamp. Moreover, there have been problems that flickers and darkening phenomenon in the discharge tube wall may be easily caused undesirably due to scattering of the electrode evaporation to be adhered onto the inner face of the discharge tube during the lighting-on operation of the lamp, and also a color temperature change may be easily caused due to the change of the lamp voltage. The progressing degree of the blackening phenomenon is deeply related to the contour design of the electrodes. In the meanwhile, when the heating of the discharge tube is excessively suppressed in temperature, there may be undesirably caused a lower-most part in temperature in the discharge tube wall behind the electrodes, which suppresses the evaporation of the metal halide in the discharge tube, resulting in deterioration of the luminous efficiency. Thus, there has been increasing a strong demand for establishing a reference guiding principle for providing a suitable design of discharge electrodes to have optimum lamp characteristics, i.e., high luminous flux retention rate, high luminance of the arc discharge portion and small lamp voltage varying rate in fabricating a metal halide lamp, under consideration of the restrictions of the lamp power, gap distance between the electrodes, and upper limit in mass of the fill of mercury. EP 0 649 164 A2 discloses a metal halide lamp comprises a sealed tube containing mercury vapor and halide, and electrodes extending to a center of the sealed tube, supported by sealed portions at both ends of the sealed tube. A notch extending in a direction perpendicular to an axis of the electrode is formed in each electrode. A transverse cross sectional area of a portion where the notch is formed is smaller than the transverse cross sectional area of another portions and functions as a heat dam portion for damming heat. Accordingly, temperature of a proximal portion from the heat dam portion to the support portion is lower than that of the same portion of the conventional electrode, and temperature of a distal end portion is higher than that of the same portion of the conventional electrode. Thus, formation of low-melting alloy due to reaction of the proximal portion of the electrode and the metal halide can be suppressed. EP 0 459 786 A2 discloses a metal halide lamp apparatus comprises a reflector and a metal halide lamp which is without an outer bulb and which has a reflecting/thermal insulating film and a frosted portion which is partially formed on the lamp outer surface within a predetermined range continued from the reflecting/thermal insulating film. This causes a reduction in an overall illuminance decrease and the attainment of a desired illuminance ratio and prevents the occurrence of irregularity in illuminance and colour. In addition, since electrodes are asymmetrically disposed, it is possible to decrease the rate of devitrification of the luminous tube and make an attempt to increase the life of the luminous tube. SUMMARY OF THE INVENTION Accordingly, in view of the above-described problems, the present inventors have studied specific mutual relations when in fabricating a metal halide lamp under consideration of restrictions of a lamp power, gap distance between oppositely disposed discharge electrodes, and upper limit of a fill of mercury. In summary, the present inventors have found that a product between a lamp electric field and a current density has mutual relations to a luminous flux retention rate and to a mean temperature value at a tip portion of each electrode where the lamp electric field and current density respectively depend on a gap distance between the oppositely disposed electrodes and a shape and size of the electrodes. Based on the inventors' study mentioned above, they have developed a novel method for fabricating an improved metal halide lamp having optimum lamp characteristics, i.e., high luminous flux retention rate and high luminance of the arc discharge portion. Moreover, the present inventors have studied and found a mutual relation between the shape and dimension of the electrodes and the lamp voltage varying rate, and found a mutual relation between the lamp electric field and the lower-most temperature of the discharge tube wall. Thus, an essential objective of the present invention is to provide an improved metal halide lamp having a high luminous flux retention rate and high luminance of an arc discharge portion, suppressing a lamp voltage varying rate. This objective is solved by a lamp as claimed in claim 1. In order to achieve the objective mentioned above, a first inventive metal halide lamp which includes a discharge tube retaining a fill of mercury and at least one metal halide added as a luminous material in an inert gas atmosphere sealed therein, comprises: a pair of discharge electrodes oppositely disposed with a space of a gap distance defining a length of an arc discharge portion produced between the paired discharge electrodes in the discharge tube, where an energy density of the arc discharge portion represented by a product E × j is in the range of 70.0 ≤ E × j ≤ 150.0 (VA/mm 3 ) where E=V/d, j=I/S, assuming that I is a lamp current in amperes with a lamp voltage of V volts applied between the paired discharge electrodes in a stable lighting condition of the lamp and that each of the electrodes has a tip face (1a, 1a') of which a cut area in section is S mm 2 and the gap distance is d in millimeters. In a second inventive metal halide lamp, a temperature mean value (Tm) of an electrode tip portion of each electrode is set within the range of 2300 to 2700 K. By this arrangement, an improved metal halide lamp can be provided to have a high luminous flux retention rate and high luminance of an arc discharge portion with a longer life of the lamp, suppressing a lamp voltage varying rate, avoiding a change in color temperature, which remarkably improves additional merits when in utilization as a light source in various display apparatuses such as optical projection systems. Moreover, the optimum range of the temperature mean value of the electrode tip portion can be defined with a fixed value of E × j (= V/d × I/S), with the fixed values of gap distance (d) and area (S) in section of the electrode tip portion. In the construction of the present invention, a wide range of different metal halide materials to be sealed as well as different lamp powers can be adapted to fabricating metal halide lamps, and therefore degree of freedom in fabrication of the design and efficiency in development thereof can be remarkably improved. Moreover, in arranging a lamp-lighting circuit, since the securing range in applying the lamp voltage can be restricted, therefore the fabrication in design of the lamp can be facilitated advantageously. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and features of the present invention will become apparent from the following description taken in conjunction with the preferred embodiment thereof with reference to the accompanying drawings, in which: Fig. 1 is a schematic plan view showing a metal halide lamp of the first embodiment according to the present invention; Fig. 2 is a graph showing a relation between a product E × j and a luminous flux retention rate L.F.R. according to the present invention; Fig. 3 is a graph showing a relation between a product E × j and a mean value of temperature at the tip portion of an electrode according to the present invention; Fig. 4 is a graph showing a relation between a lighting time and a luminous flux retention rate according to the present invention; Fig. 5 is a graph showing a relation between a product E × j and a luminous flux per electrode gap distance (L/d) according to the present invention; Fig. 6 is a graph showing a relation between a product E × j and a luminous flux retention rate L.F.R. and a relation between a product E × j and a luminous flux per electrode gap distance (L/d) according to the present invention; Fig. 7 is a graph showing a relation between a mean value of temperature at the tip portion of an electrode and a luminous flux retention rate according to the present invention; Fig. 8 is a graph showing a relation between a product E × j and a mean value of temperature at the tip portion of an electrode according to the present invention; Fig. 9 is a schematic view showing a construction of an electrode for use in a metal halide lamp of the second embodiment according to the present invention; Fig. 10 is a graph showing a relation between a length of a protruded portion and a mean value of temperature at the tip portion of an electrode according to the present invention; Figs. 11 is a schematic view showing a modified example of an electrode for use in a metal halide lamp of the second embodiment according to the present invention; Fig. 12 is a schematic view showing another modified example of an electrode for use in a metal halide lamp of the second embodiment. Fig. 13 is a schematic view showing further another modified construction of an electrode for use in a metal halide lamp of the second embodiment, DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Before the description proceeds, it is noted that, since the basic structures of the metal halide lamps are the same in the preferred embodiments, like parts are designated by like reference numerals in the appending drawings. FIRST EMBODIMENT The following describes a first embodiment of the present invention with reference to Figs. 1 to 6. Fig. 1 shows a schematic construction of a metal halide lamp which includes a discharge tube 2 serving as a discharge envelop vessel made of e.g. a quartz glass or the like material, having a spherical-like inner bulb wall 2a retaining a fill of mercury and at least one metal halide added as a luminous material to obtain a color temperature in an inert gas atmosphere sealed therein. In the discharge tube 2, a pair of discharge electrodes 1 and 1' made of e.g. a tungsten material are oppositely disposed with a space of a gap distance of d mm which defines an arc discharge length (d). Each of the electrodes 1 and 1' of a column-like pin shape has a tip face (1a, 1a') of which a cut area in section is S mm 2 and the paired electrodes 1 and 1' are integrally connected to electrode shafts 4 and 4' respectively and protruded inward therefrom. The electrode shafts 4 and 4' inserted in sealing members 5 and 5' are connected to outer well terminals 7 and 7' respectively via metal foil portions 6 and 6' which are securely sealed in the sealing members 5 and 5'. In this construction, a lamp voltage (V) is applied between the paired discharge electrodes 1 and 1' to pass a lamp current (I) between the electrodes with use of an arc discharge generating circuit of a power source (as shown in Fig. 17 to be described later), and thus an arc discharge 3 is thereby generated between the electrodes 1 and 1' in the inert gas atmosphere in a stable lighting condition of the lamp. Now that, by combining various conditions of metal halide lamps varying the gap distance d and tip cut area S within the ranges of: d = 1.8 to 13 mm and S = 0.169 to 1.327 mm 2 (i.e., varying a diameter  of a circular cut plane in section of the tip portion of the electrode in the range of  = 0.5 to 1.3 mm), the variation of the luminous flux retention rate was measured using a light-flux meter at a time t 100 after a time lapse of 100 hours with respect to that at a light starting-up time to of the lamp while employing different kinds of metal halide materials and different lamp powers. Fig. 2 shows a variation of the luminous flux retention rate (%) on an ordinate axis of the graph at a time t 100 after a time lapse of 100 hours from the light starting-up time t 0 in relation to a product value (E × j) of a lamp electric field (E) and an electric current density (j) on an abscissa axis after a time lapse of 0 hour, i.e., at a light starting-up time to, where the lamp electric field is represented by: E = V/d (V/mm) and the current density is represented by: j = I/S (A/mm 2 ). The reason why the luminous flux retention rate is taken after the time lapse of 100 hours is because the deterioration of the luminous flux retention rate is mainly caused by attenuation of a light transmission of the discharge bulb glass due to the blackened or darkish inner wall 2a thereof. This blackening phenomenon of the discharge bulb wall 2a is caused when the electrode material is vapored and scattered therearound to be adhered onto the inner face 2a of the discharge tube 2 during the lighting-on operation of the lamp. The progressing degree of the blackening phenomenon is deeply related to the contour design of the electrodes. In Fig. 2, the measurement examples of the experiments are classified into three groups i) to iii) by changing the material of the metal halide fill and lamp power level as below: i) marks ○ represent a case when using a metal halide fill of indium (In) - holmium (Ho) with lamp power application of 200 W, ii) marks represents a case when using a metal halide fill of indium (In) - thulium (Tm) with lamp power application of 200 W, and iii) a mark ◆ represents a case when using a metal halide fill of dysprosium (Dy) - thallium (Tl) - sodium (Na) - holmium (Ho) - thulium (Tm) with lamp power application of 150 W, which is available on the market. In these examples, the measurement was carried out while the gap distance d between the paired electrodes and the area S in section of the tip portion of each electrode are both optionally changed and combined within the ranges mentioned above. The unit of the product E × j is V•A/mm 3 , i.e., W/mm/mm 2 , and this means an energy density per a unit length of an arc discharge portion 3 which is received by a unit area of the tip face (1a, 1a') of the electrode end portion (1, 1'). It is noted here that a linear solid line in this graph is a regression line Rl 1 obtained by a least square approximation of the plots on the graph. As the measurement results in Fig. 2, the larger the energy density E × j, the worse the luminous flux retention rate is reduced. This is because, when the energy density E × j is increased, the movement of the energy from the arc discharge portion to each discharge electrode is increased particularly at a front face of the tip portion of the electrode, and therefore the temperature of the electrode tip portion is excessively raised, resulting in that the electrode material is vapored, or it may be considered that photons, electrons and the like ions with some particle-like characteristics of high energy density impinge upon the electrode tip portion to thereby cause scattering of the electrode material, resulting in progressing the blackening of the inner face of the discharge bulb wall 2a. Thus, the luminous flux retention rate is deteriorated. Fig. 3 shows a relation of the temperature mean value of the electrode tip portion with respect to the energy density E × j at the light starting-up time to using the same examples of the lamps as those in Fig. 2. By this measurement results shown in Fig. 3, it is confirmed that, the larger the energy density E × j, the higher the temperature mean value of the electrode tip portion rises. In this experiment, the measurement of the temperature mean value of the electrode tip portion was carried out by a bi-color radiation temperature measuring method as disclosed in the Japanese Patent Laid-open (Unexamined) Publication (Tokkaihei) 8-152360 published on June 11, 1996. This method is based on the principle that the spectral radiation luminous ratio of different two homogeneous wavelengths emitted from an object to be measured is represented by a function in relation to a temperature of the object. In this method of the publication, in order to detect the pure thermal radiation from the electrode part while preventing the mixture with the other radiation from the arc discharge portion, the spectrum distribution in the vicinity of the electrode part is measured by a spectrophotometer having a high resolution of 0.01 nm to obtain different two homogeneous wavelengths of narrow band having very little radiation from the arc discharge portion. Thus, the luminances of the thermal radiation from the electrode part are measured by the different two wavelengths, and then the temperature of the part is obtained by the ratio between the two luminances, where a two-dimensional light-receiving unit such as a CCD camera is used as means for detecting the thermal radiation luminance from the electrode part so that the temperature mean value of the electrode tip portion is obtained. Fig. 4 shows a relation of the variation of the luminous flux retention rate with respect to the increase of the lighting time period in typical two cases A and B of metal halide lamps, where the case A designated by ○ marks is an example using a lamp having a luminous flux retention rate of 80 % after a time lapse of 100 hours from the light starting-up time t 0 , while the case B designated by ▪ marks is an example using a lamp having a luminous flux retention rate of 85 % after a time lapse of 100 hours from the light starting-up time t 0 . Even in the case A, the half life period of the luminous flux retention rate is about 5000 hours of the lighting time period, while in the case B, the half life period of the luminous flux retention rate is about 7000 hours of the lighting time period. It is noted here that the half life period of 5000 hours is an average value for a general illumination metal halide lamp having a gap distance of 10 mm or more between a pair of discharge electrodes, which the life length of 5000 hours is sufficient for the highest level of a metal halide lamp having a small gap distance of nearly 3 mm adapted to be used as a light source incorporated in a projector. Based on the acquirements of the measurement results shown in Fig. 4, when the reference value of 80 % is set up as the necessary luminous flux retention rate at the time lapse of 100 hours in Fig. 2, the energy density (E × j) must be smaller than 150 VA/mm 3 for satisfying the requirement. In a general illumination type metal halide lamp having a gap distance of 10 mm or more between the discharge electrodes, as manufactured by Matsushita Electric Industrial Co., such as the examples designated by mark ◆ having a gap distance of 10 to 80 mm with lamp power application of 70 to 1000 W in Fig. 2, the lamp of this type is operated with the energy density (E × j) in a range of 69 to 12 VA/mm 3 , and it is confirmed that there is obtained a desirable luminous flux retention rate of 90 % or higher at the time lapse of 100 hours after the light starting-up of the lamp as designated by plots in the left upper portion in Fig. 2. However, when using such a general illumination type metal halide lamp having a large gap distance of 10 mm or more between the paired discharge electrodes, since the luminance of the arc discharge portion is too small and insufficient due to a small lamp electric field, therefore such a general illumination type metal halide lamp can not be used as a light source of a projector incorporated in an optical projection system. When a luminous flux of a lamp is L (lm) and a gap distance between the discharge electrodes is d (mm), the luminous value L/d (lm/mm) per a unit arc length is correlative and nearly equal to the luminance of the arc discharge portion. Fig. 5 shows a relation between the luminance values L/d per a unit arc length represented on the ordinate axis and the product E × j represented on the abscissa axis. When employing a metal halide lamp of the above mentioned type having a gap distance of 10 to 80 mm between the electrodes operated with lamp power application of 70 to 1000 W and energy density E × j of 69 to 12 (VA/mm 3 ), the value L/d is in the range of 420 to 1060 (lm/mm) which is represented by mark ◆ plotted in a left lower portion in Fig. 5. In this relation in Fig. 5, when the value E × j is decreased, the value L/d is also decreased as shown by a regression line Rl 2 thereof. When a metal halide lamp is used as a light source for illuminating a screen of an optical projector having a size of generally 40 inches type, it is required that the lamp has the value L/d of at least 4000 lm/mm for obtaining a sufficient brightness of the screen. By this requirement, the value of E × j must be larger than 70 (VA/mm 3 ) as shown in Fig. 5 for satisfying the necessary condition. It is noted here that, in Fig. 5, the reason why the marks ○ and are dispersed up and down with respect to the regression line Rl 2 is because as below. That is, the feature of the steep incline rising rightward located in the upper portion of the regression line is formed by the plots of a group of lamp samples having the same area S in section of the electrode tip portion and different gap distances d between the paired discharge electrodes, while the feature of the gradual incline rising rightward located in the lower portion of the regression line is formed by the plots of a group of lamp samples having the same gap distance d and different areas S in section of the electrode tip portion. This means that the variation in the gap distance d exerts larger influence on the value L/d than the area S in section. However, in any case, it is always required that the value E × j be larger than 70 (VA/mm 3 ) for obtaining sufficient value L/d of at least 4000 lm/mm. Based on the experimental results shown in Figs. 2 and 5, in order to satisfying the first requirement having a luminous flux retention rate of at least 80 % at the time lapse of 100 hours together with satisfying the second requirement having the luminance value L/d of at least 4000 lm/mm, the effective product value E × j for the lamp should be in the range of 70.0 ≤ E × j ≤ 150.0 (VA/mm 3 ), which the effective range is shown in Fig. 6. The present inventors confirm that the effective range of 70.0 ≤ E × j ≤ 150.0 (VA/mm 3 ) as shown in Fig. 6 of the lamp lighting operation is not overlapped by those of the conventional metal halide lamps. This means that in the prior art there has not been taught or suggested any metal halide lamp satisfying the above two requirements, i.e., having a luminous flux retention rate of at least 80 % at the time lapse of 100 hours as well as having the value L/d of at least 4000 lm/mm. By this arrangement, a metal halide lamp can be fabricated for use as a light source having characteristics of high luminance and high luminous flux retention rate, adapted for an essential part of an optical display incorporated in e.g. an optical projection system. SECOND EMBODIMENT The following describes a second embodiment of the present invention with reference to Figs. 7 to 12. As described in the first embodiment, when the energy density E × j is decreased as shown in Figs. 2 and 6, a high luminous flux retention rate can be maintained while suppressing the deterioration thereof, and thus the half-life property of the lamp regarding the luminous flux retention rate is improved as shown in Fig. 4. However, in the case where there is used a metal halide lamp having a small gap distance of 3 mm or smaller, i.e., in a range of 1.5 mm to 3 mm, adapted to be incorporated in an optical projector and the like, it may be difficult to attain a high luminous flux retention rate merely by reducing the value of E × j in view of fabricating a contour design of the lamp because of the following reasons. That is, in this type of the lamp having such a small gap distance, the value E × j (= V/d × I/S) is defined by parameters of a lamp power (= V × I ), gap distance d between the electrodes and area S in section of the electrode tip portion, where the lamp power is restricted for providing sufficient luminous energy or quantity of light in view of provision of a lamp lighting circuit, and there have been required further restriction of the gap distance d for arc length between the pair of electrodes for increasing a luminance or brightness of an arc discharge portion in view of an optical requirement. Therefore, only a parameter S of the area in section is available in fabricating the lamp. In order to reduce the value E × j, it may be realized by increasing the parameter S. However, the parameter S also has an upper limit restricted from a viewpoint of a correlation between a dimension in diameter of the arc discharge portion and an optical configuration in design of the lamp. That is, there is a general principle that the dimension in diameter of the arc discharge portion produced between the discharge electrodes is increased when the area S in section of the electrode tip portion is increased. In particular, in the case where the lamp is used as a light source to be incorporated in an optical condensing projection system, when the diameter of the arc discharge portion is increased, the luminance of the arc discharge portion is reduced, resulting in reduction of the resultant quantity of light to be taken out of the optical projecting system. Therefore, there may be a case that the parameter S should be restricted small to have an upper limit for suppressing the diameter of the arc discharge portion. In order to improve the luminous flux retention rate with a fixed value of E × j while fixing the parameter S of the electrode tip area in section, the present inventors have studied and attained a new method by controlling a temperature of the electrode tip portion by adjusting a power source. In more detail, Fig. 7 shows a relation of the luminous flux retention rate at a time lapse of 100 hours represented on the ordinate axis with respect to the mean value Tm of the temperature of the electrode tip portion represented on the abscissa axis using the same lamp examples as those of Figs. 2 and 3. Based on the measurement results in Fig. 7, it is confirmed that the temperature mean value Tm should be below 3000 K in order to attain a high luminous flux retention rate of more than 80 %. In particular, in order to attain higher luminous flux retention rate of 85 % or more as described in the preferred embodiment with reference to Fig. 4, the temperature mean value Tm should be within the range of 2300 to 2700 K as defined in Fig. 7. Thus, as shown by the case B designated by ▪ marks in Fig. 4, the half life period of the luminous flux retention rate of about 7000 hours can be obtained in lamp lighting time by realizing the high luminous flux retention rate of 85 % or higher. That is, as shown in Fig. 3, there is depicted dispersed difference in temperature mean values of the electrode tip portion with respect to a fixed value of E × j, which the difference in temperature mean values causes the differences in luminous flux retention rate in spite of the same value of E × j as shown in Fig. 2. Fig. 8 shows a preferred range of the temperature mean vale Tm of the electrode tip portion with respect to the optimum value of the product E × j obtained by combining the conditions of Figs. 6 and 7. By defining the optimum ranges of both the temperature mean value within the range of 2300 to 2700 K and the product value E × j within the effective range of 70.0 ≤ E × j ≤ 150.0 (VA/mm 3 ) in fabricating the metal halide lamp, a high luminous flux retention rate of more than 85 % can be realized together with a half life property of 7000 hours of lamp lighting time regarding the luminous flux retention rate. Fig. 9 shows an example of a method for defining an optimum range of the temperature mean value Tm of the electrode tip portion in order to attain a high luminous flux retention rate with a fixed value of E × j (= V/d × I/S), i.e., with the fixed values of lamp power (W), gap distance (d) and area (S) in section of the electrode tip portion. In Fig. 9, the column-like discharge electrode 1 is integrally protruded in the discharge tube 2 from the electrode shaft 4 inserted in the sealing member 5, and there is formed a diameter-increased or diameter-reduced portion between the tip end 1a and a base portion 1b thereof to have a varied area S B in section different from the area S A in section of the other portion of the protruded electrode shaft 1. As shown in Fig. 9, when a diameter-increased portion is formed in an intermediate front-ward portion of the protruded column-like electrode shaft 1, there is provided e.g. an electrode coil member 26 made of the same tungsten material is wound by welding on the protruded electrode shaft 1. In Fig. 9, the tip end portion 21 between the tip face 1a of the protruded electrode shaft 1 and the top end 1c of the electrode coil member 26 has a length of h mm, which is referred to as "tip length" hereinafter. The present inventors have studied that there is a correlation between the tip length h and the temperature mean value Tm of the electrode tip portion 21 and found that the temperature mean value can be controlled by varying the tip length h. Fig. 10 shows a relation between the temperature mean value Tm on the ordinate and the tip length h on the abscissa axis with a preferred effective energy density within the range of 100 ≤ E × j ≤ 120 VA/mm 3 while fixing the values of lamp power (V × I), gap distance d and area S in section of the electrode tip portion. As shown in Fig. 10, it is confirmed that the temperature mean value Tm is reduced as the tip length h is reduced. By this arrangement, the temperature mean value Tm can be optimized by adjusting the tip length h, i.e., by adjusting the position of providing the electrode coil member 26 on the protruded electrode shaft 1, and thus a high luminous flux retention rate can be attained with the fixed value of E × j, thereby preventing the deterioration of the luminous flux retention rate. The diameter-increased portion or diameter-reduced portion may be integrally formed by machining or cutting the protruded electrode shaft 1 as shown in Figs. 11 and 12 instead of providing a coil member. Fig. 13 shows a modified example of an electrode tip portion 31 having a curved surface 31a corresponding to a supporting part of the arc discharge portion 3. The curved surface 31a has an actual surface area S1 and a vertical section area S2 perpendicular to the arc discharge axis 37. In this case, the vertical section area S2, which is the smallest in area of the discharge supporting portion, is considered as the cut area S in section of the electrode tip portion, and with the smallest area S, the product value E × j becomes the largest, which is the lowest condition regarding the luminous flux retention rate with reference to Fig. 2. The actual surface area S1 is larger than the vertical section area S2, and when S1 is considered as the cut area S in section, the luminous flux retention rate is raised to be improved. Referring to the effects of the present invention, an improved metal halide lamp can be provided to have a high luminous flux retention rate and high luminance of an arc discharge portion with a longer life of the lamp, suppressing a lamp voltage varying rate, avoiding a change in color temperature, which remarkably improves additional merits when in utilization as a light source in various display apparatuses such as optical projection systems. In the construction of the present invention, a wide range of different metal halide materials to be sealed as well as different lamp powers can be adapted to fabricating metal halide lamps, and therefore degree of freedom in fabrication of the design and efficiency in development thereof can be remarkably improved. Moreover, in arranging a lamp-lighting circuit, since the securing range in applying the lamp voltage can be restricted, therefore the fabrication in design of the lamp can be facilitated advantageously. Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be noted here that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention as defined by the appended claims, they should be construed as included therein.

Claims: A metal halide lamp which includes a discharge tube (2) retaining a fill of mercury and at least one metal halide added as a luminous material in an inert gas atmosphere sealed therein, comprising: a pair of discharge electrodes (1, 1') oppositely disposed with a space of a gap distance defining a length of an arc discharge portion (3) produced between the paired discharge electrodes (1, 1') in the discharge tube (2); wherein an energy density of the arc discharge portion (3) represented by a product E × j is in the range of 70.0 ≤ E × j ≤ 150.0 (VA/mm 3 ); wherein a temperature mean value (Tm) of an electrode tip portion (21) of each electrode (1) is within the range of 2300 to 2700 K; and where E = V/d , j = I/S , assuming that I is a lamp current in amperes with a lamp voltage of V volts applied between the paired discharge electrodes in a stable lighting condition of the lamp and that each of the electrodes has a tip face (1a, 1a') of which a cut area in section is S mm 2 and the gap distance is d in millimeters. The metal halide lamp as claimed in claim 1, wherein the discharge tube (2) is made of a quartz glass, having a spherical-like inner bulb wall (2a) and each of the paired discharge electrodes (1, 1') is of a column-like shape which is integrally protruded from an electrode shaft (4) inserted in a sealing member (5). The metal halide lamp as claimed in claim 1 or 2, wherein each of the discharge electrodes is formed with a diameter-varied portion (26, 27, 28) between the tip face (1a, 1a') and a base portion (1b, 1b') thereof to have a varied area (S B ) in section different from the area (S A ) in section of the tip face (1a, 1a') of the protruded electrode (1). The metal halide lamp as claimed in claim 3, wherein the diameter-varied portion (26, 27, 28) is formed in an intermediate frontward portion of the protruded electrode (1). The metal halide lamp as claimed in claim 4, wherein the diameter-varied portion is of a diameter-increased portion by providing an electrode coil member (26) made of the same material as that of the electrode, which is wound by welding on the electrode. The metal halide lamp as claimed in claim 4, wherein the diameter-varied portion (27, 28) is integrally formed with the protruded electrode portion (1) by machining. The metal halide lamp as claimed in claim 4, wherein an electrode tip portion (31) of each electrode has a curved surface (31a) corresponding to a supporting part of the arc discharge portion (3).

Inventors: KAI MAKOTO, KANEKO YURIKO, TAKEDA MAMORU, KAI, MAKOTO, KANEKO, YURIKO, TAKEDA, MAMORU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1997
Main classification: H01J61
IPC codes: H01J61