Method of operating a metal halide lamp

Abstract: In a metal halide lamp in which Hg, a rare gas and InI and HoI 3 are filled, filler amount of InI is in a range of 0.1 mg/cc to 1.5 mg/cc, and filler amount of HoI 3 is in such range that an evaporable amount of said halide depending on the temperature of said halide is the minimum filler amount and 3.0 mg/cc of said halide is the maximum thereof, and thereby a lamp having a high efficacy in which an emission spectrum is well distributed over a visible range can be obtained, having also a long lifetime because devitrification of an arc tube is restricted.

Description: The present invention relates to a method of operating a metal halide lamp used as a light source of a liquid crystal projector and the like. Recently, liquid crystal projectors or the like are known as means for enlarging and projecting an image of characters, figures, etc. Since a certain optical output is required for such an image projector, a metal halide lamp, which has a high luminous efficacy, is widely used as a light source for the image projector in general. In this kind of metal halide lamp, as disclosed in Japanese Unexamined Patent Publication No. 3-219546 for example, an iodide of Nd, Dy, and Cs has been generally used as a halide to be filled in an arc tube. The lamp in which the iodide of Nd, Dy and Cs is filled (hereinafter described as Dy-Nd-Cs-I series lamp), as disclosed in Japanese Unexamined Patent Publication No. 3-219546, has an excellent luminous efficacy, however, it has the disadvantage that devitrification occurs in an early stage of its lifetime, particularly because of high reactivity between neodymium iodide (NdI 3 ) and silica glass of the arc tube. The lack of transparency decreases the intensity of light beam, reduces the luminous intensity and disperses the light beam, resulting in nonuniform illumination intensity and reduced brightness on a screen of the liquid crystal projector. Specifically, the drawback of the Dy-Nd-Cs-I lamp for use as the light source of a liquid crystal projector is that the lifetime of the liquid crystal projector is short. Recently, from the viewpoint of saving energy, there is a demand for a light source having a higher luminous efficacy than that of the Dy-Nd-Cs lamp. "Luminous characteristics of metal halide lamp containing rare earth halide" (Journal of Lighting Society, vol. 65 No. 10, 1981, page 17) discloses that a light source having a high luminous efficacy can be obtained by combining a rare earth halide with a halide of Tl or In. However, the low correlated color temperature of the light source disclosed therein is unsuitable for a light source of such things as a liquid crystal projector. An example of a light source disclosed therein is a metal halide lamp in which InI and TmI 3 are filled, and a correlated color temperature estimated from the relative spectral distribution diagram disclosed therein is about 4500 K. On the other hand, a white color reference for an image projector such as liquid crystal projectors is about 9000 K. EP 0 605 248 A2 discloses a metal halide lamp having an arc tube and a filling which comprises argon gas and halides of dysprosium and indium. The amount of indium halide in one embodiment is 0.44 mg/cc. To increase the life time of the metal halide lamp, the catephoresis effect by DC lighting is used. The wall load developed in the arc tube is substantially greater than 40 W/cm 2 . EP 0 386 601 A2 discloses a metal halide lamp having an arc tube and a filling which comprises a noble gas and halides of indium and a rare earth metal. In a specific embodiment, the amount of indium halide is set in a range between about 0.01 mg/cc to 1.5 mg/cc. This lamp is operated at 100 watts, it emits an emission spectrum having an intense line spectrum. However, there is the need and the object to provide a method for operating a metal halide lamp having an emission spectrum distributed over the entire visual inspecting range, a high luminous efficacy, an appropriate color temperature, and a long life time. This object is solved by the method as defined in claim 1. Preferred embodiments are disclosed in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view showing the construction of a metal halide lamp for use in an embodiment 1 of the invention; FIG. 2 is a diagram showing a distribution of the metal halide lamp of Fig. 1; FIG. 3 is a diagram showing the relation between amount of iodide filled in the lamp and correlated color temperature in the embodiment 1 of the invention; FIG. 4 is a view showing the construction of an optical system used for lifetime evaluation of a metal halide lamp used in the invention; FIG. 5 is a diagram showing the relation between lighting time and illuminace maintenance factor of the metal halide lamp used in the embodiment 1 of the invention; FIG. 6 is a diagram showing an emission distribution of a metal halide lamp used in an embodiment 2 of the invention; FIG. 7 is a diagram showing the relation between lighting time and illuminance maintenance factor of the metal halide lamp used in the embodiment 2 of the invention; FIG. 8 is a diagram showing the relation between amount of iodide filled in the lamp and correlated color temperature in the embodiment 2 of the invention; FIG. 9 is a diagram showing an emission distribution of a metal halide lamp used in an embodiment 3 of the invention; FIG. 10 is a diagram showing an emission distribution of a metal halide lamp used in an embodiment 4 of the invention; FIG. 11 is a diagram showing an emission distribution of a metal halide lamp used in an embodiment 5 of the invention; FIG. 12 is a diagram showing an emission distribution of a metal halide lamp used in an embodiment 6 of the invention; FIG. 13 is a diagram showing an emission distribution of a metal halide lamp used in an embodiment 7 of the invention; FIG. 14 is a diagram showing a relation between the tube wall load and the correlated color temperature in the embodiment 1 of the invention. 1 arc tube 2a electrode 2b electrode 10 light source 11 light beam condensing mirror 12 projection lens system 13 screen PREFERRED EMBODIMENTS Referring to the drawings, preferred embodiments of the invention are described in detail below. A first problem of the prior art Dy-Nd-Cs-I series lamp is that, since the reactivity of NdI 3 and silica glass of an arc tube is intense, devitrification of a arc tube occurs at an early stage of its lifetime. This problem can be solved if a material having a lower devitrification level (weaker reactivity with silica glass) than that of NdI 3 is used for the filler of the lamp. From this viewpoint, devitrification levels of various metal halides are evaluated firstly by the following devitrification evaluation test. In the devitrification test, an ampoule composed of a tube made of silica glass, having a content volume of 5 cc and filled with 10 mg of metal halide, is heated at 1, 100 °C for 100 hours, and then a total transmittance of the ampoule is measured in order to evaluate the devitrification characteristic of the metal halide. The ratios (%) of the total transmittances of the ampoules after heating (after the test) to the total transmittances thereof measured before heating are shown in Table 1. A larger percentage means a lower level devitrification. Blanks in the table indicate that evaluation was not conducted. Material Total transmittance (%) Material Total transmittance (%) NdI 3 94.6 NdBr 3 97.0 TbI 3 98.0 TbBr 3 DyI 3 94.8 DyBr 3 99.0 HoI 3 98.1 HoBr 3 ErI 3 98.6 ErBr 3 TmI 3 98.7 TmBr 3 99.0 InI 99.0 InBr SnI 3 99.0 SnBr 3 99.0 As shown in Table 1, the total transmittances of ampoules filled with TbI 3 , DyI 3 , HoI 3 , ErI 3 , TmI 3 , InI, SnI 3 , DyBr 3 , TmBr 3 , InBr and SnBr 3 , respectively, are larger than the total transmittance of an ampoule filled with NdI3, and indicate a low level devitrification characteristic. Accordingly various characteristics of combinations of these materials with a low level devitrification characteristic are investigated. As a result, as described below, excellent lifetime and luminous efficacy characteristics are obtained by adding a halide of Tb, Dy, Ho, Er, or Tm, or a mixture of these, to a halide of In. (Embodiment 1) FIG. 1 is a view showing a metal halide lamp used in a first embodiment of the invention. In FIG. 1, reference numeral 1 denotes a light transmitting container as an arc tube made of quartz, at both ends of which sealing portions 6a, 6b are formed. To the sealing portions 6a, 6b, respectively, metal foil conductors 3a, 3 b made of molybdenum are tightly attached. To these metal foil conductors 3a, 3 b, respectively, electrodes 2a, 2b, and outer lead wires 4a, 4b made of molybdenum are electrically connected, . The electrodes 2a, 2b are composed of tungsten rods 7a, 7b and tungsten coils 8a, 8b, respectively. The coils 8a, 8b are electrically fixed to tip portions of the tungsten rods 7a, 7b by welding, and serve as radiators of the electrodes 2a, 2b. The electrodes 2a, 2b are arranged in the arc tube 1 so as to face each other and maintain a distance of 3. 5 ± 0.5 mm therebetween. The arc tube 1 is nearly spherical, and has an inner diameter of about 10.8 mm, an inner volume of about 0.7 cc, and an inner surface area of about 3.6 cm 2 , in which 0.4 mg (0.57 mg/cc) of InI as a filling material, 1 mg (1.43 mg/cc) of HoI 3 as a rare earth iodide, 35 mg of Hg as a buffer gas, and 200 mbar of Ar as a start-up rare gas are filled. The metal halide lamp having the above-mentioned construction was supplied with electric power via external lead wires 4a, 4b to be lit with a rated lamp power of 200 W (tube wall load: 55 W/cm 2 ) and the emission characteristic was evaluated. FIG. 2 is a diagram showing a spectrum distribution of the metal halide lamp used in this embodiment. The correlated color temperature and luminous efficacy of this case were respectively about 5500 K and about 87 lm/W. Rich emission is seen all across the visible range. The emission in the red color range is particularly rich. For the purpose of comparison, a lamp in which 1 mg of DyI 3 , 1 mg of NdI 3 , and 1 mg of CsI are filled in lieu of InI and HoI 3 , and the other constructive details are the same as those of the metal halide lamp of FIG. 1 (hereinafter described as Dy-Nd-Cs-I lamp) was evaluated. The luminous efficacy of the comparative metal halide lamp when being lit with the rated power was 77 lm/W. These results indicate that the metal halide lamp used in this embodiment has a good luminous efficacy. Next, various different lamps were fabricated wherein filler amounts of InI and HoI 3 differed and the other constructive details were the same as those of the metal halide lamp of FIG. 1. Examination was conducted on luminous efficacy of these lamps at the time they are operated with the rated power and further, on basic characteristics of the metal halide lamp in which InI and HoI 3 are filled. The results will be described referring to FIGs. 3, 4 and 5. FIG. 3 is a diagram showing the relation between the filler amount (mg) of InI per unit volume (axis of abscissa) and correlation color temperature (K) (axis of ordinate) wherein filler amounts of HoI 3 are taken as a parameter. Three marks , ○, □ in FIGs. indicate lamps in which the filler amounts of HoI 3 are 0.57, 1.43 and 2.86 mg/cc, respectively. As seen in FIG. 3, it was found that the correlated color temperature highly depends on the filler amounts of InI, as shown in curve 3A in the figure. However, the effect of filler amounts of HoI 3 on the correlation color temperature is relatively low. This is caused by the fact that InI generally conducts an unsaturated action, and HoI 3 generally conducts a saturated action. The correlated color temperature required for use as a light source depends on the usage purpose . When it is used for a light source of a liquid crystal projector or like apparatus, it is preferable that the correlated color temperature of the light source is about 4500 K or more . When the temperature is less than 4500 K, such bad conditions is occurred that the white color temperature on a screen becomes slightly yellow color temperature. It is preferable that the correlated color temperature of the light source is about 9000 K which is used as a white color reference in many liquid crystal projector or like apparatus. Results provided in FIG. 3show that a preferable filler amount of InI in correspondence to the requirement of such various relatively high correlated color temperatures is 0.1 mg/cc to 1.5 mg/cc in the metal halide lamp used in the embodiment. Additionally, it is apparent from the following description, that the range of preferable filler amounts of InI in correspondence to the requirement of such various relatively high correlated color temperature can apply to other lamp which has different tube wall load from that of the lamp used in this embodiment. Lamps in which each amount of InI is different and the other constructive details are the same as those of the metal halide lamp of FIG. 1, are operated by various powers and the correlated color temperatures of the lamps are detected . The results are shown in FIG. 14. In the FIG. 14 the curves 14A, 14B, 14C show respectively the relations between the correlated color temperature and the filler amount of InI under 175 W (tube wall load: about 48 W/cm 2 ), 200 W (tube wall load: about 55 W/cm 2 ), and 225 W (tube wall load: about 62 W/cm 2 ). As a result it is apparent that preferable correlated color temperature of about 4500 K - 9000 K can be obtained by such filler amount of InI of 0.1 mg/cc to 1. 5 mg/cc even for different tube wall loads. But the higher the tube wall load is , the lower the correlated color temperature becomes. While the correlated color temperature is about 6800 K at a tube wall load of 55 W/cm 2 when the filler amount of InI is 0.57 mg/cc, the correlated color temperature is about 5800 K at a tube wall load of 62 W/cm 2 . But the more the filler amount of InI is , the less such lowering of the color temperature becomes. When the filler amount of InI is about 1.5 mg/cc, the variation ratio of the correlated color temperature against such change of tube wall load of 55 W/cm 2 to 62 W/cm 2 is less than 5 % and it can be negligible. Such results show that the correlated color temperature of about 4500 K or more can be obtained by making the filler amount of InI being 1. 5 mg/cc or less irrespective of amount of the tube wall load. Next, the metal halide lamp used in the embodiment was used as a light source for an optical system as shown in FIG. 4 to evaluate a maintenance factor of screen 13 illuminance to lighting time. In FIG. 4, reference numeral 10 denotes a light source, reference numeral 11 denotes a converging mirror which reflects and converges light radiated from the light source 10 and reference numeral 12 denotes a projection lens system which projects the light converged by the converging mirror on the screen 13. Evaluation results are shown in FIG. 5 (curve 5A). In FIG. 5, the axis of abscissa indicates lighting time and the axis of ordinate indicates the maintenance factor of an average of illuminances of 13 points on the screen 13. For the purpose of comparison, the results on the Dy-Nd-Cs-I lamp are also indicated (curve 5B). From these results, it is newly demonstrated that the metal halide lamp comprising InI and HoI 3 has a longer lifetime than that of the prior art lamp wherein NdI 3 is filled. These results are shown in Table 1 above, which support the results of the devitrification evaluation test. (Embodiment 2) Next, a lamp wherein 1 mg (1.43 mg/cc) of TmI 3 is filled in lieu of HoI 3 , and the other constructive details are the same as those of the metal halide lamp of FIG. 1 will be described below. FIG. 6 is a diagram showing a spectrum distribution of the metal halide lamp used in the embodiment. The correlated color temperature and luminous efficacy in the case are about 6400 K and about 94 lm/W, respectively. The lamp wherein TmI 3 replaces HoI 3 is characterized by having a higher luminous efficacy than that used in the embodiment. On the other hand, emission in the red color range is less satisfactory. With respect to the correlated color temperature, the lamp is equivalent to the lamp wherein HoI 3 is filled. The correlated color temperatures of the lamp used in the present embodiment plotted in FIG. 3 fit to curve 3A which relates filler amounts of InI to correlated color temperatures. Accordingly, a preferable filler amount of INI in the lamp in which TmI 3 is used in lieu of HoI 3 in the present embodiment (in which a relatively high correlated color temperature is attained, which can not be attained in the prior art In-Tm-I series lamp) is 0.1 mg/cc to 1. 5 mg/cc, the same as in embodiment 1. The lamp used in the present embodiment is also characterized by a long lifetime. The same as FIG. 5, FIG. 7 is a diagram (curve 7A) showing changes in maintenance factor of an average of illuminances of 13 points on the screen 13 to lighting time when the lamp used in the present embodiment was used as a light source for the optical system shown in FIG. 4. In addition, the results on a lamp used in the present embodiment wherein 2 mg of TmI 3 is filled, and the prior art Dy-Nd-Cs-I lamp are also indicated (curves 7B and 7C, respectively). The average illuminance of the screen in the prior art Dy-Nd-Cs-I lamp after about 1 400 hours of lighting decreased to 50 % of that of an early stage of use, while the average illuminance of the lamp of the present embodiment, even after 2000 hours of lighting, maintains 60 % of that of the early stage of use. However, as shown in FIG. 7B, it was newly found that lifetime is shortened with increase of filler amount of TmI 3. The tendency was also observed in the HoI 3 -filled lamp, which is described in embodiment 1. Accordingly as small as possible filler amount of TmI 3 is preferable for lifetime. The minimum amount of TmI 3 is such amount being possible to evaporate (As the vapor pressures of TmI 3 and HoI 3 are low, the whole amount of filler does not evaporate). Meawhile an evaporable amount of said halide is determined by the temperature of said halide. The coolest temperature of the lamp used in the embodiment is about 1000 K as like other general metal halide lamps and the saturation vapor pressure of TmI 3 at this temperature is about 4 × 10 -6 atm and therefore the amount of the evaporated TmI 3 within the lamp of 0.7 cc is about 0.0001 mg on the basis equation of gas state . However, because such an ultra small amount can not be weighed, an amount of 0.01 mg is a practical value. The maximum limitation amount is 2 mg (= about 3 mg/cc) from FIG. 7. This preferable range of filler amount of TmI 3 can be also applied to the HoI 3 -filled lamp which is described in the foregoing embodiment 1, from the same reason. Additionally, even when a different amount of TmI 3 or HoI 3 is filled, TmI 3 and HoI 3 conduct saturation action, so that luminous efficacy is not adversely affected. FIG. 8 is a diagram showing the relationship between the filler amount of HoI 3 and the luminous efficacy in a lamp wherein HoI 3 replaced TmI 3 in the present embodiment. (Embodiment 3) Next, a lamp wherein 1 mg (1.43 mg/cc) of TbI 3 is filled in lieu of HoI 3 , and the other constructive details are the same as those of the metal halide lamp of FIG. 1 will be described below. FIG. 9 is a diagram showing a spectrum distribution of the metal halide lamp used in the present embodiment. The correlated color temperature and luminous efficacy in the case are about 7000 K and about 82 lm/W, respectively. The lamp wherein TbI 3 replaces HoI 3 is characterized by having a higher correlation color temperature indicated by the present embodiment. The emission of around 500 nm wavelength, which is richer than the red color range, contributes to the higher correlation color temperature. Additionally, the correlated color temperature and luminous efficacy of another lamp used in the present embodiment, wherein 0.6 mg (0.86 mg/cc) of InI, and 2 mg (2.86 mg/cc) of TbI 3 are filled, are about 6300 K and about 80 lm/W, respectively. In the lamp used in the present embodiment, a preferable filler amount of InI in the lamp is 0.1 mg/cc to 1.5 mg/cc, the same as in the lamp used in embodiment 1. From the results of the devitrification test shown in Table 1 and the results of embodiments 1, 2, it is apparent that the TbI 3 - filled lamp used in the present embodiment also has a long lifetime. (Embodiment 4) Next, a lamp wherein 0.6 mg (0.86 mg/cc) of InI and 2 mg (2.86 mg/cc) of ErI 3 in lieu of HoI 3 are filled, and the other constructive details are the same as those of the metal halide lamp of FIG. 1, will be described below. FIG. 10 is a diagram showing a spectrum distribution of the metal halide lamp used in the embodiment The correlated color temperature and luminous efficacy in the case are about 5000 K and about 86 lm/W, respectively. The lamp used in the present embodiment wherein ErI 3 is filled indicates the same emission distribution as that with HoI 3 . Given this characteristic, ErI 3 can completely replace HoI 3 . In the lamp used in the present embodiment, a preferable filler amount of InI is 0.1 mg/cc to 1.5 mg/cc, the same as in the lamp used in embodiment 1. A similar emission characteristic can also be obtained in a lamp used in the present embodiment wherein Dy 3 replaces ErI 3 . However, as apparent from Table 1 of devitrification test results, devitrification property is relatively strong, so that replacement with DyI 3 slightly diminishes a prominent effect to lifetime. (Embodiment 5) Next, a lamp will be described below wherein 0.6 mg (0.86 mg/cc) of InI and 1mg (1.43 mg/cc) of HoI 3 are filled, and 1 mg (1.43 mg/cc) of TbI 3 is added, and the other constructive details are the same as those of the metal halide lamp of FIG. 1. FIG. 11 is a diagram showing a spectrum distribution of the metal halide lamp used in the present embodiment. The correlated color temperature and luminous efficacy in the case are about 6100 K and about 831 m/W, respectively. The lamp wherein TbI 3 is added to HoI 3 is characterized by emission distribution in which characteristics of both TbI 3 and HoI 3 are added. Emission around 500 nm wavelength, which is richer than that of the lamp in which only TbI 3 is filled, is an effect produced by TbI 3 , and emission around the red color range, which is richer than that of the lamp in which only TbI 3 is filled, is an effect produced by HoI 3 . The effects of such addition can be also attained by other combinations than the combination of HoI 3 and TbI 3 . By appropriately combining TbI 3 , DyI 3 , HoI 3 , ErI 3 and TmI 3 , a lamp which has unique characteristics can be attained. For example, in the case of a combination of HoI 3 and TmI 3 , emission of TmI 3 in the red color range, which is slightly less satisfactory, is improved by HoI 3 , and a higher luminous efficacy than that of a lamp in which only HoI 3 is filled is produced by TmI 3 . (Embodiment 6) Next, a lamp wherein 0. 4 mg (0.57 mg/cc) of InBr is filled in lieu of InI, and the other constructive details are the same as those of the metal halide lamp of FIG. 1, will be described below. FIG. 12 is a diagram showing a spectrum distribution of the metal halide lamp used in the present embodiment. The correlated color temperature and luminous efficacy in the case are about 5300 K and about 801 m/W, respectively. As indicated by the present embodiment, change in emission distribution due to the replacement by InBr was not observed. With respect to this point, replacement of InI by InBr is possible. Additionally as apparent from devitrification test results in Table 1, no adverse effect on lifetime was found. However, luminous efficacy is slightly lessened. The replacement of InI by InBr is not limited to the case of the combination with HoI 3 , as indicated in the present embodiment. It is possible in combinations with TbI 3 , DyI 3 , ERI 3 , TmI 3 as well as a mixture of these iodides. Additionally, bromides may be used in lieu of iodides, and iodides may be combined with bromides. (Embodiment 7) Next, a lamp wherein 1 mg (1.43 mg/cc) of HoBr 3 is filled in lieu of HoI 3 and the other constructive details are the same as those of the metal halide lamp of FIG. 1 will be described below. FIG. 13 is a diagram showing a spectrum distribution of the metal halide lamp used in the present embodiment. The correlated color temperature and luminous efficacy in the case are about 7200 K and about 74 1m/W, respectively. A lamp in which HoBr3 replaced HoI 3 is characterized by a higher correlated color temperature than that of the present embodiment. Emission around 440 nm wavelength largely increased. The luminous efficacy decreased by 10 %, to a level equivalent to that of the conventional Dy-Nd-Cs-I lamp. With respect to the TmI 3 -filled lamp described in embodiment 2, another lamp in which 1 mg (1.43 mg/cc) of TmBr 3 was filled in lieu of TmI 3 , it has a correlated color temperature of about 8600 K and a luminous efficacy of about 81 lm/W. The spectrum distribution of the lamp is shown in FIG. 14. As mentioned above, alteration of iodides of Ho and Tm to the bromides thereof causes a higher correlated color temperature and a decrease of about 10 % in luminous efficacy as a result. Despite the decrease in luminous efficacy, a longer lifetime is obtained than in the case of the iodides. It is demonstrated in the devitrification evaluation test results in Table 1 that Tm has a lower devitrification property in its bromide than in its iodide. Additionally, it is apparent that even in Dy and Nd, whose iodides indicated high devitrifications, the bromides thereof indicate low devitrifications which are comparable to that of TmBr 3 , and that the bromides of Ho, Tb and Er also result in a comparably long lifetime. It is also the case with these bromides, alteration of InI to InBr causes no detrimental effect As mentioned above, by combining a halide of 0.1 mg/cc to 1. 5 mg/cc of In with a halide of rare earth of Tb, Dy, Ho, Er, Tm or a mixture of elements selected from these rare earth elements, a highly efficient metal halide lamp satisfying the requirement for various relatively high correlated color temperatures can be realized. Additionally, when the filler amount of said halide of the rare earth is in such range that an evaporable amount of said halide depending on the temperature of said halide is the minimum filler amount and 3.0 mg/cc of said halide is the maximum thereof, a metal halide lamp having a further extended lifetime can be realized. In the embodiments the arc length is 3.5 ± 0.5mm, but when the arc length is 5 mm or less , same merits are obtained. When the arc length is more than 5 mm, the efficacy increases but the correlated color temperature decreases. In the case where In was replaced by Ga, which also belongs to 3B, the luminous efficacy was less than 70 lm/W. In the case where a halide of Sn, which indicated a low devitrification in the devitrification test results, and a iodide of In were combined, a luminous efficacy of 701 m/W or more could be obtained, but because of the low luminance, the lamp was unsuitable as a light source for a liquid crystal projector or the like. Although in the aforementioned embodiments, examples of metal halide lamps having electrodes were described, a similar effect can be also obtained even in a so- called "no electrode" metal halide lamp in which no electrode is provided and a filler in the arc tube is excited and emits light Although, in the above, preferred embodiments were described, the present invention is not restricted to the description mentioned above, and it is to be understood that modifications will be apparent to those skilled in the art without departing from the spirit of the invention. A similar effect can be obtained even in a lamp having a different wattage and different sizes, and also in the case where a material containing Na, Cs or the like is added to a filler in order to stabilize arc, a similar effect can be obtained. On the basis of the embodiments, it is possible, according to the invention, to obtain an economical light source having a good luminous efficacy and an emission spectrum covering the visible range, which can satisfy the requirement for various relatively high correlated color temperatures. This is made possible by, in addition to a start-up rare gas , combining at least a halide of In and a halide of a rare earth element including Tb, Dy, Ho, Er and Tm, or a halide of a mixture of those selected from these elements. Additionally, since the progress of the reaction between a transparent container material and an additive metal is decelerated, devitrification is restricted as a consequence , and thereby a light source can be attained having a long lifetime, suitable to a light source for a liquid crystal projector, for example. Meanwhile the currents applied to the lamps in the above mentioed embodiments are rectangular waveform of 270 Hz. Accurents are applied to the lamps in the present invention.

Claims: A method of operating a metal halide lamp comprising a light transmitting container (1) with a fill including, in addition to a start-up rare gas, at least a halide of In, and a halide of Tb, Dy, Ho, Er, Tm, or a mixture of said Tb, Dy, Ho, Er, Tm; wherein 0.1 mg/cc to 1.5 mg/cc of the halide of In is filled, the method comprising the step of: operating the lamp by AC current, wherein lighting is conducted with a lamp power having a tube wall load of 48 W/cm 2 to 62 W/cm 2 . The method of claim 1, wherein the halide of Tb, Dy, Ho, Er, Tm, or the mixture of said Tb, Dy, Ho, Er, Tm is filled to such an extent that the minimum filler amount is the amount of said halide that is completely evaporatable depending on the temperature of said halide and 3.0 mg/cc of said halide is the maximum thereof. The method of claim 2, wherein filled material in the light transmitting container (1) are excited by electromagnetic wave externally supplied and begin to emit. The method of any one of claims 1 to 3, wherein a halogen of the halide of In is iodine or bromine. The method of any one of claims 1 to 4, wherein a halogen of the halide of a rare earth element is iodine or bromine. The method of claim 1 or 2, wherein the lamp comprises a pair of electrodes (2a,2b) electrically connected to an external power supply so that a distance between the pair of electrodes is 5 mm or less.

Inventors: HORIUCHI MAKOTO, TAKAHASHI KIYOSHI, TAKEDA MAMORU, HORIUCHI, MAKOTO, TAKAHASHI, KIYOSHI, TAKEDA, MAMORU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1996
Main classification: H01J61
IPC codes: H01J61