Patent Publication Number: US-2010127609-A1

Title: Coating liquid for diffusing film of high-pressure discharge lamp and high-pressure discharge lamp

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2008-298714, filed Nov. 21, 2008, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a coating liquid for a diffusing film that diffuses light emitted from a luminous tube of a high-pressure discharge lamp and a high-pressure discharge lamp provided with the diffusing film. 
     2. Description of the Related Art 
     Many high-pressure discharge lamps are currently used in the fields of outdoor illumination and have been increasingly used in the fields of indoor illumination for stores and the like in recent years. As high-pressure discharge lamps in indoor illumination fields, those having high efficiency and high color rendering properties are primarily used. 
     Generally, the surface of the outer bulb of such a high-pressure lamp is subjected to frost processing with hydrofluoric acid to diffuse the light emitted from a luminous tube for preventing glare. Also, other diffusing methods are known. For example, there is known a method of roughening the surface by sandblast or chemical etching, or a method comprising steps of applying a coating solution obtained by adding a hydrophobic plasticizer to an organic metal compound solution to the surface of the glass tube and drying and then baking to make a rough-surfaced filter, thereby obtaining a diffusing surface (Jpn. Pat. Appln. KOKAI Publication No. 2001-342037). Moreover, a metal halide lamp made to have a diffusing effect by applying SiO 2  microparticles to the outer surface of the outer bulb is also known (Jpn. Pat. Appln. KOKAI Publication No. 7-320687). 
     However, the method such as the sandblast that mechanically abrades the surface of the glass tube for frost processing has a problem of reducing original strength of the glass tube. Also, since the frost processing by chemical treatment uses hydrofluoric acid, this method is inadequate taking environmental problem of waste water into account. Moreover, when these methods are used for providing diffusing properties, there is brought about a problem of reducing light flux and thus reducing efficiency. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above problems, and it is an object of the invention to provide a coating liquid for a diffusing film of a high-pressure discharge lamp that properly diffuses light emitted from a luminous tube and also suppresses reduction in light flux and efficiency and to provide a high-pressure discharge lamp comprising the diffusing film. 
     According to an aspect of the present invention, there is provided a high-pressure discharge lamp comprising a luminous tube, an outer bulb housing the luminous tube, and a diffusing film formed on at least one of inner and outer surfaces of the outer bulb, wherein the diffusing film comprises first silica particles having shapes different in surface curvature from each other and hollow second silica particles. 
     According to another aspect of the present invention, there is provided a high-pressure discharge lamp comprising a luminous tube, an inner bulb surrounding the luminous tube and being sealed vacuum-tightly, an outer bulb enclosing the inner bulb, and a diffusing film formed on at least one of inner and outer surfaces of the outer bulb, wherein the diffusing film comprises first silica particles having shapes different in surface curvature from each other and hollow second silica particles. 
     The term “hollow second silica particles” means that, for example, the second silica particles have a spherical outer shape and an inner cavity forming a hollow portion. 
     Since the second silica particles have a hollow shape, they act to raise total light transmittance of the diffusing film without impairing light diffusing characteristics. 
     The second silica particles preferably have a spherical outer shape having an average particle size of 2 to 10 μm. The second silica particles more preferably have an average particle size of 2 to 5 μm. When the average particle size of the second silica particles is less than 2 μm, diffusion by the diffusing film is weakened, bringing about a rise in linear transmittance, whereas when the average particle size exceeds 10 μm, the total light transmittance is lowered. 
     The percentage by volume of the hollow portion of the hollow second silica particles is preferably 30 to 70% by volume. When the percentage by volume of the hollow portion of the hollow second silica particles is less than 30% by volume, diffusion by the diffusing film is weakened, bringing about a rise in linear transmittance. When the percentage by volume of the hollow portion of the hollow second silica particles exceeds 70% by volume, the second silica particles cannot have strength required to hold the hollow portion in a process of preparing coating liquid, and thus, the diffusing film comprising the second silica particles cannot be formed. 
     Also, the second silica particles are preferably partially provided with projection parts which project from the inner surface or outer surface in order to further enhance diffusion. 
     The term “first silica particles having shapes different in surface curvature from each other” means that a shell fragment constituting first silica particle has different surface curvatures on the inner and outer surfaces, or shell fragments constituting first silica particles have different surface curvatures on their outer surfaces. 
     The first silica particles are specifically produced in the following manner. For example, hollow silica particles having an average particle size of 3 μm and having 60% by volume of hollow portion are fractured by a mill such as a pin mill, jet mill and blade. When the volume occupied by hollow silica particles before fracturing is defined as 100% by volume, the silica particles are fractured such that the volume occupied by the first silica particles to be produced is 5 to 80% by volume on average. The hollow silica particles are fractured at random by this treatment and are made into first silica particles made of shell fragments of the hollow silica particles and have shapes different in surface curvature from each other. Such first silica particles having shapes different in surface curvature from each other have superior light diffusing characteristics to hollow silica particles having a spherical shape. Then, the first silica particles are classified as needed, and the obtained silica particles are dispersed in water containing a dispersant with a bead mill to obtain a dispersed product of the first silica particles having shapes different in surface curvature from each other. 
     The reason why hollow silica particles are fractured such that the volume occupied by the fractured particles is designed to be 5 to 80% by volume on average to obtain the first silica particles is as follows. When hollow silica particles are fractured until the volume occupied by the fractured particles becomes less than 5% by volume, clearances between second silica particles are filled with the produced first silica particles, and thus, diffusion by the diffusing film is weakened. When the volume of particles after the hollow silica particles are fractured exceeds 80% by volume, the produced first silica particles are not so different from the non-fractured hollow silica particles, and thus, diffusion by the diffusing film is weakened. 
     The average particle size of the hollow silica particles used as the raw material of the first silica particles is preferably 2 to 5 μm. When first silica particles produced from hollow silica particles having an average particle size of less than 2 μm are used, diffusion by the diffusing film is weakened and a haze value (Hz) is lowered. When first silica particles produced from hollow silica particles having an average particle size exceeding 5 μm are used, the total light transmittance of the diffusing film is lowered. 
     The percentage by volume of the hollow portion of the hollow silica particles used as the raw material of the first silica particles is preferably 30 to 70% by volume like that of the aforementioned second silica particles. 
     In the high-pressure discharge lamp of the present invention, the mixing ratio by weight of the second silica particles to the first silica particles in the diffusing film is preferably 1/5 to 10/1. When the mixing ratio is less than 1/5, the total light transmittance of the diffusing film is lowered. When the mixing ratio exceeds 10/1, diffusion by the diffusing film is weakened and the haze value is lowered. 
     In the high-pressure discharge lamp of the present invention, the diffusing film preferably has characteristics of a linear transmittance of 5 to 50% in a wavelength range of 300 to 800 nm and a linear transmittance of 30% or less at a wavelength of 550 nm and has a total light transmittance of 90% or more. Here, the total light transmittance is measured using an integrating sphere using BaSO 4  where the total light transmittance of the substrate glass is defined as 100%. Also, the linear light transmittance is measured on the premise that the linear light transmittance of the substrate glass is defined as 100%. 
     The diffusing film has preferably a total light transmittance of 85% or more and a haze value of 50% or more, and more preferably a total light transmittance of 90% or more and a haze value of 80% or more. 
     In the high-pressure discharge lamp of the present invention, the diffusing film preferably has a thickness of 2 to 20 μm. When the thickness of the diffusing film is less than 2 μm, this brings about weak diffusion and a low haze value. When the thickness exceeds 20 μm, the total light transmittance is lowered. 
     The coating liquid for a diffusing film of a high-pressure discharge lamp according to the present invention comprises first silica particles having shapes different in surface curvature from each other and hollow second silica particles as major components, and a silicate polymer as a binder. 
     The coating liquid for a diffusing film of a high-pressure discharge lamp according to the present invention may be produced, for example, by blending the first silica particles and the second silica particles with a silicate polymer obtained by hydrolyzing ethyl silicate, followed by subjecting to dehydration condensation. The diffusing film may be formed by applying the coating liquid to at least one surface of the outer bulb of the high-pressure discharge lamp and drying the coating film, followed by subjecting to heat treatment. The silicate polymer used as the binder is almost converted into silica by the heat treatment. 
     The amount of the binder made of the silicate polymer in the coating liquid is preferably 3 to 20% by weight. When the amount of the binder is less than 3% by weight, the strength of the diffusing film is reduced and therefore, the film tends to be peeled off. When the amount of the binder exceeds 20% by weight, the binder filled in the clearances between silica particles is increased, bringing about low diffusion and a reduction in total light transmittance. 
     Using the coating liquid for a diffusing film of a high-pressure discharge lamp according to the present invention, a diffusing film containing the first silica particles having shapes different in surface curvature from each other and the hollow second silica particles is formed, enabling the production of a diffusing film which satisfactorily diffuses the light emitted from a luminous tube, limits a reduction in diffusion transmittance to suppress a reduction in efficiency and is also improved in light distribution characteristics. 
     Also, since the silicate polymer is added to the coating liquid as the binder, a high-pressure discharge lamp diffusing film less degraded and having sufficient strength can be formed. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a cross-sectional view of a high-pressure discharge lamp according to a first embodiment of the present invention; 
         FIG. 2  is a cross-sectional view of a high-pressure discharge lamp according to a second embodiment of the present invention; 
         FIGS. 3A and 3B  are secondary electron photographic images of the surface and section of the diffusing film formed on the high-pressure discharge lamp of  FIG. 2 ; and 
         FIG. 4  is a diagram showing a light distribution curve of the high-pressure discharge lamp of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the invention will be described with reference to the drawings. 
     First Embodiment 
       FIG. 1  is a schematic cross-sectional view of a high-pressure discharge lamp according to a first embodiment of the present invention. A metal halide lamp  1  as the high-pressure discharge lamp comprises a luminous tube  2 , a glass cylinder  3  enclosing the luminous tube  2 , a reinforcing member (not shown) made of alumina fiber strings wound around the outer periphery of the glass cylinder  3 , an outer bulb  4  housing the luminous tube  2  and the glass tube  3 , a support member  5 , a metal plate member  6  and a feeder unit  7 . 
     The luminous tube  2  comprises a translucent ceramics sealed container  8 , a pair of electrodes (not shown), a pair of current introduction conductors  9   a  and  9   b  and a discharge medium sealed in the sealed container  8 . The current introduction conductor  9   a  disposed on the upper side of the luminous tube  2  is supported by the support member  5  and connected to an internal lead  10   a  through the support member  5 . Also, the current introduction conductor  9   b  disposed on the lower side of the luminous tube  2  is connected to connecting conductors  11  and  12  and also connected to an internal lead  10   b  through the connecting conductors  11  and  12 . The outer bulb  4  is provided with a flare stem  13  at the neck part positioned at the lower part thereof. A diffusing film  14  having a thickness of 4 μm and comprising first silica particles having shapes different in surface curvature from each other and hollow second silica particles is formed on the inner surface of the outer bulb  4 . 
     The diffusing film  14  is formed in the following manner. Hollow silica particles having an average particle size of 3 μm and having 60% by volume of hollow portion inside thereof, for example, are fractured such that the produced particles have an average volume of 5 to 80 vol % with a mill such as a pin mill to provide first silica particles having random curvatures which are aggregates of fine shell fragments. The first silica particles have a non-spherical shape and have a nearly flat shape since they are made of shell fragments and are therefore superior in light diffusing characteristics. Next, the first silica particles are dispersed in water containing dispersant with a bead mill and classified as needed to produce a dispersed product of the first silica particles having random curvatures. Subsequently, hollow second silica particles having an average particle size of 3 μm and containing 60% by volume of hollow portion inside thereof, for example, are prepared. Because the second silica particles have a hollow shape, they act to raise the diffusion transmittance without impairing the light diffusing characteristics. Then, the first and second silica particles are mixed with a silicate polymer obtained by hydrolyzing ethyl silicate followed by subjecting to dehydration condensation, to prepare a coating liquid. Further, the liquid is applied to the inner surface of the outer bulb  4  by flow coating, dried and then heat-treated at about 500° C. to form the diffusing film  14 . At this time, it is preferable to form the diffusing film  14  such that the second silica particles are exposed and, in some cases, projected from the surface of the film. 
     The support member  5  comprises a support frame  15  and a band conductor  16 . The support frame  15  is provided with a falling object-receiving plate  17 . The support frame  15  is made of a stainless steel rod and shaped by bending a part of the rod with its bended portion being connected to the internal lead  10   a . The band conductor  16  is welded to the support frame  15  bridge-wise and the current introduction conductor  9   a  disposed on the upper side (in the drawing) of the luminous tube  2  is welded to the band conductor  16  to arrange the luminous tube  2  at a predetermined position. Reference numeral  18  in the drawing represents a UV enhancer, reference numerals  19   a  and  19   b  represent getters, and reference numeral  20  represents a conductor that connects the external electrode of the metal halide lamp  1  with the connecting conductor  12 . 
     The metal halide lamp of the first embodiment has a structure in which the diffusing film  14 , comprising the first silica particles having shapes different in surface curvature from each other and the hollow second silica particles, is formed on the inner surface of the outer bulb  4 . Therefore, the metal halide lamp properly diffuses the light emitted from the luminous tube  2  and also, suppresses a reduction in light flux to thereby limit a reduction in efficiency, making it possible to improve the light distribution characteristics. 
     Second Embodiment 
       FIG. 2  is a schematic cross-sectional view showing a high-pressure discharge lamp according to a second embodiment of the present invention. 
     Reference numeral  31  in the drawing represents a 150-W metal halide lamp as the high-pressure discharge lamp and comprises a ceramics luminous tube  32 . An inner bulb  33  is disposed around the luminous tube  32  as a transparent protective tube that protects the luminous tube  32 . An E-type cap  34  as a feeder unit which conducts with the luminous tube  32  is attached to the inner bulb  33 . 
     The luminous tube  32  comprises a luminous part  35  and narrow tubes  36   a  and  36   b  extending in directions opposite to each other along the axis of the luminous part  35 . The luminous part  35  is air-tightly sealed and has a discharge space formed inside thereof. In the discharge space, a pair of electrodes (no shown) inserted from the narrow parts  36   a  and  36   b  are disposed opposite to each other. Feeder bodies  37   a  and  37   b  each joined with an electrode at its end are disposed and sealed by a glass frit or the like in the narrow tubes  36   a  and  36   b . A discharge medium made of, for example, a metal halide or rare gas (mercury is added as needed) is sealed in the luminous tube  32 . Power feeder lines  38   a  and  38   b  are electrically connected to the feeder bodies  37   a  and  37   b . A pinch seal  39  that seals the power feeder lines  38   a  and  38   b  is formed on the case side of the inner bulb  33 . 
     The inner bulb  33  is enclosed by a transparent cylindrical outer bulb (cover bulb)  40  with an opened lower end. The lower end of the outer bulb  40  is fixed to a ceramic holder  42  by an outer bulb caulking metal ring  41 . A diffusing film  43 , comprising first silica particles different in curvature from each other and hollow second silica particles, is formed on the inner surface of the outer bulb  40 . The diffusing film  43  is formed in the following manner. Specifically, a coating liquid in which the first silica particles different in curvature from each other and hollow second silica particles having an average particle size of 3 μm are dispersed is applied to the inner surface of the outer bulb, dried and then baked under heating at about 500° C. to form a white and opaque hard film. 
     The formed diffusing film  43  has a thickness of 3 μm at the center in the direction of the tube axis of the outer bulb  40  and has characteristics that the linear transmittance at a wavelength of 300 to 800 nm is 5% to 50% or less, the linear transmittance at a wavelength of 550 nm is 30% or less and the diffusion transmittance is 90% or more. The linear transmittance is measured using an integrating sphere type measuring device using BaSO 4  as the standard sample. 
     Reference numeral  44  in the drawing is a protective tube support that supports the pinch seal part  39  of the inner bulb  33  and is integrated with the ceramic holder  42 . 
     The metal halide lamp of the second embodiment has a structure in which the diffusing film  43 , comprising the first silica particles different in surface curvature from each other and the hollow second silica particles, is formed on the inner surface of the outer bulb  40 . Therefore, the metal halide lamp suppresses a reduction in light flux of emission from the luminous tube  32  to thereby limit a reduction in efficiency, making it possible to improve the light distribution characteristics. 
     Third Embodiment 
     A coating liquid to be used for a diffusing film of a high-pressure discharge lamp according to a third embodiment contains first silica particles having shapes different in surface curvature from each other, hollow second silica particles and a silicate polymer. The liquid is prepared by blending the first silica particles and second silica particles with a silicate polymer obtained by hydrolyzing ethyl silicate followed by subjecting to dehydration condensation. The diffusing film is formed by applying the liquid to the inner surface of outer bulb (inner surface of the lamp), drying the coating film and heat-treating at 500° C. 
     Since the coating liquid of the third embodiment comprises a silicate polymer superior in heat resistance, even if the diffusing film is formed on the inner surface of the outer bulb of the high-pressure discharge lamp heated to a high temperature during lighting, the diffusing film suffers less thermal degradation, is resistant to cracking and is free from problems such as film peeling, keeping sufficient strength. 
     Examples 1 to 16 and Comparative Examples 1 to 15 
     Examples 1 to 16 and Comparative Examples 1 to 15 are shown in Tables 1 and 2 below. With regard to these Examples and Comparative Examples, Table 1 shows the particle size D (μm) of the hollow second silica particles and the volume percentage Vh (vol %) of their hollow portion, the particle size D (μm) of the hollow silica particles which are the raw material of the first silica particles and the volume percentage Vh (vol %) of their hollow portion, the volume ratio (vol %) occupied by the fractured first silica particles, the amount of the first and second silica particles Wp (wt %) mixed in the liquid and the ratio by weight S 2 /S 1  of the second silica particles to the first silica particles. With regard to these Examples and Comparative Examples, Table 2 shows the amount of the silicate polymer Wb (wt %) mixed in the liquid as a binder, the thickness t (μm) of the diffusing film, the total light transmittance (Tt) and haze value (Hz). The haze value was measured using a haze meter (trade name: NDH-2000, manufactured by Nippon Denshoku Industries Co., Ltd.) after a coating liquid to be a diffusing film was applied to a slide glass by flow coating. The haze value is a value measured in a circumstance containing 0% of atmosphere where the diffusing film is separated from the glass substrate. The total light transmittance (Tt) was measured by forming a diffusing film on a glass substrate having the same optical characteristics as the outer bulb used in the above Examples where the total light transmittance of the glass substrate was defined as 100%. The transmittance was measured using an integrating sphere type measuring device using BaSO 4  as the standard sample. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 2nd silica 
                 1st silica 
                 Amount 
                   
               
               
                   
                 particles 
                 particles 
                 of parti- 
                 Weight 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 D 
                 Vh 
                 D 
                 Vh 
                 Vr 
                 cles Wp 
                 ratio 
               
               
                   
                 (μm) 
                 (vol %) 
                 (μm) 
                 (vol %) 
                 (vol %) 
                 (wt %) 
                 S 1 /S 2   
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Ex. 1 
                 3 
                 50 
                 3 
                 50 
                 30 
                 10 
                 5/1 
               
               
                 Ex. 2 
                 2 
                 50 
                 3 
                 50 
                 30 
                 10 
                 5/1 
               
               
                 Ex. 3 
                 5 
                 50 
                 3 
                 50 
                 30 
                 10 
                 5/1 
               
               
                 Ex. 4 
                 3 
                 30 
                 3 
                 50 
                 30 
                 10 
                 5/1 
               
               
                 Ex. 5 
                 3 
                 50 
                 2 
                 50 
                 30 
                 10 
                 5/1 
               
               
                 Ex. 6 
                 3 
                 50 
                 5 
                 50 
                 30 
                 10 
                 5/1 
               
               
                 Ex. 7 
                 3 
                 50 
                 3 
                 30 
                 30 
                 10 
                 5/1 
               
               
                 Ex. 8 
                 3 
                 50 
                 3 
                 50 
                 10 
                 10 
                 5/1 
               
               
                 Ex. 9 
                 3 
                 50 
                 3 
                 50 
                 70 
                 10 
                 5/1 
               
               
                 Ex. 10 
                 3 
                 50 
                 3 
                 50 
                 30 
                 3 
                 5/1 
               
               
                 Ex. 11 
                 3 
                 50 
                 3 
                 50 
                 30 
                 20 
                 5/1 
               
               
                 Ex. 12 
                 3 
                 50 
                 3 
                 50 
                 30 
                 10 
                 1/5 
               
               
                 Ex. 13 
                 3 
                 50 
                 3 
                 50 
                 30 
                 10 
                 10/1  
               
               
                 Ex. 14 
                 3 
                 50 
                 3 
                 50 
                 30 
                 10 
                 5/1 
               
               
                 Ex. 15 
                 3 
                 50 
                 3 
                 50 
                 30 
                 10 
                 5/1 
               
               
                 Ex. 16 
                 3 
                 50 
                 3 
                 50 
                 30 
                 10 
                 5/1 
               
               
                 Comp. 1 
                 1 
                 50 
                 3 
                 50 
                 30 
                 10 
                 5/1 
               
               
                 Comp. 2 
                 8 
                 50 
                 3 
                 50 
                 30 
                 10 
                 5/1 
               
               
                 Comp. 3 
                 3 
                 25 
                 3 
                 50 
                 30 
                 10 
                 5/1 
               
               
                 Comp. 4 
                 3 
                 50 
                 1 
                 50 
                 30 
                 10 
                 5/1 
               
               
                 Comp. 5 
                 3 
                 50 
                 8 
                 50 
                 30 
                 10 
                 5/1 
               
               
                 Comp. 6 
                 3 
                 50 
                 3 
                 25 
                 30 
                 10 
                 5/1 
               
               
                 Comp. 7 
                 3 
                 50 
                 3 
                 50 
                  3 
                 10 
                 5/1 
               
               
                 Comp. 8 
                 3 
                 50 
                 3 
                 50 
                 90 
                 10 
                 5/1 
               
               
                 Comp. 9 
                 3 
                 50 
                 3 
                 50 
                 30 
                 2 
                 5/1 
               
               
                 Comp. 10 
                 3 
                 50 
                 3 
                 50 
                 30 
                 25 
                 5/1 
               
               
                 Comp. 11 
                 3 
                 50 
                 3 
                 50 
                 30 
                 10 
                  1/10 
               
               
                 Comp. 12 
                 3 
                 50 
                 3 
                 50 
                 30 
                 10 
                 15/1  
               
               
                 Comp. 13 
                 3 
                 50 
                 3 
                 50 
                 30 
                 10 
                 5/1 
               
               
                 Comp. 14 
                 3 
                 50 
                 3 
                 50 
                 30 
                 10 
                 5/1 
               
               
                 Comp. 15 
                 3 
                 50 
                 3 
                 50 
                 30 
                 10 
                 5/1 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Amount of 
                 Thickness of 
                   
                   
               
               
                   
                 binder 
                 diffusing film 
                 Tt 
                 Hz 
               
               
                   
                 Wb(wt %) 
                 t(μm) 
                 (%) 
                 (%) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Ex. 1 
                 10 
                 6 
                 95 
                 98 
               
               
                   
                 Ex. 2 
                 10 
                 6 
                 94 
                 76 
               
               
                   
                 Ex. 3 
                 10 
                 6 
                 89 
                 92 
               
               
                   
                 Ex. 4 
                 10 
                 6 
                 93 
                 71 
               
               
                   
                 Ex. 5 
                 10 
                 6 
                 92 
                 79 
               
               
                   
                 Ex. 6 
                 10 
                 6 
                 91 
                 90 
               
               
                   
                 Ex. 7 
                 10 
                 6 
                 92 
                 78 
               
               
                   
                 Ex. 8 
                 10 
                 6 
                 92 
                 63 
               
               
                   
                 Ex. 9 
                 10 
                 6 
                 92 
                 51 
               
               
                   
                 Ex. 10 
                 10 
                 6 
                 95 
                 58 
               
               
                   
                 Ex. 11 
                 10 
                 6 
                 86 
                 91 
               
               
                   
                 Ex. 12 
                 10 
                 6 
                 85 
                 92 
               
               
                   
                 Ex. 13 
                 10 
                 6 
                 93 
                 53 
               
               
                   
                 Ex. 14 
                 20 
                 6 
                 89 
                 61 
               
               
                   
                 Ex. 15 
                 10 
                 2 
                 95 
                 57 
               
               
                   
                 Ex. 16 
                 10 
                 20 
                 85 
                 98 
               
               
                   
                 Comp. 1 
                 10 
                 6 
                 90 
                 48 
               
               
                   
                 Comp. 2 
                 10 
                 6 
                 79 
                 85 
               
               
                   
                 Comp. 3 
                 10 
                 6 
                 87 
                 47 
               
               
                   
                 Comp. 4 
                 10 
                 6 
                 87 
                 44 
               
               
                   
                 Comp. 5 
                 10 
                 6 
                 81 
                 89 
               
               
                   
                 Comp. 6 
                 10 
                 6 
                 86 
                 45 
               
               
                   
                 Comp. 7 
                 10 
                 6 
                 88 
                 37 
               
               
                   
                 Comp. 8 
                 10 
                 6 
                 89 
                 28 
               
               
                   
                 Comp. 9 
                 10 
                 6 
                 95 
                 32 
               
               
                   
                 Comp. 10 
                 10 
                 6 
                 77 
                 87 
               
               
                   
                 Comp. 11 
                 10 
                 6 
                 79 
                 89 
               
               
                   
                 Comp. 12 
                 10 
                 6 
                 90 
                 31 
               
               
                   
                 Comp. 13 
                 25 
                 6 
                 86 
                 43 
               
               
                   
                 Comp. 14 
                 10 
                 1 
                 95 
                 32 
               
               
                   
                 Comp. 15 
                 10 
                 25 
                 76 
                 98 
               
               
                   
                   
               
            
           
         
       
     
       FIGS. 3A and 3B  are secondary electron photographic images of the surface and section of a diffusing film used in the metal halide lamp of  FIG. 2  as the high-pressure discharge lamp. It was found from  FIG. 3  that the hollow second silica particles  102  are dispersed in the first silica particles  101  and projected from the inner surface. In  FIG. 3 , although cracks are produced on the diffusing film by drying aggregation, these cracks have no influence on the performance of the diffusing film. Specifically, even in the case where there are cracks, problems such as film peeling or the like do not arise until the end of film life and the film can keep sufficient strength where the silicate polymer is added to the coating liquid. 
     Also, the almost spherical second silica particles have a hollow portion, and the hollow portion is different from the shell in refractive index. For this reason, the visible light incident on the particles is refracted by the shell and further by the inside hollow portion to thereby obtain diffusing characteristics. For the same reason, the second silica particles have a hollow portion in the diffusing film and therefore, they have a higher diffusion transmittance than silica particles having no hollow portion. Thus, in the metal halide lamp in which the diffusing film is formed, reduction in initial light flux can be prevented. Moreover, the first silica particles and the second silica particles are made of the same metal oxides and therefore have the same refractive index, making it possible to reduce the optical loss caused by light interference and difference in refractive index. 
       FIG. 4  is a characteristic diagram showing the light distribution of the metal halide lamp shown in  FIG. 2  when the lamp is turned on at a vertical position where the cap is set upward. In  FIG. 4 , the symbol A (dotted line) shows the light distribution characteristics of the metal halide lamp (present invention) in which the diffusing film is formed on the inner surface of the outer bulb, and the symbol B (solid line) shows the light distribution characteristics of the conventional metal halide lamp in which no diffusing film is formed.  FIG. 4  shows that, since the diffusing film is formed on the inner surface of the outer bulb in the metal halide lamp of the present invention, the light emitted from the luminous tube is diffused and therefore, smooth and uniform light distribution characteristics are obtained. 
     The relative initial light flux of the metal halide lamp when the lamp was turned on at a vertical position where the cap is set upward was measured. Where the initial light flux of a lamp in which no diffusing film was formed on the inner surface of the outer bulb was defined as 100%, the initial light fluxes of the lamps in which a diffusing film was formed in a thickness of 2.5 μm, 3.5 μm and 5.8 μm on the inner surface of the outer bulb were 99.0%, 98.3% and 98.1%, respectively. It was confirmed from this test that the initial light flux of the lamp provided with the diffusing film formed on the inner surface of the outer bulb was reduced by less than 2% compared to the initial light flux of the lamp provided with no diffusing film and therefore, a reduction in efficiency could be prevented. 
     The present invention is not limited to the above embodiments and the constituting elements of these embodiments may be modified in practical stages within the scope and the spirit of the present invention. Specifically, in the above embodiments, the diffusing film is formed on the inner surface of the outer bulb (cover bulb). However, the diffusing film may be formed on the outer surface or on each of the inner and outer surfaces. Also, the thickness of the diffusing film and the ratio of the second and first silica particles to be blended are not limited to the above range as long as the efficiency and light distribution characteristics are satisfactorily kept.