Abstract:
A projection type cathode ray tube which has excellent optical characteristics and endurance thanks to the enhanced cooling effect and is manufactured efficiently at low cost. The thickness of a panel is uniform throughout the panel except its peripheral portion contacting a coupler (a cooling liquid holder). The cooling effect on the panel by a cooling system is uniform throughout the entire panel, enhancing the optical characteristics and endurance of the panel. In the manufacturing process applying heat to the panel, a reduction in the cooling temperature and the cooling time causes no difference in temperature, achieving uniform cooling. The panel except its peripheral portion is a curved portion in the concave form. For the interest of higher adhesion, it is sufficient to polish only the peripheral portion adhering to the edge of the opening on the rear side of the coupler, which reduces the polishing time.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to a projection type cathode ray tube used in, for example, a projector for projecting a picture on a screen.  
           [0003]    2. Description of the Related Art  
           [0004]    A projector in accordance with the related art comprises, as shown in FIG. 1A and FIG. 1B, single-color projection type cathode ray tubes  101  arranged side by side for producing red (R), green (G) and blue (B) images. In the front face of each of the cathode ray tubes  101 , provided is a projection lens  102 . In the projector, each of the cathode ray tubes  101  produces a picture, which is enlarged by the projection lens  102  for image formation on a large-size screen  103  forward of the projector. The projector thus achieves color display on a large-size screen. FIG. 1A and FIG. 1B illustrate a projector of the front projection type in which a picture is projected on the front face of the screen  103 ; some projectors, however, are of the rear projection type in which a picture is projected on the rear face of a screen via mirrors.  
           [0005]    [0005]FIG. 2 illustrates the structure of a common projection type cathode ray tube used in the projector in accordance with the related art. The projection type cathode ray tube requires high intensity for the purpose of projecting a picture with sufficient intensity on the large-size screen  103  (see FIG. 1A and FIG. 1B). It is therefore necessary to operate the main body  110  of the cathode ray tube at a high voltage to emit an electron beam having high energy from a high intensity electron gun (riot shown) toward a panel  111 , on which a fluorescent screen  112  is formed. However, this makes the panel  111  in the form of a glass plate hot and causes thermal displacement, resulting in a deterioration in optical characteristics and the degradation of endurance of a device. Moreover, the possibility arises that the panel  111  will be broken.  
           [0006]    To avoid this problem, the cathode ray tube comprises a cooling system  120  on a front surface  111   a  of the panel  111 . The purpose of the cooling system  120  is to cool the panel  111 , which has become hot. The cooling system  120  comprises a cooling liquid holder  121 , which is referred to as a coupler, mounted on the front surface  111   a  of the panel  111 . The hermetic seal between the cooling liquid holder  121  and the panel  111  is formed by an adhesive  130  such as silicone rubber. A cooling liquid  122  having a refractive index equal to the refractive index of glass is sealed in the cooling liquid holder  121 . Thus, an opening  121   a  on the rear side of the cooling liquid holder  121  is blocked by the front surface  111   a  of the panel  111  of the main body  110  of the cathode ray tube. On the front side of the cooling liquid holder  121 , a concave incidence lens  140  is mounted with a hermetic seal formed by an O ring  150 , and thereby, another opening on the front side of the cooling liquid holder  121  is blocked.  
           [0007]    On the other hand, in an attempt at minimizing the projection distance to meet the demands for a reduction in the size and depth dimension of a projector, the inner surface  111   b  of the panel  111  of the main body  110  of the cathode ray tube is a curved surface (or spherical surface) radiused inwardly. The curved surface of the inner surface  111   b  offers orientation with respect to the light, thereby serving as a lens. This results in a reduction in the focal length of the entire lens system including the projection lens  102  (see FIG. 1A and FIG. 1B).  
           [0008]    [0008]FIG. 3 illustrates in section the panel  111  processed in accordance with the related art. During the process of formation of the panel  111 , in the step of exhausting the inside of the panel  111  to a vacuum, the center portion of the front surface  111   a  is pressure-formed into the recessed portion  111   c  curved inwardly. Then, in the step of sealing the front surface (outer surface)  111   a  of the panel  111  with the rear side of the cooling liquid holder  121  using the adhesive  130  (see FIG. 2), the entire front surface  111   a  is polished and planarized for enhancing its adhesion.  
           [0009]    The processing of the panel  111  as described above causes some problems as follows. Firstly, the inner surface  111   b  of the panel  111  of the main body  110  of the cathode ray tube is a curved surface, but the front surface  111   a  is a planar surface. The thickness of the panel  111  is large in the center portion and small in the peripheral portion. The difference between the thickness of the panel  111  in the center portion and in the peripheral portion is considerably large. Therefore, in the center portion of the panel  111  with a large thickness, sufficient cooling effect of the cooling system  120  is not achieved. This causes insufficient intensity and the degradation of the fluorescent screen  112 , resulting in problems such as a deterioration in optical characteristics and the degradation in endurance of a device.  
           [0010]    Secondly, in the step of attaching the rear side of the cooling liquid holder  121  to the front surface  111   a  of the panel  111  of the main body  110  of the cathode ray tube using the adhesive  130  such as silicone rubber, it is necessary to heat silicone rubber or the like to high temperatures for setting with ultraviolet radiation. Since the panel  111  is also heated in this step, it is necessary to cool the panel  111 . However, because of the considerably large difference between the thickness of the panel  111  in the center portion and in the peripheral portion, the rapid cooling of the panel  111  causes a difference in temperature, resulting in the possibility that strain will occur inside the glass-made panel  111  and the panel  111  will be broken. It is possible to avoid this problem by cooling the panel  111  slowly instead of rapid cooling. However, slow cooling requires long cooling time, and therefore decreases the manufacturing efficiency.  
           [0011]    Thirdly, the panel  111  is also heated in the step of exhausting the inside of the main body  110  of the cathode ray tube to a vacuum and in the step of glass fusing the panel  111  of the main body  110  of the cathode ray tube to a funnel  113 . Thus, long cooling time is further required, causing a similar problem of poor manufacturing efficiency.  
           [0012]    Moreover, in the projector in accordance with the related art, in the step of sealing the front surface (outer surface)  111   a  of the panel  111  with the rear side of the cooling liquid holder  121  by adhesion, it is necessary to polish the front surface  111   a  of the panel  111  for enhancing its adhesion. Since the front surface  111   a  after pressure-forming is a substantially plane surface except for the recessed portion  111   c  in the center portion, it is necessary to polish the entire front surface  111   a . This requires a number of man-hours for processing, causing a problem of poor manufacturing efficiency and high manufacturing cost.  
           [0013]    An example of attempts to solve such a problem is Publication of Japanese Unexamined Patent Application No. Hei 5-13023 proposing a cathode ray tube having a panel with a uniform thickness. However, the proposal does not involve sufficient consideration in applying the proposal to an actual projection type cathode ray tube; for example, little thought is given to cooling by the cooling system. Moreover, according to the proposal, the thickness of the panel is uniform across the panel, without special consideration in polishing the front surface (outer surface) of the panel. Therefore, the proposal fails to provide satisfactory solution to the problem such as poor manufacturing efficiency and high manufacturing cost.  
         SUMMARY OF THE INVENTION  
         [0014]    The invention has been made to overcome the foregoing problems. An object of the invention is to provide a projection type cathode ray tube which exhibits excellent optical characteristics and endurance by enhancing the cooling effect and is manufactured with high manufacturing efficiency at low manufacturing cost.  
           [0015]    A projection type cathode ray tube in accordance with the invention comprises: a main body comprising a panel on which at least phosphor is provided, the panel being formed in the inwardly curved form with a uniform thickness; and a cooling system which has an opening facing the panel, and is filled with a cooling liquid for cooling the panel so that the cooling system makes contact with the panel through the opening.  
           [0016]    In a projection type cathode ray tube in accordance with the invention, the thickness of the panel of the main body of the cathode ray tube is almost uniform across the panel except for the portion which is in contact with the sealing member. Therefore, the uniform cooling effect on the panel by the cooling liquid holder is obtained. Furthermore, in cooling the panel heated in the manufacturing process, lowering the cooling temperature to reduce the cooling time causes no difference in temperature, which enables uniform cooling. In addition, only the portion of the front surface of the panel which is in contact with the sealing member is polished, which contributes a reduction in the polishing time and the number of man-hours for processing.  
           [0017]    Other and further objects, features and advantages of the invention will appear more fully from the following description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1A is a plan view of a projector in accordance with the related art.  
         [0019]    [0019]FIG. 1B is a side elevational view of the projector in accordance with the related art.  
         [0020]    [0020]FIG. 2 is a sectional view of part of a projection type cathode ray tube in accordance with the related art.  
         [0021]    [0021]FIG. 3 is a sectional view for illustrating processing of a panel of the projection type cathode ray tube shown in FIG. 2 in accordance with the related art.  
         [0022]    [0022]FIG. 4 is a sectional view of part of a projection type cathode ray tube in accordance with the invention.  
         [0023]    [0023]FIG. 5 is a sectional view for illustrating processing of a panel of the projection type cathode ray tube shown in FIG. 4. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    Preferred embodiments of the invention will now be described in detail below with reference to the drawings.  
         [0025]    As shown in FIG. 4, a projection type cathode ray tube  1  in accordance with the embodiment comprises a main body  10  of the cathode ray tube, a cooling system  20  and an incidence lens  40 . The cooling system  20  is mounted on the side of the front surface (outer surface)  11   a  of the main body  10  of the cathode ray tube. The incidence lens  40  is provided on the front side of the cooling system  20 .  
         [0026]    The main body  10  of the cathode ray tube is assembled by fusing a panel  11 , a funnel  13  and a neck (not shown). The panel  11  is made of glass. The funnel  13  is in the form of a funnel and is made of a glass tube. The neck is in the cylindrical form and is made of a glass tube. The neck incorporates electron guns (not shown) for emitting electron beams for producing red, green and blue images based on color signals outputted from a color signal output circuit. The inside of the main body  10  of the cathode ray tube is in a high vacuum.  
         [0027]    The panel  11  is pressure-formed as shown in FIG. 5. A front surface (outer surface)  11   a  except for a peripheral portion  11   d  is a curved surface (or spherical surface) having a curvature equal to the curvature of an inwardly-radiused curved surface (or spherical surface) of an inner surface  11   b . The front surface (outer surface)  11   a  except for the peripheral portion  11   d  forms a curved portion  11   e  in the concave form. However, the outer surface  11   a  and the inner surface  11   b  may have different curvatures. In the curved potion  11   e , the outer surface  11   a  of the panel  11  has a uniform glass thickness. The thickness of the panel  11  is in the order of, for example, 4 to 11 mm in consideration of prevention against explosion. It is to be noted that the outer surface  11   a  is easily pressure-formed into the curved portion  11   e  in the concave form as in the embodiment, since the center portion of the outer surface  111   a  (see FIG. 3) is also in the concave form in the related art. The form of the panel  11  is not adversely influenced when the panel  11  is taken off from a mold.  
         [0028]    On the inner surface  11   b  of the panel  11 , formed is a single-color fluorescent screen  12  for producing a red, green or blue image. The fluorescent screen  12  is formed by, for example, precipitation method. The fluorescent screen  12  emits light when struck by an electron beam emitted from an electron gun (not shown). The purpose of forming the inner surface  11   b  as a curved surface is, as in the related art, to reduce the focal length.  
         [0029]    The peripheral portion lid of the front surface  11   a  of the panel  11  is a substantially planar surface. The width of the peripheral portion  11   d  is in the order of, for example, 6 to 10 mm. Since the peripheral portion  11   d  of the panel  11  adheres to and is sealed with a cooling liquid holder  21  in the cooling system  20 , the peripheral portion  11   d  is polished using, for example, cerium oxide for enhancing its adhesion. The detailed description of the cooling liquid holder  21  and the cooling system  20  will be given below.  
         [0030]    The cooling system  20  comprises the cooling liquid holder  21  and a cooling liquid  22 . The cooling liquid holder  21  is a holder for a cooling liquid. The cooling liquid  22  fills and is sealed in the cooling liquid holder  21 . The cooling liquid holder  21  is made by, for example, die casting of alloys of, for example, aluminum and zinc. The cooling liquid holder  21  is hollow. The cooling liquid holder  21  is in the form of a frame body. On the rear side of the cooling liquid holder  21 , formed is an opening  21   a . The opening  21   a  has a diameter a little smaller than the outside dimensions of the panel  11 . On the front side of the cooling liquid holder  21 , another opening  21   b  is formed. The opening  21   b  has a diameter almost equal to the lens diameter of the incidence lens  40 . The cooling liquid  22  is a clear liquid having a refractive index almost equal to the refractive index of glass; for example, a mixed solution of ethylene glycol and glycerin.  
         [0031]    The incidence lens  40  is a concave lens comprised of a spherical plate curved inwardly in the form of a hemisphere. The incidence lens  40  is positioned at the latest stage of the projection lens system comprising a plurality of lenses (not shown).  
         [0032]    To the front surface (outer surface)  11   a  of the panel  11  of the main body  10  of the cathode ray tube, attached is the rear side of the cooling liquid holder  21 . The polished peripheral portion  11   d  of the front surface  11   a  of the panel  11  adheres to the edge of the opening  21   a  on the rear side of the cooling liquid holder  21  using the adhesive  30  as a sealing member. The adhesive  30  is made of silicone resin such as silicone rubber. The adhesive  30  sets by, for example, ultraviolet radiation to make completely absolute contact between the peripheral portion lid of the outer surface  11   a  of the panel  11  and the edge of the opening  21   a  on the rear side of the cooling liquid holder  21 , thereby forming a hermetic seal therebetween. Thus, the opening  21   a  on the rear side of the cooling liquid holder  21  is blocked by the front surface  11   a  of the panel  11 .  
         [0033]    To the front side of the cooling liquid holder  21 , attached is the incidence lens  40 . A lens securing plate  41  in the ring form is fixed with screws to the mounted position (not shown) formed on the front side of the cooling liquid holder  21 . Thereby, the incidence lens  40  is sealed with and mounted on the front side of the cooling liquid holder  21  with an O ring  50  in between. Thus, the opening  21   b  on the front side of the cooling liquid holder  21  is blocked by the incidence lens  40 .  
         [0034]    As noted above, both the front side and the rear side of the cooling liquid holder  21  is hermetically sealed, and the inside space thus sealed is filled with the cooling liquid  22 . The cooling liquid  22  changes in viscosity when the panel  11  becomes hot due to an electron beam having high energy emitted from a high intensity electron gun (not shown), causing a change in temperature. The changes in viscosity and temperature cause natural convection of the cooling liquid  22  in the cooling liquid holder  21 . Thus, the cooling liquid  22  cools the panel  11  and makes the temperature uniform. In the meanwhile, the cooling liquid  22  serves as a medium for applying heat to the entire cooling liquid holder  21 . The cooling liquid holder  21  thus heated dissipates heat as a heat sink. Moreover, the cooling liquid  22  is a clear liquid having a refractive index substantially equal to the refractive index of glass. The cooling liquid is, therefore, capable of suppressing reflection at the interface between the panel  11  and the incidence lens  40 , and thereby enhancing the contrast of the picture.  
         [0035]    As described above, in accordance with the projection type cathode ray tube of the embodiment, the thickness of the panel  11  of the main body  10  of the cathode ray tube is uniform except for the peripheral portion  11   d  where the panel  11  adheres to the cooling liquid holder  21 . This enables the cooling system  20  to cool the panel  11  uniformly, enhancing the cooling effect. Even if the panel  11  is irradiated with electron beams having high energy, the uniform and sufficient cooling effect is obtained, and therefore, the degradation of the fluorescent screen  12  is minimized. This makes it possible, for example, to project a picture of higher intensity, enabling the enhancement of the optical characteristics and endurance of the panel  11 .  
         [0036]    The thickness of the panel  11  of the main body  10  of the cathode ray tube is uniform except for the peripheral portion lid. This produces another beneficial effect as follows. The manufacturing process involves, for example, a step of attaching the rear side of the cooling liquid holder  21  to the front side  11   a  of the panel  11  with the adhesive  30  such as silicone rubber, or a step of exhausting the inside of the main body  10  of the cathode ray tube to a vacuum, or a step of fusing the panel  11 , the funnel  13  and the neck (not shown) of the main body  10  of the cathode ray tube. The panel  11  is heated in these steps in the manufacturing process. In cooling the panel  11  heated in these steps, lowering the cooling temperature to reduce the cooling time causes no difference in temperature, achieving uniform cooling. This enables an enhancement in the manufacturing efficiency.  
         [0037]    Furthermore, the front surface  11   a  of the panel  11  except for the peripheral portion  11   d , that is, the curved portion  11   e , is in the concave form. Therefore, higher adhesion is obtained by polishing only the peripheral portion  11   d , where the panel  11  adheres to the edge of the opening  21   a  on the rear side of the cooling liquid holder  21 . This enables a reduction in the polishing time, a reduction in the number of man-hours for processing, an enhancement of the manufacturing efficiency, and a cost reduction. The curved portion  11   e  remains unpolished and constitutes part of the optical system. However, this causes no problems or influences because slight projections or recesses in the front surface  11   a  of the panel  11  is filled in with the cooling liquid  22  having a refractive index substantially equal to the refractive index of glass, and because a picture is formed on a screen forward of the front surface  11   a.    
         [0038]    The invention has been described by referring to the embodiment above. However, the invention is not limited to the above-described embodiment but various changes and modifications are possible. For example, the above-described embodiment adopts the silicone sealing technique in which the cooling system  20  is mounted on the main body  10  of the cathode ray tube with the adhesive  30  as a sealing member made of silicone resin such as silicone rubber. However, it may be also possible to adopt the mechanically sealing technique in which the cooling system  20  and the main body  10  of the cathode ray tube are arranged face to face with a sealing member such as an O ring in between and then closely attached to each other by pressing them with a flat spring in the case of adopting the mechanically sealing technique, the portion of the front surface  11   a  which contacts a sealing member such as an O ring constitutes the peripheral portion  11   d  to be polished.  
         [0039]    Moreover, the above-described embodiment utilizes optical coupling in which the opening  21   b  on the front side of the cooling liquid holder  21  is blocked by the incidence lens  40 . However, it may be possible to utilize air coupling in which the opening  21   b  on the front side of the cooling liquid holder  21  is blocked by a glass plate.  
         [0040]    Furthermore, in the above-described embodiment, the curvature of the curved portion  11   e  of the front surface  11   a  of the panel  11  is equal to the curvature of the curve of the inner surface  11   b  so that the thickness of the panel  11  is uniform. However, it is not always necessary for the curvature of the curved portion  11   e  to be equal to the curvature of the curve of the inner surface  11   b , so long as the thickness of the panel  11  is substantially uniform.  
         [0041]    In addition, in the above-described embodiment, the entire front surface  11   a  of the panel  11  except for the peripheral portion  11   d  forms the curved portion  11   e . However, it is not always necessary for the entire front surface  11   a  of the panel  11  except for the peripheral portion  1   d  to form the curved portion  11   e . For example, for the interest of prevention of the degradation of the fluorescent screen  12 , it is sufficient for at least the portion of the front surface  11   a  which faces the portion of the inner surface  11   b  where the fluorescent screen  12  is formed to form the curved portion  11   e.    
         [0042]    Furthermore, the invention is applicable not only to a projector of the front projection type in which a picture is projected on the front face of a screen but also to a projector of the rear projection type in which a picture is projected on the rear face of a screen via mirrors.  
         [0043]    Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.