Abstract:
Provided are a lens barrel part and a method of producing the same, such that the generation of grinding debris due to friction with lenses is suppressed while maintaining the air permeability of porous ceramics. Those areas of a lens barrel part  1  which are in contact with the lenses are ground in the water by a polishing tool  25,  with the result that in those regions of surfaces in contact with the lenses where fine projections exist, protection film portions  28  consisting of hydrates or oxides are created due to the generation of tribochemical reactions, and that in those regions of surfaces in contact with the lenses which are other than the regions where fine projections are present, there exist porous portions  29  where no tribochemical reaction occurs and where a porous state remains.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a lens barrel part into which lenses are incorporated and a method of producing the same and particularly to a lens barrel part made of porous ceramic and a method of producing the same. The present invention also relates to a lens assembly and an imaging device provided with such lens barrel part. 
       BACKGROUND ART 
       [0002]    Imaging devices used in, for example, portable telephones and automobiles are required to have excellent performance stability and durability at high and low temperatures, and other harsh environmental conditions. Accordingly, Patent Literatures 1 and 2, for example, propose using a ceramic lens barrel. Patent Literature 3 proposes using porous ceramic excelling in air permeability and durability for optical parts. 
         [0003]    A lens barrel part generally comprises, as described in, for example, Patent Literature 4, a lens barrel body containing a plurality of lenses, spacing rings for defining the distances between the lenses in the lens barrel body, and a holder ring provided at an end of the lens barrel body. The lenses incorporated in the lens barrel part are in contact with the lens barrel body and thus secured to ensure that the optical axes and focal points of the lenses remain in alignment. On the other hand, a lens barrel part of porous ceramic generally has fine asperities on the surfaces thereof. Therefore, a problem may arise that when, for example, lenses are placed in the lens barrel part, the friction between lenses and the part produces grinding debris from the lens barrel part. 
         [0004]    On the other hand, Patent Literature 5, for example, proposes coating the whole surface of a ceramic with a protection film by tribochemical reaction to smooth the surface. 
       CITATION LIST 
     Patent Literature 
       [0005]    Patent Document 1: JP 2006-284991 A 
         [0006]    Patent Document 2: JP 2006-292927 A 
         [0007]    Patent Document 3: JP 2007-238430 A 
         [0008]    Patent Document 4: JP 2007-279557 A 
         [0009]    Patent Document 5: JP 2003-145416 A 
       SUMMARY OF THE INVENTION 
     Technical Problems 
       [0010]    However, coating the whole surface of a lens barrel part to smooth out the asperities of the surface restricts air permeability and hence increases the temperature difference between inside and outside of a lens assembly where lenses are incorporated in the lens barrel part, causing moisture to form on the inside thereof. 
         [0011]    The present invention has been made in order to solve the above problems in the art and has an object of providing a lens barrel part and a method of producing the same wherein the air permeability of porous ceramic is maintained while generation of grinding debris due to friction with the lenses is restrained. 
         [0012]    The present invention also has an object of providing a lens assembly and an imaging device comprising such a lens barrel part. 
       Solution To Problems 
       [0013]    To achieve the above objects, a lens barrel part according to the present invention is a lens barrel part made of porous ceramic with lens contact surfaces, wherein the lens contact surfaces including: protection film portions made of a hydrate or an oxide generated in a tribochemical reaction caused by polishing the lens contact surfaces with a polishing tool in water; and porous portions that are not subjected to the tribochemical reaction and therefore remain porous. 
         [0014]    The protection film portions preferably have a surface area of 20% to 80% of the lens contact surfaces. 
         [0015]    The protection film portions may be generated by polishing the lens contact surfaces with the polishing tool having a surface roughness of 3 μm or less. 
         [0016]    The lens barrel part is preferably made of silicon nitride. 
         [0017]    The lens barrel part comprises a lens barrel body containing a plurality of lenses, at least one spacing ring for defining spacings between the lenses in the lens barrel body, and a holder ring located at an end of the lens barrel body, wherein each of lens contact surfaces of the lens barrel body, the at least one spacing ring, and the holder ring may have the protection film portions and the porous portions. 
         [0018]    A lens assembly of the invention has lenses incorporated in the above-mentioned lens barrel part. 
         [0019]    The imaging device of the invention comprises the lens assembly described above. 
         [0020]    A method of producing a lens barrel part according to the invention is a method of producing a lens barrel part made of porous ceramic with lens contact surfaces, the method comprising polishing the lens contact surfaces with a polishing tool in water and thereby, in fine projection areas of the lens contact surfaces, inducing a tribochemical reaction caused by friction heat that accompanies the polishing to generate protection film portions made of a hydrate or an oxide, while in other areas than the fine projection areas of the lens contact surfaces, inducing no tribochemical reaction to allow the other areas to be porous portions remaining porous. 
         [0021]    The polishing tool may have a smaller surface roughness than those of the lens contact surfaces. 
         [0022]    The polishing tool may have a surface roughness of 3 μm or less. 
       Advantageous Effects of Invention 
       [0023]    The present invention enables the air permeability of porous ceramic to be maintained while generation of grinding debris due to friction with lenses is restrained. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0024]      FIG. 1  is a cross sectional view illustrating a configuration of a lens barrel part according to an embodiment of the invention. 
           [0025]      FIG. 2  is a cross sectional view illustrating a configuration of a protection film producing device. 
           [0026]      FIG. 3  is an enlarged view of a lens barrel part where protection film portions have been formed by the protection film producing device. 
           [0027]      FIG. 4  is a cross sectional view showing how a lens assembly is completed. 
           [0028]      FIG. 5  is a cross sectional view illustrating a configuration of an imaging device. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0029]    The present invention is described in detail below with reference to preferred embodiments illustrated in the attached drawings. 
         [0030]      FIG. 1  illustrates a lens assembly  10  wherein four lenses L 1  to L 4  are incorporated in a lens barrel part  1 . The lens barrel part  1  comprises a lens barrel body  2  housing the lenses L 1  to L 4  in this order, spacing rings  6   a  to  6   c  defining the distances between the lenses L 1  to L 4  in the lens barrel body  2 , and a holder ring  7  provided on the outer periphery of the front end of the lens barrel body  2 . 
         [0031]    The lens barrel body  2  has the shape of a hollow cylinder with open ends, with a rear end opening  3  on the rear end (image forming side) and a front end opening  4  on the front end (side closer to a subject). The lens barrel body  2  has a configuration such that the inner periphery of the rear end portion is in contact with the outer periphery of the lens L 1 , the inner periphery of the middle portion is in contact with the outer periphery of the lenses L 2  and L 3 , and the inner periphery of the front end portion is in contact with the outer periphery of the lens L 4 . The lenses L 2  and L 3  have a larger diameter than the lens L 1 , and the lens L 4  has a larger diameter than the lenses L 2  and L 3 . Accordingly, the rear end portion, the middle portion, and the front end portion of the lens barrel body  2  have different inner diameters and are formed concentrically to align the optical axes of the lenses L 1  to L 4 . 
         [0032]    The rear end opening  3  of the lens barrel body  2  has a rim ridge  5  having a smaller inner diameter than the diameter of the lens L 1  and supporting the lens L 1  from behind the lens L 1 . The front end opening  4  of the lens barrel body  2  has an inner diameter corresponding to the diameter of the lens L 4  and is so formed as to allow sequential insertion of the lenses L 1  to L 4 . 
         [0033]    The spacing rings  6   a  to  6   c  are inserted between the lenses L 1  to L 4  incorporated in the lens barrel body  2 . The spacing rings  6   a  to  6   c  have an annular shape, and the outer peripheries thereof are in contact with the inner periphery of the lens barrel body  2 . 
         [0034]    The rear and the front surfaces of the spacing ring  6   a  are in contact with the lenses L 1  and L 2  respectively to define the spacing between them. The rear and the front surfaces of the spacing ring  6   b  are in contact with the lenses L 2  and L 3  respectively to define the spacing between them. The rear and the front surfaces of the spacing ring  6   c  are in contact with the lenses L 3  and L 4  respectively to define the spacings between them. 
         [0035]    On the outer periphery of the front end of the lens barrel body  2 , there is provided the holder ring  7  having a cylindrical shape. The front end portion of the holder ring  7  has a rim ridge  8  having a smaller inner diameter than the diameter of the lens L 4  and securing the lens L 4  from the front side of the lens L 4 . The holder ring  7  is secured to the lens barrel body  2  by, for example, threaded or fitted engagement. 
         [0036]    The lens barrel part  1  (lens barrel body  2 , spacing rings  6   a  to  6   c,  and holder ring  7 ) are made of porous ceramic such as, for example, silicon nitride and have lens contact surfaces including the inner periphery of the lens barrel body  2  in contact with the outer peripheries of the lenses L 1  to L 4 , the front and the rear surfaces of the spacing rings  6   a  to  6   c  in contact with the front and the rear surfaces of the lenses L 1  to L 4 , the rim ridge  5  of the lens barrel body  2 , and the rim ridge  8  of the holder ring  7 . Porous ceramic herein is ceramic having a porosity of 10% or more. 
         [0037]      FIG. 2  illustrates a configuration of the protection film producing device  21  for forming protection film portions on the lens contact surfaces of the lens barrel part  1 . The protection film producing device  21  comprises a tank  22  storing a reaction liquid R such as water for causing a tribochemical reaction. 
         [0038]    In the reaction liquid R in the tank  22 , there is provided a member fastener means  23  for fastening the lens barrel body  2 . The member fastener means  23  is connected via a drive shaft  24  to a rotation actuator  25  for turning the member fastener means  23  at a given rotation speed in a given direction. In the reaction liquid R in the tank  22 , there is provided a polishing tool  26  for polishing the lens contact surfaces of the lens barrel body  2 . The polishing tool  26  is made of, for example, ceramic or metal having a higher hardness than the lens barrel body  2 . The surface roughness of the polishing tool  26  is smaller than those of the lens contact surfaces of the lens barrel body  2  and may be, for example,  3  pm or less. The polishing tool  26  is connected via a drive shaft  27  to a rotation actuator  28 . The rotation actuator  28  moves the polishing tool  26  vertically and turns it at a given speed in the opposite direction to the rotation direction of the member fastener means  23 . 
         [0039]    First, after the lens barrel body  2  is secured to the member fastener means  23  in the tank  22 , the reaction liquid R is supplied into the tank  22  so that the lens barrel body  2  is filled therewith. The polishing tool  26  is positioned so as to be in contact with the lens contact surfaces of the lens barrel body  2 , then the rotation actuator  25  turns the member fastener means  23  at a given speed whereas the rotation actuator  28  turns the polishing tool  26  at a given speed in the opposite direction to the rotation direction of the member fastener means  23 . Thus, the lens contact surfaces of the lens barrel body  2  made of silicon nitride are polished by the polishing tool  26  in the reaction liquid. Friction heat generated by polishing causes a tribochemical reaction whereby silicon generated on the surface of the lens barrel body  2  reacts with the surrounding reaction liquid R to produce a protection film made of a hydrate (Si—OH) and an oxide (Si—O). 
         [0040]    The polishing tool  26 , having a smaller surface roughness than the lens contact surfaces of the lens barrel body  2 , grinds only the projections of the fine asperities of the lens barrel body  2 . Therefore, the protection film is generated only with fine projections of the lens barrel body  2 . The protection film generated is crushed by the polishing tool  26  to form smoothed-out protection film portions in given areas of fine projections. Thus, the fine projection areas of the lens barrel body  2  can be smoothed out. The other areas than the fine projection areas of the lens barrel body  2 , free from tribochemical reaction, are porous portions made of silicon nitride remaining porous. Since the porosity remains, the lens barrel body  2  has air permeability as well. 
         [0041]    Moving the polishing tool  26  over the lens contact surfaces of the lens barrel body  2  vertically and turning the member fastener means  23  about the drive shaft  24  enable lens contact surfaces having protection film portions and porous portions to be obtained over the whole inner periphery of the lens barrel body  2 . 
         [0042]    Likewise, causing a tribochemical reaction on the lens contact surfaces of the spacing rings  6   a  to  6   c  and the holder ring  7  using a polishing tool having a smaller surface roughness than the lens contact surfaces makes it possible to obtain lens contact surfaces on the spacing rings  6   a  to  6   c  and the holder ring  7  wherein protection film portions are generated in fine projection areas while the other areas than the fine projection areas are porous portions. 
         [0043]      FIG. 3  shows a photograph of a lens contact surface of the lens barrel part  1  having protection film portions  29  and porous portions  30 . The photograph shows protection film portions formed on a lens contact surface of the lens barrel part  1  using the protection film producing device  21  enlarged under a scanning electron microscope at a magnification of 1000 times. Thus, protection film portions are produced in the porous areas formed with fine projections, and the other areas than the fine projection areas are porous portions remaining porous. 
         [0044]    The ratio in surface area of the protection film portions  29  to the lens contact surfaces of the lens barrel part  1  can be controlled depending on the surface roughness of the polishing tool  26 , a higher ratio is obtained with the polishing tool  26  having a larger surface roughness, and a lower ratio with the polishing tool  26  having a smaller surface roughness. To restrain generation of grinding debris while maintaining air permeability of the lens barrel part  1 , it is desirable to provide protection film portions  29  having a surface area of preferably 20% to 80% of the lens contact surfaces. 
         [0045]    Next, a description will be made to explain how the lens assembly  10  illustrated in  FIG. 1  is completed using the lens barrel body  2  where the lens contact surfaces are formed with protection film portions and porous portions, the spacing rings  6   a  to  6   c,  and the holder ring  7 . 
         [0046]    First, as illustrated in  FIG. 4 , the lens L 1  is inserted from the front end opening  4  of the lens barrel body  2  and moved, with the outer periphery of the lens L 1  kept in contact with the protection film portions formed on the inner periphery of the lens barrel body  2  at a given ratio, until the rear surface of the lens L 1  comes into contact with the rim ridge  5  of the lens barrel body  2 . Protection film portions are also formed on the rim ridge  5  at a given ratio and the rim ridge  5  is in contact with the rear surface of the lens L 1  through the protection film portions. Next, the spacing ring  6   a  is inserted from the front end opening  4  of the lens barrel body  2  and moved, with the outer periphery of the spacing ring  6   a  kept in contact with the protection film portions formed on the inner periphery of the lens barrel body  2 , until the rear surface of the spacing ring  6   a  comes into contact with the front surface of the lens L 1 . The spacing ring  6   a  is now in contact with the lens L 1  through the protection film portions formed at a given ratio on the rear surface thereof. Likewise, the lens L 2 , the spacing ring  6   b,  the lens L 3 , the spacing ring  6   c,  and the lens L 4  are sequentially inserted from the front end opening  4  of the lens barrel body  2  and moved until the front surface of the spacing ring  6   a  and the rear surface of the spacing ring  6   b  come into contact with the lens L 2 , the front surface of the spacing ring  6   b  and the rear surface of the spacing ring  6   c  come into contact with the lens L 3 , and the front surface of the spacing ring  6   c  comes into contact with the lens L 4 . In the above process also, the spacing rings  6   b  and  6   c  are moved, with the outer peripheries thereof kept in contact with the protection film portions formed on the inner periphery of the lens barrel body  2 , and are now in contact with the lenses L 2  to L 4  through the protection film portions formed at a given ratio on the rear and the front surfaces. 
         [0047]    Because, as described above, the lenses L 1  to L 4  are moved while kept in contact with the smoothed protection film portions of the lens barrel body  2 , generation of grinding debris caused by the friction between the lenses L 1  to L 4  and the porous portions of the lens barrel body  2  can be prevented. Likewise, because the smoothed protection film portions of the spacing rings  6   a  to  6   c  are in contact with the lenses L 1  to L 4 , generation of grinding debris caused by the friction between the porous portions of the spacing rings  6   a  to  6   c  and the lenses L 1  to L 4  can be prevented. 
         [0048]    When the lenses L 1  to L 4  have been contained in the lens barrel body  2 , the holder ring  7  is mounted at the front end portion of the lens barrel body  2 , and the rim ridge  8  of the holder ring  7  holds the lens L 4  from the front side thereof, so that the lens barrel body  2  and the holder ring  7  are secured to each other. Thus, the lenses L 1  to L 4  are sandwiched between the rim ridge  5  of the lens barrel body  2  and the rim ridge  8  of the holder ring  7 , and the space defined by the lens barrel body  2 , the lens L 1 , and the lens L 4  is closed to complete the lens assembly  10 . 
         [0049]    Because the rim ridge  8  of the holder ring  7  is in contact with the lens L 4  through the protection film portions formed at a given ratio, generation of grinding debris caused by the friction between the porous portions of the holder ring  7  and the lens L 4  can be prevented. 
         [0050]    According to this embodiment, because the lens contact surfaces of the lens barrel part  1  comprise the protection film portions and the porous portions at a given ratio, generation of grinding debris caused by the friction between the porous portions of the lens barrel part  1  and the lenses L 1  to L 4  can be prevented with maintaining the air permeability of the lens barrel part  1 . Further, even after the lenses L 1  to L 4  are installed in the lens barrel part  1 , because the lens contact surfaces of the lens barrel part  1  remain in contact with the lenses L 1  to L 4  through the protection film portions, generation of grinding debris caused by vibration and movement can be prevented, and because the air permeability is maintained by the porous portions, the temperature difference that might arise between inside and outside from the airtightness can be reduced. 
         [0051]    The lens assembly  10  thus constructed may be combined with, for example, an image sensor, to configure an imaging device. As illustrated in  FIG. 5 , for example, an imaging device  31  may be assembled by bonding and securing a camera body frame  32  having a cylindrical shape to the rear end portion of the lens barrel body  2  of the lens assembly  10  and bonding and securing a board  34  having an image sensor  33  and other components mounted thereon to the rear end side of the camera body frame  32 . 
         [0052]    In this embodiment, the protection film portions are provided only on the lens contact surfaces of the lens barrel part  1 . However, the protection film portions may also be provided at a given ratio on areas, such as the outer peripheries of the spacing rings  6   a  to  6   c  that come into contact with the lens barrel body  2 , where grinding debris are generated as porous portions are ground. 
         [0053]    The lens barrel part  1  is essentially required to be capable of causing a tribochemical reaction to generate the protection film portions and may be made of, for example, a substance containing silicon such as silicon nitride or silicon carbide. 
         [0054]    The spacing rings  6   a  to  6   c  are essentially required to be capable of defining the spacings between the lenses L 1  to L 4 , and their outer peripheries need not necessarily be in contact with the inner periphery of the lens barrel body  2 . 
         [0055]    The polishing of the lens barrel part  1  implemented with the protection film producing device  21  is essentially required to be capable of generating the protection film portions through a tribochemical reaction caused by friction heat accompanying the polishing in the fine projection areas of the lens barrel part  1  and may be achieved through, for example, barrel polishing using a polishing tool such as ceramic balls. 
         [0056]    The lenses incorporated in the lens barrel part  1  are essentially required to be capable of forming an image by the light traveling from a subject and need not necessarily be four lenses such as L 1  to L 4 . 
       REFERENCE SIGNS LIST 
       [0057]      1  lens barrel part,  2  lens barrel body,  3  rear end opening,  4  front end opening,  5  rim ridge,  6   a  to  6   c  spacing rings,  7  holder ring,  8  rim ridge,  10  lens assembly,  21  protection film producing device,  22  tank,  23  member fastener means,  24 ,  27  actuator shafts,  25 ,  28  rotation actuators,  26  polishing tool,  29  protection film portions,  30  porous portions,  31  imaging device,  32  camera body frame,  33  image sensor, L 1  to L 4  lenses, R reaction liquid.