Abstract:
A lens barrel for holding a lens system including at least a plastic lens disposed between two glass lenses separated from each other by a spacer ring comprises a cylindrical lens holding barrel fixedly holding the two glass and the plastic lenses, a cylindrical spacer ring fitted in the cylindrical lens holding barrel for elastically supporting the plastic lens therein and positioning the two glass lenses at a predetermined axial distance on opposite sides of the plastic lens.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a lens barrel for use with a projector, a camera or an endoscope and, more particular, to a lens barrel having a cylindrical spacer ring for regulating an axial distance between two lens components.  
         [0003]     2. Description of Related Art  
         [0004]     Practically, because of a strong demand for reduction in cost for a lens comprising a lens barrel and a lens system, it is usual to use plastic lens components for the lens system. Such a plastic lens component causes a greater change in dimension than glass lens components due to a change in ambient temperature and a change in ambient humidity. On the other hand, the lens barrel in which a lens system including a plastic lens component or plastic lens components are installed comprises a lens receiving barrel for receiving one or more plastic lens components, a lens holding ring for securely holding the plastic lens component in the lens receiving barrel and a cylindrical spacer ring for defining an axial distance of the plastic lens component from another lens component. These parts of the lens barrel are typically made of a metal which is less affected by a dimensional change due to a change in ambient temperature and/or a change in ambient humidity in order to be finished with high accuracy. If adjacent plastic lens components cause thermal expansion, the lens system encounters deterioration in optical performance due to a change in axial distance between the adjacent plastic lens components. Further, if firmly forcing the cylindrical spacer ring against a periphery of the plastic lens component, the remaining portion of the plastic lens component causes deformation and, in consequence, optical distortion, which leads to deterioration in its own optical performance. On the other hand, if loosely forcing the cylindrical spacer ring against a periphery of the plastic lens component, the plastic lens component is apt to become loose. In an attempt to solve the problems, there have been proposed various plastic lens holding devices. Japanese Patent Publication No. 3-32044 describes a lens holding device in which an elastic ring member is inserted between a plastic lens component and a lens holding ring so as thereby to absorb expansion of the plastic lens component. Unexamined Japanese Patent Publication No. 62-245209 describes a lens holding device in which a buffer ring having bits of elastic adhesive is positioned between a plastic lens component and another lens component or a stationary portion of a lens barrel so as to fixedly hold the plastic lens component. Further, Unexamined Japanese Patent Publication No. 5-188253 describes a lens holding mechanism in which a lens holding ring having tongue-shaped resiliently deformable claws is used to engage in recesses formed in a flange of a plastic lens component so as thereby to force the plastic lens component against a shoulder formed in a lens holding barrel. However, the use of elastic members for respective plastic lens components forming a lens system drives up costs. In addition, it is disadvantageous in light of a complex internal shape and a tapering bore to form a number of concave parts in a lens holding barrel for receiving plastic lens components therein. The lens holding barrel thus formed also drive up a manufacturing cost.  
         [0005]     Consequentially, Unexamined Japanese Patent Publication No. 59-68710 describes a noble method for assembling a lens system into a lens barrel. In this method, lens components for forming the lens system are previously arranged in given axial positions using cylindrical spacer rings between respective adjacent lens components, respectively. The lens components is put in a metal mold for molding a lens barrel as they are remained in their axial positions and, subsequently, a molten resin is poured into the metal mold so as thereby to form a lens barrel with the lens components separately positioned by the cylindrical spacer rings therein as one whole.  
         [0006]     However, in the case where a plastic lens component is fixed in position by a cylindrical spacer ring, a periphery or an edge of the plastic lens component against which the cylindrical spacer ring is forced expands in an axial direction upon a rise in temperature. This expansion of the plastic lens component entails a change in axial distance with respect to a lens component adjacent thereto as a necessary consequence, and hence a change in air space between the two adjacent lens components (which is defined as a distance between vertices of opposed surfaces of the two adjacent lens components). In consequence, since it is hard to make the optical performance of the lens system fall within tolerable limits due to changes in two optical factors, i.e. a change in axial distance and a change in air space, the mechanical feature in which a plastic lens component is forced against a cylindrical spacer ring, or vice versa, can not be employed in the conventional lens barrel.  
       SUMMARY OF THE INVENTION  
       [0007]     It is therefore an object of the present invention to provide a lens barrel for holding a lens system including at least a plastic lens component which maintains an optical performance of the lens system within tolerable limits even though a spacer ring is used.  
         [0008]     The foregoing object of the present invention is accomplished by a lens barrel for holding an optical lens system including at least two glass lens components spaced by a predetermined axial distance and a plastic lens component disposed between the two glass lens components which comprises a cylindrical lens holding barrel forming a part of the lens barrel which fixedly holds the two glass lens components separated from each other by spacer means fitted in the cylindrical lens holding barrel which positions the two glass lens components at a predetermined axial distance from each other, the spacer means comprising a cylindrical spacer ring for receiving the plastic lens component therein and a retaining member for retaining the plastic lens component in the cylindrical spacer ring. With the lens barrel, deterioration in optical performance encountered by the optical lens system falls within a tolerance.  
         [0009]     The elastic retaining means may comprise an internal flange extending radially inward from the cylindrical spacer ring on which the plastic lens component is seated and elastic securing means such as an elastic retainer ring or a spring retainer secured in the cylindrical spacer ring for elastically forcing the plastic lens component against the inner flange so as thereby to fixedly hold the plastic lens component in the cylindrical spacer ring. The elastic securing means forces the plastic lens component against the internal flange and, at the same time, secures it in the cylindrical spacer ring when fitted in the cylindrical spacer ring, and besides absorbing axial deformation of the plastic lens component due to thermal expansion.  
         [0010]     Furthermore, the elastic securing means may comprise an elastic adhesive lying in a slight cylindrical space provided between the cylindrical spacer ring and the plastic lens component Such the elastic adhesive used as the securing means absorbing axial and radial deformation of the plastic lens component due to thermal expansion. An example of the elastic adhesive is a UV cure adhesive which is elastic after curing. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The foregoing and other objects and features of the present invention will be clearly understood from the following detailed description when reading with reference to the accompanying drawings wherein same or similar parts or mechanisms are denoted by the same reference numerals throughout the drawings and in which:  
         [0012]      FIG. 1  is a schematic view of a rear projection type projector in which a projection lens unit including a lens barrel according to an embodiment of the present invention is installed;  
         [0013]      FIG. 2  is a schematic sectional view of the projection lens unit; and  
         [0014]      FIG. 3  is a schematic sectional view of a second lens holding barrel of the projection lens unit 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     In the following description, parts or mechanisms of a projector which are not direct importance to the invention and parts or mechanisms of a projector which are purely of conventional construction will not be described in detail since their construction and operation can be easily be arrived at by those skilled in the art  
         [0016]     Referring to the accompanying drawings in detail, and in particular, to  FIG. 1  showing a projector  10  of a rear projection type in which a lens barrel according to an embodiment of the present invention is equipped, the projector  10  comprises a projector housing  14  generally shaped in an inverted trapezoid, a projection unit  12  including a projection lens unit, a reflective mirror  13  and a screen  11 . Specifically, the projector housing  14  has a front wall  14   a  in which an opening  15  is formed and a rear wall  14   b  sloping with respect to the front wall  14   a . The screen  11  is fitted in the opening  15 . The reflective mirror  13  is mounted to the rear wall  3  with a reflective surface opposed to the screen  11 . The projection unit  12  is disposed on a front bottom corner of the projector housing  14  so as to project an optical image toward the reflective mirror  13  and then onto the screen  11 . The optical image projected on the screen  11  is viewed from front.  
         [0017]     Referring to  FIG. 2 , the projection lens unit  12  comprises an optical image generator  17 , only schematically shown, for generating an optical image and a projection lens  16  for projecting the optical image generated by the optical image generator  17  onto the reflective mirror  13  and then onto the screen  11  from behind. The optical image generator  17 , which is known in various types and may take any type well known in the art, generates an optical image by modulating light that emanates from a light source (not shown) and uniformizing it in energy distribution by, for example, a rod integrator. The projection lens  16  comprises a projection lens system consisting of a plurality of, for example seven in this embodiment, lens components  22  to  28 , first and second cylindrical lens holding barrels  18  and  19 , first and second cylindrical spacer rings  20  and  21  and a tap ring  29 . The first cylindrical lens holding barrel  18  is stationary with respect to the projection lens unit  12 , and the second cylindrical lens holding barrel  19  is received, fixedly or movably, in the first cylindrical lens holding barrel  18 . The projection lens system is installed partly in the first cylindrical lens holding barrel  18  and partly in the second cylindrical lens holding barrel  19 . More specifically, the first to third lens components  22  to  24  are arranged in this order from a screen side (a front side) to an image source side (a rear side) and fixedly held in the first cylindrical lens holding barrel  18 . the fourth to seventh lens components  25  to  29  are arranged in this order from the front side) to the rear side and fixedly held in the second lens holding barrel  18 . The first and second cylindrical spacer rings  20  and  21  have axial lengths predetermined according to design specifications of the lens system. At least the second cylindrical spacer ring  21  is made of metal. Each of the lens components  22  to  29  may consist of a single lens component or multiple lens components cemented one another. In his embodiment, the sixth lens component  28  is made of plastic and the remaining lens components  22  to  26  and  28  are made of glass.  
         [0018]     Referring to  FIG. 3 , the second cylindrical lens holding barrel  19  has a front barrel portion  19   a  and a rear barrel portion  19   b  having an inner diameter larger than the front barrel portion  19   a  which are formed as an integral piece. The second cylindrical lens holding barrel  19 , more specifically the front barrel portion  19   a , has an annular abutting shoulder  30  extending laterally inward near a front end opening. Before installing and holding the forth to seventh lens components  25  to  28  in the second cylindrical lens holding barrel  19 , the second cylindrical spacer ring  21  is united with the sixth lens component  27 . The second cylindrical spacer ring  21  has a cylindrical ring portion  32  and an annular flange portion  33  forming a part of retaining means which extends laterally inward so as to form an opening defined by an edge  41 . The cylindrical ring portion  32  has an outer cylindrical wall  34  snugly fits in an inner cylindrical wall  35  of the front barrel portion  19   a  of the second cylindrical lens holding barrel  19  and an inner cylindrical wall  36  in which an outer wall  37  of the sixth lens component  27  snugly fits. It is desirable that the inner cylindrical wall  36  of the cylindrical ring portion  32  of the second cylindrical spacer ring  21  has an inner diameter slightly larger than an outer diameter of the sixth lens component  27  and that a clearance formed between the sixth lens component  27  and the inner cylindrical wall  36  of the cylindrical ring portion  32  is filled with an elastic adhesive. The annular flange portion  33  has a circular edge  41  and an annular abutting shoulder  42  working as front and rear backing means, respectively, for supporting a rear surface  26   a  of the fifth lens component  26  and a front flange portion  43  of the sixth lens component  27 , respectively.  
         [0019]     Preparatory to installing the lend components  25  to  28  in the second cylindrical lens holding barrel  19 , the sixth lens component  27  is dropped in the cylindrical ring portion  32  of the second cylindrical spacer ring  21  until the front flange portion  43  of the sixth lens component  27  is brought into abutment with the annular abutting shoulder  42  of the annular flange portion  33  and, subsequently, an elastically deformable securing member  45  formed in the shape of a ring in this embodiment which forms a part of the retaining means is snugly fitted in the cylindrical ring portion  32  so as to force and hold down the rear flange portion  46  of the sixth lens component  27  against the annular abutting shoulder  42 . It is desirable to join the elastic securing ring  45  to the inner cylindrical wall  36  of the cylindrical ring portion  32  of the second cylindrical spacer ring  21  at a plurality of circumferential positions with an adhesive. Thereafter, the forth lens component  25  is dropped in the front barrel portion  19   a  of the second cylindrical lens holding barrel  19  and, then, the first cylindrical spacer ring  20  is fitted in the same so as to hold down the forth lens component  25  against the annular abutting shoulder  30 . Subsequently, after dropping the fifth lens component  26  in the front barrel portion  19   a  of the second cylindrical lens holding barrel  19  until the fifth lens component  26  is supported on the rear end of the first cylindrical spacer ring  20 , the second cylindrical spacer ring  21  with the sixth lens component  27  and the elastic securing ring  45  united together therewith is fitted in the rear barrel portion  19   b  of the second cylindrical lens holding barrel  19  so that the circular edge  41  of the annular flange portion  33  is brought into abutment with the rear surface  26   a  of the fifth lens component  26 . Thereafter, the seventh lens component  28  is dropped in the rear barrel portion  19   b  of the second cylindrical lens holding barrel  19  until a front surface  28   a  of the seventh lens component  28  is supported on the rear end  40  of the second cylindrical spacer ring  21 . Finally, the tap ring  29  is fitted onto a rear end portion of the rear barrel portion  19   b  of the second cylindrical lens holding barrel  19  so as to hold down the seventh lens component  28  in an axial direction In this way, the forth to seventh lens components  25  to  28  are fixedly held in the second cylindrical lens holding barrel  19 . As was previously described, the second cylindrical spacer ring  21  is made of metal, and the sixth lens component  27  is made of plastic. In this instance, the plastic lens component  27  is apt to expand and contract more easily than a metal spacer  21  due to environmental variations. When the plastic lens component  27  expands due to a change in temperature, axial expansion of the plastic lens component  27  is absorbed through elastic deformation of the retaining means, more specifically the elastic securing ring  45  thereof and, at the same time, radial expansion of the plastic lens component  27  is absorbed by a clearance provided between the plastic lens component  27  and the metal cylindrical spacer ring  21  or by elastic deformation of an elastic adhesive if the clearance is filled with the elastic adhesive.  
         [0020]     The elastic securing ring  45  for holding down the sixth lens component  27  against the annular abutting shoulder  42  of the second cylindrical spacer ring  21  may be replaced with an elastic adhesive such as a UV cure adhesive. The UV cure adhesive allows for accurate joining and fixing of the sixth lens component  27  to the second cylindrical spacer ring  21  in a short period of time. The UV cure adhesive causes initial cure in several tens of seconds under ultraviolet light in a wavelength range from 350 to 380 nm and completely cures with appropriate elasticity after a few minutes. It is desirable to apply an adhesive to several circumferential locations, more specifically three to six locations, of an interface between the inner cylindrical wall  36  of the second cylindrical spacer ring  21  and the rear surface  27   a  of the sixth lens component  27 . It is desirable to apply an adhesive to diametrically opposed locations.  
         [0021]     Although the front and rear backing means for supporting the rear surface  26   a  of the fifth lens component  26  and the front flange portion  43  of the sixth lens component  27 , respectively, are provided together by the annular flange portion  33  of the second cylindrical spacer ring  21 , nevertheless, these backing means may be provided by two annular flange portions formed separately from each other. Further, although, in the above embodiment, the lens component, such as the fifth lens component  26 , the sixth lens component  27  or the seventh lens component  28 , is abutted by a continuous edge or a continuous surface of the second cylindrical spacer ring  21 , it may be abutted by discontinuous edges or discontinuous surfaces.  
         [0022]     It is to be understood that although the present invention has been described with regard to preferred embodiments thereof, various other embodiments and variants may occur to those skilled in the art, which are within the scope and spirit of the invention, and such other embodiments and variants are intended to be covered by the following claims.