Abstract:
A plastic lens comprises a lens portion and a flange portion which are cast as an integral piece by injection molding using separate molds. A positioning reference surface is formed as an integral part of the lens portion between the lens portion and the flange portion by a casting mold for forming the lens portion. When the plastic lens is installed in a lens barrel and adjusted in position, a fixture holds the plastic lens at the positioning reference surface.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a plastic lens element having a flange portion and, more specifically, to a plastic lens element which is molded by the use of a split mold assembly comprising a lens portion split mold and a flange portion split mold. 
         [0003]    2. Description of Related Art 
         [0004]    As shown in  FIG. 7 , a plastic lens element  50  comprises an effective lens portion  51  functioning as an optical lens and a flange portion  52  surrounding the effective lens portion  51  which are formed as an integral piece. The plastic lens element  50  is installed in a lens barrel by holding the flange portion  52  directly by the lens barrel or through a lens holder. Typically, the plastic lens element  50  is formed by split and solidifying a molten resin into a mold cavity  58  formed in a split mold  57  which comprises a stationary mold half  55  and a movable mold half  57 . The stationary mold half  55  comprises a lens portion mold half  59  and a flange portion mold half  61  which are separately provide. Similarly, the movable mold half  57  comprises a lens portion mold half  60  and a flange portion mold half  62  which are separately provided. Because the plastic lens element is formed by the split mold  57  comprising the stationary mold half  55  and the movable mold half  57  each of which comprises two discrete parts, namely a lens portion mold half and a flange portion mold half, it is not improvable that lens portion  51  has an optical axis out of line with a mechanical central axis of the flange portion  52 . In the case where the optical axis of the lens portion  51  is in disagreement with the mechanical central axis of the flange portion  52 , the plastic lens element  50  encounters the problem that the plastic lens element  50  installed in a lens barrel is not coaxial with respect to the lens barrel. 
         [0005]    In order to avoid an occurrence of the problem, it is typical to carry out optical axis adjustment when installing the plastic lens element into a lens barrel. The optical axis adjustment is performed in such a way that is disclosed in, for example, Japanese Unexamined Patent Publication No. 2000-121902. Specifically, while the plastic lens element is kept in position in a lens barrel by holding the flange portion by a fixture, a deflection of the optical axis of the plastic lens element with respect to the central axis of the lens barrel is optically detected. Then, the plastic lens element is adjusted in position so as to remove the deflection and, thereafter, fixed in the lens barrel by an adhesive. 
         [0006]    The optical axis adjustment is time consuming work due to delicate operation and causes a cost rise of a lens device due to a requirement of a precise fixture. 
       SUMMARY OF THE INVENTION 
       [0007]    It is therefore an object of the present invention to provide a plastic lens element which facilitates optical axis adjustment. 
         [0008]    The foregoing object of the present invention is accomplished by a plastic lens element comprising a lens portion which is formed by a lens casting split mold and a flange portion surrounding the lens portion which is formed as an integral part of the lens portion by a flange casting split mold provided separately from the lens casting split mold. The plastic lens element comprises a first lens surface forming an entrance aperture of the lens portion, a second lens surface forming an effective exit aperture of the lens portion, and a positioning surface serving as a reference surface for adjusting an optical axis of the plastic lens element with respect to a central axis of a lens barrel in which the plastic lens element is installed, wherein the positioning surface is formed as an integral part of the lens portion formed between the lens portion and the flange portion by the lens casting split mold. 
         [0009]    The positioning surface preferably comprises a beveled annular surface continuing from the second lens surface and may further comprise a cylindrical surface continuing from the beveled annular surface. 
         [0010]    According to an embodiment, because the reference surface is formed by the same mold as for the lens portion, the optical axis of the lens portion is precise in position with respect to the reference surface. Further, because the plastic lens element can be installed in a lens barrel on the basis of the reference surface, a deflection of the optical axis of the plastic lens element with respect to the central axis of the lens barrel is made small, so that the optical axis adjustment is facilitated and simplified or dispensed. 
     
     
       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 in which same or similar parts or portions are denoted by the same reference numerals for avoiding repetitive descriptions, and wherein: 
           [0012]      FIG. 1  is a cross-sectional view of a lens barrel in which a plastic lens element according to an embodiment is installed; 
           [0013]      FIG. 2  is a cross-sectional view of a split mold assembly for molding the plastic lens element shown in  FIG. 1 ; 
           [0014]      FIG. 3  is a cross-sectional view of the plastic lens element shown in  FIG. 1  which is supported by a fixture; 
           [0015]      FIG. 4  is a cross-sectional view of a plastic lens element according to another embodiment; 
           [0016]      FIG. 5  is a cross-sectional view of a split mold assembly for molding the plastic lens element shown in  FIG. 4 ; 
           [0017]      FIG. 6  is a cross-sectional view of the plastic lens element shown in  FIG. 4  which is supported by a fixture; 
           [0018]      FIG. 7  is a cross-sectional view of a prior art plastic lens element; and 
           [0019]      FIG. 8  is a cross-sectional view of a split mold assembly for molding the prior art plastic lens element shown in  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    Referring to the accompanying drawings in detail and, in particular, to  FIG. 1  showing a plastic lens element  2 A according to an embodiment installed in a cylindrical lens barrel  3 , the plastic lens element  2  comprises a lens portion  4  and an annular flange portion  5  surrounding the lens portion  4  which are formed as an integral piece. The lens portion  4  has a convex front or first lens surface  6  for forming an entrance aperture thereof and a concave rear or second lens surface  7  for forming an effective exit aperture thereof. The flange portion  5  has a rear surface  5   a  and a front working face  5   b.  The plastic lens element  2 A is provided with an beveled annular shoulder  14  formed between the rear surface  5   a  of the flange portion  5  and the concave rear lens surface  7  of the lens portion  4  and out of an effective aperture area of the lens portion  4 . The beveled annular shoulder  14  serves as a reference surface for axial positioning of the plastic lens element  2 A with respect to the lens barrel  3  and has no effect on an optical function of the lens portion  4 . The lens barrel  3  has an inner annular flange  3   b  for positioning the plastic lens element  2 A in an axial direction and a plurality of radial bores  3   c  formed in a shell thereof at regular angular intervals. The plastic lens element  2 A is installed in the lens barrel  3  and fixed to an inner wall  3   a  of the lens barrel  3  by an adhesive  11  filled in a circumferential clearance  9  between the flange portion  5  of the plastic lens element  2 A and the inner wall  3   a  of the lens barrel  3  through the radial bores  3   c.    
         [0021]    Referring to  FIG. 2  showing a split mold assembly  16 A for injection molding the plastic lens element  2 A, the split mold assembly  16 A comprises a stationary mold half  17  and a movable mold half  20  between which a mold cavity  16   a  is formed. The movable mold half  20  is movable between a clamped position (shown in  FIG. 2 ) in which the movable mold half  20  is clamped down to the stationary mold half  17  and a split position in which the movable mold half  20  is split apart from the stationary mold half  17 . The stationary mold half  17  comprises a lens portion mold half  18 A and a flange portion mold half  19 A which are separately provided. Similarly, the movable mold half  20  comprises a lens portion mold half  21  and a flange portion mold half  22  which are separately provided. The lens portion mold half  18 A of the stationary mold half  17  has a beveled corner  18   a  for forming the beveled annular surface  14  of the plastic lens element  2 A. Because the lens portion  4  and the beveled annular shoulder  14  are cast in one integral body by the split mold assembly  16 A, geometrical position of the beveled annular shoulder  14  with respective to the optical axis of the lens portion  4  is highly accurate. 
         [0022]      FIG. 3  shows the plastic lens element  2 A injection molded in the split mold assembly  16 A before installation into the lens barrel  3  (see  FIG. 1 ). In this state, the plastic lens element  2 A is held by, for example, a cylindrical fixture  25 A comprising a cylindrical shell  25   a  which has a beveled annular corner  25   b  in conformity in shape with the beveled annular shoulder  14  of the plastic lens element  2 A. The fixture  25 A is desirably adapted to stick fast to the beveled annular shoulder  14  of the plastic lens element  2 A at the beveled annular corner  25   b  by electrostatic sticking or vacuum sticking so as thereby to firmly hold the plastic lens element  2 A. The fixture  25 A is capable of coaxially holding the plastic lens element  2 A with high accuracy through the beveled annular corner  25   b  thereof which is in conformity in shape with the beveled annular shoulder  14  of the plastic lens element  2 A. 
         [0023]    The plastic lens element  2 A held by the fixture  25 A is inserted into the lens barrel  3  until the front working face  5   b  is brought into contact to the inner annular flange  3   b.  Then, after carrying out adjustment of the optical axis of the plastic lens element  2 A held by the fixture  25 A and inserted into the lens barrel  3  to bring the optical axis of the plastic lens element  2 A in line with the geometrical central axis of the lens barrel  3 , an adhesive  11  is filled in the circumferential clearance  9  between the flange portion  5  of the plastic lens element  2 A and the inner wall  3   a  of the lens barrel  3  through the radial bores  3   c  so as thereby to fix the plastic lens element  2 A to the lens barrel  3 . In this instance, it is enabled to save time spent on the optical axis adjustment by keeping the fixture  25 A positioned coaxially in the lens barrel  3  as precisely as possible. 
         [0024]      FIG. 4  shows a plastic lens element  2 B according to another embodiment, the plastic lens element  2 B comprises a lens portion  4  and an annular flange portion  5  surrounding the lens portion  4  which are formed as an integral piece. The lens portion  4  has a convex front or first lens surface  6  and a concave rear or second lens surface  7 . The flange portion  5  has a front working face  5   b  and a rear surface  5   a.  The plastic lens element  2 B is provided with a beveled annular shoulder  14   a  and a straight cylindrical shoulder  14   b  continuing from an annular edge of the beveled annular shoulder  14   a  which are formed between the rear surface  5   a  of the flange portion  5  and the concave rear lens surface  7  of the lens portion  4  and out of an effective aperture area of the lens portion  4 . 
         [0025]      FIG. 5  shows a split mold assembly  16 B for injection molding the plastic lens element  2 B, the split mold assembly  16 B comprises a stationary mold half  17  and a movable mold half  18  between which a mold cavity  16   b  is formed. The movable mold half  18  is movable between a clamped position (shown in  FIG. 5 ) in which the movable mold half  18  is clamped down to the stationary mold half  17  and a split position in which the movable mold half  18  is split apart from the stationary mold half  17 . The stationary mold half  17  comprises a lens portion mold half  18 B and a flange portion mold half  19 B which are separately provided. Similarly, the movable mold half  18  comprises a lens portion mold half  21  and a flange portion mold half  22  which are separately provided. The lens portion mold half  18 B of the stationary mold half  17  has a beveled corner  18   a  for forming the beveled annular surface  14   a  and a cylindrical wall  18   b  forming the straight cylindrical shoulder  14   b.  Because the lens portion  4  and the beveled annular shoulder  14   a  are cast in one integral body by the split mold assembly  16 B, the beveled annular shoulder  14   a  is accurately positioned with respective to the optical axis of the lens portion  4 . 
         [0026]      FIG. 6  shows the plastic lens element  2 B held by a cylindrical fixture  25 B. The fixture  25 B comprises a cylindrical shell  25   a  having substantially the same outer diameter as the inner diameter of the cylindrical shoulder  14   b  and a beveled annular corner  25   b  in conformity in shape with the beveled annular shoulder  14   a  of the plastic lens element  2 B. Since there is formed a fitting structure between the fixture  25 B and the cylindrical shoulder  14   b  of the flange portion  5 , the fixture  25 B is capable of firmly holding the plastic lens element  2 B as well as capable of coaxially holding the plastic lens element  2 B with high accuracy through the beveled annular corner  25   b  thereof. 
         [0027]    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.