Patent Publication Number: US-2009225405-A1

Title: Wide-Angle Lens, Optical Device Using the Wide-Angle Lens, and Method for Fabricating the Wide-Angle Lens

Description:
TECHNICAL FIELD 
     The present invention relates to a wide-angle lens, an optical device using a wide-angle lens, and a method for manufacturing a wide-angle lens. 
     BACKGROUND ART 
     A lens whose angle of view is not smaller than 60° is called a wide-angle lens, and a lens whose angle of view is not smaller than 100° is called a super-wide-angle lens. An example of super-wide-angle lens is disclosed in Patent Document 1. 
     The super-wide-angle lens disclosed in Patent Document 1 includes a concave lens unit arranged on an object side and a converging lens unit arranged on an image-forming side. The concave lens unit comprises a concave lens group made up of a plurality of concave lenses. Generally, the angle of view of the wide-angle lens increases as the number of the concave lenses constituting the concave lens group increases. Further, to achieve a wide angle of view, as the concave lenses for constituting the concave lens group, use is made of a lens having a light incident surface comprising a convex surface and a light emitting surface comprising a concave surface. The light diverged by the concave lens unit and traveling toward an image-forming surface is converged by the converging lens unit to form an image on the image-forming surface.
         Patent Document 1: JP-A-2005-345577       

     As to a wide-angle lens or a super-wide-angle lens having the above-described structure, efforts have been made to increase the angle of view and eliminate chromatic aberration. However, sufficient efforts have not been made to eliminate image distortion. As a wide-angle lens is commonly called a fish-eye lens, the image obtained by a wide-angle lens is considerably distorted particularly at the periphery. For instance, when an image of a rectangular object is captured by a conventional wide-angle lens in such a manner as to fill the screen, the image obtained is generally distorted largely into the form of a barrel. In this case, the distortion at the periphery of the image can reach 20% or more. 
     A wide-angle lens having the above-described structure is often used for a back monitor camera to be mounted on a vehicle or a security camera. However, due to the large image distortion caused by the wide angle of view, it is difficult to grasp the actual situation from the captured image. 
     If the image distortion of a wide-angle lens is considerably reduced, the application of a wide-angle lens greatly increases. For instance, by using a wide-angle lens, a thin image reader can be produced which is capable of instantaneously reading the two-dimensional image of a document placed on a document table without scanning, which can replace a scanner for reading a two-dimensional image by scanning a line sensor. 
     DISCLOSURE OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a wide-angle lens with reduced image distortion. Another object of the present invention to provide an optical device using such a wide-angle lens. Still another object of the present invention to provide a method for easily manufacturing such a wide-angle lens. 
     According to a first aspect of the present invention, there is provided a wide-angle lens comprising an object-side lens group, an image-forming side lens group, and at least one converging lens arranged between the object-side lens group and the image-forming side lens group. The object-side lens group comprises at least one concave lens including a convex light incident surface on the object side. The light incident surface of the concave lens is a convex aspheric surface whose curvature increases from a portion adjacent to a central optical axis toward a periphery of the lens. The concave lens further includes a light emitting surface which is a substantially concave spherical surface. 
     In a preferred embodiment, the image-forming side lens group is designed to be capable of forming an image of an object on an image-forming surface by itself with reduced image distortion. 
     In a preferred embodiment, the object-side lens group consists of the single concave lens, and the wide-angle lens has an angle of view in a range of 60° to 100° and image distortion of not more than ±3%. 
     In a preferred embodiment, the object-side lens group consists of the two concave lenses, and the wide-angle lens has an angle of view in a range of 100° to 130° and image distortion of not more than ±3%. 
     In a preferred embodiment, the object-side lens group consists of the three concave lenses, and the wide-angle lens has an angle of view in a range of 100° to 170° and image distortion of not more than +3%. 
     Preferably, when the object-side lens group consists of two or more concave lenses, the concave lens arranged closest to an object is made of resin, whereas the other concave lens or lenses are made of molded glass whose Abbe number is not smaller than 70. 
     In a preferred embodiment, the Abbe number of the converging lens is smaller than the Abbe number of the concave lens constituting the object-side lens group. Preferably, in this case, the concave lens constituting the object-side lens group and the converging lens are made of resin. 
     According to a second aspect of the present invention, there is provided an optical device comprising a wide-angle lens according to the first aspect of the present invention and a two-dimensional area sensor which is so arranged that its light receiving surface is located on the image-forming surface. 
     According to a third aspect of the present invention, there is provided a camera module comprising a wide-angle lens according to the first aspect of the present invention and a two-dimensional area sensor which is so arranged that its light receiving surface is located on the image-forming surface. The camera module is designed to obtain a two-dimensional image of an object by the two-dimensional area sensor. 
     According to a fourth aspect of the present invention, there is provided an image reader comprising a transparent document table and a camera module according to the third aspect of the present invention arranged below the document table. The image reader is designed to obtain a two-dimensional image of a document placed on the document table by the two-dimensional area sensor. 
     According to a fifth aspect of the present invention, there is provided a method for manufacturing a wide-angle lens comprising an object-side lens group, an image-forming side lens group, and at least one converging lens arranged between the object-side lens group and the image-forming side lens group. The object-side lens group comprises at least one concave lens including a convex light incident surface on the object side. The light incident surface of the concave lens comprises a convex aspheric surface. The concave lens further includes a light emitting surface which is a substantially concave spherical surface. The method comprises the steps of preparing the image-forming side lens group to be capable of forming an image of an object on an image-forming surface by itself with reduced image distortion, and determining specifications of the lens for constituting the object-side lens group and the converging lens so that an image with reduced image distortion is to be formed on the image-forming surface. 
     According to a sixth aspect of the present invention, there is provided a method for manufacturing a wide-angle lens comprising an object-side lens group; an image-forming side lens group, and at least one converging lens arranged between the object-side lens group and the image-forming side lens group. The object-side lens group comprises one or a plurality of concave lenses each including a light incident surface which is a convex aspheric surface and a light emitting surface which is a substantially concave spherical surface. The method is capable of producing wide-angle lenses of different angles of view by appropriately selecting the number of the concave lenses for constituting the object-side lens group. The method comprises preparing the image-forming side lens group by using a lens group of predetermined specifications which make it possible to form an image of an object on an image-forming surface by itself with reduced image distortion. In this method, when the object-side lens group is to consist of a single concave lens, specifications of the single concave lens and the converging lens are so determined that an image with reduced image distortion is to be formed on the image-forming surface. When the object-side lens group is to consist of two or more concave lenses, the specifications of the single concave lens for the structure in which the object-side lens group consists of a single concave lens is used as it is, and one or more additional concave lenses are arranged on the object side of the concave lens, and specifications of the additional concave lens or lenses and the converging lens are so determined that an image with reduced image distortion is to be formed on the image-forming surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic structural view of a cameral module incorporating a wide-angle lens according to a first embodiment of the present invention. 
         FIG. 2  is a schematic structural view of a cameral module incorporating a wide-angle lens according to a second embodiment of the present invention. 
         FIG. 3  is a schematic structural view of a cameral module incorporating a wide-angle lens according to a third embodiment of the present invention. 
         FIG. 4  is a schematic structural view of a cameral module incorporating a wide-angle lens according to a fourth embodiment of the present invention. 
         FIG. 5  is a sectional view of an image reader structured using a wide-angle lens and a cameral module according to the present invention. 
         FIG. 6  is a sectional view taken along lines VI-VI in  FIG. 5 . 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. 
       FIG. 1  shows the overall structure of a camera module  100 A incorporating a wide-angle lens  10 A according to a first embodiment of the present invention. 
     The wide-angle lens  10 A includes an object-side lens  200  comprising a concave lens  210 , an image-forming side lens group  400  comprising a plurality of lenses  410  and  420 , and a converging lens  300  arranged between the object-side concave lens  210  and the image-forming side lens group  400 . The lenses  210 ,  300 ,  410  and  420  are arranged along a common central axis Lc and held by a lens holder  550 . A two-dimensional area sensor  600 , which may be a CCD sensor, is so arranged that its light receiving surface  610  is located on an image-forming surface  500 . The two-dimensional area sensor  600  is mounted on a substrate  700 . The lens holder  550  is also mounted on the substrate  700  to define the positional relationship among the lenses  210 ,  300 ,  410 ,  420  and the positional relationship between each of the lenses  210 ,  300 ,  410 ,  420  and the image-forming surface  500 . In the figure, the reference number  520  indicates a diaphragm for limiting the diameter of the light traveling toward image-forming side lens group  400  to a predetermined range, whereas the reference number  510  indicates an infrared light filter. 
     The object-side concave lens  210  includes a light incident surface  211  on the object side and a light emitting surface  212  on the image-forming side. The light incident surface  211  comprises a convex aspheric surface  211   a , whereas the light emitting surface  212  includes a concave spherical surface region. In this embodiment, the central angle of the concave spherical surface region is approximately 180°. The convex aspheric surface  211   a  constituting the light incident surface  211  is configured to have a larger curvature at a portion farther from the central axis Lc of the lens. However, since the object-side lens  210  is a concave lens as a whole, at any point of the light incident surface  211 , the curvature is smaller than that of the light-emitting surface  212 . It is preferable that the light emitting surface  212  is an accurate concave spherical surface. However, the light emitting surface may be coated with resin for achromatizing or slightly varied for correcting various aberrations as long as the surface keeps a substantially concave spherical configuration. This holds true for other embodiments shown in  FIG. 2  and the subsequent drawings. 
     In this embodiment, the image-forming side lens group  400  comprises the convex aspheric lens  410  and the concave aspheric lens  420 . The image-forming side lens group  400  is designed to be capable of forming an image of an object on the image-forming surface by itself with reduced aberration. As shown in  FIG. 1 , the light rays entering the image-forming side lens group  400  to reach the image-forming surface  500  are parallel or generally parallel with each other, which indicates that the image-forming side lens group  400  is capable of forming an image of an object on the image-forming surface  500  by itself. That is, in the wide-angle lens  10 A according to the present invention, a known lens group or a lens group designed to be usable by itself as a standard lens is used as the image-forming side lens group  400 , and the object-side concave lens  210  and the converging lens  300  are added to the image-forming side lens group to increase the angle of view and reduce the image distortion. 
     Since the object-side lens  200  is a concave lens, the light rays diverge from the light emitting surface  212  of the object-side lens  200 . The converging lens  300  changes the diverging light rays to parallel or generally parallel light rays to cause the light rays to impinge on the image-forming side lens group  400  (see  FIG. 1 ). Thus, the converging lens  300  is a convex lens as a whole. 
     As noted before, the light incident surface  211  of the object-side concave lens  210  is configured to have a larger curvature at a portion farther from the central axis Lc of the lens. Thus, as compared with the structure in which the entire light incident surface is a convex spherical surface in which the curvature adjacent to the central axis is maintained throughout the surface, the structure of this embodiment ensures that an image of an object located close to the central axis Lc is formed at a peripheral point P of the image-forming surface  500 . This indicates that the distortion of the image formed on the image-forming surface  500  reduces. 
     In this embodiment, the wide-angle lens  10 A with reduced image distortion is provided by configuring the light incident surface  211  of the concave lens  210 , which is closest to the object, as the convex aspheric surface  211   a.    
     In the first embodiment shown in  FIG. 1 , by employing the single concave lens  210  as the object-side lens  200 , the angle of view up to about 90° and image distortion of not more than ±3% are achieved. 
     To manufacture the wide-angle lens  10 A, the specifications of each lens  210 ,  300 ,  410 ,  420  need to be determined. The profile of the aspheric surface is determined using a known formula. The structure of this embodiment includes four lenses. If the light incident surfaces and light emitting surfaces of all the four lenses are aspheric, a large amount of calculation needs to be performed to determine the eight aspheric profiles so as to make the image distortion be not more than a predetermined level, which is not practical. 
     However, in the wide-angle lens  10 A according to this embodiment, the aspheric profile of each lens  410 ,  420  constituting the image-forming side lens group  400  is already determined. Further, as to the light emitting surface  212  of the object-side concave lens  210 , it is not necessary to determine the aspheric profile, because it is a concave spherical surface. Thus, it is only necessary to determine the aspheric profile of the light incident surface  211  of the object-side concave lens  210  and additionally determine the aspheric profile of the converging lens  300 , so that a large amount of calculation is not necessary. 
       FIG. 2  shows the overall structure of a camera module  100 B incorporating a wide-angle lens  10 B according to a second embodiment of the present invention. 
     The wide-angle lens  10 B of the second embodiment differs from the wide-angle lens  10 A of the first embodiment shown in  FIG. 1  in that the wide-angle lens  10 B includes two concave lenses  210  and  220  which constitute an object-side lens group  200 . The wide-angle lens  10 B of the second embodiment is the same as that of the first embodiment in that it includes an image-forming side lens group  400 , a converging lens  300  is arranged between the object-side lens group  200  and the image-forming side lens group  400 , the lenses  210 ,  220 ,  300 ,  410 ,  420  are supported by a lens holder  550  mounted on a substrate  700 , a diaphragm  520  and an infrared light filter  510  are arranged at predetermined positions, and a light receiving surface  610  of a two-dimensional area sensor  600  mounted on the substrate  700  is arranged on an image-forming surface  500 . 
     Of the two concave lenses  210  and  220  constituting the object-side lens group  200  in the wide-angle lens  10 B, the lens  210  positioned on the image-forming side comprises a lens having the same specification as that of the concave lens  210  of the first embodiment shown in  FIG. 1 . Further, as the image-forming side lens group  400 , a lens group having the same specification as that of the image-forming side lens group  400  of the first embodiment is employed. That is, the wide-angle lens  10 B of the second embodiment is obtained by adding a second object-side concave lens  220  to the wide-angle lens  10 A shown in  FIG. 1  and replacing the converging lens  300  of the wide-angle lens  10 A with a different one. 
     Due to addition of the concave lens  220  for constituting the object-side lens group  200 , the degree of divergence of the light rays emitted from the object-side lens group  200  increases as compared with the first embodiment. This is the reason why the converging lens  300  is replaced. In this embodiment again, the converging lens  300  serves to change the diverging light rays to parallel or generally parallel light rays to cause the light rays to impinge on the image-forming side lens group  400 . 
     The concave lens  220  added to the object side includes a light incident surface  221  comprising a convex aspheric surface  221   a  and a light emitting surface  222  comprising a concave spherical surface. In this embodiment again, the central angle of the concave spherical light-emitting surface  222  is approximately 180°. The convex aspheric surface  211   a  of the light incident surface  221  is configured to have a larger curvature at a portion farther from the central axis Lc of the lens. This arrangement reduces the image distortion, as noted before with respect to the object-side concave lens  210  of the wide-angle lens  10 A shown in  FIG. 1 . 
     The structure of the second embodiment includes five lenses  210 ,  220 ,  300 ,  410  and  420 . If the light incident surfaces and light emitting surfaces of all the five lenses are aspheric and the aspheric profiles of all these surfaces need to be determined so as to make the image distortion be not more than a predetermined level, a large amount of calculation needs to be performed, which is not practical. However, as noted before, the aspheric profile of each lens constituting the image-forming side lens group  400  is already determined. Further, of the two concave lenses  210  and  220  constituting the object-side lens group  200 , the profile of the concave lens  210  on the image-forming side is already determined. Moreover, as to the light emitting surface  222  of the additional concave lens  220  constituting the object-side lens group  200 , It is not necessary to determine the aspheric profile, because it is a concave spherical surface. Thus, to manufacture the wide-angle lens  10 B of the second embodiment, it is only necessary to determine the aspheric profile of the light incident surface  221  of the additional concave lens  220 , so that a large amount of calculation is not necessary. 
     In the second embodiment shown in  FIG. 2 , by employing two concave lenses  210  and  220  as the object-side lens group  200 , the angle of view up to about 110° and image distortion of not more than +3% are achieved. 
       FIG. 3  shows the overall structure of a camera module  100 C incorporating a wide-angle lens  10 C according to a third embodiment of the present invention. 
     The wide-angle lens  10 C of the second embodiment differs from the wide-angle lens  10 A of the first embodiment shown in  FIG. 1  in that the wide-angle lens  10 C includes three concave lenses  210 ,  220  and  230  which constitute an object-side lens group  200 . The wide-angle lens  10 C of the third embodiment is the same as that of the first embodiment shown in  FIG. 1  in that it includes an image-forming side lens group  400 , a converging lens  300  is arranged between the object-side lens group  200  and the image-forming side lens group  400 , the lenses  210 ,  220 ,  230 ,  300 ,  410 ,  420  are supported by a lens holder  550  mounted on a substrate  700 , a diaphragm  520  and an infrared light filter  510  are arranged at predetermined positions, and a light receiving surface  610  of a two-dimensional area sensor  600  mounted on the substrate  700  is arranged on an image-forming surface  500 . 
     Of the three concave lenses  210 ,  220  and  230  constituting the object-side lens group  200  in the wide-angle lens  10 C, the two concave lenses  210  and  220  positioned on the image-forming side comprise lenses having the same specification as that of the two concave lenses  210  and  220  of the second embodiment shown in  FIG. 2 . Further, as the image-forming side lens group  400 , a lens group having the same specification as that of the image-forming side lens group  400  of the first embodiment shown in is employed. That is, the wide-angle lens  10 C of the third embodiment is obtained by adding a third object-side concave lens  230  to the wide-angle lens  10 B shown in  FIG. 2  and replacing the converging lens  300  of the wide-angle lens  10 B with a different one. 
     Due to addition of the concave lens  230  for constituting the object-side lens group  200 , the degree of divergence of the light rays emitted from the object-side lens group  200  increases as compared with the first and the second embodiments. This is the reason why the converging lens  300  is replaced. In this embodiment again, the converging lens  300  serves to change the diverging light rays to parallel or generally parallel light rays to cause the light rays to impinge on the image-forming side lens group  400 . 
     The third concave lens  230  added to the object side includes a light incident surface  231  comprising a convex aspheric surface  231   a  and a light emitting surface  232  comprising a concave spherical surface. In this embodiment again, the central angle of the concave spherical light-emitting surface  232  is approximately 180°. The convex aspheric surface  231   a  of the light incident surface  231  is configured to have a larger curvature at a portion farther from the central axis Lc of the lens. This arrangement reduces the image distortion, as noted before with respect to the embodiments shown in  FIGS. 1 and 2 . 
     The structure of the third embodiment includes six lenses  210 ,  220 ,  230 ,  300 ,  410  and  420 . If the light incident surfaces and light emitting surfaces of all the six lenses are aspheric and the aspheric profiles of all these surfaces need to be determined so as to make the image distortion be not more than a predetermined level, a large amount of calculation needs to be performed, which is not practical. However, as noted before, the aspheric profile of each lens constituting the image-forming side lens group  400  is already determined. Further, of the three concave lenses  210 ,  220  and  230  constituting the object-side lens group  200 , the profile of the two concave lenses  210  and  220  on the image-forming side is already determined. Moreover, as to the light emitting surface  232  of the additional concave lens  230  constituting the object-side lens group  200 , it is not necessary to determine the aspheric profile, because it is a concave spherical surface. Thus, to manufacture the wide-angle lens  10 C of the third embodiment, it is only necessary to determine the aspheric profile of the light incident surface  231  of the additional concave lens  230 , so that a large amount of calculation is not necessary. 
     In the third embodiment shown in  FIG. 3 , by employing three concave lenses  210 ,  220  and  230  as the object-side lens group  200 , the angle of view up to about 160° and image distortion of not more than ±3% are achieved. 
     In this way, the above-described lenses  10 A,  10 B and  10 C achieve a reduction in image distortion, although they are wide-angle lenses. As described before, each of the wide-angle lenses includes an object-side lens group  200  and an image-forming side lens group  400 . The image-forming side lens group  400  is designed to be capable of forming an image of an object on the image-forming surface by itself. The light incident surfaces  211 ,  221 ,  231  of the concave lenses  210 ,  220 ,  230  constituting the object-side lens group  200  comprise convex aspheric surfaces  211   a ,  221   a ,  231   a . With this arrangement, the calculation for determining the aspheric profile of the convex aspheric surfaces is performed easily using a formula so as to achieve a reduction in image distortion. 
     In the wide-angle lenses  10 A,  10 B and  10 C, the light rays incident on the periphery of the lens need to be greatly refracted by each concave lens. Thus, it is preferable that the index of refraction of the material of the concave lenses  210 ,  220  and  230  is as high as possible. Further, in capturing a color image, as the angle of view increases, color blurring is more likely to occur at the periphery of an object image. Such color blurring occurs in such a manner that blue deviates inward while red deviates outward. To reduce such color blurring, it is preferable to make a lens using a material whose change in index of refraction depending on the light wavelength is as small as possible, i.e., a material having a large Abbe number. 
     Generally, glass-based materials have a larger Abbe number than that of resin-based materials. However, molding of a concave lens using such a glass-based material requires long time for heating and cooling. Thus, in terms of productivity, molding of a concave lens using glass-based materials is inferior to that using resin-based materials. 
       FIG. 4  shows a wide-angle lens  10 D according to a fourth embodiment of the present invention, which is designed to achieve a high productivity and reduce the color blurring in the image. 
     The wide-angle lens  10 D is the same as the wide-angle lens  10 C of the third embodiment in arrangement of the lenses. The wide-angle lens  10 D includes three concave lenses  210 ,  220  and  230  which constitute an object-side lens group  200 . The wide-angle lens  10 D further includes an image-forming side lens group  400 , and a converging lens  300  arranged between the object-side lens group  200  and the image-forming side lens group  400 . The lenses  210 ,  220 ,  230 ,  300 ,  410 ,  420  are supported by a lens-holder  550  mounted on a substrate  700 . The three concave lenses  210 ,  220 ,  230  and the converging lens  300  include flange portions  213 ,  223 ,  233  and  303  for facilitating proper holding by the holder  550 . A diaphragm  520  and an infrared light filter  510  are arranged at predetermined positions. A light receiving surface  610  of a two-dimensional area sensor  600  mounted on the substrate  700  is arranged on an image-forming surface  500 . Similarly to the concave lenses of the foregoing embodiments, the light incident surfaces  211 ,  221  and  231  of the three concave lenses  210 ,  220  and  230  constituting the object-side lens group  200  comprise convex aspheric surfaces  211   a ,  221   a  and  231   a  designed to have a larger curvature at a portion farther from the central axis of the lens. The light emitting surfaces  212 ,  222  and  232  are substantially concave spherical surfaces. The image-forming side lens group  400  is designed to be capable of forming an image of an object on the image-forming surface by itself with reduced image distortion. 
     In this embodiment, the central angle of the concave spherical surface region of the light emitting surface  212 ,  222 ,  232  of each concave lens  210 ,  220 ,  230  is smaller than that of the foregoing embodiments. However, due to the use of three concave lenses, the angle of view of not smaller than 100° and image distortion of not more than ±3% are achieved. 
     To provide a wide angle of view, the lens diameter of the concave lens  230 , which is closest to the object among the three concave lenses  210 ,  220  and  230  constituting the object-side lens group  200 , is made larger than the lens diameter of other two concave lenses  210  and  220 . As noted before, to reduce the color blurring of the captured image, it is desirable to make a concave lens by using a glass-based material whose Abbe number is large. However, when the lens diameter of the lens to be formed is relatively large, to form the lens by molding a glass-based material requires a long time for heating and cooling, which is not efficient. Thus, the lenses  210  and  220  having a relatively small lens diameter may be made of a glass-based material whose Abbe number is relatively large, whereas the lens  230  having a relatively large lens diameter may be made of a resin-based material. In this case, a high productivity is achieved. 
     As the glass-based material, use may be made of a material whose Abbe number is not smaller than 70. Specifically, for instance, use may be made of “PKF 80” (Abbe Number: 81.5, index of refraction: 1.497) available from SUMITA Optical Glass, Inc. or “FCD1” (Abbe Number: 81.6, index of refraction: 1.497) available from HOYA CORPORATION. 
     As to the resin-based material for molding the concave lens  230 , it is preferable that the Abbe number is as large as possible. For instance, use may be made of “ZEONEX 480R” (Abbe Number: 56.2, index of refraction: 1.525) available from ZEON CORPORATION or “ARTON-F” (Abbe Number: 56.3, index of refraction: 1.513) available from JSR Corporation. 
     Alternatively, all the three concave lenses  210 ,  220  and  230  may be made of a resin-based material having a large Abbe number like the above-described “ZEONEX 480R” or “ARTON-F”. In this case, the productivity of the object-side lens group  200  is further enhanced. 
     In this way, by making the concave lenses  210 ,  220  and  230  of the object-side lens group  200  by using a material whose Abbe number is large, a lens is produced which reduces image distortion and color blurring in capturing a color image while achieving a large angle of view. With this arrangement, however, color blurring cannot be eliminated completely, because light rays are greatly refracted particularly at the periphery of the object-side lens group  200  before the light rays impinge on the converging lens  300 . 
     To solve this problem, in the wide-angle lens  10 D according to the fourth embodiment, the converging lens  300 , which is a convex lens, is made of a material whose Abbe number is smaller than that of the concave lenses  210 ,  220  and  230 . That is, as the material of the converging lens  300  which refracts light rays in the converging direction after the light rays are divergently refracted by the concave lenses  210 ,  220  and  230 , a material whose Abbe number is relatively small is employed unlike the material of the concave lenses  210 ,  220  and  230 . By this arrangement, the color blurring caused by the passing of light rays through the concave lenses  210 ,  220  and  230  is eliminated. 
     For instance, when the above-described “ZEONEX 480R” (Abbe Number: 56.2, index of refraction: 1.525) or “ARTON-F” (Abbe Number: 56.3, index of refraction: 1.513) is used as the material of the concave lenses  210 ,  220  and  230 , polycarbonate whose Abbe number is relatively low (Abbe Number: 31.1, index of refraction: 1.585) may be used as the material of the converging lens  300 . 
       FIGS. 5 and 6  show a schematic structure of an image reader  800  which is an example of optical device incorporating the wide-angle lens  10 C and the camera module  100 C of the foregoing embodiment. 
     The image reader  800  includes a box-shaped case  810 , a document table  820  made of e.g. transparent glass and arranged on top of the case  810 , and the camera module  100 C arranged on a bottom plate  830  of the case  810 . As the camera module, the use of the camera module  100 C shown in  FIG. 3  which includes three concave lenses as the object-side lens group  200  is most desirable, because it provides the advantages such as the utilization of a wide angle of view of about 160° and a reduction in thickness of the case and hence a reduction in thickness of the image reader  800 . However, the camera modules  100 A,  100 B and  100 D shown in  FIGS. 1 ,  2  and  4  may be employed. 
     A cover  840  for covering the document table  820  is supported at an end by an edge of the upper surface of the case  810  to be pivotable for opening and closing movement. An LED element  730  as the illumination light source and semiconductor devices  710  and  720  for obtaining an image from the two-dimensional area sensor  600 , controlling light emission of the LED element  730 , transferring image data and soon are mounted on an extended portion of the substrate  700  of the camera module  100 C. 
     The image reader  800  is capable of instantaneously capturing a two-dimensional image of a document Dc placed on the document table  820  with reduced image distortion. 
     The present invention is not limited to the foregoing embodiments, and all the variations within the scope of the following claims are intended to be included in the scope of the present invention. 
     Although the number of the lenses constituting the object-side lens group  200  is three at the most in the foregoing embodiments, the object-side lens groups  200  may consist of four or more lenses. 
     Although a single convex lens is used as the converging lens  300  in the foregoing embodiments, a plurality of lenses may be used as the converging lenses when the number of lenses constituting the object-side lens group  200  is increased. 
     For instance, instead of an image reader as described above, the optical device incorporating a camera module according to the present invention may be designed as a back monitor camera to be mounted on a vehicle to capture the rear view of the vehicle as a two-dimensional video image, a monitoring camera to be installed in a building or a construction site, or an optical identification device to be set in an ATM in a financial institution for performing personal identification based on a palm vein pattern.