Patent Publication Number: US-9417447-B2

Title: Rod lens array and image sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation application of U.S. patent application Ser. No. 14/337,869 filed on Jul. 22, 2014, which claims a priority of Japanese Patent Application No. 2013-153063 filed on Jul. 23, 2013. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a rod lens array used for an image sensor in image reading devices, such as image scanners, or other various optical devices, and to an image sensor that includes the rod lens array. 
     2. Description of Related Art 
     In conventional image reading devices, such as facsimile machines, copiers, image scanners, or printers, and other various optical devices, a contact-type image sensor (CIS) of an equal-magnification image-forming optical system is widely used to optically read an image on a document and convert the image into electric signals. The CIS includes a rod lens array in which one or more rows of many cylindrical rod lenses are arranged between two substrates in such a way that the central axes of the lenses become parallel to each other (For example, see Jpn. Pat. Appln. Laid-Open Publication No. 2012-244344). 
     The rod lens is so designed as to have a refractive index distribution in which the refractive index decreases continuously from the central axis thereof to an outer periphery. Most of the rod lens were originally glass lenses, which were produced by carrying out a spinning molding of a rod-shaped glass material and giving the refractive index distribution through ion-exchange treatment or cation heat interchange (For example, see Jpn. Pat. Appln. Laid-Open Publication No. 10-139472). 
     Relatively low-cost plastic rod lenses whose refractive index distributions can be precisely controlled are now frequently employed (For example, see Jpn. Pat. Appln. Laid-Open Publication No. 2012-78750). 
     The CIS could have an adverse effect on the reading of images or outputting of sensors if dust gets into the CIS from the outside or if processing debris comes off from components inside the CIS. In particular, in order to prevent dust from getting into a space between a transparent member or platen glass, on which a document is placed, and a rod lens array, what is known is the CIS in which the rod lens array is put between the platen glass and a support member without any gap therebetween to eliminate the space which dust can get into (For example, see Jpn. Pat. Appln. Laid-Open Publication No. 05-344276). 
     When the CIS is assembled, the rod lens array needs to be placed at a predetermined position with high precision to get optimal optical performance. To eliminate the need for precise positioning or fine tuning of the rod lens array and to make it easier to put the rod lens array into the CIS, what is proposed is a micro lens array structure in which, to one lens end surface of the rod lens array, a reed-shaped transparent light guide member having an optical length equal to an operating distance of the rod lens is attached, and a light receiving element array is integrally joined to the other surface of the transparent light guide member (For example, see Jpn. Pat. Appln. Laid-Open Publication No. 05-134104). 
     Basically, there is a strong call for the above optical devices to be miniaturized. Similarly, there is a call for the contact-type image sensor to be made smaller in size by reducing the distance between an object, such as a document, and an image, or the image-forming distance. On the other hand, in the contact-type image sensor, in order to enable the sensor to read a clear image even if the distance between the surface of the document and the rod lens is somewhat changed due to floating of the document or the like, the depth of focus of the rod lens needs to be set as deeper as possible. 
       FIG. 9  schematically shows how an image is formed by a conventional rod lens of an upright equal-magnification image-forming system. In the diagram, the rod lens  1  is a cylindrical lens with a constant radius of r 0  and a lens length of Z 0 ; at the incident and emission ends thereof, there are an incident surface  2  and emission surface  3  that are polished to be flat. The refractive index of the rod lens  1  continuously decreases from a refractive index N 1  at a central axis thereof in a radial direction. The light coming from a point image P 0  on a document surface  4  enters the incident surface  2  of the rod lens  1 , and meanders through the rod lens at a constant frequency in an optical axis direction. Then, the light comes out through the emission surface  3 , and a point image I 0  is formed on a light receiving surface  5  of a light receiving element. In this case, the distance between the point image P 0  and the incident surface  2 , or the operating distance L 0 , is equal to the distance between the point image I 0  and the emission surface  3 . 
     The depth of focus of the rod lens  1  is inversely proportional to a numerical aperture, and the numerical aperture is proportional to the refractive index N 1  of the center, the refractive index distribution constant, and the radius r 0  of the lens. Accordingly, if the refractive index N 1  of the center and the refractive index distribution constant remain constant, the radius r 0  of the lens needs to be smaller to make the depth of focus deeper. However, if the radius r 0  of the lens is made smaller, the handling and processing of the rod lens becomes difficult when the rod lens is produced. Moreover, the brightness of the rod lens  1  sharply decreases in proportion to the square of the numerical aperture. As a result, there might be a decrease in the image reading performance. 
     Moreover, the operating distance L 0  of the rod lens  1  changes in a tangent manner with respect to the lens length Z 0 , and is inversely proportional to the refractive index N 1  of the center and the square of the refractive index distribution constant. Therefore, if the refractive index N 1  of the center and the lens radius r 0  are kept constant, and the refractive index distribution constant is made smaller, the operating distance L 0  becomes longer when the lens length Z 0  is constant. As a result, the conjugation length of the rod lens  1  (the distance between the object and the image =Z 0 +2L 0 ) becomes longer, and the entire optical system becomes longer. Therefore, the rod lens array and the image sensor that includes the rod lens array cannot be made smaller in size. If the lens length Z 0  is made smaller to prevent the operating distance L 0  from becoming longer, the field of view of the rod lens  1  and the radius thereof become smaller, possibly leading to a periodic light intensity variation. Therefore, such a configuration is not preferred. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above problems of the prior art. The object of the present invention is to provide a rod lens array and an image sensor including the rod lens array in which a depth of focus can be set deep and an entire optical system can become shorter without the image reading performance being reduced. 
     Another object of the present invention is to provide a contact-type image sensor including the rod lens array to reduce the size of the contact-type image sensor. 
     According to the first aspect of the present invention, a rod lens array comprises a plurality of columnar rod lenses each having a refractive index distribution in which a refractive index continuously decreases from a central axis thereof to an outer periphery, and arranged in at least one row to align the central axes in parallel to each other. Each of the plurality of columnar rod lenses includes an emission-side end portion region, an incident-side end portion region, and an intermediate region between the emission-side end portion region and the incident-side end portion region, each having a central refractive index. The central refractive index of the incident-side end portion is equal to that of the emission-side end portion region in an optical axis direction; and the central refractive index at the intermediate region is higher than those of the emission-side and incident-side end portion regions. 
     According to the second aspect of the present invention, an image sensor comprises the rod lens array according to the first aspect, and a light receiving sensor to receive light of an image formed through each of the plurality of columnar rod lenses of the rod lens 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic perspective view of an image scanner on which an image sensor unit of the present invention is mounted; 
         FIG. 2  is a schematic exploded perspective view of the image sensor unit of the present invention; 
         FIG. 3  is a schematic partially-broken perspective view of a rod lens array shown in  FIG. 2 ; 
         FIG. 4  is a diagram illustrating how an image is formed through a rod lens shown in  FIG. 3 ; 
         FIG. 5  is a schematic cross-sectional view of an image sensor unit according to the present embodiment; 
         FIGS. 6A and 6B  are schematic diagrams illustrating how the rod lens array shown in  FIG. 2  is mounted on an image sensor unit with a conventional structure; 
         FIG. 7  is a schematic cross-sectional view of an image sensor unit according to a modified example of  FIG. 5 ; 
         FIG. 8  is a schematic cross-sectional view of an image sensor unit according to a modified example of  FIG. 7 ; and 
         FIG. 9  is a diagram illustrating how an image is formed through a conventional rod lens. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. 
       FIG. 1  is a schematic perspective view of an image scanner on which an image sensor unit of a preferred embodiment of the present invention is mounted. An image scanner  20  of the present embodiment is an image reading device of a flatbed type. The image scanner  20  includes a main body  21 , which is substantially in the shape of a rectangular box, and a platen cover  22 , one end of which is mounted on the main body through a hinge (not shown) in such a way that the platen cover  22  can be freely opened or closed. 
     The main body  21  includes a platen glass  23 , which is a large rectangular transparent glass plate fixed on an upper surface thereof, and a contact-type image sensor unit  24  of the present invention. On an upper surface of the platen glass  23 , a to-be-read document is placed with a document surface thereof down. When the platen cover  22  is closed, the document surface comes in close contact with the upper surface of the platen glass due to a pressing member  22   a  that is provided on an inner surface thereof. 
     The image sensor unit  24  is disposed immediately below the platen glass  23 . The image sensor unit  24  is held by a holding member  25  in such a way that the upper surface thereof is in close contact with a lower surface of the platen glass. During reading of the document, the image sensor unit is driven by a drive motor  26  via a wire  27  and the like in such a way as to move in a document reading direction or a sub-scanning direction along a slide shaft  28 . 
       FIG. 2  schematically shows the basic configuration of the image sensor unit  24  of the present embodiment. The image sensor unit  24  includes a lighting device  29 , a rod lens array  30 , and a light receiving sensor  31 . The image sensor unit  24  further includes a housing in which the lighting device, the rod lens array, and the light receiving sensor are mounted and held at predetermined positions. 
     The light receiving sensor  31  includes a large number of photoelectric conversion elements  33 , which are mounted in a line on a sensor substrate  32 . As the photoelectric conversion elements  33 , for example, solid-state imaging elements, such as CMOS image sensors or CCD image sensors, are used. The rod lens array  30  is disposed in such a way that a lower-end emission surface thereof is aligned with the photoelectric conversion elements. The illumination light emitted from the lighting device  29  is reflected by a document surface of a document  34  before entering an upper-end incident surface of the rod lens array  30 . Then, the light comes out through a lower-end emission surface, and an image is formed on the photoelectric conversion elements  33 . 
     The lighting device  29  includes light-emitting elements such as LEDs as light sources, and a light guide rod, which guides the light emitted from the light-emitting elements. The light guide rod is made of a transparent material with a high level of translucency, such as glass, acrylic resin, or epoxy resin; the light guide rod has a length corresponding to a reading line width of the image sensor unit  24 . The light guide rod includes an emission surface and a light scattering surface, which face each other across almost the entire length of a longitudinal direction thereof. The light that enters an end surface of the light guide rod from the light-emitting elements is guided in the longitudinal direction while being reflected by an inner surface of the light guide rod. Then, the light is reflected by the light scattering surface, and is emitted to the document  34  as an illumination light having a uniform amount of light in the longitudinal direction. 
       FIG. 3  schematically shows the configuration of the rod lens array  30  for line-scanning, which is used in the image sensor unit  24 . The rod lens array  30  includes one row of rod lenses: a large number of rod lenses  40  are arranged between two rectangular substrates  41  and  42  and between the spacers  43  and  44  in a main scanning direction of the image sensor unit  24  in such a way that the central axes of the rod lenses, or optical axes  40   a , become parallel to each other. A gap between the rod lenses  40 , the substrates  41  and  42 , and the spacers  43  and  44  is filled with a thermosetting black silicon resin  45 , for example. In this manner, the rod lenses are bonded and fixed. 
     The two substrates  41  and  42  have extension portions  41   a  and  42   a , respectively, which extend a predetermined length toward the emission side thereof in the optical axis direction of the rod lenses  40 . The predetermined length of the extension portions  41   a  and  42   a  may be set based on an operating distance of the rod lenses  40 , for example. 
     According to the present embodiment, the extension portions  41   a  and  42   a  are equal in length in the optical axis direction of the rod lenses. Moreover, the extension portions  41   a  and  42   a  are so formed as to have a constant length in the longitudinal direction of the rod lens array. According to another embodiment, the optical-axis-direction lengths of the extension portions  41   a  and  42   a  may be different. Moreover, the optical-axis-direction length of each extension portion may not necessarily be constant in the longitudinal direction of the rod lens array. 
     According to the present embodiment, the adjacent rod lenses  40  are disposed in such a way as to be in close contact with each other, as shown in the diagram. According to another embodiment, a certain gap may be provided between the rod lenses  40  that are disposed. Moreover, two or more rows of rod lenses may be arranged. 
     The rod lenses  40  each are a cylindrical lens, which is uniformly circular in cross-section along the central axis or the optical axis with a radius of r 1 , and which has both end surfaces that are perpendicular to the optical axis and have been polished to be flat. Each rod lens  40  has a refractive index distribution in which the refractive index thereof decreases continuously from the central axis to an outer periphery. Furthermore, the rod lenses  40  of the present embodiment have the following refractive index distribution characteristics: the refractive index thereof continuously changes in the optical axis direction. 
     Moreover, the size and shape of the circular cross-section of the rod lenses  40  may be changed in an axis line direction. Furthermore, the cross-section of the rod lenses  40  may be formed into various shapes except for the circular shape, such as a polygon or cross, for example. 
     As shown in  FIG. 4 , a rod lens  40  has a refractive index distribution Q 1  in which, in the optical axis direction, an incident-side end portion region X 1  and an emission-side end portion region X 2  have the same refractive index N 1 , i.e., the refractive indices of the central axes of those regions, or the central refractive indices of those regions, are equal. However, in an intermediate region X 3  between the above regions, the central refractive index N 2  is greater than N 1 ; the intermediate region X 3  is so designed as to have a refractive index distribution Q 2  that is different from those of the incident-side and emission-side end portion regions X 1  and X 2 . Accordingly, the light that is reflected off a point image P 1  on the document surface  34   a  enters an incident surface  46  of the rod lens  40 , and meanders through the incident-side end portion region X 1  in accordance with the refractive index distribution Q 1 . After entering the intermediate region X 3 , the light meanders through the region in accordance with the refractive index distribution Q 2 . Furthermore, the light meanders through the emission-side end portion region X 2  in accordance with the first refractive index distribution Q 1 , before being emitted through an emission surface  47 . As a result, a point image I 1  is formed on light-receiving surfaces  33   a  of the photoelectric conversion elements  33 . 
     The optical-axis-direction refractive index distribution can be set in such a way that the refractive index gradually changes between the incident-side and emission-side end portion regions X 1  and X 2  and the intermediate region X 3 . According to another embodiment, the refractive index can be set in such a way as to rapidly change between the incident-side and emission-side end portion regions X 1  and X 2  and the intermediate region X 3 . Moreover, the refractive index may be set in such a way as to continuously change in the optical axis direction in the intermediate region X 3 . In this case, the peak central refractive index N 2  may not necessarily come at an optical-axis-direction central position of the rod lens  40 . 
     As described above, since the optical-axis-direction refractive index distribution is given, the frequencies of the light that meanders through the incident-side and emission-side end portion regions X 1  and X 2  in the optical axis direction are equal. The frequency of the light that meanders through the intermediate region X 3  in the optical axis direction is shorter than the frequencies for the incident-side and emission-side end portion regions X 1  and X 2 . In this case, the distance between the point image P 1  and the incident surface  46 , or the operating distance L 1 , is equal to the distance between the point image I 1  and the emission surface  47 . 
     This is compared with the conventional rod lens  1  shown in  FIG. 9 . Both rod lenses  1  and  40  have the same lens radius r 0  or r 1 , and have the same depth of focus. Therefore, assume that the operating distances L 0  and L 1  are equal. In the rod lens  40  of the present embodiment, the length of the intermediate region X 3  is shorter than a corresponding intermediate region of the conventional rod lens  1 . Accordingly, the lens length Z 1  is shorter than the lens length Z 0  of the conventional rod lens  1 . As a result, while keeping the same depth of focus, the conjugation length of the rod lens  40  (=Z 1 +2L1) is shorter than that of the conventional rod lens  1 . Therefore, the rod lens array  30  and the image sensor  24  can be made smaller in size. Moreover, since the lens radiuses are equal, the image reading performance does not drop. 
     The rod lenses  40  can be made by conventional techniques. For example, if a germanium-doped silica glass material is used for the production, it is known that different optical-axis-direction refractive index distributions can be given by changing the intensity of an emitted ultraviolet light in the optical axis direction to offer the refractive index distributions. Moreover, in the case of a plastics material, it is known that the refractive index can be changed in the optical axis direction by adjusting a light condensing region of a laser beam emitted to the polymer material. 
       FIG. 5  is a cross-sectional view of the image sensor unit according to the present embodiment. As shown in the diagram, the light receiving sensor  31  is fixed to a predetermined position through an appropriate fixing means, such as screws, fasteners, or adhesives, for example, at a lower end of a housing  48  of the image sensor unit  24  in such a way that the photoelectric conversion elements  33  face an inner side or an upper side in the diagram. 
     The rod lens array  30  is disposed above the light receiving sensor  31  in such a way that the extension portions  41   a  and  42   a  of the substrates  41  and  42  face a lower side, and that the photoelectric conversion elements  33  are positioned between the extension portions. The position of the rod lens array  30  is determined in such a way that both lower ends of the extension portions  41   a  and  42   a  come in contact with an upper surface of the sensor substrate  32 , and that the optical axes  40   a  of the rod lenses  40  pass through the centers of the photoelectric conversion elements  33 . In this manner, the rod lens array  30  is fixed to the housing  48 . The rod lens array  30  may be directly fixed to the sensor substrate  32  at the lower ends of the extension portions  41   a  and  42   a , and may be formed integrally with the light receiving sensor  31 . 
     In that manner, a narrow space is established as being closed by the extension portions  41   a  and  42   a  of the substrates  41  and  42  in the horizontal direction in the diagram, in such a way that the photoelectric conversion elements  33  are enclosed between the emission surface of the rod lens array  30  and the sensor substrate  32 . Therefore, even if the operating distance of the rod lenses  40  becomes longer, it is possible to prevent, in an effective manner, dust from getting into the optical path between the rod lenses  40  and the photoelectric conversion elements  33 , or from adhering to the emission surfaces of the rod lenses  40  or the light receiving surfaces of the photoelectric conversion elements  33 . 
     Furthermore, the rod lens array  30  can be positioned accurately and easily at a desired height and planar position with respect to the sensor substrate  32 .  FIG. 6A  shows a comparative example: a rod lens array  30 ′ having a conventional structure, in which rod lenses  40  of  FIG. 4  with a long operating distance are sandwiched between two substrates that are equal in length to the rod lenses  40 , is mounted on a housing  48 ′ in the same way as the conventional structure of  FIG. 9 . In this case, if the sensor substrate  32  of the light receiving sensor  31  is mounted on the housing  48 ′ in such a way as to be slightly inclined in the vertical direction, displacement occurs in terms of position and inclination between the incident ranges D 1  of the rod lenses  40  and the light-receiving ranges S 1  of the photoelectric conversion elements  33 . 
       FIG. 6B  shows the case where, in the image sensor unit  6  of  FIG. 10  in which the conventional rod lens array  8  with a short operating distance is mounted on the housing  7 , the sensor substrate  11  of the light receiving sensor  9  is similarly mounted on the housing in such a way as to be slightly inclined in the vertical direction at the same angle as that of  FIG. 6A . Even in this case, displacement occurs in terms of position and inclination between the incident ranges D 0  of the rod lenses  8  and the light-receiving ranges S 0  of the photoelectric conversion elements  12 . When  FIG. 6A  is compared with  FIG. 6B , the same displacement occurs in terms of inclination. However, the positional displacement in  FIG. 6A  is far larger than that in  FIG. 6B . The positional displacement between the incident ranges D 1  of the rod lenses and the light-receiving ranges S 1  of the photoelectric conversion elements  33  may lead to a significant drop in the quality of an output image. 
     According to the present embodiment, the lower ends of the extension portions  41   a  and  42   a  of the substrates of the rod lens array  40  come in contact with the upper surface of the sensor substrate  32 . Therefore, even if the sensor substrate is mounted in such a way as to be inclined with respect to the housing  48 , the above positional displacement does not occur between the incident ranges of the rod lenses  40  and the light-receiving ranges of the photoelectric conversion elements  33 . Therefore, a stable, excellent output image can be obtained at any time. 
     Moreover, the substrates  41  and  42  of the rod lens array are appropriately rigid. Therefore, it is possible to prevent deformation of the sensor substrate  32 . 
       FIG. 7  shows an image sensor unit  24  according to a modified example of the present embodiment. As shown in the diagram, in this modified example, on the sensor substrate  32  of the light receiving sensor  31 , through-holes  49  and  50  are provided at positions corresponding to the extension portions  41   a  and  42   a  of the substrates of the rod lens array  40 . The through-holes  49  and  50  are formed into a single groove shape that fits the shape and size of the extension portions  41   a  and  42   a.    
     The rod lens array  40  is fixed in a predetermined location as the extension portions  41   a  and  42   a  of the substrates  41  and  42  are fitted into the corresponding through-holes  49  and  50 . In this manner, the rod lens array  40  and the light receiving sensor  31  are formed as one unit, and both can be positioned more reliably. 
       FIG. 8  shows an image sensor unit  24  according to a modified example of  FIG. 7 . As shown in the diagram, in this modified example, on the sensor substrate  32  of the light receiving sensor  31 , instead of the through-holes, bottomed holes  51  and  52  are provided at positions corresponding to the extension portions  41   a  and  42   a  of the substrates of the rod lens array  40 . The bottomed holes  51  and  52  are formed into a groove shape that fits the shape and size of the extension portions  41   a  and  42   a.    
     The rod lens array  40  is fixed in a predetermined location as the extension portions  41   a  and  42   a  of the substrates  41  and  42  are fitted into the corresponding bottomed holes  51  and  52 . In this manner, the rod lens array  40  and the light receiving sensor  31  are similarly formed as one unit, and both can be positioned more reliably. 
     In particular, in the examples shown in  FIGS. 7 and 8 , the extension portions  41   a  and  42   a  of the substrates  41  and  42  may not have a constant length across the longitudinal-direction entire length of the rod lens array  40 . For example, on the sensor substrate  32 , the through-holes or bottomed holes may be formed as a large number of grooves or holes, not in a single, continuous groove shape. In this case, only portions of the extension portions  41   a  and  42   a  of the substrates that correspond to the through-holes or bottomed holes may protrude in such a way as to become longer depending on the depth of each hole. 
     The present invention has been described in connection with preferred embodiments. However, the present invention is not limited to the above embodiments. Needless to say, various changes or modifications may be made for the embodiments within the technical scope thereof.