Patent Publication Number: US-2023152552-A1

Title: Imaging device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from Japanese Patent Application No. 2020-071278 (filed on Apr. 10, 2020), the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to an imaging device. 
     BACKGROUND OF INVENTION 
     In recent years, smaller imaging devices with a greater number of pixels have been desired. A reduction in pixel pitch has led to a reduction in allowable assembly errors, for example, between lenses and between an imaging element and an objective lens. 
     For example, Patent Literature 1 discloses an imaging device including two units, each unit including lenses and a holding frame (lens barrel). When the units are assembled together, one unit can be moved with respect to the other along an optical axis to focus an objective lens on an imaging element and then the units can be fixed together with a thermosetting resin. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: International Publication No. 2015/064614 
       
    
     SUMMARY 
     In an embodiment of the present disclosure, an imaging device includes a first lens barrel, an imaging element, and a frame. The first lens barrel holds a first lens of an imaging optical system. The imaging element is disposed on an image side of the imaging optical system. The frame is attached to a substrate. The imaging element is mounted on the substrate. The frame is attached to the first lens barrel at a position on an object side of the first lens. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is an exploded view illustrating main elements of an imaging device according to an embodiment of the present disclosure. 
         FIG.  2    is an external view of the imaging device according to the embodiment of the present disclosure. 
         FIG.  3    is a sectional view illustrating the main elements of the imaging device according to the embodiment of the present disclosure. 
         FIG.  4    is a flowchart of a method for manufacturing the imaging device according to the embodiment of the present disclosure. 
         FIG.  5    illustrates an example in which a vehicle includes the imaging device according to the embodiment of the present disclosure. 
         FIG.  6    is a sectional view of an imaging device according to a comparative example. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     (Configuration of Imaging Device) 
       FIG.  1    is an exploded view illustrating main elements of an imaging device  10  according to an embodiment of the present disclosure.  FIG.  2    is an external view of the imaging device  10  according to the present embodiment.  FIG.  3    is a sectional view illustrating the main elements of the imaging device  10  according to the present embodiment.  FIG.  3    is a sectional view illustrating the main elements contained in, for example, a housing of the imaging device  10  taken along line A-A in  FIG.  2   . As illustrated in  FIGS.  1  to  3   , an orthogonal coordinate system is defined to correspond to the orientation of the imaging device  10 . A z-axis direction is a direction parallel to an optical axis of the imaging device  10 . The imaging device  10  captures an image of a subject. The subject is an object located in a positive z-axis direction from the imaging device  10 . The positive z-axis direction may be referred to as a direction toward an object side or front. A negative z-axis direction may be referred to as a direction toward an image side or rear. A y-axis direction corresponds to a width direction of the imaging device  10 . An x-axis direction corresponds to a height direction of the imaging device  10 . In the following description, positional relationships may be described using axes or planes of the orthogonal coordinate system. 
     As illustrated in  FIG.  1   , the imaging device  10  includes a second lens barrel  22 , an imaging optical system  20 , a first lens barrel  21 , an imaging element cover  33 , a frame  30 , a spacer  40 , an imaging element  31 , and a substrate  32 . As illustrated in  FIG.  2   , the imaging device  10  also includes a front housing  12 , a rear housing  13 , and a wiring unit  11 . As illustrated in  FIG.  3   , the imaging device  10  also includes a joining member  23 . Thus, the imaging device  10  includes a structure in which the front housing  12  and the rear housing  13  illustrated in  FIG.  2    cover the elements illustrated in  FIGS.  1  and  3   . Components of the imaging device  10  are described in detail below. Note that  FIGS.  1  to  3    are illustrative. The imaging device  10  does not necessarily include all of the components illustrated in  FIGS.  1  to  3   . The imaging device  10  may include one or more components other than the components illustrated in  FIGS.  1  to  3   . 
     The front housing  12  is positioned at the front of the imaging device  10  and protects components contained in the front housing  12  from, for example, an impact. The front housing  12  includes an opening that does not block light that travels toward the imaging optical system  20 . The opening in the front housing  12  exposes portions of the imaging optical system  20  and the second lens barrel  22 . The front housing  12  may be connected to the second lens barrel  22  through, for example, the fit between a projection  22 A of the second lens barrel  22  and the opening in the front housing  12 . As another example, the front housing  12  and the second lens barrel  22  may be connected to each other by another method, for example, by an adhesive or welding. The front housing  12  also has a function of pressing the imaging optical system  20  rearward so that the imaging optical system  20  does not fall through the opening. 
     The material of the front housing  12  may be, but is not limited to, a resin. Examples of the resin usable as the material of the front housing  12  include polyphenylene sulfide (PPS), polyetherimide (PEI), polyether ether ketone (PEEK), polycarbonate (PC), cyclo-olefin polymer (COP), ABS resin, polyethylene terephthalate (PET), and polystyrene (PS). However, the material is not limited to the examples mentioned above. 
     The rear housing  13  is positioned at the rear of the imaging device  10  and protects components contained in the rear housing  13  from, for example, an impact. The position of the rear housing  13  may be fixed with respect to the front housing  12  while the elements illustrated in  FIGS.  1  and  3    are disposed between the rear housing  13  and the front housing  12 . The rear housing  13  and the front housing  12  may be connected to each other by, for example, an adhesive or by another method, such as welding, fitting, or screwing. The material of the rear housing  13  may be, but is not limited to, a resin. Examples of the resin usable as the material of the rear housing  13  include, but are not limited to, the resins mentioned in the description of the front housing  12 . 
     The wiring unit  11  includes wires including a line for supplying electric power to the imaging device  10  and a signal line for outputting an image signal from the imaging element  31 . The wires of the wiring unit  11  may extend to the outside of the imaging device  10  through an opening in the rear housing  13  and be connected to an electronic device disposed outside the imaging device  10 . 
     The imaging optical system  20  includes at least one optical member and is designed to have desired optical characteristics, such as a focal length and a focal depth. In the present embodiment, the imaging optical system  20  includes a first lens  201 , a lens  202 , a lens  203 , a second lens  204 , and a lens  205  as optical members. The imaging optical system  20  may also include a diaphragm and an optical filter. The first lens  201 , the lens  202 , the lens  203 , the second lens  204 , and the lens  205  may be, for example, plastic lenses, but one or more thereof may be a glass lens or lenses. The number of lenses included in the imaging optical system  20  may be one or more and not more than four, or six or more. 
     The first lens barrel  21  holds one or more of the lenses included in the imaging optical system  20 . The first lens barrel  21  is disposed on the image side of the second lens barrel  22 , that is, behind the second lens barrel  22 . In the present embodiment, the first lens barrel  21  holds the first lens  201 . The first lens barrel  21  also holds the lens  202  and the lens  203 . In the present embodiment, of the lenses held by the first lens barrel  21 , the first lens  201  is positioned closest to the object side, that is, closest to the front end. The first lens  201 , the lens  202 , and the lens  203  may converge light that passes through the first lens  201 , the lens  202 , and the lens  203 . A lens group held by the first lens barrel  21  is to be attached with a higher positional accuracy than a lens group held by the second lens barrel  22  described below. In other words, when the lens group held by the first lens barrel  21  moves in an optical axis direction, the imaging position changes by a greater amount than when the lens group held by the second lens barrel  22  moves in the optical axis direction. 
     The first lens barrel  21  includes a tubular portion and a flange portion  21 A. The tubular portion surrounds an optical axis of the imaging optical system  20 . The flange portion  21 A projects in directions crossing the optical axis of the imaging optical system  20 . The flange portion  21 A may project in directions along a plane (xy plane) orthogonal to the optical axis of the imaging optical system  20 , or in directions along a plane inclined with respect to the xy plane toward the z axis. The flange portion  21 A is positioned at an end portion of the first lens barrel  21  at the object side. As described in detail below, the frame  30  is attached to the flange portion  21 A. The flange portion  21 A may include screw holes through which the flange portion  21 A can be screwed to the frame  30 . The first lens barrel  21  and the frame  30  may be connected to each other by another method, such as welding or fitting, instead of screwing. 
     The material of the first lens barrel  21  may be, but is not limited to, a resin. Examples of the resin usable as the material of the first lens barrel  21  include, but are not limited to, the resins mentioned in the description of the front housing  12 . The resin used as the material of the first lens barrel  21  preferably has a low hygroscopicity. In another example, the material of the first lens barrel  21  may be a metal, such as an aluminum alloy, a magnesium alloy, or a zinc alloy. 
     The second lens barrel  22  holds one or more of the lenses included in the imaging optical system  20 . The second lens barrel  22  is disposed on the object side of the first lens barrel  21 , that is, in front of the first lens barrel  21 . In the present embodiment, the second lens barrel  22  holds the second lens  204 . The second lens barrel  22  also holds the lens  205 . In the present embodiment, of the lenses held by the second lens barrel  22 , the second lens  204  is positioned closest to the object side, that is, closest to the front end. The second lens barrel  22  has a tubular shape that surrounds the optical axis of the imaging optical system  20 . An end portion of the second lens barrel  22  at the object side is a projection  22 A projecting in the positive z-axis direction, and is connectable to the front housing  12  as described above. The second lens  204  and the lens  205  may diffuse light that passes through the second lens  204  and the lens  205 . 
     The material of the second lens barrel  22  may be, but is not limited to, a resin. Examples of the resin usable as the material of the second lens barrel  22  include, but are not limited to, the resins mentioned in the description of the front housing  12 . The resin used as the material of the second lens barrel  22  preferably has a low hygroscopicity. In another example, the material of the second lens barrel  22  may be a metal, such as an aluminum alloy, a magnesium alloy, or a zinc alloy. 
     The joining member  23  joins the first lens barrel  21  and the second lens barrel  22  together. The joining member  23  adjusts the position of the second lens barrel  22  with respect to the first lens barrel  21  in six-axis directions so that the imaging element  31  can receive an image focused by the imaging optical system  20 . The adjustment in the six-axis directions means an adjustment in the x-axis direction, the y-axis direction, and the z-axis direction illustrated in  FIGS.  1  to  3    and rotation directions around these axes (pan, tilt, and roll). The joining member  23  joins an object-side end portion of the first lens barrel  21  to an image-side end portion of the second lens barrel  22 . The joining member  23  may be an adhesive capable of providing a predetermined interval between the first lens barrel  21  and the second lens barrel  22 . In other words, the second lens barrel  22  may be attached to the first lens barrel  21  with the adhesive. As described below, the predetermined interval is a small value less than a thickness adjustable by the spacer  40 . When the joining member  23  is an adhesive, the adhesive is preferably cured with ultraviolet light so that no optical displacement occurs due to contraction when the adhesive is cured. However, the adhesive is not limited to this, and may be a thermosetting adhesive. As another example, when the first lens barrel  21  and the second lens barrel  22  are each made of a metal, the joining member  23  may be a solder. As another example, the first lens barrel  21  and the second lens barrel  22  may be screwed to each other, and the joining member  23  may be a screw or a screw with a spring. 
     The frame  30  is attached to the first lens barrel  21  and the substrate  32  on which the imaging element  31  is mounted. The frame  30  includes an interior space that contains at least a portion of the first lens barrel  21 . As illustrated in  FIG.  1   , an object-side end portion of the frame  30  is screwed to the flange portion  21 A of the first lens barrel  21 . The first lens  201  may have various shapes depending on the design of the imaging optical system  20 . Therefore, in the following description, positional relationships are described with reference to a center of gravity G of the first lens  201 . As illustrated in  FIG.  3   , an attachment position C at which the frame  30  and the flange portion  21 A are joined together is on the object side of the center of gravity G of the first lens  201 . In other words, the frame  30  is attached to the first lens barrel  21  at a position on the object side of the first lens  201 . Such a structure facilitates calculation of influence of an environmental change on the imaging device  10  and allows for easier optical design. That is, a change in the shape of the first lens barrel  21 , a resulting change in performances of the lenses held by the first lens barrel  21 , and a change in the shape of the frame  30  can be calculated using the attachment position C as a common reference position. In response to an environmental change, such as changes in temperature and humidity, the first lens barrel  21  and the frame  30  each expand or contract with reference to the attachment position C, and therefore the lenses held by the first lens barrel  21  may be easily maintained focused on the imaging element  31 . A predetermined interval, that is, a gap, is provided between the frame  30  and a portion of the first lens barrel  21  excluding the flange portion  21 A. Even when the shape of the first lens barrel  21  changes due to a change in an operating environment, the optical performance is maintained as long as the change in shape occurs within the gap. In other words, when the shape of the first lens barrel  21  changes beyond the gap, a portion of the first lens barrel  21  other than the flange portion  21 A comes into contact with the frame  30 , and the optical performance may be degraded. 
     The frame  30  is attached to the substrate  32  with the spacer  40  disposed between the frame  30  and the substrate  32 . As illustrated in  FIG.  1   , the frame  30  may be screwed to the substrate  32  with the imaging element cover  33  and the spacer  40  disposed between the frame  30  and the substrate  32 . 
     The material of the frame  30  may be, but is not limited to, a metal. Examples of the metal used as the material of the frame  30  include an aluminum alloy, such as ADC12, a magnesium alloy, and a zinc alloy. The frame  30  may be a die casting to ensure high dimensional accuracy. 
     The imaging element cover  33  includes an opening that does not block a subject image that travels from the imaging optical system  20  to a light-receiving surface of the imaging element  31 . Due to the imaging element cover  33 , ambient light other than the subject image is not incident on the light-receiving surface of the imaging element  31 . The material of the imaging element cover  33  may be, but is not limited to, a resin. 
     The spacer  40  is positioned between the frame  30  and the substrate  32 , and serves to adjust the interval between the frame  30  and the substrate  32  in one-axis direction (z-axis direction). One or more spacers  40  are inserted between the frame  30  and the substrate  32 . The interval between the frame  30  and the substrate  32  in the z-axis direction is adjusted by the thickness or number of the one or more spacers  40 . In other words, the one or more spacers  40  may serve to adjust the interval between the frame  30  and the substrate  32  in the z-axis direction to position the imaging element  31  so that the imaging element  31  can receive the image focused by the imaging optical system  20 . The interval in the z-axis direction may be adjusted by the change in the number of the one or more spacers  40  that are inserted. In one example, the thickness of each spacer  40  in the z direction may be 50 μm. In this case, the interval in the z-axis direction may be adjusted in steps of 50 μm. The thickness of each spacer  40  in the z direction is preferably as small as possible, and is preferably not more than 100 μm, for example. More preferably, the thickness of each spacer  40  in the z direction is not more than 50 μm. Still more preferably, the thickness of each spacer  40  in the z direction is not more than 20 μm. Two or more spacers  40  with different thicknesses in the z direction may be used. The thickness of each spacer  40  may be set by various processes, such as polishing or etching. 
     The material of the one or more spacers  40  may be, but is not limited to, a metal. The material of the one or more spacers  40  preferably has a low coefficient of linear expansion. The metal used as the material of the one or more spacers  40  may be, for example, a stainless steel having a coefficient of linear expansion of not more than 16. As another example, the material of the one or more spacers  40  may be a ceramic. The one or more spacers  40  may be made of a resin as long as the resin has a low coefficient of linear expansion. 
     The imaging element  31  is disposed on the image side of the imaging optical system  20 . The imaging element  31  is capable of receiving the subject image focused by the imaging optical system  20 . The imaging element  31  captures the subject image focused on the light-receiving surface, converts the subject image into an image signal, and outputs the image signal. The imaging element  31  may be, for example, a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) image sensor. 
     The substrate  32  is a circuit board. Electronic components including at least the imaging element  31  are mounted on the substrate  32 . The imaging element  31  is mounted on a front surface of the substrate  32 . In other words, the imaging element  31  is mounted on the substrate  32  so that the light-receiving surface of the imaging element  31  is capable of receiving the subject image focused by the imaging optical system  20 . The position of the substrate  32  is fixed with respect to the frame  30  while the imaging element cover  33  and the one or more spacers  40  are disposed between the substrate  32  and the frame  30 . The substrate  32  may include screw holes having a diameter greater than the diameter of screws with which the substrate  32  can be screwed to the frame  30 . When the substrate  32  is screwed to the frame  30 , the position of the substrate  32  is adjusted in two-axis directions, which are the x-axis direction and the y-axis direction, and the subject image is focused on the light-receiving surface of the imaging element  31 . 
     (Manufacturing Method) 
       FIG.  4    illustrates an example of a method for manufacturing the imaging device  10  according to the embodiment of the present disclosure. The imaging device  10  including the above-described configuration may be manufactured in accordance with the flowchart of  FIG.  4   . First, the first lens barrel  21  is attached to the frame  30  (step S 1 ). More specifically, the flange portion  21 A of the first lens barrel  21  is screwed to the front of the frame  30 . 
     After the first lens barrel  21  is attached to the frame  30 , the substrate  32  is attached to the frame  30  with the one or more spacers  40  disposed between the substrate  32  and the frame  30  (step S 2 , substrate attaching step). More specifically, the substrate  32  on which the imaging element  31  is mounted is screwed to the rear of the frame  30  with the one or more spacers  40  disposed between the substrate  32  and the frame  30 . In step S 2 , the position of the imaging element  31  on the substrate  32  with respect to the frame  30  in the z-axis direction may be adjusted by the thickness or number of the one or more spacers  40  that are inserted. In addition, in step S 2 , the position of the imaging element  31  with respect to the frame  30  in the x-axis and y-axis directions may be adjusted by the position at which the substrate  32  is screwed to the frame  30 . The thickness in the z-axis direction or number of the one or more spacers  40  to be inserted may be determined in advance through measurement using a measurement device. For example, the focal position of the lenses on the first lens barrel  21  attached to the frame  30  may be measured with the measurement device to determine a target thickness of the one or more spacers  40  in the z-axis direction. Alternatively, the imaging optical system  20  may be measured while the second lens barrel  22  is temporarily placed on the frame  30  to which the first lens barrel is attached. The target thickness of the one or more spacers  40  in the z-axis direction is a thickness that enables the imaging optical system  20  to focus the subject image on the light-receiving surface of the imaging element  31  when the second lens barrel  22  is attached to the frame  30 . The second lens barrel  22  is attached to the frame  30  with the joining member  23  having a predetermined thickness after the first lens barrel  21  and the substrate  32  are attached to the frame  30 . 
     Then, an attachment position at which the second lens barrel  22  is to be attached to a combined body is determined (step S 3 ). The combined body is composed of the frame  30  to which the first lens barrel is attached. More specifically, the combined body includes the frame  30  to which the first lens barrel  21  and the substrate  32  are attached. The attachment position is determined so that the light-receiving surface of the imaging element  31  can receive the subject image focused by the imaging optical system  20 . The attachment position may be determined so that an optical axis of the second lens  204  and the lens  205  held by the second lens barrel  22  coincides with an optical axis of the first lens  201 , the lens  202 , and the lens  203  held by the first lens barrel  21 . 
     Then, the second lens barrel  22  and the combined body are attached to each other with the joining member  23  (step S 4 ). The joining member  23  attaches the second lens barrel  22  and the frame  30  to each other after an adjustment in the six-axis directions. 
     The joining member  23  provides a predetermined interval between the second lens barrel  22  and the first lens barrel  21  to adjust an interval that cannot be adjusted by the one or more spacers  40 . As described above, each spacer  40  preferably has a thickness of not more than 100 μm. In this case, the joining member  23  may have a thickness of less than 100 μm that is not adjustable by the one or more spacers  40 . For example, the joining member  23  may be a small amount of adhesive with a thickness of less than 100 μm. In general, the volume of an adhesive changes due to temperature increase and moisture absorption. However, when the joining member  23  is composed of a small amount of adhesive, the volume changes only by a small amount in response to temperature increase and moisture absorption. In this manufacturing method, the second lens barrel  22  and the combined body are attached to each other with the joining member  23  after the frame  30  and the substrate  32  are attached to each other with the one or more spacers  40  whose thickness or number is set. The positional relationship between the first lens barrel  21  and the imaging element  31  is determined based on the thickness or number of the one or more spacers  40 . Therefore, the distance between the second lens barrel  22  and the position at which the frame  30  is attached is roughly determined. Accordingly, the joining member  23  may be composed of a predetermined small amount of adhesive. In this case, an assembly clearance is substantially constant. 
     (Installation of Imaging Device in Vehicle) 
     The imaging device  10  including the above-described configuration may be installed in a vehicle  1  as, for example, an onboard camera. The imaging device  10  may be fixed externally to the front of the vehicle  1  to record the behavior of a vehicle in front of the vehicle  1 . The imaging device  10  may be fixed externally to the rear of the vehicle  1  to record the behavior of a vehicle behind the vehicle  1 . In another example, as illustrated in  FIG.  5   , for example, the imaging device  10  may be disposed in a side mirror and constitute an electronic mirror. In such a case, the imaging device  10  may capture an image of an area behind the vehicle  1  and provide a driver with the image as driving assistance information. The image captured by the imaging device  10  may be displayed on a display device disposed in a cabin of the vehicle  1 . The display device may be disposed on, for example, a rear-view mirror or an instrument panel so that the driver can see the display device while driving the vehicle  1 . 
     (Influence of Environmental Change) 
     The imaging device  10  installed in the vehicle  1  as an onboard camera is used in an environment with larger temperature and humidity variations compared to an indoor environment. Large changes in temperature and humidity may cause the components of the imaging device  10  to expand or contract. As described below, in the present embodiment, the imaging device  10  has an optical performance with less degradation due to a change in the operating environment. Therefore, the imaging device  10  is suitable for use as an onboard camera. 
     In the present embodiment, the imaging device  10  includes a nesting structure in which the first lens barrel  21  is disposed in the interior space of the frame  30  with a gap provided between the first lens barrel  21  and the frame  30  (see  FIG.  3   ). As described above, even when the shape of the first lens barrel  21  changes due to a change in the operating environment, the optical performance is maintained as long as the change in shape occurs within the gap. Therefore, even when the shape of the first lens barrel  21  changes due to an environmental change, such as changes in temperature and humidity, the optical performance can be maintained unless the shape of the first lens barrel  21  changes beyond the gap. 
     Since the first lens barrel  21  and the frame  30  form a nesting structure, the shapes of the first lens barrel  21  and the frame  30  change in the same direction in response to an environmental change. For example, when the first lens barrel  21  expands in the z-axis direction due to a temperature increase, the frame  30  also expands in the z-axis direction (see  FIG.  3   ). When the frame  30  expands in the z-axis direction, the position of the imaging element  31  mounted on the substrate  32  attached to the frame  30  moves in the negative z-axis direction. Therefore, the image focused on the light-receiving surface of the imaging element  31  is not easily displaced. Thus, even when the shapes of the first lens barrel  21  and the frame  30  change due to a temperature increase, the shapes change in the same direction. Therefore, the light-receiving surface of the imaging element  31  can receive the subject image focused by the imaging optical system  20 . 
     The attachment position C at which the frame  30  and the flange portion  21 A are joined together is on the object side of the center of gravity G of the first lens  201 . Of the lenses included in the imaging optical system  20  and attached to the first lens barrel  21 , the first lens  201  is positioned closest to the object side. More specifically, the attachment position C may be at end portions of the first lens barrel  21  and the frame  30  at the object side. Therefore, when the first lens barrel  21  and the frame  30  expand or contract in the z-axis direction due to a temperature change, the first lens barrel  21  and the frame  30  individually expand or contract in the same direction from the same reference point (attachment position C). Accordingly, the lenses of the imaging optical system  20  are maintained focused on the imaging element  31  despite the expansion or contraction of the first lens barrel  21 . Thus, degradation in the optical performance due to an environmental change can be reduced. 
     The frame  30  may have a coefficient of linear expansion less than that of the first lens barrel  21 . The frame  30  is attached to the first lens barrel  21  and the substrate  32  on which the imaging element  31  is mounted. Accordingly, a change in shape of the frame  30  affects the positions of the first lens barrel  21  and the imaging element  31 , and is therefore preferably small. A change in shape (expansion) of the first lens barrel  21  is relatively greater than that of the frame  30 . However, as described above, the first lens barrel  21  and the frame  30  are spaced from each other by a gap and do not come into contact with each other in an environment in which the imaging device  10  is normally used. 
     In the imaging device  10  according to the present embodiment, the frame  30  is attached to the substrate  32  with the one or more spacers  40  disposed between the frame  30  and the substrate  32 . Therefore, the position of the imaging element  31  is adjusted by the thickness or number of the one or more spacers  40  so that the imaging element  31  can receive the image focused by the imaging optical system  20 . Each spacer  40  is a member made of, for example, a metal or a ceramic having a thickness of, for example, not more than 100 μm. Thus, in the imaging device  10  according to the present embodiment, the position of the imaging element  31  can be finely adjusted by the one or more spacers  40 . Since the material of the one or more spacers  40  is, for example, a metal or a ceramic, the coefficient of linear expansion of the material is less than that of a resin, and a change in volume caused by an environmental change, such as changes in temperature and humidity, can be reduced. 
     In the imaging device  10  according to the present embodiment, the second lens barrel  22  and the frame  30  (more specifically, the first lens barrel  21  attached to the frame  30 ) are joined together with the joining member  23 . As described above, the joining member  23  has a small thickness that is less than the thickness of each spacer  40  and is composed of, for example, a small amount of adhesive. Therefore, the volume of the joining member  23  changes only by a small amount in response to temperature increase and moisture absorption. 
       FIG.  6    is a sectional view of an imaging device  110  according to a comparative example including no frame  30 . In the comparative example, the imaging device  110  includes a second lens barrel  22  and a substrate  32  on which an imaging element  31  is mounted. The second lens barrel  22 , the substrate  32 , and the imaging element  31  are the same as those included in the imaging device  10  according to the present embodiment. However, in the imaging device  110  according to the comparative example, the second lens barrel  22  and the substrate  32  are connected to a first lens barrel  121  including no flange with an adhesive portion  123  and an adhesive portion  124 , respectively. The adhesive portion  123  and the adhesive portion  124  are adhesives. 
     In the imaging device  110  according to the comparative example, when the shape of the first lens barrel  121  changes due to an environmental change, such as changes in temperature and humidity, the change in shape of the first lens barrel  121  affects the positional relationship between the lens group on the first lens barrel  121 , the second lens barrel  22 , and the imaging element  31  along the optical axis. As illustrated in  FIG.  6   , when the first lens barrel  121  expands in the z-axis direction, the imaging optical system  20  is deformed, and the imaging element  31  moves away from an incident portion. In the imaging device  110  according to the comparative example, the position of the substrate  32  is adjusted not by the one or more spacers  40  but by the adhesive portion  124 , which is an adhesive. Therefore, the adhesive portion  124  is deformed due to temperature increase and moisture absorption, and the deformation affects the reception of the image by the imaging element  31 . In the comparative example, the imaging device  110  includes the adhesive portion  123 , which is an adhesive, but includes no spacers  40 . Therefore, the amount of adhesive cannot be reduced. Accordingly, the adhesive portion  123  is deformed due to the influence of temperature increase and moisture absorption, and the deformation affects an optical path of the imaging optical system  20 . Thus, in the imaging device  110  according to the comparative example, the optical performance is inevitably degraded due to an environmental change, such as changes in temperature and humidity. 
     As is clear from a comparison with the comparative example, in the present embodiment, the imaging device  10  includes the above-described configuration and therefore has an optical performance with less degradation due to an environmental change, such as changes in temperature and humidity. The above-described manufacturing method enables manufacture of the imaging device  10  having an optical performance with less degradation. 
     Although the present disclosure has been described with reference to drawings and an embodiment, note that various changes and modifications are possible by those skilled in the art based on the present disclosure. Therefore, note that those changes and modifications are included in the scope of the present disclosure. For example, functions and the like included in each means or the like may be rearranged without any logical inconsistencies, and a plurality of means or the like may be combined together or divided. 
     For example, the imaging device  10  may include a processor that executes a process based on the image signal from the imaging element  31 . The processor may output the processed image signal to the outside of the imaging device  10  through the wiring unit  11 . The process performed by the processor based on the image signal may be, for example, an image process for adjusting brightness in accordance with external light, or an image process for displaying an image emphasizing a specified object included in the captured image. Examples of the specified object include a traffic sign and a white line on the road. 
     For example, the imaging device  10  may include a heat transfer member for dissipating heat generated by the imaging element  31 . The heat transfer member may be disposed between the front housing  12  and the imaging element  31  or between the rear housing  13  and the imaging element  31 . The heat transfer member is, for example, a flexible heat transfer sheet. The material of the heat transfer member may be, for example, silicone. 
     However, the material is not limited to this, and may be another material that transfers heat. 
     REFERENCE SIGNS 
     
         
         
           
               1  vehicle 
               10  imaging device 
               11  wiring unit 
               12  front housing 
               13  rear housing 
               20  imaging optical system 
               21  first lens barrel 
               21 A flange portion 
               22  second lens barrel 
               22 A projection 
               23  joining member 
               30  frame 
               31  imaging element 
               32  substrate 
               33  imaging element cover 
               40  spacer 
               110  imaging device 
               121  first lens barrel 
               123  adhesive portion 
               124  adhesive portion 
               201  first lens 
               202  lens 
               203  lens 
               204  second lens 
               205  lens