Abstract:
Low cost constructions of vehicular cameras employ various means for aligning and mounting the camera lens with respect to the imager. Such means include adhesive mounting using a UV curable adhesive, wherein the lens may be focused prior to cure of the adhesive. Other means include directly attaching the lens to the imager by adhesive; integrating the lens barrel and camera lens holder; dropping the lens barrel onto the surface of the imager; focusing the lens utilizing PCB mounting and focusing Screws; and focusing the lens by the relative positioning of camera front and back housings. Costs can also be contained by utilizing matching the resolution of the lens in conformance to human contrast sensitivity function, and by replacing optical chromatic aberration with digital chromatic aberration.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/163,240 filed Mar. 25, 2009 and U.S. Provisional Application No. 61/232,544, filed Aug. 10, 2009, the contents of both of which are incorporated herein. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to vehicular cameras, and more particularly, to low cost construction and assembly of such cameras. 
     BACKGROUND OF THE INVENTION 
     Vehicular cameras are used for a variety of purposes, such as to assist a driver in avoiding obstacles behind a vehicle when backing up, and to detect imminent collisions ahead of the vehicle when driving forward. A vehicular camera includes a lens that focuses video input on an image sensor provided on an imager. In general, the position of the lens relative to the image sensor can impact the quality of the video input received by the image sensor. For example, if the lens is positioned such that the video input is not in focus, then the video information passed to the driver may be blurry, and other vehicular systems, such as a collision detection system for example, may not function as well as they otherwise could. As the size of the camera is reduced, the positioning of the lens relative to the image sensor may be relatively more critical, at least because small variations in position can result in relatively large changes in angular offset. Therefore, the positioning of the lens relative to the image sensor may be particularly critical for vehicular rearview cameras. Furthermore, it is important that the camera be capable of holding the lens in position over a selected period of time under certain operating conditions, so that the performance of the camera is maintained over a useful operating life. 
     Several aspects of the camera may contribute to the overall tolerance in the position of the lens relative to the image sensor. For example, for lenses and lens holders that are threaded, the threaded connection therebetween has a tolerance associated with it. The angle of cast of the lens holder has a tolerance associated with it. The position of the imager has a tolerance associated with it. 
     It is desirable to provide a more integrated, lower cost camera assembly with means for positioning the lens relative to the imager within tolerance. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention is directed to a vehicular camera having a lens that is mounted to a lens holder and is held in position by a selected adhesive. The adhesive is capable of being initially cured relatively quickly by exposure to UV light for supporting the lens relative to the lens holder. The adhesive is further capable of being cured by exposure to a secondary curing condition, such as by exposure to heat, to achieve a fully cured strength, which may take a relatively longer period of time, such as minutes or hours. By providing an adhesive that is initially curable quickly but that reaches a selected fully cured strength and selected performance characteristics, the camera lends itself to having the lens positioned by a robot and then having the adhesive cured quickly to fix the position of the lens so that the camera can be transferred from the robot to a curing fixture for exposure to the secondary curing condition to fully cure the adhesive. Thus, the robot, which may be a relatively expensive component of a system used to manufacture the camera, can be used to adjust the lens of another camera, which may then be transferred to another curing fixture. 
     In a particular embodiment, the invention is directed to a vehicular camera including a lens, a lens holder, and an imager. The lens is connected to the lens holder by an adhesive. The adhesive is curable by UV light sufficiently to support the lens in the lens holder. The adhesive is further curable to a fully cured strength when exposed to a secondary curing step. The adhesive is configured to provide at least a selected strength of bond between the lens and lens holder when exposed to at least one selected operating condition for a selected period of time. The imager includes an image sensor positioned for receiving video input from the lens. The camera is configured to transmit to at least one other vehicular device signals relating to the video input received by the imager. In a further particular embodiment, the adhesive may be referred to as adhesive AD VE 43812 by Delo Industrial Adhesives of Windach, Germany. 
     In another aspect, a vehicular camera is provided which includes a first camera housing having an integrated barrel portion for holding optical components; optical components mounted in the barrel portion so as to form a lens; a retainer cap mounted to the barrel portion for containing and vertically positioning the optical components in the barrel portion; imaging circuitry including an image sensor positioned for receiving optical images from the lens; and a second camera housing, connected to the first camera housing so as to encase the imaging circuitry. 
     In another aspect, a vehicular camera is provided which includes a lens including a lens barrel holding optical components therein; an imager for receiving images from the lens; and a housing encasing the imager and a portion of the lens barrel. The lens barrel includes a feature for guiding and seating a periphery of the lens barrel onto the surface of the imager. Means such as adhesive or solderable retainer pins are provided for securing the lens barrel to the imager. And means are provided for ensuring focus between the lens and imager. The lens barrel may also be integrated with at least a portion of the housing. 
     In another aspect, a vehicular camera is provided which includes a lens including a lens barrel holding optical components therein; an imager for receiving images from the lens; a printed circuit board (PCB) for mounting the imager; a lens holder for mounting the lens barrel, the lens holder including a feature for guiding the lens barrel transversely relative to the imager; set screws for mounting the PCB to the lens holder; and means such as compressive gaskets, wave washers or lock washer in combination with the set screws to hold the axial position of the PCB and imager relative to the lens. 
     In another aspect, a vehicular camera is provided which includes a first camera housing having an integrated barrel portion for holding optical components; optical components mounted in the barrel portion so as to form a lens; an imager for receiving images from the lens; a printed circuit board (PCB) for mounting the imager; and a second camera housing to which the PCB is mounted, where the first and second camera housings in combination encasing the imager and PCB. The first and second camera housing are secured via UV-cured adhesive that is cured with UV light only after the position of the second camera housing relative to the first camera housing is set to bring the lens in focus and optically center-aligned with the imager. 
     In another aspect, a vehicular camera is provided which includes a camera housing having an integrated barrel portion for holding optical components; optical components mounted in the barrel portion so as to form a lens; an imager for receiving images from the lens; a printed circuit board (PCB) for mounting the imager. The PCB is secured to the camera housing by a UV-cured adhesive that is cured only after the position of PCB relative to the housing is set to bring the lens in focus and optically center-aligned with the imager. 
     In another aspect, a vehicular camera is provided which includes a lens including a lens barrel holding optical components therein; an imager for receiving images from the lens; and a printed circuit board (PCB) for mounting the imager. The lens barrel is directly secured to the imager by a transparent UV-cured adhesive fixing the lens barrel to at least one of the imager and the PCB. The adhesive is cured only after the position of lens barrel relative to the imager is set to bring the lens in focus and optically center-aligned with the imager. 
     In another aspect, an improved vehicular camera system is provided where the lens resolution is selected to meet but not substantially exceed a resolution determined from the size of a display, a distance between an observer and the display, a selected point on a contrast sensitivity function, and the size of an imager sensing surface. 
     In another aspect, an improved vehicular camera system is provided where wherein the lens omits achromatic lenses and employs digital chromatic correction based on a predetermined chromatic aberration measurement. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described by way of example only with reference to the attached drawings in which: 
         FIG. 1  is an exploded perspective view of a vehicular camera in accordance with an first embodiment of the invention wherein a lens barrel is adhesively secured to a lens holder via a UV-curable adhesive; 
         FIG. 2  is a cutaway side view of the vehicular camera shown in  FIG. 1 , in an assembled state; 
         FIG. 3  is a schematic cross-sectional view of a variant of the first embodiment; 
         FIG. 4  is a cross-sectional view of a prior art lens; 
         FIG. 5  is a schematic cross-sectional view of a second embodiment of the invention wherein a lens barrel is integrated with a camera lens holder; 
         FIG. 6  is a schematic cross-sectional view of a third embodiment of the invention wherein a lens barrel is dropped on a surface of an imager; 
         FIG. 6A  is a detail view of a portion of  FIG. 6 ; 
         FIG. 7  is a schematic cross-sectional view of a variant of the third embodiment; 
         FIG. 8  is a detail cross-sectional view of the third embodiment; 
         FIG. 9  a schematic cross-sectional view of a fourth embodiment of the invention wherein a lens is focused by PCB mounting screws; 
         FIG. 10  is a schematic cross-sectional view of the fourth embodiment including a back housing; 
         FIG. 11  is a schematic cross-sectional view of a fifth embodiment of the invention wherein a lens is focused by the selective positioning of camera front and back housings; 
         FIG. 12  is a schematic cross-sectional view of a variant of the fifth embodiment; 
         FIG. 13  is a schematic cross-sectional view of a variant of the fifth embodiment, wherein a PCB is selectively positioned; 
         FIG. 14  is a schematic cross-sectional view of a sixth embodiment of the invention wherein a lens is focused by directly attaching a lens to an imager through a transparent adhesive; 
         FIG. 15  is a graph of a contrast sensitivity function; and 
         FIG. 16  is a graph of an example of lens chromatic aberration. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiment 1 
     Use of UV-Curable Adhesive to Mount Lens to Holder 
       FIG. 1  shows an exploded view of a vehicular camera  10  in accordance with a first embodiment of the invention. The vehicular camera  10  includes an imager  20 , a lens holder such as a front camera housing  14  and a lens  16 . The vehicular camera  10  may include other components such as additional circuitry for processing the video input received by the imager  20 , e.g., circuitry for providing graphic overlay to the video input. The vehicular camera  10  may further be configured to transmit the video input to other vehicular devices, such as a display controller (not shown) for a cabin-mounted display (not shown). 
     The imager  20  may be a charge-coupled device (CCD) or a complimentary metal-oxide semiconductor (CMOS) sensor. Referring additionally to  FIG. 2 , the imager  20  is mounted to a printed circuit board (PCB)  12 . The imager  20  is positioned to receive optical images from the lens  16 . In the exemplary embodiment shown in  FIG. 1 , the imager  20  is connected to the lens holder  14  by a plurality of threaded fasteners  22 . 
     The lens  16  is mounted to the lens holder/front camera housing  14  at a selected position for focusing video input onto the sensing surface of the imager  20 . The lens  16  may be any suitable type of lens known in the art. The lens  16  may have an exterior surface  24  that is configured to be received in a cylindrical aperture  26  having an aperture wall  28  on the lens holder/front camera housing  14 . The exterior surface  24  and the aperture wall  28  may have a selected amount of clearance therebetween, shown by a gap G. An adhesive  30  is provided for holding the lens  12  in a specific position relative to the lens holder/front camera housing  14 . More particularly, the adhesive  30  may be applied between a first axial face  32  on the lens holder/front camera housing  14 , and a second axial face  34  on the lens  16 . 
     The position of the lens  16  relative to the imager  20  impacts the degree of focus present in the optical images received by the imager  20  and thus the performance of the camera  10  and the optical alignment of the optical image on the imager. 
     To control the position of the lens  16 , a positioning system (not shown) may be provided that includes a robot (not shown). The robot holds and adjusts the position of the lens  16  relative to the lens holder/front camera housing  14  until a target object appears in suitable focus and at a suitable position on the imager  20 , prior to hardening of the adhesive  30 . The adjustment of the lens  16  relative to the lens holder/front camera housing  14  is facilitated by providing the selected amount of clearance between the exterior surface  24  of the lens  16  and the aperture wall  28  of the lens holder/front camera housing  14 . Additionally, the thickness of the layer of adhesive  30  between the lens  16  and lens holder/front camera housing  14  may be selected to provide a suitable amount of relative angular adjustment between the lens  16  and lens holder  14 /front camera housing. The thickness of the layer of adhesive may be approximately 0.75 mm prior to adjustment of the lens  16 . 
     Once the lens  16  has been suitably positioned by the robot, the adhesive  30  is initially cured by exposure to UV light while the robot holds the lens  16  in position. The UV light may be provided from a plurality of UV sources about the periphery of the camera  10 . The initial curing of the adhesive  30  may result in the adhesive being strong enough to hold the lens  16  in the lens holder/front camera housing  14  without needing the robot to grip the lens  16 , and may take less than 7 seconds. However, the lens  16  may be susceptible to movement if it incurs a relatively small disturbance at this stage. After the initial curing, the camera  10  may be placed by the robot relatively gently on a conveyor (not shown) and moved to a UV curing station (not shown) for a further UV curing period, such as, for example, 25 seconds. Another UV curing station (not shown) may optionally be provided to further cure the adhesive  30  for another period, such as 25 seconds, after the camera  10  leaves the first UV curing station. Subsequent to the UV curing, the camera  10  may be transferred to another curing station where the adhesive  30  can be thermally cured, or may be cured by exposure to some other secondary curing condition, to achieve its fully cured strength so that it can hold the lens  16  in position during use on a vehicle. The step of initially curing the adhesive  30  using UV light may be relatively instantaneous. This step of thermally curing the adhesive may take several minutes or hours. As an additional or alternative curing measure, the adhesive  30  may be moisture cured. 
     Providing an adhesive  30  that has an initial curability by UV light is advantageous in that the robot is not needed to hold the lens  16  in position over the period of time that it would take for the secondary curing condition to sufficiently harden the adhesive  30  to be self-supporting. Once the camera  10  is transferred from the robot to the curing fixture, the robot can be used for the positioning of another lens  16  in another lens holder  14 /front camera housing. Because the task of positioning the lens  16  and initially curing the adhesive  30  using UV light can take less time than fully thermally curing of the adhesive  30 , a single robot can feed cameras  10  with initially cured lenses to a plurality of curing fixtures, thereby providing the capability of achieving a relatively high rate of production per robot. 
     Once fully cured, the adhesive  30  may be capable of holding the lens  16  in position with at least a selected strength of bond between the lens  16  and lens holder/front camera housing  14  under one or more selected operating conditions. For example, the adhesive  30  may be capable of holding the lens  16  in position after a selected time period of 1000 hours of exposure to a selected temperature of 85 degrees Celsius and optionally a humidity of approximately 85%. Any of the aforementioned selected values may be selected to suit the particular environment that the camera  10  is expected to experience during use. The selected time period may, for example, be some other time period, such as approximately 1200 hours. The selected adhesive  30  may be further capable of holding the lens  16  in position after a selected time period exposed to a selected temperature of −40 degrees Celsius. The fully cured adhesive  30  may have other performance characteristics including: maintaining at least 70% of its strength (e.g. tensile strength) during exposure to temperatures ranging from −40 degrees Celsius to 95 degrees Celsius, having a tensile strength of at least 1000 psi, having a Shore D hardness value of at least 50, having a viscosity of between about 30000 and 70000 centipoise, being non-hygroscopic (so that it does not swell significantly when exposed to moisture), having a cure depth of at least 3 mm, having the capability to bond to Polybutylene Terephtalate/Polycarbonate and/or Polyphenylene Sulfide and/or liquid crystal polymer and/or anodized aluminum, having a bond shear strength of at least 1000 psi with less than a 60% reduction in its bond shear strength at 85 degrees Celsius, little or no outgassing, being capable of withstanding exposure to salt fog, being capable of withstanding typical automotive chemicals, such as gasoline and automotive cleaning agents, having a glass transition temperature that is at least 90 degrees Celsius and being ‘automotive-grade’ (i.e. being generally applicable for use in a vehicle). 
     The adhesive  30  may be applied by the robot itself prior to adjustment of the lens  16  relative to the lens holder/front camera housing  14 . Alternatively, the adhesive  30  may be applied by some other device prior to (or during) possession of the camera  10  by the robot. 
     Aside from fixing the position of the lens  16  relative to the lens holder/front camera housing  14 , the adhesive  30  may also hermetically seal the interior of the camera  10  against the outside environment. 
     Numerous adhesives were attempted for use as the adhesive  30 . For example, it was found that some adhesives, such as some UV-cure free radical acrylates that have the capability of being initially cured using UV light, have a reduced strength (e.g. tensile strength) under exposure to elevated operating temperatures such as 85 degrees Celsius over a selected period of time. It was further found that adhesives, such as some UV-curable free radical epoxy hybrids also have a reduced strength (e.g. tensile strength) under exposure to elevated operating temperatures such as 85 degrees Celsius over a selected period of time. Some cationic epoxies that were tried also lost their strength when exposed to a temperature of 85 degrees Celsius and a humidity of 85% over a selected period of time. Some anionic cyanoacrylates that were tried were unsuitable as they produced too much outgas for optical use. Other adhesives, such as some UV-cure free radical silicones have a relatively low dimensional stability and are thus not suitable. 
     Surprisingly, it was found that a suitable adhesive that can be used for the adhesive is adhesive AD VE 43812 manufactured by Delo Industrial Adhesives of Windach, Germany. This adhesive is a low-temperature cure, epoxy-amine adhesive that can be cured initially relatively quickly by exposure UV light. 
       FIG. 3  shows a variant  100  of the rear view camera  10 . This embodiment incorporates a lens  112 , a front housing/lens holder  130 , a back housing  132  and an imager  140 . As shown in greater detail in  FIG. 4 , the lens  112  includes a lens barrel  114  in which lens optical elements  120 , O-ring  122 , spacers  124  and IR cutoff filter  126  are mounted and held in place by a retainer cap  116 . The front housing  130  holds the lens barrel  114  via a threaded connection, or an adhesive flange as discussed above. A printed circuit board (PCB)  138  with imager  140  is mounted in the housing defined by the front and back housing parts  130 ,  132 . Screws  134  are used for this purpose. In order to mount the lens  112 , it is first positioned in the housing  130 ,  132  by a robot or multi-axis focusing machine (not shown) so as to provide a focused image relative to the imager  140  and once properly aligned the lens  112  is thereafter fixedly attached to the front housing  130 . The sealing between the lens  112  and front housing  130  is preferably provided by the adhesive discussed above, or by utilizing a thread lock device. Then, the back housing  132  is attached to the front housing  130  by laser or ultrasonic welding, adhesive, or via a press fitting. 
     Embodiment 2 
     Integration of Lens Barrel and Camera Lens Holder 
       FIG. 5  shows another embodiment  110  of a vehicular camera, wherein the lens barrel  114  housing the optical components of the lens  112  and the camera front housing  130  form a single integrated piece  150 . The lens optical elements  120 , O-rings and spacers  122 ,  124  and IR cutoff filter  126  ( FIG. 4 ) are placed inside a lens barrel portion  114 ′ of the integrated lens barrel and camera upper housing piece  150  as part of the conventional lens assembly process to provide a lens  112 ′ ( FIG. 5 ). The integrated piece  150  can be formed by plastic injection molding or metal machining. Plastic injection molding is preferred for lower cost and ease of attaching the back housing  132  to the integrated piece  150  by gluing, laser or ultrasonic welding. 
     The PCB  138  with imager  140  is mounted to the integrated piece  110 . Lens  112 ′ is focused relative to the imager  140  by applying techniques described in embodiments 3 to 6. 
     The advantages of this embodiment  110  include a savings in tooling cost as one expensive upper housing plastic molding tool is eliminated; material cost savings since less plastic material is used and no expensive adhesive or thread lock epoxy is needed; and a more simplified camera assembly process since the step of attaching the lens to the upper housing is eliminated. 
     Embodiment 3 
     Lens Barrel Dropped on Surface of Imager 
       FIG. 6  shows another embodiment  200  of a vehicular camera wherein the lens barrel  114 ′ of the integrated piece  150  is dropped onto and sits directly on top of the surface of the imager  140 . During the camera assembly process, the lens barrel  114 ′ is dropped directly onto the imager  140  as shown in  FIG. 6 . The lens barrel  114 ′ includes a special designed mechanical feature such as rebate  202  (see detail view of  FIG. 6A ) so that, while the lens barrel  114 ′ is dropped to onto the imager  140 , the rebate  202  guides the lens  112 ′ to have proper horizontal alignment such that the lens optical axis is in line with the center of the imager sensing area. 
     (The alignment of optics axis to the center of the imager can also be achieved by digital shifting of image sensing window on imager. This digital center shifting feature can be found in some imagers, e.g. Aptina MT9V 126 CMOS imager.) 
     As shown in  FIG. 6 , the lens  112 ′ can be secured by applying adhesive  204  (such as UV-cured adhesive) around the interface of the lens barrel  114 ′ with the imager  140  and PCB  138 , thus fixing the lens focus position. In a variant  200 ′ shown in  FIG. 7 , an alternative way to fix the lens  112 ′ to the imager  140  is to include metal insert pins  206  in the lens barrel  114 ′. The metal insert  206  is then soldered to the PCB  138  during PCB reflow process to fix the lens  112 ′ to the PCB  138 . 
     As shown in  FIG. 8 , the distance from the lens principal plane LPP to the lens seating surface H 1  (which is defined by a cover glass  158  that is spaced apart from imaging sensor surface  160 ), and the distance H 2  between the imaging sensor surface  160  to the top surface of cover glass  158  need to satisfy the relation H 1 +H 2 =F+ΔF, where F is the effective focal length of the lens, and ΔF is the focus tolerance range. 
     ΔF multiplied by two ΔF*2) is also called depth of focus, which can range from a few micrometers to hundreds of micrometers. For a typical automotive camera, the depth of focus is about 40 to 70 micrometers. H 1  and H 2  are the two sources that contribute to the variation of focus. The lens barrel  114 ′ may be designed to have a tightly controlled length tolerance. The barrel length can be designed such that when it is dropped on the imager cover glass  158 , the lens is focused right to the imager sensing surface  160  nominally. The imager  140  can also be designed such that the distance H 2  between the sensing surface  160  and the top cover glass surface of the imager has a tight tolerance. However, the lenses and imagers manufactured will always have variations from their designed nominal values. The variation of H 1  and H 2  can stack up and drive the lens imager pair out of focus. 
     To control the focus tolerance and increase manufacturing yield, one or more of the following methods can be employed: 
     First, use optical technology such as wavefront coding as promoted by OmniVision. The technology uses specially designed lens elements and image processing algorithm to increase the depth of focus (ΔF) of the lens. The wider lens depth of focus allows more tolerate of focus position variation. The manufacturing yield and product focus quality can be maintained high. 
     Second, use a laser or other means to cut or ablate extra lens barrel material in the bottom of the lens barrel  114 ′ so that the correct lens barrel length can be altered to achieve good focus. A pre-laser ablation focus measurement is performed to determine how much barrel material to ablate. To address the case that the lens being too short, one can design the lens barrel so that it is always in the longer side. 
     Third, bin and match lens  112 ′ and imager  140  to achieve good drop-on focus. The idea is to measure and sort lenses and imagers. Bin the lenses and imagers to matching groups. For example, a lens group with Plus 20 to 30 micrometer too long of flange focal length is matched with an imager group with Minus 20 to 30 micrometer too short of silicon to top glass distance. The two groups will form a good focus camera. 
     It will thus be seen that by directly dropping the lens  112 ′ to the image sensor  140 , it is possible to avoid a time-consuming assembly step in the camera manufacturing process which requires actively searching for best focus position. It results in a reduced cycling time and increased production efficiency, and avoids the use of a very expensive multi-axis focus machine. 
     Embodiment 4 
     Lens Focus by PCB Mounting and Focusing Screws 
       FIG. 9  shows another embodiment  300  of a vehicular camera where a camera front housing  330  includes a mechanical guidance feature such as wall  302  for guiding the lens  112  to proper horizontal alignment with the imager  140  so that the lens optical axis is in line with the center of the imager sensing surface. In this embodiment, the PCB  138  with imager  140  is attached to the front housing  330  by screws  304  but also utilizing compressive gaskets, wave washers or lock washers  306  held between the PCB  138  and body of the front housing  330 . The focusing between the lens  112  to imager  140  is accomplished by turning these screws  304  and actively monitoring camera output. 
     The alignment of the lens optical axis and imager center can be achieved by digitally shift the image window on the imager. 
     Referring additionally to  FIG. 10 , the attachment of the camera back housing  132  to the front housing  330  (not drawn in this drawing) can be achieved by laser or ultrasonic welding, glue, press fitting, screw together or other means. 
     The camera front housing in this embodiment may also employ an integrated lens barrel as discussed in with reference to embodiment  110 . 
     Embodiment 5 
     Lens Focused by Positioning of Camera Front and Back Housings 
       FIG. 11  shows another embodiment  400  of a vehicular camera in which the integrated lens barrel and camera upper housing piece  150  of embodiment  110  is attached to the camera back housing  132  by UV cured glue  402 . The glue is applied before focus. An active focus and alignment (utilizing a multi-axis focusing machine) is performed to reach optimum lens focus and optical axis alignment to the imager center. While holding the integrated lens barrel and camera front housing piece  150  in the position achieving the best focus and alignment, a robot applies UV illumination to the adhesive to cure it and fix the position of the lens  112 ′ and seal the camera. In this embodiment, the PCB  138  is mounted to back housing by screws  134 , glue between PCB and back housing or other means. 
     In a variant  400 ′ shown in  FIG. 12 , the UV cured adhesive  402 ′ also replaces the screws  134  used to mount the PCB  138  to the housing. The adhesive  402 ′ thus attaches the PCB  132  to the back housing  138 , fixes the integrated piece  150  to the back housing  132 , and seals the camera. 
     In another variant  400 ″ shown in  FIG. 13 , the imager PCB is focused and aligned and then fixed to the lens barrel and camera front housing piece  150  by UV cured adhesive  402 ″ applied on and between the PCB  138  and standoff parts  404 ″ of the integrated piece  150 . During the focus assembly process, the imager PCB  138  is grabbed and moved in x, y and z direction(s), and optionally in two rotational directions, to achieve optimum focus and alignment. While the imager PCB  138  is being held in the position, UV illumination is applied to cure the adhesive  402 ″. 
     Embodiment 6 
     Direct Attachment of Lens and Imager by Adhesive 
       FIG. 14  shows another embodiment  500  of a vehicular camera in which transparent UV-curable adhesive  502  is applied directly between lens  112  and imager  140  and/or PCB  138 . The adhesive  502  is provided as a relatively large blob to bonds the lens  112  to the imager  140  and/or the PCB  138 . The focus and alignment of the lens  112  is performed before UV light cures the adhesive. The adhesive preferably encapsulates the imager  140  and acts a protective shield for it. 
     In a preferred method of assembly, adhesive is applied on and around the imager in a controlled amount. A 5-axis robot (not shown, with motions in x, y, z and two orthogonal rotations) also grips and dips the lens into a batch of adhesive. The robot then focuses and aligns the lens to the imager, whereupon UV light is applied to cure the adhesive. The robot then releases the lens. 
     This embodiment simplifies the lens barrel design and reduces the lens size. This embodiment can also be more advantageous than embodiments that utilize a threaded lens, which can be slow to focus or difficult to hold, or a press-fit lens, which provides only coarse movement and thus can be difficult to control. Thus, a more accurate alignment can be obtained. 
     In all of the foregoing embodiments its also desired to reduce the cost of the lens itself. This can be accomplished in one or more of the following ways. 
     First plastic may be used for the lens barrel  114  and retainer cap  116 . The barrel and cap are preferably made by injection molding of plastic material like PPS. This material is dense, nonporous, rigid and has ultra low hygroscopic characteristics and thus it meets the special environmental and durability requirements for a rear view camera lens. 
     Second, the lens  112 ′ may be formed to incorporate only one glass element as the outer-most element  120   a  ( FIG. 4 ) of the lens, and utilize two or three plastics lens (made by injection molding) for the inner optical elements. An alternative configuration may include two glass elements and one or two plastic lenses. Minimizing the number of glass elements reduces cost of the optical components. 
     In addition, cost savings can be realized by eliminating the lens IR cutoff filter  126  which is conventionally provided as a glass plate. Instead, the IR cutoff filter can be moved to the imager cover glass  120   a . One added benefit of eliminating the IR cutoff filter in the lens is that it reduces or eliminate light multi-reflection between the flat IR cutoff filter and imager cover glass  120   a . This multi-reflection can cause lens flare and ghost images. 
     Third, lens cost can be reduced by lowering the lens resolution. The lens resolution can be reduced to a level that fits the application requirement of the camera. More particularly, the human eye resolution perception can be represented by a contrast sensitivity function (CSF) as shown in  FIG. 15 . The CSF peaks within a range of 1 to eight cycles per degree, where a cycle is defined as a one transition from black to white (or vice versa), which may be referred to in the literature as a “line pair”. Thus, a required resolution can be determined from the display size, the distance between the observer and the display, the selected CSF, and the size of the imager sensing surface. 
     For example, consider a 7-inch diagonal display (with a 16×9) aspect ratio. It has a horizontal dimension of 155 mm. Assume the distance between the observer and the display is 600 mm, which is the average distance between a driver&#39;s eyes and a display in the vehicle center console. Select a CSF of 7 cycles per degree, which is a reasonable compromise between machine vision and human vision requirements. And assume that the imager has a horizontal sensing width of 3.58 mm. One angular degree represents a width of 10.5 mm at distance of 600 mm. The display resolution required is 0.67 line pairs/mm. The required camera resolution is thus 28.9 line pairs per mm. Thus, a camera can produce a sufficient resolution is its lens yields a camera level modulation transfer function of 28.9 line pairs per mm. 
     Other examples of sufficient camera resolutions are provided in the chart below: 
     
       
         
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Display diagonal size (inch) 
               
             
          
           
               
                   
                 8 
                 7 
                 6 
                 3.5 
                 2.5 
               
               
                   
                   
               
             
          
           
               
                 Aspect Ratio 
                 16 × 9 
                 16 × 9 
                 16 × 9 
                 4 × 3 
                 4 × 3 
               
               
                 Horizontal Dimension 
                 177 
                 155 
                 133 
                 71.1 
                 50.8 
               
               
                 (mm) 
               
               
                 Eye to Display Distance 
                 600 
                 600 
                 600 
                 500 
                 500 
               
               
                 (mm) 
               
               
                 mm per 1 degree at 
                 10.5 
                 10.5 
                 10.5 
                 8.7 
                 8.7 
               
               
                 Display 
               
               
                 At display resolution 
                 0.668 
                 0.668 
                 0.668 
                 0.802 
                 0.802 
               
               
                 (lp/mm) 
               
               
                 Required camera 
                 33.0 
                 28.9 
                 24.8 
                 15.9 
                 11.4 
               
               
                 resolution (lp/mm) 
               
               
                   
               
             
          
         
       
     
     Thus, lens resolution can be reduced to the limits dictated by the CSF in order to reduce cost. Prior art lenses may have too high resolution for human visual perception, and high resolution lenses can adversely cause a negative consequence called the “Moire Effect”. Some of prior art camera designs utilized an optical low pass filter to lower the image sharpness of the lens to eliminate the “Moire Effect”. The optical low pass filter adds cost to camera along with the higher cost high resolution lens. 
     Fourth, lens cost can be reduced by not optically addressing any chromatic aberration in lens. Lens chromatic aberration can cause the resultant image to have color fringes at the edges of objects, as well as lower image resolution. Lens chromatic aberration can typically be fixed or mitigated by a pair of glass lens cemented together, the so-called achromat pair. However, for a low cost lens solution, the chromatic aberration is not fixed in the lens, rather, the imager system-on-chip (SOC) or an adjunct digital processor applies digital correction to correct the chromatic aberration. The chromatic aberration typically has fixed amount of spatial separation among different colors at a specific off-axis angle, as shown in the lateral color diagram example of  FIG. 16 . 
     The basic principle of digital correction of chromatic aberration is as follows. 
     Every pixel of an imager has individual values of red, green and blue colors. By shifting one pixel colors to one or more other pixels, and repeat the process to the whole imager, it is possible to correct or reduce the effect of lens chromatic aberration. Based on the lateral color separation of the lens, like the example graph shown in  FIG. 16 , the separation of the color as a function of the distance from the center of the imager is known. For each imager pixel, it is possible to calculate the distance needed to shift every individual colors of the pixel. The shift happens in a radial direction because of the lens&#39; symmetry to its axis. In each pixel, new position coordinates of each color is re-calculated. Then this color value will be sent to the new pixel whose coordinates were calculated. The other two colors of this pixel are also calculated and sent to new pixels. 
     This shifting or redistribution of the pixel colors can be performed in System-On-Chip (SOC) part of imager, or a separate processor after the imager. The processor can be a microprocessor, a DSP, or a FPGA or other digital devices. Adding some gates or logical units to an existing digital processing unit most likely is less expensive than adding achromat glass elements in lenses. The lens chromatic aberration is typically symmetric over the optical axis, which lowers the complexity of digital chromatic aberration in the SOC or processor. 
     Lens manufacturing variation may cause the chromatic aberration to not be totally cylindrically symmetric. The spectral response of every imager pixel may thus have variations. To correct the negative effect to digital chromatic aberration caused by these two variations, one can apply calibration procedures. During a calibration procedure, a special target, an image acquisition and image processing algorithms are used to calculate lateral color separation at every pixel. Then the pixel related lateral color values are used in digital chromatic aberration correction process described above. 
     While the above describes particular embodiment(s) of the invention, it will be appreciated that modifications and variations may be made to the detailed embodiment(s) described herein without departing from the spirit of the invention.