Abstract:
A method and camera assembly for use in a machine vision system, the assembly comprising a support structure forming a mounting flange that is configured for coupling with any of a plurality of exchangeable electrically controllable adjustable focal length lens assemblies, a two dimensional image sensor supported by the support structure and forming a sensor plane spaced from the mounting flange by a flange focal distance and a processor programmed with a flange focal distance error and to use the flange focal distance error to generate lens control signals to compensate for the flange focal distance error when a lens is mounted to the mounting flange.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    The invention relates to calibration of imaging systems and more specifically to calibration to compensate for a flange focal distance error due to manufacturing tolerances. 
         [0004]    The machine vision industry has developed digital camera systems for obtaining high quality images used for code identification and decoding as well as for vision inspection systems. A digital camera typically includes a two dimensional CMOS or CCD sensor array, a lens assembly, a lens mounting flange and a camera processor. The lens assembly attaches to the lens mounting flange which is supposed to position the lens assembly at a specific distance from the sensor. For instance, in the case of a C-mount camera, a specified distance between the mounting flange and the sensor plane is 17.526 millimeters. Similarly, in the case of a CS-mount camera, a specified distance between the mounting flange and the sensor plane is 12.5 millimeters. The processor is programmed to control the lens in a manner calculated to control the focal distance of the camera where control characteristics are based at least in part on the specified distance between the mounting flange and the sensor array. 
         [0005]    One type of lens assembly is a motorized lens type wherein a motor is used to move lens components along an optical axis to adjust focal distance. Another type of lens assembly is a variable optical power type assembly such as a liquid lens where, instead of moving the lens along the optical axis to adjust the focal distance, the shape of the lens is modified to adjust the distance. To control a motorized lens, the processor adjusts control signals to the lens to drive a lens motor to modify the focal distance. To control a liquid lens, the processor adjusts a voltage applied to the lens to change the shape of the lens thereby adjusting the focal distance. 
         [0006]    Some camera systems have been developed that can be used with many different types of lenses. Where several different types of liquid lenses may be used with a single camera assembly, the different lenses typically have different lens specific operating characteristics that are stored in a memory device mounted to the lens that, among other things, can be used to calculate how the processor is to control the lens to adjust focal distance. When a liquid lens is attached to the mounting flange, the processor reads the operating characteristics from the lens memory device and thereafter controls the lens in a manner consistent with the operating characteristics and the camera mount type (e.g., C-mount, CS-mount, other). For instance, the operating characteristics and characteristics of a specific camera mount type may be useable to calculate a voltage to apply to the lens to control the shape of the lens and cause a specific optical power to occur. 
         [0007]    One problem with existing CCD or CMOS based camera systems is that manufacturing tolerances related to the CCD or CMOS sensor array often result in a flange focal distance error which in turn causes a focal distance error. For example, a CCD sensor array includes a CCD array mounted on a printed circuit board which is then mounted within a system support structure. The thickness of the PCB and array can vary appreciably and result in a flange focal distance error (i.e., a deviation from the specified or ideal flange focal distance for a specific lens type). Tolerance in the position of a sensor die in its package also contributes appreciably to the flange focal distance error. It has been empirically determined that the flange focal distance error can result in a focusing error that is greater than the depth of field in certain vision applications such that the error substantially impacts performance of an overall system. 
         [0008]    One solution to the problems associated with the flange focal distance error is to factory calibrate the combination of a sensor array (e.g., a CCD array) and a specific lens (e.g., a liquid lens of a specific type) prior to shipping the combination. While this solution works well in cases where only the factory installed lens will be used with a sensor array, this solution does not allow other lenses with varying capabilities to be swapped for the factory installed lens. 
         [0009]    Another solution is to integrate a calibration target into a camera assembly at a known distance from the sensor array or into an application environment at a known distance from the sensor array and program the system to recalibrate itself each time the assembly is booted up during a commissioning procedure. A similar solution is to field calibrate a sensor array and lens combination with a target placed a known distance from the sensor array after the system is installed. Either of these solutions, unfortunately, require additional commissioning procedure steps. In addition, these solutions include processes that must be repeated every time one type of lens is swapped for a different type of lens. 
         [0010]    Another solution to the problems associated with the flange focal distance error would be to provide a mechanical adjustment mechanism for adjusting the flange focal distance between the mounting flange and the sensor plane after manufacture to compensate for or eliminate the focal distance error. This solution, while possible, would require an extremely precise mechanical adjustment assembly and therefore would require additional system components and would increase overall cost. 
         [0011]    One other solution is to design a closed loop autofocus system where a sequence of images are obtained and the system adjusts the lens to set an optimal focal distance based on measured image sharpness. This solution does not work well in fast moving applications where there is insufficient time to analyze a series of images and adjust focus between each obtained image to hunt for a focused setting. 
         [0012]    Thus, it would be advantageous to have a camera system that could automatically compensate for flange focal distance error regardless of the type of lens used with the system. 
       BRIEF SUMMARY OF THE INVENTION 
       [0013]    It has been recognized that the flange focal distance error for a specific sensor/flange assembly can be measured after manufacture and stored in a memory device physically associated with the sensor/flange assembly. Then, when a lens is mounted to a mounting flange, the system processor may be programmed to automatically adjust the lens to compensate for the flange focal distance error. For instance, where the flange focal distance error on a C-mount sensor/flange assembly is 250 microns short (i.e., 250 microns less than the specified 17.526 millimeters), the processor can be programmed to automatically drive the lens in a motorized lens assembly 250 microns in the opposite direction to extend the sensor to lens distance by 250 microns thereby eliminating the error. 
         [0014]    In the case of a variable optical power lens such as a liquid lens, the amount of optical power required to compensate for a flange focal distance error depends on the effective focal length of the lens. In many cases the effective focal length of a lens is stored as a lens operating parameter in a lens memory. When a lens is mounted to a camera, the effective focal length value can be used by a camera processor along with the flange focal distance error value to identify an optical power required to compensate for the error. The optical power can be signaled to a lens driver causing the driver to control a voltage on the lens to change lens shape and achiever the optical power required to compensate for the error. 
         [0015]    Consistent with the above comments, at least some inventive embodiments include a camera assembly for use in a machine vision system, the assembly comprising a support structure forming a mounting flange that is configured for coupling with any of a plurality of exchangeable electrically controllable adjustable focal length lens assemblies, a two dimensional image sensor supported by the support structure and forming a two dimensional sensor plane spaced from the mounting flange by a flange focal distance and a processor programmed during a commissioning procedure prior to normal operation with a flange focal distance error and to use the flange focal distance error to generate lens control signals to compensate for the flange focal distance error when a lens is mounted to the mounting flange wherein the flange focal distance error is the difference between an ideal flange focal distance and the flange focal distance. 
         [0016]    Some embodiments further include a memory supported by the support structure and storing a flange focal distance error prior to normal operation of the assembly and for use thereafter with multiple adjustable focal length lens assemblies. In some cases the flange focal distance error is determined using a first lens assembly during the commissioning procedure and is thereafter used during normal operation with at least a second lens assembly. 
         [0017]    In some cases the processor is supported by the support structure that forms the mounting flange. Some embodiments further include electrical contacts adjacent the mounting flange and linked to the processor, the contacts for linking to contacts on a lens assembly when the lens assembly is mounted to the mounting flange to provide the lens control signals to the lens assembly. In some cases the processor is further programmed to, when a lens assembly is mounted to the mounting flange, obtain lens characteristics from the lens assembly and use the lens characteristics to generate the lens control signals to compensate for the flange focal distance error. In some cases the lens includes a fixed glass imager lens and a liquid lens. In some cases the lens characteristics include an effective focal length of the imager lens. 
         [0018]    Some embodiments further include a distance determiner for determining a target distance from the camera to a target to be imaged, the processor further programmed to use the target distance to generate the lens control signals to compensate for the flange focal distance error. In some cases the lens assembly is a motorized lens assembly and the control signals cause the lens assembly to adjust by an amount equal to the flange focal distance error. In some cases the flange focal distance error is coded into software run by the processor to generate the lens control signals. 
         [0019]    Other embodiments include a camera assembly for use in a machine vision system, the assembly comprising a support structure forming a mounting flange that is configured for coupling with any of a plurality of exchangeable electrically controllable adjustable focal length lens assemblies, a two dimensional image sensor supported by the support structure and forming a two dimensional sensor plane spaced from the mounting flange by a flange focal distance, a memory supported by the support structure and storing a flange focal distance error prior to normal operation of the assembly and for use thereafter with multiple adjustable focal length lens assemblies wherein the flange focal distance error is the difference between an ideal flange focal distance and the flange focal distance and a processor supported by the support structure and programmed to obtain the flange focal distance error from the memory and use the flange focal distance error to generate lens control signals to compensate for the flange focal distance error when a lens is mounted to the mounting flange. 
         [0020]    In some cases the processor is further programmed to, when a lens is mounted to the support structure, obtain at least one lens characteristic from a lens memory and use the flange focal distance error and the lens characteristic to generate the lens control signals to compensate for the flange focal distance error. Some embodiments further include a distance determiner for determining a target distance from the camera to a target to be imaged, the processor further programmed to use the target distance to generate the lens control signals to compensate for the flange focal distance error. 
         [0021]    Still other embodiments include a method for use with a camera assembly for use in a machine vision system where the camera assembly includes a support structure forming a mounting flange that is configured for coupling with any of a plurality of exchangeable electrically controllable adjustable focal length lens assemblies, a two dimensional image sensor supported by the support structure and forming a two dimensional sensor plane spaced from the mounting flange by a flange focal distance and a processor, the method comprising the steps of during a commissioning procedure prior to normal operation of the camera, measuring a flange focal distance error which is the difference between an ideal flange focal distance nd the flange focal distance and programming the processor to use the flange focal distance error to generate lens control signals to compensate for the flange focal distance error when a lens is mounted to the mounting flange for use with the assembly during normal operation of the assembly. 
         [0022]    In some cases the camera assembly further includes a memory supported by the support structure, the step of programming including storing the flange focal distance error in the memory where the processor is programmed to retrieve the flange focal distance error from the memory. In some cases the flange focal distance error is determined using a first lens assembly during a commissioning procedure and is thereafter used during normal operation with at least a second lens assembly. In some cases the processor is further programmed to, when a lens assembly is mounted to the mounting flange, obtain lens characteristics from the lens assembly and use the lens characteristics to generate the lens control signals to compensate for the flange focal distance error. 
         [0023]    In some cases the lens characteristics include an effective focal length associated with the lens assembly. In some cases the lens assembly mounted to the flange is a motorized lens assembly and the control signals cause the lens assembly to adjust by an amount equal to the flange focal distance error. In some cases the flange focal distance error is coded into software run by the processor to generate the lens control signals. 
         [0024]    Other embodiments include a method for use with a camera assembly for use in a machine vision system where the camera assembly includes a support structure forming a mounting flange that is configured for coupling with any of a plurality of exchangeable electrically controllable adjustable focal length lens assemblies, a two dimensional image sensor supported by the support structure and forming a two dimensional sensor plane spaced from the mounting flange by a flange focal distance, a memory supported by the support structure and a processor, the method comprising the steps of during a commissioning procedure prior to normal operation of the camera, measuring a flange focal distance error which is the difference between an ideal flange focal distance and the flange focal distance, storing the flange focal distance error in the memory, programming the processor to retrieve the flange focal distance error from the memory and to use the flange focal distance error to generate lens control signals to compensate for the flange focal distance error when a lens is mounted to the mounting flange for use with the assembly during normal operation of the assembly. 
         [0025]    In some cases the processor is further programmed to, when a lens is mounted to the support structure, obtain at least one lens characteristic from a lens memory and use the flange focal distance error and the lens characteristic to generate the lens control signals to compensate for the flange focal distance error. Some embodiments further include a distance determiner for determining a target distance from the camera to a target to be imaged, the processor further programmed to use the target distance to generate the lens control signals to compensate for the flange focal distance error. 
         [0026]    These and other objects, advantages and aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefore, to the claims herein for interpreting the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0027]      FIG. 1  is a perspective view a camera/lens assembly that is consistent with at least some aspects of the present invention; 
           [0028]      FIG. 2  is a partial cross-sectional view of the assembly shown in  FIG. 1 , albeit with a lens subassembly spaced apart from a camera subassembly; 
           [0029]      FIG. 3  is a similar to  FIG. 2 , albeit showing the lens subassembly mounted to the camera subassembly; 
           [0030]      FIG. 4  is a schematic view illustrating the camera/lens assembly shown in  FIG. 3 ; 
           [0031]      FIG. 5  is a flow chart illustrating a process for use with a liquid lens subassembly that is consistent with at least some aspects of the present invention; and 
           [0032]      FIG. 6  is a flow chart illustrating a process for use with a motorized lens subassembly that is consistent with at least some aspects of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0033]    One or more specific embodiments of the present invention will be described below. It should be appreciated that in the development of any actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve a developer&#39;s specific goals, such as compliance with system-related and business related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
         [0034]    The present invention will be described in the context of the exemplary camera/lens assembly  10  shown in  FIGS. 1 through 3  and schematically in  FIG. 4 . Assembly  10  includes a camera subassembly  11  and a lens subassembly  14 . Camera subassembly  11  includes a camera housing  12 , a processing unit or processor  24  and an image sensor array  26 . In the illustrated embodiment, the housing  12  is shown as several components that together form the housing structure. In other embodiments, other subsets of components may be combined to form the housing structure and in at least some cases, a single molded component may form the housing structure. 
         [0035]    Referring still to  FIGS. 1 through 4 , housing  12  forms a housing cavity  20  that is generally open to one side (e.g., the left side as illustrated in  FIGS. 2 and 3 ). Processor  24  (see  FIG. 4 ) is mounted to a PCB  27  within cavity  20 . Image sensor  26  includes a CCD, CMOS, or other type 2D planar sensor array which is mounted to a surface of a PCB  27  and the sensor/PCB assembly is mounted within cavity  20  so that the sensor  26  faces the open end of the cavity. The sensor  26  is arranged along a camera imaging axis  35 . Referring to  FIGS. 2 and 3 , after installation, a sensing surface of image sensor  26  is located within a sensing plane  37 . Image sensor  26  is linked to processor  24 . 
         [0036]    Referring still to  FIGS. 2 and 3 , housing  12  also forms a flange surface or flange  41  that faces out the open end of cavity  20  as well as a threaded female coupling opening  30  that opens into cavity  20  through flange surface  41 . Opening  30  is symmetrically formed around imaging axis  35  and adjacent the sensing surface of image sensor  26 . A flange focal distance (FFD) exists between the sensing plane  37  of image sensor  26  and the flange surface  39  as labeled in  FIGS. 2 and 3 . 
         [0037]    Referring to  FIGS. 1 and 4 , camera subassembly  11  forms a female electrical coupler or port  16  adjacent coupling opening  30 . Port  16  is designed to securely receive and mechanically couple to a male electrical plug member  14  that forms part of the lens subassembly  14  so that lens control signals can be transmitted from processor  24  to lens subassembly  14  and so that, in at least some embodiments, lens characteristics can be read by processor  24  from a lens memory. 
         [0038]    Referring to  FIG. 4 , in addition to the components described above, camera subassembly  11  includes a power supply  53 , a memory  42  and a distance measurement device  40 . In at least some embodiments power supply  53  is a battery. Processor  24  is linked to power supply  53  to receive power there from. Processor  24  is also linked to memory  42 . Algorithms performed by processor  24  to carry out methods that are consistent with at least certain aspects of the present invention are stored in memory  42 . In addition, data required to perform inventive methods or processes is also stored in memory  42 . 
         [0039]    Distance measurement device  40  includes a device or subassembly that can be use to determine the distance between camera assembly  10  and a target surface to be imaged. Various distance measurement devices are well known in the art and therefore will not be described here in detail. Device  40  is linked to processor  24  to provide instantaneous camera to target distance values (see d in  FIG. 4 ) to processor  24  during assembly operation. Although the embodiment described here includes measurement device  40  as part of camera assembly  10 , device  40  may be provided separate from assembly  10 . 
         [0040]    Referring again to  FIGS. 1 through 4 , in at least some embodiments, lens subassembly  14  includes a lens support structure  17 , a liquid lens assembly  32 , a fixed glass imager lens  55  (or group of fixed lenses), a lens memory device  44  (see specifically  FIG. 4 ), a driver  51  and the male electrical plug  18  (see specifically  FIG. 1 ). Support structure  17  is typically formed of a rigid plastic material and, as the label implies, provides support for the other components that comprise lens subassembly  14 . While structure  17  is shown as including several components in the illustrated example, in at least some embodiments, structure  17  may be formed of a different subset of components or even by a single component member. Each of the driver  51 , memory  44 , plug  18 , imager lens  55  and liquid lens assembly  32  are supported by structure  17 . 
         [0041]    Structure  17  has other features for coupling lens subassembly  14  to camera subassembly  11  and that cooperate with features of the camera subassembly in an attempt to precisely position the lens subassembly components with respect to the camera components. To this end, referring again to  FIGS. 3 and 4 , support structure  17  forms a generally cylindrical passageway in which the fixed and liquid lenses are mounted along and aligned with an optical axis. An external surface of structure  17  forms a cylindrical threaded male coupling surface  34  designed to couple to the threaded female coupling opening  30  formed by camera housing  12 . Structure  17  also forms a stop surface  43  that extends perpendicular to threaded surface  34  and that is designed to cooperate with flange surface  41  to limit the position of lens subassembly  14  with respect to camera subassembly  12  upon mounting. 
         [0042]    Liquid lenses like lens  32  are well known in the art and therefore lens subassembly  32  will not be described here in detail. Here it should suffice to say that liquid lens  32  is a variable focus liquid lens assembly that includes a fluid chamber defined by two parallel windows facing each other, and a body to which the windows are fixed. The windows are preferably transparent plates formed of an optical transparent material such as glass. The fluid chamber contains two immiscible liquids of similar density and having different optical indices, which form an optical interface in the form of a meniscus. One of the liquids is preferably an insulating liquid, for example comprising oil and/or an oily substance, and the other is preferably a conductive liquid comprising, for example, an aqueous solution. The liquid lens also comprises a cap and a gasket that is sandwiched between the cap and the body ensuring the tightness of the lens structure. The conductive liquid is in contact with an electrode formed by the cap, and the liquid-liquid interface contacts a conical part of the body that comprises an insulated electrode. Through electrowetting phenomena it is possible to modify the curvature of the liquid-liquid interface, according to a voltage V applied between the electrodes formed by the cap and the body. For example, the curvature may change from a concave first shape to a relatively more concave second shape. Thus, a beam of light passing through the fluid chamber will be focused to a greater or lesser extent according to the applied voltage. 
         [0043]    Referring to  FIG. 4 , an actuator (not shown) of liquid lens assembly  32  is electrically controlled by driver  51  that applies driving voltages such that the focal length of the optical lens assembly  14  can be changed in a controlled fashion. Driver  51  is linked to plug  18  so that driver  51  can receive control signals from processing unit  24  when plug  18  is connected to port  16  (see again  FIG. 1 ). In at least some embodiments lens memory  44  is also electrically linked to plug  18  so that lens characteristics can be read by processor  24  when plug  18  is coupled to port  16 . 
         [0044]    Memory  44 , in at least some embodiments, includes an Eeprom and is used to store various characteristics of the lens subassembly  14 . For instance, in at least some embodiments memory  44  will store and effective focal length of the fixed lens  55  (see again  FIG. 4 ). In addition, memory  44  may store slope and offset values that relate optical power of liquid lens  32  to applied voltage values required to change lens shape to produce different optical powers. 
         [0045]    Referring again to  FIG. 2 , to mount lens subassembly  14  to camera subassembly  11 , structure  17  is positioned to align the threaded surface  34  of the lens male coupler with the threaded surface of the camera housing female coupler  30  and is moved toward camera subassembly  11  until the threads engage. The lens subassembly  14  is rotated until stop surface  43  abuts flange surface  41 . At this point surfaces  41  and  43  cooperate to position the components of assembly  14  in a specific juxtaposition with respect to the camera subassembly components. More specifically, surfaces  41  and  43  and other support structure of the lens subassembly and the housing  12  cooperate to position the lenses  55  and  32  along the imaging axis  35  (see also  FIG. 3 ) and in an attempt to set the lenses  32  and  55  at known positions with respect to the sensing surface of image sensor  26 . 
         [0046]    The calculations performed by processor  24  to generate the liquid lens control signals for controlling lens assembly  32  are based on the presumption that lenses  32  and  35  are at precise positions with respect to sensor  26 . Any variation in the juxtapositions between the lenses and sensor can therefore adversely affect ability to focus on a target surface (see  69  in  FIG. 4 ) at a distance from the assembly  10 . Thus, the flange focal distance FFD (see again  FIGS. 2 and 3 ) is a particularly important dimension when manufacturing the camera subassembly  11 . 
         [0047]    As described above, for various reasons, the FFD is difficult to control during manufacturing so that an FFD error (e FFD ) often occurs which can adversely affect ability of camera assembly  10  to focus images of targets  69  on the sensing surface of sensor  26 . 
         [0048]    According to at least some embodiments of the present invention, to compensate for the e FFD , the e FFD  may be calculated for each camera subassembly  11  after manufacturing is complete and the e FFD  may be stored in the camera memory  42  for use by processor  24  to compensate for the error. Then, during normal operation, after a lens subassembly is mounted to the camera subassembly  11 , the processor may use the e FFD  to control the variable focus lens subassembly to compensate for the e FFD . 
         [0049]    In the case of a liquid lens like lens  32 , the manner in which lens control signals have to be altered to compensate for the e FFD  depends on the effective focal length of the lens. As described above, many lens subassemblies that include a liquid lens come with the focal length of the lens stored in the lens memory  44  which can be read out by processor  24  upon lens mounting. 
         [0050]    Regarding calculation of the lens control signals, the distance d at which a target is in focus for an exemplary camera assembly can be expressed by the following general equation: 
         [0000]      1 /d=LL   op   +IL   op −1/( FFD   i   +e   FFD )  Equation 1
 
         [0000]    where LL op  is the optical power of liquid lens  32 , IL op  is the optical power of imager lens  55 , both expressed in diopters, FFD i  is an ideal flange focal distance (i.e., the flange focal distance if there was no e FFD ) and e FFD  is the flange focal distance error. The optical power IL op  of the imager lens can be expressed as the inverse of the effective focal length of the imager lens as in equation 2: 
         [0000]        IL   op =1 /e   fl   Equation 2
 
         [0051]    Equations 1 and 2 can be combined and rewritten to calculate a required optical power of the liquid lens  32  for focusing on a target at a specific distance d as follows: 
         [0000]        LL   op =1 /d− 1 /e   fl +1/( FFD   i   +e   FFD )  Equation 3
 
         [0052]    Effective focal length e fl  is stored in liquid lens camera memory  44 . The flange focal distance error e FFD  is measured after manufacture and stored in camera memory  42 . In addition, camera subassembly  11  is of a specific type (e.g., C-mount, CS-mount, etc.) and therefore is characterized by an ideal flange focal distance FFD i  which can be programmed into the algorithm performed by processor  24 . Thus, when camera to target distance d is measured via distance measurement device  40  and is provided to processor  24 , processor  24  can calculate the liquid lens optical power LL op  required for focusing on the target at distance d by reading the effective focal distance and flange focal distance error from memories  44  and  42 , respectively, and solving equation 3. Once optical power LL op  is determined, processor  24  generates control signals that are provided to lens driver  51  to specify the liquid lens optical power required to focus at distance d. In at least some embodiments the control signals are provided as PWM signals although other control signal types are contemplated. 
         [0053]    Referring still to  FIG. 4 , driver  51  is programmed with various liquid lens parameters that enable driver  51  to control liquid lens  32  to adjust the optical power of lens  32  to match the power value calculated using equation 3. To this end, as well known in the industry, lens  32  is characterized by a slope and an offset value that relate optical power to the voltage level applied across the lens. The slope and offset are determined for the lens and are stored in the lens memory  44  for use by driver  51 . Driver  51  provides the voltage to the lens actuator to control the liquid lens optical power. 
         [0054]    After manufacture of a camera assembly  11 , any of various methods may be used to measure the flange focal distance error e FFD . For instance, in at least some cases a liquid lens may be mounted to a camera for which error e FFD  is to be measured and a target may be located a known distance from the camera assembly. The processor may calculate an optical power LL op  for the liquid lens by solving Equation 3 with the known distance d and assuming a zero flange focal distance error e FFD . The camera assembly may be controlled to obtain target images while changing the e FFD  value in equation 3 to thereby change the optical power of the liquid lens until a sharply focused image results. The error e FFD  corresponding to the focused image may then be stored in the camera memory  42  for subsequent use with other lenses. 
         [0055]    As another instance, a liquid lens may be mounted to a first camera that is known to have an ideal flange focal distance FFD i  and the first camera/lens assembly may be positioned with respect to a target so that a resulting image is sharp with the liquid lens set to a specific optical power. A first distance between the first camera/lens assembly and the target is measured when the image is sharp. Next, without changing the focus settings on the lens, the lens is mounted to a second camera for which the e FFD  is to be determined. The second camera/lens assembly is moved with respect to the target until a sharp image results and a second camera/lens to target distance is measured. The known effective focal length and difference between the first and second distances can be used to calculate the error e FFD  (e.g., by solving a version of equation 3) which is then stored for subsequent use. 
         [0056]    Referring now to  FIG. 5 , a flow chart  50  is illustrated that shows a method that is consistent with at least some aspects of at least some embodiments of the present invention. Referring also to  FIG. 4 , at block  52 , the effective focal length e fl  of the imager lens  55  is stored in lens memory  44 . In addition, although not illustrated in  FIG. 5 , the slope and offset values associated with the optical power and lens voltage relationship that are used by driver  51  to control the liquid lens are also stored at block  52 . At block  54 , the flange focal distance error e FFD  is measured after a camera assembly has been manufactured and is stored in camera memory  42 . At block  56 , any of a plurality of different liquid lens assemblies (e.g., see exemplary assembly  14  in  FIG. 2 ) is mounted to camera subassembly  11 . In at least the illustrated embodiment, mounting includes mechanical mounting of lens subassembly  14  to the camera subassembly  11  as well as reception of plug  18  in port  16  to form an electrical connection between processor  24  and driver  51  as well as a data communication link between lens memory  44  and processor  24 . 
         [0057]    Referring still to  FIG. 5 , after a lens subassembly  14  is mounted to camera subassembly  11 , processor  24  reads data from the lens memory. In the embodiment described above, processor reads the effective focal length e fl  value from memory  44 . At block  60 , during normal operation when the camera/lens assembly  10  is used to obtain an image of a target (see  69  in  FIG. 4 ) at a distance d, at block  60 , distance measurement device  40  first determines the camera to target distance d and provides value d to processor  24 . At block  62 , processor  24  reads flange focal distance error e FFD  from camera memory  42 . At block  64 , processor  24  uses the flange focal distance error e FFD , the effective focal length e fl  and the measured distance d from block  60  to calculate the liquid lens optical power required to focus an image of the target at distance d on sensor  26  (see again  FIG. 4 ). At block  66 , processor  24  generates and provides lens control signals to liquid lens driver  51  indicating the required optical power. At block  68 , driver  51  uses the control signals as well as the slope and offset values associated with liquid lens  32  to set the liquid lens voltage at a level calculated to result in the required optical power. 
         [0058]    In at least some embodiments, it is contemplated that, instead of a liquid lens subassembly, a motorized lens subassembly may be used with a camera subassembly  11  as described above. Where a motorized lens assembly is employed, a flange focal distance error as described above has similar effect on the ability of a camera assembly to focus on a target. In the case of a motorized lens subassembly, a flange focal distance error can be compensated without obtaining any information such as the effective focal distance from the lens subassembly. Instead, processor  24  can be programmed to simply adjust control of a motorized lens by an amount equal to the flange focal distance error but in an opposite direction thereby moving the motorized lens assembly along the imaging axis  35 . For example, referring again to  FIG. 2 , where an actual flange focal distance FFD is 250 microns greater than an ideal flange focal distance FFD i  so that the flange focal distance error e FFD  is +250 microns, processor  24  can be controlled to simply adjust a motorized lens subassembly to move the subassembly lenses along imaging axis  35  by 250 microns toward image sensor  26  to directly compensate for the error. Similarly, where the flange focal distance error is −250 microns, processor  24  can be programmed to move the lenses in the lens subassembly away from image sensor  26  by 250 microns along axis  35 . 
         [0059]    Referring again to  FIG. 4 , in the case of a camera assembly including a motorized lens, the  FIG. 4  schematic would not include lens memory  44  and the liquid lens  32  and fixed lens  55  would be replaced by a motorized lens assembly driven by a suitable driver  51 . 
         [0060]    Referring now to  FIG. 6 , an exemplary method  70  is illustrated that may be used to compensate for a flange focal distance error when a motorized lens subassembly is mounted to a camera subassembly  11 . Referring also to  FIG. 4 , at block  72 , a flange focal distance error e FFD  for a camera is measured and stored in camera memory  42 . At block  72  a motorized lens subassembly is mounted to the camera subassembly. At block  78 , during normal operation, when an image of a target at a distance d is to be obtained, distance measurement device  40  is used to determine distance d between camera subassembly  11  and the target  69 . At block  80 , processor  24  reads the flange focal distance error e FFD  from memory  42 . At block  84 , processor  24  adjusts the lens position by the flange focal distance error e FFD  and a function of distance d to compensate for the error. 
         [0061]    While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. For example, while the processor for controlling the lenses in the above examples is mounted to the camera housing, it should be appreciated that the processor could be mounted directly to a lens or within a lens housing. In this case, when a lens is connected to a camera subassembly, the lens based processor may read the flange focal distance error from the camera memory and perform one of the above processes. As another example, in the case of a liquid lens, other lens characteristics such as the slope and offset values that define the voltage and optical power relationship may be read along with an effective focal distance from a lens memory by a camera processor and the processor may generate a lens voltage signal as opposed to the control signals to affect the required optical power. 
         [0062]    As still one other example, while the embodiments described above include a camera memory in which a measured flange focal distance error is stored after manufacture and prior to normal operation of the camera assembly, in other embodiments, after the flange focal distance error is calculated, equation 3 may be modified to reflect the error so that the error is, in effect, programmed into the software run by processor  24 . Thus, the last factor in equation 3 above may collapse into a constant value as, after the error is calculated, both the ideal flange focal distance FFD i  and the error e FFD  would be known. In this way memory  42  in  FIG. 4  could be eliminated as the error e FFD  would be coded directly into the software run by processor  24 . 
         [0063]    Thus, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims. 
         [0064]    To apprise the public of the scope of this invention, the following claims are made: