Patent Publication Number: US-11385385-B2

Title: System and method for reduction of drift in a vision system variable lens

Description:
RELATED APPLICATION 
     This application is a continuation of co-pending U.S. patent application Ser. No. 15/847,868, entitled SYSTEM AND METHOD FOR REDUCTION OF DRIFT IN A VISION SYSTEM VARIABLE LENS, filed Dec. 19, 2017, which is a continuation-in-part of co-pending U.S. patent application Ser. No. 14/271,148, entitled SYSTEM AND METHOD FOR REDUCTION OF DRIFT IN A VISION SYSTEM VARIABLE LENS, filed May 6, 2014, the teachings of each of which applications are expressly incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This application relates to cameras used in machine vision and more particularly to automatic focusing lens assemblies. 
     BACKGROUND OF THE INVENTION 
     Vision systems that perform measurement, inspection, alignment of objects and/or decoding of symbology (e.g. bar codes, or more simply “IDs”) are used in a wide range of applications and industries. These systems are based around the use of an image sensor, which acquires images (typically grayscale or color, and in one, two or three dimensions) of the subject or object, and processes these acquired images using an on-board or interconnected vision system processor. The processor generally includes both processing hardware and non-transitory computer-readable program instructions that perform one or more vision system processes to generate a desired output based upon the image&#39;s processed information. This image information is typically provided within an array of image pixels each having various colors and/or intensities. In the example of an ID reader, the user or automated process acquires an image of an object that is believed to contain one or more IDs. The image is processed to identify ID features, which are then decoded by a decoding process and/or processor to obtain the inherent information (e.g. alphanumeric data) that is encoded in the pattern of the ID. 
     Often, a vision system camera includes an internal processor and other components that allow it to act as a standalone unit, providing a desired output data (e.g. decoded symbol information) to a downstream process, such as an inventory tracking computer system or logistics application. 
     An exemplary lens configuration that can be desirable in certain vision system applications is the automatic focusing (auto-focus) assembly. By way of example, an auto-focus lens can be facilitated by a type of “variable lens” assembly (defined further below), known as a so-called liquid lens assembly. One form of liquid lens, available from Varioptic of France uses two iso-density liquids—oil is an insulator while water is a conductor. The variation of voltage passed through the lens by surrounding circuitry leads to a change of curvature of the liquid-liquid interface, which in turn leads to a change of the focal length of the lens. Some significant advantages in the use of a liquid lens are the lens&#39; ruggedness (it is free of mechanical moving parts), its fast response times, its relatively good optical quality, and its low power consumption and size. The use of a liquid lens can desirably simplify installation, setup and maintenance of the vision system by eliminating the need to manually touch the lens. Relative to other auto-focus mechanisms, the liquid lens has extremely fast response times. It is also ideal for applications with reading distances that change from object-to-object (surface-to-surface) or during the changeover from the reading of one object to another object—for example in scanning a moving conveyor containing differing sized/height objects (such as shipping boxes). In general, the ability to quickly focus “on the fly” is desirable in many vision system applications. 
     A recent development in liquid lens technology is available from Optotune AG of Switzerland. This lens utilizes a movable membrane covering a liquid reservoir to vary its focal distance. A bobbin exerts pressure to alter the shape of the membrane and thereby vary the lens focus. The bobbin is moved by varying the input current within a preset range. Differing current levels provide differing focal distances for the liquid lens. This lens advantageously can provide a larger aperture (e.g. 6 to 10 millimeters) than competing designs (e.g. Varioptic of France) and operates faster. However, due to thermal drift and other factors, there may be variation in calibration and focus setting during runtime use, and over time in general. A variety of systems can be provided to compensate and/or correct for focus variation and other factors. However, such compensation routines can require processing time (within the camera&#39;s internal processor) that slows the lens&#39; overall response time in arriving at a new focus. Likewise, such compensation routines, (e.g. thermal drift) can be standardized, and not customized to the lens&#39; intrinsics, rendering them less reliable for the specific drift conditions that a lens may encounter over time. Note that drift in a liquid lens can be, for example, approximately 0.15 Diopter/° C. (i.e. for certain Varioptic liquid lenses currently in production and/or specified in commercially available products). Some vision applications, especially when small features at a large distance are to be detected, require a stability in optical power of the imager lens of +/−0.1 diopter. 
     Also it is recognized generally that a control frequency of at least approximately 1000 Hz may be required to adequately control the focus of the lens and maintain it within desired ranges. This poses a burden to the vision system&#39;s processor, which can be based on a DSP or similar architecture. That is, vision system tasks would suffer if the DSP were continually preoccupied with lens-control tasks. All of these disadvantages make drift compensation a challenge in many applications. 
     SUMMARY OF THE INVENTION 
     This invention overcomes disadvantages of the prior art by providing a vision system that is arranged to compensate for optical drift that can occur in certain lens assemblies capable of varying optical power, wherein the optical power (and hence, varying focal length/distance where focal length=1/optical power) is varied by controlling lens shape and/or lens refractive index. Such lens assemblies include, but are not limited to, liquid lens arrangements employing, for example, two iso-density fluids or a flexible membrane—also generally termed a “variable lens” assembly herein. The system includes an image sensor operatively connected to a vision system processor, and a variable lens assembly that is controlled (e.g. by the vision processor or another range-determining device) to vary a focal distance thereof. A positive lens assembly is configured to weaken an effect of the variable lens assembly over a predetermined operational range of the object from the positive lens assembly. The variable lens assembly illustratively comprises a liquid lens assembly, and such a liquid lens assembly can be inherently variable over approximately 20 diopter. Illustratively, the positive lens assembly and the variable lens assembly are collectively housed in a removable lens barrel with respect to a camera body and the image sensor. The image sensor is illustratively located within the camera body. Likewise, the vision processor can be all, or in part, located in the camera body. In an embodiment, the lens barrel has a C-mount lens base, and the positive lens assembly comprises a doublet, which includes a front convex lens and rear concave lens. The positive lens assembly can define an effective focal range of 40 millimeters. Illustratively, the usable focal length of the lens (e.g. a doublet) is between approximately 10 and 100 millimeters. Additionally, the variable lens assembly (e.g. liquid lens assembly) is typically located adjacent to, but remote from, a focal point of the positive lens assembly, which can be the front, or more typically, the back/rear focal point of the positive lens assembly. The distance between the variable lens assembly and the focal point can be between approximately 0.1 and 0.5 times a focal length F of the positive lens assembly. In this manner, the positive lens assembly and the variable lens assembly are part of an overall lens assembly focusing light on the image sensor. The optical power of the positive lens assembly, thus, “predominantly defines” an overall optical power of the overall lens assembly—in other words, the majority of magnification/optical power is provided by the positive lens assembly, thereby minimizing the effect of drift in the variable lens assembly. 
     In an illustrative embodiment a vision system that compensates for drift is provided. The vision system includes an image sensor operatively connected to a vision system processor, a variable lens assembly that varies a shape or a refractive index thereof, and a fixed lens assembly configured to weaken an effect of the variable lens assembly over a predetermined operational range of the object. Illustratively, the variable lens assembly comprises a liquid lens assembly. It can be positioned between the image sensor and the fixed lens assembly and can be variable over approximately 20 diopter. Additionally, the fixed lens assembly can define a positive optical power. Illustratively, the fixed lens assembly and the variable lens assembly are housed in a removable lens barrel with respect to a camera assembly body and the image sensor, the image sensor can be located within the camera assembly body. The camera assembly body can be electrically connected to the variable lens assembly to provide at least one of power and control thereof, by at least one of contact pads and a cable assembly. The fixed lens assembly can comprise one of: (a) a front lens with a front concave surface and a rear convex surface and a central biconvex lens spaced from the front lens, (b) a front biconvex lens and a rear stacked lens assembly with a front positive lens, center biconcave lens and rear positive lens, (c) a front planoconcave lens and a negative lens, a central stacked lens assembly with a biconvex lens and a planoconvex lens, and a rear biconvex lens and positive lens, (d) a front planoconvex lens and positive lens and a rear positive lens and negative lens, and (e) a front stacked lens assembly with a biconvex lens and biconcave lens and a rear planoconvex lens and negative lens. Also, at least one lens of the fixed lens assembly can comprise a polymer material. By way of example, the fixed lens assembly can define an effective usable focal range of between approximately 0.3 to 8 meters. Also by way of example, the variable lens assembly can be located adjacent to a focal point of the fixed lens assembly. The focal point is one of either a front focal point or a back focal point of the fixed lens assembly. In embodiments, the fixed lens assembly can comprise a front lens assembly and a rear lens assembly with the variable lens assembly positioned therebetween, in which the rear lens assembly can define a positive optical power. Also in such embodiments, the front lens assembly can have a pair of lenses, each having convex front surfaces and concave rear surfaces and a lens having opposing concave surfaces, and the rear lens assembly can have a lens having opposing convex surfaces. Illustratively, the fixed lens assembly and the variable lens assembly are part of an overall lens assembly focusing light on the image sensor, in which an optical power of the fixed lens assembly predominantly defines an overall optical power of the overall lens assembly. 
     In another illustrative embodiment, a variable lens system for a vision system having an image sensor that transmits image data to a processor is provided. The system includes a variable lens assembly (such as a liquid lens assembly. The system includes a fixed lens assembly having a focal point. The variable lens assembly is located adjacent to the focal point. The fixed lens assembly and the variable lens assembly can be part of an overall lens assembly focusing light on the image sensor. The optical power of the positive lens assembly can predominantly define the overall optical power of the overall lens assembly. Illustratively, the liquid lens assembly is variable over approximately 20 diopter. In embodiments, the fixed lens and the variable lens assembly are housed in a removable lens barrel with respect to a camera assembly body and the image sensor. The image sensor is located within the camera assembly body. The camera assembly body can be electrically connected to the variable lens assembly, to provide at least one of power and control thereof, by at least one of contact pads and a cable assembly. Illustratively, the lens system can comprise one of: (a) a front lens with a front concave surface and a rear convex surface and a central biconvex lens spaced from the front lens, (b) a front biconvex lens and a rear stacked lens assembly with a front positive lens, center biconcave lens and rear positive lens, (c) a front planoconcave lens and a negative lens, a central stacked lens assembly with a biconvex lens and a planoconvex lens, and a rear biconvex lens and positive lens, (d) a front planoconvex lens and positive lens and a rear positive lens and negative lens, and (e) a front stacked lens assembly with a biconvex lens and biconcave lens and a rear planoconvex lens and negative lens. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention description below refers to the accompanying drawings, of which: 
         FIG. 1  is a diagram of an illustrative vision system arrangement having a vision system camera with associated vision processor, with a lens assembly that compensates for inherent drift over time, shown acquiring images of an exemplary object in a scene according to an illustrative embodiment; 
         FIG. 2  is a diagram of the ray trace for an exemplary lens system that includes a variable lens assembly imaging an object; 
         FIG. 3  is a diagram of the ray trace for an illustrative lens system including a variable lens assembly and a positive lens assembly is positioned along the optical axis at a predetermined distance from the variable lens assembly, to thereby provide a drift-tolerant lens system; 
         FIG. 4  is a side cross section of a lens unit including a variable lens assembly and positive lens exhibiting drift tolerance according to an illustrative embodiment, showing relative dimensions of the lens barrel and components associated therewith; 
         FIG. 4A  is a side cross section of the lens unit of  FIG. 4 , showing the relative placement of components along the optical axis; 
         FIG. 5  is a diagram of the ray trace for the illustrative lens unit of  FIG. 4 , shown imaging an object at a first distance; 
         FIG. 6  is a diagram of the ray trace for the illustrative lens unit of  FIG. 4 , shown imaging an object at a second distance, longer than the first distance; 
         FIG. 7  is a diagram of the ray trace for the illustrative lens unit of  FIG. 4 , shown imaging an object at a first distance; 
         FIG. 8  is a diagram of the relationship between the positive lens assembly, variable lens assembly and positive lens focal point according to embodiments herein; 
         FIG. 9  is a diagram of an arrangement of lenses for a drift-tolerant lens system in which a variable lens assembly is located between the optics and the image sensor according to an embodiment; 
         FIG. 10  is a diagram of an arrangement of lenses for a drift-tolerant lens system in which a variable lens assembly is located between two groups of optics, placed ahead of an image sensor according to an embodiment; 
         FIG. 11  is a diagram of an arrangement of lenses for a 12-millimeter, drift-tolerant lens system in which a variable lens assembly is located between the optics and the image sensor according to another embodiment; 
         FIG. 12  is a perspective view of a lens assembly that includes the lens arrangement of  FIG. 11 ; 
         FIG. 13  is a side cross section of the lens taken along line  13 - 13  of  FIG. 12 ; 
         FIG. 14  is a diagram of an arrangement of lenses for a 16-millimeter, drift-tolerant lens system in which a variable lens assembly is located between the optics and the image sensor according to another embodiment; 
         FIG. 14A  is a diagram of the image circles a rectangular image sensor that can be employed in the vision system according to an embodiment; 
         FIG. 15  is a diagram of an arrangement of lenses for a 25-millimeter, drift-tolerant lens system in which a variable lens assembly is located between the optics and the image sensor according to another embodiment; 
         FIG. 16  is a diagram of an arrangement of lenses for a 35-millimeter, drift-tolerant lens system in which a variable lens assembly is located between the optics and the image sensor according to another embodiment; 
         FIG. 17  is a is a perspective view of a lens assembly that includes a version of the lens arrangement of  FIG. 16 ; and 
         FIG. 18  is a side cross section of the lens taken along line  18 - 18  of  FIG. 17 . 
     
    
    
     DETAILED DESCRIPTION 
     I. System Overview 
       FIG. 1  details a vision system  100  that includes a vision system camera assembly  110  and associated lens unit/assembly  120 . The construction of the lens unit  120  is described further below. In an embodiment, the lens unit  120  is fixed to the camera, or can be removable using a custom or conventional mount base, such as the well-known Cine or “C-mount”. The camera includes a body/housing that can house a plurality of operational components including an image sensor or imager  130  (shown in phantom). In this embodiment, the imager  130  is operatively connected with an on-board vision processor  140  that operates a variety of hardware and/or software processes, generally termed a vision process  142 . The vision process  142  can include a plurality of software applications that are adapted to perform general purpose or specialized vision system tasks, for example, ID (code) finding and decoding tasks, edge detection, blob analysis, surface inspection, robot manipulation and/or other operations. See, for example exemplary ID  144 . The processes  142  can include various image acquisition and image manipulation applications as well—which place image data into a form more appropriate for use in vision system tasks—e.g. histogramming, thresholding, etc. These tasks and processes are known to those of skill in the art and can be sourced from a commercial vision system supplier—such as Cognex Corporation of Natick, Mass. As shown, the illustrative vision system processor  140  is contained within the camera body. Vision system data in “raw”, pre-processed (e.g. found, undecoded ID image data), or fully processed (e.g. decoded ID data) form can be provided over a wired and/or wireless link  144  to an appropriate data-handling system or processor, such as a standalone PC or server system. Alternate systems, such as mobile computing devices, cloud-based devices, and the like can be provided in alternate implementations. The data-handling system stores and manipulates the image-based data as desired by the user—e.g. quality or inventory control. In alternate embodiments, some or all of the vision system processors/processes can be instantiated and/or performed in a remote processor (e.g. the computing device/processor  150 ) that is interconnected to the camera  110  by an appropriate wired and/or wireless link (e.g. link  144 ) in a manner known to those of skill in the art. 
     Note, as used herein, the terms “process” and/or “processor” should be taken broadly to include a variety of electronic hardware and/or software based functions and components. Moreover, a depicted process or processor can be combined with other processes and/or processors or divided into various sub-processes or processors. Such sub-processes and/or sub-processors can be variously combined according to embodiments herein. Likewise, it is expressly contemplated that any function, process and/or processor herein can be implemented using electronic hardware, software consisting of a non-transitory computer-readable medium of program instructions, or a combination of hardware and software. In a system arrangement, such processes/process functions can be termed as occurring/existing in a corresponding “module” or “element”. For example, an “ID-reading module”, which performs the functions associated with reading and/or decoding of ID codes. 
     The lens assembly  120  is shown aligned along the optical axis OA (with the plane of the sensor  130 ) typically arranged perpendicularly to the axis. The lens assembly  120  and sensor  130  image an object O. The object O, by way of example, can be any two-dimensional (2D) or three-dimensional (3D) surface or shape that partially or fully fits within the field of view (FOV). In the depicted example, range/distance (do) of the object O from the camera  110  (e.g. from the focal plane of the sensor  130 ) can be varied, but defines a predetermined operating range (according to an illustrative embodiment) within which to image the object O. 
     Illustratively, this embodiment compensates for potential optical drift over time in a variable lens (e.g. a liquid lens) that is part of the overall lens assembly  120  by defining an operating range for the vision system at which the influence of the optical power of the variable lens on the optical power of the overall lens assembly (including any fixed lenses therein) is reduced. In this manner, drift is a small component of the overall focal performance of the lens assembly. This illustrative arrangement provides benefits where the adjustable focus range can be reduced. Thus, this system is useful in various embodiments—such as those where the distance (do) of the object surface from the focal plane is relatively constant, or this distance (do) varies over a small relative distance. Illustratively, the system can be employed in vision system applications that read at larger distances, wherein the required optical range is only a small fraction (approximately 2 diopter) of the specified range of commercially available liquid lenses (20 diopter). As described above, the variable lens assembly of the embodiments contemplated herein can include a variety of lens types that are capable of varying optical power. More particularly, in embodiments, the optical power (and hence, varying focal length/distance where focal length=1/optical power) is varied by controlling the lens shape and/or the lens refractive index. Such variable lens assemblies include, but are not limited to, liquid lenses, and a variety of liquid lens types can be employed including iso-density fluid types (Varioptic), membrane types (Optotune), etc. Likewise, variable lenses that operate using other mechanisms, such as electro-mechanical actuation, can be employed. 
     II. Drift-Reduction Lens Arrangement 
     By way of further illustration of the concepts of an embodiment,  FIG. 2  depicts a ray trace diagram of a basic optics arrangement for an exemplary vision system  200  with an exemplary object O 1 , image sensor  230  and generalized variable lens (e.g. a liquid lens (LL 1 )). The object O 1  is positioned at a distance dl from the variable lens LL 1 . This system is free of additional lenses and the rays  240  reflected from the object O 1  pass through the variable lens LL 1  and are focused directly on the image sensor  130  as shown. Thus, any minor variation (for example, from drift) in the focus of the variable lens LL 1  results in a potentially significant out-of-focus condition that can affect the ability of the vision system to render a proper result. 
     To address such sensitivity to drift and other focal variations in e.g. a liquid lens, reference is now made to  FIG. 3 , which shows a generalized optical arrangement for a vision system  300  according to an embodiment. A fixed (non-variable) positive lens PL is located at a predetermined distance d in front of the variable (e.g. liquid) lens assembly LL 2 , along the optical path between the system and imaged object O 2 . 
     Thus, the optical power A of this system  300  (where A 1  is the optical power of the positive lens assembly PL, A 2  is the optical power of the variable lens assembly LL 2  and d is the distance between the positive lens PL and the variable lens LL 2 ) is:
 
 A=A 1+ A 2− d*A 1* A 2
 
     If the distance between the variable lens LL 2  and the positive lens PL is relatively large, (e.g. d=k/A 1  (where k=0.5 . . . 0.9, and represents the product of the power of the positive lens A 1  and distance d; i.e. k=d*A 1 )), then the overall optical power A of the above-defined system of lenses with powers A 1  and A 2 , and relative distance d can be written as:
 
 A=A 1+(1− k )* A 2
 
and the drift, represented as a differential of lens optical power (dA) per unit temperature (dT) (dA/dT) of the system is:
 
 dA/dT=dA 1/ dT +(1− k )* dA 2/ dT  
 
meaning that the drift of the over system dA/dT equals the sum of the drift of the positive lens dA1/dT and (1−k) times the drift of the variable lens dA2/dT.
 
     In an embodiment, the fixed positive lens PL can be chosen as a glass lens with inherently low drift (i.e. dA1/dT≈0), so compared to the original setup in  FIG. 2 , it follows that the overall drift dA/dT of the system of  FIG. 3  is effectively reduced by a factor 1−k (=0.1× . . . 0.5×) using the positive lens PL, and the larger power the positive lens (i.e. larger k), the greater the drift reduction in the variable lens. 
     Reference is now made to  FIG. 4 , which details a cross section of an integrated lens unit/assembly  120  for use in the illustrative vision system camera  110  of  FIG. 1 . This lens assembly  120  can include various electrical connections and/or leads (shown schematically in phantom as cable  410  and connector  412 ) that extend from the variable (e.g. liquid) lens assembly  420  to a location on the body of the camera  110  in communication with appropriate control processors/components that are associated with the vision processor  140 . Note that the exemplary liquid lens assembly  420 , which can be a membrane-type, iso-density fluid-type or equivalent, is contained within a barrel  430  and the lead  410  is constructed to extend from a location on the barrel  430 . This connection allows control signals to power the liquid lens assembly (e.g. current and/or voltage modulation) to enable the variation and setting of the focus of the liquid lens assembly  420  in response to commands of the processor. Proper focus can be determined and/or set using a variety of techniques known to those of skill—for example using the crispness of imaged edges after stepping through various focus settings and/or using an external range-finding device. While the use of a separate cable link, with associated connector on the body of the camera is employed in the depicted embodiment, the connection arrangement can be internal to the barrel  430 —for example consisting of aligned contact pads and/or contact rings (on the lens and camera body) that interconnect when the lens assembly  120  is secured to the camera body. 
     The lens assembly barrel  430  is sized and arranged in this embodiment with the form factor of a conventional C-mount lens, having an appropriately threaded base  440 . The depicted external thread of the barrel base (flange)  440  is adapted to mate with a corresponding internal thread (not shown) on the camera body. The thread size is conventional (e.g. 1 inch×32). Note that the camera body can include a variety of accessories and functional components, such as a ring illuminator surrounding the lens and/or connections for an external illumination assembly. Such accessories and/or components can be applied to the camera to accomplish specific vision system tasks. The barrel  430  can be constructed from a variety of materials such as cast or machined aluminum alloy. The threaded base allows the barrel, and associated overall lens assembly contained therein, to be removably attached to the camera body and replaced with other types of lenses at the option of either the manufacturer or user. While the form factor of a C-mount base is used in this embodiment, any acceptable lens base form that allows accommodation of a liquid lens or other appropriate variable lens can be employed in alternate embodiments. For example, an F-mount lens base can be employed. 
     The dimensions of the lens barrel  430  are shown by way of non-limiting example in  FIG. 4 . As depicted, the barrel outer diameter ODL can be approximately 28-29 millimeters in an embodiment. This addresses the general size constraints/parameters of a C-mount lens. Likewise, the length OLL of the barrel  430  from front end  432  to the threaded base  440  is illustratively, approximately 32-34 millimeters. The distance DS from the lens base  440  to the focal plane of the image sensor  130  is approximately 17.5 millimeters. Note that these dimensions are illustrative of a wide range of possible relationships that are known to those of skill. 
     With further reference to  FIG. 4A , the positioning of the internal optical components of the lens is described in detail. A positive lens assembly  450 , having a relatively large diameter with respect to the variable lens ( 420 ) diameter, is located adjacent to the front end  432  of the barrel  430 . This positive lens assembly (also termed the “positive lens”)  450  is seated within a recess  454  formed at the front end of the barrel. The positive lens  450  is secured at its front side by a threaded ring  456 . Note that this arrangement is highly variable in alternate embodiments, and a variety of mounting and/or attachment mechanisms can be employed in alternate embodiments. The positive lens  450  is an achromatic doublet, defining an effective focal length (f) of 40 mm and a back focal length of 33.26 millimeters. The clear aperture is 24 millimeters. The overall lens assembly diameter is 25 millimeters. Illustratively, it consists of a front, convex lens  458  and a rear, concave lens  459 . The convex lens  458  defines a front radius RL 1  of 27.97 millimeters and rear radius RL 2  of −18.85 millimeters (where positive and negative radii represent directions with respect to the orientation of the imaged object, with positive radii oriented toward the object and negative radii oriented toward the image sensor). The concave lens  459  defines a front radius (also RL 2 ) of 18.85 millimeters (complimenting the mating surface of the convex lens  458 ) and a rear radius RL 3  of 152.94 millimeters. The convex lens  458  has a center thickness TC 1  (along the optical axis OA) of 9.5 millimeters and the concave lens has a center thickness TC 2  of 2.5 millimeters. These dimensions are highly variable in alternate embodiments. The above-described embodiment and associated dimensions of a positive lens (e.g. doublet) assembly  450  is commercially available from Edmund Optics Inc. of Barrington, N.J. as stock number 32321. In this embodiment, the lens front-to-sensor plane distance ODLF is approximately 49 millimeters according to an embodiment. It should be clear that the positive lens&#39; dimensions and/or the arrangement of components are highly variable in alternate embodiments. 
     The variable (e.g. liquid) lens assembly (which can be sourced from a variety of manufacturers)  420  is positioned adjacent to the rear end of the lens barrel  430 . In this embodiment, and by way of non-limiting example, the variable lens assembly  420  can comprise a model Arctic 416 liquid lens available from Varioptic of France. The exemplary variable lens assembly has a focus range of approximately 20 diopter (i.e. 5 centimeters to infinity), a diameter of 7.75 millimeters and a thickness (along the optical axis) of 1.6 millimeters. The depicted, exemplary, liquid lens assembly  420  consists of the lens unit  470 , which is mounted on a controller circuit board  472 , having a central aperture  474 , aligned along the optical axis through which focused light passes onto the sensor  130 . 
     The lens assembly  130  can be supported within the barrel  430  using an integral or unitary spacer, shoulder arrangement and/or support structure  460 . The support structure  460  ensures that the variable lens assembly  420  remains fixed in an appropriate alignment with respect to the optical axis OA. The distance DLR from the positive lens rear to the front of the variable lens unit  470  is 18.0 millimeters in this embodiment. Note that the image sensor  130  can define a conventional ½ inch-size CMOS sensor (6.9 millimeters (horizontal) by 5.5 millimeters (vertical)—SW in  FIG. 5 ) in an embodiment. 
     Reference is now made to  FIGS. 5-7 , which show the vision system and lens assembly in operation at a plurality of focal distances within the operational range of the system. The object O is thus located at three exemplary distances DO 1 , DO 2  and DO 3  in each of ray trace diagrams of  FIGS. 5, 6 and 7 , respectively. By way of example, DO 1  is approximately 219 millimeters, DO 2  is approximately 430 millimeters, and DO 3  is approximately 635 millimeters. Within this range, the optical power of the variable lens assembly is varied from +10.73 diopter for F=37.4 millimeters ( FIG. 5 ); to +0.32 diopter for F=39.8 millimeters ( FIG. 6 ); to −3.81 diopter for F=42.3 millimeters. This 219 to 635-millimeter focal range is associated with a 6.9 diopter variation. By way of comparison, a system mounting with the depicted variable lens assembly in a conventional arrangement with the variable lens attached at close distance of the front lens typically requires a 3.3 diopter variation. Thus, the illustrative system effectively reduces potential drift by more than a factor of 2 relative to a conventional arrangement. 
     More generally, the variable lens assembly (e.g. liquid lens assembly) is located adjacent to, but remote from, a focal point of the positive lens assembly, which can be the front, or more typically, the back/rear focal point of the positive lens assembly. It is understood that the positioning adjacent to the focal point allows for the variable lens to contribute to the total power of the lens system. The distance between the variable lens assembly and the focal point can be between approximately 0.1 and 0.5 times a focal length F of the positive lens assembly. By way of illustration, reference is made to the diagram of  FIG. 8 , where a positive lens assembly PL is positioned along the optical axis OA with a variable lens VL adjacent to the positive lens focal point FP. The focal length F between the positive lens PL and focal point FP is depicted. The distance (1−k)*F is characterized as the distance between the variable lens VL and the focal lens and the focal point FP with k=0.9 to 0.5 (i.e. 0.9*F to 0.5*F). Thus, the distance between the positive lens PL and the variable lens VL is k*F (i.e. 0.1*F to 0.5*F). In this manner, the positive lens assembly PL and the variable lens VL assembly are part of an overall lens assembly LA focusing light on the image sensor, and the optical power of the positive lens assembly “predominantly defines” an overall optical power of the overall lens assembly—in other words, the majority of magnification/optical power is provided by the positive lens assembly, thereby minimizing the effect of drift in the variable lens assembly. 
     Reference is now made to  FIGS. 9 and 10 , which show two embodiments of drift-reducing lens arrangements according to embodiments. The tables hereinbelow also provide, respectively, exemplary parameters for each lens element.  FIG. 9  is an arrangement  900  of lenses in association with an image sensor  910  of (e.g.) conventional design. In this embodiment, variable lens comprises a liquid lens assembly  920 . The depicted rays  930  are shown reflected into the lens arrangement  900  from an object (not shown) that can be placed at a distance of (e.g.) 200 millimeters from the first lens  940 . By way of non-limiting example, this lens  940  comprises a front concave surface  942  and a rear convex surface  944 . This lens can comprise a polymer, such as polycarbonate (or another optically suitable material). A central lens assembly includes a front composite lens  950  with a convex lens  952  having a front surface  954  and a rear surface  956 . This mates with a concave lens  958  with convex surface of similar radius and a rear convex surface  960 . Note that the lens element(s) ( 950 ) can also be constructed from polycarbonate (or another optical material). A disk-shaped optical element  970  (e.g. an IR filter) with infinite radii on each side (e.g. parallel planes) is located behind the composite lens assembly  950 . The rays  930  converge from the disk  970  at the variable (liquid) lens assembly  920 . This exemplary assembly can be based around the Arctic 416 lens from Varioptic of France, or another appropriate (e.g.) liquid lens. It comprises a front cover disk  980 , lens element  982 , interconnected with the lens control circuitry  990 , aperture stop (with associated radius of 341.763 millimeters)  984  and rear cover disk  986 . The lens control circuitry can be operatively connected with the vision system described above. This element is adjusted to maintain focus on the image sensor  910  and resists drift based on a variety of conditions described above. The spacing between the liquid lens assembly  920  and imager can be approximately 13-14 millimeters and the spacing between the disk  970  and liquid lens assembly  920  can be approximately 3-4 millimeters. Note that the size and shape of the exemplary lenses can be modified in accordance with skill in the art, as well as the spacing therebetween. Likewise, the variable lens assembly can comprise a variety of different types, operating on differing physical principles. For example a membrane-type liquid lens available from Optotune of Switzerland can be substituted, as well as mechanical lens types. 
     Referring now to  FIG. 10 , another embodiment of a lens arrangement  1000  capable of low-drift over a given working range, is depicted. This arrangement includes an image sensor  1010  of conventional design and a variable (liquid) lens assembly  1020 . Rays  1030  reflect from an object (not shown) at a range of (e.g.) 80-100 millimeters to a front convex lens  1040 . By way of non-limiting example, the front lens  1040  includes a front convex surface  1042  and a rear concave surface  1044 . The next lens  1046  defines a front convex surface  1048  and a rear concave surface  1050 . The next lens  1060  is concave on both surfaces  1062  and  1064 , with a front surface  1062  and a rear surface  1064 . Rays  1030  exit this lens  1060  and are directed into the variable (e.g. liquid) lens assembly  1020 , which, in this embodiment, is in the middle of the optical arrangement, with additional lenses  1080  and  1088  positioned between it and the image sensor  1010 . In this example, the liquid lens assembly  1020  is similar in model and construction to the assembly  920  described above (with an aperture stop having a radius of 10.101 millimeters), and is controlled by a lens controller  1090  that can operate similarly to the controller  990  also described above. Alternate embodiments of the variable lens assembly can be employed where appropriate, as described with reference to the arrangement  900  above. Rays  1030  from the variable (e.g. liquid) lens assembly  1020  are directed into a convex lens  1080  spaced at approximately 1.2 millimeters from the liquid lens assembly. The convex lens  1080  includes a front convex surface  1082  and a rear convex surface  1084 . A disk-shaped (e.g.) IR filter  1088  can be located behind the convex lens  1080 . It is spaced approximately 10-12 millimeters in front of the image sensor in this embodiment. The lenses are, by way of non-limiting example, constructed from optical glass in this embodiment, but one or more of the lenses (or other optical elements) can be constructed from another acceptable material, such as polycarbonate, or an appropriate equivalent material. 
     The lens arrangements  900  and  1000  described above can be adapted to be enclosed in a lens package with (e.g. a conventional camera base mount, such as a C-mount, as described above). Appropriate electrical connectors can be provided between the lens body and the camera base to enable control of the variable lens assembly. The electronics of the control circuit can reside in whole or in part with respect to the lens body, or within the camera body as appropriate. 
     By way of non-limiting example, the lenses of the various embodiments herein can define the specified parameters in each of the tables presented below. The parameters for the lens assembly  900  ( FIG. 9 ) are provided in the first table hereinbelow, with the associated front and rear surfaces (as applicable) of each structure or element in the overall assembly ordered (left-to-right) respectively from 0-13: 
                                                                     Thickness or       Semi-                        Distance to       diam-       Sur-       Ref.   Radius   Next Surface       eter       face   Structure   #   (mm)   (mm)   Material   (mm)                                                            0   Object            200.414                   (not                               shown)                           1   lens 940   942   −5.758   3.543   Polycarbonate   3.28       2       944   −6.977   4.121       4.00       3   lens 950   954   11.027   3.012   480R + PC   4.00       4       956   −5.976   0.792       4.00       5       960   −48.925   0.100       4.00       6   filter 970       infinty   1.300   B270   4.00                   (flat)                   7           infinity   3.000       4.00                   (flat)                   8   liquid lens       infinity   0.550   multiple   2.00           920       (flat)                   9           infinity           2.00                   (flat)                   10           variable           1.15       11           infinity   0.300       2.00                   (flat)                   12           infinity   13.884       2.00                   (flat)                   13   Image 910                    
The table below is for the lens assembly  1000  ( FIG. 10 ), with the associated front and rear surfaces (as applicable) of each structure or element in the overall assembly ordered (left-to-right) respectively from 0-14:
 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
               
               
                   
                   
                   
                   
                 Thickness or 
                   
                 Semi- 
               
               
                   
                   
                   
                   
                 Distance to  
                   
                 diam- 
               
               
                   
                   
                 Ref. 
                 Radius 
                 Next Surface 
                   
                 eter 
               
               
                 Surface 
                 Structure 
                 # 
                 (mm) 
                 (mm) 
                 Material 
                 (mm) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 0 
                 Object 
                   
                   
                 82.081 
                   
                   
               
               
                   
                 (not 
                   
                   
                   
                   
                   
               
               
                   
                 shown) 
                   
                   
                   
                   
                   
               
               
                 1 
                 lens 1040 
                 1042 
                 13.078 
                 1.461 
                 N-LASF9 
                 4.00 
               
               
                 2 
                   
                 1044 
                 49.620 
                 0.256 
                   
                 4.00 
               
               
                 3 
                 lens 1046 
                 1048 
                 19.922 
                 1.770 
                 N-SF6 
                 4.00 
               
               
                 4 
                   
                 1050 
                 18.444 
                 1.066 
                   
                 4.00 
               
               
                 5 
                 lens 1060 
                 1062 
                 −25.000 
                 1.495 
                 N-SF6 
                 4.00 
               
               
                 6 
                   
                 1064 
                 25.000 
                 2.000 
                   
                 4.00 
               
               
                 7 
                 liquid lens 
                   
                 infinity 
                 0.550 
                 multiple 
                 2.00 
               
               
                   
                 1020 
                   
                 (flat) 
                   
                   
                   
               
               
                 8 
                   
                   
                 infinity 
                   
                   
                 2.00 
               
               
                   
                   
                   
                 (flat) 
                   
                   
                   
               
               
                 9 
                   
                   
                 variable 
                   
                   
                 1.15 
               
               
                 10 
                   
                   
                 infinity 
                 0.300 
                   
                 2.00 
               
               
                   
                   
                   
                 (flat) 
                   
                   
                   
               
               
                 11 
                   
                   
                 infinity 
                 1.200 
                   
                 2.00 
               
               
                   
                   
                   
                 (flat) 
                   
                   
                   
               
               
                 12 
                 lens 1080 
                 1082 
                 23.854 
                 1.400 
                 N-LASF9 
                 3.00 
               
               
                 13 
                   
                 1084 
                 −18.000 
                 1.433 
                   
                 3.00 
               
               
                 14 
                 filter 1088 
                   
                 infinity 
                 1.300 
                 B270 
                 3.00 
               
               
                   
                   
                   
                 (flat) 
                   
                   
                   
               
               
                 15 
                   
                   
                 infinity 
                 11.100 
                   
                 3.00 
               
               
                   
                   
                   
                 (flat) 
                   
                   
                   
               
               
                 16 
                 Image 
                 1010 
               
               
                   
               
            
           
         
       
     
     It is further contemplated that the drift-compensating lens arrangement of the embodiments herein can be employed in combination with other drift-reducing methods, such as temperature stabilization of the variable lens or optical feedback systems. By way of non-limiting example, and incorporated herein by reference as useful background information, such arrangements are shown and described in commonly assigned U.S. Pat. No. 8,576,390, entitled SYSTEM AND METHOD FOR DETERMINING AND CONTROLLING FOCAL DISTANCE IN A VISION SYSTEM CAMERA, by Nunnink. Reference is also made to U.S. patent application Ser. No. 14/139,867, entitled CONSTANT MAGNIFICATION LENS FOR VISION SYSTEM CAMERA, by Nunnink; U.S. patent application Ser. No. 13/800,055, entitled LENS ASSEMBLY WITH INTEGRATED FEEDBACK LOOP FOR FOCUS ADJUSTMENT, by Nunnink et al. Illustratively, this application provides a removably mountable lens assembly for a vision system camera that includes an integral auto-focusing, liquid lens unit, in which the lens unit compensates for focus variations by employing a feedback control circuit that is integrated into the body of the lens assembly. The feedback control circuit receives motion information related to and actuator, such as a bobbin (which variably biases the membrane under current control) of the lens from a position sensor (e.g., a Hall sensor) and uses this information internally to correct for motion variations that deviate from the lens setting position at a target lens focal distance setting. The position sensor can be a single unit, or a combination of discrete sensors located variously with respect to the actuator/bobbin to measure movement at various locations around the lens unit. Illustratively, the feedback circuit can be interconnected with one or more temperature sensors that adjust the lens setting position for a particular temperature value. In addition, the feedback circuit can communicate with an accelerometer that senses the acting direction of gravity, and thereby corrects for potential sag (or other orientation-induced deformation) in the lens membrane based upon the spatial orientation of the lens. 
     III. Drift-Reduction Lens Assembly 
       FIGS. 11-18  variously describe embodiments of a drift-reduction lens allowing for extended range reading of object features (e.g. ID codes) for use in a variety of camera assemblies and associated applications, including handheld and fixed-mount units. Focal ranges of up to approximately 8 meters can be imaged using the lens arrangements herein. In general, the illustrative arrangements provide a variable-focus (e.g. liquid) lens positioned behind the remaining, fixed lens optics package—such that the variable lens is generally at the rear of the lens assembly, and between the fixed optics package and camera image sensor. With reference to  FIG. 11 , a lens arrangement  1100  is shown. This arrangement is applicable to a 12-millimeter (f′=12) lens. As shown, the relative scale  1110  of the overall lens arrangement (in millimeters) is provided. The lens&#39; fixed optics (shown in dashed box  1120 ) consists of a front plate element  1130 , followed by a biconvex lens  1132 . A set of three, smaller diameter lenses  1134  (positive),  1136  (biconcave) and  1138  (positive—opposite facing) are provided behind the biconvex lens  1132 . In this embodiment, the fixed optics package  1120  is provided in a separate lens housing, while a variable-focus lens assembly  1140  is mounted within the framework of the vision system housing (e.g. a handheld ID reader, such as described in commonly assigned U.S. patent application Ser. No. 14/550,709, entitled IMAGE MODULE INCLUDING MOUNTING AND DECODER FOR MOBILE DEVICES, filed Nov. 21, 2014). By way of non-limiting example, the variable lens assembly  1140  can comprise the above-described liquid lens mechanism, available from Optotune of Switzerland. The variable lens assembly can alternatively comprise any acceptable, manually or electronically adjustable lens arrangement, including those described above, available from Varioptic of France. The variable lens assembly  1140  can be interconnected (via a cable, printed circuit traces, etc.) to the vision system processor or another controller that allows the focal length of the lens to be adjusted. This controller can be integrated with the above-described feedback system. The variable lens assembly  1140  is optionally arranged with one or more filters and/or dust covers as appropriate. An aperture stop  1142  is also provided in this embodiment. The variable lens assembly  1140  focuses light (rays  1150 ) onto the image sensor  1152  for transmission to the vision system processor. The overall length  1160  of arrangement  1100  between the front surface of the plate  1130  and image sensor  1152  is approximately 15.2 millimeters. The distance  1162  between the rear face of the rear lens  1138  and the image plane (image sensor  1152 ) is approximately 10.26 millimeters. By way of example, the approximate parameters of the arrangement  1100  define F/# of 7; an image radius of 3 millimeters (i.e. ⅓ inch at 1.2-Megapixel sensor, up to a 5.0-Megapixel sensor); an RMS spot radius of 1.7 μm for 3 mm image height; and a measured distortion of less than 3%-4%. 
     The table below is for the lens assembly  1100  ( FIG. 11 ), with the associated front and rear surfaces (as applicable) of each structure or element in the overall assembly ordered (left-to-right) respectively from 0-16: 
     
       
         
           
               
               
               
               
               
               
             
               
                   
               
               
                   
                   
                   
                 Thickness or 
                   
                   
               
               
                   
                   
                   
                 distance to 
                   
                 Semi- 
               
               
                   
                   
                 Radius 
                 next surface 
                   
                 diameter 
               
               
                 Surface 
                 Structure 
                 (mm) 
                 (mm) 
                 Material 
                 (mm) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 0 
                 Object 
                   
                 500 
                   
                   
               
               
                   
                 (not shown) 
               
               
                 1 
                 filter 1130 
                 infinity 
                 0.650 
                 B-270 
                 2.50 
               
               
                   
                   
                 (flat) 
               
               
                 2 
                   
                 infinity 
                 0.200 
                   
                 2.50 
               
               
                   
                   
                 (flat) 
               
               
                 3 
                 lens 1132 
                 11.175 
                 1.300 
                 N-SK16 
                 2.50 
               
               
                 4 
                   
                 −11.175 
                 0.200 
                   
                 2.50 
               
               
                 5 
                 lens 1134 
                 3.765 
                 0.850 
                 N-SK16 
                 1.50 
               
               
                 6 
                   
                 5.228 
                 0.336 
                   
                 1.00 
               
               
                 7 
                 lens 1136 
                 −5.227 
                 0.800 
                 N-SF2 
                 1.50 
               
               
                 8 
                   
                 5.227 
                 0.150 
                   
                 1.20 
               
               
                 9 
                 aperture stop 
                   
                 0.150 
                 multiple 
                 0.63 
               
               
                   
                 1142 
               
               
                 10 
                 lens 1138 
                 −8.538 
                 0.800 
                 N-BAF10 
                 1.20 
               
               
                 11 
                   
                 −3.331 
                 1.100 
                   
                 1.50 
               
               
                 12 
                 liquid lens 
                 variable 
                   
                 multiple 
                 1.60 
               
               
                 13 
                 1100 
                 infinity 
                   
                   
                 1.60 
               
               
                   
                   
                 (flat) 
               
               
                 14 
                 sensor window 
                 infinity 
                 0.400 
                 D263T 
               
               
                   
                 1152 
                 (flat) 
               
               
                 15 
                   
                 infinity 
                 0.125 
               
               
                   
                   
                 (flat) 
               
               
                 16 
                 Image 1160 
               
               
                   
               
            
           
         
       
     
     Note that the various tables of lens parameters presented above and further below are only by way of example of a wide range of possible implementations. It should be clear to those of skill that any or all of the lenses and/or optical components herein can be altered by employing different parts, sizes, focal lengths, thicknesses, etc. as appropriate to the mechanical and optical needs of the imaging application. 
       FIGS. 12 and 13  show a lens assembly  1200  corresponding to the fixed optics package  1120 . The lens elements are contained within a barrel housing  1210  constructed from aluminum, or another acceptable material. The lens elements are similarly numbered to their counterparts in the assembly  1120  of  FIG. 11 . The base  1230  of the lens  1200  can be any acceptable format—for example, a C-mount threaded base (i.e. 1 inch×32 threads-per-inch) can be specified for the full length of the barrel. Alternatively, an M8×0.5 thread can be specified for the full length of the barrel, or in either case, an appropriate portion thereof. As shown in the cross section of  FIG. 13 , an aperture stop  1310  can be located between the biconcave lens  1136  and rearmost positive lens  1138 . The lenses  1130 - 1138  contained within the barrel  1210  are retained by a front retaining ring  1240  with an outside diameter  1330  of 10 millimeters, which is threaded onto the front end of the barrel  1210 . A threaded spacer ring  1250  is also threaded onto the barrel, and is located therealong so as to set the focal distance of the lens assembly with respect to the image plane. In an embodiment, when the ring  1250  is properly located on the lens barrel  1210 , it can be permanently/semi-permanently secured to the barrel using thread-locking compound, adhesive or another fixing mechanism (e.g. a set screw, pin, etc.). When the lens is threaded into the device&#39;s lens mount, the ring  1250  bears against the mounting and provides a desired spacing. In an embodiment, the overall lens length  1340  is approximately 6.9 millimeters, and the set distance  1350  between the rear face of the retaining ring  1250  and the image plane  1360  is approximately 12.15 millimeters. 
       FIGS. 14-18  variously depict versions of a drift-reduction lens assembly that can include the variable lens within its overall structure and that can be employed in (e.g.) fixed mount vision systems—for example, ID readers used in logistics and object tracking applications. 
     With reference to  FIG. 14 , a 16-millimeter lens arrangement  1400  is shown. This arrangement can be constructed with a housing  1410  that includes the variable (e.g. liquid) lens assembly  1430  within the overall package. The lens assembly is connected via a cable  1432  or other modality to a connector/contacts on the camera, or other vision system housing, which communicates with the processor so that the focal distance of the lens assembly  1430  can be controlled. Note that a variety of circuitry can be built into/onto the lens housing to perform some or all of the variable-lens-control functions. 
     The lens arrangement  1400  includes a front negative lens  1440 , followed by a smaller-diameter negative lens  1442 , another biconvex lens  1444 , and a smaller-diameter doublet  1445  consisting of a biconvex lens  1446  and planoconcave lens  1448 . A second, smaller diameter doublet  1450 , consisting of a positive lens  1452  and biconvex lens  1454 , is provided behind the first doublet  1445 , and a positive lens  1456  is provided between the doublet  1450  and variable (liquid) lens assembly  1430 . An aperture stop  1458  can also be provided at the rear surface of the last positive lens  1456 . The relative scale  1470 , in millimeters, is depicted and the back focal length  1480  between the rear of the variable lens  1430  and the image plane on the surface of the image sensor  1490  is set at approximately 8.5 millimeters—using (e.g.) appropriate adjustment rings, bases, mounts, etc. that should be clear to those of skill. As shown briefly in  FIG. 14A , this generates an image circle of approximately 8 millimeters in diameter. This lies within the maximum image circle  1496  (approximately 8.83 millimeters) of the depicted, exemplary sensor  1490  is an IMX265 model image CMOS sensor (by Sony Corporation of Japan). That is, the image circle  1496  circumscribes the corners of the rectangular perimeter  1495 , which represents the useable array of image pixels for the sensor  1490 . Other image sensors, such as that having a pixel array defined by the rectangle  1494 , are characterized by a different (in this example, smaller—e.g. 7.66 millimeters) image circle dimension ( 1493 ). Such a smaller-dimension sensor is available from Teledyne e2v, Ltd. (UK). 
     Other exemplary optical parameters of the lens assembly  1400  can include a focal length of approximately 16.2 to 16.6 millimeters, aperture size of F8, a total track of approximately 27.9 millimeters, a focus range of 1.0-4.0 meters and a working range for the variable lens of between approximately 0.0 and 2.5 diopters. Within this range, there is theoretically 2.5-times less drift than a conventional design. The RMS spot radius is below 2.2 microns in the extreme field of view (FOV) position. 
     The table below is for the lens assembly  1400  ( FIG. 14 ), with the associated front and rear surfaces (as applicable) of each structure or element in the overall assembly ordered (left-to-right) respectively from 0-20: 
     
       
         
           
               
               
               
               
               
               
             
               
                   
               
               
                   
                   
                   
                 Thickness or 
                   
                   
               
               
                   
                   
                   
                 Distance to 
                   
                 Semi- 
               
               
                   
                   
                 Radius 
                 Next Surface 
                   
                 diameter 
               
               
                 Surface 
                 Structure 
                 (mm) 
                 (mm) 
                 Material 
                 (mm) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 0 
                 Object 
                   
                 1828.571 
                   
                   
               
               
                   
                 (not shown) 
               
               
                 1 
                 lens 1440 
                 25.720 
                 1.143 
                 N-BK7 
                 3.886 
               
               
                 2 
                   
                 6.657 
                 1.000 
                   
                 3.000 
               
               
                 3 
                 lens 1442 
                 14.130 
                 2.000 
                 N-SK16 
                 3.000 
               
               
                 4 
                   
                 5.789 
                 1.000 
                   
                 2.500 
               
               
                 5 
                 lens 1444 
                 20.400 
                 0.914 
                 N-SK16 
                 2.500 
               
               
                 6 
                   
                 −32.069 
                 0.229 
                   
                 2.266 
               
               
                 7 
                 doublet 1445 
                 10.082 
                 1.234 
                 N-SK16 + 
                 2.237 
               
               
                 8 
                   
                 −32.093 
                 1.097 
                 N-SF2 
                 2.129 
               
               
                 9 
                   
                 32.093 
                 0.200 
                   
                 1.988 
               
               
                 10 
                 doublet 1450 
                 5.850 
                 1.097 
                 N-SF2 + 
                 2.000 
               
               
                 11 
                   
                 5.000 
                 1.500 
                 N-BK7 
                 2.000 
               
               
                 12 
                   
                 −12.411 
                 0.229 
                   
                 2.000 
               
               
                 13 
                 lens 1456 
                 6.259 
                 1.000 
                 N-SF2 
                 2.000 
               
               
                 14 
                   
                 3.130 
                 0.229 
                   
                 0.920 
               
               
                 15 
                 aperture stop 
                   
                 1.000 
                 multiple 
                 0.898 
               
               
                   
                 1458 
               
               
                 16 
                 liquid lens 
                 variable 
                   
                 multiple 
                 1.600 
               
               
                 17 
                 1430 
                 infinity 
                 9.740 
                   
                 1.600 
               
               
                   
                   
                 (flat) 
               
               
                 18 
                 sensor window 
                 infinity 
                 0.400 
                 D263T 
               
               
                   
                 1490 
                 (flat) 
               
               
                 19 
                   
                 infinity 
                 0.125 
               
               
                   
                   
                 (flat) 
               
               
                 20 
                 Image 1494 
               
               
                   
               
            
           
         
       
     
     With reference to  FIG. 15 , a 25-millimeter lens arrangement  1500  is shown. This arrangement can be constructed with a housing  1510  that includes the variable (e.g. liquid) lens assembly  1530  within the overall package. The lens assembly is connected via a cable  1532  or other modality to a connector/contacts as described above. The lens arrangement  1500  includes a front lens  1540  with a slightly concave front face, followed by a smaller-diameter planoconvex lens  1542 , and a doublet  1544  consisting of a biconvex lens  1546  and biconcave lens  1548 . A second, smaller-diameter doublet  1550 , consisting of a first positive lens  1552  and second lens  1554 , is provided behind the first doublet  1544 , and a negative lens  1556  is provided between the doublet  1550  and variable (liquid) lens assembly  1530 . An aperture stop  1558  can also be provided at the rear surface of the last positive lens  1556 . The relative scale  1570 , in millimeters, is depicted and the back focal length  1580  between the rear of the variable lens  1530  and the image plane on the surface of the image sensor  1590  (e.g. approximately an 8-millimeter image circle) is set at approximately 8.5 millimeters—using (e.g.) appropriate adjustment rings, bases, mounts, etc. that should be clear to those of skill. Other parameters of the lens assembly include a focal length of approximately 24.2 to 25.2 millimeters, aperture size of F8, a total track of approximately 27.6 millimeters, a focus range of 1.0-4.0 meters and a working range for the variable lens of between approximately 0.0 and 4.0 diopters. Within this range, there is theoretically four-times less drift than a conventional design. The RMS spot radius is below 1.9 microns in the extreme field of view (FOV) position. 
     The table below is for the lens assembly  1500  ( FIG. 11 ), with the associated front and rear surfaces (as applicable) of each structure or element in the overall assembly ordered (left-to-right) respectively from 0-16: 
     
       
         
           
               
               
               
               
               
               
             
               
                   
               
               
                   
                   
                   
                 Thickness or 
                   
                   
               
               
                   
                   
                   
                 Distance to 
                   
                 Semi- 
               
               
                   
                   
                 Radius 
                 Next Surface 
                   
                 diameter 
               
               
                 Surface 
                 Structure 
                 (mm) 
                 (mm) 
                 Material 
                 (mm) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 0 
                 Object 
                   
                 variable 
                   
                   
               
               
                   
                 (not shown) 
               
               
                 1 
                 lens 1540 
                 141.523 
                 1.786 
                 N-SK16 
                 6.071 
               
               
                 2 
                   
                 infinity 
                 0.300 
                   
                 6.071 
               
               
                   
                   
                 (flat) 
               
               
                 3 
                 lens 1542 
                 24.589 
                 1.429 
                 N-SK16 
                 4.286 
               
               
                 4 
                   
                 179.888 
                 0.357 
                   
                 4.286 
               
               
                 7 
                 doublet 1544 
                 11.789 
                 1.929 
                 N-SK16 + 
                 4.286 
               
               
                 8 
                   
                 −13.653 
                 1.714 
                 N-SF2 
                 4.286 
               
               
                 9 
                   
                 13.653 
                 1.434 
                   
                 3.571 
               
               
                 10 
                 doublet 1550 
                 6.888 
                 1.714 
                 N-SF2 + 
                 2.500 
               
               
                 11 
                   
                 8.747 
                 1.071 
                 N-BK7 
                 2.500 
               
               
                 12 
                   
                 14.691 
                 0.357 
                   
                 1.786 
               
               
                 13 
                 lens 1556 
                 8.245 
                 0.714 
                 N-BK7 
                 2.500 
               
               
                 14 
                   
                 4.122 
                 0.357 
                   
                 1.786 
               
               
                 15 
                 aperture stop 
                   
                 1.429 
                 multiple 
                 0.850 
               
               
                   
                 1558 
               
               
                 16 
                 liquid lens 
                 variable 
                   
                 multiple 
                 1.600 
               
               
                 17 
                 1530 
                 infinity 
                 8.500 
                   
                 1.600 
               
               
                   
                   
                 (flat) 
               
               
                 14 
                 sensor window 
                 infinity 
                 0.400 
                 D263T 
               
               
                   
                 1590 
                 (flat) 
               
               
                 15 
                   
                 infinity 
                 0.125 
               
               
                   
                   
                 (flat) 
               
               
                 16 
                 Image 1594 
               
               
                   
               
            
           
         
       
     
     With reference to  FIG. 16 , a 35-millimeter lens arrangement  1600  is shown. The lens arrangement in this embodiment is adapted for use in large-scale camera assemblies, such as those employed in high-volume logistics operations (e.g. fulfillment services, mass shipping, etc.) involving various-size objects. This lens arrangement  1600  can be constructed with a housing  1610  (shown in  FIG. 16  as a dashed box, and described in further detail below), which includes the variable (e.g. liquid) lens assembly  1630  within the overall package. The lens assembly is connected via a cable  1632  or other modality to a connector/contacts as described above. 
     The lens arrangement  1600  includes a front large-diameter planoconvex lens  1640 , followed by a smaller-diameter biconvex lens  1642 , stacked with a doublet  1644  consisting of a positive lens  1646  and biconcave lens  1648 . A second, smaller-diameter doublet  1650 , consisting of a first positive lens  1652  and second planoconvex lens  1654 , is provided behind the first doublet  1644 , and a negative lens  1656  is provided between the doublet  1650  and variable (liquid) lens assembly  1630 . An aperture stop  1658  can also be provided on the rear surface of the last negative lens  1656 . The relative scale  1670 , in millimeters, is depicted and the back focal length  1680  between the rear of the variable lens  1630  and the image plane on the surface of the image sensor  1690  (e.g. an 8-millimeter image circle) is set at approximately 8.5 millimeters. Other parameters of the lens assembly include a focal length of approximately 32.4 to 34.8 millimeters, aperture size of F8, a total track of approximately 49.6 millimeters, a focus range of 1.0-4.0 meters and a working range for the variable lens of between approximately 0.0 and 6.5 diopters. Within this range, there is theoretically 6.5-times less drift than a conventional design. The RMS spot radius is below 3.4 microns in the extreme field of view (FOV) position. 
     The table below is for the lens assembly  1600  ( FIG. 16 ), with the associated front and rear surfaces (as applicable) of each structure or element in the overall assembly ordered (left-to-right) respectively from 0-20: 
                                                             Thickness or                           Distance to       Semi-               Radius   Next Surface       diameter       Surface   Structure   (mm)   (mm)   Material   (mm)                                                        0   Object       variable                   (not shown)       1   lens 1640   55.586   2.500   N-SK16   8.500       2       infinity   10.000       8.500               (flat)       3   lens 1642   20.856   3.000   N-SK16   6.000       4       −24.197   0.500       6.000       7   doublet 1644   −20.861   2.700   N-SK16 +   6.000       8       17.704   2.400   N-SF2   6.000       9       17.704   7.479       5.000       10   doublet 1650   8.787   2.400   N-SF2 +   3.500       11       9.034   1.500   N-BK7   3.500       12       —   0.500       2.500               2195.069       13   lens 1656   11.052   1.000   N-BK7   3.500       14       5.526   0.500       2.500       15   aperture stop       2.000       0.892           1658       16   liquid lens   variable       multiple   1.600       17   1630   infinity   8.500       1.600               (flat)       18   sensor window   infinity   0.400   D263T           1690   (flat)       19       infinity   0.125               (flat)       20   Image 1694                    
The table below is for the lens assembly  1800  ( FIG. 18 ), with the associated front and rear surfaces (as applicable) of each structure or element in the overall assembly ordered (left-to-right) respectively from 0-22:
 
     
       
         
           
               
               
               
               
               
               
             
               
                   
               
               
                   
                   
                   
                 Thickness or 
                   
                   
               
               
                   
                   
                   
                 Distance to 
                   
                 Semi- 
               
               
                   
                   
                 Radius 
                 Next Surface 
                   
                 diameter 
               
               
                 Surface 
                 Structure 
                 (mm) 
                 (mm) 
                 Material 
                 (mm) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 0 
                 Object 
                   
                 variable 
                   
                   
               
               
                   
                 (not shown) 
               
               
                 1 
                 filter 1840 
                 infinity 
                 2.000 
                 multiple 
                 10.750 
               
               
                   
                   
                 (flat) 
               
               
                 2 
                   
                 infinity 
                 3.500 
                   
                 10.750 
               
               
                   
                   
                 (flat) 
               
               
                 3 
                 lens 1842 
                 infinity 
                 2.000 
                 N-SK16 
                 9.500 
               
               
                   
                   
                 (flat) 
               
               
                 4 
                   
                 −46.710 
                 1.000 
                   
                 9.500 
               
               
                 5 
                 doublet 1844 
                 19.310 
                 2.700 
                 N-SK16 + 
                 7.500 
               
               
                 6 
                   
                 infinity 
                 2.400 
                 N-SF2 
                 7.500 
               
               
                   
                   
                 (flat) 
               
               
                 7 
                   
                 62.220 
                 1.500 
                   
                 7.000 
               
               
                 8 
                 lens 1850 
                 −30.000 
                 2.000 
                 N-SF2 
                 7.500 
               
               
                 9 
                   
                 30.000 
                 1.000 
                   
                 7.000 
               
               
                 10 
                 lens 1852 
                 24.550 
                 3.400 
                 N-SK16 
                 7.500 
               
               
                 11 
                   
                 300.000 
                 1.000 
                   
                 7.000 
               
               
                 12 
                 lens 1854 
                 −300.000 
                 3.400 
                 N-SF2 
                 7.000 
               
               
                 13 
                   
                 −24.550 
                 1.000 
                   
                 7.500 
               
               
                 14 
                 doublet 1856 
                 18.000 
                 2.400 
                 N-SF2 + 
                 4.000 
               
               
                 15 
                   
                 9.000 
                 1.500 
                 N-SK16 
                 4.000 
               
               
                 16 
                   
                 12.700 
                 1.000 
                   
                 3.500 
               
               
                 17 
                 lens 1858 
                 30.000 
                 1.500 
                 N-SK16 
                 4.000 
               
               
                 18 
                   
                 17.140 
                 0.500 
                   
                 3.000 
               
               
                 19 
                 aperture stop 
                   
                   
                   
                 0.987 
               
               
                   
                 1859 
               
               
                 20 
                 liquid lens 
                 variable 
                   
                   
                 1.600 
               
               
                 21 
                 1860 
                 infinity 
                 9.103 
                 multiple 
                 1.600 
               
               
                   
                   
                 (flat) 
               
               
                 22 
                 Image 
               
               
                   
                 (not shown) 
               
               
                   
               
            
           
         
       
     
     As shown in  FIGS. 17 and 18 , a further embodiment and/or implementation of the 35-millimeter reduced-drift lens  1700  is shown in further detail. This lens assembly  1700  includes an outer housing  1710  that encloses a series of lenses that are functionally similar or identical to the above-described arrangement  1600  in  FIG. 16 . The housing can be constructed from any acceptable material (e.g. aluminum alloy), and in a variety of shapes. As shown, the housing  1710  includes a front end  1720 , main barrel  1730  and rear end  1740 . With further reference to  FIG. 18 , the lens front  1720  is threaded into an internal thread, which is formed in a widened flange  1820  of the main barrel  1730 . Note that an optional filter (e.g. a red bandpass filter)  1840  can be fitted to the lens front in this or other embodiments. The overall diameter DF of the lens front end  1720  is approximately 27.5 millimeters. In general, the filter  1840  can be a commercially available, threaded filter of appropriate optical specifications (e.g. wavelength bandpass for visible color, IR, UV, etc.). The main barrel  1730  houses a planoconvex lens  1842  in front of a doublet  1844  consisting of a planoconvex lens  1846  and planoconcave lens  1848  that together generate a positive lens geometry. A biconcave lens  1850  is stacked behind the doublet  1844 . A smaller-diameter pair of opposing planoconvex lenses  1852  and  1854  is provided behind the biconcave lens  1850 . A smaller-diameter doublet  1856  defines a negative lens behind the lenses  1852  and  1854 . This doublet  1856  resides behind a rearmost positive lens  1858 . The variable (i.e. liquid) lens  1860  resides behind the lens  1858 . It is retained in the smaller-diameter rear end  1740  by a threaded, annular retaining ring  1862  that sits inside a rear collar  1864  with (e.g.) an M13×0.5 internal thread. The inner diameter IDC of the collar is approximately 13 millimeters (threaded) and the outer diameter ODC is approximately 14 millimeters, and its axial length LC can be approximately 3.1 millimeters. The retaining ring  1862  can include a slot  1866  for in tightening by a blade-shaped tool of appropriate side and shape. Note that the lens arrangement  1700  can also include an aperture stop  1859  within the optical path at an appropriate location—for example adjacent to the liquid lens assembly  1860  on the rear surface of the lens  1858 . 
     The barrel  1730  can be threaded at the back end  1734  to mate with an internal thread on a camera assembly lens mount. The depth of mounting is controlled by an adjustment sleeve  1736  that slides over the barrel  1730 . One or more keyways (not shown) formed between the inner surface of the sleeve  1736  and outer surface of the barrel  1730  can be used to restrict rotation of the sleeve relative to the barrel, while allowing an axial sliding motion (axial being parallel to the optical axis OA. The sleeve  1736  is retained in a desired position by one or more set screws  1760 . The threaded back end  1734  of the barrel can define a standard C-mount size thread in an embodiment. Hence the outer diameter of the barrel  1730  is approximately 25 millimeters. The depicted lens can handle at least 3-Megapixel resolution in the presence of moderate drift. 
     IV. Conclusion 
     It should be clear that the above-described embodiments, provide a system that is particularly useful for imaging a small feature (or feature set), such as an ID code, over a relatively large distance. The effect of the variable lens assembly is weakened using the positive lens assembly according to an embodiment. This arrangement is acceptable within the desired operational range and feature size. In further embodiments, the (e.g. removable) lens arrangement places the variable lens behind the fixed optical components, which generate the reduced drift characteristic. The variable lens, thus provides the rearmost optical component of the arrangement before the sensor. The variable lens can be included in the lens arrangement/housing, or can be part of the camera assembly. 
     The foregoing has been a detailed description of illustrative embodiments of the invention. Various modifications and additions can be made without departing from the spirit and scope of this invention. Features of each of the various embodiments described above may be combined with features of other described embodiments as appropriate in order to provide a multiplicity of feature combinations in associated new embodiments. Furthermore, while the foregoing describes a number of separate embodiments of the apparatus and method of the present invention, what has been described herein is merely illustrative of the application of the principles of the present invention. For example, as used herein various directional and orientational terms such as “vertical”, “horizontal”, “up”, “down”, “bottom”, “top”, “side”, “front”, “rear”, “left”, “right”, and the like, are used only as relative conventions and not as absolute orientations with respect to a fixed coordinate system, such as gravity. Also, while the depicted lens assembly is incorporated in a removable lens unit, it is contemplated that the system can be employed in a fixed and/or permanently mounted lens. Likewise, while the above-described lens sizes and spacing distances are employed for the exemplary operational range, such sizes and distances can be scaled upwardly or downwardly in arrangements that have similar relative parameters but a larger or smaller overall size. Additionally, where a “lens assembly” is employed and/or described herein, it can consist of one or more discrete lenses that provide a desired optical effect. Accordingly, this description is meant to be taken only by way of example, and not to otherwise limit the scope of this invention.