Patent Publication Number: US-11030430-B2

Title: Constant magnification lens for vision system camera

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 61/745,927, filed Dec. 26, 2012, entitled CONSTANT MAGNIFICATION LENS FOR VISION SYSTEM CAMERA, the entire disclosure of which is herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to machine vision systems and more particularly to optics for use in handheld symbology readers, and methods for use of such optics. 
     BACKGROUND OF THE INVENTION 
     Vision systems that perform measurement, inspection, alignment of objects and/or decoding of symbology (e.g. bar codes) 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 remote, 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 a symbology (barcode) reader, the user or automated process acquires an image of an object that is believed to contain one or more barcodes. The image is processed to identify barcode features, which are then decoded by a decoding process and/or processor obtain the inherent alphanumeric data represented by the code. In other types of vision systems, various vision system tools (e.g. edge detectors, calipers, blob analysis) are employed by the system processor to detect edges and other features that allow for recognition of object features, and the determination of desired information based upon these features—for example whether the object is defective or whether it is properly aligned. 
     In a vision system, a key component is the vision system camera assembly. The camera assembly includes a lens (optics) and an imager (or “sensor”) that provides the array of image pixel information. The vision system processor, as described above, receives the pixel data from the imager/sensor and processes it to derive useful vision system information about the imaged scene and/or object. The vision system processor and related components (e.g. data memory, decoders, etc.) can be provided within the camera assembly&#39;s housing or enclosure, or some or all of these components can be mounted remotely (e.g. within a PC, or other remote, self-contained processing system), and linked by a wired or wireless interconnect. Likewise, the camera assembly can include an on-board (internal) illuminator that typically surrounds the lens, and/or another illumination arrangement that provides light to the imaged scene. 
     In some vision system cameras, it is desirable to provide an automatic focus (“auto-focus”) capability. Many auto-focus arrangements rely upon electromechanical actuation to move a fixed lens, while others increasingly rely upon other forms of varioptic lens designs, such as a so-called liquid-lens. 
     In the particular field of symbology reading using sensor-based vision systems, a common reader arrangement employs a handheld unit that is directed at an object containing a symbol (e.g. a 1D or 2D barcode). Such handheld systems are commonly employed to track inventory, for example in a warehouse or factory floor. In such environments, the distance between a symbol and the reader can be highly variable, as some objects can reside relatively close to a user, while others are disposed at a distance (e.g. an object located on a high shelf). While a ready may include a conventional auto-focus mechanism to allow it to generate a sharp image of both the close object and the far object, the symbol in the far object can appear small relative to the overall field of view given this long focal distance as the opening angle of the optics is too large. As such, the small size of the symbol in the overall image may render it difficult to properly decode due to lack of sufficient resolution when compared to the overall field that is captured by the sensor (i.e. the feature of interest/symbol is too small at distance). 
     It is therefore desirable to provide a vision system camera assembly that can more effectively resolve a symbol or other feature of interest at both short focal distances and long focal distance. This camera assembly should be adaptable to a handheld device and/or to a fixed-mount device. 
     SUMMARY OF THE INVENTION 
     This invention overcomes disadvantages of the prior art by providing a lens assembly for a vision system, such as a handheld symbology reader, which allows for a constant magnification at both short and long focal distances. The lens assembly resides movably and/or adjustably along the optical axis at a predetermined distance from the image sensor (located generally perpendicular to the axis). In an illustrative embodiment, the lens assembly consists of two lenses L 1  and L 2  separated from each other along an optical axis. In embodiments, L 1  and L 2  can be represented by groups of lenses. The two lenses L 1  and L 2  define respective focal lengths f 1  and f 2 . The lenses satisfy the following relationships: (a) the focal points of L 1  and L 2  coincide; (b) the aperture stop of the assembly is between the back surface of L 1  and the focal point of L 1 ; (c) the magnification of the assembly is constant and equal to f 2 /f 1 ; and (d) the shift in focal position of the assembly is (f 1 /f 2 ) 2 *(movement of the assembly along the optical axis). In an embodiment, the lens is moved to a selected position along the optical axis, and with respect to the sensor, by an actuator (e.g. a geared stepper or servo motor). The actuator moves in response to commands from the vision processor that employs a conventional or custom auto-focus process to resolve a sharp image of the feature of interest (e.g. a symbol) on an object at the prevailing focal distance. The feature of interest will appear at approximately the same resolution at each of a maximum and minimum focal distance and at all ranges therebetween. The sensor&#39;s inherent pixel resolution is sufficient at the range of operating distances to provide sufficient detail to identify and decode the symbol. 
     In an illustrative embodiment, a vision system for acquiring images of objects over a range of focal distances within a field of view includes an image sensor operatively connected to a vision processor. A constant magnification lens assembly, oriented along an optical axis, and including a front lens assembly, receives light from a scene and transmits the light to the image sensor. The front lens assembly is smaller in area (or associated dimensions) than an area (or associated dimensions) of the field of view, making of a practical and relatively compact package. The constant magnification lens assembly also includes a rear lens assembly. The front lens assembly and the rear lens assembly are arranged in a fixed spatial relationship therebetween. Illustratively, the front lens assembly and the rear lens assembly are constructed and arranged so that (a) a focal point of the front lens assembly and a focal point of the rear lens assembly coincide, (b) an aperture stop of the constant magnification lens assembly is between a back surface of the front lens assembly and a focal point of the front lens assembly, (c) a magnification of the assembly is constant and equal to a focal length (f 2 ) of the rear lens assembly/a focal length (f 1 ) of the front lens assembly, and (d) the shift in focal position of the assembly is (f 1 /f 2 ) 2 *(movement of the constant magnification lens assembly along the optical axis). This arrangement (i.e. item (b)) allows the front lens assembly to define an area and/or dimensions that are smaller than those of the imaged field of view. The front lens assembly and the rear lens assembly can be mounted in a barrel that is moved toward and away from the image sensor by an actuator responsive to a focus process. Alternatively, the constant magnification lens assembly and its components can be fixed with respect to the camera body/frame, and the sensor assembly (or a portion thereof containing the sensor) can be moved toward and away from (along the optical axis) the constant magnification lens assembly by an appropriate actuator in response to the focus process. 
     In an illustrative embodiment, a vision system for acquiring images of an object over a range of focal distances within a field of view comprises an image sensor operatively connected to a vision processor. A constant magnification lens assembly is oriented along an optical axis that receives light from a scene and transmits the light to the image sensor. The constant magnification lens assembly includes a liquid lens assembly oriented between the image sensor and a front lens assembly. This lens assembly can be based upon the use of at least two iso-density liquids that vary interaction based upon the principle of electro wetting, or the lens can include an actuator that changes the shape of a liquid-filled membrane. The front lens assembly comprises one or more fixed lenses, and the (rear) liquid lens assembly including an interface that employs input electrical energy to vary a magnification m 2  of the liquid lens assembly. A controller selectively adjusts the magnification m 2  of the liquid lens assembly to maintain focus on the object at a constant system magnification M at each focal distance of the range of focal distances. Illustratively, the front lens assembly and the rear lens assembly are constructed and arranged so that (a) a focal point of the front lens assembly and a front principal plane of the liquid lens assembly coincide, and (b) the magnification is constant and equal to a ratio between the distance (d 2 ) from the liquid lens assembly to the image sensor and the distance (d 1 ) between the front lens assembly and the liquid lens assembly. The controller is also arranged to iteratively adjust the magnification m 2  of the liquid lens assembly until a desired focus at the constant system magnification M is provided. 
     Illustratively, in any embodiment herein, the front lens assembly and the liquid (or rear) lens assembly are constructed and arranged so that (a) a focal point of the front lens assembly and the front principal plane of the rear lens assembly coincide (d 1 =f 1 ), (b) an aperture stop of the constant magnification lens assembly is located between a back surface of the front lens assembly and a focal point of the front lens assembly, (c) the magnification is constant and equal to a ratio between the distance (d 2 ) from the liquid lens assembly to the image sensor and the distance (d 1 ) between the front lens assembly and the liquid lens assembly. 
     In a further embodiment, a method for acquiring images of an object over a range of focal distances within a field of view includes the steps of providing an image sensor operatively connected to a vision processor and a constant magnification lens assembly oriented along an optical axis that receives light from a scene and transmits the light to the image sensor. The constant magnification lens assembly includes a front lens assembly. The constant magnification lens assembly is iteratively adjusted until the object achieves a desired focus at the image sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention description below refers to the accompanying drawings, of which: 
         FIG. 1  is a diagram of a handheld symbology reader, and associated data processing and storage system, employing a constant magnification lens assembly according to an illustrative embodiment to acquire images of objects at a short focal distance and a long focal distance; 
         FIG. 2  is an image of a scene acquired by the image sensor of the reader of  FIG. 1 , showing resolution of a feature of interest at a short focal distance; 
         FIG. 3  is an image of a scene acquired by the image sensor of the reader of  FIG. 1 , showing resolution of a feature of interest at a comparatively long focal distance; 
         FIG. 4  is a cross section of the lens assembly for use in the reader of  FIG. 1 , according to an illustrative embodiment, showing the relative position of the assembly along the optical axis with respect to the image sensor for a relatively short focal distance; 
         FIG. 5  is a cross section of the lens assembly shown in  FIG. 4 , in which the relative position of the assembly along the optical axis with respect to the image sensor is set for a longer focal distance; 
         FIG. 6  is a diagram of a lens focus mechanism and associated processors for processing image data and controlling focus of the lens assembly in the reader of  FIG. 1 ; 
         FIG. 7  is a flow diagram of an exemplary focus process using the lens assembly in the reader of  FIG. 1 ; 
         FIG. 8  is a diagram of a lens assembly for a constant magnification vision system including a liquid lens assembly according to an illustrative embodiment; 
         FIGS. 9 and 10  are thin-line ray-trace diagrams of the lens assembly for the constant magnification vision system of  FIG. 8 , shown maintaining constant magnification on an exemplary object at a nearer and more-distant positioning, respectively, by controlling magnification of the liquid lens assembly; 
         FIG. 11  is a diagram of a lens assembly for a constant magnification vision system including a liquid lens assembly and an aperture stop located between the liquid lens assembly and a front lens assembly according to another illustrative embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     I. General Considerations 
       FIG. 1  shows a vision system  100 , which includes at least one symbology reader  110  that can be handheld as shown, or fixed in a position with respect to an imaged scene. The reader can define any acceptable housing, including the depicted main body  112  and grip  114 . In this embodiment, the reader includes a front window  116 , which can include an external and/or internal illumination system (illuminator). The illuminator can comprise any arrangement and/or combination of lighting elements in any acceptable arrangement. In this embodiment, and by way of example, light elements (e.g. high-output LEDs)  120 ,  122  are employed and allow for differing color/wavelength, angle and/or intensity of illumination. The illuminator can include conventional aiming LEDs (not shown) that project a beam onto a field of view to ensure that features of interest (e.g. barcodes or other symbols, also termed “IDs”) are properly and fully imaged. The reader  110  can include an indicator and interface panel  130 , located at the rear of the body  112  in this embodiment. This panel can include on/off and other switches as well as lights to indicate a “good” or “failed” symbol read (i.e. success or failure in reading/decoding the symbol). The grip  114  can include one or more trigger buttons  132  that trigger illumination and image capture among other functions, such as toggling of aiming LEDs. The reader also includes one or more processing circuits, memory and the like, that are collectively shown (in phantom) as a vision processor  136 . This processor performs various image processing, and image data handling/storage functions. Illustratively, the processor  136  receives captured image frame rate in the form of color or grayscale pixels (among other formats) from the image sensor (also shown in phantom). The processor searches for ID features (or other features of interest) in the image, and then passes appropriate data to a decoding process that generates codes from the ID features. These codes are stored and/or passed via a communication link (which can be wired, or wireless as shown)  140  to a receiver  142  that is interconnected via a network or other link with a data processing and storage system  144 . This system  144  can comprise a conventional server or PC running appropriate applications for handling and storing code data transmitted from the reader  110 . Such applications and the architecture of the system  144  should be clear to those of skill in the art. 
     The reader also includes a lens assembly  150  (shown in phantom behind window  116 ) that provides for a constant magnification over a range of focal distances. By way of example, an object O 1  having a symbol S 1  is imaged by the reader  110  with the lens  150  focusing upon a field of view FOV 1  in which the symbol S 1  occupies a relatively prominent place/scale therewithin. This scale is sufficient to allow sufficient detail for an acceptable ID reading. The focal distance D 1  along optical axis OA 1  is within an operating range of least approximately 350 mm, and for the purposes of the example is at a distance of approximately 500 mm. Likewise, the reader  110  can be focused (as shown in phantom) on another object O 2  located at a significantly shorter focal distance D 2  along optical axis OA 2  that, for the purposes of the example, is approximately 50 mm. Notably, using the constant magnification lens assembly  150 , in accordance with an illustrative embodiment, the scale of the second symbol S 2  within the associated field of view FOV 2  is approximately the same as that of S 1  and FOV 1 . Hence, regardless of distance within a predetermined distance range, the size of the field of view and symbol therewithin remains the same, allowing for sufficient detail to obtain a good read. 
     With reference to  FIGS. 2 and 3 , the principle of constant magnification is further illustrated by respective exemplary images  200  and  300  that simulate the appearance of images acquired, respectively, at focal distances of approximately 50 mm and 500 mm. The field of view of each image  200 ,  300  is defined by the outside edges of the depicted image. Using a constant magnification arrangement, both images should ideally present approximately the same boundaries relative to the scene. Likewise, each exemplary symbol region  210 ,  310  appears to be relatively similar in size with respect to the field of view, allowing sufficient detail for the processor to find and decode the symbol. 
     II. Constant Magnification with Mechanically Driven Lens 
     Reference is now made to  FIGS. 4 and 5 , which show the constant magnification lens assembly  150  in further detail, according to an illustrative embodiment. The assembly  150  consists of a front lens L 1  (with a focal length f 1 ) and a rear lens L 2  (with a focal length f 2 ) aligned along the optical axis OA with respect to a sensor  410 , which typically defines an image plane perpendicular to the axis OA. The lenses L 1 , L 2  in this embodiment are positioned at a fixed distance SL with respect to each other with in barrel  420 , or other supporting structure that maintains their relative alignment and spacing. Each of the two lenses L 1 , L 2  is particularly designed to establish a set of relationships that ensure constant magnification over a range of focal distances. More particularly, these relationships are as follows:
         (a) the focal points of L 1  and L 2  coincide at the depicted plane (d 1 =f 1 +f 2 );   (b) the aperture stop of the assembly (AS) is between the back surface of L 1  and the focal point of L 1 ;   (c) the magnification of the assembly is constant and equal to f 2 /f 1 ; and   (d) the shift in focal position of the assembly is (f 1 /f 2 ) 2 *(movement of the assembly along the optical axis).       

     Note that the placement of the aperture stop at a position defined in item (b) above is advantageous in that the size of the front lens can be smaller in diameter, area, etc., than an area, length, width, etc., the imaged object and associated field of view. Conversely, placement of the aperture elsewhere (e.g. at focal point f 1 ), could necessitate use of a front lens approximately the size of the desired field of view—for example, in the manner of a telecentric lens. Such a large lens is typically disadvantageous where size and placement constraints exist. 
     Note, it is also expressly contemplated that the depicted lenses L 1  and/or L 2  can be defined by group(s) of lenses having similar or the same optical power as a single lens element. In various embodiments, such groups of lenses can provide improved correction of optical aberrations relative to single, discrete lens elements. Thus as used herein the term “lens” should be taken broadly to include an arrangement of a plurality of discrete lenses. 
     One of skill in the art of lens design should understand the construction of a lens assembly that satisfies the above relationships (a)-(d). In an embodiment, the value f 2 /f 1  is approximately 0.1, but other ratios are expressly contemplated. Illustratively, both groups of lenses L 1  and L 2  define a positive optical power. By way of example, lens L 1  can define a focal length between approximately 30 and 60 millimeters and lens L 2  can define a focal length between approximately 6 and 10 millimeters. As shown, at a relatively short focal distance FD 1  ( FIG. 3 ), the arrangement of lenses L 1 , L 2  in the assembly  150  defines a ray pattern  430  that diverges at a steeper angle for a given distance DS 1  between the sensor  410  and the rear lens  410 . This more divergent pattern defines a focus on a field of view  440  that is a desired size for appropriately imaging a feature of interest (e.g. a symbol/ID) therein. 
     To achieve a similarly sized field of view  540  ( FIG. 5 ) to the field  440  at a longer focal distance FD 2 , the distance DS 2  between rear lens L 2  and sensor  410  is shortened with respect to the above distance DS 1 . Thus the ray pattern  530  is less divergent. Note at both focal distance the aperture stop AS is the same. Thus, by appropriately moving the distance between the rear lens and the sensor, the optical system can be brought into focus on a similarly sized field of view at a wide range of focal distances. 
     With reference now to  FIG. 6 , an exemplary focus mechanism  610  for moving the lens assembly  150  along the optical axis OA, toward and away from the sensor  138  (shown mounted on an associated imager circuit board that defines the base of a sensor assembly  630 ) is depicted. Note that the depicted mechanism  610  is exemplary of a wide variety of possible focus-adjustment mechanisms, others implementations of which should be clear to those of skill in the art. In this exemplary embodiment, the mechanism  610  includes an actuator in the form of a motor (e.g. a servo or stepper motor)  620  having appropriate torque, and where desired, gear reduction to rotate a pinion gear  622  in each of opposing rotational directions (double arrow  624 ). The motor rotates a large gear  626  having an inner perimeter that is enmeshed with the outer surface of the lens assembly by mating threads  628  that can include a relatively shallow pitch. As the gear  626  rotates based upon the drive of the motor, it causes the lens assembly to move toward or away (double arrow  631 ) from the sensor  630  assembly and associated sensor image plane. In this embodiment, an anti-rotation pin  632 , which is fixed to the housing body  112 , engages an axially-aligned slot  634  in the lens assembly  150  to prevent rotation of the lens. In this manner rotation of the gear is fully translated intro linear motion of the lens assembly along the axis OA. A variety of alternate anti-rotation arrangement can be employed in this implementation. Alternatively the lens can be rotated and a fixed, threaded base (substituting for a rotating gear  626 ) can be used to generate linear translation in the lens assembly  150  with respect to the sensor. In further embodiments, the lens can be fixed and the sensor can be provided on an axially moving base. In general, the system defines relative motion between the lens assembly  150  and sensor/image plane ( 138 ). 
     The axial position of the lens assembly  150  is determined by the proper focus of the projected image on the sensor  138 . In an embodiment, the on-board vision processor  136  includes a focus process  640  that can employ conventional techniques to determine when an image comes into sharp focus. For example, the contrast fall-off at edges in acquired images can be employed. In the focus process  640 , the constant magnification lens assembly  150  is moved (in its entirety as a fixed front lens L 1 , rear lens L 2  and aperture stop AS) by the motor  620  through a multiplicity of position steps, and the process  640  determines the best focus position based upon certain metrics in the acquired images. Other techniques for focusing the lens are expressly contemplated—for example, sweeping through lens positions or employing a distance sensor and/or range finder can be employed to determine the distance to the object/imaged scene and move the lend to a predetermined setting. The lens position setting can be based, for example, on a formula or look-up table that uses the sensed distance to determine a lens position setting. 
     With further reference to  FIG. 6 , it is expressly contemplated that the constant magnificent lens assembly  150  can be fixed with respect to body/frame of the camera. In such embodiments, the sensor assembly  630  (or a portion thereof (e.g. the sensor  138 )) can be moved along the optical axis OA toward and away from the fixed-position constant magnification lens assembly  150 . An appropriate sensor actuator (SA) (shown in phantom as box  648 ) can be used to move the sensor assembly  630  as indicated by the associated double arrow. Any acceptable actuation mechanism can be used including (but not limited to) electrically powered worm drives, gear drives and/or linear motors. Focus processes, as described generally above (e.g., stepping, sweeping, sensing distance, etc.), can be used to set the appropriate sensor position with respect to the fixed, constant magnification lens assembly. 
     III. Constant Magnification Focus Process 
     With brief reference to  FIG. 7 , an illustrative procedure  700  for adjusting focus of the constant magnification lens assembly is shown. This procedure  700  is a simplified example of any acceptable procedure for adjusting focus during setup and/or runtime. In step  710 , the sensor acquires one or more image frames of the scene and transmits these image frames to the vision processor. Among other processes, the processor performs the focus process ( 640  in  FIG. 6 ) to determine whether the image is sufficiently resolved (step  720 ). If the image is sufficiently resolved (for example, a symbol can be decoded), then decision step  730  allows focus to be set at the current setting (step  740 ). If focus is unacceptable, or worse than that achieved in a previous adjustment of the lens assembly, then the focus process readjusts the lens focus in a predetermined direction over an adjustment increment (step  750 ). Selection of the predetermined direction form movement of the lens assembly can be based upon a prediction as to which direction will improve focus, or it can be based upon an analysis of the image indicating which direction will achieve better focus. If focus is worse than the previous adjustment, then the direction of adjustment in the next cycle is reversed accordingly. While adjustment can occur in increments, using an iterative process, it is contemplated that a larger adjustment can be made initially based upon a determination as to how “out-of-focus” the image is, and then smaller-sized adjustments can be made until a final focus is achieved. Again, this procedure ( 700 ) is exemplary of a wide range of focus-adjustment procedures and/or techniques that should be clear to those of skill in the art. 
     IV. Constant Magnification using Liquid Lens 
     An exemplary lens configuration that can be desirable in certain vision system applications is a so-called liquid lens assembly. One form of commercially available liquid lens, available, for example from Varioptic of France, uses two iso-density liquids—oil is an insulator while water is a conductor—and the principle (phenomenon) of electro wetting to vary the optical power setting of the lens. On example provides an 18-diopter (1/focal length) variable range of optical power. 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 autofocus 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. 
     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. This lens advantageously provides a larger aperture than competing designs and operates faster. The focal length/distance of an optical system employing liquid lens technology can be varied within a predetermined range (e.g. 20 diopters) based upon the setting of the liquid lens element. This setting is varied by applying force to the perimeter of the membrane using electromagnetic actuation in accordance with known techniques. 
       FIG. 8  depicts a generalized lens system configuration  800  for a vision system (See  FIG. 1 ) including a liquid lens element L 2 L oriented along an optical axis OAL. In this illustrative arrangement, the overall lens system  800  consists of at least two lenses or two groups of lenses. The first (front) group L 1 L has a fixed optical power, the second (rear) group L 2 L consists of, or includes, a liquid lens element with variable optical power. Note that the front lens assembly and the (rear) liquid lens assembly are arranged at a fixed spatial relationship along the optical axis OAL in this embodiment. This liquid lens L 2 L is driven by a driver or similar processor  810  that provides current to the lens to adjust its focus based upon a lens focus process  820 . A complete range of exemplary parameters for the system are provided below. If an exemplary object  830  is placed at a distance Sobj (in this example, approximately 20.710 mm) from the optical plane of the first lens (group) L 1 L with optical power A 1 , this lens will project an image at a distance:
 
 S _2 =Sobj/ ( A 1* Sobj− 1)   (Eq. 1)
 
     By way of non-limiting example, the distance d 1  between the respective optical planes of lenses L 1 L and L 2 L, and the distance d 2  between the optical plane of the liquid lens L 2 L and image sensor  840  can be fixed. Liquid lens L 2 L thereby projects this intermediate image onto the sensor  840  if the liquid lens&#39; optical power A_LL is equal to (set to):
 
 A _ LL= 1/( S _2− d 1)+1/ d 2   (Eq. 2)
 
     The geometrical magnification m 1  of the first lens (group) is equal to:
 
 m 1=1/( A 1* S _ obj− 1)   (Eq. 3)
 
and the magnification m 2  of the second lens (group) is equal to:
 
 m 2=1/( A _ LL* ( S _2− d 1)−1)   (Eq. 4)
 
Now the liquid lens L 2 L is placed into the back focal point of lens L 1 L, this expression can also be written as d 1 =1/A 1 .
 
     Substituting this into the equations (2) and (4), the total magnification M of this system reduces to:
 
 M=m 1* m 2= d 2/ d 1   (Eq. 5)
 
where M is the system magnification, m 1  is the magnification of the fixed lens group/assembly, m 2  is the magnification of the liquid lens group/assembly, d 1  is the distance between the respective optical planes of lenses L 1 L and L 2 L, and d 2  is the distance between the optical plane of the liquid lens L 2 L and image sensor  840 .
 
     Thus, this arrangement produces a constant magnification that is free of dependence on (independent of) the object distance.  FIGS. 9 and 10  each respectively show an exemplary thin-lens ray-trace though the lens arrangement  800  of the system for two different object distances S 3  (closer to the optical plane of L 1 L) and S 4  (further from the optical plane of L 11 ). The focal distance F 2 L and F 2 L′ of the liquid lens L 2 L is appropriately adjusted by the driver  810  and process  820  to project the focused image at constant magnification M throughout the anticipated range of object distances Sobj. 
     Thus, by varying the optical power of L 2 L (m 2 ), the value of M can be maintained at a predetermined level over varying distances of object from the system (Sobj). The value for m 2  can be set using a variety of techniques. The above-described focus process  700  can be used to set m 2 . That is, the power of the lens L 2 L can be adjusted incrementally (iteratively) until appropriate focus for the selected constant value for M is achieved for the object  830  at a given distance. 
     Reference is now made to  FIG. 11 , which shows a constant magnification lens system  1100  with a liquid lens assembly L 2 L according to a further embodiment. Elements that are similar in structure and/or function as those described above with reference to  FIGS. 8-10  have been provided with like reference numbers. Illustratively, an aperture stop ASL (see also the description above for mechanically moved lenses) can be positioned between the front lens assembly L 1 L and the rear, liquid lens assembly L 2 L. The position of this aperture stop ASL along the optical axis OAL defines the size of the lenses (groups/assemblies) L 1 L and L 2 L. If the aperture stop ASL is positioned toward the front lens assembly L 1 L, then the size (diameter) of the liquid lens assembly L 2 L should be increased. If the aperture stop ASL is, conversely, positioned toward the liquid lens assembly L 2 L, then the size (diameter) of the front lens assembly L 1 L should be increased. Currently, commercially available liquid lenses are generally available in smaller diameters, so the aperture stop is generally positioned placed nearer to the liquid lens, or the liquid lens assembly (itself) acts as the aperture stop in the system. 
     Some generalized parameters for an operational example of a constant magnification lens assembly employing a membrane-type liquid lens are shown and described in the Table as follows: 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Focal distance of Lens 1 
                 f1 = 100 mm  
               
               
                   
                 Closest object distance: 
                 S_near = −200 mm  
               
               
                   
                 Largest object distance 
                 S_far = −400 mm  
               
               
                   
                 Intermediate Near Image S2 
                 S2_near = 200 mm  
               
               
                   
                 Intermediate FAR*Image S2 
                 S2_far = 133.333 mm  
               
               
                   
                 Distance between lenses 
                 d1 = f1 mm  
               
               
                   
                 L2L to sensor distance 
                 d2 = 20 mm  
               
               
                   
                 Focal Length of L2L for Near object 
                 f_ll_near = 16.667 mm  
               
               
                   
                 Optical Power of L2L (Diopter)@Near 
                 A_ll_near = 60 diopter  
               
               
                   
                 Focal Length of L2L for FAR object 
                 f_ll_far = 12.5 mm  
               
               
                   
                 Optical Power of L2L (Diopter)@Far 
                 A_ll_Far = 80 diopter  
               
               
                   
                 Required Optical Power Range of L2L  
                 R = 20 diopter  
               
               
                   
                 (Diopter) 
                   
               
               
                   
                 Magnification at NEAR distance 
                 m1_near = −1 
               
               
                   
                   
                 m2_near = 0.2 
               
               
                   
                   
                 M_near = m1*m2 = −0.2 
               
               
                   
                 Magnification at FAR distance 
                 m1_far = −0.333 
               
               
                   
                   
                 m2_far = 0.6 
               
               
                   
                   
                 M_far = m1*m2 = −0.2 
               
               
                   
                   
               
            
           
         
       
     
     It should be clear that the vision system with constant magnification lens described herein advantageously allows for acquisition of images of an area of interest at a wide range of focal distances with adequate detail and relatively straightforward adjustment of the lens assembly. This increases acquisition speed as the distance changes between objects, rendering the system highly suited to handheld vision systems and to fixed-mount vision systems (for example in a moving conveyor line) that can encounter objects of different size and shape (with associated differences in focal distance). 
     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 orientation 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. Moreover, 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 here 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. Moreover, while the lens assembly is shown as a unit with at least two spatially fixed lenses, and an external actuator, it is expressly contemplated that additional lenses and/or other optical elements e.g. filters) can be provided in alternate embodiments. Also, the lenses of the lens assembly can be individually actuated by separate actuation devices (or a set of gears linked to a common motor. Additionally, actuation can be achieved by alternative mechanisms, such as a linear motor. Moreover, the lens assembly can be removable and/or include a self-contained actuator that is linked to the camera by an appropriate link. In embodiments employing, for example, a liquid lens element, the positioning of the fixed lens assembly at the front and liquid lens assembly at the rear is illustrative only. Where appropriately sized liquid lens assemblies are available, such can be arranged at the front of the assembly, and a fixed (or other) lens assembly can be located at the rear (i.e. more-adjacent to the image sensor and more-distant from the imaged object/scene). Accordingly, this description is meant to be taken only by way of example, and not to otherwise limit the scope of this invention.