CN113311519B

CN113311519B - System and method for reducing zoom lens drift in a vision system

Info

Publication number: CN113311519B
Application number: CN202110410506.XA
Authority: CN
Inventors: L·努恩宁克; E·O·洛扎诺; J·F·多拉多
Original assignee: Cognex Corp
Current assignee: Cognex Corp
Priority date: 2017-12-19
Filing date: 2018-12-19
Publication date: 2023-01-24
Anticipated expiration: 2038-12-19
Also published as: CN113311519A; CN109946769B; CN109946769A; DE102018132699B4; DE102018132699A1

Abstract

The present invention provides a vision system configured to compensate for optical drift that may occur in certain variable focus lens assemblies, including but not limited to liquid lens arrangements. The system comprises: an image sensor operably connected to a vision system processor; and a variable focus lens assembly controlled (e.g., by a vision processor or other distance measuring device) to vary its focal length. The positive lens component is configured to mitigate an effect of the variable focus lens component relative to an object from a predetermined operating range of the positive lens component. The zoom lens component is positioned near the front focus or the back focus of the positive lens; illustratively, the variable focus lens assembly comprises a liquid lens assembly that is inherently variable focus at a diopter of about 20D. In one embodiment, the lens barrel has a lens mount with a C-interface.

Description

System and method for reducing zoom lens drift in a vision system

Divisional application

The application is divisional application entitled "system and method for reducing drift of zoom lens of visual system" of application No. 201811553924.9, application date 2018, 12 and 19.

Cross Reference to Related Applications

This application is a partial continuation OF the co-pending U.S. patent application entitled "SYSTEM AND METHOD FOR REDUCTION OF IN a VISION SYSTEM VARIABLE LENS" filed on 6.5.2014, application number 14/271,148, the teachings OF which are expressly incorporated herein by reference.

Technical Field

The present application relates to cameras used in machine vision, and more particularly to an autofocus lens assembly.

Background

Vision systems for object measurement, detection and decoding of bits and/or symbols (e.g., bar codes or more simply ID codes) are widely used in industry. The vision system operates based on an image sensor that captures images (typically grayscale, color, and one-, two-, or three-dimensional) of an object or subject and processes the captured images using an onboard or networked vision system processor. The processor typically includes processing hardware and non-transitory computer readable program instructions for executing one or more vision system processes to generate a desired output based on the processed information of the image. This image information is typically contained in an array of image pixels, each having a variety of different colors and/or intensities. In the example of an ID reader, a user or automated process program acquires images of objects that are deemed to contain one or more IDs. The image is processed to identify its ID characteristics and then decoded by a decoding process and/or processor to obtain the intrinsic information (e.g., alphanumeric data) encoded in the ID mode.

Vision system cameras typically include an internal processor and other components that allow it to operate as a standalone unit, providing the required output data (e.g., decoded symbol information) to downstream processes (e.g., inventory tracking computer systems or logistics applications).

An exemplary lens that may be required in certain vision system applications needs to be an auto-focus (auto-focus) component. For example, an autofocus lens may be facilitated by a "zoom lens" assembly (also referred to as a so-called liquid lens assembly, as further defined below). One of these is the liquid lens of variaplastic, france, which uses two liquids of equal density-oil as an insulator and water as a conductor. The lens changes the curvature of the liquid-liquid interface under the influence of the change of the peripheral conducting circuit voltage, thereby changing the focal length (focal length) of the lens. Some of the significant advantages of liquid lenses are ruggedness (no mechanical moving parts), fast response, relatively good optical quality, low power consumption and small size. The liquid lens does not require manual contact and, therefore, installation, setup and maintenance of the vision system is ideally simplified. The response of the liquid lens is very fast compared to other autofocus mechanisms. Liquid lenses are also well suited for reading variations in distance between objects (between facets) or from one object to another, for example, scanning a moving conveyor (e.g., a transport box) containing objects of different sizes/heights. In general, in many vision system applications, it is desirable for the vision system to have fast "on-the-fly" focusing capabilities.

The latest state of the art liquid lenses is available from Optotune AG, switzerland. Such lenses change the focal length (focal distance) by covering the liquid cavity with a moving film. The spool applies pressure to change the shape of the membrane and thus the lens focus. The bobbin may be moved by varying the input current within a preset range. The focal length of the liquid lens will be different for different values of the current. The lens has the advantages of large aperture and fast running speed compared to the design of its competitive products (e.g., variopic, france). However, due to thermal drift and other factors, such lenses typically exhibit variations in alignment and focus settings over time during operation. While focus variations and other factors can be compensated and/or corrected by configuring various systems, such compensation procedures require time to run and process (in the camera's internal processor), thereby slowing the overall response time of the lens to the new focus. Likewise, such compensation procedures (e.g., for thermal drift) may be standardized rather than customized for lens characteristics, which makes them less reliable in dealing with specific drift conditions of the lens over time. For example, it is noted that liquid lenses can drift by up to about 0.15D/deg.C (e.g., certain Variopic liquid lenses currently being produced and/or commercially available specialty products). In some vision applications, particularly for detecting small features at greater distances, it is desirable that the power of the imager lens be stabilized at +/-0.1D diopters.

In addition, it is generally believed that the lens requires a control frequency of at least about 1000Hz to adequately control the focus of the lens and maintain the focus within a desired range. This will burden the vision system processor based on a DSP or similar architecture. That is, if the DSP continues to preempt the lens control task, the tasks of the vision system will be affected. All of these drawbacks make drift compensation a difficult problem in many applications.

Disclosure of Invention

The present invention overcomes the disadvantages of the prior art by providing a vision system configured to compensate for optical drift that may occur in certain lens assemblies capable of changing optical power, wherein the optical power is changed (and thus the focal length/focal length, wherein focal length = 1/power) by controlling the lens shape and/or lens refractive index. Such lens assemblies include, but are not limited to, liquid lens arrangements that employ, for example, two equal density liquids or variable membranes (also commonly referred to herein as "zoom lens" assemblies). The system, comprising: an image sensor operably connected to a vision system processor; and a variable focus lens assembly controlled, for example, by a vision processor or other distance measuring device to vary its focal length. The positive lens assembly is configured to mitigate an effect of the variable focus lens assembly relative to an object from a predetermined operating range of the positive lens assembly. Illustratively, the variable focus lens assembly comprises a liquid lens assembly, and the liquid lens assembly may be inherently variable focus at a diopter of about 20D. Illustratively, the positive lens component and the zoom lens component are co-located within a detachable lens barrel with respect to the camera body and the image sensor. Illustratively, the image sensor is located within the camera body. Likewise, the vision processor may be located wholly or partially within the camera body. In one embodiment, the lens barrel has a lens mount with a C-interface, and the positive lens component includes a doublet including a front convex lens and a rear concave lens. The positive lens component may define an effective focal length range of 40 mm. Illustratively, the usable focal length of a lens (e.g., a doublet) may range from about 10 mm to about 100 mm. Additionally, the variable focus lens assembly (e.g., liquid lens assembly) is typically located near the positive lens assembly, but away from its focal point, which may be the front focal point, or more typically the rear/rear focal point, of the positive lens assembly. The distance between the variable focus lens package and the focal point may be about 0.1 to 0.5 times the focal length F of the positive lens package. In this way, the positive lens component and the variable focus lens component become part of a lens assembly that focuses light onto the image sensor. Thus, the power of the positive lens component "primarily defines" the total power of the lens assembly, in other words, most of the magnification/power is provided by the positive lens component, such that the effect of the drift of the variable focus lens component is minimized.

In one illustrative embodiment, a vision system is provided that compensates for drift. The vision system includes: an image sensor operably connected to the vision system processor; a variable focus lens package, the shape or refractive index of which can be varied; and a fixed focus lens assembly configured to mitigate the effect of the variable focus lens assembly on a predetermined operating range of the object. Illustratively, the variable focus lens package comprises a liquid lens package. The liquid lens assembly may be located between the image sensor and the fixed focus lens assembly and may zoom over approximately 20D of optical power. Further, the fixed focus lens assembly may define a positive power. Illustratively, the fixed focus lens assembly and the variable focus lens assembly are disposed within a detachable lens barrel with respect to the camera apparatus body and an image sensor, which may be located within the camera apparatus body. The camera apparatus body may be electrically connected to the zoom lens assembly, to which at least one of power and control is provided through at least one of a contact pad and a cable assembly. The fixed focus lens assembly may comprise one of: (a) A front lens having a front concave surface and a rear convex surface and a middle biconvex lens spaced apart from the front lens; (b) A front biconvex lens and a stacked rear lens assembly, wherein the stacked rear lens assembly has a front positive lens, a middle biconcave lens and a rear positive lens; (c) A front plano-concave lens and a negative lens, a stack-type intermediate lens having a biconvex lens and a plano-convex lens, and a rear biconvex lens and a positive lens; (d) A front plano-convex lens and a positive lens, and a rear positive lens and a negative lens; (e) A stacked front lens assembly having a double convex lens and a double concave lens, and a rear plano-convex lens and a negative lens. Additionally, at least one lens of the fixed focus lens assembly may comprise a polymeric material. For example, a fixed focus lens assembly may define a usable range of effective focal lengths between about 0.3m and 8 m. For another example, the variable focus lens assembly is positioned near the focal point of the fixed focus lens assembly. The focal point is one of a front focal point or a back focal point of the fixed focus lens assembly. In an embodiment, the fixed focus lens assembly may comprise a front lens assembly and a rear lens assembly with the variable focus lens assembly therebetween, wherein the rear lens assembly may define a positive optical power. Likewise, in these embodiments, the front lens assembly may have a pair of lenses, each lens having a front convex surface and a rear concave surface, and one lens having an opposing concave surface, while the rear lens assembly has a lens with an opposing convex surface. Illustratively, the fixed focus lens assembly and the variable focus lens assembly are part of a lens assembly that focuses light onto the image sensor, wherein the optical power of the fixed focus lens assembly primarily defines the total optical power of the lens assembly.

In another illustrative embodiment, a zoom lens system for a vision system having an image sensor that communicates image data to a processor is provided. The system includes a variable focus lens assembly (e.g., a liquid lens assembly). The system includes a fixed focus lens assembly having a focal point. The variable focus lens assembly is located near the focal point. The fixed focus lens assembly and the variable focus lens assembly may be part of a lens assembly that focuses light onto the image sensor. The power of the positive lens component may primarily define the total power of the lens assembly. Illustratively, the liquid lens assembly may be zoomed at about 20D of optical power. In an embodiment, the fixed focus lens assembly and the variable focus lens assembly are disposed within a detachable lens barrel with respect to the camera apparatus body and the image sensor. The image sensor is located within the camera device body. The camera apparatus body is electrically connected to the zoom lens assembly to be provided with at least one of power and control through at least one of a contact pad and a cable assembly. Illustratively, the lens system may comprise one of: (a) A front lens having a front concave surface and a rear convex surface and a middle biconvex lens spaced apart from the front lens; (b) A front biconvex lens and a stacked rear lens assembly, wherein the stacked rear lens assembly has a front positive lens, a middle biconcave lens and a rear positive lens; (c) A front plano-concave lens and a negative lens, a stack-type intermediate lens having a biconvex lens and a plano-convex lens, and a rear biconvex lens and a positive lens; (d) A front plano-convex lens and a positive lens, and a rear positive lens and a negative lens; (e) A stacked front lens assembly having a double convex lens and a double concave lens, and a rear plano-convex lens and a negative lens.

Drawings

The invention is described below with reference to the accompanying drawings, in which:

FIG. 1 is a configuration diagram of an illustrative vision system with a vision system camera having an associated vision processor and lens assembly that compensates for inherent drift over time, showing acquiring images of an exemplary object in a scene, in accordance with an illustrative embodiment;

FIG. 2 is a ray trace diagram of an exemplary lens system including a variable focus lens assembly for imaging an object;

FIG. 3 is a ray trace diagram of an exemplary lens system including a variable focus lens package and a positive lens package disposed along an optical axis at a predetermined distance from the variable focus lens package to provide a drift-tolerance (drift-tolerant) lens system;

FIG. 4 is a side cross-sectional view of a lens group including a variable focus lens assembly and a positive lens (illustrating drift tolerance) showing the relative dimensions of a lens barrel and its associated components, according to an exemplary embodiment;

FIG. 4A is a side cross-sectional view of the lens assembly shown in FIG. 4, illustrating the relative positions of the components along the optical axis;

FIG. 5 is a ray trace diagram of the illustrative lens group shown in FIG. 4 illustrating imaging of an object at a first distance;

FIG. 6 is a ray trace diagram of the illustrative lens group shown in FIG. 4 illustrating imaging of an object at a second distance, wherein the second distance is longer than the first distance;

FIG. 7 is a ray trace diagram of the illustrative lens group shown in FIG. 4 illustrating imaging of an object at a first distance;

FIG. 8 is a diagram of the relationship between a positive lens component, a variable focus lens component and a positive lens focus according to an embodiment of the present description;

FIG. 9 is a lens arrangement of a drift tolerance lens system according to an embodiment, wherein a variable focus lens assembly is located between an optical device and an image sensor;

FIG. 10 is a lens arrangement of a drift tolerance lens system according to an embodiment, wherein a variable focus lens assembly is positioned between two sets of optics, in front of an image sensor;

FIG. 11 is a lens arrangement of a 12mm drift tolerance lens system according to another embodiment, wherein a variable focus lens assembly is located between an optical device and an image sensor;

FIG. 12 is a perspective view of a lens assembly incorporating the lens arrangement shown in FIG. 11;

FIG. 13 is a side cross-sectional view of the lens taken along line 13-13 of FIG. 12;

FIG. 14 is a lens arrangement diagram of a 16mm drift tolerance lens system in accordance with another embodiment, wherein the variable focus lens assembly is located between the optics and the image sensor;

fig. 14A is a circle diagram of a rectangular image sensor that may be used in a vision system according to an embodiment;

FIG. 15 is a lens arrangement of a 25mm drift tolerance lens system according to another embodiment, wherein a variable focus lens assembly is located between an optical device and an image sensor;

FIG. 16 is a lens arrangement diagram of a 35mm drift tolerance lens system in accordance with another embodiment, wherein a zoom lens assembly is located between an optical device and an image sensor;

FIG. 17 is a perspective view of a lens assembly incorporating the lens arrangement shown in FIG. 16; and

fig. 18 is a side cross-sectional view of the lens taken along line 18-18 of fig. 17.

Detailed Description

1. Overview of the System

Fig. 1 shows a detailed view of a vision system 100, the vision system 100 including a vision system camera device 110 and associated lens group/assembly 120. The results of lens assembly 120 are further described below. In one embodiment, lens group 120 is fixed to the camera or may be removably mounted with a custom or conventional mount (e.g., the well-known lens mount of the Cine or "C-interface"). The camera includes a body/housing that can house a number of operational components, including an image sensor or imager 130 (shown in phantom). In this embodiment, imager 130 is operably connected to an on-board vision processor 140, and on-board vision processor 140 may run a variety of hardware and/or software processes (commonly referred to as vision processes 142). The vision process 142 may include a plurality of software applications adapted to perform general or specialized vision system tasks, such as ID (encoding) look-up and decode tasks, edge detection, blob analysis, surface inspection, robotic operations, and/or other operations. See, for example, exemplary ID 144. The process 142 may also include various image acquisition and image processing applications that save image data in a form more suitable for vision system tasks, such as histograms and image thresholds. These tasks and procedures are known to those skilled in the art and are available through vision systems commercial suppliers (e.g., connaissance corporation, nattek, mass.). As shown, an illustrative vision system processor 140 is included in the camera body. The vision system data may be transmitted in "raw", pre-processed (e.g., the located undecoded ID image data) or fully processed data (e.g., the decoded ID data) over a wired and/or wireless link 144 to a suitable data processing system or processor, such as a personal computer or server system. In alternative embodiments, alternative systems may be employed, such as mobile computing devices and cloud devices, among others. The data processing system stores and processes the image-based data according to the needs of the user (e.g., quality or inventory control). In alternative embodiments, some or all of the vision system processors/processes may be instantiated and/or executed in a remote processor (e.g., computing device/processor 150) interconnected to the camera 110 by a suitable wired and/or wireless link (e.g., link 144) in a manner known to those skilled in the art.

Note that the terms "process" and/or "processor" as used in this specification should be taken broadly to include various electronic hardware and/or software based functions and components. Further, the processes or processors illustrated may be combined with other processes and/or processors or divided into various sub-processes or sub-processors. These sub-processes and/or sub-processors may be variously combined according to embodiments in the present specification. Likewise, it is expressly contemplated that any of the functions, processes, and/or processors in this specification can be performed by electronic hardware, software containing non-transitory computer-readable medium program instructions, or a combination of hardware and software. In a system configuration, these processes/process functions may be named by the respective "module" or "element" in which they occur/exist. For example, an "ID-reading module" which performs the functions associated with reading and/or decoding an ID identification code.

As shown, the lens assembly 120 is aligned along an optical axis OA (with the plane of the sensor 130), generally disposed perpendicular to that axis. The lens assembly 120 and the sensor 130 image the object O. For example, the object O may be any two-dimensional (2D) or three-dimensional (3D) surface or shape that partially or fully covers the field of view (FOV). In the example shown, the range/distance (do) of object O from camera 110 (e.g., the focal plane of distance sensor 130) may vary, but (according to an illustrative embodiment) defines a predetermined working range in which object O may be imaged.

Illustratively, this embodiment compensates for optical drift that may occur over time in a zoom lens (e.g., a liquid lens) that is part of the lens assembly 120 by defining an operating range of the vision system at which the vision system can reduce the effect of the optical power of the zoom lens on the optical power of the lens assembly (including all fixed focus lenses in the lens assembly). In this way, drift is made a small part of the overall focusing performance of the lens assembly. An advantage of the illustrative apparatus is that the adjustable focus range can be reduced. Thus, the system is suitable for embodiments where the distance (do) of the object surface from the focal plane is relatively constant, or where the distance (do) varies over a relatively small range of distances. Illustratively, the system can be used for vision system applications that read at greater distances, where the required optical distance is only a small fraction (about 2D) of a commercially available liquid lens (20D). As noted above, the various lens assemblies in the embodiments of the present description may include various lenses of varying optical power. More specifically, in an embodiment, the optical power is changed (thereby changing the focal length/focal length, wherein focal length = 1/optical power) by controlling the lens shape and/or the lens refractive index. Such variable focus lens assemblies include, but are not limited to, liquid lenses, and various types of liquid lenses can be employed, including equal density liquid type lenses (varioptical) and thin film type liquid lenses (Optotune), among others. Likewise, zoom lenses operated by other mechanisms, such as electromechanical driven lenses, may also be employed.

2. Lens arrangement with reduced amount of drift

To further illustrate the concepts of the embodiments, a ray trace diagram for the basic optics of an exemplary vision system 200 having an exemplary object O1, an image sensor 230, and a common zoom lens (e.g., liquid lens (LL 1)) is shown in FIG. 2. The object O1 is located at a distance d1 from the zoom lens LL 1. As shown, the system has no additional lenses, and light rays 240 reflected from object O1 pass through zoom lens LL1 and are focused directly onto image sensor 130. Thus, any slight change (e.g., drift) in the focus of the zoom lens LL1 may cause a significant defocus condition to occur, which may affect the ability of the vision system to provide correct results.

To address the problem of sensitivity of lenses (e.g., liquid lenses) to drift and other focus variations, reference is now made to fig. 3, which illustrates a generic optical arrangement for a vision system 300, according to an embodiment. A fixed focus (non-zoom) positive lens PL is disposed along the optical path between the system and the object O2 to be imaged at a predetermined distance d in front of the zoom (e.g. liquid) lens assembly LL 2.

Thus, the power A of the system 300 (where A1 is the power of the positive lens component PL, A2 is the power of the variable focus lens component LL2, and d is the distance between the positive lens PL and the variable focus lens LL 2) is:

A＝A1+A2-d×A1×A2

if the distance between the zoom lens LL2 and the positive lens PL is relatively large (e.g. d = k/A1, where k =0.5 … … 0.9.9, and k represents the product between the positive lens A1 and the distance d, e.g. k = d × A1), the total optical power and the relative distance d of the lens system defined above having powers A1 and A2 can be expressed as:

A＝A1+(1-k)×A2

and the amount of drift is expressed as the difference (dA/dT) in lens power (dA) per unit temperature (dT) of the system:

dA/dT＝dA1/dT+(1-k)×dA2/dT

this indicates that the drift amount dA/dT of the above system is the sum of the positive lens drift amount dA1/dT and the (1-k) -fold zoom lens drift amount dA 2/dT.

In one embodiment, the fixed focus positive lens PL may be selected to be a glass lens with low intrinsic drift (e.g., dA1/dT ≈ 0), so the total amount of drift dA/dT of the system in fig. 3 is effectively reduced by 1-k (= 0.1x … … 0.5 x) by the positive lens PL compared to the original setting shown in fig. 2, and the larger the power of the positive lens (e.g., the larger k), the more the drift of the zoom lens is reduced.

Referring now to fig. 4, a cross section of the integrated lens group/assembly 120 for the illustrative vision system camera 110 of fig. 1 is shown in detail. The lens assembly 120 may include various electrical wiring and/or leads (the cable 410 and connector 412 are shown schematically in phantom) that extend from the variable focus lens assembly (e.g., liquid lens) 420 to locations on the body of the camera 110 in communication with a corresponding control processor/assembly of the associated vision processor 140. Note that an exemplary liquid lens assembly 420 (which may be a thin film type, an equal density liquid type, or an equivalent type) is contained within barrel 430, and lead 410 is configured to extend from a location on barrel 430. The connection is such that the control signal can drive the liquid lens assembly (e.g., current and/or voltage modulation) such that the focus of the liquid lens assembly 420 is varied and set in response to instructions from the processor. Various techniques known to those skilled in the art may be used to determine and/or set the appropriate focus, for example, using image edge sharpness after stepping through each focus setting and/or using an external ranging device. Although a separate cable link is used in the illustrated embodiment, with an associated connector on the camera body, the connection arrangement may be placed within lens barrel 430, for example, consisting of aligned contact pads and/or contact rings (on the lens and camera body) when lens assembly 120 is secured to the camera body.

In this embodiment, lens assembly barrel 430 is configured with a conventionally sized and shaped C-interface lens having a suitable threaded mount 440. The external threads shown of the barrel base (flange) 440 mate with corresponding internal threads (not shown) on the camera body. The threads are of conventional size (e.g., 1 inch x 32). Note that the camera body may include various accessory and functional components, such as a ring light source surrounding the lens and/or wiring for an external lighting component. These accessories and/or components may be applied to a camera to accomplish the tasks of a vision system. Lens barrel 430 may be constructed from a variety of materials, such as cast aluminum alloy or machined aluminum alloy. The threaded mount allows the lens barrel and associated lens assembly contained therein to be removably attached to the camera body and replaceable with other types of lenses as selected by the manufacturer or user. Although a C-interface mount is used in this embodiment, any acceptable lens mount that can accommodate a liquid lens or other suitable variable focus lens may be used in alternative embodiments. For example, an F-shaped lens mount may be used.

The dimensions of the lens barrel 430 are shown by way of non-limiting example in fig. 4. As shown, in one embodiment, the barrel outer diameter ODL may be approximately 28-29mm. This addresses the general size/parameter limitations in a C-interface lens mount. Likewise, the length OLL of the lens barrel 430 from the front end 432 to the threaded base 440 is about 32-34 mm, for example. The distance DS between the lens mount 440 and the focal plane of the image sensor 130 is about 17.5mm. Note that these dimensions indicate various possible relationships known to those skilled in the art.

With further reference to FIG. 4A, the positioning of the internal optical components of the lens is shown in detail. A positive lens component 450 is located near the front end 432 of the barrel 430, the positive lens component 450 having a larger diameter relative to the zoom lens (420). The positive lens assembly (also referred to as "positive lens") 450 is disposed in a recess 454 formed at the front end of the lens barrel. The positive lens 450 is fixed at its front side by a threaded ring 456. Note that this arrangement is highly variable in alternative embodiments, and various mounting and/or attachment mechanisms may be employed in alternative embodiments. The positive lens 450 is an achromatic doublet, and defines the effective focal length (f) as 40mm and the back focal length as 33.26mm. The aperture is 24mm. The diameter of the lens assembly is 25mm. Illustratively, the lens assembly is comprised of a front convex lens 458 and a rear concave lens 459. Convex lens 458 defines a front radius RL1 of 27.97mm and a rear radius RL2 of-18.85 mm (where positive and negative radii indicate directions relative to the orientation of the imaged object, positive radius toward the object and negative radius toward the image sensor). The concave lens 459 defines a front radius (also RL 2) of 18.85mm (which fits the mating surface of the convex lens 458) and a rear radius RL3 of 152.94mm. The convex lens 458 has a center thickness TC1 (along the optical axis OA) of 9.5mm and the concave lens has a center thickness TC2 of 2.5mm. These dimensions are highly variable in alternative embodiments. The positive lens (e.g., doublet) assembly 450 of the above-described embodiment and the associated dimensions is commercially available from Edmund Optics inc, of barlington, new jersey (order number: 32321). In this embodiment, the distance ODLF from the front of the lens to the sensor plane is about 49mm, according to one embodiment. It will be apparent that in alternative embodiments, the size of the positive lens and/or the arrangement of the components is highly variable.

A variable focus (e.g., liquid) lens assembly (available from a variety of manufacturers) 420 is disposed near the rear end of lens barrel 430. In this embodiment, and as a non-limiting example, the variable focus lens assembly 420 may comprise a liquid lens of the type Arctic416 by Variopic corporation, france. The exemplary variable focus lens assembly has a focal range of about 20D (i.e., 5cm to infinity), a diameter of 7.75mm, and a thickness (along the optical axis) of 1.6mm. The exemplary liquid lens assembly 420 shown is comprised of a lens assembly 470 mounted on a controller circuit board 472 having a central aperture 474 aligned along the optical axis such that focused light is transmitted through the optical axis to the sensor 130.

Lens assembly 130 may be supported within barrel 430 using an integral or unitary spacer, shoulder arrangement, and/or support structure 460. The support structure 460 ensures that the variable focus lens package 420 remains fixed and correctly aligned with respect to the optical axis OA. In this embodiment, the distance DLR from the rear of the positive lens to the front of the zoom lens group 470 is 18.0mm. Note that in one embodiment, image sensor 130 may define a 1/2 inch conventional CMOS sensor (swing (SW): 6.9mm (horizontal) × 5.5mm (vertical) shown in FIG. 5).

Referring now to fig. 5-7, a vision system and lens assembly that operates at multiple focal lengths within the system operating range is shown. Thus, in each of the ray trace diagrams of fig. 5, 6, and 7, the object O is located at 3 exemplary distances DO1, DO2, and DO3, respectively. For example, DO1 is approximately 219mm, DO2 is approximately 430mm, and DO3 is approximately 635mm. The optical power of the variable focus lens package changes within this range from +10.73D for F =37.4mm (fig. 5) to +0.32D for F =39.8mm (fig. 6) and-3.81D for F =42.3 mm. The focus range 219 to 625 relates to a diopter change of 6.9D. Mounting the shown zoom lens assembly in a conventional arrangement typically requires a 3.3D refractive change compared to an arrangement where the system attaches the zoom lens closer to the front lens. Thus, the illustrative system effectively reduces the amount of potential drift by more than a factor of 2 relative to conventional arrangements.

In general, a variable focus lens assembly (e.g., a liquid lens assembly) is located near, but far from, the focal point of the positive lens assembly, which may be the front focal point, or more generally the rear/rear focal point, of the positive lens assembly. It is well known that placement near the focal point allows the zoom lens to contribute to the total optical power of the lens system. The distance between the variable focus lens package and the focal point may be about 0.1 to 0.5 times the focal length F of the positive lens package. For example, reference is made to the diagram in fig. 8, where the positive lens component PL is arranged along the optical axis OA, and the zoom lens VL is located near the positive lens focus FP. The figure shows the focal length F between the positive lens PL and the focal point FP. The distance (1-k) × F is expressed in terms of the distance between the zoom lens VL and the focus lens and the focal point FP, where k =0.9 to 0.5 (i.e., 0.9 × F to 0.5 × F). Therefore, the distance between the positive lens PL and the zoom lens VL is k × F (i.e., 0.1 × F to 0.5 × F). In this way, the positive lens component PL and the zoom lens VL component become part of the lens assembly LA which focuses the light onto the image sensor, and the power of the positive lens component "mainly defines" the total power of the lens assembly, in other words, most of the amplitude/power is provided by the positive lens component, thereby minimizing the effects of drift in the zoom lens component.

Referring now to fig. 9-10, there are shown 2 lens arrangements for reducing the amount of drift, according to an embodiment. Exemplary parameters for each lens element are also provided separately in the table below. Fig. 9 shows a lens arrangement 900 associated with an image sensor 910 of, for example, conventional design. In this embodiment, the zoom lens comprises a liquid lens assembly 920. The illustrated ray 930 is reflected into the lens device 900 from an object (not shown) that may be positioned 200mm from the first lens 940 (for example). In a non-limiting example, the lens 940 includes a front concave surface 942 and a rear convex surface 944. The lens may comprise a polymer, such as polycarbonate (or other suitable optical material). The intermediate lens assembly includes a front compound lens 950 having a convex lens 952, the convex lens 952 having a front surface 954 and a rear surface 956. This fits with a concave lens 958 and a convex back surface 960 having convex surfaces of similar radii. Note that the lens element (950) may also be made of polycarbonate (or other optical material). A disc-shaped optical element 970 (e.g., an infrared filter) with an infinite radius on each side (e.g., parallel planes) is positioned behind the composite lens assembly 950. Light rays 930 converge from the dish element 970 at the variable focus (liquid) lens assembly 920. The exemplary assembly may be based on an Arctic 416-type lens from Variopic, france or other suitable, e.g., liquid, lens. The assembly includes a front cover disk 980, a lens element 982 interconnected to lens control circuitry 990, an aperture stop (corresponding to a radius of 341.763 mm) 984, and a rear cover disk 986. The lens control circuit is operatively connected to the vision system. This element is adjusted to maintain focus on the image sensor 910 and to control drift tolerance based on the various conditions described above. The spacing between liquid lens assembly 920 and the imager may be about 13-14 mm and the spacing between disk element 970 and liquid lens assembly 920 may be about 3-4 mm. Note that the size and shape of the exemplary lenses and their spacing can be modified by one skilled in the art. Likewise, the variable focus lens package may comprise a variety of different types of lenses operating on different physical principles. For example, a thin film type liquid lens of Optotune, switzerland, and a mechanical type lens may be used instead.

Referring now to FIG. 10, there is shown another embodiment of a lens apparatus 1000 that reduces the amount of drift in a given operating range. The apparatus comprises an image sensor 1010 of conventional design and a variable focus (liquid) lens assembly 1020. Light ray 1030 reflects from an object (not shown) to the forward convex lens 1040 in the range of, for example, 80-100 mm. In a non-limiting example, front lens 1040 includes a front convex surface 1042 and a rear concave surface 1044. The posterior group lens 1046 defines a front convex surface 1048 and a rear concave surface 1050. The back group lens 1060 has a front surface 1062 and a back surface 1064, both

surfaces

1062 and 1064 being concave. Light rays 1030 exit the lens 1060 and enter a variable focus (e.g., liquid) lens assembly 1020. In this embodiment, variable focus lens assembly 1020 is positioned in the middle of the optics, and

additional lenses

1080 and 1088 are positioned between the variable focus lens assembly and image sensor 1010. In this example, liquid lens assembly 1020 is modeled and configured similar to assembly 920 described above (having an aperture stop with a radius of 10.101 mm) and is controlled by lens controller 1090, whose operation is also similar to controller 990 described above. As with the apparatus 900 described above, alternative embodiments of variable focus lens packages may be employed, where appropriate. Light rays 1030 from the variable focus (e.g., liquid) lens assembly 1020 strike a convex lens 1080 located approximately 1.2mm from the liquid lens assembly. The convex lens 1080 includes a front convex surface 1082 and a rear convex surface 1084. A dish-shaped, for example, infrared filter 1088 may be located behind the convex lens 1080. In this embodiment, the dish-shaped infrared filters are disposed at a distance of about 10 to 12mm in front of the image sensor. In this embodiment, the lenses are composed of optical glass, as a non-limiting example, but one or more of the lenses (or other optical elements) may be composed of other acceptable materials, such as polycarbonate or suitable equivalent materials.

The lens devices 900 and 1000 described above may be adapted to be enclosed within a lens by, for example, a conventional camera mount, such as the lens mount of the C-interface described above. Suitable electrical connectors may be provided between the lens body and the camera mount to enable control of the variable focus lens assembly. The electronics of the control circuit may be disposed wholly or partially with respect to the lens body or, where appropriate, within the camera body.

By way of non-limiting example, the lens of the various embodiments of the present description may define each of the specified parameters in the following table. The parameters of lens assembly 900 (fig. 9) are given in the following first table, wherein the respective front and rear surfaces (if applicable) of the structures or elements in the overall assembly are ordered in the order of 0 to 13 (left to right), respectively:

the following table is for lens assembly 1000 (fig. 10), where the respective front and back surfaces (if applicable) of each structure or element in the overall assembly are ordered from 0 to 14 (left to right), respectively:

it is further contemplated that the drift compensation lens arrangements of the embodiments herein may be used in conjunction with other methods of reducing the amount of drift, such as temperature stabilization of a zoom lens or an optical feedback system. Such a device is incorporated herein by reference as useful background information by way of non-limiting example, as shown and described in commonly assigned U.S. patent No. 8576390; commonly assigned: nunnik) entitled "SYSTEM AND METHOD FOR DETERMINING AND CONTROLLING FOR DISTANCE IN A VISION SYSTEM CAMERA". This application also refers to U.S. patent application entitled "CONSTANT MAGNIFICATION LENS FOR VISION SYSTEM CAMERA" (patent No. 14/139867; inventor: nunnink) and U.S. patent application entitled "ASSEMBLY WITH INTEGRATED FEEDBACK LOOP FOR FOCUS ADJUSTMENT" (patent No. 13/800055; inventor: nunnink et al). Illustratively, the present application provides a removable lens assembly for a vision system camera comprising an auto-focusing integrated liquid lens group, wherein the lens group compensates for focus variation by using a feedback control circuit integrated in the lens assembly body. The feedback control circuit receives motion information from a position sensor (e.g., a hall sensor) relating to a lens actuator (e.g., a bobbin that deflects the membrane under current control) and uses this information internally to correct for motion variations, i.e., operational variations that deviate from a set lens position at a set lens target focal length. The position sensor may be a single machine or a combination of discrete sensors whose positions vary with respect to the actuator/bobbin in order to measure movement values at multiple positions around the lens group. For example, the feedback circuit may be interconnected with one or more temperature sensors that adjust the set position of the lens for a particular temperature value. In addition, the feedback circuit may be in communication with an accelerometer that senses the direction of the gravitational force acting to correct for potential sag (or other orientation-induced deformation) in the lens film based on the spatial orientation of the lens.

3. Lens assembly with reduced amount of drift

Fig. 11-18 illustrate various embodiments of a drift amount reducing lens that may be used in various camera devices and related applications, including hand-held and stationary mounted units, to extend the read range of object features, such as ID identification codes. Imaging at focal lengths up to about 8m is possible using the lens arrangement of the present description. The illustrative apparatus typically positions the zoom (e.g., liquid) lens behind the remaining fixed lens optical assembly, such that the zoom lens is typically located behind the lens assembly and between the fixed optical assembly and the camera image sensor. Referring to fig. 11, a lens apparatus 1100 is shown. The device is suitable for 12mm (f' = 12) lenses. As shown, a relative scale 1110 for the entire lens arrangement (in mm) is provided. The lens holding optics (shown in dashed box 1120) consists of a front plate element 1130 followed by a lenticular lens 1132. Behind the lenticular lens 1132 are arranged a set (3) of smaller diameter lenses 1134 (positive lens), 1136 (double concave lens) and 1138 (positive lens — negative). In this embodiment, fixed optical assembly 1120 is disposed in a separate lens housing, while variable focus lens assembly 1140 is mounted within the frame of the vision system housing (e.g., a hand-held ID reader, such as the commonly assigned U.S. patent application entitled "IMAGE Module organizing movement AND DECODER FOR MOBILE DEVICES", filed 11, 21/2014, with patent number 14/550709). In a non-limiting example, the variable focus lens assembly 1140 may comprise the liquid lens mechanism described above, which is commercially available from Optotune, switzerland. Alternatively, the variable focus lens package may comprise any acceptable manual or motorized focus lens arrangement, including the lenses described above and available from Variopic, france. The variable focus lens assembly 1140 may be interconnected (by cabling, printed circuit wiring, etc.) to a vision system processor or other controller that can adjust the focal length of the lens. The controller may be integrated with the feedback system described above. Optionally, variable focus lens assembly 1140 is provided with one or more optical filters and/or dust covers (if applicable). An aperture stop 1142 is also provided in this embodiment. The variable focus lens assembly 1140 focuses the light (ray 1150) onto an image sensor 1152 for transmission to the vision system processor. The overall length 1160 of the device 1100 between the front surface of the light panel 1130 and the image sensor 1152 is about 15.2mm. The distance 1162 between the rear surface of the rear lens 1138 and the image plane (image sensor 1152) is about 10.26mm. For example, the approximate parameters of the apparatus 1100 define an F/# of 7; an image radius of 3mm (i.e., 1/3 inch for a sensor up to 1.2-5.0 megapixels); when the image height is 3mm, the RMS spot radius is 1.7 μm; and the measurement distortion is less than 3% -4%.

The following table applies to lens assembly 1100 (fig. 11), where the respective front and back surfaces (if applicable) of each structure or element in the overall assembly are ordered in the order of 0 to 16 (left to right), respectively:

note that the various lens parameter tables provided further above and below are used only as examples of possible broad implementations. It will be apparent to those skilled in the art that any or all of the lenses and/or optical components described herein may be modified by using different components, sizes, focal lengths, thicknesses, etc., to meet the mechanical and optical requirements of the imaging application.

Fig. 12 and 13 illustrate a lens assembly 1200 corresponding to a fixed optical assembly 1120. The lens elements are contained within a barrel housing 1210 constructed of aluminum or other acceptable material. The numbering of the lens elements is similar to that of the corresponding elements of the assembly of fig. 11. The base 1230 of the lens 1200 may be of any acceptable form, for example, it may be specified to use a C-interface threaded base (i.e., 1 inch x 32 threads/inch) over the entire length of the barrel. Alternatively, it may be specified that the M8 × 0.5 thread is used on the entire length of the lens barrel, or in either case, on the appropriate portion of the lens barrel. As shown in the cross-sectional view of fig. 13, the aperture stop 1310 may be located between the biconcave lens 1136 and the rearmost positive lens 1138. Lenses 1130 to 1138 included in the lens barrel 1210 are held by a front snap ring 1240, and the front snap ring 1240 is screwed to the front end of the lens barrel 1210 with an outer diameter 1330 of the front snap ring 1240 of 10 mm. A threaded spacer ring 1250 is also screwed onto the barrel and positioned along the barrel to set the focal length of the lens assembly relative to the image plane. In one embodiment, when the ring 1250 is properly placed on the barrel 1210, it can be permanently/semi-permanently fixed to the barrel using a threaded locking glue, adhesive, or other securing mechanism (e.g., set screws and pins, etc.). When the lens is screwed into the lens interface (mount) on the device, the ring 1250 rests against the mount and provides the desired spacing. In one embodiment, the total lens length 1340 is about 6.9mm, and the set distance 1350 between the rear surface of the snap ring 1250 and the image surface 1360 is about 12.15mm.

Fig. 14-18 illustrate various drift reduction lens assemblies including a zoom lens in its overall structure, and which may be used, for example, in fixed mount vision systems, such as ID readers used in logistics and object tracking applications.

Referring to fig. 14, a 16mm lens apparatus 1400 is shown. The apparatus may be formed of a housing 1410, the housing 1410 including a variable focus (e.g., liquid) lens assembly 1430 throughout the assembly. The lens assembly is connected via a cable 1432 or other form of connection to a connector/contact on the camera or other vision system housing, in communication with the processor, to control the focal length of the lens assembly 1430. Note that various circuits may be built/provided in the lens housing to perform some or all of the zoom lens control functions.

The lens arrangement 1400 includes a front negative lens 1440, followed by a smaller diameter negative lens 1442, a double convex lens 1444, and a smaller diameter doublet 1445 consisting of a double convex lens 1446 and a plano-concave lens 1448. A second, smaller diameter, doublet 1450, consisting of a positive lens 1452 and a double-convex lens 1454, is arranged behind the first doublet 1445, and a positive lens 1456 is arranged between the doublet 1450 and the variable focus (liquid) lens assembly 1430. An aperture stop 1458 may also be provided on the rear surface of the last positive lens 1456. A relative scale 1470 (in millimeters) is shown and a back focal length 1480 between the back of the zoom lens 1430 and an image plane on the surface of the image sensor 1490 is set to about 8.5mm by using, for example, suitable adjustment rings, mounts, interfaces, etc. known to those skilled in the art. This produces an image circle of about 8mm in diameter, as shown briefly in fig. 14A. The sensor is located within the maximum image circle 1496 (about 8.83 mm) shown, an exemplary sensor 1490 is an IMX265 model image CMOS sensor (manufactured by sony corporation, japan). That is, image circle 1496 surrounds corners of rectangular perimeter 1495, which represents an available image pixel array of sensor 1490. Other image sensors (e.g., image sensors having a pixel array defined by rectangle 1494) are characterized by different image circle sizes (1493) (in this example, the image circle size is smaller, i.e., 7.66 mm). Such smaller size sensors are available from Teledyne e2v corporation (uk).

Other exemplary optical parameters of lens assembly 1400 may include a focal length of about 16.2-16.6 mm, an aperture size F8, a total footprint of about 27.9mm, a focus range of 1.0-4.0 m, and an operating range of the variable focus lens assembly of about 0.0D-2.5D. Within this range, the amount of drift can theoretically be 2.5 times less than that of the conventional design. At extreme field of view (FOV) positions, the RMS spot radius is below 2.2 μm.

The following table is for lens assembly 1400 (fig. 14), where the respective front and back surfaces (if applicable) of each structure or element in the overall assembly are ordered in the order of 0 to 20 (left to right), respectively:

referring to fig. 15, a 25mm lens apparatus 1500 is shown. The apparatus may be comprised of a housing 1510, the housing 1510 including a variable focus (e.g., liquid) lens assembly 1530 within the overall assembly. As described above, the lens assembly is connected to the connectors/contacts via cable 1532 or other form. The lens arrangement 1500 includes a front lens 1540 having a slightly concave front face, and a rear, smaller diameter, plano-convex lens 1542 and doublet lens 1544 consisting of a double convex lens 1546 and a double concave lens 1548. A second, smaller diameter, doublet 1550, consisting of a first positive lens 1552 and a second lens 1554, is disposed behind the first doublet 1544, and a negative lens 1556 is disposed between the doublet 1550 and the zoom (liquid) lens assembly 1530. An aperture stop 1558 may also be disposed on the back surface of the last positive lens 1556. A relative scale 1570 (in millimeters) is shown and the back focal length 1580 between the back of the zoom lens 1530 and the image plane on the surface of the image sensor 1590 (e.g., the image circle is about 8-mm) is set to about 8.5mm by using, for example, suitable adjustment rings, mounts, interfaces, etc., as known to those skilled in the art. Other parameters of the lens assembly may include a focal length of about 24.2-25.2 mm, an aperture size F8, a total track of about 27.6mm, a focus range of 1.0-4.0 m, and a zoom lens operating range of about 0.0D-4.0D. Within this range, the amount of drift can theoretically be 4 times less than conventional designs. At extreme field of view (FOV) positions, the RMS spot radius is below 1.9 μm.

The following table is for lens assembly 1500 (fig. 11), where the respective front and back surfaces (if applicable) of each structure or element in the overall assembly are ordered in the order of 0 to 16 (left to right), respectively:

referring to FIG. 16, a 35mm lens apparatus 1600 is shown. The lens apparatus in this embodiment is suitable for use in large camera apparatus, such as those used in high volume logistics operations involving objects of various sizes (e.g., logistics service, high volume shipping, etc.). The lens apparatus 1600 may be formed from a housing 1610 (as shown by the dashed box in fig. 16 and described in further detail below), the housing 1610 including a variable focus (e.g., liquid) lens assembly 1630 within the overall assembly. As described above, the lens assembly is connected to the connectors/contacts via cables 1632 or other forms.

The lens device 1600 includes a large-diameter front plano-convex lens 1640 and a rear smaller-diameter lenticular lens 1642, stacked with a doublet 1644, the doublet 1644 being composed of a positive lens 1646 and a biconcave lens 1648. A second, smaller diameter doublet 1650, consisting of a first positive lens 1652 and a second plano-convex lens 1654, is disposed behind the first doublet 1644, and a negative lens 1656 is disposed between the doublet 1650 and the variable focus (liquid) lens assembly 1630. An aperture stop 1658 may also be disposed on the back surface of the last negative lens 1656. A relative scale 1670 (in millimeters) is shown and the rear focal length 1680 between the rear of the zoom lens 1630 and the image plane (e.g., with an image circle of about 8-mm) on the surface of the image sensor 1690 is set to about 8.5mm. Other parameters of the lens assembly include a focal length of about 32.4-34.8 mm, an aperture size F8, a total track of about 49.6mm, a focus range of 1.0-4.0 m, and a zoom lens operating range of about 0.0D-6.5D. Within this range, the amount of drift can theoretically be 6.5 times less than conventional designs. At extreme field of view (FOV) positions, the RMS spot radius is below 3.4 μm.

The following table is for lens assembly 1600 (fig. 16), where the respective front and back surfaces (if applicable) of the various structures or elements in the overall assembly are ordered from 0 to 20 (left to right), respectively:

the following table is for lens assembly 1800 (fig. 18), where the respective front and back surfaces (if applicable) of each structure or element in the overall assembly are ordered in the order of 0 to 22 (left to right), respectively:

as shown in fig. 17 and 18, another embodiment and/or implementation of a 35mm lens 1700 that can reduce the amount of drift is shown in greater detail. The lens assembly 1700 includes a housing 1710, the housing 1710 enclosing a plurality of lenses that function similarly or identically to the apparatus 1600 described above and shown in FIG. 16. The housing may be constructed of any acceptable material (e.g., aluminum alloy) and may have various shapes. As shown, the housing 1710 includes a front end 1720, a main barrel 1730, and a rear end 1740. With further reference to fig. 18, the lens front 1720 is screwed into an internal thread formed in a widened flange 1820 of the main barrel 1730. Note that in this or other embodiments, an optional filter (e.g., a red band pass filter) 1840 may be mounted to the front of the lens. The overall diameter DF of the lens front 1720 is about 27.5mm. The filter 1840 is typically a commercially available threaded filter having appropriate optical specifications (e.g., visible color, infrared, and ultraviolet, etc. wavelength band pass filters). The main barrel 1730 places a plano-convex lens 1842 in front of a doublet 1844, which doublet 1844 is composed of a plano-convex lens 1846 and a plano-concave lens 1848, which together create a positive lens geometry. A biconcave lens 1850 is stacked behind doublet 1844. A pair of smaller diameter opposing plano-

convex lenses

1852 and 1854 are disposed behind the biconcave lens 1850. The smaller diameter doublet 1856 defines a negative lens behind

lenses

1852 and 1854. The doublet 1856 is positioned behind the last positive lens 1858. A zoom (e.g., liquid) lens 1860 is positioned behind lens 1858. The lens is held at the smaller diameter rear end 1740 by a threaded annular snap ring 1862, the snap ring 1862 being located within a rear collar 1864 having (for example) M13 x 0.5 internal threads. The inner diameter IDC of the collar is about 13mm (threads), the outer diameter ODC is about 14mm, and its axial length LC may be about 3.1mm. The snap ring 1862 may include a catch 1866 for fastening by a lobed tool having the appropriate side and shape. Note that lens arrangement 1700 also includes an aperture stop 1859 in the light path at an appropriate location, e.g., adjacent to liquid lens assembly 1860 on the rear surface of lens 1858.

Lens barrel 1730 may be threaded at rear end 1734 to mate with internal threads on a camera device lens interface. The depth of installation is controlled by an adjustment sleeve 1736 that slides over a barrel 1730. One or more splines (not shown) formed between the inner surface of sleeve 1736 and the outer surface of barrel 1730 can be used to limit rotation of the sleeve relative to the barrel while allowing axial sliding movement (axial) parallel to optical axis OA. The sleeve 1736 is held in the desired position by one or more set screws 1760. In one embodiment, the threaded rear end 1734 of the lens barrel may define the thread specifications of a standard C-interface. Accordingly, the outer diameter of the lens barrel 1730 is about 25mm. The lens shown can handle a resolution of at least 300 million pixels with moderate drift.

4. Conclusion

It will be apparent that the above-described embodiments provide a system that is particularly well-suited for imaging small features (or sets of features), such as ID tags, at relatively large distances. The use of a positive lens package according to embodiments reduces the effectiveness of the variable focus lens package. This arrangement is acceptable within the desired operating range and feature size. In other embodiments, a lens arrangement (e.g., a removable lens arrangement) places the zoom lens behind a fixed optical assembly, resulting in reduced drift characteristics. The zoom lens thus provides the rearmost optical components of the device in front of the sensor. The zoom lens may be contained in a lens arrangement/housing or may be part of a camera arrangement.

Illustrative embodiments of the invention have been described above in detail. Various modifications and additions may be made thereto without departing from the spirit and scope thereof. The features of each of the embodiments described above may be combined with the features of the other described embodiments as appropriate to provide a variety of combinations of features in the new embodiments concerned. Furthermore, while separate embodiments of the apparatus and method of the present invention have been described above, the description herein is merely illustrative of the application of the principles of the invention. For example, various directional and orientational terms used herein, such as "vertical," "horizontal," "upward," "downward," "bottom," "top," "side," "front," "rear," "left," and "right," are relative terms only and should not be construed as absolute orientations (e.g., the direction of gravitational force) relative to a fixed coordinate system. Further, while the lens assembly is shown as being incorporated into a removable lens group, it is contemplated that the system may be used with fixed and/or permanently mounted lenses. Likewise, while the above-described lens sizes and spacing distances are employed within the exemplary operating ranges, these sizes and distances may be scaled up or down in devices having similar relative parameters but larger or smaller overall sizes. Additionally, where a "lens assembly" is employed and/or described herein, the lens assembly may be comprised of one or more discrete lenses that provide the desired optical effect. Accordingly, this description is to be construed as illustrative only and is not intended to limit the scope of the present invention.

Claims

1. A vision system that compensates for drift, comprising:

an image sensor operatively connected to the vision system processor;

a variable focus lens package, the shape or refractive index of which can be varied;

a first lens group disposed in front of the variable focus lens assembly and configured to compensate for drift of the variable focus lens assembly, wherein the first lens group comprises: a first front lens comprising a first front convex surface and a first back concave surface; a second front lens comprising a second front convex surface and a second back concave surface; a third front lens comprising a third rear concave surface; and

a second lens group disposed behind the variable focus lens assembly and configured to direct light from the variable focus lens assembly to the image sensor.

2. The vision system of claim 1, wherein said variable focus lens assembly comprises a liquid lens assembly.

3. The vision system of claim 1, wherein said third front lens comprises a third front concave surface.

4. The vision system of claim 1, wherein a radius of curvature of said second front convex surface is greater than a radius of curvature of said first front convex surface.

5. The vision system of claim 3, wherein a radius of curvature of said third anterior concave surface is equal to a radius of curvature of said third posterior concave surface.

6. The vision system of claim 1, wherein the outer diameters of the first, second and third front lenses are equal.

7. The vision system of claim 1, wherein the light rays are reflected from an object to the first front lens, wherein the object is located within 80-100mm from said first front lens.

8. The vision system of claim 1, wherein said second lens group comprises:

a first rear lens comprising a front convex surface and a rear convex surface; and

an infrared filter.

9. A zoom lens system for a vision system having an image sensor that communicates image data to a processor, the zoom lens system comprising:

10. Zoom lens system of claim 9, wherein the zoom lens assembly comprises a liquid lens assembly.

11. Zoom lens system of claim 9, wherein the third front lens comprises a third concave front surface.

12. Zoom lens system of claim 9, wherein the radius of curvature of the second front convex surface is greater than the radius of curvature of the first front convex surface.

13. Zoom lens system of claim 11, wherein the radius of curvature of the third front concave surface is equal to the radius of curvature of the third back concave surface.

14. Zoom lens system of claim 9, wherein outer diameters of the first, second, and third front lenses are equal.

15. Zoom lens system of claim 9, wherein the light rays reflect from an object to the first front lens, wherein the object is located within 80-100mm from the first front lens.

16. Zoom lens system of claim 9, wherein the second lens group comprises:

an infrared filter.

17. A vision system that compensates for drift, comprising:

an image sensor operatively connected to the vision system processor;

a first lens group disposed in front of the variable focus lens assembly and configured to compensate for drift of the variable focus lens assembly, the first lens group comprising:

a first front lens comprising a first front convex surface and a first back concave surface;

a second front lens comprising a second front convex surface and a second back concave surface;

a third front lens comprising a third rear concave surface; and

a second lens group disposed behind the zoom lens assembly and configured to direct light from the zoom lens assembly to the image sensor, the second lens group comprising a first rear lens comprising a rear convex surface.