Abstract:
The exemplary bar code reader has an auto-focusing component positioned between a photo-detector array and an object having target indicia. The auto-focusing component has a lens system for focusing the target indicia at an imaging plane that generally co-incides with a plane of the photo-detector array. An electromagnetic coil actuator is coupled to an adjustable aperture such that activation of the coil actuator moves the adjustable aperture into and out of the imaging field of view to adjust a focusing on the target indicia. The autofocus component is fixed when the moveable aperture is within the field of view for focusing on objects close to the reader and when the aperture is removed, the autofocus is effective in imaging objects at a greater distance from the reader.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to a system comprising a method and apparatus for improving an imaging quality in an imaging bar code reader. 
       BACKGROUND 
       [0002]    Existing portable barcode readers may be hand held so they can be moved with respect to a target barcode, to image and decode the bar code. Target objects, e.g., a product package that includes a target barcode, are brought within a field-of-view (“FOV”) of the barcode reader by aiming a visible aiming pattern to strike the package at a region of the barcode. In stationary bar code readers the situation is reversed, i.e. the product is moved through a stationary field of view. The barcode reader typically provides an audible and/or visual signal to indicate the target barcode has been successfully imaged and decoded. 
         [0003]    Both stationary and portable imaging-based barcode readers include at least one camera. The camera has an array of photosensitive elements such as a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device. The bar code reader also typically includes an illumination system that directs illumination toward a target object, e.g., a target bar code. Light reflected from the target bar code is focused through a lens of the camera by an imaging system such that focused light is concentrated onto the array of photosensitive elements. Picture elements or pixels of the array are sequentially read out by the scan engine, generating an analog signal representative of a captured image frame. The analog signal is amplified by a gain factor and the amplified analog signal is digitized by an analog-to-digital converter. Decoding circuitry and/or software of the bar code reader processes the digitized signals and decodes the imaged bar code. 
         [0004]    Some existing bar code readers include an autofocus lens system in their imaging camera. These autofocus lens systems are most useful for imaging at far distances. They typically use a relatively large fixed size aperture that lets in more light which is useful for imaging at large distances. These prior art autofocus systems have a fixed size aperture and the depth of field is reduced as the camera focuses on objects more closely positioned with respect to the camera. The travel distance of the lens increases exponentially with decreasing distance to the target to be imaged, so the time to focus on objects close to the camera is longer. 
       SUMMARY 
       [0005]    An exemplary imaging system improves the quality of images captured by an image based bar code reader. The bar code reader acquires an image within a bar code reader field of view so that a controller can identify a bar code within that field of view. 
         [0006]    The exemplary imaging system includes a light source for illuminating a target indicia on an object and a photo-detector array that receives light reflected from the object in a region of the target indicia. 
         [0007]    An auto-focusing component is positioned between the photo-detector array and the object. The auto-focusing component has a lens system for focusing the target indicia at an imaging plane that generally co-incides with a plane of the photo-detector array. An electromagnetic drive is coupled to a moveable aperture. Activation of the electromagnetic drive provides controlled movement of the adjustable aperture within the imaging field of view to adjust a depth of field of an image of the target indicia. 
         [0008]    In an exemplary embodiment of the disclosed system an electromagnetic coil actuator moves an aperture in and out of the optical path. The autofocusing component uses a large aperture and a small aperture is selectively moved in place by the coil actuator. In one embodiment, if the target object is at a near distance, the small aperture is moved into place enabling large depth of field and the lens position is fixed. With a fixed lens position and small aperture, the camera is configured best for near distance imaging. When the target is at a far distance, the small aperture is moved out of the optical path and the autofocus component is enabled. The large aperture enables the collection of more light that is needed at a far distance, and the lens travel at far distances is small. Thus the camera has the best characteristics for distance imaging. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The foregoing and other features and advantages of the present disclosure will become apparent to one skilled in the art to which the present disclosure relates upon consideration of the following description of the invention with reference to the accompanying drawings, wherein like reference numerals, unless otherwise described refer to like parts throughout the drawings and in which: 
           [0010]      FIG. 1  is a perspective view of a portable scanner having at least one camera for imaging a target object; 
           [0011]      FIG. 2  is a block diagram of the portable scanner shown in  FIG. 1 ; and 
           [0012]      FIGS. 3A and 3B  schematically depict imaging optical paths used with an exemplary system at different imaging ranges. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    The present disclosure relates to a system for improving image quality in a bar code reader. In one example embodiment illustrated in  FIG. 1 , an imaging system  10  comprises a hand held portable reader  26  that can be carried. One use of the imaging system  10  is by a user walking or riding through a store, warehouse, or plant, while reading various symbology codes for stocking and inventory control purposes. The reader  26  of  FIG. 1  includes a housing  13  having a head  28 , handle  30 , and trigger  32 . Located in the housing is a protective window  34  for protecting an imaging subsystem  18 . 
         [0014]    The imaging subsystem  18  projects an aiming pattern  36  toward a target bar code  24  located on a product  20  or product&#39;s packaging during operation for decoding the bar code in the image of the target object. The operation of the decoding of the bar code by the imaging system  18  is well known in the prior art. 
         [0015]    The imaging subsystem  18  has several optical components ( FIG. 2 ) that include, illumination optics  42 , aiming optics  44  for generating the aiming pattern  36 , and imaging optics  46 . Each of the optical components have a designed field-of-view for projecting or receiving light directed during operation at the target object  20 . The imaging optics  46  includes a focusing lens or lenses  48  that focus the reflected image from the target object  24  onto a sensor array  50  located behind the focusing lens(es). The aiming optics  44  include a refractive or diffractive optical element that facilitates in the projection of an aiming pattern  36  for aligning the imaging optics for capturing an image that contains the bar code  24 . The object  20  shown in  FIG. 1  includes a 1D barcode  24  and a 2D bar code is shown on the object  20  in  FIG. 2 . 
         [0016]    When enabled by a controller  60  ( FIG. 3 ), the imaging optics  46  captures an image frame of a field of view FV of the imaging system. When imaging a target bar code  24 ,  25 , the imaging process may need to capture and store in a memory  64  a series of image frames  66  ( FIG. 3 ) in response to multiple user actuations of the trigger. A decoding system  70  analyzes each image frame of the series of image frames  66  and attempts to decode the imaged bar code. All or portions of the images may be stored in the memory  64  based as results  72  of this decoding. Alternately, the results  72  may be the contents in ascii form of the barcode as interpreted by the decoding system  70 . 
         [0017]    As depicted in  FIG. 2 , the imaging optics  46  is coupled to the controller  60 . The imaging optics  46  includes a housing supporting focusing optics including a focusing lens  48  and a 2D photosensor or pixel array  50 . The imaging optics  46  is enabled during an imaging session to capture images of the field of view FV of the focusing lens  48  and that make up the image frames  124 . 
         [0018]    Auto-focusing is achieved by controlled movement of one or more lenses  48  positioned between the photo-detector array  50  and the object  20 . In the exemplary system autofocus is achieved by a micro-electromechanical or MEMS device or drive  80  coupled to the lens or lenses  48 . The camera assembly  46  also includes two apertures  82 ,  84  for controlling light transmission from the object to the sensor array  50 . An adjustable or moveable aperture  82  is coupled to a coil drive  86  and in the exemplary embodiment is moved into and out of an imaging field of view to adjust a depth of field of an image of the target indicia, principally with the object located in a near range of imaging distances such as R 1  (typically having a range of 4 to 20 inches) shown in  FIG. 3B . 
         [0019]    The exemplary actuator for moving the aperture  82  has a coil attached to the MEMs device  80 . The moving aperture actuator is a coil that surrounds a housing for the lens  48  (several millimeters in diameter). When the coil is energized in one direction or sense the magnetic field set up by the energized coil moves a plate (not shown) supporting the aperture  82  along a linear path  88  away from one magnet into proximity to a second magnet. When the coil is energized in the opposite direction, the aperture moves back to its original position. 
         [0020]    With a fixed position of the lens or lenses  48  and small aperture  82  (about 7 millimeters diameter) within the camera field of view the camera is configured best for near distance imaging. When the target is at a far distance (typical range of 2-50 feet), the small aperture  82  is moved out of the optical path and the autofocus is enabled. The large aperture  84  (about 1.2 millimeters in diameter) enables the collection of more light that is needed at a far distance, and the lens travel at far distances is small. Thus the camera has the best characteristics for distance imaging. 
         [0021]    A range finder is able to determine a distance between the object and the reader  10 . The range finder is implemented in one of three ways. The MEMs autofocus device  80  has a capacitive signal feedback system that allows the controller  60  to determine where the autofocus is attempting to image an object. When the lens position indicates the autofocus is attempting to focus at a near distance, the autofocus is disabled and the moveable aperture positioned within the field of view. A second alternative is to use the aiming pattern  36  for determining object distance. The source for the aiming pattern is offset from the imaging optical axis. Images of the aiming pattern can be obtained by the controller and evaluated to determine the object spacing from the bar code reader. For objects close to the reader the moveable aperture is moved into the field of view and the autofocus is disabled. A third option is to provide a user actuatable switch or control that sends a signal to the controller, causing the controller to configure the reader for either far field or close in imaging. 
         [0022]    A suitable MEMs device  80  is commercially available from Tessera Corporation having a place of business in Arcadia, Calif. under part number SF23XS. 
         [0023]    The bar code reader circuitry within the housing  13  is electrically coupled to a power supply, which may be in the form of an on-board battery or a connected off-board power supply. If powered by an on-board battery, the reader  10  may be a stand-alone, portable unit as depicted in  FIG. 1 . If powered by an off-board power supply, the reader  10  may have some or all of the reader&#39;s functionality provided by a connected host device. Circuitry associated with the imaging and decoding systems may be embodied in hardware, software, firmware, electrical circuitry or any combination thereof and may be disposed within, partially within, or external to the reader housing  13 . 
         [0024]    The sensor array  50  may comprise a charged coupled device (CCD), a complementary metal oxide semiconductor (CMOS), or other imaging pixel array, operating under the control of the imaging circuitry  24 . In one exemplary embodiment, the pixel array  50  comprises a two dimensional (2D) mega pixel array with a typical size of the pixel array being on the order of 1280×1024 pixels. 
         [0025]    During an imaging session, multiple images of the field of view FV may be obtained by the imaging system  10 . An imaging session may be instituted by an operator, for example, pressing the trigger  32  to institute an imaging. Alternately, for a stationary imaging system, an imaging session might start when a lower or bottom edge of an item begin to move through a portion of the field of view FV. After an exposure period, some or all of the pixels of pixel array  50  are successively read out by the controller  60 , thereby generating an analog signal which is converted by an analog to digital converter that forms part of the controller  60 . In some sensors, particularly CMOS sensors, all pixels of the pixel array  50  are not exposed at the same time, thus, reading out of some pixels may coincide in time with an exposure period for some other pixels. 
         [0026]    The analog image signal represents a sequence of photosensor voltage values, the magnitude of each value representing an intensity of the reflected light received by a photosensor/pixel during an exposure period. The analog signal from the array  150  is amplified by a gain factor, generating an amplified analog signal. The amplified analog signal is digitized by an A/D converter generating a digitized signal. The digitized signal comprises a sequence of digital gray scale values typically ranging from 0-255 (for an eight bit processor, i.e., 2 8 =256), where a 0 gray scale value would represent an absence of any reflected light received by a pixel (characterized as low pixel brightness) and a  255  gray scale value would represent a very intense level of reflected light received by a pixel during an integration period (characterized as high pixel brightness). 
         [0027]    Successful decoding of the indicia, typically a bar code, can be accompanied by user or machine perceptible outputs from the reader  26 . To achieve such notification the reader  26  includes a light emitting diode  90 , speaker  92  and display  94 . Successful decoding may also be accompanied by signal transmissions via an output port  96  to other devices using a communication protocol. 
         [0028]    What have been described above are examples of the present invention. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the present invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims.