Abstract:
The disclosed bar code reader has an imaging system that includes a light monitoring pixel array and a focusing lens that is fixed with respect to the pixel array for transmitting an image of the target object onto the pixel array. The bar code reader also includes an illumination system for illuminating the target having a light source for emitting a diverging light outwardly from a light source location and a lens for bending the light including a first surface facing the light source and a second surface facing the target for bending the light emitted by the light source to more uniformly illuminate a rectangular, generally planar region at the target within the barcode reader&#39;s field of view.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an imaging-based bar code reader having a field of view illuminating system including a light emitting diode. 
       BACKGROUND OF THE INVENTION 
       [0002]    Various electro-optical systems have been developed for reading optical indicia, such as bar codes. A bar code is a coded pattern of graphical indicia comprised of a matrix or series of bars and spaces of varying widths, the bars and spaces having differing light reflecting characteristics. Systems that read and decode bar codes employing CCD or CMOS-based imaging systems are typically referred to as imaging-based bar code readers or bar code scanners. 
         [0003]    Imaging systems include CCD arrays, CMOS arrays, or other imaging pixel arrays having a plurality of photosensitive elements or pixels. Light reflected from a target image, e.g., a target bar code is focused through a lens of the imaging system onto the pixel array. Output signals from the pixels of the pixel array are digitized by an analog-to-digital converter. Decoding circuitry of the imaging system processes the digitized signals and attempts to decode the imaged bar code. 
         [0004]    The ability of an imaging system to successfully decode an imaged bar code is dependent upon the ability to satisfactorily capture a clear image of the target bar code that is focused onto the pixel array and this in turn may be dependent on uniform illumination of a target bar code. 
         [0005]    For many imaging applications, a useful field of view (FOV) is rectangular as dictated by the pixel array&#39;s aspect ratio. The illumination pattern emitted by the bar code reader should cover the rectangular FOV with good uniformity in light intensity. 
         [0006]    Without a lens, a light emitting diode generates a much wider illumination pattern than is necessary thus wasting light. Furthermore, the pattern is not uniform enough. In the prior art a lens is used to match the LED to the bar code reader&#39;s field of view. This still produces a round illumination pattern and to cover the entire rectangular FOV light is still wasted reducing the energy efficiency and causing stray light that provides no useful function. 
       SUMMARY 
       [0007]    An exemplary system is used with an imaging based barcode reader for imaging a target and has an imaging system that includes a light monitoring pixel array and an optical system having one or more focusing lenses positioned with respect to the pixel array to transmit an image of a target object toward the pixel array. 
         [0008]    The exemplary system includes an illumination system having a light source such as a light emitting diode (LED) for illuminating a target within a field of view defined by the optical system. A drive circuit coupled to the light emitting diode energizes the light emitting diode. A lens has a first surface facing the light emitting diode and a second surface facing the target. In the exemplary system the lens has one input surface positioned with respect to a light source such as the light emitting diode and a generally non-rotationally symmetric polynomial output surface facing the target. This lens bends the light to create a field of view with good efficiency and having uniformity across the field of view. By adjustment of the lens input and output surfaces uniformity across a generally rectangular region is obtained. 
         [0009]    These and other objects advantages and features of the invention will become further understood from reference to the accompanying description of an exemplary embodiment of the invention when considered in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic block diagram of an imaging-based bar code reader of the present invention having an automatic focusing system; 
           [0011]      FIG. 2  is a depiction of a housing for supporting the components depicted in  FIG. 1 ; 
           [0012]      FIG. 3  and  FIG. 4  are perspective view of a lens which in conjunction with a light emitting diode illuminate a reader field of view; and 
           [0013]      FIG. 5A-5C  are depictions illustrating irradiance uniformity achieved through practice of the exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    A block diagram of an imaging-based bar code reader  10  is shown schematically in  FIG. 1 . The bar code reader  10 , in addition to imaging and decoding both 1D and 2D bar codes and postal codes, is also capable of capturing images and signatures. In one preferred embodiment of the present invention, the bar code reader  10  is a hand held portable reader components of which are supported within a housing  11  ( FIG. 2 ) that can be carried and used by a user walking or riding through a store, warehouse or plant for reading bar codes for stocking and inventory control purposes. 
         [0015]    A bar code reader of the present invention, however, may be advantageously used in connection with any type of imaging-based automatic identification system including, but not limited to, bar code readers, signature imaging acquisition and identification systems, optical character recognition systems, fingerprint identification systems and the like. It is the intent of the present invention to encompass all such imaging-based automatic identification systems. 
         [0016]    The bar code reader  10  includes a trigger  12  coupled to bar code reader circuitry  13  for initiating reading of a target bar code  15  positioned on an object when the trigger  12  is pulled or pressed. The bar code reader  10  includes an imaging component  20  including imaging optics  21  and a CCD imager  24 . 
         [0017]    One or more lenses focus light reflected from the target bar code  15  onto an array of photosensors or pixels  28  of the CCD imager  24 . In one embodiment, the reader  10  includes an auto focus system  50  that moves at least one lens with a motor  29  having an output transmission coupled to the lens and whose movement is monitored with a position encoder  27 . The pixels of the pixel array  28  are read out generating an analog signal at an output  30  representative of an image of whatever is focused by the imaging optics  21  onto the pixel array  28 , for example, an image of the bar code  15  intersected by the reader&#39;s optical axis OA. The analog image signal at the output  30  is then digitized by an analog-to-digital converter  70  and a digitized signal at an output  74  is decoded by decoder circuitry  80 . Decoded data  90 , representative of the data/information coded in the bar code  15  is then output via a data output port  100  and/or displayed to a user of the reader  10  via a display  108 . Upon achieving a good “read” of the bar code  15 , that is, the bar code  15  was successfully imaged and decoded, a speaker  120  is activated by the circuitry  13  to indicate to the user that the bar code has been successfully read. 
         [0018]    The reader  10  further includes an aiming pattern generator  40  that generates a visible aiming pattern  43  to aid the user in properly aiming the reader at the target bar code  15 . In one preferred embodiment, the aiming generator  40  is a laser aiming apparatus. Alternatively, the aiming apparatus  40  may utilize an LED or another source of illumination known to those of skill in the art. The pattern  43  may be a pattern comprising a crosshair formed from a thick horizontal line  43   a  and a perpendicular thin vertical line  43   b . In one preferred embodiment, the laser aiming apparatus  40  includes a laser diode  42  and a diffractive lens  44 . 
         [0019]    In the illustrated embodiment, in addition to the aiming pattern generator  40 , the reader  10  includes a separate illumination system  51  having a light emitting diode  152  for shining illumination light onto the target bar code  15 . 
         [0020]    The CCD or CMOS sensors that make up the imager  24  sense light reflected back from the target surface and form pixel data corresponding to an image of the target. It is advantageous to use an array sensor that has the capability to output a portion of pixels upon request, so that the transfer time and processing time can be shortened when only a portion of the array is properly exposed. One such sensor is a CMOS array made by Micron having part number MT9M001. The pixel data from the array is converted into digital data by an A/D converter  70  that is decoded by decoding system  80  that includes a microprocessor controller. An output port or display  108  provides the results of decoding to a peripheral device (not shown) or displays them to the user. The scanner  10  also includes an illumination source (not shown) that is capable, within a prescribed scanner range, of illuminating a portion of the target surface sufficient to fill the entire two-dimensional array of sensors with data. The scanner includes an aiming pattern generator  40  that includes one or more laser diodes  42  and a focusing lens  44  (see  FIG. 1 ) that is activated by a user actuated trigger  12 . 
       High Intensity Illumination Source 
       [0021]    The illumination system  51  includes a lens  140  ( FIG. 3 ) for redirecting or bending light from a single light emitting diode  152  and illuminating the target  15  within a field of view at a focus distance D defined by the imaging optics  21 . In the exemplary embodiment the light emitting diode  152  is supported by a printed circuit board  150  so that the light emitting diode  152  is positioned on a centerline of a z axis of a co-ordinate system depicted in the figures. The light emitting diode  152  is most preferably energized by a power supply (not shown) in response to actuation of the trigger  12 , but could alternately be activated by a signal from a controller that co-ordinates and controls operations of the bar code reader including the decoding functions. 
         [0022]    The lens  140  is positioned in relation to the LED  152  such that an entrance surface S 1  faces the LED  152  and a surface S 2  faces the target. But for the presence of the lens, light from the light emitting diode would disperse and a planar region of the target would intersect the generally spherical coverage with a non uniform coverage of light, requiring compensation and possible misidentification by the decoding electronics of the reader. A generating curve used to generate the surface S 1  is rotationally symmetric and can be aspherical, such as equation 1: 
         [0000]    
       
         
           
             
               
                 
                   
                     z 
                     = 
                     
                       
                         
                           Cr 
                           2 
                         
                         
                           ( 
                           
                             1 
                             + 
                             
                               
                                 1 
                                 - 
                                 
                                   
                                     ( 
                                     
                                       k 
                                       + 
                                       1 
                                     
                                     ) 
                                   
                                    
                                   
                                     C 
                                     2 
                                   
                                    
                                   
                                     r 
                                     2 
                                   
                                 
                               
                             
                           
                           ) 
                         
                       
                       + 
                       
                         
                           ∑ 
                           i 
                         
                          
                         
                           
                             a 
                             i 
                           
                            
                           
                             r 
                             i 
                           
                         
                       
                     
                   
                   , 
                   
                     
 
                   
                    
                   
                     
                       where 
                        
                       
                           
                       
                        
                       r 
                     
                     = 
                     
                       
                         
                           x 
                           2 
                         
                         + 
                         
                           y 
                           2 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   1 
                 
               
             
           
         
       
     
         [0000]    C is the radius of curvature of the base or reference sphere, k is a conic constant and the a i  are the departure terms from the base or reference sphere. 
         [0023]    Where suitable parameters for this equation are listed below: 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 C 
                 k 
                 i 
                 a 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 0 
                 0 
                 2 
                 0.06 
               
               
                   
                   
                   
                 4 
                 −0.02 
               
               
                   
                   
               
             
          
         
       
     
         [0024]    A more generic version of equation 1 is a polynomial surface which can include rotational symmetric or asymmetric aspheres governed by equation 2: 
         [0000]    
       
         
           
             
               
                 
                   
                     z 
                     = 
                     
                       
                         
                           Cr 
                           2 
                         
                         
                           ( 
                           
                             1 
                             + 
                             
                               
                                 1 
                                 - 
                                 
                                   
                                     ( 
                                     
                                       k 
                                       + 
                                       1 
                                     
                                     ) 
                                   
                                    
                                   
                                     C 
                                     2 
                                   
                                    
                                   
                                     r 
                                     2 
                                   
                                 
                               
                             
                           
                           ) 
                         
                       
                       + 
                       
                         
                           ∑ 
                           i 
                         
                          
                         
                           ( 
                           
                             
                               
                                 a 
                                 i 
                               
                                
                               
                                 r 
                                 i 
                               
                             
                             + 
                             
                               
                                 b 
                                 i 
                               
                                
                               
                                 x 
                                 i 
                               
                             
                             + 
                             
                               
                                 c 
                                 i 
                               
                                
                               
                                 y 
                                 i 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                   , 
                   
                     
 
                   
                    
                   
                     where 
                      
                     
                         
                     
                      
                     
                       b 
                       i 
                     
                      
                     
                         
                     
                      
                     and 
                      
                     
                         
                     
                      
                     
                       c 
                       i 
                     
                      
                     
                         
                     
                      
                     are 
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   2 
                 
               
             
           
         
       
     
         [0000]    coefficient of the polynomial expansion terms. 
         [0025]    The generating curve used to generate the surface S 2  is non-rotationally symmetric polynomial asphere, which is a subset of a polynomial surface as described in equation 2, is also governed by and can be generated by the a polynomial such as described by equation 3: 
         [0000]    
       
         
           
             
               
                 
                   
                     z 
                     = 
                     
                       
                         
                           Cr 
                           2 
                         
                         
                           ( 
                           
                             1 
                             + 
                             
                               
                                 1 
                                 - 
                                 
                                   
                                     ( 
                                     
                                       k 
                                       + 
                                       1 
                                     
                                     ) 
                                   
                                    
                                   
                                     C 
                                     2 
                                   
                                    
                                   
                                     r 
                                     2 
                                   
                                 
                               
                             
                           
                           ) 
                         
                       
                       + 
                       
                         
                           ∑ 
                           i 
                         
                          
                         
                           ( 
                           
                             
                               
                                 a 
                                 i 
                               
                                
                               
                                 r 
                                 i 
                               
                             
                             + 
                             
                               
                                 b 
                                 i 
                               
                                
                               
                                 x 
                                 i 
                               
                             
                             + 
                             
                               
                                 c 
                                 i 
                               
                                
                               
                                 y 
                                 i 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                   , 
                   
                     
 
                   
                    
                   
                     
                       where 
                        
                       
                           
                       
                        
                       r 
                     
                     = 
                     
                       
                         
                           x 
                           2 
                         
                         + 
                         
                           y 
                           2 
                         
                       
                     
                   
                   , 
                   
                     
 
                   
                    
                   
                     C 
                      
                     
                         
                     
                      
                     is 
                      
                     
                         
                     
                      
                     the 
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   3 
                 
               
             
           
         
       
     
         [0000]    radius of curvature of the base sphere, k is a conic constant, a i  are the coefficient of departure terms from the reference sphere, and b i  and c i  are coefficient of the polynomial departure terms from reference sphere. 
         [0026]    Where suitable parameters for this equation (n th  degree polynomial) are listed below: 
         [0000]    
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 C 
                 k 
                 i 
                 a 
                 b 
                 c 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 −0.39 
                 −1 
                 2 
                 −0.195 
                 −0.015 
                 −0.073 
               
               
                   
                   
                   
                 4 
                 −0.004 
                 −0.004 
                 −0.017 
               
               
                   
                   
                   
                 6 
                 −0.001 
                 −0.0025 
               
               
                   
                   
                   
                 10 
                 −0.00003167 
                   
                 −0.0003 
               
               
                   
                   
               
             
          
         
       
     
         [0027]    Note, in equations 2 and 3 for the base sphere, the C and k are single constants. In a more general case, the equation is biconic so that the equation 4 representation is satisfied: 
         [0000]    
       
         
           
             
               
                 
                   z 
                   = 
                   
                     
                       
                         
                           
                             C 
                             x 
                           
                            
                           
                             x 
                             2 
                           
                         
                         + 
                         
                           
                             C 
                             y 
                           
                            
                           
                             y 
                             2 
                           
                         
                       
                       
                         ( 
                         
                           1 
                           + 
                           
                             
                               1 
                               - 
                               
                                 
                                   ( 
                                   
                                     
                                       k 
                                       x 
                                     
                                     + 
                                     1 
                                   
                                   ) 
                                 
                                  
                                 
                                   C 
                                   x 
                                   2 
                                 
                                  
                                 
                                   x 
                                   2 
                                 
                               
                               - 
                               
                                 
                                   ( 
                                   
                                     
                                       k 
                                       y 
                                     
                                     + 
                                     1 
                                   
                                   ) 
                                 
                                  
                                 
                                   C 
                                   y 
                                   2 
                                 
                                  
                                 
                                   y 
                                   2 
                                 
                               
                             
                           
                         
                         ) 
                       
                     
                     + 
                     
                       
                         ∑ 
                         i 
                       
                        
                       
                         ( 
                         
                           
                             
                               a 
                               i 
                             
                              
                             
                               r 
                               i 
                             
                           
                           + 
                           
                             
                               b 
                               i 
                             
                              
                             
                               x 
                               i 
                             
                           
                           + 
                           
                             
                               c 
                               i 
                             
                              
                             
                               y 
                               i 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   4 
                 
               
             
           
         
       
     
         [0028]    The co-ordinate axis shown in  FIG. 3  has its origin in a plane where the two surfaces intersect. A curved line  154  of intersection of the surfaces S 1 , S 2  is shown in  FIG. 3 . One point P 1  that is common to both surfaces lies on this line of intersection and a second point P 2  lies on the surface S 2 . To determine the co-ordinates in the z direction of these points one merely inserts their x and y co-ordinates into the generating equation for the particular surface and solve for z. As a straightforward example, the point P 3  has x and y co-ordinates of 0.0 so that their z co-ordinate is a 2 +a 4 =0.04. Proper scaling of the parameters can produce results in any units desired. 
         [0029]    The surfaces described in equations 1 and 2 are representative and other forms are suitable so long as they bend the generally radially directed light emitted from the LED  152  to provide uniform intensity over a rectangular (possible square) region of interests within the sensor field of view. The surfaces are produce in the exemplary embodiment by diamond turning technology which produces a mold which is then used to plastic injection mold the lens  140 .  FIGS. 5A-5C  are depictions of data gathered from a light emitting diode/lens combination at a distance of 100 mm in the x and y direction at the target plane. This data was obtained with the distance D from the from surface S 2  of the lens  140  to the target of 100 mm. The LED was spaced a distance 0.2 mm from the center of the surface S 1  in this test. The data illustrates good uniformity across the reader&#39;s field of view and almost complete cropping of the light outside the field of view. 
         [0030]    The lens  140  functions to focus the diffuse or diverging light from the single source LED  152  to provide more parallel paths for the light emitted by the LED. 
         [0031]    There are substantial advantages to use of aspheres in the illuminating system. Aspheres reduce the number of optical surfaces while eliminating aberrations and distortions while providing a compact lower mass system. Aspheres can be made using Diamond-Turning technology which makes it possible to cut precision aspheric surfaces for use in molding the lens. 
         [0032]    While the present invention has been described with a degree of particularity, it is the intent that the invention includes all modifications and alterations from the disclosed design falling with the spirit or scope of the appended claims.