Abstract:
An exemplary imaging based barcode reader has an imaging system that includes a photosensitive array, a focusing lens fixed with respect to the light detecting array which creates an image of a target object on the light detecting array; and an aperture stop fixed in relation to the focusing lens having an opening that allows light from the target object to impinge upon the lens for focusing onto the light detecting array. The focusing lens has a surface facing the aperture stop that is toroidal and in one exemplary embodiment approximates a cylinder that balances or optimizes. Astigmatism of the lens in the imaging plane located in the close proximity to the photosensitive array.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an imaging-based bar code reader. 
       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 then attempts to decode the imaged bar code. 
         [0004]    Usually an imaging lens for a camera consist of a few lens elements. Simple cost effective designs are very challenging and difficult to achieve with an adequate performance. The simplest well known camera design is a pinhole camera. No lens is required in this design. The main problem with the pinhole design is a low light throughput and a limited resolution. The low light throughput can be compensated by improving the illumination system, but this may significantly increase the cost of the imaging optics. It would be beneficial to increase the aperture size for a better light throughput. The resolution is limited due to the diffraction of light at the aperture and cannot be further improved. Manufacturing of an accurate pinhole aperture in mass production could also be very challenging and costly. 
         [0005]    A single lens design gives more flexibility for the design but finding the right arrangement of the lens and an aperture stop is challenging. In general a single lens design yields poor off axis performance if proper arrangement has not been implemented. Astigmatism in the resulting image is one concern. 
         [0006]    Astigmatism is a lens aberration that results in a cone of light from an object point not being converged to a point image at any place behind the lens. Rather, the cone is converged in one direction (for example, horizontal\) at a certain location and in the other direction (for example, vertical) at a different location. 
         [0007]    This phenomenon can be caused by asymmetry in the lens. However, even in a perfectly symmetrical lens, the phenomenon will still occur for object points not on the optical axis of the lens. 
       SUMMARY 
       [0008]    In the present invention it is proposed a single lens element design with an aperture and cylindrical surface, which balances performance over the entire field, improves field curvature, and allows a bigger aperture stop for a higher light throughput. 
         [0009]    An exemplary imaging based barcode reader has an imaging system that includes a light detecting array, a focusing lens fixed with respect to the light detecting array which creates an image of a target object on the light detecting array; and an aperture stop fixed in relation to the focusing lens having an opening that allows light from the target object to impinge upon the lens for focusing onto the light detecting array. 
         [0010]    The focusing lens has a surface facing the aperture stop that is toroidal and in one exemplary embodiment approximates a cylinder that balances or optimizes for astigmatism in images formed on the pixel array. 
         [0011]    These and other objects, advantages and features of the exemplary barcode reader are understood from the following detailed description which is described in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a schematic block diagram of an imaging-based bar code reader of the present invention; 
           [0013]      FIG. 2  is a top plan view of imaging optics of a bar code reader of the present invention; 
           [0014]      FIG. 3  is a side elevation view of imaging optics of a bar code reader of the present invention; 
           [0015]      FIG. 4  is a top view of an alternate embodiment of imaging optics for use with the invention; 
           [0016]      FIG. 4A  is a side elevation view of the imaging optics of  FIG. 4 ; and 
           [0017]      FIGS. 5 and 6  illustrate imaging effects of balanced (or optimized) astigmatism on axis and off axis spots with and without use of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    A block diagram of an imaging-based bar code reader  10  is shown schematically in  FIG. 1 . The bar code reader  10  decodes 1D and 2D bar codes and postal codes, and can also capture 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. The depicted scanner is for reading barcodes in close proximity to a housing exit window, and could for example be built into household appliances such as coffeemaker or the like. 
         [0019]    The 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 identification systems. 
         [0020]    The bar code reader  10  includes a trigger  12  coupled to bar code reader circuitry  13  for initiating reading of a target bar code  150  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 . 
         [0021]    A fixed lens (described in greater detail below) focuses light reflected from the target bar code  150  onto alight monitoring array  28  of photosensors or pixels of the CCD imager  24 . The pixels of pixel array  28  are read out generating an analog signal at an output  30  representative of an image of whatever is focused by the lens on the pixel array  28 , for example, an image of the bar code  150 . 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. In one illustrative example, after successful decode of a coffee disc, a coffeemaker automatically selects mode of operation to brew coffee accordingly prescribed by the barcode. 
       Imaging Optics 
       [0022]      FIGS. 2 and 3  schematically depict imaging optics  21  which projects an image of the barcode  150  onto an active area of the light monitoring array  28 . Light from the barcode passes through an entrance window  151  and then through an aperture stop  152  and a fixed lens  154  A first surface  160  of the lens  154  facing the aperture stop  152  is a section of a cylinder and balances astigmatism across the field of view (along the sensor array). The cylindrical surface facing the aperture stop is concave ( FIG. 3 ) or convex ( FIG. 4 ). More generally, the surface  160  can be a torus or surface or a toroid. The shape of a toroid is generated by revolving a circle around an axis external to the circle. 
         [0023]    In a preferred embodiment there is an air gap  162  between the aperture and the cylindrical surface. A preferred radius of curvature of the cylindrical surface is from 5-25 MM and more preferably about 12 MM and the distance between the aperture and the lens is from 0 to 2 MM and more preferably about 1 MM. Typically in a rotationally symmetrical imaging system astigmatism is zero on axis and could be quite large off axis, at the edge of the field of view. If one images a small point, its image on the sensor surface on axis will appear as a small round dot  170  ( FIG. 5 ), however, if the point is located significantly off axis, at the edge of the field of view, the image will look like an line  172  as show in the  FIG. 5 , the spot will be elongated in vertical direction. 
         [0024]    On axis barcodes with defects such as scratches and voids on the surface of the barcode  150  are difficult to read. If a projected image of a defect is the size of the sensor pixel, it might be incorrectly interpreted by the decoder as an actual bar or space. It will be beneficial to have vertically elongated spot uniformly over the entire sensor. The cylindrical surface integrated into the lens between the aperture stop and aspherical surface produces a vertically elongated spot over the entire linear sensor as shown in the  FIG. 6 . 
         [0025]    This effect can be described in terms of sagital and tangential astigmatisms. In case of no cylindrical surface the difference between sagital and tangential astigmatism is zero on axis, which results in a round spot, however off axis there is a difference between the sagital and tangential astigmatisms, which result in elongated spot. 
         [0026]    The origin and meaning of the terms sagital and tangential astigmatisms are found in a book entitled Modern Optical Engineering by Warren J. Smith Second Edition, McGraw Hill Copyright 1990 at pages 65-67 which is incorporated herein by reference. With a cylindrical surface entrance surface to the lens, the difference between sagital and tangential astigmatism is relatively constant, which results in a substantially consistent elongated spot on the sensor  28 . 
         [0027]    A second surface of the lens faces the sensor and has a rotationally symmetrical spherical or aspherical surface. In a preferred embodiment the surface is aspherical. An aspheric lens in this context is a lens whose surface that faces the array  28  is neither a portion of a sphere nor of a circular cylinder. An aspherical surface allows better compensation for aberrations, less field curvature and yields better lens performance. Usually aspherical lenses can be well compensated for spherical aberration therefore the diameter of the aperture stop  152  can be large. A large aperture transmits more light onto the sensor array therefore the signal to noise ratio is improved. In this case less light is necessary from the illumination system, which can reduce cost of the scanner. Typically the aperture size is about 1 MM width and 2 MM height to compare with pinhole aperture width 0.1 MM. 
         [0028]    The aperture can be round or elongated along the vertical direction, i.e. elliptical or rectangular. It gives additional light throughput through the imaging lens. In a preferred embodiment the aperture can be elliptical. 
         [0029]    The lens  212  shown in  FIG. 4  is integral with a support  210 . A concave forward facing lens surface  232  is spaced from an aperture stop  214  by a gap  230 . The lens  212  and support  210  are molded in one piece from clear plastic such as acrylic or polycarbonate. The support has a generally annular portion  210   a  which supports a printed circuit board  211  which in turn supports the sensor array  28 . In the illustrated embodiment, a transparent glass cover  220  protects the array  28  although this is an optional feature of the system. This mounting arrangement allows better positioning accuracy of the imaging lens with respect to the sensor  28 . In this type of optics assembly no active focusing is required, which further reduces the manufacturing cost. 
         [0030]    It is known that focal distance of the plastic lens varies significantly with temperature. At higher temperatures the focal distance of the lens increases. In some cases it may compromise adequate product performance. In the proposed design, due to the thermal expansion of the annular portion  210   a , the distance from the lens  212  to the sensor  28  increases/decreases with temperature, which partially compensates the thermal variation of the focal length of the lens. (if the temperature increases the lens expands, if the temperature decreases, the lens contracts.) The lens surface moves further away from the sensor  28  when the temperature rises and by this means partially compensate increased in the focal distance of the lens. 
         [0031]    The lens may have mechanical features, such as pins or keys, to align accurately an aperture  214  attached to the support  210  with respect to the lens surface to make it easier to consistently make the product. In the  FIG. 4  embodiment, the lens is injection molded out of a plastic material. This embodiment has cost advantages in fabricating. No additional cost is required in mass producing this embodiment in quantity if an aspherical design is used but performance is improved over a spherical surface. A surface adjacent to the aperture  214  may have a cylindrical or toroidal curvature as well, which can give an additional design flexibility to project the best possible image to the sensor. 
         [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.