Abstract:
A method and apparatus for illuminating a target object such as a bar code having areas of differing light intensity on the target. A bar code reader has a light source that is selectively activated, a screen having a slit aperture for creating a narrow light beam, a reflecting mirror for reflecting light transmitted through the slit aperture and a lens for shaping light reflected by the mirror to form an elongated target/illumination pattern.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an illumination system for a bar code reader and, more particularly, to an illumination system for creating a visible aiming target on an object. 
       BACKGROUND ART 
       [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 series of bars and spaces of varying widths, the bars and spaces having differing light reflecting characteristics. Some of the more popular bar code symbologies include: Uniform Product Code (UPC), typically used in retail stores sales; Code 39, primarily used in inventory tracking; and Postnet, which is used for encoding zip codes for U.S. mail. Systems that read and decode bar codes employing charged coupled device (CCD) or complementary metal oxide semiconductor (CMOS) based imaging systems are typically referred to hereinafter as imaging-based bar code readers. 
         [0003]    Bar code readers electro-optically transform the graphic indicia of the bar code into electrical signals, which are decoded into alphanumerical characters that are descriptive of the article containing the bar code. The characters are then typically represented in digital form and utilized as an input to a data processing system for various end-user applications such as point-of-sale processing, inventory control and the like. 
         [0004]    Imaging systems used in bar code readers include charge coupled device (CCD) arrays, complementary metal oxide semiconductor (CMOS) arrays, or other imaging pixel arrays having a plurality of photosensitive elements (photosensors) or pixel array. An illumination system directs illumination toward a target object, e.g., a target bar code and light reflected from the target bar code is focused through a lens of the imaging system onto the pixel array. 
         [0005]    Imaging-based bar code readers typically employ an illumination system to flood a target object with illumination from a light source such as a light emitting diode (LED) in the reader. Light from the light source or LED is reflected from the target object. The reflected light is then focused through a lens of the imaging system onto a two dimensional pixel array. In a linear imaging bar code reader, the sensor array is much wider in one dimension than another. The sensor array can capture a wide (few inches) field of view that is very narrow (one or only a few pixels) in an orthogonal direction so that only a narrow strip of pixels is captured by the reader. 
         [0006]    Bar code readers often have an illumination system that facilitates aiming the bar code reader. One challenge in designing bar code readers is a way to provide simple and cost effective illumination optics to generate a sharp illumination/aiming scan line having brightness without substantial loss due to coupling efficiency between the light source and a lens element that transmits light from the source to a target object. Published U.S. patent application US 2008/0156876 to Vinogradov discloses an illumination system and a focusing lens to generate an illumination pattern. The disclosure of this application is incorporated herein by reference in its entirety. 
       SUMMARY 
       [0007]    The present disclosure is directed to a bar code reading having an illumination system for generating an illumination/aiming pattern and has particular utility for use with a linear imaging bar code reader. 
         [0008]    A representative system has a fold mirror with an optical power that is unequal in orthogonal directions for matching the emitting angle of a light source to the numerical aperture of an illumination lens. In addition, the illumination lens has an aspherical toroidal surface, which allows it to yield more uniform illumination along the scan line with brighter light intensity at the edges of a scan line for better perception of the scan line by the user. 
         [0009]    These and other objects, advantages, and features of the exemplary embodiments are described in detail in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic side elevation view of an exemplary embodiment of an imaging-based bar code reader; 
           [0011]      FIG. 2  is a schematic front elevation view of the bar code reader of  FIG. 1 ; 
           [0012]      FIG. 3  is a schematic view of an imaging assembly of the bar code reader of  FIG. 1 ; 
           [0013]      FIG. 4  is a depiction of light from a source generating a target or aiming pattern within a field of view of a bar code reader; 
           [0014]      FIG. 5  is a top plan depiction of the apparatus of  FIG. 4 ; 
           [0015]      FIG. 6  is a schematic depiction of a source, aperture, positive and negative lens for creating an illumination/aiming pattern; and 
           [0016]      FIG. 7  is a schematic perspective view of an exemplary illumination system for use with a bar code reader. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    An exemplary embodiment of an imaging-based bar code reader of the present invention is shown schematically at  10  in the Figures. The bar code reader  10  includes an imaging system  12  ( FIG. 3 ) and a decoding system  14  supported in a housing  16 . The imaging and decoding systems  12 ,  14  are capable of reading, that is, imaging and decoding both 1D and 2D bar codes and postal codes. The present disclosure emphasizes a reader  10  that reads 1D bar codes  34  affixed to a target object  32 . Such a reader  10  is configured as a linear imager for capturing only a narrow pixel array. 
         [0018]    The decoding system  14  is adapted to decode encoded indicia within a selected captured image frame. The housing  16  supports reader circuitry  11  within an interior region  17  of the housing  16 . The reader circuitry  11  includes a microprocessor  11   a  and a power supply  11   b.  The power supply  11   b  is electrically coupled to and provides power to the circuitry  11 . The housing  16  also supports the imaging and decoding systems  12 ,  14  within the housing&#39;s interior region  17 . The depicted reader  10  includes a docking station  30  adapted to receive the housing  16 . The docking station  30  and the housing  16  support an electrical interface to allow electric coupling between circuitry resident in the housing  16  and circuitry resident in the docking station  30 . 
         [0019]    The imaging and decoding systems  12 ,  14  operate under the control of the microprocessor  11   a.  The imaging and decoding systems  12 ,  14  may be separate assemblies which are electrically coupled or may be integrated into a single imaging and decoding system. When removed from the docking station  30  of the reader  10 , power is supplied to the imaging and decoding systems  12 ,  14  by the power supply  11   b.  The circuitry of the imaging and decoding systems  12 ,  14  may be embodied in hardware, software, firmware or electrical circuitry or any combination thereof. Moreover, portions of the circuitry  11  may be resident in the housing  16  or the docking station  30 . 
         [0020]    In a hand-held or point-and-shoot mode of operation ( FIG. 2 ), the reader  10  is carried and operated by a user walking or riding through a store, warehouse or plant for reading target bar codes for stocking and inventory control purposes. In the hand-held mode, the housing  16  is removed from a docking station  30  so the reader  10  can be carried by the user. The user grasps a housing gripping portion  16  a and positions the housing  16  with respect to the target bar code  34  such that the target bar code is within a field of view of the imaging system  12 . 
         [0021]    In the hand-held mode, imaging and decoding of the target bar code  34  is instituted by the user depressing a trigger switch  16   e  which extends through an opening near the upper part  16   c  of the gripping portion  16   a.  When the trigger  16   e  is depressed, the imaging system  12  generates a series of image frames ( 54   a - 54   f  for example) until either the user releases the trigger  16   e,  an image  34 ′ of one frame ( 54   d  for example) the target bar code  34  has been successfully decoded or a predetermined period of time elapses, whereupon the imaging system  12  awaits a new trigger signal. 
         [0022]    In a fixed position or hands-free mode ( FIG. 1 ), the reader  10  is received in the docking station  30  which is positioned on a substrate, such as a table or counter  19 . The docking station  30  is plugged into an AC power source and provides regulated DC power to the circuitry  11  of the reader  10 . The bar code reader  10  includes an illumination system  36  to illuminate the target bar code  34  with an illumination/aiming light pattern  40 . The illumination system  36  typically includes one or more illumination LEDs  38  which are energized to direct illumination light to a reflecting mirror  42  which reflects light through a lens  44  to the bar code to form the illumination/aiming pattern  40  which can be aligned by the user with respect to the bar code  34 . A center line  40   a  of the target pattern  40  from the illumination system  36  can be moved from side to side and up and down as the user manipulates the scanner. 
         [0023]    The aiming pattern forms a line of illumination having a width W and length L. When imaging a 2D bar code, the reader uses a sensor having a large number of pixels in two orthogonal directions. The aiming pattern could have use with a raster scanner bar code reader as well. This construction using a light source with an oscillating mirror that scans vertically across a bar code. The aiming pattern may distort the imaged bar code and complicate the decoding of the imaged bar code so that the aiming system may be intermittently energized in a flash mode such that at least some of the captured image frames  54   a - 54   f  do not include an image of the aiming pattern  40 . 
         [0024]    The imaging system  12  has an imaging camera assembly  20  and associated imaging circuitry  22 . The imaging camera  20  includes a housing  24  supporting focusing optics including a focusing lens  26  and a sensor or pixel array  28 . The sensor array  28  is enabled during an exposure period to capture image pixels. The field of view of the imaging system  12  is a function of both the configuration of the sensor array  28  and the optical characteristics of the focusing lens  26 . For a linear imager, the field of view is a narrow swatch of pixels in one direction, possible only one pixel wide. 
         [0025]    The camera housing  24  is positioned within an interior region  17  of the scanning head  16   b.  The housing  24  is in proximity to a transparent window  50  defining a portion of a front wall  16   h  of the housing scanning head  16   b.  Reflected light from the target bar code  34  passes through the transparent window  50 , is received by the focusing lens  26  and focused onto the imaging system sensor array  28 . 
         [0026]    In an exemplary embodiment, the illumination assembly  36  of the LED  38  and the mirror  42  are positioned behind the window  50 . Illumination from the illumination LED  38  and an aiming pattern also pass through the window  50 . 
         [0027]    The imaging system  12  includes the sensor array  28  of the imaging camera assembly  20 . The sensor array  28  comprises a charged coupled device (CCD), a complementary metal oxide semiconductor (CMOS), or other imaging pixel array, operating under the control of the imaging circuitry  22 . In the hand-held mode of operation, (possibly aided by the aiming system), the user points the housing  16  at the target bar code  34  and, assuming the target bar code  34  is within the field of view FV of the imaging module  12 , each image frame  54   a,    54   b,    54   c,  . . . of the series of image frames  54  includes an image  34 ′ of the target bar code  34  (shown schematically in  FIG. 4 ). The decoding system  14  selects an image frame from the series of image frames  54  and attempts to locate and decode a digitized version of the image bar code  34 ′. 
         [0028]    Electrical signals are generated by reading out some or all of the pixels of the pixel array  28  after an exposure period generating an analog signal  56  ( FIG. 3 ). 
         [0029]    The analog image signal  56  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  46  is amplified by a gain factor, generating an amplified analog signal  58 . The imaging circuitry  22  further includes an analog-to-digital (A/D) converter  60 . The amplified analog signal  58  is digitized by the A/D converter  60  generating a digitized signal  62 . The digitized signal  62  comprises a sequence of digital gray scale values  63  typically ranging from 0-255 (for an eight bit A/D converter, i.e., 2 8 =256), where a 0 gray scale value would represent an absence of any reflected light received by a pixel during an exposure or integration period (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 exposure period (characterized as high pixel brightness). 
         [0030]    The digitized gray scale values  63  of the digitized signal  62  are stored in a memory  64 . The digital values  63  corresponding to a read out of the pixel array  28  constitute the image frame  54 , which is representative of the image projected by the focusing lens  26  onto the pixel array  28  during an exposure period. If the field of view FOV of the imaging assembly  24  includes the target bar code  34 , then a digital gray scale value image  14 ′ of the target bar code  34  would be present in the image frame  54 . 
         [0031]    The decoding circuitry  14  then operates on the digitized gray scale values  63  of the image frame  54  and attempts to decode any decodable image within the image frame, e.g., the imaged target bar code  14 ′. If the decoding is successful, decoded data  66 , representative of the data/information coded in the bar code  34  is then output via a data output port  67  and/or displayed to a user of the reader  10  via a display  68 . Upon achieving a good “read” of the bar code  34 , that is, the bar code  34  was successfully imaged and decoded, a speaker  70  and/or an indicator LED  72  is activated by the bar code reader circuitry  13  to indicate to the user that the target bar code  14  has successfully read, that is, the target bar code  34  has been successfully imaged and decoded. 
       Aiming Pattern 
       [0032]      FIGS. 6 and 7  illustrate use of a curved mirror  42 , light source such an LED  38  and lens  44  for creating a rectangular aiming pattern  40 . The illumination system  36  of  FIGS. 4 and 5  have no mirror and show an image of a slit aperture  110  of a screen  112  and projected into the far field using a lens  44  with curvatures in tangential (vertical or y direction as seen in  FIG. 4 ) and sagittal (x-z) planes to create a slit-like illumination pattern  40  within a reader field of view focused at a distance D from the screen  112 . The lens  44  is configured to focus diverging light  120  ( FIG. 4 ) from the narrow side of the slit in the tangential direction, thus creating small y-spot radius with small y-field of view to cover the vertical field of view of an imager assembly  24  having a sensor only one or a few pixels wide. It is desired that the lens  44  be as far away as possible from the aperture  110  to match the numerical aperture of the lens to the width of the aperture to maximize the light throughput. 
         [0033]    The diverging light  130  ( FIG. 5 ) emitted from the long side of the slit aperture is further diverged (in the x direction) and optimized to provide uniform, and wide angle illumination  132  to match the imaging field of view for different size barcodes. Since the width of the beam is greater, this means that the clear aperture of the lens also needs to be larger, and the resulting size of the lens is not compact and typically will not conform to mechanical constraints of a typical bar code reader. 
         [0034]    An alternate approach is to use a shorter focal distance in the saggital direction but this would imply that the lens needs to move closer to the aperture or a substantially thick lens is used. Unfortunately, moving the lens  44  closer to the screen  112  contradicts the requirement for the tangential case that a a longer focal length is desired, and making the lens thick (typically tapered) would either create total internal reflections within the lens element itself that corrupt the angular spread and the uniformity of the illumination pattern, or make the entrance face too small so much of the light is truncated and lost. 
         [0035]    The exemplary system depicted in  FIG. 7  has a mirror  42  that is curved in the saggital direction to better match the numerical aperture of the source and constrain its angular extent so that the light throughput is maximized for both the sagittal and tangential direction when projected from the slit  110 . This system retains the compactness of the illumination system. The preferred mirror is spherical, aspherical, biconic, toroidal or polynomial. One or more set ups can be integrated together to provide a desired radiant flux (power seen by the solid state detector) or luminous flux (power perceived by the human eye). 
         [0036]    Advantages of use of the mirror  42  are depicted in  FIG. 6 . In that figure a light source  38  such as an LED directs diverging light toward a slit aperture  110 . An amount of light passes through the aperture but is still diverging. The positive element  113  focuses light toward a centerline  114 . Without this positive element much of the light would be unusable and miss a lens  44 , for example, which further bends the light downstream from the positive element. 
         [0037]    Returning to  FIG. 7 , the mirror  42  has a reflecting surface  42   a  such as a surface coated with a reflective material or a plastic or glass element with features to reflect light by the means of total internal reflection. Light striking the surface  42   a  is reflected off from the surface at an angle defined by the angle at which it strikes the surface and the shape of the surface. The lens  44  has an entrance surface  44   a  and an exit surface  44   b  spaced apart a distance H. The path of a representative light beam is bent in accordance with the shape of these two surfaces where the beam enters and exits the as well as a length L of the lens  44 . 
         [0038]    In the exemplary embodiment of the disclosure the surface  42   a,  the surface  44   a,  and the surface  44   b  are all toroidal surfaces or they approximate toroidal surfaces. In the embodiment of  FIG. 7 , the mirror surface  42   a  is a segment of a cylinder which is a special case of a toroidal surface having a rotation radius of zero. 
         [0000]    Toroidal surfaces 
         [0039]    Toroidal surfaces are formed by defining a curve in the Y-Z plane, and then rotating this curve about an axis parallel to the y axis ( FIG. 4 ) and intersecting the z axis. Toroids are defined using a base radius of curvature c, in the Y-Z plane, as well as a conic constant k, and polynomial aspheric coefficients. The curve in the Y-Z plane is defined by: (note, rotation radius=0 for cylinder) 
         [0000]    
       
         
           
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         [0000]    This curve is then rotated about an axis a distance R from the vertex. This distance R is referred to as the radius of rotation, and may be positive or negative. Through suitable choices of the coefficients for this generating curve, the combination of the mirror and the lens can be adjusted to produce a suitable aiming/illumination light pattern at a desired focal length from the reader. One suitable structure has an entrance surface  44   a  constructed using a radius of curvature=0.0 mm, a rotation radius of 100 mm and a 2 =−2.90×10 −3 . The exit surface  44   b  is constructed using a radius of curvature c of 6.7 mm, a rotation radius of −20 mm, a 4 =−2.04×10 −3 . The lens  44  has a height of 2.5 mm, width of 10 mm and thickness of 4.0 mm. 
         [0040]    While a preferred embodiment of the invention has been described with a degree of particularity, it is the intent that the invention include all modifications and alterations from the disclosed design falling within the spirit or scope of the appended claims.