Abstract:
An exemplary barcode reader includes an imaging system that includes a light monitoring pixel array for converting light reflected from a target into electrical signals, 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 said pixel array. An illumination system comprising a light source for illuminating a the target within a field of view defined by the optical system. A filter is disposed adjacent the focusing lens and passing illumination with a wavelength less than about 700 nanometers to the pixel array and impedes the passage of light having a wavelength greater than this value.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a filter for an imaging-based bar code reader. 
       BACKGROUND 
       [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 having differing light reflecting characteristics. The pattern of the bars and spaces encode information. In certain bar codes, there is a single row of bars and spaces, typically of varying widths. Such bar codes are referred to as one dimensional bar codes. Other bar codes include multiple rows of bars and spaces, each typically having the same width. Such bar codes are referred to as two dimensional bar codes. Devices that read and decode one and two dimensional bar codes utilizing imaging systems that image and decode imaged bar codes are typically referred to as imaging-based bar code readers or bar code scanners. 
         [0003]    Imaging systems include charge coupled device (CCD) arrays, complementary metal oxide semiconductor (CMOS) arrays, or other imaging pixel arrays having a plurality of photosensitive elements or pixels. An illumination system comprising light emitting diodes (LEDs) or other light source 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 imaging system onto the pixel array. Thus, an image of a field of view of the focusing lens is focused on the pixel array. Periodically, the pixels of the array are sequentially read out 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 of the imaging system processes the digitized signals and attempts to decode the imaged bar code. 
         [0004]    United States published patent application entitled “Ambient Light Shield and Color Filter for Imaging-Based Bar Code Reader”, publication no 2007/0199996 describes an ambient illumination shielding apparatus. That application also describes a filter disposed in proximity to an imaging system that passes illumination within a predetermined wavelength range to a sensor pixel array. This published patent application is incorporated herein by reference. 
       SUMMARY 
       [0005]    Many CMOS sensors are sensitive to deep red and infrared wavelengths. This high sensitivity is undesired because imaging lens are optimized for visible light of shorter wavelength. Since infrared light has longer wavelength than visible light, the lenses tend to focus at greater lengths resulting in color separating and bigger diffraction spot size. In addition, longer wavelengths deplete farther in the pixel substrate, resulting in more leakage current, and thus reduce effective pixel resolution per sensor module. Moreover, longer wavelengths result in pixel cross talk since they are focused at a greater distance. The net result is reduced image contrast making the resultant image less sharp. 
         [0006]    An exemplary barcode reader includes an imaging system that includes a light monitoring pixel array for converting light reflected from a target into electrical signals, 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 said pixel array. An illumination system comprising a light source for illuminating a the target within a field of view defined by the optical system. 
         [0007]    A filter is disposed in proximity to the imaging system for passing illumination within a predetermined wavelength range to the pixel array and impeding the passage of illumination outside of the predetermined wavelength range. The exemplary filter passes light in the visible range and impedes or blocks the passage of light in the infrared range. 
         [0008]    These and other objects, advantages, and features of the exemplary embodiment of the invention are described in detail in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic side elevation view of an imaging-based bar code reader of the present invention; 
           [0010]      FIG. 2  is a schematic front elevation view of the imaging-based bar code reader of  FIG. 1 ; 
           [0011]      FIG. 3  schematic sectional view of a portion of the imaging-based bar code reader of  FIG. 1  showing the scanner head and one embodiment of an ambient illumination shielding apparatus of the present invention; 
           [0012]      FIG. 4  is a block diagram of an imaging-based bar code reader of  FIG. 1 ; 
           [0013]      FIG. 5  is a graph showing quantum efficiency for a CMOS imaging sensor; and 
           [0014]      FIGS. 6-8  depict a bar code reader  10  lens assembly having a filter coated on to a sensor facing surface of that lens assembly. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    An imaging-based reader, such as an imaging-based bar code reader, is shown schematically at  10  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. The bar code reader  10  includes an imaging system  20  and a decoding system  40  ( FIGS. 3 and 4 ) for capturing image frames of a field of view FV of the imaging system  20  and decoding encoded indicia within a captured image frame. The bar code reader  10  includes a housing  11  supporting the imaging and decoding systems  20 ,  40  within an interior region of the housing  11 . 
         [0016]    The imaging system  20  has an imaging camera assembly  22  and associated imaging circuitry  24 . The imaging camera  22  includes a housing  25  supporting a focusing lens  26  and an imager  27  comprising a pixel array  28 . The imager  27  is enabled during an exposure period to capture an image of the field of view FV of the focusing lens  26 . 
         [0017]    In one preferred embodiment of the present invention, the bar code reader  10  is a hand held portable reader encased in the pistol-shaped housing  11  adapted to be carried and maneuvered by a user. As is best seen in  FIGS. 1 and 2 , the bar code reader housing  11  includes a generally upright gripping portion  11   a  adapted to be grasped by a user&#39;s hand and a horizontally extending scanning head  11   b  which supports the imaging assembly  20 , an illumination assembly  60  and an aiming apparatus  70 . At the intersection of gripping portion  11   a  and the scanning head  11   b  is a trigger  12  coupled to bar code reader circuitry  13  for initiating reading of target indicia, such as a target bar code  14 , when the trigger  12  is pulled or pressed. The bar code reader circuitry  13 , the imaging system  20  and the decoding circuitry  40  are coupled to a power supply  16 , which may be in the form of an on-board battery or a connected off-board power supply. If powered by an off-board power supply, the scanner  10  may be a stand-alone unit or have some or all of the scanner&#39;s functionality provided by a connected host device. When actuated to read the target bar code  14 , the imaging system  20  images the target bar code  14  and the decoding system  40  decode a digitized image  14 ′ (shown schematically in  FIG. 4 ) of the target bar code  14 . 
         [0018]    The imaging system  20  includes the imaging circuitry  24  and decoding circuitry  40  for decoding the imaged target bar code  14 ′ (shown schematically in  FIG. 4 ) within an image frame  42  stored in a memory  44 . The imaging and decoding circuitry  24 ,  40  may be embodied in hardware, software, firmware, electrical circuitry or any combination thereof. The imaging circuitry  24  may be disposed within, partially within, or external to the camera assembly housing  25 . Shown schematically in  FIG. 4 , the imaging camera housing  25  is supported with the scanning head  11   b  of the housing  11  and receives reflected illumination from the target bar code  14  through a transparent window  17  supported by the scanning head  11   b . The focusing lens  26  is supported by a lens holder  26   a . The camera housing  25  defines a front opening  25   a  that supports and seals against the lens holder  26   a  so that the only illumination incident upon the sensor array  28  is illumination passing through the focusing lens  26 . Depending on the specifics of the camera assembly  22 , the lens holder  26   a  may slide in and out within the camera housing front opening  25   a  to allow dual focusing under the control of the imaging circuitry  24  or the lens holder  26   a  may be fixed with respect to the camera housing  25  in a fixed focus camera assembly. The lens holder  26   a  is typically made of metal. A back end of the housing  25  may be comprised of a printed circuit board  25   b , which forms part of the imaging circuitry  24  and may extend beyond the housing  25  to support the illumination system  60  and the laser aiming apparatus  70 . 
         [0019]    The imaging system  20  includes the imager  27  of the imaging camera assembly  22 . The imager  27  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  24 . In one exemplary embodiment, the pixel array  28  of the CCD imager  27  comprises a two dimensional (2D) mega pixel array with a typical size of the pixel array being on the order of 1280×1024 pixels. The pixel array  28  is secured to the printed circuit board  25   b , in parallel direction for stability. 
         [0020]    As is best seen in  FIG. 3 , the focusing lens  26  focuses light reflected from the target bar code  14  through an aperture  26   b  onto the pixel/photosensor array  28  of the CCD imager  27 . Thus, the focusing lens  26  focuses an image of the target bar code  14  (assuming it is within the field of view FV) onto the array of pixels comprising the pixel array  28 . Electrical signals are generated by reading out of some or all of the pixels of the pixel array  28  after an exposure period. After the exposure time has elapsed, some or all of the pixels of pixel array  28  are successively read out thereby generating an analog signal  46  ( FIG. 4 ). In some sensors, particularly CMOS sensors, all pixels of the pixel array  28  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. 
         [0021]    The analog image signal  46  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  48 . The imaging circuitry  24  further includes an analog-to-digital (A/D) converter  50 . The amplified analog signal  48  is digitized by the A/D converter  50  generating a digitized signal  52 . The digitized signal  52  comprises a sequence of digital gray scale values  53  typically ranging from 0-255 (for an eight bit processor, i.e., 28=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). 
         [0022]    The digitized gray scale values  53  of the digitized signal  52  are stored in the memory  44 . The digital values  53  corresponding to a read out of the pixel array  28  constitute the image frame  42 , 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 FV of the focusing lens  26  includes the target bar code  14 , then a digital gray scale value image  14 ′ of the target bar code  14  would be present in the image frame  42 . 
         [0023]    The decoding circuitry  40  operates on the digitized gray scale values  53  of the image frame  42  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  56 , representative of the data/information coded in the bar code  14  is then output via a data output port  57  and/or displayed to a user of the reader  10  via a display  58 . Upon achieving a good “read” of the bar code  14 , that is, the bar code  14  was successfully imaged and decoded, a speaker  59   a  and/or an indicator LED  59   b  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  14  has been successfully imaged and the imaged bar code  14 ′ has been successfully decoded. 
         [0024]    The bar code reader  10  further includes the illumination assembly  60  for directing a beam of illumination to illuminate the target bar code  14  and the aiming apparatus  70  for generating a visible aiming pattern  72  ( FIG. 5 ) to aid the user in properly aiming the reader at the target bar code  14 . The illumination assembly  60  and the aiming apparatus  70  operate under the control of the imaging circuitry  24 . As can best be seen in  FIGS. 2 and 3 , in one preferred embodiment, the illumination assembly  60  is a single LED  62  producing a wide illumination angle to completely illuminate the target bar code  14 . 
         [0025]    The LED  62  is supported within the scanning head  11   b  just behind the transparent window  17  and face forwardly, that is, toward the target bar code  14 . The LED  62  is positioned away from the focusing lens  26  to increase the illumination angle (shown schematically as I in  FIG. 4 ) produced by the LED  62 . Preferably, the illumination provided by the illumination assembly  60  is intermittent or flash illumination as opposed to continuously on illumination to save on power consumption. 
         [0026]    In one exemplary embodiment, the aiming apparatus  70  is a laser aiming apparatus. The aiming pattern  72  may be a pattern comprising a single dot of illumination, a plurality of dots and/or lines of illumination or overlapping groups of dots/lines of illumination ( FIG. 5 ). The laser aiming apparatus  70  includes a laser diode  74 , a focusing lens  76  and a pattern generator  77  for generating the desired aiming pattern  77 . The laser diode  74 , the lens  76  and the pattern generator are supported by a lens holder  78  which extends from the printed circuit board  25   b . The aiming apparatus  70  is supported in the scanning head  11   b  and the aiming pattern exits the head through the transparent window  17 . 
         [0027]    Operating under the control of the imaging circuitry  24 , when the user has properly aimed the reader  10  by directing the aiming pattern  72  onto the target bar code  14 , the aiming apparatus  70  is turned off when an image of the target bar code  14  is acquired such that the aiming pattern  72  does not appear in the captured image frame  42 . Intermittantly, especially when the scanner imaging circuitry  24  is transferring the captured image frame  42  to memory  44  and/or when processing the image, the aiming apparatus  70  is turned back on. If the decoding circuitry  40  cannot decode the imaged bar code  14 ′ and the user in the mean time has not released the trigger  12 , the process of acquiring an image of the target bar code  14  set forth above is repeated. 
       Infrared Filter 
       [0028]    Infrared light has longer wavelengths than visible light and less photon energy. For a fixed amount of energy there are more infrared photons than visible photons. Assuming the quantum efficiency curve is flat; this would mean higher sensor sensitivity for infrared light. The curve in  FIG. 5  shows typical quantum efficiency for a CMOS sensor. Notice that when it is superimposed on top of a linearly increasing photon count curve for longer wavelength light, the resulting sensor sensitivity is higher for longer wavelength light. 
         [0029]    If the lens  26  is not optimized for both visible and infrared light, light with longer wavelengths will focus farther than those with short wavelengths. Thus, the effective spot size is larger and image contrast is lower. The situation is worse with higher sensor sensitivity for longer wavelength light. 
         [0030]    Moreover, long wavelength light, particularly infrared, will deplete farther in the pixel substrate, resulting in more leakage current. This will reduce effective pixel resolution or pixel per module. Sensors have also pixel crosstalk issues as result of the use of lenslet arrays that focus steep angle light bundles and missing the proper pixel. This issue is also made worse with longer wavelength light since it focuses farther than light in the visible range. An infrared cutter or short-pass filter  34  reduces exposure to long wavelength light, in particular the infrared light. 
         [0031]    The exemplary bar code reader  10  has a filter  34  positioned between the focusing lens  26  and the photosensor array  28 . Positioning the filter  34  in space between the photosensor array  28  and the focusing lens  26  does not detrimentally affect the functioning of the focusing lens  26  (although the lens  26  may have to be positioned slightly further away from the photosensor array  28  to maintain the same focus onto the photosensor array  28 ). The filter  34  would most preferably block light having a wavelength of greater than 700 nm. An appropriate interference filter may be obtained from various optical suppliers such as Edmund Optics, Barrington, N.J. 08007 (www.edmundoptics.com). 
         [0032]    The filter  34  is shown in  FIGS. 3 and 4  located between the focusing lens  26  and the photosensor array  28 . It should be appreciated, however, that the filter  34  may be disposed upstream, that is, outwardly of the focusing lens  26 . Additionally, the filter  34  may be incorporated into the transparent window  17  (or a portion of the transparent window  17  adjacent the reader housing) thereby eliminating the need for having two separate components for the window  17  and the filter  34 . 
       Imaging Lens Assembly  130   
       [0033]    A presently preferred bar code reader  10  has a filter coated on to a sensor facing surface of a lens assembly  130  shown in  FIGS. 6-8 . The focusing lens assembly  130  focuses light reflected and scattered from the object of interest such as the target bar code  14  onto the sensor array  28 , thereby focusing an image of the target bar code  14  (assuming it is within the field of view FV) onto the sensor array  28 . The imaging lens assembly  130  depicted in the figures is advantageously compact. A similar compact lens assembly is a described more fully in pending U.S. patent application Ser. No. 11/731,835 entitled “Compact Imaging Lens Assembly for an Imaging Based Bar Code Reader” filed Mar. 30, 2007 which is assigned to the assignee of the present application and which is incorporated herein by reference. 
         [0034]    As seen in the Figures, the imaging lens assembly  130  includes four lenses  132 ,  133 ,  134 ,  135  and an intermediate front aperture stop  136  mounted in a holder  140 . The aperture stop  131  defines a circular or rectangular opening, which limits the light impinging upon the sensor array  28 . Additional details of the functionality of a similar lens assembly is described in the aforementioned pending United States patent application. 
         [0035]    The four lenses  132 - 135  of the lens assembly  130  are supported in a generally cylindrical lens holder  140 , which may be fabricated of metal or plastic. The lens holder  37 , in turn is supported by the camera housing  25  which extends to the printed circuit board  25   b . In addition to supporting the lens holder  37 , the camera housing protects the sensor array  28  from ambient illumination. Annular seals  142 - 144  adhesively seal the lenses to the holder  140 . 
         [0036]    The rearmost lens includes an outer coating  150  that covers an entire generally planar rear surface of the lens assembly  130 . The coating  150  is applied using the techniques of thin-film filter fabrication. A thin film filter is a multi-layer, light filtering coating that is built up layer by layer on a substrate such as clear plastic by evaporative deposition or other method. When complete, the thin film coating has appropriate wavelength blocking characteristics. Specifics on fabricating a thin film bandpass filter may be found in a book entitled  Thin - Film Optical Filters,  3 rd  Edition, by H. Angus Macleod, Institute of Physics Publishing, Dirac House, Temple Back, Bristol, UK Bs1 6BE, copyright 2110, ISBN 0 7503 06882. The aforementioned book is incorporated in its entirety herein by reference. 
         [0037]    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 within the spirit or scope of the appended claims.