Abstract:
An imaging-based bar code reader that includes an imaging and decoding system. Focusing optics and a sensor array define a field of view. An exemplary system has an image sensor includes multiple configuration registers. With multiple configuration registers a video mode can be implemented where the video consists of a continuous sequence of frames with a fixed sequence a number of configurations. The configurations can vary the frame size, exposure time, gain, etc. Compared to a sensor with only one set of configuration registers, successive frames can be captured with different configurations without synchronization issues or frame lag.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an imaging-based bar code reader and, more particularly, to a bar code reader that facilitates capturing images. 
       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 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 (1D) bar codes. Other bar codes include multiple rows of bars and spaces, each row typically having the same width. Such bar codes are referred to as two dimensional (2D) bar codes. 
         [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 decodes the imaged bar code. 
         [0004]    Existing two dimensional imaging scanners use image sensors such as sensors provided by Micron Technology Inc. A product specification entitled “½ Inch 1.3 Megapixel CMOS Active-Pixel Digital Image Sensor” (copyright, 2003) describes part no. MT9M001 and is incorporated herein by reference. Such sensors produce an electronic image of the focused light from a target such as a label. Prior art sensors include a set of programmable registers that can store one sensor configuration for a specific operation. Common registers include bits that set the exposure time (length of time over which the image is collected), electronic gain, row start, column start, row sizes and column size. The row and column start and size are used to specify the rectangular region of interest to be read from the 2D sensor array. For example, a 640×480 sensor has 640 columns and 480 rows. The sensor can be configured to start reading at column 50, row 100 and read 200 columns and 300 rows. This would produce an image size of 200×300 (60,000 pixels) that start with pixel 50, 100 of the array. 
         [0005]    An image sensor operating in video mode with only a single set of configuration registers produces a sequence of images sharing one configuration. If multiple configurations are desired, then the configuration registers must be reprogrammed during an image capture sequence and there is typically one or more frame delays before an image with the updated configuration is produced. This presents a synchronization issue because the timing of the register updates may be difficult to predict. 
         [0006]    Automatic exposure control (AEC) is used in a 2D imager for determining the proper exposure time to collect an image with good contrast. To perform AEC with a prior art system, an image is acquired, analyzed and the configuration is updated with an estimate of the best exposure. The system iterates until a good exposure is found. 
       SUMMARY OF THE INVENTION 
       [0007]    An exemplary system has an image sensor that includes multiple configuration registers. With multiple configuration registers a video mode can be implemented where the video consists of a continuous sequence of frames with a fixed sequence of a number of configurations. The configurations can vary the frame size, exposure time, gain, etc. Compared to a sensor with only one set of configuration registers, successive frames can be captured with different configurations without synchronization issues or frame lag. 
         [0008]    An exemplary system for evaluating a target image includes an imaging and decoding system for capturing a sequence of target images at a frame rate from a target including focusing optics defining a field of view for focusing reflected illumination from an image. A sensor array defines an array of picture elements including a plurality of configuration memories for storing image capture configuration instructions. A processor has a memory for storing an image gathered from the sensor array from a target located within the field of view and programming the configuration memories. 
         [0009]    These and other objects, advantages, and features of the exemplary system are described in detail in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a perspective view of a bar code scanner supported on a stationary stand; 
           [0011]      FIG. 2  is a schematic sectional view of a portion of the imaging-based bar code reader showing the scanner head; 
           [0012]      FIG. 3  is a block circuit diagram of the imaging-based bar code reader of  FIG. 1 ; 
           [0013]      FIG. 4  is a block diagram of an image sensor used with the exemplary system; and 
           [0014]      FIG. 5  is a timing diagram showing capture of two successive image frames that are captured in timed relation to a target illumination as shown. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    An imaging-based scanner or reader that is capable of reading bar codes is shown schematically at  10  in the Figures. One example of a 2D bar code  14  is shown in  FIG. 3 . Additionally, the reader  10  is also capable of capturing images such as an image of a document  12 . The bar code reader  10  includes a housing  11  supporting an imaging system  20  and a decoding system  40  ( FIG. 3 ). The housing  11  supports a transparent window  17  through which reflected illumination from the target document is received by the imaging system  20 . 
         [0016]    When enabled, the imaging system  20  captures an image frame  42  of a field of view FV of the imaging system which is stored in a memory  44 . The imaging process captures an image of the target bar code. The decoding system  40  analyzes a captured image frame  42  and attempts to decode decodable portions of the image frame  42 . The decoded portions of the image frame  42  are stored in a buffer memory  44   a.  Alternately, a series of image frames  43  are captured and using a stitching method the decoding system  40  attempts to combine or stitch the decoded portions stored in buffer memory to achieve a full decode of the document  12 . 
         [0017]    The imaging system  20  includes an imaging camera  22  ( FIG. 2 ) and associated imaging circuitry  24 . The imaging camera  22  includes a housing supporting focusing optics including a focusing lens  26  and a 2D photosensor or pixel array  28 . The imaging camera  22  is coupled to a controller  101  and may be enabled during an imaging session to capture a sequence of images having different characteristics from the field of view FV of the focusing lens  26 . 
         [0018]    In one mode of operation, the bar code reader  10  is a hands-free reader including a housing having a flat base portion that can be placed on a counter or tabletop. The scanner  10  of  FIG. 1  is supported by a support stand  100 . When so mounted, the exposure operation mode of the camera can be altered to enhance the image quality of the resulting image produced by the scanner  10 . 
         [0019]    The housing  11  defines an interior region  11   a.  Disposed within the interior region are the imaging and decoding systems  20 ,  40  and an illumination assembly  60  including one or more light emitting diodes  62 . When enabled, the LEDs  62  direct illumination through the transparent window  17  and onto a target. Circuitry  13  within the housing  11  is electrically coupled to a power supply, which may be in the form of an on-board battery or a connected off-board power supply. If powered by an on-board battery, the reader  10  may be a stand-alone, portable unit. If powered by an off-board power supply, the reader  10  may have some or all of the reader&#39;s functionality provided by a connected host device. 
         [0020]    Circuitry associated with the imaging and decoding systems  20 ,  40 , including the imaging circuitry  24 , may be embodied in hardware, software, electrical circuitry or any combination thereof and may be disposed within, partially within, or external to the camera assembly housing  25 . In the illustrated embodiment, the functions of the reader are controlled and co-ordinated by a microprocessor controller  101 . The controller  101  also manages outputs from the decoding system  40  such as an output  56  to a display  58  and communications output port  57  (coupled to a cable  104 ) and visual and audible signals from an LED  59   b  and speaker  59   a.  The imaging camera housing  25  is supported with an upper or scanning head portion  11   c  of the housing and receives reflected illumination from the target document through the transparent window  17  supported by the scanning head  11   c.  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 . 
         [0021]    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  24   b,  which forms part of the imaging circuitry  24  and may extend beyond the housing  25  to support the illumination system  60 . 
         [0022]    The imaging system  20  includes the sensor array  28  which may comprise a charged coupled device (CCD), a complementary metal oxide semiconductor (CMOS), or other imaging pixel array, operating under the control of the imaging circuitry  24 . In one exemplary embodiment, the pixel array  28  comprises a two dimensional (2D) mega pixel array with a typical size of the pixel array being on the order of 1280×1024 pixels. 
         [0023]    As is best seen in  FIG. 2 , the focusing lens  26  focuses light reflected from the target bar code  14  through an aperture  26   b  onto the pixel/photosensor array  28 . Thus, the focusing lens  26  focuses an image of the target document within the field of view FV onto the array of pixels comprising the pixel array  28 . The focusing lens  26  field of view FV includes both a horizontal and a vertical field of view, the vertical field of view being shown schematically as FV in  FIG. 2 . 
         [0024]    During an imaging session, one or more images in the field of view FV of the reader  10  may be obtained by the imaging system  20 . An imaging session may be instituted by an operator, for example, pressing a trigger to institute an imaging session. Alternately, the imaging system  20  may institute an imaging session when a lower or bottom edge of the item  15  moves through an upper portion of the field of view FV. Yet another alternative is to have the imaging system  30  always operational. In such a video mode image after image is captured and analyzed for the presence of data within an imaged target. In any event, the process of capturing an image  42  of the field of view FV during an imaging session is known in the scanner art. 
         [0025]    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 . 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 other pixels from a different part of the array  28 . 
         [0026]    The analog image signal  46  from the pixel array 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., 2 8 =256), where a 0 gray scale value would represent an absence of any reflected light received by a pixel (characterized as low pixel brightness) and a 255 gray scale value would represent a very intense level of reflected light received by a pixel during an integration period (characterized as high pixel brightness). 
       Frame Configurations 
       [0027]    The imaging and decoding systems  20 ,  40  of the exemplary scanner  10  can capture a sequence of target images at a video frame rate from a target. The reader includes a sensor S ( FIG. 4 ) that defines the array of picture elements  28  and includes a plurality of individually configurable memories or registers  150 ,  152 ,  154  for storing image capture configuration instructions. 
         [0028]    As noted above, the sensor S responds to the controller  101  in configuring these registers or alternatively may include its own sensor controller. The memory  44  stores an image gathered from the sensor array from a target located within the field of view. The processor  101  executes a control program that updates the configuration memories for capturing images having different characteristics. Different image frames from the field of view are then captured at least as fast as the video frame rate of the imaging and decoding system. 
         [0029]    With multiple configurations, the preferred embodiment of the system cycles through a sequence of exposures over a range and the information collected from multiple regions of interest (ROI) can be used, for example, to rapidly compute the best exposure. The same method can be used to adjust gain, or the gain/exposure can be adjusted concurrently. This has the effect of dramatically decreasing the automatic exposure control (AEC) response time. 
         [0030]    In many cases, such as under normal office lighting conditions, a scanner requires the use of active illumination. It is highly desirable that whether illumination must be used be determined together with AEC. For determination of whether such illumination is required, a test flash from the LED  62  can be used on some parts of the image only. The test flash is timed such that it can be “seen” by some pixels on the sensor array, while cannot be seen by others. For example for many sensors the different pixels must finish their exposure at the same time. We can therefore provide timing signals such that pixels in different ROIs start their exposures at different times, but finish together. This is co-ordinated with an illumination pulse that occurs at the beginning of the longer exposure times, thus masking its influence on some pixels. 
         [0031]    The timing diagram of  FIG. 5  illustrates this use.. Here ROI  1  would expose with the illumination is on, and ROI 2  with it off. If the ambient is dark and illumination is required, ROI  2  would be significantly under exposed, while ROI  1  may be properly exposed. To test for other conditions, more ROIs can be used with different gain values. For example, a third region of interest may be used with the same exposure timing as ROI  2 , but with twice the gain. 
         [0032]    Motion detection is sometimes used to determine the presence of an object to be scanned. Multiple regions of interest (ROI) are used to detect motion of an object into the field of view. The performance of this type of system is dramatically improved by programmed a sequence of ROI&#39;s into the sensor such that any delays or synchronization issues due to reprogramming are avoided. 
         [0033]    Returning to  FIG. 4 , the sensor S includes a serial interface  162  through which the controller  101  programs the multiple banks of registers  150 ,  152 ,  154 . Data is presented at an input  166  and this data is clocked into a buffer which is coupled to the three registers  150 ,  152 ,  154 . Although three registers are depicted it is appreciated that more such registers can be utilized. The controller selects a given register using its binary address and then clocks the data from the serial interface  162  into the selected register. In this way different fields of view are programmed into the registers. In the exemplary embodiment when the trigger input to the sensor is activated the registers begin to grab frames starting with a frame whose region of interest is controlled by the first register  150 . The sensor then cycles through the other registers for each subsequent frame. 
         [0034]    The exposure time is programmed for each ROI. The read-out time is determined by the number of pixels in the ROI. The registers are programmed through a standard 2-wire serial interface known as  12 C (I-squared C). A chip select is not needed because the  12 C protocol requires that a device address precede any register read or write. 
         [0035]    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.