Patent Publication Number: US-8118226-B2

Title: High-resolution optical code imaging using a color imager

Description:
RELATED APPLICATION 
     This application claims benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 61/151,768, titled “High-Resolution Optical Code Imaging Using a Color Imager,” filed Feb. 11, 2009, the entire contents of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The field of this disclosure relates generally to systems and methods of data reading, and more particularly but not exclusively to reading of optical codes (e.g., bar codes). 
     BACKGROUND INFORMATION 
     Optical codes encode useful, optically-readable information about the items to which they are attached or otherwise associated. Perhaps the most common example of an optical code is the bar code. Bar codes are ubiquitously found on or associated with objects of various types, such as the packaging of retail, wholesale, and inventory goods; retail product presentation fixtures (e.g., shelves); goods undergoing manufacturing; personal or company assets; and documents. By encoding information, a bar code typically serves as an identifier of an object, whether the identification be to a class of objects (e.g., containers of milk) or a unique item. A typical linear or one-dimensional bar code, such as a UPC code, consist of alternating bars (i.e., relatively dark areas) and spaces (i.e., relatively light areas). The pattern of alternating bars and spaces and the widths of those bars and spaces represent a string of binary ones and zeros, wherein the width of any particular bar or space is an integer multiple of a specified minimum width, which is called a “module” or “unit.” Thus, to decode the information, a bar code reader must be able to reliably discern the pattern of bars and spaces, such as by determining the locations of edges demarking adjacent bars and spaces from one another, across the entire length of the bar code. 
     Bar codes are just one example of the many types of optical codes in use today. Higher-dimensional optical codes, such as, two-dimensional matrix codes (e.g., MaxiCode) or stacked codes (e.g., PDF 417), which are also sometimes referred to as “bar codes,” are also used for various purposes. 
     Different methods and types of optical code reading devices are available for capturing an optical code and for decoding the information represented by the optical code. For example, image-based readers are available that include imagers, such as charge coupled devices (CODs) or complementary metal oxide semiconductor (CMOS) imagers, that generate electronic image data that represent an image of a captured optical code. Image-based readers are used for reading one-dimensional optical codes and higher-dimensional optical codes. Because optical codes most often include dark and light patterns (e.g., black and white) that represent binary data, imagers of image-based readers are typically monochrome so that uniform sensitivity for each pixel of the imager is achieved. Also, typical image-based readers include light sources that illuminate the image-based reader&#39;s field of view with narrowband visible light to achieve high optical resolution by avoiding chromatic aberration and polychromatic diffraction effects. Narrowband light sources typically used for imaging include laser diodes, having a bandwidth on the order of 5 nanometers (nm), and light emitting diodes (LEDs), having a bandwidth on the order of 50 nm. 
     Common imagers made for image capturing devices, such as still cameras and video cameras, however, are color imagers—not monochrome. Because imagers made for many image capturing devices are color, color imagers are generally made in higher volume and have become less expensive than monochrome imagers. Some image-based readers have included color imagers, but the present inventors have recognized that those readers do not effectively achieve high optical resolution comparable to monochrome image-based readers with the same number and size of pixels. 
     SUMMARY OF THE DISCLOSURE 
     This disclosure describes improved optical reading devices and associated methods. 
     One embodiment is directed to an optical code reading device that includes a color image sensor array positioned to sense light reflected from an object in a field of view of the optical code reading device and to produce from the sensed reflected light image data representing an image of the object. The color image sensor array has a first set of sensor elements that are sensitive to a first visible wavelength band of light, and a second set of sensor elements that are sensitive to a second visible wavelength band of light. The first and second sets of sensor elements are also sensitive to light within an infrared wavelength band. The optical code reading device includes an artificial illumination source positioned to illuminate the field of view of the optical code reading device with light that is incident upon and reflected from the object in the field of view toward the image sensor array. The illumination source is operable to produce infrared light having wavelengths within the infrared wavelength band so that, upon illumination of the field of view, at least some sensor elements of each of the first and second sets are sensitive to the infrared light and contribute to production of the image data. 
     Additional aspects and advantages will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of an imaging system according to a preferred embodiment. 
         FIG. 2  is a diagram of a color filter pattern of a color image sensor array of the imaging system of  FIG. 1 . 
         FIG. 3  is a graph of the sensitivity of blue, green, and red sensor elements as a function of light wavelength of an illustrative color image sensor array used in the imaging system of  FIG. 1 . 
         FIG. 4  is a flowchart showing the operational steps of the imaging system of  FIG. 1 . 
         FIG. 5  is a flowchart of an illumination calibration method according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to the above-listed drawings, this section describes particular embodiments and their detailed construction and operation. 
       FIG. 1  is a diagram of an imaging device  100 , such as an optical code reading device, according to one embodiment. The imaging device  100  includes a color image sensor array  102  contained within a housing  103  of the imaging device  100 . The color image sensor array  102  includes a first set of sensor elements  104  and a second set of sensor elements  106 . The sensor elements of the color image sensor array  102  may be arranged in a one-dimensional array or, preferably, a two-dimensional array. The color image sensor array  102  may be a CCD, such as a frame-transfer or interline-transfer CCD. The color image sensor array  102  may alternatively be a CMOS imager, such as a global shuttered or rolling-reset CMOS imager. Suitable imagers or image sensor arrays are available, for example, from Aptina Imaging Corporation of San Jose, Calif., USA including, but not limited to, a model MT9V022 VGA color imager. Imagers are available from many manufacturers and are available in various resolutions (numbers of pixels). For higher resolution applications, other imagers from Aptina Imaging Corporation are suitable, including the model MT9M001 with 1.3 megapixels, the model MT9M002 with 1.6 megapixels, and the model MT9P001 with 5 megapixels. As imaging technology advances, imager resolution increases and other imagers may also be suitable. 
     The color image sensor array  102  may include more than two sets of sensor elements. For example, the color image sensor array  102  may include three sets of sensor elements  104 ,  106 , and  202  arranged in a Bayer pattern as shown in  FIG. 2 . Each set of sensor elements corresponds to a different color. For example, the first set  104  may be sensitive to light having wavelengths that correspond to the color green (G) (wavelengths ranging between about 500 nanometers (nm) and about 600 nm), the second set  106  may be sensitive to light having wavelengths that correspond to the color red (R) (wavelengths ranging between about 600 nm and about 750 nm), and the third set  202  may be sensitive to light having wavelengths that correspond to the color blue (B) (wavelengths ranging between about 400 nm and about 500 nm). Moreover, a color filter associated with each sensor element of the different sets appreciably filters out visible light that does not correspond to its color (i.e., the color filters associated with the first set  104  appreciably block out red and blue light). 
       FIG. 3  is a graph depicting an example of the quantum efficiency percentage versus the wavelength of light incident upon red, green and blue sensor elements of the model MT9V022 VGA color imager available from Aptina Imaging Corporation that may be used as the color image sensor array  102 . A curve  104 ′, corresponding to the spectral sensitivity of the sensor elements of the first set  104 , has a local peak  104   a ′ at a wavelength corresponding to the color green. A curve  106 ′, corresponding to the spectral sensitivity of the sensor elements of the second set  106 , has a local peak  106   a ′ at a wavelength corresponding to the color red. A curve  202 ′, corresponding to the spectral sensitivity of the sensor elements of the third set  202 , has a local peak  202   a ′ at a wavelength corresponding to the color blue. The curves  104 ′,  106 ′, and  202 ′ also have respective local peaks  104   b ′,  106   b ′, and  202   b ′ near a common wavelength that corresponds non-visible light—in this case infrared light. In other words, the first set  104 , the second set  106 , and the third set  202  are not only sensitive to green, red, and blue light, respectively, but also to light within an infrared wavelength band. The infrared wavelength band, to which the first set  104 , the second set  106 , and the third set  202  are sensitive, may be relatively narrow (e.g., no more than about 100 nm) and may include 850 nm. Also, the quantum efficiency percentage associated with the local peaks  104   b ′,  106   b ′, and  202   b ′ may be substantially the same or within a narrow percentage range. In other words, the sensitivity of the sets  104 ,  106 , and  202  to infrared light may be substantially equal or within a narrow sensitivity range (e.g., about five percent in quantum efficiency) so that an average intensity value of light sensed by the first set  104  may be substantially equal to an average intensity value of light sensed by the second set  106  and an average intensity value of light sensed by the third set  202 . 
     The color image sensor array  102  need not be limited to three sets of sensor elements or the colors red, green, and blue, and the color image sensor array  102  may include color filter patterns other than the Bayer pattern. For example, the color image sensor array  102  may include a cyan, yellow, green, and magenta (CYGM) filter or a red, green, blue, and emerald (RGBE) filter in which the sensor elements of the different colors are also sensitive to light within an infrared wavelength band. The color filter pattern used on the color filter array  102  may be chosen to achieve accurate color rendition or to improve sensitivity in a color photograph application. While these distinctions are not necessary in the present embodiment, the imaging device  100  and its associated methods are flexible to compensate for the effects of these various filters. 
     The imaging device  100  may also include one or more artificial illumination sources  108  (two illumination sources are depicted in  FIG. 1 ). The artificial illumination sources  108  may be mounted to a printed circuit board  110  upon which the color image sensor array  102  is also mounted. In a first embodiment, the artificial illumination sources  108  are operable to emit infrared illumination. The infrared illumination emitted by the artificial illumination sources  108  may be narrowband infrared illumination (e.g., illumination having a bandwidth less than about 100 nm). Also, the wavelength bandwidth of light emitted by the artificial illumination sources  108  preferably includes 850 nm, when using a color image sensor array with characteristics shown in  FIG. 3 . 
     The imaging device  100  typically includes a suitable optical system  112  positioned to focus light upon the color image sensor array  102 . The optical system  112  may include conventional optical components, such as one or more lenses, an aperture, and, in some cases, a mechanical shutter. As an alternative to a mechanical shutter, the color image sensor array  102  may include electronic shuttering means. The optical system  112  may also include one or more optical filters to block out certain wavelengths of light. In one example, when infrared illumination sources are selected for the artificial illumination sources  108 , the optical system  112  excludes an infrared filter, which is operable to block out infrared light, and may include one or more optical filters that are operable to block out light that does not have wavelengths within the infrared wavelength band. Although the artificial illumination sources  108  are shown as being mounted on the printed circuit board  110 , the artificial illumination sources  108  may be positioned in other convenient locations to provide illumination of the object  114 . 
     A preferred operation of the imaging device  100  will now be described with reference to a flowchart  400  of  FIG. 4 . The artificial illumination sources  108  illuminate the field of view  116  with infrared illumination (step  402 ) or illumination of another non-visible frequency at which all of the sensor elements have an acceptable response. If an object  114  (e.g., an optical code) is within the field of view  116  of the imaging device, infrared light reflects off the object  114  toward the optical system  112 . Infrared light that is incident on the optical system  112  is focused by the optical system  112  onto the sensor elements of the color image sensor array  102  (step  404 ). Sensor elements of the first set  104 , the second set  106 , and the third set  202  sense the focused infrared light (steps  406   a ,  406   b , and  406   c ). The color image sensor array  102  produces image data based upon the infrared light that is incident on, and sensed by, the sensor elements of the first, second, and third sets  104 ,  106 , and  202  (step  408 ). An enclosure  118  may cover the color image sensor array  102  except where the optical system  112  is located, so that an appreciable amount of light from sources other than the artificial illumination sources  108  does not reach the color image sensor array  102 . Because each of the sets  104 ,  106 , and  202  are sensitive to infrared light, each of the sets  104 ,  106 , and  202  contribute to production of the image data and high-resolution infrared imaging of the object  114  may be achieved. Each of the sets  104 ,  106 , and  202  may contribute to the production of the image data to a sufficiently equal extent that no one of the set  104 ,  106 , or  202  contributes to the image data appreciably more than the other two sets. The resolution of an infrared image represented by the image data may be substantially equal to a resolution produced by a monochrome image sensor array having the same size of sensor elements and the same number of sensor elements as the sum of the number of sensor elements in the sets  104 ,  106 , and  202 . In other words, when illuminated with infrared light, the color image sensor array  102  may achieve a resolution substantially equivalent to a monochrome imager. 
     In a second alternative embodiment, the artificial illumination sources  108  emit visible light having red, green, and blue light components. For example, the artificial illumination sources  108  emit visible polychromatic (white) light, or a combination of monochromatic or quasi-monochromatic lights having wavelengths corresponding to the colors red, green and blue. In one configuration, the artificial illumination sources  108  include a red light, a green light and a blue light (e.g., red, green and blue light emitting diodes). Light emission intensities of each of the red, green and blue light components are calibrated to compensate for the behavior of the color image sensor array  102  described below. In a conventional optical code reading device that includes a monochrome imager, data representing an image captured by the monochrome imager are converted to grayscale where the shade of gray produced by a sensor element of the monochrome imager depends upon the light intensity level captured by it. For the color image sensor array  102 , the patterned color filter that covers the array of sensor elements effects the transmittance of light, and, thus, the intensity of light that is incident on the sensor elements of the sets  104 ,  106 , and  202 . Also, the transmittance of light associated with the filter portions may be different between colors such that the filter portions of one color may transmit more (or less) light than the filter portions of the other colors. Moreover, the sensor elements behind the filter portions may be inherently more or less sensitive to certain wavelengths of light (e.g., the sensor elements may be more sensitive to red wavelengths than to blue and green wavelengths). The effects of the color dependent differences in light transmittances and pixel sensitivities can be seen in the example of  FIG. 3  in which the quantum efficiencies associated with the local peaks  104   a ′,  106   a ′ and  202   a ′ are different from one another. 
     Prior to operation, the differences in the quantum efficiencies of the sets  104 ,  106  and  202  may be determined and the light sources  108  may be calibrated according to an illumination calibration method  500  shown in the flowchart of  FIG. 5 . First, the color image sensor array  102  is uniformly illuminated with a light source, such as the sources  108  or another light source (step  502 ). In one example, the color image sensor array  102  is directly illuminated with the light source. In another example, a white background (such as a white piece of paper) is illuminated, and an image of the white background is captured by the color image sensor array  102 . Each of the sets  104 ,  106  and  202  produce a set of image data representing light intensity levels captured by the sensor elements. The light intensity levels represented in the sets of image data are compared to determine the relative sensitivity of each of the sets  104 ,  106  and  202  (step  504 ). In one example, an average intensity level for each of the sets  104 ,  106  and  202  is computed from the sets of image data and the averages are compared to determine the relative differences in quantum efficiencies between the sets  104 ,  106  and  202 . 
     After the image data are analyzed, the light emission intensity levels of the red, green and blue light components of the artificial illumination sources  108  are adjusted to compensate for the sensitivity differences between the sets  104 ,  106  and  202  (step  506 ). For example, if the quantum efficiency of the second set  106  is greater than the quantum efficiency of the first set  104 , the intensity level of the green component emitted by the artificial illumination sources  108  is selected to be greater than the intensity level of the red component by an amount proportional to the difference between the quantum efficiencies of the first and second sets  104  and  106 . In one example, the intensity levels of the red, green and blue light components are selected so that the average intensity levels of light captured by the sets  104 ,  106  and  202  are substantially the same. Adjustment of the light emission intensity levels can be implemented by varying an amount of current supplied to the red, green, and blue lights of the artificial illumination sources  108 . Alternatively, the color image sensor array may include an analog or digital gain for each of the sets  104 ,  106  and  202 , which are adjusted instead of the light emission intensity levels of the artificial illumination sources  108  to provide a uniform image output. This alternative embodiment may allow the artificial illumination sources  108  to provide a more pleasing color of illumination, such as white. 
     In operation, the calibrated red, green and blue light components illuminate the object  114 , the sensor elements of the sets  104 ,  106  and  202  capture an image of the object  114  and produced image data, and the image data are converted to grayscale. Similar to the first embodiment, high-resolution imaging may be realized because each of the sets  104 ,  106 , and  202  contributes to the production of the image data. 
     In a third alternative embodiment, the optical system  112  includes a dispersive (intentionally uncorrected chromatic aberration) lens assembly. For example, U.S. Pat. No. 7,224,540 (“the &#39;540 patent”), the entire contents of which are incorporated herein by reference, describes a dispersive lens assembly. Artificial illumination sources  108  emit white light or multiple quasi-monochromatic lights that includes red, green, and blue light. As discussed in the &#39;540 patent, because of the lens assembly&#39;s chromatic aberration, a wavelength dependent focal shift (i.e., longitudinal aberration) of the reflected light occurs. The data from each of the sets  104 ,  106 , and  202  of the color image sensor array  102  are processed as separate color sub-images, called color planes (such as red, green, and blue color planes). The depth of field of each color plane is distinct and typically overlapping. The focus quality of each color plane depends on the distance between the object  114  and the imaging device  100 . For example, if the distance between the object  114  and the imaging device  100  provides the best green focus, the green color plane of the first set  104  will be well focused while the red and blue color planes of the second set  106  and the third set  202  will be blurred. Thus, by comparing the color planes of the first, second, and third sets  104 ,  106 , and  202 , distance between the object  114  and the imaging device  100  can be estimated by determining which color plane is in best focus. 
     One of several different methods may be used for comparing the sharpness of the color planes. For example, techniques used in passive auto-focusing systems may be used. As additional examples, the amount of high spatial frequency content of the color planes may be measured (such as by computing a Fourier transform), or edge sharpness for the color planes may be measured (i.e., measuring the intensity difference between neighboring pixels). By knowing which color plane is in best focus, and by knowing the depth of field associated with the best focused color plane, distance between the object  114  and the imaging device  100  may be estimated. Distance estimation using an optical code reading device may be useful in a number of applications. For example, estimating distance may be used for measuring the dimensions of an optical code to ascertain whether the optical code is of acceptable size (i.e., whether the optical code was printed properly). The third embodiment also provides an increased overall depth of field for the imaging device  100  because the first, second, and third sets  104 ,  106 , and  202  form focused images at different distances, although the resolution of the image from each color plane is lower in the case of infrared illumination, since only one pixel set is capturing a well focused image. 
     As skilled persons will appreciate in light of this disclosure, certain embodiments may be capable of achieving certain advantages, including (1) enabling utilization of lower cost color imagers in optical code reading devices; (2) achieving higher image resolution by utilizing all of an image sensor&#39;s pixels to contribute to an image; (3) avoidance of visible illumination from the illumination source, which can be discernable and annoying to human observers; (4) extending the depth of field of an optical code reader or measuring distance to an object using a chromatically aberrated lens, and (5) compensation for manufacturing variation in the sensitivity of different colored pixels. Other advantages of various embodiments will be apparent upon reading the above sections. Combinations of the above embodiments, and other embodiments not specifically described herein will be apparent to skilled persons upon reviewing the above description. 
     Though the present invention has been set forth in the form of its preferred embodiments, it is nevertheless intended that modifications to the disclosed systems and methods may be made without departing from inventive concepts set forth herein.