Abstract:
A 2D bar code reader with improved motion tolerance is presented. The 2D bar code reader includes a lens configuration that enhances chromatic aberration to separate the focal planes on which bar code images from two different color components of a light will be focused. An imager interprets one of the focal planes as the image and the other one as noise to be ignored. In addition, the increased depth of field generated by the chromatic aberration allows for a larger aperture stop setting and quicker shutter speed, thereby improving motion tolerance.

Description:
BACKGROUND 
     One-dimensional (1D) bar codes are machine-readable objects used to store information about a product, package, or other item upon which the 1D bar code is affixed. 1D bar codes store information only in the horizontal direction, and laser scanners are often used to read and decode these bar codes. Such laser, or “point,” scanners exhibit a high motion tolerance making them ideal for accurately decoding 1D bar codes. 
     But 1D bar codes are limited in application due to the small amount of information they contain. 1D bar codes store information only in one dimension, merely encoding a number or other identifier that, after decoding, must be compared to an external database containing relevant information. For example, after decoding 1D bar code encoding a product identifier, a computer may retrieve the price, quantity, or other relevant information about the product from an external database. 
     Due to the limited application of 1D bar codes, two-dimensional (2D) bar codes have recently grown in popularity. Unlike 1D bar codes, 2D bar codes contain information in both the horizontal and vertical directions, storing more information than 1D bar codes. This requires sophisticated 2D imagers to decode the entire image at once. Such imagers have a low motion tolerance, as any movement of the bar code or imager during image capture causes the image as a whole to blur, resulting in sluggish read rates. 
     SUMMARY 
     Embodiments of the invention are defined by the claims below, not this summary. A high-level overview of various aspects of embodiments of the invention are provided here for that reason, to provide an overview of the disclosure and to introduce a selection of concepts that are further described below in the detailed-description section. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in isolation to determine the scope of the claimed subject matter. 
     In brief and at a high level, this disclosure describes, among other things, ways to provide an improved 2D imager that achieves increased motion tolerance over present 2D imagers. Specifically, one embodiment of the invention uses chromatic aberration generated by a specially chosen lens configuration to increase the depth of field (DOF) of an optical system used in a 2D imager. Chromatic aberration is a lens distortion caused by dispersion caused by light passing through a lens. The chromatic aberration causes different wavelengths of light to be focused at different distances. By using a lens configuration with chromatic aberration, a greater DOF is achieved. The improved 2D imager may “give up” its improved DOF and instead incorporate an optical system with a larger aperture to achieve the same DOF as standard 2D imagers, allowing the improved 2D imager to use a quick shutter speed to capture an image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Illustrative embodiments of the invention are described in detail below with reference to the attached drawing figures, and wherein: 
         FIG. 1  depicts a handheld 2D imager scanning a 2D bar code; 
         FIG. 2  is a cutaway of a 2D imager according to an embodiment of the invention; 
         FIG. 3  is a schematic representing the intensity versus frequency across the visible spectrum of light emitted from a white LED, illustrating high intensities at the frequencies associated with the blue and yellow components of white light; 
         FIG. 4  is schematic representing two different focal planes for a light source having two predominant wavelengths through a lens with inherent chromatic aberration; 
         FIG. 5  depicts a lens configuration with increased chromatic aberration according to an embodiment of the invention; 
         FIG. 6  shows a ray-tracing diagram for a blue component of a white LED through a lens configuration with high chromatic aberration according to an embodiment of the invention; and 
         FIG. 7  shows a ray-tracing diagram for a yellow component of a white LED through a lens configuration with high chromatic aberration according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the invention use a lens configuration that causes chromatic aberration of an image to provide improved motion tolerance. The improved 2D imager can capture bar code images using a faster shutter speed than standard 2D imagers, reducing the time needed to capture an image of a bar code and ultimately increasing motion tolerance. 
     Bar codes encode information in a machine-readable format. For example, a bar code may comprise a series of low reflective bars (e.g., black) that are separated from a series of highly reflective spaces (e.g., white). The bars and spaces are arranged in unique groups in order to encode data. These simple bar codes provide data in one dimension (horizontal) and are accordingly often referred to as “linear” bar codes or “1D” bar codes. A scanner passing over a 1D bar code shines a light on the bars and spaces. When the light source passes over the lowly reflective bars, the light is mostly absorbed. And when the light source passes over the highly reflective spaces, the light is mostly reflected back to the scanner. The scanner, using decoding circuitry, decodes the reflected light, interpreting the alternating bars and spaces as a number or other identifier. 
     Recently 2D bar codes have grown in popularity due to the increased information they convey. Unlike their 1D counterparts, 2D bar codes store information in both the vertical and horizontal directions, dramatically increasing the amount of data contained within the bar code itself. Information that was once stored in a database and externally referenced is now directly contained within the 2D bar code. 
     Sophisticated imagers are needed to capture 2D bar codes and decode the image as a whole.  FIG. 1  depicts one example of a handheld 2D imager  110  scanning a 2D bar code  120 . In  FIG. 1 , an operator  100  holds the 2D imager  110  and aims it at a 2D bar code  120 . When the operator activates the 2D imager  110 , a light source contained within the imager illuminates the entire 2D bar code  120  at once. The lowly reflective portions of the 2D bar code  120  (the dark spaces) absorb most of the light source and the highly reflective portions (the white spaces) reflect most of the light source back toward the 2D imager  110 . Unlike traditional 1D scanners that use a flying spot to travel horizontally across a 1D bar code, the 2D imager  110  captures the entire image at once to decode the image as a whole. Embodiments of the present invention may take the form of a handheld 2D imager or a fixed imager (not shown). A fixed imager may read bar codes that are moving or stationary. For example, a fixed imager that is attached to a conveyor system may read bar codes affixed to passing objects. 
       FIG. 2  presents a cutaway of the 2D imager  110 , revealing the major components required to capture and decode the 2D bar code  120  according to embodiments of the present invention. Similar components are present in a fixed embodiment. Specifically, the 2D imager  110  contains a viewing window  200 , a light source  210 , a shutter  214 , an aperture  218 , a lens module  220 , an image sensor  230 , a processor  240  (depicted as a central processing unit or CPU), and an on/off operator interface  250  (depicted as a trigger). The components in  FIG. 2  are merely representative of the appearance of actual components used to build the 2D imager. The actual appearance of components and the imager  110  could vary. Further, the components may be rearranged in some cases. 
     The 2D imager  110  may include a variety of computer-storage media. By way of example, and not limitation, computer-storage media may comprise Random Access Memory (RAM); Read Only Memory (ROM); Electronically Erasable Programmable Read Only Memory (EEPROM); flash memory or other memory technologies; Compact Disk Read-Only Memory (CDROM), digital versatile disks (DVDs) or other optical or holographic media; magnetic cassettes, magnetic tape, magnetic disk storage, or other magnetic storage devices. The computer-storage media may be nontransitory. 
     An operator activates the 2D imager  110  by toggling the on/off operator interface  250 . In a fixed setting, a 2D imager may be activated by a proximity sensor or other mechanism. The 2D imager  110  captures an image of the 2D bar code  120  by using the light source  210  to illuminate the 2D bar code  120 . The white spaces of the 2D bar code reflect most of the light back toward the 2D imager, and the black spaces absorb most of the light. The reflected light travels through the viewing window  200 , the shutter  214 , the aperture  218 , and then passes through a lens module  220  where it is refracted, and ultimately forms a negative image of the 2D bar code  120  on the image sensor  230 . Though lens module  220  is depicted as a single lens, embodiments of the present invention may use a series of lenses that are configured to generate a chromatic aberration through the lens module  220 . The processor  240  then decodes the image, interpreting the information that the 2D bar code  120  conveys. Although depicted inside 2D imager  110  in  FIG. 2 , components, including the light source  210  and processor  240 , may be located externally. In some embodiments, the light source is ambient light. 
     Because the entire image of the 2D bar code  120  is formed on the image sensor  230  at once, any movement of the 2D imager  110  or the 2D bar code  120  during image capture creates a blurry image. Unlike flying point scanners used for 1D bar code decoding—where movement only causes skewing in a small portion of the image—movement of the 2D imager  110  or 2D bar code  120  creates simultaneous skewing at all portions of the image, resulting in failure to decode the image. And unlike 1D bar codes where the information contained in one dimension is relatively small allowing for rapid image capture, 2D bar codes require more time to effectively capture the image. 2D imagers are thus subject to sluggish read rates as images must be repeatedly captured until there is little movement and a crisp image is formed on the image sensor. 
     The shutter  214  comprises a movable mechanical barrier that opens and closes to allow light into the 2D imager  110 . The mechanical barrier may be a leaf, diaphragm, or other mechanism. The shutter  214  also comprises a timer that controls the amount of time the shutter is in the open position. The timer may by coupled to the processor or other controller for the 2D imager. This allows the processor to specify the shutter speed. Shutter speed refers to how long an optical system remains open during image capture. If a slow shutter speed is used, the optical system remains open for a long time, and the image is susceptible to blurring upon movement of the 2D imager or the 2D bar code. If a quick shutter speed is used, the optical system remains open for a short time, and thus any movement of the 2D imager or 2D bar code is less detrimental to image capture. 
     The aperture  218  is a mechanical device with an opening through which light passes. The aperture  218  may have an adjustable barrier it adjusts to form different size openings. The adjustable barrier may be manipulated by a controller. The aperture  218  is communicatively coupled to a processor, which can specify a particular aperture setting. Aperture is a measure of how large an opening light passes through into an optical system. Aperture is traditionally measured in terms of standardized “aperture stops,” ranging typically from f/2 up to f/16 and beyond. The smaller a numeral in a given aperture stop, the larger the aperture, and thus more light will enter the optical system for a given period of time. For example, an optical system with an aperture stop of f/7 will allow more light in during a given period of time than an optical system with an aperture stop of f/9. The aperture stop and shutter speed work together to regulate the amount of light that enters the imaging system. 
     As aperture increases, DOF decreases. DOF refers to the distance within an image being captured that appears in focus. For example, assume an optical system is capturing an image of three objects located at three different distances from the optical system. For an optical system having a shallow DOF, one object within the captured image may appear in focus, but the other two objects located behind or in front of the in-focus object will appear blurry. For an optical system having a deep DOF, all three objects within the captured image will appear in focus. Designing an appropriate optical system for a 2D imager requires consideration of the tradeoff between aperture and DOF. A large aperture will allow more light into the system in a short period of time, providing for a quick shutter speed. But increasing aperture reduces DOF, requiring an operator to get very near a 2D bar code to capture a crisp image for decoding. 
     An embodiment of the invention uses a larger aperture than a standard 2D imager (allowing for a quick shutter speed) while maintaining the same DOF as a standard imager. In one embodiment, this is accomplished by using a light source composed predominantly of two distinct frequency ranges and a lens with chromatic aberration. The color of visible light perceived by the human eye is a function of the frequency of waves composing the light. Waves with the highest frequencies (and thus shortest wavelengths) in the visible spectrum appear violet. And waves with the lowest frequencies (and thus longest wavelengths) in the visible spectrum appear red. 2D imagers use a wide range of light sources, including light-emitting diodes (LEDs), incandescent bulbs, lasers, external light sources, and other light sources well known in the art. Each light source&#39;s color is representative of the predominant frequencies emitted by the source. 
     One embodiment of the invention uses a white LED as a light source. A white LED emits light across the spectrum of visible light with especially high intensity distributions in the blue and yellow frequency ranges.  FIG. 3  presents a representative graph depicting light intensity versus frequency across the frequency range of a typical white LED  300 . The frequencies composing blue visible light (the blue component)  310  and the frequencies composing yellow visible light (the yellow component)  320  of the frequency range of a typical white LED  300  are much more intense than other frequencies across the visible spectrum of light. Returning again to  FIG. 2 , if a typical white LED as depicted in  FIG. 3  is used as the light source  210 , the two frequency ranges that would predominantly form an image on the image sensor  230  would be the blue component  310  and the yellow component  320  of the frequency range of typical white LED  300 . 
     Chromatic aberration is a lens distortion that arises due to dispersion (i.e., a variation of the refractive index of the lens material as a function of wavelength). Refraction is the change of direction of a wave resulting from the change in velocity when the wave passes from one medium to another. In an optical system, when a light wave enters a lens, the different density of the lens from the surrounding air causes the wave to slow and bend, or refract. Some lenses are vulnerable to a variation of refractive indexes as a function of wavelength, creating a dispersion of the wavelengths leaving the lens. This dispersion is known as chromatic aberration. If light is composed of two predominant frequencies and enters a lens vulnerable to chromatic aberration, the two frequencies will leave the lens at different angles and ultimately focus in different focal planes, resulting in blurry images. 
       FIG. 4  illustrates a lens with chromatic aberration, forming two different focal planes for two different frequencies of light. In  FIG. 4 , a light source  400  is reflected off an object  410 . The light source  400  is composed of two predominant frequencies, illustrated by rays  420  and  430 , representing a smaller-wavelength component and a larger-wavelength component, respectively. The reflected light refracts through a lens  440 , but due to chromatic aberration, the two predominant frequencies refract at different angles from one another. Focal points  450  and  460  represent the points where rays  420  and  430  come to a perfect focus, respectively. Due to the chromatic aberration in lens  440 , the rays  420  and  430  come to a perfect focus at different focal distances from the lens  440 . The shorter-wavelength ray  420  refracts more, coming to a perfect focus closer to the lens  440  at focal point  450 . The longer-wavelength ray  430  refracts less, coming to a perfect focus further from the lens  440  at focal point  460 . This results in blurry images: If an image sensor is placed at point  450 , the shorter-wavelength ray  420  is in perfect focus, but the longer-wavelength ray  430  would be out of focus, causing a blur. And if the image sensor is placed at point  460 , the longer-wavelength ray  430  is in perfect focus, but the shorter-wavelength ray  420  is out of focus, also causing a blur. 
     2D imagers in the prior art use an optical system that corrects for such chromatic aberration. For example, in the case of a standard 2D imager using a white LED portrayed in  FIG. 3 , the optical system would correct for chromatic aberration to ensure the blue component  310  and the yellow component  320  of the white LED ultimately focus on the same plane. Without such correction, an image of a 2D bar code formed on the image sensor would be blurry, making decoding of the image difficult. 
     Embodiments of the invention do not correct for chromatic aberration, but rather uses such chromatic aberration to improve motion tolerance. In one embodiment, the chromatic aberration inherent is intentionally increased.  FIG. 5  illustrates a lens configuration according to one embodiment of the invention that increases chromatic aberration. In  FIG. 5 , lenses  500  and  520  are convex lenses made of flint glass (denoted “F” in  FIG. 5 ), and lens  510  is a concave lens made of crown glass (denoted as “C” in  FIG. 5 ). Crown glass has a low refractive index and low dispersion while flint glass has a high refractive index and high dispersion. The lens configuration in  FIG. 5  is reversed from the arrangement of lens in an optical system aiming to reduce chromatic aberration. A configuration of crown, flint, crown normally corrects for chromatic aberration. But the reverse arrangement of flint, crown, flint, as depicted in  FIG. 5 , increases the chromatic aberration of the optical system. Returning to  FIG. 2 , if the lens module  220  is comprised of a flint lens  500 , crown lens  510 , and flint lens  520  as depicted in  FIG. 5 , an image of a bar code  120  would always appear blurry on the image sensor  230  to the naked eye due to chromatic aberration. 
     However, only one frequency of light is needed to read a 2D bar code. Because a bar code merely encodes information by distinguishing between areas of low reflectivity (bars) and areas of high reflectivity (spaces) without relying on the multiple colors of a source image, a monochromatic image sensor can be used as the image sensor  230  in  FIG. 2 . The monochromatic image sensor will receive in-focus light (one of the two predominant frequencies) and out-of-focus light (the other of the two predominant frequencies). Normally, such a combination would result in a blurry, undecodable image. However, when the optical system increases the chromatic aberration, by, for example, using the lens structure of  FIG. 5 , the second predominant frequency produces a consistent noise rather than a disruptive, slightly out-of-focus image. With knowledge of this predictable noise, a processor may quickly decode the image, resulting in a motion tolerant 2D imager. 
     Specifically, in one embodiment of the invention, an improved 2D imager uses a white LED, an optical system with high chromatic aberration, and a monochromatic sensor. As presented in  FIG. 3 , the blue component  310  and yellow component  320  have the highest intensities across the frequency range of a white LED  300 . In this embodiment, if the blue component  310  is focused on an image sensor, the yellow component  320  will be out of focus due to chromatic aberration of the optical system. However, because the chromatic aberration is intentionally increased, the yellow component  320  is a consistent noise, and a processor can effectively decode the image formed by the blue component  310  on the image sensor. And if the yellow component  320  is focused on an image sensor, the blue component  310  will be out of focus due to chromatic aberration, however the blue component  310  is a consistent noise, and the processor can effectively decode the image formed by the yellow component  320 . 
       FIG. 6  presents a ray diagram tracing rays of the blue component of a white LED  600  through an optical system according to one embodiment of the invention, and  FIG. 7  presents a ray diagram tracing rays of the yellow component  700  of a white LED through the identical optical system. The optical system in  FIG. 6  and  FIG. 7  is composed of a first flint lens  610 , a crown lens  620 , a second flint lens  630 , and a monochromatic image sensor  640 . The monochromatic image sensor  640  contains a front surface  642  and a rear surface  644 . As illustrated in  FIG. 6 , the rays of the blue component  600  are refracted through the three lenses  610 ,  620 , and  630 , and come to a perfect focus on the front surface of the image sensor at points  660 ,  662 ,  664 ,  666 , and  668 . But, as illustrated in  FIG. 7 , the rays of the yellow component  700  are out of focus at each of these points due to chromatic aberration. Although the rays enter the optical system at the same point in both  FIG. 6  and  FIG. 7 , the rays of the yellow component  700  do not come to a perfect focus until they reach line  646 , located behind the rear surface  644  of the image sensor  640 , at points  670 ,  672 ,  674 ,  676 , and  678 . But due to the increased chromatic aberration of the optical system, the yellow component  700  is a consistent noise at points  660 ,  662 ,  664 ,  666 , and  668 , and thus a 2D imager using this optical system can effectively decode the image focused by the blue component  600  on the front surface  642  of the image sensor  640  without undue interference from the yellow component  700 . 
     If instead the front surface  642  of image sensor  640  were located at line  646 , then the rays of the yellow component  700  would come to a perfect focus on the front surface  642  at points  670 ,  672 ,  674 ,  676 , and  678 . And the rays of blue component  600  would be out of focus at each of these points. Again, this out-of-focus component is a consistent noise, and the processor is able to easily decode the image formed by the rays of the yellow component  700 . 
     Using an optical system with increased chromatic aberration presents several benefits. First, by using an optical system with increased chromatic aberration, an increased DOF is realized. For example, current standard 2D imagers may use a small aperture, typically an aperture stop of f/9, which produces a depth of field of about 50 cm. However, using an optical system with increased chromatic aberration in the same imager would increase the DOF by about 30% while still using a small aperture, for example an aperture stop of f/9. 
     To improve motion tolerance, however, an improved 2D imager may “give up” the increased DOF and trade it in for a larger aperture, allowing the optical system to use a quick shutter speed thus improving motion tolerance. As presented above, there is a tradeoff between aperture and DOF: a larger aperture reduces DOF. In the previous example, an imager using the improved optical system could “trade in” its 30% increase in DOF for a larger aperture. If the improved optical system sought to retain a DOF of 50 cm, the system could increase the aperture of the lens configuration, for example, to an aperture stop of f/7. This larger aperture allows more light through the lens, allowing the optical system to reduce the amount of time a shutter must remain open to capture the image of the bar code. Returning to our previous example, an improved imager using an aperture of f/7 and having a DOF of about 50 cm could capture an image of the bar code using a shutter speed of about 2 ms. 
     The result is dramatic, resulting in quick and accurate readings of 2D bar codes. With such a quick shutter speed, a 2D imager is tolerant to movement by either the 2D imager or the 2D bar code during image capture. And by using such an optical system there is no increase in power consumption by the 2D imager. A 2D imager equipped with the above disclosed optical system thus produces quick and reliable decoding of 2D bar codes affordably and efficiently. 
     Many different arrangements of the various components depicted, as well as components not shown, are possible without departing from the spirit and scope of embodiments of the invention. Embodiments of the invention have been described with the intent to be illustrative rather than restrictive. Certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations and are contemplated to be within the scope of the claims. 
     Many different arrangements of the various components depicted, as well as components not shown, are possible without departing from the spirit and scope of embodiments of the invention. Embodiments of the invention have been described with the intent to be illustrative rather than restrictive. Certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations and are contemplated to be within the scope of the claims.