Abstract:
An optical scanner utilizes two linear CCD detectors and a bandpass means to improve the ability of the scanner to discriminate against specular reflection. A coded symbology is illuminated by a noncoherent light source and light reflected from the coded symbology along a first path strikes the front face of the bandpass means. The bandpass means, functioning as a notch filter, transmits a select bandwidth of light while reflecting all other light onto a first CCD detector. Simultaneously, light reflected from the bar code symbol travels along a second path, at a different angle with respect to the plane of the coded symbology than the first path, is reflected from a mirror onto the back face of the bandpass means. The bandpass means transmits the select bandwidth of light onto a second CCD detector and reflects all other light. The second CCD detector has a notch filter which permits the detection of only the select bandwidth. Since specular reflection is only experienced at a single angle, with respect to the plane of the coded symbology and each CCD detector detects an image at a different angle with respect to the plane of the coded symbology, a complete image can be reconstructed by combining information obtained from both CCD detectors.

Description:
This application is a continuation-in-part of U.S. patent application Ser. No. 08/790,956, now U.S. Pat. No. 5,942,762 filed Jan. 29, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to optical scanning systems. More particularly, this invention relates to a system and method capable of imaging targets in the presence of specular reflection. 
     2. Description of Related Art 
     Coded symbologies are being used in an increasingly diverse array of applications. The ability to track a large amount of items quickly and efficiently has led coded symbologies to be used in applications such as retail checkout, warehousing, inventory control and document tracking. As the volume of items tracked by coded symbologies has increased, the need for optical scanners which operate at high speeds has likewise increased. 
     Various optical scanning systems have been developed for reading and decoding coded symbologies. Scanning systems include optical laser scanners and optical charge-coupled device (CCD) scanners. Optical laser scanners generally employ a laser diode, a multifaceted polygonal mirror, focusing optics and a detector. The scanning rate of an optical laser scanner is limited by the number of facets on the mirror and the available motor speed. 
     CCD scanners may incorporate a non-laser light source and a CCD light detecting means, such as a CCD linear sensor. A portion of the light which is reflected from the coded symbology is detected by the CCD linear sensor and converted into an electrical signal which is the basis for a digital image of the coded symbology that has been scanned. The digital image is then processed and decoded according to the specific type of coded symbology. 
     One disadvantage with current CCD scanners is that they are susceptible to specular reflection which saturates areas of the CCD linear sensor and prohibits the detection of a portion of the optically coded information. This is particularly a problem when the coded symbology is printed under a highly reflective surface, such as a plastic coating. 
     Specular reflection is only a problem at a single angle, known as the “critical angle”, between the light source, the reflective surface and the CCD linear sensor. Current methods of coping with specular reflection include placing separate scanners at different angles with respect to the surface. However, providing duplicate CCD scanners for this purpose is extremely expensive. Techniques involving light polarizers have also been used. However, due to the light losses introduced by the materials used to make light polarizers, they are extremely inefficient. 
     Accordingly, there exists a need for an efficient and inexpensive scanning system with the speed of a CCD scanner that can accurately read and decode coded symbologies in the presence of specular reflection. 
     SUMMARY OF THE INVENTION 
     The present invention utilizes two CCD linear sensors and a bandpass means to improve the ability of an optical scanner to discriminate against specular reflection. A coded symbology is illuminated by a noncoherent light source and light reflected from the coded symbology travels along a first path and strikes the front face of the bandpass means. The bandpass means, functioning as a notch filter, transmits a select bandwidth of light while reflecting all other light onto a first CCD linear sensor. Simultaneously, light reflected from the bar code symbol travels along a second path, at a different angle with respect to the plane of the coded symbology than the first path, and is reflected from a mirror onto the back face of the bandpass means. The bandpass means transmits the select bandwidth of light onto a second CCD linear sensor and reflects all other light. The CCD linear sensors each have a notch filter which permits the detection of only a select bandwidth. Since specular reflection is only experienced at a single angle with respect to the plane of the coded symbology, and each CCD linear sensor detects an image at a different angle with respect to the plane of the coded symbology; a complete image of the coded symbology is obtained by one or both of the CCD linear sensors, or can be reconstructed by combining information obtained from both CCD linear sensors. 
     Accordingly, it is an object of the invention to provide a CCD scanner which can read and decode coded symbologies in the presence of specular reflection. 
     Other objects and advantages will become apparent to those skilled in the art after reading the detailed description of a presently preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a coded symbology scanning system made in accordance with the present invention; 
     FIG. 2A is a diagram showing the spectrum of light; 
     FIG. 2B is a more detailed diagram of the CCD detectors; 
     FIG. 3 illustrates the method of using valid information from two views and selectively combining the information; 
     FIG. 4 is a block diagram of the coded symbology logic unit; 
     FIG. 5 is a flow diagram of the method of the present invention; and 
     FIG. 6 is a first alternative embodiment of the present invention. 
     FIG. 7 is a second alternative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The preferred embodiment will be described with reference to the drawing figures wherein like numerals represent like elements throughout. Referring to FIG. 1, a coded symbology scanning system  10  made in accordance with the present invention is shown. The coded symbology scanning system  10  is able to scan any type of coded symbology. However, for simplicity, reference hereinafter will be made to a particular type of coded symbology, i.e. a bar code symbol. The scanning system  10  includes a non-coherent light source  12 , a bandpass means  14 , a planar mirror  22 , focusing optics  17 , two CCD linear sensors  16 A and  16 B, two filters  19 A and  19 B, a logic unit  32  and an output means  34 . 
     The light source  12  facilitates detection of a subject bar code symbol  18  by illuminating the bar code symbol  18  located on a package  8  or other object. Preferably, the package  8  is supported by a moving conveyor belt  7 . The planar mirror  22  and the bandpass means  14  are aligned such that light reflected from the bar code symbol  18  along a first path  20 A strikes the front of the bandpass means  14 , while light traveling along a second path  20 B reflects off the planar mirror  22  and strikes the rear of the bandpass means  14 . It should be recognized by those skilled in the art that FIG. 1 is illustrative only and is not drawn to scale. For example, the angle θ A  between the light source  12  and the bar code symbol  18  is typically 77°. The angle θ B  between the first path  20 A and the second path  20 B is approximately 3-5°. However, it should be recognized by those skilled in the art that these angles are approximate and may vary widely depending upon the specific application and the mounting of the system  10  in relation to the bar code  18 . 
     The bandpass means  14  permits light of predetermined wavelengths around η A , striking either its front or rear surface, to pass through the bandpass means  14 , and reflects the remainder of the light spectrum. The spectrum of light η 20A  traveling along the first path  20 A strikes the front of the bandpass means  14 . Light having wavelengths around η A  passes through the bandpass means  14 , while the remainder of the spectrum of light η 20A −η A ± is reflected toward the CCD detectors  16 A,  16 B. The spectrum of light η 20B  traveling along the second path  20 B is reflected off the planar mirror  22  and strikes the back of the bandpass means  14 . Light having wavelengths around η A  passes through the bandpass means  14  toward the CCD detectors  16 A,  16 B, while the remainder of the light spectrum η 20B −η A ± is reflected off the back of the bandpass means  14 . 
     It should be appreciated that the bandpass means  14  may function as a filter wherein the bandpass means  14  transmits a small bandwidth of light while reflecting the remainder of the light spectrum. Alternatively, the bandpass means  14  may function as a mirror, wherein the bandpass means  14  reflects a small bandwidth of light while transmitting the remainder of the light spectrum. Preferably a mirrored dichroic filter is used. 
     The composite spectrum η S  of light which reaches the focusing optics  17  comprises predetermined wavelengths around η A  from the second path  20 B and the remainder of the spectrum η 20A −η A ± from the first path  20 A. The composite spectrum η S  passes through the focusing optics  17 , through the filters  19 A,  19 B and onto the CCD linear array detectors  16 A,  16 B. Both filters  19 A,  19 B permit the respective detector  16 A,  16 B to detect non-overlapping bands of light. 
     Referring to FIG. 2, the second CCD detector  16 B is filtered to detect light having wavelengths around η A . The first CCD detector  16 A is filtered to permit the detection of light around a different wavelength η B . For example, the bandpass means  14  may be calibrated to transmit light around the wavelength η A  of 650 nm±. The second CCD detector  16 B is filtered to detect light around the wavelength η A  of 650 nm± originating from the second path  20 B. The first CCD detector  16 B is filtered to detect light around wavelength η B  which originates from first path  20 A, for example 600 nm±. Accordingly, the detectors  16 A,  16 B will detect two separate images of the bar code symbol  18 . 
     Although the detectors  16 A,  16 B have been referred to as separate CCD linear sensors, preferably they comprise two of the three channels commonly found in a color CCD line scan sensor. In this embodiment, the color filters are preferably replaced with the appropriate notch filters  19 A,  19 B. Those of skill in the art should realize that the bandwidth transmitted by each notch filter  19 A,  19 B, including tolerances, should not overlap with the other notch filter  19 A,  19 B. Additionally, the notch filters  19 A,  19 B need not be of equal bandwidth. One notch filter  19 A may have a narrow bandwidth of 590-610 nm±, and the other notch filter  19 B may have a wide bandwidth of 625-675 nm±. Additionally, although two notch filters  19 A,  19 B may be employed, it is also possible to use one notch filter  19 A, wherein the other filter  19 B transmits all other wavelengths of light except for the bandwidth transmitted by the notch filter  19 A. In all of these examples, the tolerances of the filters  19 A,  19 B should be kept in mind to avoid any overlap. 
     It should be apparent to those skilled in the art that the bandpass means  14  and the filters  19 A,  19 B over the CCD detectors  16 A,  16 B may be calibrated to detect any wavelength of light that is suitable for the desired application. The above values are illustrative only and should not be viewed as a limitation of the invention. 
     The light detected by the second CCD detector  16 B comprises light from the second path  20 B having wavelengths around η A . The light detected by the first CCD detector  16 A comprises light from the first path  20 A having wavelengths around η B . By definition, specular reflections only occur at a “critical angle”. Once specular reflection occurs, this angle is defined and will be present only in one of the optical paths. Therefore, the other path will have useful information. If specular reflection “washes out” the view of the bar code symbol  18  at any point along the first path  20 A, specular reflection will not be present in the second path  20 B at the same point since the angle of the bar code symbol  18  with respect to the second path  20 B is different than the angle with respect to the first path  20 A. 
     Referring to FIG. 2B, since the lengths of the two paths  20 A,  20 B are different, the detectors  16 A,  16 B must be selectively placed to account for this difference. In FIG. 1, path  20 A is shorter than path  20 B. Preferably, the detectors  16 A,  16 B are mounted upon a common substrate which is rotated upon a center line CL to position the first detector  16 A further from the focusing optics  17  than the second detector  16 B. 
     Each of the CCD detectors  16 A,  16 B produces an electrical signal which corresponds to the detected light. Using the images  30 A,  30 B,  30 C in FIG. 3 as a visual example of the reconstruction process, comparison of images  30 A and  30 B shows that image  30 A has portions of specular reflection distortion, while image  30 B also has portions of specular reflection distortion. However, the non-distorted areas of the images  30 A,  30 B can be used to form the complete image  30 C. Although the images  30 A,  30 B,  30 C are illustrated as area images, the preferred embodiment of the present invention detects and combines multiple line scans which make up the area images. It is clearly within the scope of the present invention to utilize detectors which detect either line or area scans. 
     Processing of the data from CCD detectors  16 A,  16 B to construct a complete bar code symbol  18  will be explained with reference to FIG.  4 . The data from the CCD detectors  16 A,  16 B is output and analyzed by the logic unit  32 . Depending upon the amount of specular reflection, data from one or both of the CCD detectors  16 A,  16 B may comprise a complete image of the bar code symbol  18 . In that case, the complete image is used for further decoding in accordance with the specific type of symbology. If specular reflection is detected by the logic unit  32  in the data output from the first CCD detector  16 A the logic unit  32  replaces the data with the data from the second CCD detector  16 B. 
     Referring to FIG. 4, the logic unit  32  comprises two buffers  70 A,  70 B, a selector  72  and an arbitration unit  74 . The logic unit  32  receives the data, containing bar code information, from the CCD detectors  16 A,  16 B. The information coming from the CCD detectors  16 A,  16 B is selectively buffered depending upon the height of the package  8  upon which the bar code  18  is affixed. Referring back to FIG. 1, at a first height Y, the information is obtained simultaneously from both light paths  20 A,  20 B. Accordingly, no buffering of the data is required. However, when the package  8  to which the bar code  18  is affixed reaches height X, the bar code information from the second light path  20 B will be obtained prior to the information from the first light path  20 A. Therefore, information from the second light path  20 B must be buffered by the buffer  70 B prior to comparison with the information from the first light path  20 A. Conversely, if the height of the package  8  to which the bar code  18  is affixed only reaches height Z, information from the first light path  20 A will be detected prior to the information from the second light path  20 B. In this event, the information from the first light path  20 A will be buffered by buffer  70 A. Each buffer  70 A,  70 B delays the information obtained from the respective light path  20 A,  20 B to synchronize the information with that obtained from the other light path  20 B,  20 A. 
     As discussed above, the delay is dependent upon the distance between the system  10  and the bar code symbol  18 . The distance between the system  10  and a package  8  having the bar code symbol  18  located thereon may be obtained by using a light curtain  9 , as in FIG. 1, or by any other means which is well known by those skilled in the art. From the height, or distance, the delay value may be calculated, or a look up table may be used. The delay value is then input into the desired buffer  70 A,  70 B. 
     After the signal output from either detector  16 A,  16 B has been buffered as necessary, the signals are compared by the arbitration unit  74 . The signals comprise values which represent the intensity of light detected by the pixels of the CCD detectors  16 A,  16 B. If the CCD detectors  16 A,  16 B have eight-bit resolution, the number of gray scale levels will be 255 (2 8 −1). Depending upon the application, it may be assumed that a valid signal will have a gray scale value between 0 and 240. If the gray scale value exceeds a predetermined threshold of 240, specular reflection is present. This threshold may be adjusted depending upon the particular application. In the preferred embodiment the arbitration unit  74  controls the selector  72  to select the output from the second CCD detector  16 B when the value from the output from the first CCD detector  16 A exceeds  240 . In this manner, a complete image of the bar code symbol  18  is obtained. 
     The logic unit  32  forwards a complete digital image, corresponding to the information encoded in the bar code symbol  18 , to an image processor  34  for decoding, storage or display, as is well known by those skilled in the art. 
     The scanning system  10  shown in FIG. 1 may be embodied in a mobile hand-held unit, or may be a stationary unit wherein an object carrying the bar code symbol  18  is passed under the light source  12  via a conveyor  7 . 
     In operation, the scanning system  10  executes the bar code symbol reading and decoding procedure  200  shown in FIG.  5 . The light source  12  illuminates a subject bar code symbol  18  (step  210 ). Light is reflected from the bar code symbol  18  along a first path  20 A toward the front of the bandpass means  14  (step  220 ). The bandpass means  14  transmits light around a first predetermined wavelength η A  (step  230 ) and reflects the remainder of the light spectrum η 20A −η A  toward the CCD detectors  16 A,  16 B (step  240 ). The first CCD detector  16 A detects light around a second predetermined wavelength η B  from the first light path  20 A. (step  250 ). 
     Simultaneously, light is reflected from the bar code symbol  18  along a second path  20 B (step  270 ) toward the back of the bandpass means  14  (step  280 ). The bandpass means  14  passes light around the first predetermined wavelength η A  to the CCD detectors  16 A,  16 B (step  290 ) and reflects the remainder of the light spectrum η 20B −η A  away from the CCD detectors  16 A,  16 B (step  300 ). Light originating from the second path  20 B comprises only light around the first predetermined wavelength η A . Accordingly, it will be detected by the second CCD detector  16 B (step  310 ). 
     The CCD detectors  16 A,  16 B convert the detected light into electrical signals which are output to the logic unit  32  (steps  260 ,  320 ). The delay, if any, between scan lines of the paths is then determined based on target height (step  325 ). The information from both light paths  20 A,  20 B is then synchronized. The logic unit  32  compares the signals (step  330 ) and the valid data is selected (step  340 ). This data is used to provide a complete bar code symbol  18 . In the event that both signals comprise non-distorted data, the non-distorted data of either signal may be used. The logic unit  32  then arbitrates the data representing the complete bar code symbol  18  (step  350 ) and forwards the data to the output means  34  (step  350 ). 
     Referring to FIG. 6, an alternative embodiment of the scanning system  110  is shown in which additional mirrors  124 ,  126  are added to the system  110  to direct the paths of light along a modified route. The modified route permits alignment of the components in cases where manufacturing or other considerations require that the components be placed in a configuration other than that shown in FIG.  1 . 
     Referring to FIG. 7, a second alternative embodiment of the scanning system  210  is shown. In this embodiment the light source  12  is repositioned to be located over a package  8  or other object as was shown in the embodiment of FIG.  1 . Placement of the light source  12  however, may be varied. Also, it can be appreciated by those reasonably skilled in the art that the light source may be comprised of several broadband light sources positioned at various locations in order to flood an object on the transport means  7 . A mirror  224 , which is preferably a planar mirror, is added and positioned such that light along a first path  220 A strikes the mirror  224  and is reflected toward the front of the bandpass means  14 . The first path  220 A is defined as extending from the surface of the transport means  7  to the mirror  224  and then to the bandpass means  14 . The bandpass means  14  permits light of predetermined wavelengths in a range surrounding η A , striking either its front or rear surface, to pass through the bandpass means  14 , and reflects the remainder of the light spectrum. The spectrum of light η 220A  traveling along the path of  220 A strikes the front of the bandpass means  14 . Light having wavelengths around η A  passes through the bandpass means  14 , while the remainder of the spectrum of light η 220A −η A  is reflected toward optics  17 . The spectrum of light η 20B  traveling along the second path  20 B is reflected from the planar mirror  22  and strikes the back of the bandpass means  14 . The second path  20 B is defined as extending from the surface of the transport means  7  to the mirror  22  and then to the bandpass means  14 . Light having wavelengths around η A  passes through the bandpass means  14  toward the optics  17 , while the remainder of the light spectrum η 20B −η A  is reflected off of the back of the bandpass means  14 . The composite spectrum η S  of light which reaches the focusing optics  17  comprises predetermined wavelengths approximately at η A  from the second path  20 B and the remainder of the spectrum η 220A −η A  from the first path  220 A. 
     It should be recognized by those skilled in the art that FIG. 7 is illustrative only and is not drawn to scale. For example, the angle θ A  between the axis  213  and the first reflected path  220 A is equal to the angle θ B  between the axis  213  and the second path  20 B. Axis  213  is defined as being orthogonal to the surface of the transport means  7 . It should be recognized by those skilled in the art that these angles θ A , θ B  may vary while remaining equal to each other depending upon the specific application and the mounting of the system  210  in relation to the transport means  7 . Because θ A =θ B , the path lengths  220 A,  20 B originating from the intersection point  212 , (or on a plane parallel to the transport means  7 ), to the bandpass means  14  are equal. Equalizing the path lengths  220 A and  20 B provides an advantage in that light traveling along both paths  220 A, 20 B will have the same magnification and focus point at the detectors  116 A,  116 B. 
     This embodiment also shows an alternate detection scheme. Light traveling through the focusing optics  17  may be alternatively detected utilizing a pair of simple line detectors  116 A,  116 B and a second bandpass means  214 . The use of this detection scheme eliminates the need for notch filters  19 A,  19 B as shown in the embodiment of FIG.  1 . The second bandpass means  214  permits light of predetermined wavelengths in a range surrounding η A , striking either its front or rear surface, to pass through the second bandpass means  214 , and reflects the remainder of the light spectrum. It should be noted here that the second bandpass means  214  has the same transfer characteristics as bandpass means  14 . Therefore, the composite spectrum η S  consisting of predetermined wavelengths in a range surrounding η A  from the second path  20 B and the remainder of the spectrum η 220A −η A  from the first path  220 A may be separated once again through this second bandpass means  214 . It should also be noted here that to achieve optimum wavelength separation, both bandpass means  14  and  214  should be formed of matched components having identical transfer characteristics. For example, it is preferred that the bandpass means  14 ,  214  be formed of a dichroic mirror or other suitable filter which has been coated and then separated into two components to ensure they have consistent bandpass properties or transfer characteristics. 
     The second bandpass means  214  will pass the predetermined wavelength η A  to the detector  116 A and reflect the remainder of the light spectrum consisting of η 220A −η A  toward detector  116 B to achieve the spectrum separation. It should be noted here that the bandpass means  14 ,  214  may be manufactured to pass any selected wavelength or group of wavelengths not necessarily contiguous in the spectrum with each other. For example, an optical “comb-type” filter may be utilized whereby selected wavelengths of light throughout the spectrum are permitted to pass, and other selected wavelengths are not permitted to pass. Accordingly, the bandpass means  14 ,  214  will reflect the remainder of the spectrum. As long as both bandpass means  14 ,  214  are manufactured from the same lot, the selected wavelengths to be passed versus the wavelengths to be reflected may be selected from a broad spectrum. However, it is preferred that the bandpass means  14 ,  214  have a transfer characteristic such that normalized photonic energy as seen by detectors  116 A and  116 B is equal. Therefore, the bandpass means  14 ,  214  should be designed to send approximately equal amounts of normalized photonic energy towards each detector  116 A,  116 B based upon the detector materials sensitivity to the received spectrum. 
     It should be appreciated that the second bandpass means  214  may function as a filter wherein the second bandpass means  214  transmits a bandwidth of light while reflecting the remainder of the light spectrum. Alternatively, the second bandpass means  214  may function as a mirror, wherein the second bandpass means  214  reflects a bandwidth of light while transmitting the remainder of the light spectrum. It should be noted here that bandpass means  14 ,  214  are therefore defined as a means for efficiently passing a selected bandwidth and for efficiently reflecting a different selected bandwidth. Preferably, both bandpass means  14 ,  214  should be of the same type and calibrated to each other. This can be achieved by producing them in the same lot as described above. 
     Although this alternate detection scheme has been shown as part of the alternative embodiment of FIG. 7, it should be understood by those reasonably skilled in the art that it is interchangeable with the detection scheme of FIG.  1 . Therefore, either detection scheme may be utilized with any of the scanning system embodiments presented here. 
     An advantage of this system is that because the light paths  220 A and  20 B are of equal length, there is no need to selectively place detectors in an angular orientation as was shown in FIG. 2B to account for unequal path lengths. 
     An additional advantage of this embodiment is that it provides higher detection efficiency because of removal of notch filters  19 A,  19 B. Since the notch filters  19 A,  19 B of the first embodiment limit the detected spectrum to a narrow bandwidth, the photonic energy arriving at each detector  116 A,  116 B is therefore limited. 
     Removal of the notch filters  19 A,  19 B in this embodiment provides maximum efficiency in that the sum of the spectrum detected by detector  116 A and the spectrum detected by detector  116 B is substantially equal to the composite spectrum η S . It can be appreciated however by those reasonably skilled in the art that some loss will occur at the bandpass means  214 . 
     An additional advantage of this embodiment is that it may be utilized in scanning systems where there is a short working distance between the object and the bandpass means  14 . It can be appreciated however that by utilizing an auto focus/zoom lens at the focusing optics  17  the working range  250  may be adjusted. 
     It should be understood that various additional components and configurations can be employed to alter the light paths and the intensity and precision of the light without departing from the spirit and scope of the invention. Although the invention has been described in part by making detailed reference to the preferred embodiment, such detail is intended to be instructive rather than restrictive. Similarly, although the preferred embodiment was described as detecting coded symbologies such as bar codes, the invention is not so limited and encompasses the imaging of other targets as well. It will be appreciated by those skilled in the art that many variations may be made in the structure and mode of operation without departing from the teachings herein.