Abstract:
A system and method of reading an indicia and rejecting ambient light is disclosed. An imaging signal and an ambient signal are received by the system where the imaging signal corresponds to indicia information and a first portion of the ambient light, and the ambient signal corresponds to a second portion of the ambient light. The imaging signal and the ambient signal are mathematically manipulated to subtract the contribution of the first portion of the ambient light from the imaging signal.

Description:
CROSS REFERENCES 
     This application claims the benefit of U.S. Provisional Application No. 61/046,289, entitled “OPTICAL COLLECTION DEVICE UTILIZING DIFFERENTIAL AREA PHOTODIODES FOR REJECTING PARASITIC AMBIENT LIGHT AND MAXIMIZING RETROCOLLECTED MODULATED LASER LIGHT”, filed Apr. 18, 2008, and is hereby incorporated by reference. 
    
    
     BACKGROUND 
     Conventional laser bar code readers scan a laser beam across a distant bar code label and detect an optical signal reflected off the bar code. However, ambient light, from sources such as low energy lights, neon signs, and sunlight, are also present in the detected signal. The frequency range of the ambient parasitic light is wide, ranging from DC to high frequencies. The ambient light component severely degrades the signal to noise ratio of the reflected bar code signal when the reading distance between the bar code reader and the bar code increases. 
     Retro reflection systems are well known in the art for extracting a signal from random noise. However, these systems are large, have low scanning frequencies, and are costly. Thus, they are not suited for use in handheld computers or bar code scanners. 
     There is a need for a system that overcomes the above problems, as well as providing additional benefits. Overall, the above examples of some related systems and associated limitations are intended to be illustrative and not exclusive. Other limitations of existing or prior systems will become apparent to those of skill in the art upon reading the following Detailed Description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a system block diagram of a bar code reader. 
         FIGS. 2A and 2B  show an example of collection optics imaging a field of view. 
         FIG. 3  shows an example of two adjacent collection optics with photodiodes mounted on flexible circuits. 
         FIG. 4  shows a ray tracing diagram of laser light collected by two adjacent reflective optics. 
         FIG. 5  shows an example of the active areas of the photodiodes used with the present invention. 
         FIG. 6  is a circuit diagram showing an example of an electronic circuit used to remove the parasitic ambient light from a detected laser bar code signal. 
         FIG. 7  shows an example of an alternative sensor configuration. 
         FIG. 8  depicts a flow diagram illustrating a suitable process for reading an indicia and rejecting ambient light. 
     
    
    
     DETAILED DESCRIPTION 
     Described in detail below is a bar code reader that uses an optical collection system to image light onto differential area photodiodes. In a suitable example, three photodiodes may be used, including a first main photodiode that receives the laser bar code signal along with ambient light and two smaller photodiodes located on either side of the main photodiode that receive only the ambient light. In one example, the total of the active areas of the two smaller photodiodes is approximately equal to the active area of the main photodiode. 
     Various aspects of the invention will now be described. The following description provides specific details for a thorough understanding and enabling description of these examples. One skilled in the art will understand, however, that the invention may be practiced without many of these details. Additionally, some well-known structures or functions may not be shown or described in detail, so as to avoid unnecessarily obscuring the relevant description. 
     The terminology used in the description presented below is intended to be interpreted in its broadest reasonable manner, even though it is being used in conjunction with a detailed description of certain specific examples of the invention. Certain terms may even be emphasized below; however, any terminology intended to be interpreted in any restricted manner will be overtly and specifically defined as such in this Detailed Description section. 
     A bar code reader includes optics for focusing a laser beam and scanning it across a bar code and optics for collecting the laser light reflected off the bar code. The collection optics are designed to optimize the field of view, maximize the collection area, and collect the light on a photodetector. However, parasitic ambient light is present on and around the area near a bar code. The ambient light may have both a DC and a modulated frequency component. When modulated ambient light is superimposed upon the laser light reflected off a bar code, the signal to noise ratio at the photodetector of the bar code signal is degraded, especially when the reading distance between the bar code and the bar code reader increases. To improve the bar code signal, the bar code reader may capture the ambient light near the bar code and subtract it from the light reflected from the bar code that includes ambient light. 
     Conventional laser bar code readers, either retro reflective or fixed collective, detect reflected light at a point location. Alternatively, a mirror sends reflected light to a point detector, or a collective optic focuses reflected light onto a point detector. In both of these cases, it is impossible to remove ambient light with an additional detector because there is no image of the indicia, and illumination light rays reflected from the indicia would be collected in both light detecting photodiodes. 
       FIG. 1  shows an example block diagram  100  of a bar code reader used to read bar codes or other indicia at a distance. A bar code reader may include one or more light sources  110 , a scanning mirror  115 , imaging optics  120 , and bar code reader electronics  190 . 
     The light sources  110  include light source means such as laser diodes, solid state lasers, light emitting diodes (LEDs), incandescent bulbs, halogen lamps, and gas discharge lamps. A focused light source  110  such as a laser may be used for illuminating a bar code. Alternatively, a non-laser light source  110  may be used to illuminate a bar code in the present invention, provided the light source is sufficiently focused. A scanning mirror  115  may be used to scan the laser  110  across a bar code or other indicia  118 , and imaging optics  120  may be used to collect the laser light reflected from a first area surrounding and including the bar code  118  onto a primary light detector  130 . Alternatively, the scanning mirror  115  may be shaped to provide the functionality of the imaging optics  120 . The imaging optics  120  also image light from one or more areas near, but should not be overlapping, the first area onto one or more secondary light detectors  130 . The secondary light detectors  130  do not receive any light reflected from the bar code  118 . 
     The bar code reader electronics  190  may include one or more light detectors  130 , processors  140 , memory units  180 , communications modules  150 , input/output devices  160 , and power supplies  170 . 
     The light detectors  130  include light sensing means such as photodiodes, PIN diodes, photodetectors, photoconductors, charge-coupled devices (CCD) that can convert an optical signal into an electrical signal. A processor  140  may be used to decode the electrical signals from the detectors  130 . Memory units  180  may include, but are not limited to, RAM, ROM, and any combination of volatile and non-volatile memory. The memory units  180  may store the converted electrical Communications modules  150  may be used to transmit scanned bar codes either wirelessly or through electrical or optical cables to another device, a database, a memory unit, and/or a processor. Input/output devices  160  may include, but are not limited to, triggers to start and stop the bar code reader or to initiate other bar code reader functions, visual displays, speakers, and communication devices that operate through wired or wireless communications. Power supplies  170  may include, but are not limited to, a battery or an electrical wall outlet. 
       FIG. 2A  shows an example imaging diagram  200  generated by collection optics  210  used by a laser bar code reader. The collection optics  210  may include, but are not limited to, a lens or a mirror, and a plurality of photodetectors. The collection optics  210  see a field of view  220 . Three particular areas are delineated in the field of view  220 , a top rectangular area  222 , a middle rectangular area  224 , and a bottom rectangular area  228 . Within the middle rectangular area  224  lies a bar code (not shown) to be scanned. A laser in the bar code reader scans a line  226  in the field of view  220  in order to read the bar code. The sum of the areas of the top and bottom rectangular areas  222 ,  228  are approximately equal to the area of the middle rectangular area  224  in one example. Because parasitic ambient light is present throughout the field of view  220 , and the intensity of the ambient light is essentially spatially independent, the total ambient light to which the top and bottom rectangular areas  222 ,  228  are exposed is approximately equal to the ambient light to which the middle rectangular area  224  is exposed. 
     The collection optics  210  image the field of view  220  onto an image plane  230 . The top rectangular area  222  in the field of view  220  is imaged to area  232 , the bottom rectangular area  228  in the field of view  220  is imaged to area  238 , and the middle rectangular area  224  in the field of view  220  is imaged to area  234  in this example. Three separate photodiodes may be used in the image plane  230  with active areas covering each of the areas  232 ,  234 ,  238 . 
     Alternatively, a non-laser light source may be used to illuminate the bar code as long as the light source is focused to illuminate only the rectangular area  224  around the bar code and not the neighboring areas  222 ,  228 . If the bar code is not entirely contained within the rectangular area  224 , the light source must still be focused to stay within the rectangular area  224 . In another example, a scanning mechanism may use optics to spread light from a light source, such as a laser, into a narrow line of light and project the line of light onto the bar code, while remaining entirely within the area  224 . It is important that the light be confined within the area  224  because the light source must not illuminate the photodiodes  232 ,  238  that sense ambient light from, respectively, the top and bottom rectangular areas  222 ,  228  in the field of view  220 . 
       FIG. 2B  shows an expanded diagram  250  of the image plane  230 . Because the parasitic ambient light is evenly distributed over the field of view  220 , upon imaging by the collection optics  210 , the parasitic ambient light  260  is also evenly distributed in the imaging plane  230 . The line  226  scanned by the laser in the field of view  220  is imaged as line  236 . Also shown in the image plane  230  is the image of the bar code  237  scanned by the laser. Note that the bar code image  237  and the image of the laser scan line  236  are both contained in the area  234  and do not overlap the adjacent areas  238 ,  232 . It will be apparent to a person skilled in the art that the collection optics  210  may magnify or shrink the field of view  220  as it is imaged onto the image plane  230 , but the ratio of the dimensions of the areas  222 ,  224 ,  228  is substantially maintained in the image plane  230 . Each of the three photodiodes  238 ,  234 ,  232  in the image plane  230  converts the light impinging upon its surface into an electrical current. Thus, due to the fidelity of the image plane, all three photodiodes receive and convert parasitic ambient light, but only the photodiode covering area  234  receives and converts the laser signal reflected off the bar code. 
       FIG. 3  shows a front isometric view of a suitable example  300  of two juxtaposed optical collection imagers  310 ,  320 . The optical imagers  310 ,  320  each have a concave mirror designed to maximize the collecting area, optimize the optical field, and focus light onto an image plane. A rectangular hole  330  between the two optical collection mirrors  310 ,  320  permits a laser beam to pass through. The arrows near the top of example  300  indicate that the bar code or indicia to be scanned is located towards the right side of the imagers  310 ,  320 , and the light reflected from the bar code travels in the opposite direction. The laser from the bar code reader is scanned across a bar code, and the reflected light is focused and imaged by the mirrors  310 ,  320  onto photodiodes or other transducers that convert light to electricity located on the underside of flexible circuits  315 ,  325 , as indicated by the dotted lines. It will be apparent to a person skilled in the art that although two optical collection imagers are used in example  300 , any number of collection imagers may be used to image the laser light reflected off a bar code, such as one imager or three or more imagers. 
       FIG. 4  shows an example ray tracing diagram  400  of the example  300  having two juxtaposed optical collection imagers  310 ,  320 . For clarity, the rays internal to the bundle of rays depicted in diagram  400  are not shown. A laser beam  410  is seen entering from the left side of the diagram. The laser beam passes through the hole  330  (not visible) between the collection optics  310 ,  320 . The laser beam then reflects off a bar code (not shown) beyond the right side of the diagram and travels back toward the collection optics  310 ,  320 . The reflected laser signal is transmitted through the front surfaces  418 ,  428  of the collection optics  310 ,  320  and then reflects off the back surfaces  419 ,  429  of the collection optics  310 ,  320  before striking the photodiodes located on flexible circuits  315 ,  325 . 
     The optical collection imagers  310 ,  320  in  FIGS. 3 and 4  use mirrors to fold the reflected rays within a compact space. Alternatively, the optical collection imagers  310 ,  320  may use lenses rather than mirrors where the photodetectors are positioned on the opposite side of the lens from the bar code or other indicia. 
       FIG. 5  shows suitable, relative dimensions of the active areas of the photodiodes used in a suitable optical sensor  500  with the example  300  and the resulting pattern of the photodiodes on a photodiode chip  550 . Two of the sensors  500  are used with the example  300 , one on each of the flexible circuits  315 ,  325 . There are three photodiodes  510 ,  520 ,  530  in the sensor  500 , similar to the photodiodes described in diagram  250 . In this example, the middle photodiode  520  is the only one of the three photodiodes to receive the laser signal reflected off the bar code, but all three photodiodes  510 ,  520 ,  530  receive the parasitic ambient light. The lengths of the three silicon photodiodes  510 ,  520 ,  530  are approximately equal, having a length of 5.6 mm in the prototype. However, while the width of the middle photodiode  520  is 0.6 mm, the widths of each of the top  510  and bottom  530  photodiodes are 0.3 mm. Thus, the area of the middle photodiode  520  that receives the laser signal is approximately the same as the total of the areas of the top and bottom photodiodes  510 ,  530  that only receive the parasitic ambient light. 
     Electric current generated by photodiode  520  has two components, current from the reflected laser signal and current from the ambient light. Because the active area of photodiode  520  is approximately equal to the sum of the active areas of the photodiodes  510 ,  530 , the current generated by photodiode  520  due to the ambient light is approximately equal to the total current generated by the ambient light by photodiodes  510 ,  530 . The example electrical circuit  600  shown in  FIG. 6  may be used to subtract out or remove the current component generated by the ambient light from the current component generated by the laser bar code signal at photodiode  520  to obtain just the desired bar code signal. In one example, the currents generated by the two photodiodes  510 ,  530  are combined, and the two photodiodes  510 ,  530  are represented by a single photodiode circuit element  612  in the circuit diagram  600 . The photodiode  520  is represented by the photodiode circuit element  610 . Note that two of the electrical circuits  600  will be used with the example  400 , one for each set of three photodiodes on the flexible circuits  315 ,  325 , and the outputs of the two circuits  600  are combined. In a different example, the current from the two middle photodiodes would be combined, and the currents from the two top photodiodes and the two bottom photodiodes would also be combined; the latter currents would then be subtracted from the former currents using a single electrical circuit  600 . 
     Both photodiode circuit elements  610 ,  612  are DC-biased through resistors, transistors, or impedance elements  620 ,  622 ,  624 ,  626 . The currents of the photodiode elements  610 ,  612  pass through the capacitors  630 ,  640 ,  650 ,  660  located near the input terminals of the amplifier  640 . Consequently, unwanted parasitic current generated by the photodiode  520  (or equivalently the photodiode circuit element  610 ) is effectively amplified and cancelled electronically at the output to the amplifier  640  without the introduction of any additional noise or the use of any other amplifiers or circuits that might decrease the signal to noise ratio. Moreover, because the same optical collector operates upon the same local field, the efficiency of the cancellation of the parasitic ambient light is maximized. 
     It should be noted that the amount of current generated by parasitic ambient light actually removed from the current generated by the photodiode  520  by circuit  600  depends upon the level of the signal detected by the photodiode  520  relative to the levels of the signals detected by the other photodiodes  510 ,  530 , and the signal detected by the photodiodes  510 ,  520 ,  530  depends upon the spatial efficiency of the optical collection imagers used to image the light onto the photodiodes and the surface reflection coefficients of the bar code and the area near the bar code. For the dimensions of the photodiodes in the prototype  500 , where the photodiodes  510 ,  530  are approximately half the width of the photodiode  520 , spatial efficiency variations are very small. Also, typically the surfaces above and below the bar code usually have the same reflection coefficient as the bar code itself. Thus, the configuration of the photodiode prototype  500  may be a preferred implementation in certain situations. 
     It will be apparent to a person skilled in the art that other dimensions and configurations of the photodiode active areas and/or different ratios of the width to the length of the photodiode active areas may be used. For example, in  FIG. 7 , an alternative sensor configuration  700  is shown. A bar code or other indicia may be raster scanned by a laser onto a two-dimensional sensor  710  located on the image plane. The two-dimensional sensor  710  may be a CMOS-based sensor. The two-dimensional sensor  710  may be surrounded by smaller area photodetectors  720 ,  730 ,  740  or point photodetectors  750 ,  751 ,  752  that collect ambient light without collecting the laser light reflected from the bar code. It will also be apparent that any number of photodiode areas used for imaging ambient light may be used in conjunction with the photodiode area imaging the laser light reflected off the bar code, following the guidelines given above with respect to the surface area, spatial efficiency, and reflection coefficients. Also, the active areas of the detectors for detecting the laser signal and for detecting just parasitic ambient light need not be equal, but other electrical or optical accommodations would be necessary. Further, other electrical circuit configurations may also be used to cancel the photodiode current generated by the parasitic ambient light. 
       FIG. 8  depicts a flow diagram illustrating a suitable process  800  for reading an indicia and rejecting ambient light. At block  805 , the system illuminates the indicia to be read with a light source. The light source can be either a laser or non-laser source. In one example, the light source can be focused and scanned over the indicia. 
     At block  810 , the system images the light reflected from a first area immediately surrounding and including the indicia onto the active area of a primary photodetector using imaging optics. The light reflected from the first area includes light reflected from the indicia and also parasitic ambient light. Then at block  815 , the primary photodetector converts the imaged light to a primary electric current. 
     In parallel to the imaging performed by the system at block  810 , at block  812 , the system can use the same imaging optics to image ambient light from one or more additional areas near the indicia that should not be overlapping the first area onto one or more secondary photodetectors. Because the ambient light is substantially spatially independent, the ambient light per unit area imaged from the first area is substantially the same as the ambient light per unit area imaged from the one or more additional areas near the indicia that do not overlap the first area. Note that if the system uses a non-laser source to illuminate the indicia, the non-laser source should be focused by the system such that no light from the source is imaged onto the secondary photodetectors. 
     Then at block  817 , the secondary photodetectors each convert the ambient light that impinges on their respective active areas into secondary electric currents. A processor adds up all of the secondary electric currents generated by the secondary photodetectors at block  820 . 
     At block  825  the system calculates a multiplier for weighting the summed secondary currents. The multiplier is calculated by dividing the active area of the primary photodetector by the sum of the active areas of the secondary photodetectors. In some instances, the multiplier can also be dependent upon the spatial efficiency of the imaging optics and/or the surface reflection coefficients of the indicia and the areas near the indicia. At block  830 , the system weights the sum of the secondary electric currents obtained in block  820  by the multiplier obtained in block  825 . 
     At block  835 , the system subtracts the weighted secondary electric current sum from the primary electric current to obtain the electric current generated by the light reflected from the indicia, free of the influence of ambient light. At block  840 , the system optionally amplifies the signal current for further processing. The process  800  ends at block  899 . 
     The words “herein,” “above,” “below,” and words of similar import, when used in this application, shall refer to this application as a whole and not to any particular portions of this application. Where the context permits, words in the above Detailed Description using the singular or plural number may also include the plural or singular number respectively. The word “or,” in reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list. 
     The above detailed description of examples of the invention is not intended to be exhaustive or to limit the invention to the precise form disclosed above. While specific embodiments of, and examples for, the invention are described above for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. For example, while a laser bar code reader for reading bar codes is mentioned, any desired target indicia may be scanned or imaged under the principles disclosed herein, such as alphabetic, numeric, or CJK (Chinese, Japanese, Korean language character sets) characters. Further any specific numbers noted herein are only examples: alternative implementations may employ differing values or ranges. 
     The teachings of the invention provided herein can be applied to other systems, not necessarily the system described above. The elements and acts of the various examples described above can be combined to provide further examples. 
     While the above description describes certain examples of the invention, and describes the best mode contemplated, no matter how detailed the above appears in text, the invention can be practiced in many ways. Details of the system may vary considerably in its implementation details, while still being encompassed by the invention disclosed herein. As noted above, particular terminology used when describing certain features or aspects of the invention should not be taken to imply that the terminology is being redefined herein to be restricted to any specific characteristics, features, or aspects of the invention with which that terminology is associated. In general, the terms used in the following claims should not be construed to limit the invention to the specific examples disclosed in the specification, unless the above Detailed Description section explicitly defines such terms. Accordingly, the actual scope of the invention encompasses not only the disclosed examples, but also all equivalent ways of practicing or implementing the invention under the claims.