Abstract:
In an optical code detection system and method, infrared pulses are utilized to detect the presence of an object within the range of an optical imaging device, but the image of an infrared pulse reflected from the object is also analyzed to determine the distance between the object and the imaging device. An illumination pulse is then produced to illuminate the optical code on the object, and the characteristic of that pulse, such as duration, are controlled to provide appropriate exposure for an object at the detected distance.

Description:
The present patent application is the U.S. national stage of International Application No. PCT/US08/081,298, which was published in English on Apr. 1, 2010 under Publication No. WO 2010/0036277 A1. The disclosure of the International Application is hereby incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to optical code detection and reading systems and, more particularly, concerns an optical code reading system and method which provide adaptive exposure control. 
     Anyone who has shopped in a modern supermarket is familiar with optical code imaging devices, which facilitate rapid checkout by scanning bar codes imprinted on product packages. This is a relatively undemanding application of bar code reading, as a package is essentially brought to a standstill by the operator for purposes of scanning the bar code. 
     More recently, optical code readers have been utilized in production lines where items are assembled, where they are inspected, where they are packaged, and the like. This application of optical code reading is far more demanding, as products move down a production line at a relatively high speed, for example, on a conveyor belt. In order to avoid the creation of a bottle neck on the production line, it is therefore important that accurate decoding of optical codes take place without reducing the speed at which the objects move down the production line. The speed at which an optical code can be decoded accurately therefore becomes a primary concern. 
     For purposes of explanation herein, imaging devices may include imaging devices, CCD imaging devices, CMOS imaging devices, and other devices for capturing the image. 
       FIG. 1  is a timing diagram illustrating the operation of a typical, existing high speed optical code reader or imaging device. The imaging device creates an image of a scanned optical code on an imaging sensor, and that image is then decoded to recover the optical code. A sensor frame signal  10  defines the periods of time (frames, e.g. F 1 , F 2 , F 3 ) during which an image may be detected and acquired. The imaging device produces pulses of infrared LED illumination  12  and visible light illumination  14 . 
     When the image sensor senses a reflection of an infrared light pulse  16  in a frame F 1 , this is an indication that the presence of an object has been detected within the operating range of the imaging device. Thereafter, a first illumination pulse a is emitted during sensor frame F 2 , and a first image is captured at b. The exposure of this image is then evaluated, and a second illumination pulse c is emitted in the following sensor frame F 3 . The width of that pulse is calculated to produce a properly exposed image during frame f 3  this will result in the optical code being decoded at d, with a high reliability. Typically, it now takes at least 60 ms for the optical code to be decoded. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, infrared pulses are utilized to detect the presence of an object within the range of an optical imaging device, but the image of an infrared pulse reflected from the object is also analyzed to determine the distance between the object and the imaging device. An illumination pulse is then emitted to illuminate the optical code on the object, and the characteristic of that pulse, such as intensity or duration, are controlled to provide appropriate exposure for an object at the detected distance. This effectively provides a variable image depth of field, which improves reliability and speed of decoding. 
     It is a feature of the present invention that proper exposure of an optical code can be obtained in the first instance, resulting in the reduction of total decoding time below 40 ms. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features and advantages of the present invention will be understood more completely from the following detailed description of a presently preferred, but nonetheless illustrative, embodiment in accordance with the present invention, with reference being had to the accompanying drawings, in which: 
         FIG. 1  is a timing diagram illustrating the operation of a typical, existing high speed optical code imaging device; 
         FIG. 2  is a schematic representation of a system embodying the present invention; 
         FIG. 3  is a timing chart useful in describing the operation of system  20  of  FIG. 2 ; 
         FIG. 4  is a flow chart useful in describing the operation of system  20  of  FIG. 2 ; 
         FIG. 5 , comprising  FIGS. 5(A) , and  5 (B), illustrates a preferred method of estimating the distance between the object and the imaging device, with  FIG. 5(A)  schematically representing the object positioned at three different distances a, b, and c (50 mm, 100 mm and 150 mm, respectively), and  FIG. 5(B)  depicting the image of the reflected infrared radiation (a spot) obtained at each of the positions a, b, and c (from left to right); 
         FIG. 6(A)  is a graph showing how the intensity of the emitted visible illumination decreases with the distance between the object and imaging device; 
         FIG. 6(B)  is a graph illustrating a preferred method for adjusting visible illumination based upon the distance between the object and imaging device; and 
         FIG. 7  is block diagram of a preferred embodiment of system  20  incorporating the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning now to the drawings,  FIG. 2  is a schematic representation of an optical imaging device system  20  embodying the present invention.  FIG. 3  is a timing chart useful in describing the operation of system  20  of  FIG. 2 , and  FIG. 4  is a flow chart, also useful in describing the operation. 
     As may be seen in  FIG. 2 , system  20  includes an infrared light emitting diode (LED  22 ), which emits infrared radiation and an illumination LED  24 , which emits visible light. Radiation from LEDs  22  and  24  is directed at a face of an object O, which bears an optical code, preferably in an area above a point P where the infrared radiation impinges. A camera module  26  having a field of view FOV defined by the lines L 1 , L 2  monitors object O through a mirror M. Line L 3  represents an image of point P reflected from object O to camera module  26 . 
     In operation, as represented by block  100  in the flow chart of  FIG. 4 , the infrared LED  22  is pulsed during a sensor frame F 2  and, when the image of point P is detected in camera module  26 , a determination is made at A that an object is present, and estimation of its distance from system  20  is initiated (block  102 ). Upon completion of frame F 2 , it is determined at B what the appropriate illumination should be for object O (block  104 ), based upon the previous determination of the distance between object O and system  20 . In the following frame F 3 , a pulse producing the appropriate illumination (intensity and duration) is emitted at C (block  106 ). With the appropriate illumination, the optical code is decoded with a high probability of success, upon completion of frame F 3  (block  108 ). Thereafter, the charge stored in the image sensor may be erased, and control returns to block  100 , to permit decoding of the code on the next object. 
       FIG. 5 , comprising  FIGS. 5(A) , and  5 (B), illustrates a preferred method of estimating the distance between the object and the imaging device, with  FIG. 5(A)  schematically representing the object positioned at three different locations a, b, and c (50 mm, 100 mm and 150 mm, respectively), and  FIG. 5(B)  depicting the image of the reflected infrared radiation (a spot) obtained at each of the positions a, b, and c (from left to right). 
       FIG. 5(A)  is similar to  FIG. 2 , except it depicts the object O at three different distances a, b, and c from the imaging device (50 mm, 100 mm and 150 mm, respectively). As may be seen, the infrared ray R impinges on the object O at different heights at the positions a, b, and c. The reflections of the ray R from the object are represented by broken lines in  FIG. 5(A) . The reflected beams reflect off of mirror M and pass through a lens L, which forms an image of ray R on an image sensor S (lens L and image sensor S are part of camera module  26  of  FIG. 2 ). The image of the ray Ron image sensor S is a spot  70  within an otherwise dark area, and as may be seen in  FIG. 5(B) , the spot  70  is at different heights in the image, because the beam impinges on the object at different heights in each of positions a, b, and c (imaged from left to right, respectively). 
     In practice system  20  would be calibrated to place the spot  70  at the top of the image formed on sensor S when object O is at the nearest position to be measured. Thereafter, the distance between object O and sensor S can be estimated, based upon the height of spot  70  in the image. Those skilled in the art could readily program this function into the system electronics or into a look up table. 
     Once the distance between the object and the imaging device is determined, the visible illumination provided to form a well exposed image can be controlled accordingly.  FIG. 6(A)  is a graph showing how the intensity of the emitted visible illumination decreases with the distance between the object and imaging device. Basically, illumination varies inversely with the square of the distance. Therefore, more light must be provided to the object as its distance increases. 
     In the preferred embodiment, illumination is adjusted by controlling the duration of the pulse of visible illumination in relationship to the determined distance between the object and the imaging device.  FIG. 6(B)  is a graph illustrating a preferred method for adjusting visible illumination based upon the distance between the object and imaging device. As may be seen, the width of the pulse of illumination is increased with the distance between the object and imaging device. Preferably, the duration of the pulse is kept at a constant value when the distance exceeds a threshold value, 140 mm in the preferred embodiment. Those skilled in the art will appreciate that it would also be possible to control the pulse&#39;s intensity or both its intensity and duration to achieve desired illumination. 
       FIG. 7  is block diagram of a preferred embodiment of system  20 . System  20  broadly comprises and imaging portion or subsystem  30 , which forms an image of the optical code C on an object O; and illumination portion or subsystem  40 , which provides both the infrared and visible illumination to produce an image of object O; and a processor portion or subsystem  50 , which provides all the necessary processing for the operation of system  20 . 
     Imaging portion  30  includes optics  32 , such as a lens system, which focuses an image of the optical code C on an image sensor  34 , such as a CMOS array. Such devices are well known in the art. Image sensor  34  contains an array of pixel elements storing the image which can be processed to reproduce (decode) the information in optical code C in processor portion  50 . 
     Processor portion  50  includes an application specific integrated circuit (ASIC)  52 , which processes the pixel information from image portion  30  to reproduce the information encoded in optical code C. ASIC  52  has access to random access memory (RAM, preferably SDRAM)  54  and read-only memory (ROM)  56 . Processor portion  50  also includes a complex programmable logic device (CPLD)  58 , which provides control signals for the illumination portion  40 . CPLD  58  is programmed with the graph of  FIG. 6 . ASIC  52  provides a signal to CPLD  58  which represents the intensity of detected infrared radiation from point P, and CPLD  58 , making use of the programmed curve of  FIG. 6 , produces a control signal that controls the illumination provided by illumination portion  40 . 
     Illumination portion  40  includes an LED  42  which emits visible radiation and is driven by a driver  44  under control of CPLD  58 . Illumination from LED  42  is focused by optics  46 , for example a lens system. Also included in illumination portion  40  is an infrared LED  48  which is driven by a driver  49  under control of CPLD  58 . Illumination from LED  48  is focused via optics  47 , for example a lens system. 
     Operation of system  20  is as described previously with respect to  FIGS. 3 and 4 . Those skilled in the art will appreciate that the intensity or the duration of a visible light pulse, or both, may be controlled to achieve proper exposure. 
     Although a preferred embodiment of the invention has been disclosed for illustrative purposes, those skilled in the art will appreciate that many additions and modifications, and substitutions are possible without departing from the scope and spirit of the invention as defined by the accompanying claims.