Abstract:
In laser bar code scanners bright sunlight tends to overpower the reflected laser light, drastically reducing the difference between bar code and background signals. In the present bar code reader, a plurality of light detectors are utilized with different spectral response characteristics at the wavelength of the incident laser beam, such that the differential response of the detectors tends to minimize the effect of background illumination. Narrowband optical filters with offset passbands may be used with conventional light sensors. As another example, filters with long wavelength transmission characteristics and offset cutoff wavelengths may be employed if necessary with a neutral density filter to provide balanced responses under broadband illumination.

Description:
SUMMARY OF THE INVENTION 
     This invention relates to an optical reading system and method, and particularly to a laser bar code reader system. 
     It is an object of the invention to provide a bar code reader of enhanced sensitivity, and/or a reader requiring a lower power laser light source, with consequent greater safety, better adaptability to hand-held and/or portable (battery powered) operation, less heat to dissipate and longer life expectancy. 
     Another object of the invention is to provide a laser bar code reader particularly adapted to reliable operation in sunlight. 
     A further object is to provide a laser beam scanner unit for reliably reading optical information, wherein the beam is swept along an extended scanning path at an information containing surface, and which scanner unit is particularly adapted to operation under daylight conditions. 
     Other objects, features and advantages of the invention will be apparent from the following detailed description, taken in connection with the accompanying sheet of drawings, and from the respective features and relationships of the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagramatic illustration of a laser bar code reader system in accordance with the present invention; 
     FIG. 2 is a graphical illustration for explaining one embodiment of filter means for use in FIG. 1; 
     FIG. 3 illustrates another embodiment of filter means for use in FIG. 1; 
     FIG. 4 is a diagramatic end elevational view of a laser bar code scanner unit wherein the laser beam is to be swept over an extended scanning path to read a relatively wide bar code label; 
     FIG. 5 is a somewhat diagramatic view taken generally along the line V--V of FIG. 4; 
     FIG. 6 is a diagramatic view taken generally along the line VI--VI in FIG. 4 and indicating exemplary electronic circuitry for association with the swept laser beam scanner embodiment of FIGS. 4, 5 and 6; and 
     FIG. 7 illustrates a wand type bar code scanner in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is intended as a generic illustration wherein scanning of a bar code label 10 takes place by relative movement between a laser beam indicated at 11 and the bar code label 10. For example, the label may be moved in a longitudinal direction as indicated by arrow 12, or the laser beam may be moved in a scanning direction such as indicated at 14 for impingement on successive points along a scanning path such as indicated at 15. 
     By way of example, a laser light source is indicated at 20 and respective light detectors 21 and 22 are shown for receiving reflected light produced by the beam 11 at each successive point along the scanning path 15. By way of example, detectors 21 and 22 may be fixedly secured in a housing with the laser source 20 so as to be focused at a common point such as indicated at 15a at a suitable distance from an end face of the housing. In one type of embodiment with common point focus, the label 10 may be moved longitudinally as indicated at 12 so as to effect sequential scanning of the complete bar code. In another example, the housing itself may be moved in the direction of arrow 14 so that the complete bar code is sequentially scanned. In a further example, laser light source 20 and detectors 21 and 22 may be pivotally mounted within the housing so as to jointly sweep along the scanning path 15 so as to scan a complete bar code. In another type of a scanner, the laser light source 20 is equipped with scanning means for causing the beam 11 to scan along a scanning path such as indicated at 15 at a selected distance from the housing, while detectors 21 and 22 are arranged to collect reflected light from each successive point along the scanning path 15. Alternatively, the laser light source 20 may be simultaneously illuminate the entire regin 15 and the detector means 21 and 22 may be pivotally mounted to sequentially scan successive points along the region 15. 
     In a specific example in accordance with the present invention, detectors 21 and 22 comprise respective light sensors 31 and 32 which may be identical, and respective filters 41 and 42 which provide generally comparable response to sunlight but provide substantially different responses to the limited spectral band transmitted by the light source 20, such that an enhanced sensitivity is provided by a differential between the outputs from sensors 31 and 32. 
     In the embodiment of FIG. 2, laser light source 20 supplies a wavelength of light as indicated at 50 and the filters 41 and 42 have bandpass spectral properties as indicated at 51 and 52. 
     In the embodiment of FIG. 3, the wavelength of the laser light source 20 is indicated at 60 and the broadband spectral transmission properties of the respective filters 41 and 42 are indicated at 61 and 62. 
     In each of the embodiment of FIGS. 2 and 3, the ordinate axis may represent transmission between zero percent and one hundred percent. In each case, the outputs from detectors 21 and 22 are preferably substantially balanced, that is of equal amplitude in the presence of sunlight alone, the differential in a transmission at the wavelength 50 or 60, being at least fifty percent in the examples of FIGS. 2 and 3. 
     In FIG. 4, there is indicated a scanner unit 70 including a housing with an end face of 71 which is arranged to confront a bar code label such as indicated at 10 in FIG. 1 at a selected distance such as three or more inches. In the indicated example of FIG. 4, the laser beam may be scanned along the length of an elongated window indicated at 73 and may effect scanning in a plane such as indicated at 74 which would include the scanning path such as in indicated at 15 in FIG. 1. 
     In the exemplary embodiment of FIG. 4, an array of first and second light detectors is provided with the first detectors such as 21A and 21B and the second detectors such as 22A and 22B, being arranged in respective pairs such as 21A, 22A along the locus of reflected light produced by the scanning of the laser beam. For the example of two pairs as shown in FIG. 4, and for scanning of the laser beam from left to right as viewed in FIG. 4, during scanning of a left segment of the label, reflected light would predominately reach the detectors 21A and 22A. In a mid region of the label, reflected light would reach both pairs of detectors with comparable magnitude, and for a right-hand segment of the bar code label, the reflected light would predominately reach the right-hand pair of detectors 21B, 22B. For each point along the scanning path the reflected light reaching a first detector such as 21A of a pair would be of substantially equal magnitude with the reflected light reaching the second detector such as 22A of such pair. 
     In the specific example of FIG. 4, a common filter element 81 having spectral characteristics as indicated at 51 or 61 may cover all of the first light sensors such as 31A and 31B of the array, while a common filter element 82 having the spectral transmission properties 52 or 62 may be assoicated with the second light sensors of the array such as indicated at 32A and 32B. By way of example, window 73 and filter elements 81 and 82 may form part of the end face 71 of the housing of the laser bar code reader unit of FIG. 4, other portions of the end face 71 being opaque, so that light can only enter or exit the housing through window 73 and filter elements 81 and 82. 
     In FIG. 5, laser light source means 20-1 is indicated as comprising a laser source 90 and a suitable scanner system 91 which may cause a laser beam 111 of a wavelength such as indicated at 50 or 60 to be focused at a point along the scanning path 15-1 and to scan along the path as indicated by arrow 112. For beam positions between those indicated at 114 and 115, reflected light is predominately received by the pair 21B, 22B. For beam positions between 115 and 116, comparable amplitudes of reflected light may reach both pairs 21B, 22B and 21A, 22A, while for beam positions between 116 and 117, reflected light may predominate at the pair of detectors 21A, 22A. 
     In the example of FIG. 5, each detector may have an individual filter element such as filter elements 42A and 42B associated with respective second light sensors 32A and 32B. In FIG. 5, end face 71 may provide a common optical window for transmitting the laser beam 111 at a region such as 73, FIG. 4, and for admitting reflected light at regions such as indicated at 81 and 82 in FIG. 4. 
     As seen in FIG. 6, each pair of light detectors of the array, such as first and second detectors 21B and 22B are symetrically arranged with respect to the plane 74 of the scanning laser beam so that the paths for reflected light from each point such as indicated at 121 along the scanning path 15-1 to detectors 21B and 22B are equal. As in FIG. 5, each detector is shown as comprising a sensor such as 31B, 32B, and a filter element such as 41B, 42B with respective spectral transmission properties as indicated in FIG. 2 or FIG. 3. In each of the embodiments, as indicated in FIG. 6, the output of each detector pair such as 21B, 22B may be supplied to a differential amplifier means such as indicated at 130A, 130B. The outputs of the detector pair 21A, 22A may be supplied to the differential amplifier means 130A, and the outputs of differential amplifier means 130A and 130B may be suitably combined either on an analog basis or on a digital basis to provide a resultant bar code signal to be decoded. By way of example, a clock 132 may be connected with component 131, and with a beam scanner control means 133 may be constructed and operated so that component 131 can determine the position of the beam 111 with reference to the zones 114-115, 115-116, and 116-117, respectively. For example component 131 may supply pulses derived from clock pulses to component 133 to drive the scanning operation. Alternatively, beam driving pulses may be generated at component 133 and supplied also to component 131. 
     It is contemplated that the present embodiments will provide a more reliable bar code reading with a given laser source and under natural and supplementary lighting conditions, and further may allow the use of a lower power laser light source, with many attendant benefits such as greater safety, better adaptability to portable (battery powered) and hand-held use, less heat to dissipate, and therefore expected longer life. A differential between the outputs of detectors such as indicated at 21 and 22 will provide superior noise rejection properites in comparison to a detector such as 31 by itself. If necessary, a neutral density filter could be combined with one or the other of the filters such as 41 and 42 to balance the detector outputs under broadband illumination (e.g. sunlight). 
     FIG. 7 shows a hand-held wand type canner 100 according to FIG. 2 or FIG. 3 wherein a light emitting diode or other narrow band light source 120 produces a band of light for example in the infrared region. In this case, wavelength 50 or 60 may be of the order of 910 nanometers, and light sensors 131 and 132 may be particularly sensitive at this wavelength. According to the example of FIG. 2, filters 141 and 142 have passbands as indicated at 51 and 52 respectively, while according to the example of FIG. 3, the filters 141 and 142 have overlapping wideband characteristics as indicated at 61 and 62. In each case, the outputs of the light sensors 131 and 132 may be supplied to differential amplifier means such as 130B, FIG. 6, so as to provide a resultant output especially sensitive to a bar code scanned thereby even in the presence of ambient daylight illumination. 
     In one embodiment according to FIG. 7, light is transmitted from light source 120 to a light port 144 via optical fibers 145, and reflected light is transmitted via respective optical fibers such as indicated at 146 and 147 which terminate at a small-area central region of light port 144. By way of example, the reflected light transmitting fibers such as 146 and 147 may be essentially uniformly distributed at the port 144 over a central circular area which is small in comparison to the size of a minimum width bar of a bar code to be scanned, so that ambient light has less effect on resolution than where the size of the incident light spot is relied upon to define scanning resolution. 
     Where lenses are utilized, the reflected light is preferably collected by symmetrically arranged lenses focussed at a common reflected light pickup region at the bar code for high resolution scanning of the bar code. (The pickup region may have a small diameter in comparison to a minimum bar dimension.) Preferably in this case also equal amounts of reflected light are transmitted to respective photodetectors such as 131,141 and 132,142. Again it is preferred that the reflected light optics provide the required resolution independently of the size of the incident light spot from the light source such as 120 so that resolution is less affected by the presence of intense ambient light. 
     Preferably in each case the response characteristics of the detectors with respect to reflected light are so matched that the first and second light sensors such as 131 and 132 provide essentially equal signals when the light source 120 is not energized and the light port 144 is held against the bar code, for each incremental position of port 144 along the length of the bar code, even in the presence of sunlight. 
     Light source 120 may be a conventional light source for a wand type scanner such as a light emitting diode, or may be a laser light source. 
     It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts and teachings of the present invention.