Abstract:
A portable electo-optical scanner for reading a target bar code having a plurality of bar code elements. The portable scanner includes: a housing supporting a scanning module and an exit window. The scanning module scans the target bar code with a scanning beam and collecting reflected light returning from the bar code. The scanning module includes: a beam source emitting a scanning beam; photodetector circuitry; and a retro-reflective oscillating mirror including a light collection mirror and an integral scanning or beam directing mirror. The scanning mirror is positioned to intersect the scanning beam and direct the scanning beam through the exit window. Oscillation of the oscillating mirror causes the scanning beam to be repetitively scanned along a scanning direction across the target bar code. The light collection mirror has a field of view that follows the scanning beam. The light collection mirror receives reflected light from the target bar code and directs the reflected light toward the photodetector circuitry. The exit window comprises a lens having a positive optical power with respect to the scanning direction.

Description:
FIELD OF THE INVENTION 
   The present invention relates to an electro-optical scanner for reading bar codes and, more particularly, to a retro-reflective scanner including an exit window having a positive optical power. 
   BACKGROUND OF THE INVENTION 
   Electro-optical scanners are widely used for reading bar codes, including one dimensional and two dimensional bar codes. A scanner typically includes a scanning module which: generates a scanning beam; repetitively directs and scans the beam across a target object, such as a bar code; receives reflected light from the target object; and digitizes and decodes the reflected light to decode the information encoded in the bar code. The scanning module is supported in a housing of the hand held portable scanner which also supports a power supply and other electronics of the scanner. 
   The scanning module scanning beam (typically a laser beam emitted by a laser diode) is directed at an oscillating scanning mirror. The oscillating scanning mirror, in turn, directs the beam outwardly through an exit window of the scanner. The oscillation of the oscillating reflector causes the beam to oscillate across a target object such as a bar code to be read. Essentially, the beam generates a beam spot that repetitively moves across or scans the bar code. 
   The light-colored or space elements of the bar code reflect the laser beam illumination and the dark or black bar elements of the bar code absorb the laser beam. Reflected light from the target bar code is received by a second reflective surface, such as a collection mirror, and directed toward photodetector circuitry, such as a photodiode. The pattern of reflected light, as received by the photodiode of the scanning module, is a representation of the pattern of the bar code. That is, a sequence of time when the photodiode is receiving reflected light represents the laser beam spot moving across a space of the bar code, while a sequence of time when the photodiode is not receiving reflected light represents the laser moving across a dark bar. Since the scanning speed or velocity of the reciprocating movement of the laser is known, the elapsed time of the photodiode receiving reflected light can be converted into a width of a bar code element corresponding to a space, while the elapsed time of the photodiode not receiving reflected light can be converted into a width of a bar code element corresponding to a bar. 
   The photodiode is part of photodetector circuitry which converts the reflected light into an analog signal. The scanning module includes an A/D converter or digitizer to digitize the analog signal generated by the photodiode. The digitizer outputs a digital bar code pattern (DPB) signal representative of the bar code pattern. A decoder of the scanning module inputs the DPB signal and decodes the bar code. The decoded bar code typically includes payload information about the product that the bar code is affixed to. Upon successful decoding of the scanned bar code, the scanner may provide an audio and/or visual signal to an operator of the scanner to indicate a successful read and decode of the bar code. The scanner typically includes a display to display payload information to the operator and a memory to store information decoded from the bar code. 
   One type of electro-optical scanner, referred to as a retro-reflective scanner, employs retro-reflective light collection. In a retro-reflective scanner, the scanning module includes a mirror that both: 1) directs the laser beam toward the target bar code or another mirror; and 2) receives reflected light from target bar code and directs it toward the photosensor circuitry. An example of such a retro-reflective scanner is disclosed in U.S. Pat. No. 6,360,949 to Shepard et al., assigned to the assignee of the present invention. The &#39;949 patent is incorporated herein in its entirety by reference. 
   As size considerations are extremely important in portable, hand held scanners, what is needed is a retro-reflective electro-optical scanner that provides for reduced size of scanning module components, specifically the light collection mirror, while maintaining the ability to read wide bar codes. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a portable electo-optical scanner for reading a target bar code having a plurality of bar code elements. The portable scanner includes a housing supporting a scanning module for electro-optically reading the target bar code by the bar code with a scanning beam and collecting reflected light returning from the bar code. The scanning beam and the reflected light pass through an exit window supported by the housing. The scanning module includes a beam source, a retro-reflective light collecting mirror and photodetector circuitry. The beam source emits a scanning beam. The scanning beam is repetitively scanned across the target bar code by beam directing apparatus. The light collection mirror has a field of view that follows the scanning beam as it moves along the scanning direction. The light collection mirror receives reflected light from the target bar code and directs the reflected light toward the photodetector circuitry. At least a portion of the exit window comprises a lens having a positive optical power with respect to the scanning direction. 
   In one embodiment, the light collection mirror is an oscillating mirror which includes an integral scanning or beam directing mirror. The beam directing mirror is part of the beam directing apparatus and is positioned to intersect the scanning beam and direct the scanning beam through the exit window. Oscillation of the oscillating mirror causes the beam directing mirror to pivot thereby repetitively scanning the beam along a scanning direction across the target bar code. 
   These and other objects, advantages, and features of the exemplary embodiment of the invention are described in detail in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is schematic view, partly in section and partly in front elevation, of an electro-optical retro-reflective scanner of the present invention; 
       FIG. 2  is a schematic perspective view of a scanning module of the scanner of  FIG. 1 ; 
       FIG. 3  is depiction of a one dimensional (1D) bar code showing representations of the scan line and field of view of the scanner of  FIG. 1 ; 
       FIG. 4  is a schematic depiction of a portion of a scanner with a prior art exit window; 
       FIG. 5  is a schematic depiction of a portion of the scanner of  FIG. 1  having a first preferred embodiment of an exit window lens; 
       FIG. 6  is a schematic depiction of a portion of the scanner of  FIG. 1  having a second preferred embodiment of an exit window lens; 
       FIG. 7  is a schematic depiction of a portion of the scanner of  FIG. 1  having a second preferred embodiment of an exit window lens; 
       FIG. 8  is a schematic view of an oscillating retro-reflective mirror including an integral light collection mirror and a beam directing scanning mirror; and 
       FIG. 9  is a top plan view of the scanning module showing the pivoting of the retro-reflective mirror of  FIG. 8  through a scanning angle β. 
   

   DETAILED DESCRIPTION 
   A portable electro-optical scanner of the present invention is shown schematically at  10  in  FIG. 1 . The scanner  10  may be used to scan and decode bar codes, such as, 1D and 2D bar codes and postal codes. As used herein, the term “bar code” is intended to be broadly construed to cover not only bar code symbol patterns comprised of alternating bars and spaces, but also other graphic patterns, such as dot or matrix array patterns and, more generally, indicia having portions of different light reflectivity or surface characteristics that result in contrasting detected signal characteristics that can be used for encoding information and can be scanned and decoded with the scanner  10 .  FIGS. 1 and 3  depict a target 1D bar code  100  affixed to a product  102 . 
   In one preferred embodiment of the present invention, the scanner  10  is a hand held, portable bar code reader. The scanner  10  is supported in a housing  12  that can be carried and used by a user walking or riding through a store, warehouse or plant for reading bar codes for stocking and inventory control purposes. 
   However, it should be recognized that scanner  10  of the present invention may be embodied in a stationary scanner. It is the intent of the present invention to encompass all such scanners. 
   The scanner  10  includes an actuation trigger  13 , a set of user input keys  14  and a visual display  15  for displaying decoded and/or other information. A speaker (not shown) providing an audio output to a user of the scanner  10  upon successful scanning and decoding of the target bar code  100  may also be provided. Also provided are data communications ports (not shown) and/or an rf transceiver (not shown) for uploading and downloading information to a remote computer system. The actuation trigger  13 , input keys  14 , display  15  and other input/output components are coupled to the circuitry  16  supported in the housing  12 . The housing  12  also supports an exit window  18  though which a scanning beam is directed outwardly and which reflected light from the target bar code  100  is received. Advantageously, as will be further discussed below, the exit window  18  comprises a lens having a positive optical power. 
   The scanner circuitry  16 , which operates under the control of one or more microprocessors, includes a retro-reflective scanning module  20 . The scanning module  20  is supported by a housing  22  which includes a printed circuit board base  24 . The scanning module  20  includes a laser diode assembly  30  for producing a scanning beam, photodetector circuitry  40  for receiving light reflected from the target bar code  100  and converting the light to an analog signal representative of the pattern of dark bars and light spaces of the bar code  100 . The analog signal output by the photodetector circuitry  40  is digitized and decoded by an A/D converter and decoding circuitry, which are part of the scanner circuitry  16 . 
   The scanning module  20  also includes a retro-reflective optical element or mirror  50  which is oscillated by a drive mechanism  70  about an axis A—A ( FIG. 8 ) through an arc or scanning rotation angle β. The retro-reflective mirror  50  includes a concave light collection mirror  52  and a planar scanning or beam reflecting mirror  54  extending outwardly from a central portion of the mirror  50 . As can best be seen in  FIG. 9 , the beam reflecting mirror  54  is offset from but vertically parallel with the pivot axis A—A such that when the mirror  50  pivots, the beam reflecting mirror  54  always intersects the beam line BL 1  generated by the laser diode assembly  30 . 
   Preferably, the mirror  50  is a single piece of molded plastic. Alternatively, the mirror  50  may fabricated of two separate pieces of plastic and/or glass which are affixed together to form the integral mirror  50 , including the light collection mirror  52  and the beam directing mirror  54 . While the beam directing mirror  54  is shown in the Figures as protruding outwardly from a central portion the concave surface of the light collecting mirror  52  and having a planar surface, it should be understood that other designs are possible. The beam directing mirror  54  may be formed as an indentation in the light collecting mirror  54  or may be cylindrical, spherical, toroidal, etc. depending upon the desired shaping of the reflected, outgoing beam line (labeled BL 2  in  FIGS. 2 and 9 ). 
   The laser diode assembly  30  is affixed to the printed circuit board  24 . The assembly  30  includes a laser diode and focusing optics which generate a scanning beam line BL 1  that is directed at the beam reflecting mirror  52 , the beam line BL 1  is reflected and redirected by the beam reflecting mirror  54 . The reflected beam line BL 2  is repetitively scanned forming a linear scan line SL. A direction of the scan line SL as it exits the scanner  10  will be referred to as the scan direction SD, as shown in  FIGS. 2  and  3 . A length or extent E of the scan line SL is determined by the scanning angle β of the mirror  50  and the optical characteristics of the exit window  18 . The repetitively scanning beam line BL 2  forms a generally pie-shaped scanning plane emanating from the beam-directing mirror  54 . The exact shape of the scanning plane is determined by the scanning angle β and the optical characteristics of the exit window  18 . 
   The light collecting mirror  52  has a field of view that is determined by the concavity of the mirror  52  and the optical characteristics of the exit window  18 . Advantageously, the positive optical power of the exit window  18  permits reduction in size of the light collecting mirror  52  while a length of the scan line SL near a nose end N ( FIG. 5 ) of the scanner  10  is sufficient to enable reading of wide bar target bar codes. 
   Since the light collecting mirror  52  and beam directing mirror  54  are integral the field of view of the light collecting mirror  52  moves with and follows the beam line BL 2  as is repetitively scanned across the target bar code  100 . This is best illustrated in  FIG. 2 . When the beam line BL 2  is at the point labeled beam spot SP 1  in  FIG. 3 , the field of view of the light collecting mirror  52  is representatively shown as FV 1 . As can be seen field of view FV 1  surrounds beam spot SP 1 . When the beam line BL 2  moves along the scan line SL the field of view of the light collecting mirror  52  moves congruently. For example, when the beam line BL 2  moves to the point labeled beam spot SP 2 , the field of view of the light collecting mirror  52  is shown as FV 2 . The extent of the field of view of the light collecting mirror  52  is shown schematically by OFV (overall field of view) in  FIG. 3 . As the retro-reflective mirror  50  pivots back and forth through angle β, the concave curvature of the light focusing mirror  52  causes the reflected light from the field of view to be directed toward a photodiode of the photodetector circuitry  40 . 
   It should be understood that while one suitable shape for the mirror  50  is shown in  FIGS. 2 and 8 , other suitable shapes (e.g., spherical, etc.) for the light collecting mirror  52  are possible. Similarly, while the shape of the beam directing mirror  54  is planar, it should be understood that other shapes such as cylindrical, spherical, toroidal, etc. depending on the need to appropriately shape the scanning beam line BL 2 . 
   The retro-reflective mirror  50  includes a pivot rod  60  which defines the pivot axis A—A. The pivot rod  60  is supported for pivotal movement in a bearing  62  affixed to the scanning module housing base  22   a . The mirror  50  is oscillated by the drive mechanism  70 . The drive mechanism  70  includes an electromagnetic coil  72  and a permanent magnet  74  coupled to the mirror  50  by a flexure support assembly  76 . The flexure support assembly includes a pair of flexible bands  76   a ,  76   b  attached to opposite sides of the retro-reflective mirror  50 . The opposite end of the band  76   a  is affixed to the magnet  74 . The opposite end of the band  76   b  is attached to a post  76   c . When an appropriate alternating current driving signal is applied to the coil  72  the magnet  74  repetitively moves inwardly and outwardly with respect to the coil opening. The movement of the magnet  74  inwardly into the coil  72  cause the band  76   a  to pull on and pivot the mirror  50  in a counterclockwise direction (as seen from the top view shown in  FIG. 9 ). The band  76   b  provides a counteracting biasing force to pivot the mirror  50  in the clockwise direction (shown in dashed line in  FIG. 9 ). The driving signal is applied to the coil  72  via the printed circuit board  24 . 
   It should be understood that the drive mechanism  70  described herein is exemplary and one of ordinary skill in the art would understand that other types of drive mechanisms could be used to effect oscillation of the mirror  50 . Suitable drive mechanism are set forth in U.S. Pat. Nos. 5,581,067, 5,367,151 6,805,295, and the aforementioned &#39;949 patent, all of which are assigned to the assignee of the present invention and all of which are incorporated in their respective entireties by reference. 
   Exit Window  18   
   A simplified prior art scanning module is shown in  FIG. 4 . The exit window was a flat piece of glass or transparent plastic having an optic power of zero. In the present invention, the exit window  18  has a positive optic power when viewed in the scan direction SD, that is, in the direction of the scan line SL. The optic power of the exit window  18  is a sum of the optic power of each side  18   a ,  18   b  of the window  18  when viewed in the scan direction SD. As is shown schematically in  FIG. 5–7 , the exit window  18  may be convex on both sides  18   a ,  18   b  of the window (shown as  18  in  FIG. 5 ). Alternately, the exit window may be convex on one side  18   b  and flat on the other side  18   a  (shown as  18 ′ in  FIG. 6 ). Or, the exit window may be convex on one side  18   b  and concave on the other side  18   a  (shown as  18 ″ in  FIG. 7 ) provide that the sum of the optic powers of the convex and concave sides is a positive optic power. 
   While the optic power of the exit window  18  in the scanning plane will depend on the particular characteristics of the scanning module  20 , the bar codes desired to be read, ambient light conditions and other factors. The optical power P of the exit window  18  in the scanning plane and its distance L from the light collecting mirror  52  can be chosen based on formulas involving two ratios: light collection mirror size reduction (Dm/Dc) and scan angle reduction (β′/β). The formulas are:
 
 Dm/Dc= 1−( P×L )
 
β′/β=1−( P×L )
 
where
         Dm=the linear size of the light collecting mirror  52 ;   Dc=the linear size of the light collection mirror that would be used in a scanning module with an exit window having an optical power of zero;   P=optic power of the exit window in the scanning plane;   L=distance from the light collecting mirror  52  to the exit window  18  along an optical axis of the exit window  18 ;   β=scanning angle (as discussed above and shown in  FIG. 8 );   β′=perceived scanning angle after scanning beam passes through exit window  18 .       

   The perceived scanning angle β′ is shown schematically in  FIG. 1 . It is the angle between a pair of rays extending from the exit window  18  to opposite ends of the length or extent E of the scan line SL. Essentially, the perceived scanning angle β′ is the effective scanning angle from the view point of the exit window  18  looking toward the target bar code  100 . Since optical power P is the reciprocal of the focal length, whatever dimension or unit (e.g., meters) is used for the measurement of L, the reciprocal of that dimension (e.g., 1/meters) will be used for P. The “linear size” of the light collection mirror  52  referred to in Dm is a projection of the light collection mirror  52  along an axis that is perpendicular to the optical axis of the exit window  18  (see  FIG. 9  where Dm is labeled, the optical axis would be parallel to the scanning line BL 2  in  FIG. 9 ). The linear size of Dc is measured the same way. It is interesting to the note that the factor 1−(P×L) is the same for both mirror size and scan angle reductions ratios. This factor will always be less than 1.0 because P&gt;0. 
   In one embodiment, the positive optical power of the exit window  18  is only in the scan direction SD, the optical power in a direction perpendicular to the scan direction SD being zero. In an alternate embodiment, the positive optical power of the exit window  18  is in all directions, including the scan direction SD, providing even greater focusing effect. In still another alternate embodiment, the optical power of the exit window  18  is positive in the scan direction SD and a negative optical power in a direction perpendicular to the scan direction SD. The optical power in the perpendicular direction will depend on the desired shape of the scanning beam line SL 2 . 
   With a flat exit window of the prior art, the effective light collection area is substantially equal to the surface area of the light collecting mirror  52 . The positive optic power exit window has the effect of focusing the reflected laser beam light received from the target bar code  100  onto the light collecting mirror  52 . This focusing effect is shown schematically in  FIG. 5–7 . This focusing effect of the positive power exit window  18  increases the effective light collection area. The result is that a smaller light collection area, that is, a smaller light collection mirror size may be used without detrimentally affecting the scanner&#39;s ability to decode the scanned bar code. In optical terms, an entrance pupil to the light collecting mirror  52  becomes larger than the physical size of the light collecting mirror. 
   Due to the focusing effect of the positive optic power exit window  18 , for a given scanning angle β of rotation of the retro-reflective mirror  50 , as the distance d from the nose end N of the scanner  10  increases, the length or extent E of the scan line SL become smaller. However, at the nose end N of the scanner  10 , the length E of scan line SL is substantially identical for what the scan line length would be if a flat exit window (optic power zero) were used. Thus, wide target bar codes may still be read in proximity to the nose end N of the scanner  10 . Additionally, there are significant advantages to utilizing a positive power exit window compared to a flat exit window resulting from the fact that a smaller sized light collection mirror  52  is required. The smaller sized light collection mirror  52  has a smaller footprint, requires less power to oscillate and resulting in less vibration. 
   Even if the scan angle β of the retro-reflective mirror  50  must be increased slightly to compensate for the reduction in scan line length as the distance d from the nose N of the scanner  10  increases, the advantage of a smaller light collecting mirror  52  still outweighs any disadvantages. The torque generated by oscillation of the retro-reflective mirror  50  is proportional to the first power of the scan angle β, but to the third power of the size of the mirror  50 . Thus, reducing the size of the light collecting mirror  52  (and, hence, the retro-reflective mirror  50 ) has a net result of reducing unwanted vibration even with an increased scanning angle β. 
   The focusing of the scan line SL by the exit window  18  advantageously increases the brightness of the scan line. The positive optic power exit window  18  allows decreased scanning speed (i.e., the angular speed of the retro-reflective mirror  50 ), narrower system bandwidth, improved signal-to-noise ratio, and increased working ranges with compromising the ability to read wide bar codes. 
   Finally, increasing the light collection system pupil (at a given light collecting mirror size) with the positive optical power exit window  18  of the present invention advantageously reduces speckle noise. Speckle noise refers to an uneven distribution of the intensity of laser light reflected from a rough surface, due to interference of light reflected from the points with different surface height and, hence, different phase delay. Generally, the larger the size of the light collecting mirror  52 , the better the capability to catch reflected light from many diffraction maxima and minima, thus assuring better averaging and less variation of measurements. The exit window  18  of the present invention reduces speckle noise without increasing the light collecting mirror size. 
   While the present invention has been described with a degree of particularity, it is the intent that the invention includes all modifications and alterations from the disclosed design falling with the spirit or scope of the appended claims.