Abstract:
A method of and system for automatically identifying packages during manual package sortation operations, wherein, a laser scanning system is supported above a workspace environment of 3-D spatial extent, which can be occupied by a human operator involved in the manual sortation of packages bearing bar code symbols. During operation of the system, a laser scanning pattern generator projects an omnidirectional laser scanning pattern that spatially encompasses a substantial portion of the workspace environment, through which packages are transported during sorting operations. A visible scanning-zone indication pattern is projected onto the floor surface immediately beneath the omni-directional laser scanning pattern. As the human operator uses the visible scanning-zone pattern to guide the transport of a package bearing a bar code symbol through the workspace environment, the package is automatically identified by the laser scanning system supported above the workspace environment.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is Application is a continuation of application Ser. No. 09/681,606 filed on May 7, 2001 now U.S. Pat. No. 6,629,640, which is a Continuation-in-Part of U.S. application Ser. No. 09/479,780 filed Jan. 7, 2000 now U.S. Pat. No. 6,561,424, which is a Continuation of U.S. application Ser. No. 08/940,561 filed Sep. 30, 1997, now U.S. Pat. No. 6,112,990, which is a Continuation of U.S. application Ser. No. 08/886,806 filed Apr. 22, 1997, now U.S. Pat. No. 5,984,185, which is a Continuation of U.S. application Ser. No. 08/573,949 filed Dec. 18, 1995, now abandoned; U.S. application Ser. No. 09,505,239 filed Feb. 16, 2000 now U.S. Pat. No. 6,517,001, which is a continuation of U.S. application Ser. No. 08/854,832 filed May 12, 1997, now U.S. Pat. No. 6,085,978, U.S. application Ser. No. 09/505,238 filed Feb. 16, 2000 now U.S. Pat. No. 6,530,522, which is a Continuation of U.S. application Ser. No. 08/949,915 filed Oct. 14, 1997, now U.S. Pat. No. 6,158,659; U.S. application Ser. No. 09/047,146 filed Mar. 24, 1998 now U.S. Pat. No. 6,360, 947; U.S. application Ser. No. 09/157,778 filed Sep. 21, 1998 now U.S. Pat. No. 6,517,004; U.S. application Ser. No. 09/274,265 filed Mar. 22, 1999 now U.S. Pat. No. 6,382,515; U.S. application Ser. No. 09/275,518 filed Mar. 24, 1999 now U.S. Pat. No. 6,457,642; U.S. application Ser. No. 09/305,896 filed May 5, 1999 Now U.S. Pat. No. 6,287,946; U.S. patent application Ser. No. 09/243,078 filed Feb. 2, 1999 now U.S. Pat. No. 6,354,505, U.S. application Ser. No. 09/442,718 filed Nov. 18, 1999 now U.S. Pat. No. 6,481,625, and U.S. application Ser. No. 09/551,887 filed Apr. 18, 2000 now U.S. Pat. No. 6,758,402. 
    
    
     BACKRGROUND OF INVENTION 
     1. Field of Invention 
     The present invention relates to holographic laser scanning systems that produce an omni-directional scanning pattern in a three-dimensional (3-D) scanning volume wherein users manually transport an object through the 3-D scanning volume to detect physical attributes of the object (such as detecting and decoding bar code symbols on surfaces of the object). 
     2. Brief Description of the Prior Art 
     Handheld laser scanning systems typically form a single scan line which must be properly aimed over the surface of its intended target object. Handheld laser scanners such as those described in U.S. Pat. Nos. 4,603,262 and 5,296,689 were developed that used a pointer beam (or aiming light) which is visible over the intended scan distance to aid the user in aiming the handheld scanner (or orienting the target object). 
     Polygonal laser scanning systems generate a multi-directional scan pattern forming a scan volume which is typically not well-defined. U.S. Pat. No. 6,223,986 discloses a polygonal laser scanning system that employs a laser light source to generate a visible target (or image) in the scan volume at a preferred location for placement of the article to be scanned. 
     Handheld laser scanning systems and polygonal laser scanning systems are typically limited to scanning applications that require a small scan volume (because it is cost-prohibitive to use such systems to omni-directionally scan a large scan volume). 
     In contrast, laser scanning systems employing holographic optical elements can be cost-effectively designed and manufactured to produce an omni-directional pattern through a large well-defined scanning volume (preferably with multiple scanning beams with varying depths of field in the scanning volume). The present inventors have recognized the potential to facilitate scanning in 3-D omni-directional holographic laser scanning systems. 
     In 3-D omni-directional holographic laser scanning systems, such as Metrologic&#39;s HoloTrak® scanner products, it is often difficult for users to locate the position of the 3-D scanning volume (and the 3-D omni-directional scan pattern therein) without looking directly into the scanner and thus exposing the user&#39;s eyes to potentially (or assumed) harmful laser scanning beams. The reason that the 3-D scanning volume (and the 3-D omni-directional scan pattern therein) is not readily visible is due to the high speed of the scanning beams and its relatively low intensity compared to ambient light. 
     When a user of such a system is required to manually transport an object through the 3-D scanning volume (and the 3-D omni-directional scanning pattern therein) to detect physical attributes of the object (such as detecting and decoding bar code symbols on surfaces of the object), unwanted scanning errors occur in the event that the user is unable to identify the correct location of the 3-D scanning volume (and the omni-directional scan pattern therein) when attempting to transport the object through the 3-D scan volume. Such unwanted scanning errors limit the productivity of the user. Moreover, any time taken by a user in locating the 3-D scanning volume limits the productivity of the user. Such limitations in user productivity represent increased costs associated with the use of the laser scanning system. In addition, a user repetitively searching for the 3-D scanning volume of the system can potentially lead to repetitive motion strain and injury 
     Thus, there is a great need in the art for an improved holographic laser scanning system that enables users to efficiently locate the 3-D scanning volume (and the 3-D omni-directional scan pattern therein) of the 3-D omni-directional laser scanning system, while avoiding the shortcomings and drawbacks of prior art holographic scanning systems and methodologies. 
     SUMMARY OF INVENTION 
     Accordingly, a primary object of the present invention is to provide a novel 3-D omni-directional holographic laser scanning-system that is free of the shortcomings and drawbacks of prior art laser scanning systems and methodologies. 
     Another object of the present invention is to provide a 3-D omni-directional holographic laser scanning system that provides visible indicia, visibly discernable by users of the system, characterizing the location of the 3-D scanning volume (and the 3-D omni-directional scan pattern therein) of the system, relative to the physical environment in which the system is installed and operated. 
     Another object of the present invention is to provide a 3-D omni-directional holographic laser scanning system that provides visible indicia characterizing the approximate location of the center, edges or other portion of the 3-D scanning volume (and the 3-D omni-directional scan pattern therein) of the system. 
     Another object of the present invention is to provide a 3-D omni-directional holographic laser scanning system that utilizes low cost materials to provide visible indicia characterizing the location of the 3-D scanning volume (and the 3-D omni-directional scan pattern therein) of the system. 
     Another object of the present invention is to provide a 3-D omni-directional holographic laser scanning system that utilizes a visible light pattern, which is preferably distinguishable from the scanning beam(s) of the system, to provide visible indicia characterizing the location of the 3-D scanning volume (and the 3-D omni-directional scan pattern therein) of the 3-D omni-directional laser scanning system. 
     Another object of the present invention is to provide a 3-D omni-directional holographic laser scanning system that utilizes a readily-discernable visible light pattern to provide visible indicia characterizing the location of the 3-D scanning volume (and the 3-D omni-directional scan pattern therein) of the system. 
     Another object of the present invention is to provide a 3-D omni-directional holographic laser scanning system that shines a visible light pattern on a surface over which the objects are moved through the 3-D scanning volume to provide a visible indication of points substantially corresponding to the boundary of the projection of the 3-D scanning volume (and the 3-D omni-directional scan pattern therein) onto the surface. 
     A further object of the present invention is to provide a 3-D omni-directional holographic laser scanning system that uses the same laser scanning beam(s) to detect properties of surfaces passing through a 3-D scanning volume and to provide visible indicia characterizing a location of the 3-D scanning volume (and the 3-D omni-directional scan pattern therein) of the system. 
     A further object of the present invention is to provide a 3-D omni-directional holographic laser scanning system that provides visible indicia characterizing a location of the 3-D scanning volume (and the 3-D omni-directional scan pattern therein) of the system and provides an indication that the user has entered a region corresponding to the 3-D scanning volume. 
     These and other objects of the present invention will become apparent hereinafter and in the Claims to Invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       For a more complete understanding of the present invention, the following Detailed Description of the Illustrative Embodiment should be read in conjunction with the accompanying Drawings, wherein: 
         FIG. 1  is a schematic illustration of a 3-D omni-directional holographic laser scanning system  100  wherein a laser light source  101  (such as a VLD) emits laser light beams (denoted l); a holographic laser scanning subsystem  103  utilizes a plurality of holographic optical elements (preferably supported on a rotating disc) to direct portions (denoted l′) of these laser light beams to create a 3-D omni-directional scan pattern that defines a 3-D scanning volume  105 ; portions of the returning (i.e., incoming) laser light beams (denoted l″) from the 3-D scanning volume  105 , are collected by the subsystem  103  and portions (denoted l′″) of the collected light are directed to photodetector(s)  107  and signal processing and control circuitry  109  that capture and analyze the collected light to identify properties (such as bar code symbols) of surfaces (or objects) within the 3-D scanning volume  105 . 
         FIG. 2(A)  is a perspective front view of an illustrative embodiment of an overhead 3-D omni-directional holographic laser scanning system  100 ′ according to the present invention, including one or more light beams (4 shown as  111 A,  111 B,  111 C and  111 D) that provide a visible light pattern characterizing the location of the 3-D scanning volume  105 ′ (and the 3-D omni-directional scan pattern therein) of the system  100 ′. 
         FIG. 2(B)  is a perspective bottom view of the overhead 3-D omni-directional holographic laser scanning system  100 ′ of  FIG. 2(A)  including at least one light production module (4 modules shows as  115 A,  115 B,  115 C and  115 D) mounted to the bottom of the housing of the system  100 ′ that provides the visible light pattern (for example, the four visible light beams  111 A,  111 B,  111 C and  111 D as shown) characterizing the location of the 3-D scanning volume  105 ′ (and the 3-D omni-directional scan pattern therein) of the system  100 ′. 
         FIG. 3  is a perspective front view of an illustrative embodiment of an overhead 3D omni-directional holographic laser scanning system  100 ″ according to the present invention, including visible markings  112  that are affixed to a surface  113  over which the objects are moved through the 3-D scanning volume  105 ″ and that provide visible indicia characterizing the location of the 3-D scanning volume  105 ″ (and the 3-D omni-directional scan pattern therein) of the system  100 ″. 
         FIG. 4(A)  is a schematic illustration of a top view of an exemplary holographic laser scanning system  100 -A of the present invention, which produces an omni-directional laser scanning pattern having different multi-directional scan patterns at multiple focal zones (e.g., multiple focal planes) in a 3-D scanning volume which are formed by five laser scanning stations (indicated as LS 1 , LS 2 , LS 3 , LS 4  and LS 5 ) arranged about a sixteen-facet holographic scanning disc  130 . 
         FIG. 4(B)  is a schematic illustration of one (LS 1 ) of the laser scanning stations of the holographic laser scanning system  100 -A of the present invention, as illustrated in  FIG. 4(A) , including a laser beam production module  147 A mounted on an optical bench; and a beam folding mirror  142 A associated with the laser scanning station L 1 , having a substantially planar reflective surface and tangentially mounted adjacent to the holographic scanning disc  130 . 
         FIG. 4(C)  is a schematic illustration of a cross-section of the holographic laser scanning system  100 -A of the present invention as illustrated in  FIGS. 4(A) and 4(B) , wherein facets of rotating the scanning disk  130  diffract incident light beams (produced from the laser beam production module  147 A) and direct the diffracted light beams onto the associated light bending mirrors  142 A, which directs the diffracted light beams through the 3-D scanning volume, thereby producing a 3-D omni-directional scanning pattern with multiple focal zones; at least one photodetector (e.g., a silicon photocell)  152 A is mounted along the central reference plane of the laser scanning station LS 1 , above the holographic disc  130  and opposite its associated beam folding mirror  142 A so that it does not block or otherwise interfere with the returning (i.e., incoming) light rays reflecting off light reflective surfaces (e.g., product surfaces, bar code symbols, etc.) during laser scanning and light collecting operations; the electrical analog scan data signal produced from the photodetector  152 A (and other photodetectors  152 B . . .  152 E) is processed to detect properties (such as detecting and decoding bar code symbols on surfaces of objects) of the surfaces; the parabolic light collecting mirror  149 A of the laser scanning station L 1  is disposed beneath the holographic scanning disc  130 , along the central reference plane associated with the laser scanning station LS 1 ; the light collecting mirror  149 A collects incoming light rays reflected off the surfaces (e.g., bar code symbols affixed thereto) and passing through the holographic facet (producing the corresponding instant scanning beam) onto the parabolic light collected mirror  149 A, and focuses such collected light rays through the same holographic facet onto the photodetector associated with the laser scanning station. 
         FIG. 4(D)  is a schematic illustration of the scanning disk  130  of the holographic laser scanning system  100 -A of the present-invention as illustrated in  FIGS. 4(A) ,  4 (B) and  4 (C).   FIG. 4(E)  is a schematic illustration of a laser production module for one (LS 1 ) of the laser scanning stations of the holographic laser scanning system  100 -A of the present invention, as illustrated in  FIGS. 4(A) ,  4 (B) and  4 (C), including: a visible laser diode (VLD)  101 A; an aspheric collimating lens  51 , supported within the bore of a housing  53  mounted upon an optical bench  143  of the module housing, for focusing the laser light produced by the VLD  101 A; a mirror  55 , supported within the housing  53 , for directing the focused laser light produced by lens  51  to a multi-function light diffractive grating  57  supported by the housing  53 ; the multi-function light diffractive grating  57 , which has a fixed spatial frequency and is disposed at an incident angle relative to the outgoing laser beam provided by the mirror  55 , and produced a primary beam that is directed towards the facets of the rotating scanning disk  130 ; and the multi-function light diffractive grating  57 , which changes the properties of the incident laser beam so that the aspect ratio of the primary beam is controlled, and beam dispersion is minimized upon the primary laser beam exiting the holographic scanning disc  13 . 
         FIG. 4(F)  is a schematic illustration of the middle focal plane of the omni-directional scanning pattern produced by the holographic laser scanning system  100 -A of the present invention as illustrated in  FIGS. 4(A) ,  4 (B) and  4 (C). 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the figures in the accompanying Drawings, the various illustrative embodiments of the improved laser scanning system (and components therein) of the present invention will be described in great detail, wherein like elements will be indicated using like reference numerals. 
       FIG. 1  is a schematic illustration of a 3-D omni-directional holographic laser scanning system  100  including a laser light source  101  (such as a VLD) that emits laser light beams (denoted l). A holographic laser scanning subsystem  103  utilizes a plurality of holographic optical elements (preferably supported on a rotating disc) to direct portions (denoted l′) of these laser light beams thereby creating an omni-directional scan pattern that defines a 3-D scanning volume  105 . Portions of returning (i.e., incoming) light rays (denoted l″) from the 3-D scanning volume  105 , which reflect off light reflective surfaces in the 3-D scanning volume  105 , are collected by the optical subsystem  103  and portions (denoted l′″) of these returning light rays are directed to photodetector(s)  107  and signal processing and control circuitry  109  that capture and analyze the returning laser light ray portions to identify properties (such as bar code symbols and/or images) of surfaces (or objects) within the 3-D scanning volume  105 . Preferably, the omni-directional scan pattern produced by the holographic laser scanning system  100  includes different multi-directional scan patterns at varying focal zones (e.g., focal planes) within the 3-D scanning volume  105 . Moreover, such multiple focal zones may cover a depth of field greater than one foot (and preferably cover a depth of field greater than one meter). In addition, the 3-D scanning volume  105  of the omni-directional scan pattern produced by the holographic laser scanning system  100  is preferably well-defined (for example, characterized by a well-defined boundary comprised of substantially planar polygonal surfaces as illustrated in  FIGS. 2(A) and 2(B) ). 
     In a preferred embodiment, the improved omni-directional holographic laser scanning system of the present invention includes a mechanism for automatically generating visible indicia (i.e., visible scanning-zone indicators) characterizing the location of the 3-D scanning volume (and the 3-D omni-directional scanning pattern therein), and thus serving to help the user navigate the manual transport of a package therethrough during automatic identification (Auto-ID) operations carried out in a work environment. In general, the production of visible scanning zone indicators may be realized by using one or more visible light beams (visibly discernable to users of the system) which provide a visible light pattern characterizing the location of the 3-D scanning volume (and the 3-D omni-directional scanning pattern therein) generated by the system. 
     Alternatively, although less preferable in particular applications, such visible indicia may be realized by visible markings (visibly discernable to users of the system), such as reflective paint or reflective tape, affixed to a surface beneath the omni-directional 3-D laser scanning system and in such a manner that characterizes location of the 3-D scanning volume (and the 3-D omni-directional scanning pattern therein) of the system. 
     Such scanning-zone indicators may specify the location of the 3-D scanning volume (and the 3-D omni-directional scanning pattern therein) by providing an indication of the approximate location of the center of the 3-D scanning volume, of one or more edges of the 3-D scanning volume, and/or of any other portion of the scanning volume. Such visible indicia enable users to quickly identify the correct location of the 3-D omni-directional scan pattern therein when attempting to transport the object through the 3-D scanning volume, thus limiting unwanted scanning errors and increasing the productivity of the user, which represents decreased costs associated with the use of the system. Moreover, such features can potentially avoid repetitive motion strain and injury due to users repetitively searching for location of the 3-D scanning volume during manual transport of a package therethrough during automatic identification (Auto-ID) operations carried out in a work environment. 
       FIGS. 2(A)  and (B) illustrate an exemplary embodiment of an overhead (i.e. walk-under) 3-D omni-directional holographic laser scanning system  100 ′ according to the present invention which includes a mechanism for automatically generating one or more visible light beams (for example, 4 visible light beams  111 A,  111 B,  111 C and  111 D as shown) that produce a visible light pattern (i.e. scanning-zone indicator pattern) which characterizes the location (and general spatial boundaries) of a 3-D scanning volume  105 ′ (and the 3-D omni-directional scanning pattern therein) generated from the system  100 ′. In this walk-under configuration, the 3-D omni-directional holographic laser scanning system  100 ′ is stationarily mounted above a work environment. The 3-D scanning volume (and the 3-D omni-directional scanning pattern therein) projects downward toward a surface to scan objects (e.g., packages) that are moved under human control over the surface. 
     In this configuration, the visible light pattern produced by the visible light beams may characterize location (and general spatial boundaries) of the 3-D scanning volume (and the 3-D omni-directional scanning pattern therein) by providing an indication of the approximate location of the edges of the 3-D scanning volume (and the 3-D omni-directional scanning pattern therein) as shown. In addition, the shining of the visible light pattern onto the surface  113  over which the objects (e.g., packages) are manually transported by a human through the 3-D scanning volume  105 ′ provides a visible indication of points substantially corresponding to the boundary of the projection of the 3-D scanning volume  105 ′ (and the 3-D omni-directional scanning pattern therein) onto the surface  113 . Alternatively, the visible light pattern produced by the one or more visible light beams may characterize location (and general spatial boundaries) of the 3-D scanning volume (and the 3-D omni-directional scanning pattern therein) by providing an indication of the approximate location of the center of the 3-D scanning volume and/or of any other portion of the 3-D scanning volume of the system  100 ′. 
     As shown in  FIG. 2(B) , the visible light beams may be produced by one or more light production modules (for example, 4 light production modules  115 A,  115 B,  115 C,  115 D) mounted to the exterior bottom of the housing of the overhead holographic laser scanning system  100 ′. Alternatively, the laser light production modules may be mounted within the interior of the housing of the overhead holographic laser scanning system and projected through a window in the housing. The one or more light production modules produce and direct at least one visible light beam (for example, the 4 visible light beams  111 A,  111 B,  111 C and  111 D as shown) to thereby construct the visible light pattern characterizing location of the 3-D scanning volume  105 ′ (and the 3-D omni-directional scanning pattern therein) below the laser scanning system  100 ′. The light production modules may utilize a visible laser light source (such as a VLD), a light-emitting diode or a white light source to generate the visible light. Preferably, the one or more visible light beams that make up the visible light pattern are distinguishable by brightness, color or both with respect to the laser light used by the laser scanning system  100 ′ in scanning the 3-D scanning volume  105 ′. 
     In this illustrative embodiment, the system  100 ′ may utilize collimating (e.g., focusing) elements and possibly other optical elements to generate and direct visible light to thereby produce the visible light pattern constituting the scanning zone indictors which help human operators accurately navigate packages and other bar-coded objects through the 3-D scanning volume during package transport operations. For example, multiple visible light beams-may be generated by a single visible light source in cooperation with a beam splitter. 
     In addition, the one or more visible light beams that make up the visible light pattern may be pulsed (for aiding its visibility or for compliance with laser safety standards). In such instances, the visible light beams are preferably pulsed at a frequency less than the critical flicker frequency to improve the visibility of the visible light pattern to potential users. The critical flicker frequency is the point at which the one or more flickering visible light beam are no longer perceived as periodic but shifts to continuous. 
     In this illustrative embodiment, the 3-D omni-directional holographic laser scanner may utilize one or more laser light sources (e.g., VLDs) having characteristic wavelength(s) in producing the omni-directional laser scanning beams together with one or more matched optical filters that enable such characteristic wavelength(s) of light to pass therethrough to the photodetector(s)  107  (while substantially blocking light outside such characteristic wavelength(s) from reaching the photodetector(s)  107 ), thereby minimizing the ambient noise that reaches the photodetector(s)  107 . Such ambient noise, if left unblocked, potentially may interfere with the signal processing functions (and, possibly the bar-code symbol decoding functions) applied to the output of the photodetector(s)  107 . In such a system, in the event that one or more laser light sources (e.g., VLDs) are used to generate the visible light pattern that characterizes location of the 3-D scanning volume (and the 3-D omni-directional scanning pattern therein), the characteristic wavelength of such laser light sources (e.g., VLDs) is preferably different from the characteristic wavelength(s) of the laser light source(s) used to produce the 3-D omni-directional scanning pattern. With this design, the optical filters will substantially block any noise produced from the laser light sources (e.g., VLDs) that are used to generate the visible light beams that characterize location of the 3-D scanning volume (and the 3-D omni-directional scanning pattern therein) from reaching the photodetector(s)  107 ), thereby minimizing the noise that reaches the photodetector(s)  107 . 
       FIG. 3  illustrates an exemplary embodiment of an overhead 3-D omni-directional holographic laser scanning system  100 ″ according to the present invention including visible markings  112  affixed to a surface  113  over which the objects are moved. Such visible markings  112  characterize location (and general spatial boundaries) of the 3-D scanning volume  105 ″ (and the 3-D omni-directional scanning pattern therein), for example, by providing an indication of the approximate location of the edges of the 3-D scanning volume  105 ″ as shown. Alternatively, such visible markings  112  may characterize location (and general spatial boundaries) of the 3-D scanning volume  105 ″ (and the 3-D omni-directional scanning pattern therein) by providing an indication of the approximate location of the center of the 3-D scanning volume  105 ″ and/or of any other portion of the 3-D scanning volume  105 ″. Such visible markings  112  may be visible tape (such as reflective or bright colored tape) or may be visible paint (such as reflective or bright colored paint) applied to the surface  113 . 
     In another embodiment of the 3-D omni-directional holographic laser scanning system of the present invention, the laser scanning beam(s) used by the system to detect properties (such as bar-code symbols affixed thereto) of surfaces passing through the 3-D scanning volume may be used to provide such visible indicia. For example, such visible indicia may be provided by controlling the 3-D omni-directional holographic laser scanning system to repeatably scan select scan lines that pass through the 3-D scanning volume thereby providing a pulsing of such select scan lines in a manner that provides a characterization of location (and general spatial boundaries) of the 3-D scanning volume (and the 3-D omni-directional scanning pattern therein) that is visibly discernable to users of the system. The pulsing of such select scan lines may characterize location (and general spatial boundaries) of the 3-D scanning volume (and the 3-D omni-directional scanning pattern therein) by providing an indication of the approximate location of the center of the 3-D scanning volume, of one or more edges of the 3-D scanning volume, and/or of any other portion of the 3-D scanning volume. 
     The 3-D omni-directional holographic laser scanning system of the present invention may utilize holographic scanning discs supporting holographic optical elements in generating the omni-directional scanning pattern, as taught in WIPO Publication No. WO 97/22945, herein incorporated by reference in its entirety. An exemplary holographic laser scanning system  100 -A of the present invention is illustrated in detail in FIGS.  4 (A)–(F). Preferably, the omni-directional scan pattern produced by the holographic laser scanning system  100 -A includes different multi-directional scan patterns at varying focal zones (e.g., focal planes) within the 3-D scanning volume. Moreover, such multiple focal zones may cover a depth of field greater than one foot (and preferably cover a depth of field greater than one meter). In addition, the 3-D scanning volume of the omni-directional scan pattern (produced by the holographic laser scanning system  100 -A) is preferably characterized by a well-defined boundary comprised of substantially planar polygonal surfaces (as illustrated in  FIGS. 2(A) and 2(B) ). 
     The exemplary holographic laser scanning system utilizes multi-faceted holographic optical elements to direct a 3-D omni-directional scan pattern of outgoing laser light through the 3-D scanning volume and collect the incoming light for capture by the optical detector(s). The 3-D scanning volume contains an omni-directional laser scanning pattern having different scan patterns over five focal zones, which are formed by five laser scanning stations indicated as LS 1 , LS 2 , LS 3 , LS 4  and LS 5  in  FIG. 4(A) , arranged about a sixteen-facet holographic scanning disc  130  (illustrated in greater detail in  FIG. 4(D) ). The scanning pattern projected within the middle (third) focal zone (e.g., focal plane) of the holographic laser scanning system is shown in  FIG. 4(F) . 
     In general, the scan pattern and scan speeds for the holographic laser scanning system can be designed and constructed using the methods detailed in U.S. Pat. Nos. 6,158,659, 6,085,978, 6,073,846, and 5,984,185, all commonly assigned to the assignee of the present invention and each herein incorporated by reference in their entirety. The design parameters for each sixteen facet holographic scanning disc shown in  FIG. 4(D) , and the supporting subsystems used therewith, are set forth in detail in the above-referenced US Patents. 
     As described in WIPO Publication No. WO 97/22945, the holographic laser scanning system  100 -A employed herein cyclically generates from its compact scanner housing  140  shown in  FIG. 4(A) , a complex three-dimensional laser scanning pattern within a well defined 3-D scanning volume. In this illustrative embodiment, such a laser scanning pattern is generated by a rotating holographic scanning disc  130 , about which are mounted five (5) independent laser scanning stations, sometime referred to as laser scanning modules by Applicants. In  FIG. 4(A) , these laser scanning stations are indicated by LS 1 , LS 2 , LS 3 , LS 4  and LS 5 . 
     In  FIG. 4(B) , one of the laser scanning stations in the holographic scanning system  100 -A is shown in greater detail. For illustration purposes, all subcomponents associated therewith shall be referenced with the character “A”, whereas the subcomponents associated with the other four laser scanning stations shall be referenced using the characters B through E. As illustrated in  FIG. 5(B) , a beam folding mirror  142 A associated with the laser scanning station L 1 , has a substantially planar reflective surface and is tangentially mounted adjacent to the holographic scanning disc  130 . In the illustrative embodiment, beam folding mirror  142 A is supported in this position relative to the housing base (i.e. the optical bench)  143  using support legs  144 A and  145 A and rear support bracket  146 A. 
     As shown in  FIG. 4(B) , the laser scanning station L 1  includes a laser beam production module  147 A mounted on the optical bench (i.e. housing base plate  143 ). The laser beam production module  147 A is preferably mounted on the optical bench  143  immediately beneath its associated beam folding mirror  142 A. 
     As shown in  FIG. 4(A) , the five laser production modules  142 A through  142 E are mounted on base plate  143 , substantially but not exactly symmetrically about the axis of rotation of the shaft of electric motor  150 . During laser scanning operations, these laser beam production modules produce 5 independent laser beams which are directed through the edge of the holographic disc  130  at an angle of incidence A i , which, owing to the symmetry of the laser scanning pattern of the illustrative embodiment, is the same for each laser scanning station (i.e. A i =43.0 degrees for values of i). The incident laser beams produced from the 5 laser beam production modules  142 A through  142 E extend along the five central reference planes, each extending normal to the plane of base plate  143  and arranged about 72 degrees apart from its adjacent neighboring central planes. While these central reference planes are not real (i.e. are merely virtual), they are useful in describing the geometrical structure of each laser scanning station in the holographic laser scanning system  100 -A of the present invention. 
     The facets of rotating the scanning disk  130  diffract the incident light beams (produced from the laser beam production modules  147 A . . .  147 E) and directs the diffracted light beams onto the associated light bending mirrors  142 A . . .  142 E, which directs the diffracted light beams through the 3-D scanning volume, thereby producing a 3-D omni-directional scanning pattern. The middle (third) focal zone (i.e., focal plane) of this 3-D omni-directional scanning pattern is shown in  FIG. 4(F) . 
     As shown in  FIG. 4(B) , the laser scanning station L 1  includes at least one photodetector (e.g. a silicon photocell)  152 A mounted along its central reference plane, above the holographic disc  130  and opposite its associated beam folding mirror  142 A so that it does not block or otherwise interfere with the returning (i.e. incoming) light rays reflecting off light reflective surfaces (e.g. product surfaces, bar code symbols, etc) during laser scanning and light collecting operations. 
     In the illustrative embodiment, the photodetectors  152 A through  152 E are supported in their respective positions by a photodetector support frame  153 , which is stationarily mounted to the optical bench by way of vertically extending support elements (two shown as  154 A and  154 B). The electrical analog scan data signal produced from each photodetector  152 A through  152 E is processed in a conventional manner by its analog scan data signal processing circuitry  201 A through  201 E, which may be supported upon the photodetector support frame as shown. The analog scan data signal processing circuitry  201 A may be realized as an Application Specific Integrated Circuit (ASIC) chip, which is suitably mounted with the photodetector  152 A onto a small printed circuit (PC) board, along with electrical connectors which allow for interfacing with other boards within the scanner housing. With all of its components mounted thereon, each PC board may be suitably fastened to the photodetector support frame  153 , along its respective central reference frame, as shown in  FIG. 5(B) . 
     Notably, the height of the photodetector support frame  153 , referenced to the base plate (i.e. optical bench), is chosen to be less than the minimum height so that the beam folding mirrors must extend above the holographic disc in order to realize the pre-specified laser scanning pattern of the illustrative embodiment. In practice, this height parameter is not selected (i.e. specified) until after the holographic disc has been completely designed according to the design process of the present invention, while satisfying the design constraints imposed on the disc design process. As explained in detail in WIPO Publication No. WO 97/22945, the use of a spreadsheet-type computer program to analytically model the geometrical structure of both the laser scanning apparatus and the ray optics of the laser beam scanning process, allows the designer to determine the geometrical parameters associated with the holographic scanning facets on the disc which, given the specified maximum height of the beam folding mirrors Y j , will produce the pre-specified laser scanning pattern (including focal plane resolution) while maximizing the use of the available light collecting area on the holographic scanning disc. 
     As best shown in  FIG. 4(C) , the parabolic light collecting mirror  149 A of the laser scanning station L 1  is disposed beneath the holographic scanning disc  130 , along the central reference plane associated with the laser scanning station. While certainly not apparent from this figure, precise placement of the parabolic light collecting element (e.g. mirror)  149 A relative to the holographic facets on the scanning disc  130  is a critical requirement for effective light detection by the photodetector ( 152 A) associated with each laser scanning station L 1 . Placement of the photodetector  152 A at the focal point of the parabolic light focusing mirror alone is not sufficient for optimal light detection in the light detection subsystem of the present invention. As taught in WIPO Publication No. WO 97/22945, careful analysis must be accorded to the light diffraction efficiency of the holographic facets on the scanning disc and to the polarization state(s) of collected and focused light rays being transmitted therethrough for detection. As will become more apparent hereinafter, the purpose of such light diffraction efficiency analysis ensures the realization of two important conditions, namely: (i) that substantially all of the incoming light rays reflected off an object (e.g. surface, or bar code symbol affixed thereto) and passing through the holographic facet (producing the corresponding instant scanning beam) are collected by the parabolic light collecting mirror  149 A; and (ii) that all of the light rays collected by the parabolic light collecting mirror  149 A are focused through the same holographic facet onto the photodetector associated with the station, with minimal loss associated with light diffraction and refractive scattering within the holographic facet. A detailed procedure is described in WIPO Publication No. WO 97/22945 for designing and installing the parabolic light collecting mirror  149 A in order to satisfy the operating conditions for effective light collection and detection as described above. 
     The optical scan data signal D 0  focused onto the photodetector  152 A during laser scanning operations is produced by light rays of a particular polarization state (e.g., S polarization state) associated with a diffracted laser beam being scanned across a light reflective surface (e.g. the bars and spaces of a bar code symbol) and scattering thereof. Typically, the polarization state distribution of the scattered light rays is altered when the scanned surface exhibits diffuse reflective characteristics. Thereafter, a portion of the scattered light rays are reflected along the same outgoing light ray paths toward the holographic facet(s) on the scanning disc  130  which produced the scanned laser beam. These reflected light rays are collected by these facet(s) and ultimately focused onto the photodetector  152 A by its parabolic light reflecting mirror  149 A disposed beneath the scanning disc  130 . The function of each photodetector  152 A is to detect variations in the amplitude (i.e. intensity) of optical scan data signal D 0 , and to produce in response thereto an electrical analog scan data signal D 1  which corresponds to such intensity variations. When a photodetector with suitable light sensitivity characteristics is used, the amplitude variations of electrical analog scan data signal D 0  will linearly correspond to the light reflection characteristics of the scanned surface (e.g. the scanned bar code symbol). The function of the analog signal processing circuitry  201 A is to filter and amplify the electrical analog scan data signal D 0 , in order to improve the signal-to-noise ratio (SNR) of the signal D 1  for output to digital signal processing circuitry, which is preferably mounted on PC board  202 A that is disposed behind the beam folding mirror  142 A of the laser scanning station L 1  as shown in  FIG. 4(C) . 
     The digital signal processing circuitry, which is preferably mounted on the PC board  202 A as shown in  FIG. 4(C) , preferably operates to convert the analog scan data signal D 1  output by the-analog signal processing circuitry into a corresponding digital scan data signal D 2 , and processes the digital scan data signal D 2  to extract information (such as symbols or bar codes) related to surfaces of objects passing through the 3-D scanning volume based upon the characteristics of the reflected light encoded by the digital scan data signal D 2 . 
     The digital signal processing circuitry preferably includes A/D conversion circuitry that converts the analog scan data signal D 1  output by the analog signal processing circuitry into a corresponding digital scan data signal D 2  having first and second (i.e. binary) signal levels which correspond to the bars and spaces of the bar code symbol being scanned. Preferably, the A/D conversion circuitry performs a thresholding function on a second-derivative zero-crossing signal in generating the digital scan data signal D 2 . In practice, the digital scan data signal D 2  appears as a pulse-width modulated type signal as the first and second signal levels thereof vary in proportion to the width of bars and spaces in the scanned bar code symbol. 
     In addition, the digital signal processing circuitry includes digitizing circuitry whose functions are two-fold: (1) to convert the digital scan data signal D 2 , associated with each scanned bar code symbol, into a corresponding sequence of digital words (i.e. a sequence of digital count values) D 3  representative of package identification (I.D.) data; and (2) to correlate time-based (or position-based) information about the facet sector on the scanning disc  130  that generated the sequence digital words D 3  (corresponding to a scan line or portion thereof). 
     Notably, in the digital word D 3 , each digital word represents the time length duration of first or second signal level in the corresponding digital scan data signal D 2 . Preferably, the digital words D 3  are in a digital format suitable for use in carrying out various symbol decoding operations which, like the scanning pattern and volume of the present invention, will be determined primarily by the particular scanning application at hand. 
     In addition, the digital signal processing circuitry includes symbol decoding circuitry that primarily functions to receive the digital word sequence D 3  produced from its respective digitizing circuitry, and subject it to one or more bar code symbol decoding algorithms in order to determine which bar code symbol is indicated (i.e. represented) by the digital word sequence D 3 . 
     Reference is made to U.S. Pat. No. 5,343,027 to Knowles, herein incorporated by reference in its entirety, as it provides technical details regarding the design and construction of circuitry suitable for use in the holographic laser scanning system  100 -A of the present invention. 
     In addition, the digital signal processing circuitry may generate information that specifies a vector-based geometric model of the laser scanning beam (and possibly plane-sector) that was used to collect the scan data underlying the decode bar code symbol. Such information may be used with “3-D ray tracing techniques” to derive the position of the decoded bar code symbol in the 3-D scanning volume as described in detail in co-pending U.S. patent application Ser. No. 09/157,778, filed Sep. 21, 1998, co-pending U.S. patent application Ser. No. 09/327,756 filed Jun. 7, 1999, and International Application PCT/US00/15624, filed Jun. 7, 2000, all commonly assigned to the assignee of the present invention and herein incorporated by reference in their entirety. 
     In addition, the analog (or digital) signal processing circuitry may include a plurality of pass-band filter stages corresponding to different focal zones (or different scan ranges) in the 3-D scanning volume. Each pass-band filter stage is designed with particular high (and low) cut-off frequencies that pass the spectral components of the analog scan data signal produced when a bar code symbol is scanned at the corresponding focal zone (or scan range), while limiting noise outside the particular spectral pass-band of interest. When a bar code symbol is scanned by a laser beam focused within a particular focal zone in the 3-D scanning volume, the pass-band filter stage corresponding to the particular focal zone (or particular scan range) is automatically switched into operation so that the spectral components of the analog scan data signal within the particular spectral pass-band are present, while noise outside the particular spectral pass-band is limited. This selective filtering enables the signal processing circuitry to generate first and second derivative signals (which are processed to produce a corresponding digital scan data signal as described above) that are substantially free from the destructive effects of thermal and substrate noise that are outside the spectral pass-band of interest for the bar code symbol being scanned. A more detailed description of such selective filtering mechanisms (and laser scanning systems that employ such mechanisms) is described in U.S. patent application Ser. No. 09/243,078 filed Feb. 2, 1999, and U.S. application Ser. No. 09/442,718 filed Nov. 18, 1999, herein incorporated by reference in their entirety. 
       FIG. 4(E)  illustrates an exemplary embodiment of the laser production modules  147 A of  FIGS. 4(B) and 4(C)  including: a visible laser diode (VLD)  101 A, an aspheric collimating lens  51  supported within the bore of a housing  53  mounted upon the optical bench  143  of the module housing for collimating (e.g., focusing) the laser light produced by the VLD  101 A; a mirror  55 , supported within the housing  53 , for directing the focused laser light produced by lens  51  to a multi-function light diffractive grating  57  (sometimes referred to by Applicants as “multi-function HOE” or “multi-function plate”) supported by the housing  53 . The multi-function light diffractive grating  57 , having a fixed spatial frequency and disposed at incident angle relative to the outgoing laser beam provided by the mirror  55 , produces a primary beam that is directed toward the facets of the rotating scanning disk  130 . The multi-function light diffractive grating  47  changes the properties of the incident laser beam so that the aspect ratio of the primary beam is controlled, and beam dispersion is minimized upon the primary laser beam exiting the holographic scanning disc  130 . Details for designing the multi-function light diffractive grating  57  and configuring the laser scanning beam module  147 A of the illustrative embodiment is described in greater detail in Applicants&#39; prior U.S. patent application Ser. No. 08/949,915 filed Oct. 14, 1997, and incorporated herein by reference in its entirety. 
     In addition, the holographic laser scanning system  100 -A includes laser drive circuitry (not shown) which generates the electrical signals for driving the VLD  101 A of the respective laser beam production modules  147 A,  147 B, . . .  147 E. The laser drive circuitry for a respective VLD may be disposed on the PC board  202  (shown in  FIG. 4(C)  as PC board  202 A for the VLD  101 A in laser beam production module  147 A). 
     In addition, the holographic laser scanning system  100 -A preferably includes a control board (not shown) disposed with the housing  140  onto which is mounted a number of components mounted on a small PC board, namely: a programmed controller with a system bus and associated program and data storage memory, for controlling the system operation of the holographic laser scanner system  1090 A and performing other auxiliary functions; serial data channels (for example, RS-232 channels) for receiving serial data input from the symbol decoding circuitry described above; an input/output (I/O) interface circuit  248  for interfacing with and transmitting symbol character data and other information to an I/O subsystem (which may be operably coupled to a data management computer system); home pulse detector, including a photodetector and associated circuitry, for detecting the home pulse generated when the laser beam from a VLD (in home pulse marking sensing module) is directed through home-pulse gap  260  (for example, between Facets Nos. 6 and 7 on the scanning disk  130  as shown in  FIG. 4(D) ) and sensed by the photodetector; and a home-offset-pulse (HOP) generator, which is preferably realized as an ASIC chip, for generating a set of home-offset pulses (HOPs) in response to the detection of each home pulse by the home pulse detector. The programmed controller produces motor control signals, and laser control signals during system operation that enable motor drive circuitry to drive the scanning disc motor coupled to holographic scanning disc  130  and enable the laser drive circuitry to drive the VLDs of the laser beam production modules  247 A,  247 B, . . .  247 E, respectively. A more detailed description of the control board and its respective components are disclosed in co-pending U.S. patent application Ser. No. 09/047,146 filed Mar. 24, 1998, co-pending U.S. patent application Ser. No. 09/157,778, filed Sep. 21, 1998, co-pending U.S. patent application Ser. No. 09/327,756 filed Jun. 7, 1999, co-pending U.S. patent application Ser. No. 09/551,887 filed Apr. 18, 2000, International Application No. PCT/US99/06505 filed Mar. 24, 1999, and International Application PCT/US00/15624, filed Jun. 7, 2000, all commonly assigned to the assignee of the present invention and herein incorporated by reference in their entirety. 
     According to the present invention, the holographic laser scanning system  100 -A includes visible indicia characterizing location (and general spatial boundaries) of the 3-D scanning volume (and the 3-D omni-directional scan pattern therein) that is visibly discernable to users of the holographic laser scanning system. Such visible indicia may be one or more visible light beams (visibly discernable to users of the holographic laser scanning system) that provide a visible light pattern characterizing location (and general spatial boundaries) of the 3-D scanning volume (and the 3-D omni-directional scan pattern therein). Alternatively, such visible indicia may be visible markings (visibly discernable to users of the holographic laser scanning system), such as reflective paint or reflective tape, affixed to a surface (over which objects are moved through the 3-D scanning volume) in a manner that characterizes location (and general spatial boundaries) of the 3-D scanning volume (and the 3-D omni-directional scan pattern therein). Such visible indicia may characterize location (and general spatial boundaries) of the 3-D scanning volume (and the 3-D omni-directional scan pattern therein) by providing an indication of the approximate location of the center of the 3-D scanning volume, of one or more edges of the 3-D scanning volume, and/or of any other portion of the 3-D scanning volume. A more detailed description of such mechanisms is described above with respect to  FIGS. 2(A) ,  2 (B) and  3 . Such visible indicia enable users to quickly identify the correct location of the 3-D scanning volume (and the 3-D omni-directional scan pattern therein) when attempting to transport the object through the 3-D scanning volume, thus limiting unwanted scanning errors and increasing the productivity of the user, which represents decreased costs associated with the use of the holographic laser scanning system. 
     In another embodiment of the 3-D omni-directional holographic laser scanning system of the present invention, such visible indicia may be generated by controlling the system to repeatably scan select scan lines that pass through the 3-D scanning volume thereby providing a pulsing of such select scan lines in a manner that provides a characterization of location (and general spatial boundaries) of the 3-D scanning volume (and the 3-D omni-directional scan pattern therein) that is visibly discernable to users of the system. The pulsing of such select scan lines may characterize location (and general spatial boundaries) of the 3-D scanning volume (and the 3-D omni-directional scan pattern therein) by providing an indication of the approximate location of the center of the 3-D scanning volume, of one or more edges of the 3-D scanning volume, and/or of any other portion of the 3-D scanning volume. In the illustrative holographic laser scanning system  100 -A as described above, the home pulse detector (and timing signals derived therefrom) may be used to control the pulsing of select scan lines in a manner that provides a characterization of location of the 3-D scanning volume (and the 3-D omni-directional scan pattern therein) that is visibly discernable to users of the system. 
     The improved 3-D omni-directional holographic laser scanning system of the illustrative embodiments of the present invention as set forth above may include an additional mechanism that indicates when a user enters the 3-D scanning volume (or a region proximate thereto) and provides the user with visible (or audio) feedback in response thereto. Such a mechanism may employ one or more infra-red detection beams that sweep the 3-D scanning volume to detect when a user enters the 3-D scanning volume (or a region proximate thereto). Upon detection, the mechanism generates a visual signal (such as flashing light) and/or an audio signal that indicates that the user is inside (or outside) the 3-D scanning volume (or the region proximate thereto). In the event that the system employs a “good read” audio (and/or visual) indicator, such “good read” indicator is preferably distinguishable from the signals that indicate that the user is inside (or outside) the 3-D scanning volume (or the region proximate thereto). In addition, the mechanism that indicates when a user enters the 3-D scanning volume (or a region proximate thereto) may be used to selectively activate (or deactivate) generation of the visible light pattern that characterizes location of the 3-D scanning volume (and the 3-D omni-directional scanning pattern therein) in response thereto. 
     The improved 3-D omni-directional holographic laser scanning system of the illustrative embodiments of the present invention as described above can be used in various types of applications, such as for example, in package handling applications where bar codes are read to determine (a) identification of incoming packages, (b) identification of outgoing packages, and (c) to provide user instructions in manually routing and sorting packages based upon the information encoded by the bar codes. Moreover, the laser scanning system of the illustrative embodiments of the present invention as described above can read virtually any bar code symbology imaginable (e.g. Interleaved two of five, Code 128 and Code three of nine) and formats so as to sort and identify packages at various package rates required by USPS or other end-users, ZIP Codes (six digits), Package Identification Codes (PIC) (sixteen characters) and Tray bar code (ten digits) symbols. 
     For example, the housing of the improved 3-D omni-directional holographic laser scanning system of the illustrative embodiments as described above may be mounted to a base that can be moved (and locked) into different spatial positions overhead one or more manual package scanning, sorting and routing stations (e.g., a station at the end of a slide, a station adjacent a conveyor belt, a station adjacent a transport container or bin, or a station adjacent a transport vehicle such as truck or van). It is contemplated that a rolling track or a multi-cantilever arm (similar to the arm used to position a light in a dentist&#39;s office) may be used to move (and lock) the 3-D omni-directional holographic laser scanning system into the desired spatial position overhead such stations. 
     Moreover, the improved 3-D omni-directional holographic laser scanning system of the illustrative embodiments of the present invention as described above can process all types of products (e.g. trays and tubs having extremely large variance in surface types, colors, and plastics (e.g. Tyvek material, canvass, cardboard, polywrap, Styrofoam, rubber, dark packages). Some of these product types include: soft pack pillows, bags; packages having non-flat bottoms, such as flats, trays, and tubs with and without bands; cartons; rugs; duffel bags (without strings or metal clips); tires; wooden containers; and sacks. 
     It is understood that the laser scanning systems, modules, engines and subsystems of the illustrative embodiments may be modified in a variety of ways which will become readily apparent to those skilled in the art, and having the benefit of the novel teachings disclosed herein. All such modifications and variations of the illustrative embodiments thereof shall be deemed to be within the scope and spirit of the present invention as defined by the Claims to Invention appended hereto.