Patent Publication Number: US-2006000911-A1

Title: Automatic certification, identification and tracking of remote objects in relative motion

Description:
FIELD OF THE INVENTION  
      The present invention relates to the field of remote tracking systems, especially for use in determining the identity and motion of a moving remote object by means of an optical identity tag carried thereon.  
     BACKGROUND OF THE INVENTION  
      Various systems are known in the prior art that address the problem of automatically identifying tagged objects or vehicles in motion. These systems generally use radiation such as ultrasonic, radioactive, optical, magnetic or radio frequency radiation. Some of these systems have not received widespread acceptance because of excessive cost and insufficient reliability.  
      Various optical systems, such as license plate recognition systems, are sensitive to lighting variations, cannot handle massive flows and necessitate the assistance of a human operator to analyze cumbersome images of license plates that the processing software cannot recognize. Other optical systems, based on barcode reading, generally have limited contrast and spatial resolution. Commonly used barcode systems based on laser scanning are generally limited to static or quasi-static situations; in dynamic situations, where the barcode is in motion, the signals tend to smear and the resolution is degraded. Normal barcode systems are also limited to close proximity between the scanner and the barcode; at large distances, the spatial resolution is again degraded because of insufficient sampling. Yet another problem arises from the fact that the field of view of prior art, conventional barcode systems is limited to the collimated beam zone; thus the operator needs to find the optimal location of the scanner in front of the barcode, which can be a time-wasting operation. Other barcode systems adapted for large distances and high velocity reading capabilities either necessitate relatively large barcode patterns or special means to magnify the barcode patterns using special optics. These systems are complex; and may have a tendency to malfunction, or may be sensitive to harsh reading conditions. Furthermore, for use with high object velocities they tend to provide smeared signals.  
      U.S. Pat. No 6,017,125 to Vann discloses the use of a bar coded retroreflective target to measure six degrees of target position and the use of a bar coded retroreflector to provide information about the target. These designs use the object motion to scan a barcode pattern that is combined with retroreflective optics, either a cube retro reflector or a ball lens retro reflector. In addition, the designs disclosed in this patent are bulky, are probably costly to manufacture, and thus may not be suited for mass usage.  
      Furthermore, in the system described by Vann, the entire field of view of the object or objects being scanned or tracked are described as being focused onto the detector, which is alternatively described as being either a position sensitive detector, or an array of photodiode elements or a camera. The decoding of the information is determined by signal processing of the time-varying digital signals obtained from these detectors. As a result, since all of the sensors of the detection means respond to all of the tags within the reader&#39;s viewing field at any given time, it is not possible to separate between multiple responses of several tags that may appear in the volume monitored, and the method thus would appear to suffer from tag cross talk Tracking of more than one tag at a time would thus appear to be difficult using this prior art apparatus.  
      There are yet other types of system that use radio frequency waves, namely radar devices. These systems installed in urban vicinities are restricted by radiation regulations and necessitate an authority license for operation. In a lot of cases, this limits their maximum power to relatively low levels. This in turn, narrows the communication zone and worsens the electromagnetic interference noise situation, resulting in a poor signal-to-noise ratio. Furthermore, radio frequency based systems are susceptible to inter-modulation or cross talk between tags that may be addressed at the same moment in time. Finally, in applications where the position and speed are desired in addition to the vehicle identity, radar devices tend to confuse between neighboring vehicles.  
     SUMMARY OF THE INVENTION  
      The present invention seeks to provide a method and apparatus for automatic certification, identification and tracking of remote objects in relative motion to a reading system, and in particular a system comprised of a novel tag affixed to an object and novel apparatus and techniques for automatically reading the tag information, its relative velocity, angle and position. The relative motion between the reader and the tag may occur in either one of three situations: (i) a stationary reader and moving tag; (ii) a stationary tag and moving reader, as in a scanning detector; and (iii) a situation with both tag and reader moving in relative motion to each other. The system has particular application to the problem of vehicle identification, as well as the measurement of their speed and position simultaneously. Another application of the system of the present invention is for the provision of automatic and maintenance-free road signposts, where signpost data could be read from a moving vehicle and from a remote distance. Yet another application is the scanning of inventory in places such as warehouses, museums etc., where readers are installed on entrances, or may be conveyed on rail arrangements so as to scan each tagged item swiftly.  
      The present invention attempts to overcome the difficulties associated with prior art systems, as outlined in the Background section, by providing a novel optically readable system and method for the remote identification of objects in relative motion, such as vehicles, in addition to speed and position determination.  
      The system preferably comprises a separate reader unit and an optical tag unit, preferably on the moving object. The system generally comprises a light source that is preferably monochromatic, an imaging device having its optical axis and field of view exactly bore sighted with the light source, and a retroreflective tag preferably attached to the moving object. The system differs from the prior art systems described above, in that the field of view of the reader unit is imaged by the detection means, preferably a video imager, such that a complete image of the entire field of view is captured at every moment. This image, which can contain retro-reflected information from multiple tags, can be processed by means of standard image processing techniques, and temporally changing information about each tag extracted separately on each pixel, without any confusion or mixing between different tags. In the prior art system of Vann, for instance, light returning from the retro-reflector is not described as undergoing any real imaging process, but is shown as being focused onto the detector plane only by means of a cylindrical lens, which is described alternatively either as compensating the divergence of the light returning from the retro-reflector, or as focusing the returning beam on the detector, in locations that are proportional to the vertical angle. From the description given, it would thus appear that retro-reflected light from a number of tags spaced in the direction of the scanning or the motion would be focused onto the detector plane without the use of an imaging lens, which may cause a smearing of the tag differentiation.  
      Yet another object of the present invention is to provide for a system and a tag that can be read at high relative velocities. As will become apparent from the detailed description of the construction and operation of the reading apparatus and tag, the optical tag uses optical elements to image the information plane of the tag, preferably a barcode, back to the reader unit aperture plane, and uses the tag&#39;s motion to scan the tag&#39;s information plane, such that the spatial information contained in this plane is transformed to a temporal scanning signal that can be acquired by the reader&#39;s video imager.  
      In accordance with a first aspect of the invention, the present invention provides a maintenance free and low-cost optical tag that use retroreflective means to reflect and modulate the reader&#39;s light, back to the reader&#39;s imaging device, without the need for an internal source of energy.  
      In accordance with a second aspect of the invention, the present invention provides a method and a system that can automatically detect and identify a remote tag in relative motion to the scanner, utilizing the tag&#39;s unique spatio-temporal features as a trigger for the reader activity.  
      In accordance with a third aspect of the invention, the present invention provides a system that can be used in severe lighting conditions, utilizing a retroreflective tag that, together with an active illumination with monochromatic light and a suitable filtered imaging device, can suppress spurious light sources and enhance the tag reflective light.  
      In accordance with a fourth aspect of the invention, the present invention provides a system that can be read from relatively large distances, utilizing a retroreflective tag and a bore sight arrangement of the reader&#39;s light source and the reader&#39;s imaging device.  
      In accordance with a fifth aspect of the invention, the present invention allows for simultaneous identification and measurement of speed and position of multiple moving objects or vehicles. As will become apparent from the detailed description of the construction and operation of the optical tag reading apparatus, the system allows for multiple reading of neighboring tags with negligible cross talk between them such that even high flows of moving objects or high traffic flows can be read successfully without degradation in system performance.  
      In accordance with a sixth aspect of the invention, the present invention provides means to handle dirt and smudge in the optical path, by locating the tag near the front windshield of a vehicle, so that if it is covered, the driving visibility will also be degraded and steps taken to rectify the situation.  
      In accordance with a seventh aspect of the invention, the present invention provides for covert operation using light in the infrared region. In addition, as the method is based on a retro reflected radiation the tag can be detected from the reader alone and no light is scattered to another directions.  
      In accordance with an eighth aspect of the invention, the present invention provides for automatic and remote certification of tagged objects using special optical means to prevent counterfeiting.  
      In accordance with a ninth aspect of the invention, the present invention provides for the production of a cost effective, thin and lightweight tag that can be affixed easily to various objects.  
      In accordance with a tenth aspect of the invention, the present invention provides for cost effective ways for the production of the proposed tag.  
      In accordance with an eleventh aspect of the invention, the present invention provides for scanning schemes that reduce the geometrical limitations of the tag reading.  
      Other objects and advantages of this invention will become apparent as the description proceeds.  
      The disclosures of all publications mentioned in this section and in the other sections of the specification, and the disclosures of all documents cited in the above publications, are hereby incorporated by reference, each in its entirety. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Non-limiting examples of embodiments of the present invention are described below with reference to figures attached hereto and listed below. In the figures, identical structures, elements or parts that appear in more than one figure are generally labeled with a same numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are chosen for convenience and clarity of presentation and are not necessarily shown to scale.  
      For fuller understanding of the objects and aspects of the present invention, preferred embodiments of the invention are described with reference to the accompanying drawings, which show in:  
       FIG. 1 : An illustration of a first embodiment of the moving tag reader apparatus, i.e. an MTR, in accordance with a preferred embodiment of the present invention;  
       FIG. 2A : An illustration of an optional embodiment of the moving tag reader apparatus, i.e. an MTR, using fiber optics located at the center of the lens, in accordance with another preferred embodiment of the present invention;  
       FIG. 2B : An illustration of an optional embodiment of the moving tag reader apparatus, i.e. an MTR, using fiber optics on the lens optical axis, in accordance with another preferred embodiment of the present invention;  
       FIG. 2C : A side view of an optional embodiment of the moving tag reader apparatus, i.e. an MTR, using light sources distributed around the camera lens, in accordance with another preferred embodiment of the present invention;  
       FIG. 2D : An upper view of an optional embodiment of the moving tag reader apparatus, i.e. an MTR, using light sources distributed around the camera lens, in accordance with another preferred embodiment of the present invention;  
       FIG. 3A -C: A schematic illustration relating to the temporal aspects of the invention, showing the various phases of operation of the invention, in accordance with another preferred embodiment of the present invention;  
       FIG. 4A , B: illustrations of an optional embodiment of the reader and tag where the moving tag is read by a multi directional scanning system, in accordance with another preferred embodiment of the present invention;  
       FIG. 5 : illustrations of optional embodiments of the reader and tag where the moving tag is read from an arbitrary direction using a Circular Barcode pattern, in accordance with another preferred embodiment of the present invention;  
       FIG. 6 : illustrations of an optional embodiment of the tag, where the focusing optics is constructed of a lenslet array such as a Diffractive Optical Element (DOE) Array, in accordance with another preferred embodiment of the present invention;  
       FIG. 7 : A detailed illustration of an optional embodiment of the optical tag, where the tag information plane is curved along a sphere, in accordance with another preferred embodiment of the present invention;  
       FIG. 8A , B: illustrations of an optional embodiment of the tag, where the tag retro-reflection is enhanced, in accordance with another preferred embodiment of the present invention;  
       FIG. 9 : illustrations of an optional embodiment of the tag, where the tag is constructed of a single surface DOE, in accordance with another preferred embodiment of the present invention;  
       FIG. 10 : An overall illustration of a preferred embodiment of the invention being used to identify moving objects, in accordance with another preferred embodiment of the present invention;  
       FIG. 11A : A schematic illustration of the scene viewed by the reader&#39;s imager, showing the tagged objects or vehicles in motion, in accordance with another preferred embodiment of the present invention;  
       FIG. 11B : A schematic illustration relating to the filtered image acquisitioned by the reader&#39;s video imager showing the tags&#39; retroreflective responses, in accordance with another preferred embodiment of the present invention;  
       FIG. 12 : A schematic illustration depicting the process of accumulating the tag data in the reader, in accordance with another preferred embodiment of the present invention; and  
       FIG. 13 : A block diagram depicting code and data flow of the signal processing process, in accordance with another preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       FIG. 1  shows a schematic layout of the system of the present invention comprising a moving tag reader, (MTR),  10 , for automatic identification, speed assessment and position determination of moving tags, in accordance with a preferred embodiment of the present invention. The MTR  10  optionally comprises a camera  11  having a lens  12  and an imager  13 , a light source  14  and a beam splitter  17 . A controller  52  controls the light source  14  and camera  11  and also preferably comprises an image processor for processing images acquired by the camera of the entire field of view of the MTR In accordance with an embodiment of the present invention, light source  14  and camera  11  optionally have coincident optical axes  20  by means of a bore sight arrangement using beam splitter  17 , and optionally have the same field of view  21  by suitable choice of the numerical aperture of the lens  12  and the cone of light  21 A emitted by the light source. The light source can preferably be either a regular lamp source emitting a diverging beam to cover the desired field of view, or a laser source emitting a coherent beam, together with a negative lens for providing a sufficiently diverging beam if the laser is too collimated.  
      In  FIG. 1 , a tag  30 , installed on a moving object, such as a vehicle, is comprised of a lens  31  and an information plane,  32 . In accordance with a preferred embodiment of the present invention, the tag  30  and the MTR  10  are optionally arranged to have the same depth of field and the same field of view by appropriate choice of the parameters of lens  12  and lens  31  and the distances of their imaging planes  13  and  32  from their respective lenses. This assures that the MTR and tag are optimally optically coordinated to work together, having both optimal visibility and resolution.  
      A light ray,  22 A, emitted from light source  14  is reflected from the beam splitter  17  to the direction of the tag as light ray  22 . Any light ray in the cone  23 , including light ray  22 , is eventually focused to the same focus point  32 A in the tag information plane  32 . In turn, part of the light from the focal point  32 A is reflected through the light cone  33  back to the entrance pupil of the tag lens  31 , focused back to the direction of the MTR  10 , transmitted through the beam splitter  17 , enters the camera lens  12  entrance pupil and is imaged to the point  13 A on the imaging plane  13  of the camera  11 . This tag configuration is called a “retro reflector” because it retro reflects any beam in its entrance pupil back to its original direction. In addition to retro reflecting, the tag configuration has the useful feature of focusing the beam back to its point of origin, which in the layout described in  FIG. 1  is co-aligned with the camera entrance pupil. These features of the tag are the most advantageous arrangement to conserve energy and maximize efficiency of the reflected light that enters the camera&#39;s entrance pupil  12 .  
      In accordance with another preferred embodiment of the present invention, the information plane  32  is optionally comprised of a retro reflective sheet. In this way the tag reflecting efficiency is enhanced because most of the light rays incident on the focus point  32 A is reflected back to the tag lens  31  entrance pupil.  
      Furthermore, according to another preferred embodiment of the MTR of the present invention, there is shown in  FIG. 1  an optional chromatic filter  15  and two aligned polarizers  16 . These optional means are useful for enhancing tag response and rejecting responses from spurious sources. In this optional embodiment, the color filter  15  is matched to a monochromatic light source  14  and the two linear polarizers are aligned so that light coming out of the light source  14  can reach the camera  11  with minimal interference and light coming from other light sources, such as sunlight reflections or vehicle lights, is reduced substantially.  
       FIG. 2A  shows another preferred embodiment of the MTR of the present invention, in which the light source,  14 , is collimated by means of a single-fiber collimator  19  into an optical fiber,  19 A. The end of the fiber is optionally inserted into a hole,  12 A, in the imaging lens,  12 . Alternatively and preferably, the fiber end can be disposed behind the lens center, at  19 B, such that the combined numerical aperture of the paraxial portion of the lens and fiber is essentially the same as that of the full aperture of the lens. The lens hole  12 A, is then unnecessary.  
      As another option to  FIG. 2A , typical of high numerical aperture applications,  FIG. 2B  shows yet another preferred embodiment, in which the end of the fiber is optionally fixed in front of the lens,  12 , co aligned to its optical axis,  20 . In these cases the parallax between the light coming out of the fiber,  22 A, and the retro reflected light collected by the imaging lens,  22 B, is negligible.  
       FIGS. 2C and 2D  show a side view and an upper view, respectively, of another preferred embodiment of the MTR of the present invention, in which a number of light sources are distributed around the imaging lens,  12 . Such an embodiment may be realized using a ring of LED&#39;s placed around the lens.  FIG. 2C , shows a side view of a particular distribution, where two light sources,  14 A and  14 B, are located on two sides of the imaging lens,  12 , in a perpendicular direction to the scanning direction.  
      The beams  25 A and  25 B, coming of the light sources  14 A and  14 B respectively, are focused on  32 A and  32 B, respectively, on the information plane,  32  of the tag,  30 . The two focuses have point spread functions,  34 A and  34 B, accordingly and thus a combined response  34 C that in turn is retro reflected in a direction surrounding the direction  22 , back to the MTR entrance pupil.  FIG. 2D , shows an upper view of this embodiment, where the two point spread functions are located on the same vertical location along the scanning direction.  
      This embodiment of the MTR is typical of applications where the numerical aperture is especially high, and enables the parallax between the light coming out ring of LED&#39;s and the retro reflected light collected by the imaging lens,  12 , to be negligible.  
      In accordance with an embodiment of the present invention, the information plane  32  optionally comprises a barcode pattern having its chief axes, i.e. the scan axes, co aligned with the direction of the object motion.  
      Reference is now made to FIGS.  3 A-C that are a series of sequential schematic illustrations, showing the motion of a tagged object  40  across the field of view of the reader unit  10 , in accordance with another preferred embodiment of the present invention. The drawings illustrate graphically the way in which the spatially moving information on the tag  30  is transformed into meaningful and simply read temporal information by means of the optics of both the tag unit  30  and the reader unit  10 . In  FIG. 3A , the tagged object  40  is shown entering the field of view of the reader unit  10 , at which point, the mutual geometries of the imaging optics of reader and tag units are such that the first bar of information  32 A on the tag information plane  32  retro-reflects the incident illuminating beam and is imaged by the read unit on the camera image plane  13  as point  13 A. As the tagged object moves along its motion path  41 , the mutual fields of view of the reader and tagged units change such that retro-reflected rays from different bars of the tag are sequentially imaged onto the camera image plane. Thus, in  FIG. 3B , bar  32  B is imaged onto point  13 B on the camera imaging plane, and in  FIG. 3C , the bar at  32 C is imaged onto point  13 C by the camera. In this way, the entire bar code information is sequentially imaged onto the camera image plane  13  such that the system controller acquires a temporally changing image of the tag information.  
      In some prior art barcode scanning systems, a collimated laser beam, swept across the bar-code, is used in order to convert the spatial information on the bar code into temporally changing information for serial processing. The system of the present invention differs from this prior art in that the optics incorporated on the tag enlarge each bit of the information plane so that it is fully resolved by the reader even at substantially large distances, such that the tag may be kept relatively small.  
      Furthermore, the system of the present invention differs from such prior art in that the effective scanning motion of the interrogating illuminating beam across the bar-code, and its retro-reflected information-bearing beam, are generated by means of the relative motion of the limited fields of view of both reader and tagged units resulting from the use of the pre-specified optical imaging systems on both of these units. Thus there is no smearing of the signal read, which can cause the degradation of signal resolution.  
      Reference is now made to  FIGS. 4A  to  5  where various optional configurations for different geometrical readings of the tag are shown.  
       FIGS. 4A  and B: illustrate an optional preferred embodiment of the reader and tag where the moving tag is read by a multi-directional scanning system, in accordance with an embodiment of the present invention. In this configuration the tag information plane is optionally constructed of several barcode segments. Without losing generality,  FIG. 4A  represent the case of two separate barcodes located in the tag&#39;s information plane. The barcodes are located in different locations along the Y-axis, perpendicular to the reading direction, X. Alternatively and preferably the tag can comprise two identical barcodes to provide increased reliability by redundancy.  
       FIG. 4B  shows two readers positioned in the appropriate angles, each of them reading the corresponding barcode segment.  
       FIG. 5  illustrates an optional embodiment of the reader and tag combination, where the moving tag is read from an arbitrary direction using a Circular Barcode pattern, in accordance with another preferred embodiment of the present invention; this optional configuration is suggested for situations where there is no guarantee that the barcode segment in the tag&#39;s information plane is aligned to the reading direction but it certain that the tag path is crossing through the reader&#39;s optical axis. Thus, independently of whether the tag is read along direction  35 A or  35 B, for instance, the information thereon is correctly imaged and decoded.  
      In accordance with another preferred embodiment of the present invention, the tag angle versus the reader optical axis direction may be recovered using the tag&#39;s reflected color. This feature is made possible by using a multicolored plane of information,  32 , where each point on the plane features a unique color corresponding to a distinct angle of view. Having in advance knowledge of the information plane color scheme enables the retrieving of the tag angle versus the reader&#39;s optical axis direction, by identifying the tag retro reflection color. In that case, the reader may optionally be a multi spectral reader such as color video camera In accordance with another preferred embodiment of the present invention, the tag&#39;s position is related to its image in the reader&#39;s imaging plane and the velocity of the tag can be recovered by temporal derivation of the tag&#39;s position vector. Using features such as angle, position and velocity the tag can be traced or even may be used as a reference for automatic navigation.  
       FIG. 6A  shows another preferred embodiment of the tag, where the focusing optics is constructed of a Lenslet Array  31 , in accordance with an embodiment of the present invention; this embodiment is useful whenever a lightweight and thin tag is desired. The number of array cells used is dependent on the reading distance of the application, the light power needed and the reading resolution available.  
      As an optional preferred embodiment, the lenslet array  31  can be created of a Diffractive Optical Element (DOE) Array. DOE&#39;s are particularly adaptable for monochromatic illumination and imaging systems and can incorporate corrections for spherical aberrations.  
      The information plane of the tag array is constructed of a periodical pattern having the same period as the optical array. The fitting of the periodical pattern can be done in numerous ways. One way is by printing a marker in a known location within the pattern and inserting the pattern into the optical array using an automated bench, having an optical feedback mechanism.  
      As an alternative option, the fitted pattern in the optical array can be left unaligned. In this case the optical marker can be identified with the reader in real time, thus the read barcode pattern can be prearranged in a cyclic manner.  
      In cases were the physical size of the tag is not negligible relative to the reading distance there is a need to compensate for the reading parallax of the tag array. This parallax can be calculated from the equation, Δx=f*d/z, were f is the optical focus of the array optics, d is the tag size and z is the reading distance. For example, a tag of 20 mm size, with an optical focus of 1.5 mm and a reading distance of 5 meters, has a six microns parallax  FIG. 6B , shows a periodical barcode pattern that is compensated to adapt for the parallax of the predefined reading distance.  
      In accordance with another preferred embodiment of the present invention, the spatial information stored within the tag can be alternatively stored in a multi layered interference filter, and assigning to each angle of interrogating beam incidence, a different reflectance. This ensures that while the tag is in motion, the interrogating beam scans different angles of incidence and thus responds to the information coded within the tag.  
      In accordance with another preferred embodiment of the present invention,  FIG. 7  shows a detailed illustration of the optical tag configuration where the information plane  32  is curved along a sphere at the focal distance from the tag lens  31 . Using this configuration, the focus point  32 A, of the chief ray  33 , is adequately focused for each direction the tag is interrogated. This embodiment represents another option to the use of DOE for the minimization of comma The present invention provides for a system that can be used in severe lighting conditions, utilizing a retroreflective tag that, together with active illumination with monochromatic light and a suitable filtered imaging device, can suppress spurious light sources and enhance the tag reflective light.  FIGS. 8A  and B shows a further preferred configuration of the retro-reflective tag.  FIG. 8A  shows the tag&#39;s back plane, made of multiple micro-mirrors,  36 , each is directed towards the tag lens&#39;s center,  37 . The beam shown in  FIG. 8B , spanning from ray  38 A to ray  39 A, is focused at the tag back plane at point  36 A, is mirror-imaged and reflected back onto itself, thus being retro-reflected. Ray  38 A is reflected to ray  38 B that is on the same path but opposite to ray  39 A. Ray  39 A in its turn, is reflected back to the same path as ray  38 A but to the opposite direction.  
       FIG. 9  shows a single surface tag that is constructed of a single surface DOE,  44 , to encode the angular reflection spectrum of a barcode,  47 . The DOE is preferably constructed of a lens and a combination of diffraction gratings, each one have a pre specified cycle frequency and thus having a consequent diffraction direction. Together, the lines create the characteristically barcode lines. The lens is designed to focus the reader&#39;s radiation back to its origin and even more importantly, bring closer the Fraunhofer diffraction pattern, located typically at large distances, so it can be observed by the reader, as known in the art (Introduction to Fourier Optics/Joseph W. Goodman, p. 61, 83-86). The reader illuminates the tag from direction  42 . Thus, the main specular reflection,  45 , comes from the opposite side of the DOE optical axis and the diffracted rays,  46 , construct the angular spectrum,  47 , of the DOE, spanning both sides of the main reflection,  45 . It should be noted that the reader, located in direction  42 , because of its relative motion with respect to the DOE, temporally samples the diffraction spectrum across the whole of the diffracted light angle.  
      In accordance with another preferred embodiment of the present invention, the location of the image of each line of the tag&#39;s information plane is proportional to its location within the information plane and the tag&#39;s focus length, and is not affected by the velocity of the tag or its acceleration. Thus the image acquisitioned by the reader&#39;s camera is robust to change in tag velocities even at high relative velocities, or in the presence of tag accelerations. However, the light integration of the camera&#39;s detector is affected by the tag&#39;s velocity. At high tag velocities, the light response is smaller. This problem is easily solved using tag reflective enhancement properties and further selecting high-powered light source.  
      In accordance with another preferred embodiment of the present invention, the present invention provides means to handle dirt and smudge in the optical path, by locating the tag near the front windshield so that if it is covered, this is a sign that the driving visibility is also degraded and steps will be taken to rectify the situation. In order to further resolve the situation, more tags can be affixed to the front windshield such that all of them are read simultaneously in order to gain redundancy. Furthermore, the reader light source can be made adaptive to the weather conditions since drivers do not see infrared light and there is no radiation hazard using this band. Furthermore, in poor weather conditions, vehicles usually reduce their speed thus compensating for the poor visibility.  
      In accordance with another preferred embodiment of the present invention, the suppression of spurious light sources is very high relative to the reflectivity of the tag. This is made possible by the high reflective efficiency of the tags and the monochromatic and polarization filtering of the reader.  
      In accordance with another preferred embodiment of the present invention, the present invention provides for covert operation using light in the infrared region.  
      In accordance with another preferred embodiment of the present invention, the present invention provides for automatic and remote certification of tagged objects using special optical means to prevent counterfeiting, as is known in the art.  
      In accordance with another preferred embodiment of the present invention, the present invention provides for a system that can be read from relatively large distances, utilizing a retroreflective tag and bore sight arrangement of the reader&#39;s light source and the reader&#39;s imaging device. In systems necessitating large tag distances, the tag reflective efficiency can be improved by selecting larger tag aperture diameters.  
       FIG. 10  shows an overall illustration of the preferred embodiment of the invention being used to identify moving objects or vehicles,  40 , in accordance with an embodiment of the present invention. The reader,  10 , may be installed on top or on the side of the path of the object,  40 . The object may be a vehicle. The tag,  30 , positioned on the vehicle, is read by the reader,  10 , and then further transferred to a controller,  52 , for further processing. The controller,  52 , may comprise a host computer and a video frame grabber,  50 .  
       FIG. 11A  shows a schematic illustration of the scene viewed by the reader imager, showing the tagged objects or vehicles in motion,  40 , in accordance with another preferred embodiment of the present invention. The objects,  40 , carry tags,  30 , and move along the read zone,  41 , of the reader.  
       FIG. 11B  shows a schematic illustration relating to the filtered image acquisitioned by the reader&#39;s video imager showing the tag retroreflective responses,  121 , in accordance with another preferred embodiment of the present invention.  
       FIG. 12  shows a schematic illustration depicting the process of accumulating the tag data in the reader, in accordance with another preferred embodiment of the present invention. In each video frame of the reader, the tag&#39;s response,  121 , is identified and then accumulated to form the accumulated image of the barcode,  124 .  
       FIG. 13  shows a block diagram depicting code and data flow of the signal processing process, in accordance with another preferred embodiment of the present invention.  
      All the processing of this invention is digital processing. Grabbing an image by the camera, such as those of the apparatus of this invention, generates a sample image on the focal plane, which sampled image is preferably, but not a two-dimensional array of pixels, wherein to each pixel is associated a value that represents the radiation intensity value of the corresponding point of the image. For example, the radiation intensity value of a pixel may be from 0 to 255 in gray scale, wherein 0=black, 255=white, and others value between 0 to 255 represent different levels of gray. The two-dimensional array of pixels, therefore, is represented by a matrix consisting of an array of radiation intensity values.  
      Hereinafter, when an image is mentioned, it should be understood that reference is made not to the image generated by a camera, but to the corresponding matrix of pixel radiation intensities.  
      Each sampled image is provided with a corresponding coordinates system, the origin of which is preferably located at the center of the sampled image.  
      In order to adequately describe the algorithm description following, a number of definitions are necessary:  
      Pixel Segment is a group of connected pixels sharing common features or a group of features.  
      Segment labeling is the process of assigning each pixel in the image with a value of the segment to which the pixel belongs.  
      Segment feature extraction procedure is the process that assigns to each segment its features, such as segment area or number of pixels, segment mass, which is the sum of the pixel&#39;s gray levels, segment various moments, such as the moment of inertia, etc.  
      Segment classification procedure is the process of assigning a class or type to a segment according to the amount of resemblance of its features to the known features of the various classes.  
      Temporally accumulated barcode segment list is the list of all barcode-classified segments from all frames; each segment is stored with its features and its video frame origin.  
      Frame i,  52   b , is grabbed within the frame sequence  52   a . In frame i, the various segments of pixels are segmented using spatio-temporal filtering  52   c  as well as morphological filtering to form the segmented image i,  52   d , as is known in the art. The various segments are then labeled,  52   e , to form the segment list i,  52   f . To each segment a feature extraction procedure,  52   g , is than applied to form the featured segmented list i,  52   h , as is known in the art. A segment classification procedure is than applied to distinguish the signal segments from the spurious noise segments to form the temporally accumulated barcode segment list  52   j , as is known in the art. The barcode segments,  52   j , are then merged,  52   k , using the segments features, such as their locations etc. to form the merged barcode strings,  52   l . Each barcode string is than decoded,  52   m , to form the decoded tag information,  52   n.    
      The information content of the tag is limited by the spot size of the optical system of the tag and the size of the information plane. The actual capacity in bits, or the number of resolvable barcode lines is the ratio of the information plane length to the lens focus spot width.  
      In accordance with another preferred embodiment of the present invention, the unique spatio-temporal behavior of the tag is utilized to automatically detect its presence within the field of view of the reader. As the moving tag enters the reader&#39;s field of view, it will be seen flickering and thus its detection and the initiation of decoding can be done automatically.  
      In accordance with another preferred embodiment of the present invention, the sampling of the barcode signal is done in the reader camera. Generally, spatio-temporal sampling is sought; both spatial and temporal samplings are needed for simultaneous tag reading without cross talk between their respective signals. There are some tradeoffs between the spatial and the temporal sampling of the signal according to the information merits needed. The tag position can be sampled by the spatial sampling alone while the tag&#39;s information content may be sampled both spatially and temporally. Thus, the combined spatio-temporal sampling scheme resolves both the tag&#39;s information content and the position vector of the tag. The position vector provides the tag location; its temporal derivative provides the tag&#39;s speed and its scalar multiplication with the reader&#39;s direction of viewing vector provides the tag&#39;s relative angle to the reader&#39;s viewing direction The simplest situation of tag reading is the case where there is no need to resolve its position and there is only one tag that may be present at a time. In this situation, temporal sampling alone is sufficient. This sampling scheme results in relatively simple signal acquisition and processing where the reader&#39;s imaging plane is preferably comprised of a single detector, usually a single photodiode. In other cases where the tag position is needed or there may be more than just one tag present in front of the reader, spatial sampling is needed as well. In cases where the position determination is needed at relatively high resolution, the spatial resolution alone may resolve both the tag&#39;s information and position. In this case, the number of pixels in the sampling matrix limits the information content that can be resolved. In yet another case where the tagged objects are moving along a distinct line, the sampling may be one dimensional, e.g. a linear array of pixels.