Abstract:
A hand held industrial identification symbol reader projects diffuse illumination onto a region of interest to produce high contrast images of symbols. The diffuse illumination is particularly well suited for producing images of symbols that are marked directly on objects. The diffuse illumination is created through the use of an optically translucent diffuser having a generally tapered shape that distributes diffuse illumination throughout the field of view of the reader. The diffuse illumination can be supplemented with dark field illumination using selectively actuated illumination sources.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/573,402, titled “Hand Held Symbology Reader Illumination Diffuser,” filed on Oct. 5, 2009, which is a continuation of U.S. patent application Ser. No. 11/014,478, titled “Hand Held Symbology Reader Illumination Diffuser,” filed Dec. 16, 2004, each of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to illumination for image acquisition devices, and more particularly, to illumination for image acquisition devices used for reading industrial identification codes. 
     2. Description of the Related Art 
     Machine vision systems use image acquisition devices that include camera sensors to deliver information on a viewed subject. The system then interprets this information according to a variety of algorithms to perform a programmed decision-making and/or identification function. For an image to be most effectively acquired by a sensor in the visible, and near visible light range, the subject should be properly illuminated. 
     In the example of identification symbol scanning using an image sensor, good lighting is highly desirable. Identification symbol scanning entails the aiming of an image acquisition sensor (CMOS camera, CCD, etc.) at a location on an object that contains a bar code, data matrix, or other identification symbol, and retrieval of an image of that symbol. The identification symbol contains a set of predetermined patterns that represent an ordered group of characters or symbols from which an attached data processor (for example a microcomputer) can derive useful information about the object (e.g. its serial number, type, model, price, etc.). Identification symbols are available in a variety of shapes and sizes. Two of the most commonly employed symbol types are the so-called one-dimensional barcode, consisting a line of vertical stripes of varying width and spacing, and the so-called two-dimensional barcode consisting of a two-dimensional array of dots or rectangles. 
     In reading identification symbols or other subjects of interest the type of illumination employed is of particular concern. Where barcodes and other viewed subjects are printed on a flat surface with contrasting ink or paint, a diffuse, high-angle “bright field” illumination may best highlight these features for the sensor. By high-angle it is meant, generally, light that strikes the subject nearly perpendicularly (normal) or at an angle that is typically no less than about 45 degrees from perpendicular (normal) to the surface of the item being scanned. Such illumination is subject to substantial reflection back toward the sensor. By way of example, barcodes and other subjects requiring mainly bright field illumination may be present on a printed label adhered to an item or container, or on a printed field in a relatively smooth area of item or container. 
     Conversely, where a barcode or other subject is formed on a more-irregular surface or is created by etching or peening a pattern directly on the surface, the use of highly reflective bright field illumination may be inappropriate. A peened/etched surface has two-dimensional properties that tend to scatter bright field illumination, thereby obscuring the acquired image. Where a viewed subject has such decidedly two-dimensional surface texture, it may be best illuminated with dark field illumination. This is an illumination with a characteristic low angle (approximately 45 degrees or less, for example) with respect to the surface of the subject (i.e. an angle of more than approximately 45 degrees with respect to normal). Using such low-angle, dark field illumination, two-dimensional surface texture is contrasted more effectively (with indents appearing as bright spots and the surroundings as shadow) for better image acquisition. 
     To take full advantage of the versatility of a camera image sensor, it is desirable to provide both bright field and dark field illumination for selective or simultaneous illumination of a subject. However, dark field illumination must be presented close to a subject to attain the low incidence angle thereto. Conversely, bright field illumination is better produced at a relative distance to ensure full area illumination. 
     In the situation where a barcode or other subject is formed on a specular surface, non-directional totally diffuse bright field illumination is preferred. This type of illumination, like that naturally produced on a cloudy day must surround the object upon which the barcode is placed. In typical machine vision applications, non-directional totally diffuse illumination is difficult and costly to provide, usually by way of surrounding a workpiece with a light tent. 
     BRIEF SUMMARY OF THE INVENTION 
     In one aspect the present invention provides diffuse illumination in a hand held reader to produce high contrast images of symbols and barcodes. A tapered translucent diffuser is disposed between the imager and the region of interest with illumination behind the diffuser directed toward the region of interest. The tapered diffuser converts bright field illumination into totally diffused non-directional illumination. The narrow end of the tapered diffuser surrounds the lens at a position near to the lens of the reader. The wide end of the tapered diffuser surrounds the field of view near the region of interest. Accordingly, the field of view is flooded with diffuse illumination. 
     In another aspect of the invention, diffuse illumination is produced in addition to low angle dark field illumination. In this aspect of the invention, a passive light pipe that produces low angle dark field illumination is positioned around the diffuser. Alternatively, an active light pipe surrounds the diffuser, with dark field illuminators directly projecting illumination at a low angle with respect to the region of interest. The reader can be configured to selectively actuate either the diffuse illumination from the tapered diffuser, or exclusive dark field illumination from the dark field light pipes, or a combination of both diffuse illumination and dark field illumination. 
     In yet another aspect of the invention, in addition to the tapered diffuser that surrounds the lens at the narrow end of the diffuser, diffused illumination directed to a semi-transparent mirror positioned in front of the lens and aligned so that a portion of the light reflecting from the region of interest is projected into the lens, while a portion of the diffuse illumination is reflected onto the region of interest. In this aspect of the invention, uniform diffuse illumination can be projected onto the region of interest without producing a reflection of the lens in the image. 
     Other objects and advantages of the present invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The present invention is further described in the detailed description which follows, by reference to the noted drawings by way of non-limiting illustrative embodiments, in which like reference numerals represent similar part throughout the several views of the drawings, and wherein: 
         FIG. 1  is a perspective view of a handheld scanning system and an object having a symbol thereon; 
         FIG. 2  is a schematic cross sectional view of an image formation system according to the prior art; 
         FIG. 3  is a perspective view of a representative object having a two-dimensional symbol marked thereon, according to the background art; 
         FIG. 4  is an image of a two dimensional symbol on a specular cylindrical object acquired using an image formation system according to the prior art; 
         FIG. 5  is a schematic cross sectional view of an image formation system according an illustrative embodiment of the present invention; 
         FIG. 6  is a perspective view of an illustrative embodiment of an image formation system according to the present invention; 
         FIG. 7  is an image of a two dimensional symbol on a specular cylindrical object acquired using an image formation system according to the present invention; 
         FIG. 8  is a schematic cross section of an illustrative embodiment of the present invention; 
         FIG. 9  is a schematic cross section of an illustrative embodiment of the present invention; 
         FIG. 10  is a schematic cross section of an illustrative embodiment of the present invention; 
         FIG. 11  is an exploded perspective view of an image formation system according to an illustrative embodiment of the present invention; 
         FIG. 12  is a schematic cross section of an illustrative embodiment of the present invention; 
         FIG. 13  is a schematic cross section of an illustrative embodiment of the present invention; 
         FIG. 14  is a perspective view of an illustrative embodiment of the present invention with the diffuser according to the invention adapted with a cylindrical shape; and 
         FIG. 15  is a perspective view of an illustrative embodiment of the present invention with selective quadrant diffuse illumination. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a scanning system  101  adapted for handheld operation. An exemplary handheld scanning appliance or handpiece  102  is provided. It includes a grip section  104  and a body section  106 . An image formation system  151 , shown in phantom and described herein, can be controlled and can direct image data to an onboard embedded processor  109 . This processor can include a scanning software application  113  by which lighting is controlled, images are acquired and image data is interpreted into usable information (for example, alphanumeric strings derived from the barcode images). The decoded information can be directed via a cable  111  to a PC or other data storage device  112  having (for example) a display  114 , keyboard  116  and mouse  118 , where it can be stored and further manipulated using an appropriate application  121 . Alternatively, the cable  111  can be directly connected to an interface in the scanning appliance and an appropriate interface in the computer  112 . In this case the computer-based application  121  performs various image interpretation and lighting control functions as needed. The precise arrangement of the handheld scanning appliance with respect to an embedded processor, computer or other processor is highly variable. For example, a wireless interconnect can be provided in which no cable  111  is present. Likewise, the depicted microcomputer can be substituted with another processing device, including an onboard processor or a miniaturized processing unit such as a personal digital assistant or other small-scale computing device. 
     The scanning application  113  can be adapted to respond to inputs from the scanning appliance  102 . For example, when the operator toggles a trigger  122  on the hand held scanning appliance  102 , an internal camera image sensor ( 110 , shown and described further below) acquires an image of a region of interest  131  on an object  105 . The exemplary region of interest includes a two-dimensional symbol  195  that can be used to identify the object  105 . Identification and other processing functions are carried out by the scanning application  113 , based upon image data transmitted from the hand held scanning appliance  102  to the processor  109 . A visual indicator  141  can be illuminated by signals from the processor  109  to indicate a successful read and decode of the symbol  195 . 
       FIG. 2  depicts an image formation system  151  typically used in a hand held scanning appliance  102 .  FIG. 2  shows a cross sectional view of a prior art imaging system  100  for reading marks or symbols on an object  105 . The imaging system  100  has an imager  110  that is one of a variety of types of photosensitive imaging sensors, such as a CMOS or CCD device. A lens  130  disposed in front of the imager  110  is configured to direct illumination onto the imager  110 . The imager  110  and lens  130  together form an optical axis  140  that defines the center of a field of view, which is bound by the polyhedral volume defined as lines  150  in  FIG. 2 . 
     At a given focus setting, a sharply focused image of the surface of an object  105  will be projected by the lens  130  onto the imager  110  when the object is positioned at an appropriate distance from the imaging system  100 . Optical properties of the system will affect the range of distance from which the object  105  can be positioned in front of the system  100  while projecting an image of the object  105  onto the imager  110  that exhibits features of sufficient sharpness and contrast so as to be perceived as a focused image. The range upon which a focused image can be formed on the imager  110  is a called the depth of field  125 . 
     Illuminators  120  are shown in the system  100  for directing illumination towards the object  105 . Typically, a diffuser  180  is disposed in front of the illuminators  120  to diffuse the illumination so that a uniform distribution of illumination can be spread over the surface of the object. Without the diffuser  180 , the illumination from the illuminators  120  will appear as areas of high intensity illumination surrounded by areas of low intensity illumination. As shown in  FIG. 2 , the field of view of an image of a cylindrical object  105  is the portion of the surface of the object from point B  135  to point A  145 . 
     Although the entire field of view is illuminated, only a portion of the reflective surface (from point P  175  to point Q  165 ) will reflect light into the imaging system  100 . This results from the maximum angle of illumination, depicted as lines  155  in  FIG. 2 , that can be reflected into the imaging system  100 , depicted as lines  160  in  FIG. 2 , being significantly less than the field of view. The image of a cylindrical object using the prior art imaging system  100  will depict a brightly illuminated band over the area defined by lines through points P and Q. 
       FIG. 3  depicts a two-dimensional data matrix symbol  195  etched on the surface of a cylindrical object, e.g., a drill bit  190 . Presenting the exemplary drill bit  190  to the imaging system  100  will produce an image of the symbol  195  as shown in  FIG. 4 . A brightly illuminated band can be observed between the regions defined as lines P  175  and Q  165 . The illumination that strikes the remaining portion of the object does not reflect into the optics of the imaging system  100 , and therefore, the image is too dark to produce a clear representation of the data matrix symbol so that decoding algorithms can be applied to decode the symbol. 
       FIG. 5  depicts an improved imaging system  200  according to the present invention that can be used as an image formation system  151  in a hand held scanning appliance  102 . The improved system  200  has an imager  110  that is one of a variety of types of photosensitive imaging sensors, such as a CMOS or CCD device. A lens  130  disposed in front of the imager  110  is configured to direct illumination onto the imager  110 . The imager  110  and lens  130  together form an optical axis  140  that defines the center of a field of view, which is bound by the polyhedral area defined as lines  150  in  FIG. 4   
     At a given focus setting, a sharply focused image of the surface of the object  105  will be projected by the lens  130  onto the imager  110  when the object is positioned at an appropriate distance from the improved system  200 . Optical properties of the system will affect the range of distance from which the object  105  can be positioned in front of the improved system  200  while projecting an image of the object  105  onto the imager  110  that exhibits features of sufficient sharpness and contrast so as to be perceived as a focused image. The range upon which a focused image can be formed on the imager  110  is a called the depth of field  125 . 
     Illuminators  120  are shown in the improved system  200  for directing illumination towards the object  105 . Optionally, a diffuser  180  can be used to diffuse the bright field illumination. In an illustrative embodiment of the present invention, a conical diffuser  300  is disposed between the lens  130  and the object  105 . One skilled in the art will appreciate that while the illustrative embodiment describes a conical shaped diffuser  300 , other embodiments can be employed with alternative shapes without departing from the scope of the invention. For example, parabolic, cylindrical, and hemispherical shaped diffusers can be used as a diffuser  300 . 
       FIG. 6  depicts a perspective view of the basic elements of the illustrative embodiment of the present invention. The conical diffuser  300  is constructed from an translucent plastic material, such as polymethyl methacrylate (PMMA) with a roughened surface to achieve the optical property of translucency. In an embodiment of the invention, the optical properties of the diffuser  300  has translucency like that of ordinary white paper, in that illumination will cause the surface of the diffuser to appear bright. The conical diffuser  300  can be molded with a rough mold surface, or sandblasted to roughen the normally clear material to be translucent. The diffuser  300  is configured so that a first distal end, the narrow opening, of the diffuser surrounds the opening of the lens  130  and aligned with the imager  110  (shown in phantom). The illuminators  120  are distributed around the lens  130  so that an even distribution of illumination can be projected. 
     Referring back to  FIG. 5 , the entire field of view is illuminated with diffuse illumination since the second distal end of the diffuser  300  surrounds the field of view near the object  105 . The maximum angle of illumination from the diffuser  300 , shown as line  310  strikes the object  105  within the region defined as the field of view between point A  145  and point B  135 , reflecting toward the lens  130  and the imager  110  as lines  150 . Illumination from the diffuser that strikes the object outside of that region is outside the field of view, and therefore, not captured by the imager  110 . 
     Presenting the exemplary drill bit  190  ( FIG. 3 ) to the improved imaging system  200 , an image of the data matrix symbol  195  will be formed like that shown in  FIG. 7 . The entire data matrix symbol  195  is visible, since the region defined as the field of view between point A  145  and point B  135  receive diffuse illumination, and therefore, decoding algorithms operating on the image can readily extract the encoded information. 
       FIG. 8  shows another implementation of the diffuser  300  with a passive light pipe  800 , shown in cross section relative to the lens  130 , to provide diffuse illumination in combination with, or in the alternative to dark field illumination. Dark field illumination  810  is directed into the light pipe  800  that is internally reflected at a chamfered portion  815  to be directed at the object  105  at a low angle. Further information regarding the design and implementation of passive light pipes with selectively actuated illumination to provide dark field illumination can be found in commonly assigned U.S. patent application Ser. No. 10/693,626, incorporated herein by reference. Bright field illumination  820  is converted into totally diffuse illumination by the diffuser  300 , to project diffuse illumination on the object  105  within the field of view, depicted as the region defined by lines  150 . Optionally, the diffuser  300  can be removably attached to the hand held scanning appliance  102  so that a user can operate the hand held scanning appliance using non-diffuse bright field illumination. The removability of the diffuser  300  can be achieved by incorporating snap-fit clearances and/or features in the diffuser and light pipe that permit removable assembly (not shown). 
     When dark field illumination is exclusively applied to the object  105  in the embodiment of the invention according to  FIG. 8 , scattered illumination may reflect from the object  105  and then from the diffuser  300  to unintentionally cause diffuse illumination on the object  105 . This condition may be undesirable for reading certain parts that are best imaged with low angle dark field illumination, such as peened data matrix marks on a highly specular surface. 
       FIG. 9  shows an alternate embodiment of the implementation of the diffuser  300  with a passive light pipe  800 , shown in cross section relative to the lens  130 , to provide selectively exclusive dark field illumination or diffuse illumination. In this embodiment, a color specific dark field illumination  835  is projected into the light pipe  800 . The diffuser  300  is fabricated with a material  840  having a color characteristic different from the color of the color specific dark field illumination  835 , and illuminated with color specific bright field illumination  825  that matches the color characteristic of the material  840 . In this embodiment, when the color specific dark field illumination is exclusively actuated, the low angle dark field illumination is projected onto the object  105 , and any scattered illumination reflecting from the object  105  and then the diffuser  300  will not be converted into totally diffuse illumination. 
     In an illustrative embodiment according to  FIG. 9 , the color specific dark field illumination  835  is red, and the color specific bright field illumination is blue, with the material  840  having a blue color. In this embodiment, if only low angle dark field illumination is desired, the red dark field illumination  835  is turned on, with the blue bright field illumination turned off. The low angle red illumination striking the object, if scattered to reflect off the blue colored material  840  of the diffuser  300 , will not appear bright in an image, and therefore not contribute to the illumination of the object with totally diffuse illumination. Conversely, if the blue bright field illumination  825  is turned on, the blue colored material  840  of the diffuser will transmit and diffuse the illumination to provide totally diffuse illumination of the object  105  within the field of view, depicted as the region defined by lines  150 . One skilled in the art will appreciate that alternative color characteristics, such as red/infrared can be employed according to the illustrative embodiment of  FIG. 9 . 
       FIG. 10  shows another illustrative embodiment of the implementation of the diffuser  300  with a passive light pipe  800 , shown in cross section relative to the lens  130 , to provide selectively exclusive dark field illumination or diffuse illumination. In this illustrative embodiment, dark field illumination  810  is projected into the light pipe  800  to produce low angle dark field illumination while the bright field illumination  820  is turned off. A light absorbing foil  860 , such as a neutral density filter material is applied to the interior surface of the diffuser  300 . Low angle dark field illumination that reflects off the object  105  is absorbed by the foil filter  860  so that totally diffuse illumination is not reflected back onto the object. If diffuse illumination is desired, either instead of dark field illumination, or in combination with dark field illumination, the bright field illumination  820  is turned on. The intensity of the bright field illumination  820  would need to be increased to compensate for the absorption of such illumination by the foil filter  820 . With the bright field illumination actuated, totally diffuse illumination will transmit from the diffuser  300 , through the foil filter  860 , and onto the object within the field of view, depicted as the region defined by lines  150 . 
       FIG. 11  depicts another illustrative embodiment of the implementation of the diffuser  300  as an image formation system  151  in a hand held scanning appliance  102 . In the illustrative embodiment according to  FIG. 11 , the diffuser  300  is inserted into an active light pipe  870 . The active light pipe  870  provides low angle dark field illumination from a ring of dark field illuminators  875 , as disclosed in the commonly assigned International Patent Application WO02/075637, incorporated herein by reference. The dark field illuminators  875  are held in place with a threaded collar  880 , and flexible sleeve  890 . Bright field illuminators  120  project bright field illumination that is diffused by an optional diffuser  180 , that is diffused over the entire field of view by the diffuser  300 . A transparent window  895  protects the lens  130  and other internal components from the operating environment. 
     In the illustrative embodiment according to  FIG. 11 , the conical diffuser  300  can be positioned to diffuse the dark field illumination from the dark field illuminators  875 , or optionally constructed to permit illumination from the dark field illuminators  875  to project onto the object at low angle without being diffused. This optional configuration can be achieved by inserting the conical diffuser  300  further into the active light pipe  870  toward the lens  130  to expose the dark field illuminators  875 , or by constructing the conical diffuser  300  to have a transparent optical property at the distal end where it is disposed in front of the dark field illuminators  875 . When the hand held scanning appliance having an image formation system  151  according to the illustrative embodiment according to  FIG. 11  is operated, the user can configure or select exclusive dark field illumination or exclusive diffuse illumination, or a combination of both modes of illumination thereof. 
     In the illustrative embodiment according to  FIG. 11 , the conical diffuser  300  can be optionally removed from the image formation system  151  so that the hand held scanning appliance  102  can be operated without diffuse illumination. This condition may be desired if high intensity non-diffuse bright field illumination, or if exclusive dark field illumination is required. The removable aspect of the diffuser  300  can be achieved by unscrewing the threaded collar  880  to remove the diffuser  300 , and re-threading the collar  880  without the diffuser in place. 
       FIG. 12  shows a schematic representation of the effect of illuminating an object  105  having a highly specular surface  910  according to the present invention. The resulting image  920  will exhibit features  925  associated with a reflection of the lens  130 . Though the field of view  930 , as defined as the region bound by lines  150 , will be illuminated with totally diffuse illumination from the diffuser  300 , the image  920  will exhibit a darkened region of features  925  that is a reflection of light not emanating from the diffuser  300 . 
       FIG. 13  shows an illustrative embodiment of the invention that further improves the uniform distribution of diffuse illumination over the field of view according to the present invention. Bright field illumination  820  is converted into totally diffuse illumination by the diffuser  300 , to project diffuse illumination on the object  105  within the field of view, depicted as the region defined by lines  150 . A semi-reflecting mirror  950  is disposed in front of the lens  130 , and behind the diffuser at an appropriate angle so that the image of the field of view is projected into the lens  130 . A second bright field illumination source  935  is directed toward the semi-reflecting mirror  950 , with a second diffuser  940  disposed between the second illumination source  935  and the semi-reflecting mirror  950 . The second diffuser  940  is composed of the same material as the diffuser  300 , with the same translucent optical properties. One skilled in the art will appreciate that a single diffuser can be fabricated or molded that can provide the functionality of both the diffuser  300  and the second diffuser  940 . The intensity of the second illumination source  935  may require an increased level during operation since a portion of the illumination  935  will not be reflected onto the object  105 , but rather through the semi-reflecting mirror into the body of the hand held scanning appliance  102 . By adjusting the intensity level of the second illumination  935 , an even distribution of diffuse illumination can be achieved over the entire field of view  930 . 
       FIG. 14  shows an illustrative embodiment of the invention that provides a cylindrical shape  990  in the diffuser  300  so that an object  105  having an extended length can be presented to the improved imaging system  200 . The cylindrical shape  990  of the diffuser provides relief so that the diffuser  300  can increase an area of diffuse illumination pattern on a reflective cylindrical object. Further, the cylindrical shape  990  provides an aiming assistance to the user. 
       FIG. 15  shows a plurality of selectively actuated illuminators  985  that can provide selective illumination of the diffuser  300 . In this illustrative embodiment, the North illuminator  996  can be actuated with the South illuminator  998  while the East illuminator  994  and the West illuminator  992  are not actuated. Conversely, the East illuminator  994  and the West illuminator  992  can be actuated while the North illuminator  996  and the South illuminator  998  are not actuated. Selective quadrant illumination according to this illustrative embodiment permits an adjustment, or compensation, of the uniformity of the illumination that is converted into totally diffuse illumination by the diffuser  300 , on an axis parallel with the axis of a specular object  105 . 
     While the invention has been described with reference to certain illustrated embodiments, the words which have been used herein are words of description rather than words of limitation. Changes may be made, within the purview of the appended claims, without departing from the scope and sprit of the invention and its aspects. Although the invention has been described herein with reference to particular structures, acts and materials, the invention is not to be limited to the particulars disclosed, but rather extends to all equivalent structures, acts, and materials, such as are within the scope of the appended claims.