Patent Publication Number: US-2010116889-A1

Title: Imaging reader with efficient laser illumination

Description:
DESCRIPTION OF THE RELATED ART 
     Solid-state imaging systems or imaging readers, as well as moving laser beam readers or laser scanners, have both been used to electro-optically read targets, such as one-dimensional bar code symbols, particularly of the Universal Product Code (UPC) type, each having a row of bars and spaces spaced apart along one direction, as well as two-dimensional symbols, such as Code 49, which introduced the concept of vertically stacking a plurality of rows of bar and space patterns in a single symbol, as described in U.S. Pat. No. 4,794,239. Another two-dimensional code structure for increasing the amount of data that can be represented or stored on a given amount of surface area is known as PDF417 and is described in U.S. Pat. No. 5,304,786. 
     The imaging reader includes an imaging module having a solid-state imager with a sensor array of cells or photosensors, which correspond to image elements or pixels in a field of view of the imager, and an imaging lens assembly for capturing return light scattered and/or reflected from the symbol being imaged in a range of working distances from the imager, and for projecting the return light onto the sensor array to initiate capture of an image of the symbol. Such an imager may include a one- or two-dimensional charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device and associated circuits for producing and processing electronic signals corresponding to a one- or two-dimensional array of pixel information over the field of view. 
     It is therefore known to use the imager for capturing a monochrome image of the symbol as, for example, disclosed in U.S. Pat. No. 5,703,349. It is also known to use the imager with multiple buried channels for capturing a full color image of the symbol as, for example, disclosed in U.S. Pat. No. 4,613,895. It is common to provide a two-dimensional CCD with a 640×480 resolution commonly found in VGA monitors, although other resolution sizes are possible. 
     In order to increase the amount of the return light captured by the imager, especially in dimly lit environments and/or at long working distance range reading, the imaging module generally also includes an illuminating light assembly having one or more light emitting diodes (LEDs) for illuminating the symbol with illumination light for reflection and scattering therefrom. In many applications, it is desirable to increase the efficiency of the illumination light and to increase the range of working distances by replacing the LEDs with a laser source operative for emitting a laser beam that is more intense and brighter than LED light. Light of greater intensity will increase the working distance range, because there will be correspondingly more return light for the imager to detect from symbols that are further away from the imaging reader. Also, the laser beam is diffraction limited and, hence, can be directed more efficiently toward the symbol, as compared to light originating from other, non-diffraction limited light sources, such as LEDs. 
     However, the use of the laser source introduces the inherent problem of speckle noise, which can cause considerable degradation in image quality. A monochromatic (red, blue or green) laser emits a laser beam having coherent waves of the same frequency and also having spatial coherence, that is, the waves have a fixed phase relationship with one another both in space and in time. When the laser beam is incident on a target symbol, the waves are scattered by being reflected from the symbol. The scattered waves have random phase delays and propagate along different directions, but all have the same frequency. When such scattered waves meet, for example, at the imager, they produce a static distribution of constructive and destructive interference, i.e., an interference pattern, also known as speckle noise. The imager sees the speckle noise as a degraded image. Reading performance is thus corrupted. 
     SUMMARY OF THE INVENTION 
     One feature of the present invention resides, briefly stated, in an imaging reader for, and a method of, electro-optically reading a symbol by image capture. The reader includes a housing, an illuminating assembly supported by the housing and including a laser for directing an illuminating laser beam along a path to the symbol to illuminate the symbol during image capture, a diffusing assembly supported by the housing and including a movable diffuser in the path of the illuminating laser beam to diffuse the illuminating laser beam as diffused illuminating laser light, and a solid-state imager, such as a CCD or a CMOS, supported by the housing and including an array of image sensors for capturing the diffused illuminating laser light returned from the symbol in a range of working distances over a field of view. 
     In the preferred embodiment, the array is one-dimensional, i.e., linear, or is two-dimensional with an anamorphic field of view. The housing has a handle for handheld operation and also has a light-transmissive window through which the diffused illuminating laser light passes in one direction, and through which the returned captured light passes in an opposite direction. 
     Advantageously, the diffuser is a light-transmissive element having a textured or diffractive surface, or is integrated with scattering particles, for scattering the illuminating laser beam. A drive, preferably a motor, is operative for moving the diffuser, preferably by rotating the diffuser about the path. 
     In accordance with this invention, the efficiency of the illuminating light and the range of working distances for the imaging reader has been increased due to the use of the illuminating laser whose bright, intense, diffraction limited light, as compared to LED light, enables more return light to be detected by the imager. Yet, the inherent problem of speckle noise introduced by the illuminating laser is minimized by the moving diffuser, which changes the phase relationship of the illuminating laser beam, and causes averaging of different speckle patterns on the imager. 
     The method of electro-optically reading a symbol by image capture is performed by illuminating the symbol by directing an illuminating laser beam along a path to the symbol during image capture, diffusing the illuminating laser beam by moving a diffuser in the path of the illuminating laser beam to produce diffused illuminating laser light, and capturing the diffused illuminating laser light returned from the symbol in a range of working distances over a field of view. 
     The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a portable imaging reader operative in either a handheld mode, or a hands-free mode, for capturing return light from target symbols; and 
         FIG. 2  is a schematic diagram of various components of the reader of  FIG. 1  in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference numeral  30  in  FIG. 1  generally identifies an imaging reader having a generally vertical window  26  and a gun-shaped housing  28  supported by a base  32  for supporting the imaging reader  30  on a countertop. The imaging reader  30  can thus be used in a hands-free mode as a stationary workstation in which products are slid, swiped past, or presented to, the vertical window  26 , or can be picked up off the countertop and held in an operator&#39;s hand and used in a handheld mode in which a trigger  34  is manually depressed to initiate imaging of indicia, especially one-dimensional symbols, to be read at far distances from the window  26 . In another variation, the base  32  can be omitted, and housings of other configurations can be employed. A cable, as illustrated in  FIG. 1 , connected to the base  32  can also be omitted, in which case, the reader  30  communicates with a remote host by a wireless link, and the reader is electrically powered by an on-board battery. 
     As schematically shown in  FIG. 2 , an imager  24  is mounted on a printed circuit board  22  in the reader. The imager  24  is a solid-state device, for example, a CCD or a CMOS imager having a one-dimensional array of addressable image sensors or pixels arranged in a single, linear row, or a two-dimensional array of such sensors arranged in mutually orthogonal rows and columns, preferably with an anamorphic field of view, and operative for detecting return light captured by an imaging lens assembly  20  along an optical path or axis  46  through the window  26 . The return light is scattered and/or reflected from a target or symbol  38  over the field of view. The imaging lens assembly  20  is operative for adjustably focusing the return light onto the array of image sensors to enable the symbol  38  to be read. The symbol  38  is located anywhere in a working range of distances between a close-in working distance (WD 1 ) and a far-out working distance (WD 2 ). In a preferred embodiment, WD 1  is about four to six inches from the imager array  24 , and WD 2  can be many feet from the window  26 , for example, around fifty feet away. 
     An illuminating assembly is also mounted in the imaging reader and preferably includes an illuminator or illuminating light source  12 , e.g., a laser, and an illuminating lens assembly  10  to uniformly illuminate the symbol  38  with an illuminating laser beam. 
     An aiming assembly is also mounted in the imaging reader and preferably includes an aiming light source  18 , e.g., an LED or a laser, and an aiming lens assembly  16  for generating an aiming light pattern or mark on the symbol  38 . 
     As shown in  FIG. 2 , the imager  24 , the illuminating light source  12  and the aiming light source  18  are operatively connected to a controller or microprocessor  36  operative for controlling the operation of these components. A memory  14  is connected and accessible to the controller  36 . Preferably, the microprocessor is the same as the one used for processing the return light from target symbols and for decoding the captured target images. 
     In operation, the microprocessor  36  sends a command signal to energize the aiming light source  18  prior to reading, and also pulses the illuminating laser  12  for a short exposure time period, say 500 microseconds or less, and energizes and exposes the imager  24  to collect light, e.g., illumination laser light and/or ambient light, from a target symbol only during said exposure time period. A typical array needs about 16 to 33 milliseconds to acquire the entire target image and operates at a frame rate of about 30 to 60 frames per second. 
     One aspect of the present invention resides in providing a diffusing assembly in the housing  28 . The diffusing assembly includes a movable diffuser  40  positioned in the path of the illuminating laser beam to diffuse the illuminating laser beam as diffused illuminating laser light, and a drive  42  operatively connected to, and controlled by, the controller  36  for moving the diffuser  40 . The imager captures the diffused illuminating laser light returning from the symbol  38  in an extended range of working distances WD 1 -WD 2  over the field of view. 
     Advantageously, the diffuser  40  is a light-transmissive, translucent element having a textured or diffractive surface, or is integrated with scattering particles, for scattering the illuminating laser beam. The drive  42  is preferably an electric motor operative for moving the diffuser  40 , preferably by rotating the diffuser  40  about the path, to randomly scatter the illuminating laser beam. The drive  42  could also move the diffuser  40  transversely of the path. 
     In accordance with this invention, the efficiency of the illuminating light and the range of working distances for the imaging reader  30  has been increased due to the use of the laser  12  whose bright, intense, diffraction limited light, as compared to LED light, enables more return light to be detected by the imager  24 . Yet, the inherent problem of speckle noise introduced by the laser  12  is minimized by the moving diffuser  40 , which changes the phase relationship of the illuminating laser beam, and causes averaging of different speckle patterns on the imager  24 . 
     As previously noted, a laser can also be used as the aiming light source  18 . The aiming lens assembly  16  for generating the aiming light pattern can include all optical element such as a hologram to create a specific aiming pattern. This optical element could, in accordance with another aspect of this invention, be configured as a movable plate having one section that contains the hologram, and another section that contains the diffuser. When the images captures light from the symbol, the plate begins to move and intercepts the laser beam with the section that contains the diffuser, thus creating the diffuse illumination needed. Alternatively, the beam can be switched in some other way between the hologram and the diffuser. 
     It will be understood that each of the elements described above, or two or more together, also may find a useful application in other types of constructions differing from the types described above. 
     While the invention has been illustrated and described as a reader for, and a method of, reading a symbol to be read by image capture with efficient laser illumination, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. 
     Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims. 
     What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.