Patent Publication Number: US-5291008-A

Title: Optical assembly and apparatus employing same using an aspherical lens and an aperture stop

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an optical assembly that employs a single element lens to view an indicia target with an image sensor. More particularly it relates to an optical assembly in a scanning instrument that minimizes field curvature of indicia being read with a single element lens, and projects an image of the indicia onto an image sensor. 
     Most modern optical scanners and bar code readers employ optical systems to direct reflected light from the indicia being read to an image sensor. Today multi-element CCD arrays are commonly used as the image sensor, and, with such array, a bar code symbol can be read while maintaining the reader in a stationary position, or by moving the reader with respect to the indicia. 
     In such scanners it is desirable that the operational depth of field be maximized while giving appropriate attention to the optical efficiency of the lens imaging system. In an optical system attention should be given to many of the following issues; optical efficiency; well known lens faults such as spherical and chromatic aberration; minimization of image field curvature, such that a relatively wide symbol can be adequately viewed; and the well known cosine to the fourth power illumination falloff phenomenon. Dealing with these issues under the constraints of adequate depth of field and maintaining economy in materials and manufacturing imposes severe burdens on those striving to advance the bar code reading art. 
     In the prior art, correction of lens aberrations has been achieved by multi element lens systems. Representative of this approach is U.S. Pat. No. 4,766,300 to Chadima, Jr. et al, in which a bar code reader optical assembly employs five lenses constructed of polystyrene and acrylic lens material. This system further employs mirrors to establish a long, filed optical path in order to achieve suitable depth of field and image resolution for a bar code application. 
     In such systems, the optics are mechanically complex and expensive to fabricate and cling. 
     SUMMARY OF THE INVENTION 
     It is therefore a primary object of the present invention to provide an improved, economical optical assembly that achieves a minimum image field curvature over the indicia being scanned. 
     It is another object o the present invention to provide an optical assembly that provides uniformly distrusted light on an image sensor. 
     It is yet another object of the present invention to reduce the complexity of optical systems in optical scanners and bar code readers. 
     It is a further object of the present invention to enable optical scanners and bar code readers to operate with a maximum optical efficiency for a given operational depth of field. 
     These and other objects of the present invention are attained by an assembly that can be adapted to small, portable scanning instruments such as bar code readers. A single element lens, preferably an aspherical lens made of a plastic material is combined with an aperture stop. The combination is capable of reducing lens aberrations and minimizing image field curvature for an object plane that contains indicia to be scanned or read. A suitably demagnified image of the indicia is focused onto an image sensor such as a CCD array. The spatial light distribution function in the image plane can be equalized by an attenuator, such as a neutral density filter of a slit, suitably configured to compensate for the reduced optical efficiency at large field angles. 
     In another aspect of the invention, the lens may be impregnated with colored dye, or filers otherwise interposed in the optical path, so that unwanted light wavelengths are rejected. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of these and other objects of the present invention, reference is made to the detailed description of the invention which is to be read in conjunction with the following drawings, wherein: 
     FIG. 1 is a partially schematic plan view of an embodiment of the invention; 
     FIG. 2 is a partially schematic side elevation of the embodiment of FIG. 1; 
     FIG. 3 is a partially schematic plan view of a first alternate embodiment of the invention; 
     FIG. 4 is a partially schematic plan view of a second alternate embodiment of the invention; and 
     FIG. 5 is a cutaway frontal view of a portion of the embodiment of FIG. 4. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the drawings, there is shown in FIGS. 1 and 2 generally at 10 an optical assembly that is an embodiment of the invention. The invention is preferably used as a part of a CCD bar code scanner that includes conventional digitizing circuitry, producing a signal that can be displayed or further processed. It is enclosed in a housing (not shown), and the CCD output is coupled to the bar code scanner signal processing circuitry. 
     A target 18, having indicia, such as a bar code symbol, is disposed in a region of space, limited by lines 17, 19, that corresponds to the working depth of field of a bar code scanner. Light energy that is incipient of the target and then reflected is exemplified by rays 22, 23, 24. Ray 23 is aligned with the optical axis. 
     A single element landscape lens 12, disposed between image sensor 14, and target 18 accepts the reflected light rays 22, 23, 24, and forms an image of target 18 on image sensor 14. The lens is preferably composed of methyl methacrylate, available form Rohm and Haas under the designation V825-366; however glass or many other optical plastic materials are suitable. It is an important aspect of the invention that lens 12 have minimum image field curvature, so that all regions of target 18 are in focus on the image sensor. Minimum image field curvature can be achieved using well known lens design techniques, and need not be further discussed herein. Lens 12 is typically a single element, aspherical converging lens, and has a magnification of about 2.73 at the center of the depth of field. Magnifications in the range of 2.05-3.70 have been found suitable in various applications. In the preferred embodiment image sensor 14, lens 12, and target 18 are disposed such that the region of sensitivity corresponding to a pixel of image sensor 14 is approximately 0.038×0.038 mm in the plane of the bar code symbol. The lens is configured such that the total field width in the mid-plane of the working depth of field is 78.23 mm. 
     Many variants in lens construction are possible. In some applications it may be desirable to include a photochromic material in the lens, or operatively associate the lens with the photochromic material so that the optical transmissivity varies in accordance with the intensity of light passing therethrough. This may prevent saturation or overdriving of the image sensor 14 under conditions of high light intensity. In other applications the lens can be impregnated with colored substances such as dyes, so that its optical transmissivity is limited to a desired range of light wavelengths. In other applications an optical filter may be included so that the system optical transmissivity is restricted to a desired range of light wavelengths. Those skilled in the art will appreciate that optical fibers, Fresnel type lenses, and even holographic imaging devices can e substituted for the single element lens described above, with appropriate field correction. 
     In the preferred embodiment sensor 14 is a conventional multi element CCD array. Line CCDs having 1×2040 elements measuring approximately 12×12 microns are suitable. It is possible to use other kinds of image sensors, such as array CCDs, light sensitive film, or an orthocon imaging device. While image sensor 14 as shown is integral with the lens system, it is possible to focus an image of the target on an optical interface, such as a fiber optic bundle, which is coupled to a remote sensor. 
     Stop 25 is provided between lens 12 and image sensor 14 in order to reduce lens aberrations to an acceptable level. The stop might also be located between lens 12 and target 18. Using modern lens design programs that are known to the art, it is possible to tailor the size and position of stop 25, as well as the lens parameters, in order to obtain uniform sharpness over the entire field of view and depth of field. 
     It can be appreciated from inspection of FIG. 1 that light 24 originating from the bottom portion of the target 18 and reaching the top portion of the image sensor 14 is constrained to the bottom portion of lens 12. A similar constraint applies to the light 22, originating from a different part of the target. Different regions of the lens are utilized for transmitting the beams 22, 24 therethrough. 
     In the preferred embodiment, the following physical dimensions have been found to be satisfactory: lens 12 has the characteristics as shown in Tables 1 and 2. 
     
                       TABLE 1                                                     
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Surface   Radius of Curvature                                             
                        Center Thickness                                  
______________________________________                                    
A         76.200 mm. Concave                                              
                        4.166 mm.                                         
B         12.060 mm. Convex                                               
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     Surface B is aspheric, with a conic constant of -0.370, utilizing standard aspheric equations known to the optical art. 
     
                       TABLE 2                                                     
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Radius (mm)    Sag (mm)                                                   
______________________________________                                    
0.0            0.0                                                        
0.5            0.1037                                                     
1.0            0.04150                                                    
1.5            0.09351                                                    
2.0            0.16656                                                    
2.5            0.26090                                                    
3.0            0.37684                                                    
3.5            0.51480                                                    
4.0            0.67526                                                    
4.5            0.85822                                                    
5.0            1.06618                                                    
5.5            1.29816                                                    
6.0            1.55576                                                    
6.5            1.84010                                                    
7.0            2.15253                                                    
7.5            2.49464                                                    
8.0            2.86829                                                    
8.5            3.27571                                                    
______________________________________                                    
 
    
     The stop 25 is positioned 10.00 mm in front of image sensor 14, and has an aperture of 2.22 mm. Lens 12 is disposed 25.25 mm in front of image senor 14, and the distance between target 18 and image sensor 14 is 151.7 m. The resulting depth of field for scanning bar code symbol elements having a minimum lateral dimension of 0.127 mm is 12.7 mm. over a field width of 74.9 mm. Other sizings of the optical elements can be used, depending upon system needs. 
     Referring now to FIG. 3, there is shown a first alternate embodiment of the invention, in which a cold mirror 28 is interposed in the light path intermediate lens 12 and target 18. This mirror acts as a filter that reflects light of a desired subrange of wavelengths and transmits unwanted light. The provision of mirror 28 is useful for operation when ambient light level is high. Other kinds of light filers can be substituted for mirror 28 in order to limit the spectral width of light that received by light sensor 14. 
     In FIGS. 4 and 5 there is depicted a second alternate embodiment of the invention. Light attenuator 29 is disposed in the optical axis intermediate lens 12 and target 18. The purpose of attenuator 29 is to equalize the amount of light energy being reflected onto image sensor 14 from target 18 through lens 12. In the preferred embodiment attenuator 29 has a longitudinal dimension 31 and a cross-longitudinal dimension 32. A slit formed therein lies in a plane that is substantially orthogonal to the optical axis 23. To compensate for fall-off of light intensity at larger field angles, the cross-longitudinal dimension of the slit varies along the a longitudinal dimension, and increases from the optical axis in a radial direction. The slit may be configured to compensate for the consine to the fourth power falloff effect referenced above, or for other predicted or measured reductions in angular light intensity. 
     The attenuator can also be a neutral density filter that is disposed in the optical path, preferably in a plane that is orthogonal to the optical axis, and which has a graded transmissivity that increases from the optical axis in a radial direction. The transmissivity gradient can be tailored to compensate for measured or predicted falloff in angular light intensity. 
     While this invention has been explained with reference to the structure disclosed herein, it is not confined to the details set forth and this application is intended to cover any modifications and changes as may come within the scope of the following claims: