Abstract:
An imaging system (FIG.  3 ) is disclosed that has a wavelength dependent focal shift caused by longitudinal chromatic aberration in a lens assembly ( 203 ) that provides extended depth of field imaging due to focal shift ( 213,214 ) and increased resolution due to reduced lens system magnification. In use, multiple wavelengths of quasi-monochromatic illumination, from different wavelength LEDs ( 206,207 ) or the like, illuminate the target, either sequentially, or in parallel in conjunction with an imager ( 200 ) with wavelength selective (colored) filters. Images are captured with different wavelengths of illumination that have different focus positions ( 208,209 ), either sequentially or by processing the color planes of a color imager separately. Extended depth of field, plus high resolution are achieved. Additionally, information about the range to the target can be determined by analyzing the degree of focus of the various colored images.

Description:
RELATED APPLICATION DATA 
     This application is a continuation of U.S. application Ser. No. 11/048,624 filed Jan. 31, 2005, U.S. Pat. No. 7,224,540. 
     This application is related to U.S. application Ser. No. 11/045,213, filed Jan. 27, 2005, U.S. Pat. No. 7,215,493, entitled IMAGING SYSTEM WITH A LENS HAVING INCREASED LIGHT COLLECTION EFFICIENCY AND A DEBLURRING EQUALIZER, by inventors Bryan L. Olmstead and Alan Shearin, incorporated herein by this reference. This application is also related to U.S. application Ser. No. 11/045,817, filed Jan. 28, 2005, U.S. Pat. No. 7,234,641, entitled Illumination Pulsing Method for a Data Reader, by inventor Bryan L. Olmstead, incorporated herein by this reference. 
    
    
     TECHNICAL FIELD 
     This application relates generally to optical systems and elements and more particularly to optical imaging systems, such as those useful for data reading and other image capture applications. 
     BACKGROUND 
     In data reading devices, a return light signal from the object or symbol being read is focused onto a detector or detector array. In the example of a bar code scanner, reading the bars and spaces of a typical bar code, there needs to be sufficient difference in signal intensity between the signal corresponding to the light space and the signal corresponding to the dark bar in order for the processor to differentiate between them. In scanner applications there has been demand for increased depth of field, i.e., the range of distance over which the scanner can effectively scan. 
     U.S. Pat. No. 6,073,851 to Olmstead et al. discloses use of multi-focus optical systems to increase depth of field in optical reader systems. Depth of field is increased through careful design of multi-focal lenses, for example, symmetrical lenses having a series of concentric circular zones (as viewed along the optical axis), each zone providing a different focal length. See &#39;851 patent FIGS. 21 and 22 and columns 21-22. Such lenses are difficult and expensive to manufacture, and interzonal interference must be controlled. That said, they can be used advantageously in some embodiments as explained below. Multi-focus systems are also disclosed of the &#39;851 patent that employ multiple lenses, or lens arrays, as shown for example in FIGS. 35-37. 
     U.S. Pat. No. 5,945,670 to Rudeen is directed to an optical system for data reading having a large depth of field. There, a variable aperture device is deployed downstream of a laser light source focusing lens to vary the focal distance of the laser beam. The modified laser beam provides an outgoing source of (scanning) illumination of an object or target. Preferred embodiments herein, however, pertain to capturing and processing an “inbound” or return light signal reflected/refracted from a target object. Moreover, preferred embodiments pertain to 2-D optical data imaging, as distinguished from scanning (laser beam) methods. Similarly, U.S. Pat. No. 5,386,105 to Quinn et al. describes diffractive laser beam shaping methods and apparatus, “to control laser beam propagation, working range and beam cross-section in a bar code scanner” (Abstract). 
     U.S. Pat. No. 6,056,198 to Rudeen et al. describes an optical scanning system including a collection system for range enhancement. In one embodiment, an optical element such as a diffusion element or aperture is placed between a collection lens and a detector such that the amount of collected light from a far field target reaching the detector is maximized and the amount of collected light from a near field target reaching the detector is selectively inhibited, rendering the total light intensity on the detector more uniform. 
     For machine vision applications, large depth of field, high resolution, and high speed operation are often important system requirements. Estimation of the range to the target is sometimes desired. To achieve high speed operation, a large aperture is needed, in order to reduce motion blur. Unfortunately, a large aperture reduces the depth of field of the system, so speed and depth of field are conflicting requirements to the optical designer. Imager resolution, typically set by the number and size of the pixels, sets a limit on the optical magnification of the system to achieve a desired system resolution. Higher resolution is possible when the optical magnification is low, while larger depth of field is possible when the optical magnification is high. Therefore, high resolution and large depth of field are conflicting requirements to the optical designer. Nevertheless, the need remains for improvements in imaging systems to provide high speed, high resolution images with extended depth of field. 
     SUMMARY 
     Conventional imaging systems are typically intended for polychromatic (white light) illumination. As such, the lens systems are carefully designed to have very small levels of chromatic aberration. These lenses are termed color corrected. If a lens that was not color corrected was used in a white light illumination application, a very blurry image would result, as some colors of the illumination would provide in-focus images, while others would be out of focus, due to the chromatic aberration of the lens. Conventional imaging systems can estimate a range to the target by focusing the lens system to best focus and recording the lens position. 
     The present invention improves on the prior art by using chromatic aberration to an advantage in an imaging system. Where typical imaging systems try to reduce chromatic aberration to provide a better focused white light image, certain preferred embodiments described herein leverage chromatic aberration to provide a wavelength dependent focal shift, and in some embodiments use quasi-monochromatic illumination. (Quasi-monochromatic illumination behaves much like ideal monochromatic radiation. Thus, the frequencies of quasi-monochromatic light are strongly peaked about a selected frequency.) For many imaging applications, color is not an important feature and illumination relatively irrespective of wavelength can provide the necessary information. This is a reasonable limitation, for example, in many machine vision applications that look at the reflectivity profile of objects, such as optical character recognition, barcode reading, and inspection tasks. 
     In accordance with a preferred embodiment, a lens is formed of a highly dispersive material, such that the focal length changes significantly as a function of wavelength of incident light. A shorter wavelength is used to create a near field focus that lowers the optical magnification of the system, allowing the imager&#39;s resolution to be sufficient to read high density barcodes. An infrared light source can be used in some embodiments to enable high intensity illumination without disturbing the user. A second, shorter wavelength, for example visible light, can be used for high density reading as well as reading data sources such as barcodes that have little infrared contrast. In another embodiment, a doublet lens is arranged to provide a significant shift in focus as a function of wavelength. The doublet has a concave lens of crown glass in front of a convex lens of flint glass. This achieves substantial chromatic aberration to shift the focal length with wavelength. 
     As noted, a preferred embodiment is directed to exploiting two or more different wavelengths of light in a single optical data reading system. In accordance with one aspect, a “wavelength-dependent aperture” is disposed between a collection lens and the object being read or “target”. In one embodiment, the aperture includes a central zone filter arranged to block light having wavelengths below a predetermined cutoff wavelength, such as visible light. The longer wavelength, lower frequency passes through the entire aperture (such as near-infrared light). For example, in bar code data reading applications, larger (lower density) bar codes imaged using IR passing through the entire aperture. Higher density bar codes (e.g. 5 mil) can be imaged using visible light that passes only through a portion (the peripheral zone in one embodiment) of the aperture. A focal shift caused by either chromatic aberration or undercorrected spherical aberration causes the visible light to focus to near field, and provides sufficient modulation for reading the higher density images. 
     For many imaging applications, images are captured using an electronic imaging device, such as a CCD or CMOS imager, both commercially available. Suitable imagers are available, for example, from Micron Technology, Inc. of Boise, Id., USA. In one implementation, an imager is selected which is sensitive to a broad range of wavelengths, from visible to near-IR. The pixels of the imager are covered by a set of wavelength sensitive filters that divide the pixels into subsets according to wavelength sensitivity. Such an imager is commonly called a color imager, and the filter is a mosaic of pixel sized filters of different color transmission, such as red, green, and blue. Other filter choices are possible, such as a combination of visible and infrared filters. 
     A lens assembly that has intentionally uncorrected chromatic aberration is used to image the target onto the imager. Light-emitting diodes (LEDs) of at least two different wavelengths simultaneously illuminate the target in quasi-monochromatic light. (Suitable LEDs are widely available.) Because of the lens&#39;s chromatic aberration, in-focus images on the imager will occur from targets at different distances. The data from the imager is processed as separate color sub-images, called color planes (such as red, green, blue, and near-IR). The depth of field of each color plane is distinct and typically overlapping. The resolution of the closest focusing color plane is increased with respect to the furthest focusing color plane due to the reduction in optical magnification. In this embodiment, a single image is gathered that encompasses a large depth of field. Range to the target can be ascertained by determining which color plane is in best focus. Near and far focus are achieved with no moving parts. 
     In an alternative embodiment, an image is captured using an electronic imaging device, such as a CCD or CMOS imager, which is sensitive to a broad range of wavelengths, from visible to near-IR. All pixels of the imager are sensitive to the same range of wavelengths. Such an imager is termed a black and white imager. A lens assembly that has intentionally uncorrected chromatic aberration again is used to image the target onto the imager. LEDs illuminate the target in quasi-monochromatic light. In this example, at least two different wavelengths of LEDs are used that are sequentially illuminated in different image frames. In other words, one frame of image data “receives” a first wavelength, while another frame receives a different wavelength. One wavelength of illumination causes the imaging system to focus toward far field, which has a longer depth of field. Another wavelength of illumination causes the imaging system to focus toward near field, which has a lower magnification, ergo enhanced resolution. The depths of field of the two illumination settings can be made to overlap, extending the depth of field further. Range to the target can be ascertained by determining which colored image was in best focus. 
     An appropriate chromatically aberrated lens assembly may take many forms. A first illustrative lens design comprises a refractive lens system with undercorrected chromatic aberration. In other words, the lens system exhibits sufficient chromatic aberration to be useful in the manner described herein to extend depth of field in a data or image capture application. The materials used in making lenses in general have dispersion (the variation of index of refraction with wavelength), so chromatic aberration is always present. Techniques exist (such as multi element designs mixing flint and crown glass lenses) to enhance or reduce chromatic aberration. Diffractive lenses inherently have large chromatic aberration, and so are especially suitable, though other materials can be used. 
     An alternative lens assembly employs a wavelength sensitive aperture whose wavelength sensitivity is a function of aperture radius. A lens system with variable focal length with respect to aperture radius is used with the wavelength sensitive aperture. A variable focal length lens, having a focal length that is a function of aperture radius, can be created using undercorrected spherical aberration, as further described in the application entitled, “Imaging system with a lens having increased light collection efficiency and a deblurring equalizer”, cited above, or as a lens with discrete radial focal zones, as described in the background. In that regard, U.S. Pat. No. 5,770,847 is hereby incorporated herein by reference. 
     As an example, the lens assembly may be designed to have two zones, each passing a different wavelength. If a target is illuminated at those two wavelengths, a double image is formed on the imager. In focus images from each wavelength will occur at different distances. Either a color imager or sequential quasi-monochromatic illumination can be used to distinguish the images as mentioned above and further described below. 
     Additional aspects and advantages will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates certain limitations of prior art imaging systems. 
         FIG. 2  shows an imaging system according to a first embodiment. 
         FIG. 3  shows an imaging system according to a second embodiment. 
         FIG. 4  shows a plan view of the color imager according to the first embodiment. 
         FIG. 5  shows the index of refraction change vs. wavelength in typical optical glasses. 
         FIG. 6  shows the first lens design with undercorrected chromatic aberration. 
         FIG. 7  shows the second lens design with a wavelength sensitive filter. 
         FIG. 8  shows the second lens design focused on a near field distance 
         FIG. 9A  shows the wavelength sensitive filter according to the second lens design. 
         FIG. 9B  shows the transmission vs. wavelength behavior of the filter according to the second lens design. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , an imaging system typical of the prior art consists of an imager  100  composed of a plurality of pixels  109  and a lens assembly  101  composed of one or more lenses  110  and an aperture  104  spaced from the imager  100  by a distance  103 . The lens assembly provides a focused image of a target  108  onto the imager  100  when the target  108  is at distance  102  from the aperture  104 . The lens assembly  101  provides a sufficiently focused image of target  108  over a depth of field  105  determined by the size of the aperture  104 . The pixel size  106  of pixels  109  of imager  100  provides a projected resolution  107  based on the optical magnification, which is defined as the ratio of object distance  102  to image distance  103 . So the resolution of an imaging system is determined by the pixel size  106  and the optical magnification. The depth of field is determined by the aperture size  104 . 
     Referring to  FIGS. 2 and 4 , a color imager  200 , composed of pixels  201  sensitive to one set of wavelengths, and pixels  202  sensitive to another set of wavelengths serves to record an electronic representation of the image impinging on its surface. These pixels  201  and  202  may be considered different colored pixels, as they are sensitive to different wavelengths (aka colors) of light. Imager  200  may consist of more than two colors of pixels, such as red, green, blue, and near-infrared. Lens assembly  203  directs light from a target onto the imager  200 . Lens assembly  203  is composed of lenses  205  and aperture  204 . Targets are imaged by the system at various distances, such as  208  and  209 . 
     Quasi-monochromatic illumination, such as from LEDs, is directed to the target. Illumination source  206  of a first wavelength is directed to target  209 , forming a well focused image on imager  200 . Pixels  201  are sensitive to this wavelength and thus record the image of target  209  in electronic form. Illumination source  207  of a second wavelength is directed to target  208 , forming a well focused image on imager  200 . Pixels  202  are sensitive to this wavelength and thus record the image of target  208  in electronic form. Illumination sources  206  and  207  provide illumination to the target simultaneously. If the target is in position  209 , a well focused image will be formed on pixels  201  due to illumination source  206 , and a blurry image will be formed on pixels  202  due to illumination source  207 . The filtering action of the pixels  201  and  202  prevent them from seeing illumination from the other illumination source. Similarly, if the target is in position  208 , a well focused image will be recorded on pixels  202  and a blurry image on the pixels  201 . So, an extended depth of field is obtained by processing images from pixels  201  and  202  of imager  200 . 
     Referring next to  FIG. 3 , an alternative embodiment incorporates an imager  300 , with pixels  301  all sensitive to the same broad range of wavelengths of light. Such an imager  300  is termed a black and white imager. The remainder of the system is identical to the first embodiment, and contains a lens assembly  203 , and illumination sources  206  and  207 . In the alternative embodiment, however, illumination sources  206  and  207  are enabled in sequential imaging frames. Imager  300  records the image from the target illuminated by source  206  in a first frame and records the image from the target illuminated by source  207  in a second frame. If the target is located at position  208 , the image on imager  300  during illumination  207  will be well focused. If the target is located at position  209 , the image on imager  300  during illumination  206  will be well focused. So an extended depth of field is obtained by processing images from both illumination sources  206  and  207  in sequential frames. 
     The lens  203  in both the first and second embodiments can be implemented as one or more refractive lenses. Referring to  FIG. 5 , typical optical glasses have an index of refraction that is a function of wavelength—a property called dispersion.  FIG. 5  shows an example of BK-7, a so-called crown glass, and SF-11, a so-called flint glass. The focal length of a lens is a function of the index of refraction, so dispersion causes the focal length to be a function of wavelength and is termed chromatic aberration. Typically, a lens designer tries to minimize chromatic aberration. This is typically done by using multiple lenses of different dispersions. Positive and negative lenses of different dispersions can be combined to minimize the effects of chromatic aberration. In a similar manner, lenses can be combined to accentuate chromatic aberration, as may be advantageous for certain embodiments. 
     Referring to  FIG. 6 , an example of a refractive lens design using chromatic aberration is shown. Imager  300 , composed of imaging surface  601  and cover glass  602  is placed a distance  607  behind the front side of circular aperture  605  of a lens assembly  610 . A plano-concave lens  604 , that may be made of optical material BK7, is placed directly in front of circular aperture  605  of thickness  608 . Directly behind circular aperture  605  is plano-convex lens  603 , that may be made of optical material SF-11. 
     Referring to  FIG. 7 , a second lens design is shown. This lens design uses either type of imager, although imager  300  (black and white) is shown in the figure. A lens  702  with undercorrected spherical aberration is placed in front of an aperture  701 . Aperture  701  forms a wavelength filtering function according to  FIGS. 9A and 9B . Referring to  FIG. 9A , aperture  701  has a central section  802  and a peripheral section  803 . Referring to  FIG. 9B , a graph of optical transmission  904  vs. wavelength  901  is shown. Central section  802  of aperture  701  has an optical transmission of  902 , which is opaque at wavelength  905  and highly transmissive at wavelength  906 . Peripheral portion  803  of aperture  701  has a uniformly high transmission  903 . Referring back to  FIG. 7 , when an illumination of wavelength  906  is reflected from the target, light rays are collected from the full aperture of lens  702 , since aperture  701  is highly transmissive throughout its entire area. 
     Spherical aberration of lens  702  causes light rays from a variety of distances  703  to be focused onto imager  300 . At a given target distance, some light will be in focus and others will be focused either in front of or behind the imaging plane, causing a blurry but extended depth of field image, as described in the commonly-owned U.S. application Ser. No. 11/045,213 application referred to above, U.S. Pat. No. 7,215,493, entitled, “Imaging system with a lens having increased light collection efficiency and a deblurring equalizer.” 
     Referring now to  FIG. 8 , if illumination of wavelength  905  is reflected from the target, light rays are only collected from the periphery of lens  702 , since aperture  701  has high transmission only at the periphery for this wavelength. In this case, spherical aberration is greatly reduced, and the lens focuses closer, providing well focused images of targets in the range  801 . So the aperture  701  and lens  702  provides extended depth of field imaging at wavelength  906  and high resolution imaging at wavelength  905 . The lens assembly consisting of aperture  701  and lens  702  effectively has chromatic aberration, as the focal length of the assembly changes with respect to the wavelength, in this case due to the spherical aberration of lens  702  and the filtering action of aperture  701 . 
     Next, one specific implementation of a preferred embodiment is described for further illustration. Here, the challenge is achieving a large depth of field for 13 mil bar code label applications, which as noted above is in conflict with achieving 5 mil resolution necessary for reading smaller (higher density) codes. This challenge is a combination of the requirement for a large aperture (to collect a large amount of light to increase the sweep speed) and the resolution limitation of the imager. To achieve 5 mil reading, the focus must be close to the near field limit of the imaging system. To achieve 13 mil long depth of field reading, the focus must be at the mid-point of the depth of field (about 5″ from the front of a typical scanner). 
     One solution to this problem was to use two wavelengths of light: IR (850 nm) and yellow (590 nm) were selected. Leveraging the dispersive properties of various glasses, the lens doublet is designed to provide a significant shift in focus with wavelength. The lens is focused to mid-field with IR illumination and focuses near the front of the scanner with yellow light. This dual wavelength design is further useful in that the far field illumination is IR, which is invisible to the eye, allowing bright illumination to be pulsed at the imager frame rate, while avoiding visible flicker, which may be distracting to the user. The yellow light is used for near field, which does not have to be as bright (because the field of view is smaller), which is compatible with the lower efficiency (in terms of imager sensitivity and LED power) of yellow LEDs. The desired depth of field of 8 inches for 13 mil barcodes and a depth of field of 1.5 inches for 5 mil barcodes was achieved, while using a lens system with an approximate clear aperture of f/5. 
     It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. The scope of the present invention should, therefore, be determined only by the following claims.