Extended depth of field imaging system using chromatic aberration

• US 7,224,540 B2
• Filed: 01/31/2005
• Issued: 05/29/2007
• Est. Priority Date: 01/31/2005
• Status: Active Grant

• Current Assignee: PSC Scanning, Inc. (Datalogic ADC Incorporated)
• Original Assignee: Datalogic Scanning Incorporated (Datalogic S.p.A.)
• Inventors: Olmstead, Bryan L., Shearin, Alan
• Primary Examiner(s): SCHWARTZ, JORDAN MARC
• Application Number:
  US11/048,624
• Publication Number:
  US 20060171041A1
• Time in Patent Office: 848 Days
• Field of Search: 359739-741, 250/339.01, 250/339.05
• US Class Current: 359/754
• CPC Class Codes: G02B 27/0075 with means for altering, e.g. increasing, the depth of field or depth of focus
• CPC Class Codes: G06K 7/10702 Particularities of propagating elements, e.g. lenses, mirrors G06K7/10831 takes precedence

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.

Claims

1. An imaging system comprising:

an electronic imaging device;

a lens assembly having one or more lenses constructed and arranged to accentuate chromatic aberration for providing a wavelength dependent focal shift, the lens assembly being disposed between the imaging device and a target region; and

an illumination source for illuminating an optical code label on an object located in the target region so that, in use, at least some illumination from the illumination source can be reflected from the optical code label, through the lens assembly and impinge on the imaging device;

wherein the illumination source emits at least two different quasi-monochromatic wavelengths of light that are reflected from the optical code label,

wherein at least one of the two different quasi-monochromatic wavelengths of light is infrared light.

2. The imaging system of claim 1 wherein:

the electronic imaging device includes an array of pixels;

a first subset of the pixels are sensitive to a first set of wavelengths of light that includes a first one of said quasi-monochromatic wavelengths of light; and

a second subset of the pixels are sensitive to a second set of wavelengths of light that includes a different one of said quasi-monochromatic wavelengths of light.

3. The imaging system of claim 2 wherein at least one of the emitted quasi-monochromatic wavelengths of light is visible light.

6. The imaging system of claim 1 wherein the illumination source comprises a light emitting diode.

7. The imaging system of claim 1 wherein the illumination source comprises at least a plurality of visible light emitting LEDs and a plurality of infrared light emitting LEDs.

8. The imaging system of claim 1 wherein the electronic imaging device comprises an array of pixel elements and further includes wavelength selective filters over at least a subset of the array of pixel elements for distinguishing between the two different quasi-monochromatic wavelengths of light.

9. The imaging system of claim 1 wherein:

the electronic imaging device comprises an array of pixel elements and is arranged to capture sequential frames of data, each frame comprising data responsive to light incident on substantially the entire array of pixel elements.

10. The imaging system of claim 1 wherein the lens assembly comprises at least one glass lens that is highly dispersive and uncorrected for chromatic aberration.

11. The imaging system of claim 1 wherein the lens assembly comprises:

a first rotationally symmetric lens;

a second rotationally symmetric lens substantially axially aligned with the first lens; and

an aperture element positioned in between the first and second lenses.

12. The imaging system of claim 11 wherein the aperture element includes a central zone positioned adjacent an optical axis of the first and second lenses, and a peripheral zone generally surrounding the central zone;

wherein the central zone is substantially transparent to transmission of light while the peripheral zone is substantially opaque.

13. The imaging system of claim 12 wherein the aperture central zone is substantially circular, and the peripheral zone generally surrounds the central region in an annular configuration.

14. The imaging system of claim 11 wherein the said chromatic aberration is substantially caused by refractive dispersion of at least one element of the lens assembly.

15. The imaging system of claim 11 wherein the said chromatic aberration is substantially caused by diffractive dispersion of at least one surface of the lens assembly.

16. The imaging system of claim 1 wherein:

the electronic imaging device comprises an array of pixel elements and is arranged to capture sequential frames of data, each frame comprising data responsive to light incident on substantially the entire array of pixel elements; and

the illumination source controllably emits at least one of the quasi-monochromatic wavelengths of light during the capture of a frame of data.

19. The imaging system of claim 1 wherein the lens assembly exhibits at least sufficient chromatic aberration at the two different quasi-monochromatic wavelengths of light so as to extend the depth of a target field within which an image of the illuminated optical code received at the electronic imaging location is substantially in focus.

4. An imaging system comprising:

an electronic imaging device;

a lens assembly having chromatic aberration, disposed between the imaging device and a target region; and

an illumination source for illuminating an object located in the target region so that, in use, at least some illumination from the illumination source can be reflected from the object, through the lens assembly and impinge on the imaging device,

wherein the illumination source emits at least two different quasi-monochromatic wayelengths of light,

wherein the electronic imaging device includes an array of pixels;

a first subset of the pixels are sensitive to a first set of wavelengths of light that includes a first one of said quasi-monochromatic wavelengths of light; and

a second subset of the pixels are sensitive to a second set of wavelengths of light that includes a different one of said quasi-monochromatic wavelengths of light,

wherein at least one of the emitted quasi-monochromatic wavelengths of light is infrared light.

5. The imaging system of claim 4

wherein at least one of the emitted quasi-monochromatic wavelengths of light is visible light.

17. An imaging system comprising:

an electronic imaging device;

a lens assembly having chromatic aberration, disposed between the imaging device and a target region; and

an illumination source for illuminating an object located in the target region so that, in use, at least some illumination from the illumination source can be reflected from the object, through the lens assembly and impinge on the imaging device,

wherein the illumination source emits at least two different quasi-monochromatic wavelengths of light, wherein the lens assembly comprises;

a first rotationally symmetric lens having undercorrected spherical aberration; and

an aperture element substantially axially aligned along the optical axis of the first lens;

wherein the aperture element is wavelength dependent.

18. The imaging system of claim 17 wherein the wavelength dependent aperture element includes at least first and second zones having different optical transmission characteristics as a function of wavelength such that a first zone is relatively transmissive to light of a selected quasi-monochromatic wavelength while the second zone is not.

20. An extended depth of field imaging method comprising:

illuminating an optical code label on a target object with at least two different quasi-monochromatic wavelengths of light, wherein the at least two different quasi-monochromatic wavelengths of light are reflected from the optical code label, and;

wherein at least one of the two different quasi-monochromatic wavelengths of light is infrared light;

receiving light of the at least two different quasi-monochromatic wavelengths reflected from the illuminated optical code label at an electronic imaging location; and

interposing a lens assembly having chromatic aberration between the illuminated optical code label and the electronic imaging location so as to extend the depth of a target field within which an image of the illuminated optical code label received at the electronic imaging location is substantially in focus.

21. The method of claim 20 and further comprising:

providing an imager at the electronic imaging location;

operating the imager to capture a series of imaging frames;

illuminating the target with a first one of the quasi-monochromatic wavelengths of light during capture of a first one of the imaging frames; and

illuminating the target with a second one of the quasi-monochromatic wavelengths of light during capture of a second one of the imaging frames.

22. The method of claim 21 wherein the imager is a color imager and both illuminating steps are carried out during capture of a single imaging frame.

23. The method of claim 22 wherein the illuminating steps are carried out substantially simultaneously.

24. The method of claim 20 wherein the imager is a color imager and said illuminating step includes illuminating the target with both of the quasi-monochromatic wavelengths of light substantially simultaneously;

and the method further comprises processing data from the imager as separate color planes so as to distinguish between the wavelengths of illumination.

25. An extended depth of field imaging method comprising:

illuminating a target with at least two different quasi-monochromatic wavelengths of light;

receiving light reflected from the illuminated target at an electronic imaging device;

interposing a lens assembly having chromatic aberration between the illuminated target and the electronic imaging location so as to extend the depth of a target field within which an image of the illuminated target received at the electronic imaging location is substantially in focus;

providing the electronic imaging device with an array of pixels, wherein a first subset of the pixels are sensitive to a first set of wavelengths of light that includes a first one of said quasi-monochromatic wavelengths of light, and a second subset of the pixels are sensitive to a second set of wavelengths of light that includes a different one of said quasi-monochromatic wavelengths of light wherein at least one of the two different quasi-monochromatic wavelengths of light is infrared light.