Single-lens extended depth-of-field imaging systems

• US 8,488,044 B2
• Filed: 02/17/2009
• Issued: 07/16/2013
• Est. Priority Date: 02/29/2008
• Status: Expired due to Fees

• Current Assignee: Global Bionic Optics Ltd
• Original Assignee: Global Bionic Optics Ltd
• Inventors: Mathieu, Gilles
• Primary Examiner(s): HO, TUAN V
• Application Number:
  US12/735,872
• Publication Number:
  US 20100328517A1
• Time in Patent Office: 1,610 Days
• Field of Search: 348/335, 348/360, 348/361, 359/676
• US Class Current: 348/335
• CPC Class Codes: G02B 13/20 Soft-focus objectives
• CPC Class Codes: G02B 27/0025 for optical correction, e.g. distorsion, aberration
• CPC Class Codes: G02B 27/0037 with diffracting elements G02B27/0056 takes precedence; holographic optical elements G02B5/32; zone systems G02B5/1876
• CPC Class Codes: G02B 27/0075 with means for altering, e.g. increasing, the depth of field or depth of focus
• CPC Class Codes: G02B 27/4211 correcting chromatic aberrations G02B27/0056, G02B27/4222, G02B27/4227 take precedence
• CPC Class Codes: G02B 27/4216 correcting geometrical aberrations
• CPC Class Codes: G02B 5/1895 such optical elements having dioptric power
• CPC Class Codes: G06V 40/19 Sensors therefor

Abstract

An extended depth of field (EDOF) imaging system that has a corresponding extended depth of focus (EDOF′). The imaging system has an optical system consisting of a single lens element and an objectwise aperture stop arrange substantially at a zero-coma axial position. The single lens element is configured so that the optical system has a select amount of spherical aberration (SA) in the range 0.2λ≦SA≦2λ, where λ is an imaging wavelength. The single lens element generates an amount of axial chromatic aberration that increases the EDOF while decreasing the average MTF of the optical system relative to the optical system having no chromatic aberration. The imaging system has an image sensor for forming a digitized raw image of an object.

Claims

1. An imaging system for imaging an object within an extended depth-of-field (EDOF) at an imaging wavelength λ

and with an average modulation transfer function (MTF), comprising;

an optical system having an optical axis and consisting of a single lens element and an aperture stop arranged objectwise of the single lens element, the optical system having an amount of spherical aberration SA such that 0.2λ

≦

SA≦

5λ

when forming an image of the object, the single lens element being made of a glass or plastic material that generates an amount of axial chromatic aberration that increases the EDOF up to 20% while decreasing the average MTF of the optical system by no more than 25%, relative to the optical system having no chromatic aberration, the aperture stop being located at an axial position that substantially minimizes comatic aberration; and

an image sensor arranged to receive the raw image and form therefrom a digitized electronic raw image.

2. The system of claim 1, further comprising:

an image processor electrically connected to the image sensor and adapted to receive the digitized electronic raw image digitally filtered to form a digitized contrast-enhanced image.

3. The system of claim 1, wherein 0.5λ

≦

SA≦

1λ

.

4. The system of claim 1, wherein the single lens element includes at least one diffractive feature configured to reduce chromatic aberration.

5. The system of claim 1, wherein the single lens element has first and second surfaces, at least one of the first and second surfaces is aspheric.

6. The system of claim 1, wherein the optical system has an F/# such that F/1.4≦

F/#≦

F/15.

7. The system of claim 1, further comprising:

a field lens arranged immediately adjacent the image sensor and configured to reduce an amount of field curvature imparted by the single lens element.

8. The system of claim 1, wherein the single lens element is selected from the group of lens elements comprising:

a bi-convex lens element, a positive meniscus lens element arranged so as to have concave surfaces relative to the object, and a piano-convex lens element arranged to have an objectwise planar surface.

18. The system of claim 1, wherein the aperture stop is located relative to the single lens element at or near a zero-coma position.

9. A method of forming an image of an object over an extended depth of field (EDOF) at an imaging wavelength λ

, comprising;

forming a raw image of an object with an optical system consisting of a single lens element and an aperture stop arranged objectwise of the single lens element, the optical system having an amount of spherical aberration SA such that 0.2λ

≦

SA≦

5λ

;

wherein the single lens element is made of a glass or plastic material, and the glass material being selected to generate an amount of chromatic aberration that increases the EDOF by no more than 20% while decreasing the average MTF by no more than 25%, relative to the optical system having no chromatic aberration, the aperture stop being located at an axial position that substantially minimizes comatic aberration; and

using an image sensor, electronically capturing the raw image to form a digitized raw image.

10. The method of claim 9, wherein 0.5λ

≦

SA≦

1λ

.

11. The method of claim 9, further comprising:

digitally filtering the digitized raw image to form a contrast-enhanced image.

12. The method of claim 11, wherein the raw image has associated therewith a raw modulation transfer function (MTF), and further comprising:

establishing an amount of defocus in the digitized raw image based on a position of the object relative to the optical system;

forming an enhanced MTF from the raw MTF by multiplying the raw MTF by a gain function, wherein the enhanced MTF is a function of the amount of defocus; and

applying the enhanced MTF to the digitized raw image to obtain the contrast-enhanced image.

13. The method of claim 12, including:

measuring a distance between the object and the optical system; and

forming the enhanced MTF by applying a two-dimensional linear digital filter to the raw MTF according to the measured distance.

14. The method of claim 13, including measuring the raw MTF at different distances that overlap the depth of field DOF by using a slanted edge with uniform incoherent backlighting.

15. The method of claim 11, wherein the raw image has an associated raw MTF, and further comprising:

forming an enhanced MTF from the raw MTF by multiplying the raw MTF by a gain function, wherein the enhanced MTF is substantially constant as a function of focus position; and

applying the enhanced MTF to the digitized raw image to obtain the contrast-enhanced image of the object.

16. The method of claim 9, including providing at least one diffraction feature on one or more surfaces of the single lens element so as to reduce chromatic aberration.

17. The method of claim 9, including providing at least one aspheric surface to the single lens element.

19. The method of claim 9, wherein the aperture stop is located relative to the single lens element at or near a zero-coma position.