Apparatus and method for extended depth of field imaging
View Patent ↗An extended depth of field is achieved by a computational imaging system that combines a multifocal imaging subsystem for producing a purposefully blurred intermediate image with a digital processing subsystem for producing a recovered image having an extended depth of field. The multifocal imaging system preferably exhibits spherical aberration as the dominant feature of the purposeful blur. A central obscuration of the multifocal imaging subsystem renders point-spread functions of object points more uniform over a range of object distances. An iterative digital deconvolution algorithm for converting the intermediate image into the recovered image contains a metric parameter that speeds convergence, avoids stagnations, and enhances image quality.
1. An imaging system, comprising:
a circularly symmetric optical system that forms a plurality of images of an object over a focal range of the optical system, wherein said optical system predominately has a controlled amount of spherical aberration and further wherein an image quality metric is substantially invariant for the respective plurality of images; and
an image processing component that operates on the plurality of images to produce an output image over an extended depth of field associated with the focal range.
2. The system of claim 1 , wherein the optical system is a multifocal optical system.
3. The system of claim 1 , wherein the controlled amount of spherical aberration is substantially between 1.8 and 6 wavelengths (λ) in nominal wavelengths of imaging light.
4. The system of claim 1 , wherein the controlled amount of spherical aberration is predominately third-order spherical aberration.
5. The system of claim 1 , wherein the optical system includes a phase plate that produces the controlled amount of spherical aberration.
6. The system of claim 5 , wherein the optical system includes an aspheric lens.
7. The system of claim 1 , wherein the optical system includes at least one lens element having a surface modified to produce the spherical aberration.
8. The system of claim 1 , wherein the optical system includes a plurality of imaging elements and the spherical aberration is divided among more than one of the imaging elements.
9. An imaging system, comprising:
an optical system that forms a plurality of images of an object over a focal range of the optical system, wherein said optical system has a controlled amount of spherical aberration and further wherein an image quality metric is substantially invariant for the respective plurality of images;
a light obscurer disposed along an optical axis of the optical system; and
an image processing component that operates on the plurality of images to produce an output image over an extended depth of field associated with the focal range.
10. The system of claim 9 , wherein the light obscurer has a radius, δR, and the optical system has a maximum aperture radius, R, further wherein 0≦δR<R.
11. The system of claim 10 , wherein ⅙≦δR<R.
12. A method, comprising:
providing a circularly symmetric optical system having a focal range, wherein said optical system has a controlled amount of predominately spherical aberration and an impulse response that is substantially invariant over the focal range;
forming a plurality of images over the focal range, wherein an image quality metric is substantially invariant for the respective plurality of images;
image-processing said plurality of images; and
forming an output image over an extended depth of field of the optical system, wherein said extended depth of field exceeds a standard depth of field for a diffraction-limited optical imaging system.
13. The method of claim 12 , wherein providing an optical system having an impulse response that is substantially invariant over the focal range further comprises obscuring a central region of the optical system.
14. The method of claim 13 , comprising providing a central obscuration having a radius, δR, wherein 0≦δR<R, and wherein the optical system has a maximum aperture radius, R.
15. The method of claim 14 , wherein ⅙≦δR<R.
16. The method of claim 12 , wherein the controlled amount of spherical aberration is substantially between 1.8 and 6 wavelengths (λ) in nominal wavelengths of imaging light.
17. The method of claim 12 , wherein the controlled amount of spherical aberration is predominately third-order spherical aberration.
18. The method of claim 12 , wherein image-processing said plurality of images comprises performing an iterative digital deconvolution of the plurality of images using a maximum entropy algorithm.
19. The method of claim 18 , wherein the maximum entropy algorithm includes the step of estimating a new image containing a combination of directional images that include a metric parameter.
20. The method of claim 19 , wherein the maximum entropy algorithm includes the step of altering the directional images using the metric parameter to speed convergence toward the recovered image while avoiding points of stagnation.
21. The method of claim 19 in which the metric parameter has a non-integer value.
22. The method of claim 21 , in which the metric parameter has a value between 0.2 and 0.6.
23. The method of claim 19 in which the directional images include pixel values, and the metric parameter adjusts weights among the pixel values.
24. The method of claim 19 in which the metric parameter is adjusted above zero to give larger pixel values more weight.
25. The method of claim 19 in which the metric parameter has a value that results in a modulation transfer function having a shape that increases contrast at high spatial frequencies approaching a Nyquist limit.
26. The method of claim 18 , wherein the iterative digital deconvolution comprises calculating a succession of estimated images of an object based on a starting image and combination of directional images; inputting a metric parameter that modifies the directional images; evaluating the estimated images modified according to the metric parameter against a criterion of entropy maximization and a statistical noise parameter to choose a closer estimate of the object; and outputting an enhanced image of the object over the starting image.
27. The method of claim 26 , in which the estimated images contain pixel values and the metric parameter alters weights assigned to the pixel values.
28. The method of claim 26 , in which the metric parameter has a non-integer value.
29. The method of claim 28 , in which the metric parameter has a value between 0.2 and 0.6.
30. A method, comprising:
providing an optical system having a focal range, wherein said optical system has a controlled amount of predominately spherical aberration and an impulse response that is substantially invariant over the focal range;
obscuring a central region of the optical system;
forming a plurality of images over the focal range, wherein an image quality metric is substantially invariant for the respective plurality of images;
image-processing said plurality of images; and
forming an output image over an extended depth of field of the optical system.