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 exhibits spherical aberration as the dominant feature of the purposeful blur. In an aspect, a central obscuration of the multifocal imaging subsystem renders point-spread functions of object points more uniform over a range of object distances, however, the system may not include a centrally obscured aperture to achieve intended results. An iterative digital deconvolution algorithm for converting the intermediate image into the recovered image based on maximum entropy involves a metric parameter that speeds convergence, avoids stagnations, and enhances image quality.
1. An imaging system for recording an extended depth of field, comprising:
a multifocal imaging subsystem including an optical imaging element having a maximum radius, R, that produces an intermediate image of an object, wherein the multifocal imaging subsystem has a controlled amount of spherical aberration that imparts a purposeful blur to the intermediate image that is substantially uniform over the focal range of the multifocal imaging subsystem, and a phase plate that produces the controlled amount of spherical aberration, located at an aperture stop of the subsystem;
a digital processing system that receives the blurred, intermediate image and calculates a non-blurred, recovered image; and,
provided a criteria is met, the multifocal imaging subsystem including a central obscuration having a radius, δR, wherein the criteria is that ⅙≦δR/R<1, otherwise δR being zero and the central obscuration is absent.
2. The system of claim 1 in which the controlled amount of spherical aberration is between 1.6λ and 6λ in nominal wavelengths of imaging light.
3. The system of claim 1 in which the controlled amount of spherical aberration is predominately third-order spherical aberration.
4. A method of providing an image of an object over an extended depth of field than provided by a typical diffraction-limited imaging system, comprising:
providing a multifocal imaging subsystem having a maximum aperture radius, R, as a part of an integrated computational imaging system, and providing a central obscuration in the multifocal imaging subsystem having a maximum obscuration radius, δR, provided that ⅙δR/R<1;
forming an intermediate image of an object that is purposefully, uniformly blurred by a controlled amount of spherical aberration over a focal range of the multifocal imaging subsystem, such that the system impulse response is approximately invariant over the focal range; and
recovering an unblurred image from the blurred intermediate image.
5. The method of claim 4 , comprising blurring the intermediate image with a controlled amount of predominately third-order spherical aberration.
6. The method of claim 4 , wherein the spherical aberration provides a phase delay of between 1.6λ and 6λ in nominal wavelengths of imaging light.
7. The method of claim 4 , wherein recovering the unblurred image from the blurred intermediate image comprises performing an iterative digital deconvolution of the blurred, intermediate image using a maximum entropy algorithm.
8. The method of claim 7 , wherein maximum entropy algorithm includes the step of estimating a new image containing a combination of directional images that include a metric parameter.
9. The method of claim 8 , wherein 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.
10. The method of claim 8 in which the metric parameter has a non-integer value.
11. The method of claim 10 in which the metric parameter has a value between 0.2 and 0.6.
12. The method of claim 8 in which the directional images include pixel values, and the metric parameter adjusts weights among the pixel values.
13. The method of claim 8 in which the metric parameter is adjusted above zero to give larger pixel values more weight.
14. The method of claim 8 in which the maximum entropy method results in a modulation transfer function having a shape that increases contrast at high spatial frequencies approaching a Nyquist limit.
15. The method of claim 7 , wherein the iterative digital deconvolution method for image processing comprises the steps of: 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.
16. The method of claim 15 in which the estimated images contain pixel values and the metric parameter alters weights assigned to the pixel values.
17. The method of claim 15 in which the metric parameter has a non-integer value.
18. The method of claim 17 in which the metric parameter has a value between 0.2 and 0.6.