Autofocus system and method in digital holography

1. An autofocus method for determining a focal plane for at least one object, the method comprising:

reconstructing a holographic image of the at least one object such as to provide a reconstructed image at a plurality of different focal depths,

wherein the reconstructed image comprises a real component and an imaginary component for jointly encoding phase and amplitude information;

performing a first edge detection on the real component for at least two depths of the plurality of different focal depths and a second edge detection on the imaginary component for the at least two depths;

obtaining a first measure of clarity for each of the at least two depths based on a first measure of statistical dispersion with respect to the first edge detection and a second measure of clarity for each of the at least two depths based on a second measure of statistical dispersion with respect to the second edge detection; and

determining the focal plane for the at least one object based on a comparison of a scalar measure of clarity for the at least two depths,

wherein the scalar measure is based on the first measure of clarity and the second measure of clarity.

2. The method according to claim 1 , further comprising identifying the at least one object in the reconstructed image,

wherein the following steps are applied to a local region of the reconstructed image corresponding to the at least one object of:

performing the first edge detection and the second edge detection;

obtaining the first measure of clarity and the second measure of clarity; and

determining the focal plane for the at least one object.

3. The method according to claim 2 , wherein identifying at least on object in the reconstructed image comprises:

digitizing the reconstructed image;

identifying connected regions having a same digitized value; and

segmenting each of the connected regions.

4. The method according to claim 2 ,

wherein a plurality of objects are identified,

wherein a plurality of focal planes are determined corresponding to the plurality of objects, and

wherein the method further comprises stitching image regions corresponding to each of the plurality of objects in the corresponding focal plane together to form a synthetic image that contains each of the plurality of objects.

5. The method according to claim 4 , wherein obtaining the first measure of clarity or obtaining the second measure of clarity comprises:

obtaining a gradient magnitude of the result of the first edge detection or the second edge detection; and

obtaining a standard deviation value of the gradient magnitude.

6. The method according claim 1 , wherein the first measure of statistical dispersion or the second measure of statistical dispersion comprises a standard deviation.

7. The method according to claim 1 , wherein the at least two depths of the plurality of different focal depths comprise depths uniformly distributed in a predetermined range.

8. The method according to claim 7 , wherein the depths uniformly distributed in the predetermined range comprise a first quartile, a second quartile, and a third quartile of the predetermined range.

9. The method according to claim 1 , further comprising:

determining at least one further depth of the plurality of different focal depths based on the determined focal plane;

repeating the steps of performing the first edge detection and the second edge detection for the at least one further depth;

obtaining the first measure of clarity and the second measure of clarity for the at least one further depth; and

adjusting the focal plane based on a scalar measure of clarity for the at least one further depth.

10. The method according to claim 9 ,

wherein the at least two depths are uniformly distributed in a first predetermined range of depths,

wherein the at least one further depth comprises depths uniformly distributed in a second range of depths,

wherein the second range of depths is narrower than the first predetermined range of depths, and

wherein the second range of depths is determined based on the comparison of the scalar measure of clarity for the at least two depths.

11. The method according to claim 1 , further comprising:

determining at least one further depth of the plurality of different focal depths based on the determined focal plane;

performing a second-stage edge detection for the at least one further depth based on an amplitude of the reconstructed image; and

evaluating a second-stage measure of clarity for the at least one object based on a result of the second-stage edge detection.

12. The method according to claim 1 , wherein performing the first edge detection and the second edge detection comprises convolving, respectively, the real component or the imaginary component with a Laplacian mask.

13. A computing device for use in an autofocus system for determining a focal plane for at least one object, the computing device being programmed for executing a method comprising:

reconstructing a holographic image of the at least one object such as to provide a reconstructed image at a plurality of different focal depths,

wherein the reconstructed image comprises a real component and an imaginary component for jointly encoding phase and amplitude information;

performing a first edge detection on the real component for at least two depths of the plurality of different focal depths and a second edge detection on the imaginary component for the at least two depths;

obtaining a first measure of clarity for each of the at least two depths based on a first measure of statistical dispersion with respect to the first edge detection and a second measure of clarity for each of the at least two depths based on a second measure of statistical dispersion with respect to the second edge detection; and

determining the focal plane for the at least one object based on a comparison of a scalar measure of clarity for the at least two depths,

wherein the scalar measure is based on the first measure of clarity and the second measure of clarity.

14. The computing device according to claim 13 , wherein the autofocus system comprises:

a light source configured to radiate light toward a sample under test and an image; and

an imager configured to acquire a hologram of the sample.

15. A non-transitory, computer-readable medium with instructions stored thereon, wherein the instructions are executable by a processor to perform a method for determining a focal plane for at least one object, the method comprising:

reconstructing a image of the at least one object such as to provide a reconstructed image at a plurality of different focal depths,

wherein the reconstructed image comprises a real component and an imaginary component for jointly encoding phase and amplitude information;

performing a first edge detection on the real component for at least two depths of the plurality of different focal depths and a second edge detection on the imaginary component for the at least two depths;

obtaining a first measure of clarity for each of the at least two depths based on a first measure of statistical dispersion with respect to the first edge detection and a second measure of clarity for each of the at least two depths based on a second measure of statistical dispersion with respect to the second edge detection; and

determining the focal plane for the at least one object based on a comparison of a scalar measure of clarity for the at least two depths,

wherein the scalar measure is based on the first measure of clarity and the second measure of clarity.

16. The non-transitory, computer-readable medium according to claim 15 , wherein the method further comprises identifying the at least one object in the reconstructed image,

wherein the following steps are applied to a local region of the reconstructed image corresponding to the at least one object of:

performing the first edge detection and the second edge detection;

obtaining the first measure of clarity and the second measure of clarity; and

determining the focal plane for the at least one object.

17. The non-transitory, computer-readable medium according to claim 16 , wherein identifying the at least one object in the reconstructed image comprises:

digitizing the reconstructed image;

identifying connected regions having a same digitized value; and

segmenting each of the connected regions.

18. The non-transitory, computer-readable medium according to claim 16 ,

wherein a plurality of objects are identified,

wherein a plurality of focal planes are determined corresponding to the plurality of objects, and

wherein the method further comprises stitching image regions corresponding to each of the plurality of objects in the corresponding focal plane together to form a synthetic image that contains each of the plurality of objects.

19. The non-transitory, computer-readable medium according to claim 15 , wherein the first measure of statistical dispersion or the second measure of statistical dispersion comprises a standard deviation.

20. The non-transitory, computer-readable medium according to claim 15 , wherein the at least two depths of the plurality of different focal depths comprise depths uniformly distributed in a predetermined range.