Virtual guiding star for adaptive optical aberration correction

Abstract

Herein is presented a method for the adaptive correction of images comprises the steps: (a) capturing a first image with an image capture device; (b) selecting within the first image at least one candidate point and at least two test points; (c) using an iterative phase retrieval algorithm to calculate for each of the at least one candidate points an initial aberration correction for the entire image as if each of said at least one candidate points were guiding stars; (d) comparing the aberrations of each of the at least two test points in each of the initial aberrations corrections; (e) selecting the one candidate point whose initial aberration correction yields suitably similar aberrations for each of the at least two test points and designating said candidate point as a virtual guiding star; and (f) calculating phase corrections for the initial aberrations according to said virtual guiding star.

Description

[0001] VIRTUAL GUIDING STAR FOR ADAPTIVE OPTICAL ABERRATION CORRECTION

[0002] FIELD OF THE INVENTION

[0003] The present application relates to optics in general, and to adaptive optical methods in particular.

[0004] BACKGROUND OF THE INVENTION:

[0005] In virtually all applications of imaging technology, optical aberrations define the limits for resolution and image quality, making information carried by electromagnetic waves susceptible to corruption and loss even before being physically measured by a sensor of choice. Such optical aberrations may affect: microscopy due to objective lens imperfections and sample -induced optical aberrations; astronomy due to atmospheric turbulence; classic photography due to chromatic differences in high contrast regions; and imaging of biological tissues, in particular ophthalmic imaging. Optical aberrations can be described as distortions in the wavefront of light, caused either by internal factors on the imaging system itself like optical misalignment or by external factors light atmospheric aberrations.

[0006] To address the problems presented by these imposed limits, engineers have developed a range of solutions referred to as adaptive optics, wherein the distortion produced by optical aberrations in an initial image is corrected for in a final image, typically by the precise deformation of a mirror to compensate for said distortions. The approach of adaptive optics depends on two stages: (a) characterization of the optical aberration by direct or indirect wavefront measurements; and (b) correction of the distortion produced thereby. Measuring optical aberrations is not trivial however, and typically requires a wavefront reference, in order to accomplish the first stage, often referred to as a “guiding star”.

[0007] In many fields of optics, in particular in astronomical imaging, a “guiding star ” can be a real natural star, though the use of real stars is limited by a minimum brightness requirement, and as such many parts of the sky cannot be accurately imaged with this approach. In place of a natural star, many astronomical adaptive optical imagine systems utilize a laser to produce an artificial “guiding star ”, typically referred to as a “laser guiding star ”, wherein a powerful laser system provides a wavefront reference with which to measure the optical aberrations present in a raw image. These systems can accurately render images at a much higher resolution and quality than would otherwise be available for regions of the night sky without natural “guiding stars ”, by using the “laser guiding star ” to accurately determine a reference point for the measurements of optical aberrations (e.g. scintillation) induced by atmospheric turbulence, with which images can be corrected for.

[0008] Firing a high power laser onto an object being imaged is possible in the case of astronomical imaging, given the lack of any immediate known risk of the object to a high powered laser, but it is not a method possible when the object is vulnerable to laser exposure. For example, satellite imaging systems are limited by the same atmospheric turbulence, albeit in detecting light travelling in the opposite direction, but high powered lasers cannot be fired at the surface of the earth without incurring massive risk to ecology, economy, and human health. Without a “guiding star”, neither real nor laser-generated, satellite imaging systems are limited to an optical spatial resolution on the order of 10 cm due to uncorrected atmospheric turbulence. Similar limitations are present in many other fields of optics where neither real nor laser-generated “guiding stars ” are available.

[0009] This is thus the technical problem presented by the current state of the art in adaptive optics: the characterization of optical aberrations without the use of real or laser-generated “guiding stars

[0010] In different fields, different approaches to improve the accuracy of adaptive optic imaging in light of the above-defined technical problem have been developed, albeit without actually solving the problem itself. In ophthalmic imaging, a low powered laser - like that taught in US5777719A US5949521A, and US609565A - can be used without incurring too much damage on the retina, depending on the area being imaged and the conditions of the imaging. Some of the limitations of this method are addressed in the system and method taught in US2003007125A1, wherein a light is directed from a light source to a human eye and reflections therefrom utilized to align phase shifts in the image. Also taught in this publication, and further expounded in US7350920, is the generation of a “test spot pattern ” utilizing an array of lenses, a comparison between which provides references to calculate the effect of some phase shift aberrations. This method is limited in its applicability to ophthalmic imaging, due to the mechanical limitations of both the said array of lenses and the source of light, as well as the types of aberrations correctable.

[0011] In similar fields, including confocal and wide-field microscopy, methods known as “deconvolution ” may be used to correct for optical aberrations, such as the method taught in US6658142. These methods can be combined with techniques to correct for the spherical aberrations, when the refractive index of the sample is fixed and known, to provide further means of correction, for example as taught in US7764433B2. This method relies on known parameters of the object being imaged, limiting its application significantly to objects where said parameters are knowable.

[0012] Neither of the methods above approach the accuracy and robustness of the “guiding star ” technique used in astronomy, though in a limited number of similar microscope applications of biological tissues, fluorescent proteins can be used to provide “artificial guiding stars ”, such as taught in Tao et al (Optical Letters (2011) Issue 36, pp 3389-3391). There are several specific drawbacks to this technique, not least the limitations on control of the fluorescence of the protein, depth of the sample, requirements for source light. More generally, the approach of using known light source in a sample object as an “artificial guiding star ” is limited to applications where it is possible to provide said “artificial guiding stars ” within the object.

[0013] For example, it is impossible to provide “artificial guiding stars ” in every capturable frame for a satellite imaging system configured to image the surface of a planet. Neither is it practical to apply the array of lenses taught in US7350920, nor can the optical aberrations produced by atmospheric turbulence be corrected for with the technique taught in US7764433B2, nor can the refractive index of the surface of an arbitrary object on the planet be known. Numerous other systems and methods for adaptively correcting the optical aberration of images provide partial solutions to related problems, but none solve the central problem of applying the “guiding star ” approach to systems wherein no natural nor artificial “guiding stars ” can be provided. As such, there exists a need within the general field of adaptive optics to adapt the approach of “guiding star” optical aberration correction to said systems.

[0014] SUMMARY OF THE INVENTION The following embodiments and aspects thereof are described and illustrated in conjunction with systems, devices and methods which are meant to be exemplary and illustrative and not limiting in scope. In various embodiments, one or more of the above -described problems have been reduced or eliminated, while other embodiments are directed to other advantages or improvements.

[0015] A method for the adaptive correction of images comprises the steps: (a) capturing a first image with an image capture device; (b) selecting within the first image at least one candidate point and at least two test points; (c) using an iterative phase retrieval algorithm to calculate for each of the at least one candidate points an initial aberration correction for the entire image as if each of said at least one candidate points were guiding stars; (d) comparing the aberrations of each of the at least two test points in each of the initial aberrations corrections; (e) selecting the one candidate point whose initial aberration correction yields suitably similar aberrations for each of the at least two test points and designating said candidate point as a virtual guiding star; and (f) calculating phase corrections for the initial aberrations according to said virtual guiding star, whereby a virtual guiding star is selected from at least one candidate point in a first image and used to calculate the corrections required to correct for detected aberrations, and whereby a control module is utilized to operate steps (b-f).

[0016] According to another aspect of the invention, the method comprises an additional step: capturing a second image with the image capture device within a specific period of time from the first image’s capture, wherein the phase corrections calculated according to said virtual guiding star are produced by a wavefront shaping device. Persons skilled in the art will appreciate that the application of method described with steps (a) - (f) cannot be applied retroactively to existing digital images as a digital correction, given that without the phase -corrected capture of a second image, the electrical field waves would be mixed to the point that their phase could not be resolved directly.

[0017] According to another aspect of the invention, the method comprises an additional step: digitally correcting the first image according to the phase corrections calculated according to the virtual guiding star. According to another aspect of the invention, the at least one candidate point and the at least two test points are the same points in the first image, such that there are at least two points that serve as both candidate points and as test points.

[0018] According to another aspect of the invention, the steps: (b); (c); (d); and (e), are repeated until a candidate point yields suitably an initial aberration correction for said candidate point to be designated as a virtual guiding star.

[0019] According to another aspect of the invention, the at least one candidate point is a plurality of candidate points selected from isolated segments of the first images, wherein selection of said plurality of points is executed according to the shape and relative intensity thereof in the first image.

[0020] According to another aspect of the invention, the object is moving at a high speed relative to the image capture device, and the specific period of time between the capture of the first and second images is below 5 seconds and typically below 50 milliseconds.

[0021] According to another aspect of the invention, the image capture device is configured on a satellite orbiting a planet , wherein the object of which the image is being captured is a region of the planet, and wherein the satellite is moving at a speed of at least 7.5 km per second relative to the object.

[0022] According to another aspect of the invention, the aperture of the image capture device is larger than the Fried parameter defining the quality of optical transmission through the atmosphere between said satellite and said planet. Persons skilled in the art will appreciate that the Fried parameter inherently varies with the wavelength of electromagnetic radiation utilized, allowing the present invention to be advantageously adapted for a wide range of wavelengths. The present invention may be effectively employed across a variety of spectral regimes, including, without limitation, visible, near-infrared, shortwave infrared, and other optical bands as appropriate. While specific embodiments may optimize the aperture size relative to the Fried parameter at particular wavelengths, the present invention is not limited to any one spectral region or wavelength range. Accordingly, the present invention encompasses methods and systems capable of dynamic adjustment, calibration, or optimization in response to variations in wavelength-dependent atmospheric conditions, thereby enhancing imaging performance under diverse environmental scenarios.

[0023] According to another aspect of the invention, the object is living biological tissue.

[0024] BRIEF DESCRIPTION OF THE FIGURES

[0025] Some embodiments of the invention are described herein with reference to the accompanying figures. The description, together with the figures, makes apparent to a person having ordinary skill in the art how some embodiments may be practiced. The figures are for the purpose of illustrative description and no attempt is made to show structural details of an embodiment in more detail than is necessary for a fundamental understanding of the invention.

[0026] FIG. 1 constitutes a flowchart of operation for the method of the invention, according to some embodiments of the invention.

[0027] FIG. 2 constitutes a system block diagram of a system on which the invention can be operated utilizing optic transformation and a wavefront shaping device, according to some embodiments of the invention.

[0028] FIG. 3A-3D constitutes four images demonstrating the process of determining and utilizing a virtual guiding star for a satellite image, according to some embodiments.

[0029] DETAILED DESCRIPTION OF SOME EMBODIMENTS

[0030] In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well- known methods, procedures, and components, modules, units and / or circuits have not been described in detail so as not to obscure the invention. Some features or elements described with respect to one embodiment may be combined with features or elements described with respect to other embodiments. For the sake of clarity, discussion of same or similar features or elements may not be repeated. Unless explicitly stated, the method embodiments described herein are not constrained to a particular order or sequence. Additionally, some of the described method embodiments or elements thereof can occur or be performed simultaneously, at the same point in time, or concurrently.

[0031] Reference is made to FIG. 1, which constitutes a flowchart of operation forthe method ofthe invention, according to some embodiments of the invention. An order of operation 101 begins with a first image capture 110, after which an identification of virtual guiding stars 120 is attempted. If virtual guiding stars are not found immediately, then an evaluation of phase distortion for yet unknown virtual guiding stars 130 is operated, after which the required phase correction is evaluated 140. If the phase correction evaluation 140 does not yield a corrective phase, the system returns to 110 and captures a new image.

[0032] If the identification of virtual guiding stars 120 yields known virtual guiding stars, then the phase distortion therefor are evaluated 150, after which a wavefront shaping device is adjusted 160 to correct according to the corrections thereby calculated, and a new image is captured 170 with the wavefront shaping device having adjusted the phase in accordance with the corrections calculated in 150. If no virtual guiding stars were found initially in the identification of virtual guiding stars 120, but a phase correction was found when phase corrections for unknown virtual guiding stars were evaluated 140, then the corrections thereby calculated can also be used by the wavefront shaping device to correct the waveform 160 before a new image is captured 170. This order of operation includes a division - inherent to the function of the identification of virtual guiding stars 120 - between “known” and “unknown” guiding stars. In this context, a “known” guiding star refers to a region of the image whose potential for guiding star qualities is definable prior to the application of the invention, according to some embodiments. According to some embodiments, such regions may constitute “known” guiding stars because they contain reflective or otherwise illuminated objects whose position is determined prior to the application of the invention. In this context, “unknown” refers to all regions of an image not containing “known” guiding stars.

[0033] Reference is made to FIG. 2, which constitutes a system block diagram of a system on which the invention can be operated utilizing optic transformation and a wavefront shaping device, according to some embodiments of the invention. A configuration of an optical system 202 is presented, wherein an incoming planewave 210 is captured and focussed by a pair of optical lenses 220 and 230, is reflected against a waveform shaping device 230, is reflected against a mirror 240, then is captured by an image capture device. The method of the invention, for example in the embodiment presented in FIG. 1 1 in a phase estimation module 250, then the waveform shaping device controller 251 controls the waveform shaping device 230 to account for the aberrations detected, and a new incoming planewave can be first corrected by said wavefront shaping device 230 and then captured by the image capture device. According to other embodiments, the optical lenses 220, 221, and 222, are not configured as shown in FIG. 2, but rather are configured as telescopic, microscopic, or other optical configurations.

[0034] Reference is made to FIG. 3, which constitutes four images demonstrating the process of determining and utilizing a virtual guiding star for a satellite image, according to some embodiments. In FIG. 3A, a first image 301 is taken by an image capture device on a satellite without any phase corrections. The image 301 is significantly aberrate, resulting in a very low resolution relative to what the image capture device and associated optics are capable of producing. In the case presented by FIG. 3, the optical aberrations are a product of atmospheric turbulence between the surface of the Earth - which is the object being captured - and the image capture device in orbit above earth. In FIG. 3B a number of phase aberrations 302 are calculated for a given n x m pixels in image 301. The more the same phase aberration pattern is determined, as is shown in the images 305, then the possibility of deriving virtual guiding stars is significantly improved. In FIG. 3C, possible virtual guiding stars 310 are outlined throughout the image 301. In FIG. 3D, one of the virtual guiding stars 310 is used to calculate phase corrections, which are utilized by a waveform shaping device to adjust the incoming planewave to generate the corrected image 303, which has a significantly improved resolution compared to the original image 301.

[0035] Although the present invention has been described with reference to specific embodiments, this description is not meant to be construed in a limited sense. Various modifications of the disclosed embodiments, as well as alternative embodiments of the invention will become apparent to persons skilled in the art upon reference to the description of the invention. It is, therefore, contemplated that the appended claims will cover such modifications that fall within the scope of the invention.

Claims

CLAIMS1. A method for the adaptive correction of images, comprising the steps: a. capturing a first image with an image capture device; b. selecting within the first image at least one candidate point and at least two test points; c. using an iterative phase retrieval algorithm to calculate for each of the at least one candidate points an initial aberration correction for the entire image as if each of said at least one candidate points were guiding stars; d. comparing the aberrations of each of the at least two test points in each of the initial aberrations corrections; e. selecting the one candidate point whose initial aberration correction yields suitably similar aberrations for each of the at least two test points and designating said candidate point as a virtual guiding star; and f. calculating phase corrections for the initial aberrations according to said virtual guiding star, whereby a virtual guiding star is selected from at least one candidate point in a first image and used to calculate the corrections required to correct for detected aberrations, and whereby a control module is utilized to operate steps (b-f).

2. The method claim 1, comprising an additional step: capturing a second image with the image capture device within a specific period of time from the first image’s capture, wherein the phase corrections calculated according to said virtual guiding star are produced by a wavefront shaping device.

3. The method of claim 1, comprising an additional step: digitally correcting the first image according to the phase corrections calculated according to the virtual guiding star.

4. The method of claims 2 or 3, wherein the at least one candidate point and the at least two test points are the same points in the first image, such that there are at least two points that serve as both candidate points and as test points.

5. The method of claim 2 or 3, wherein steps: (b); (c); (d); and (e), are repeated until a candidate point yields suitably an initial aberration correction for said candidate point to be designated as a virtual guiding star.

6. The method of claims 2 or 3, wherein the at least one candidate point is a plurality of candidate points selected from isolated segments of the first images, wherein selection of said plurality of points is executed according to the shape and relative intensity thereof in the first image. .

7. The method of claims 2, or 3, wherein the object is moving at a high speed relative to the image capture device, and the specific period of time between the capture of the first and second images is below 5 seconds and typically below 50 milliseconds.

8. The method of claim 7, wherein the image capture device is configured on a satellite orbiting a planet , wherein the object of which the image is being captured is a region of the planet, and wherein the satellite is moving at a speed of at least 7.5 km per second relative to the object.

9. The method of claim 6, wherein the aperture of the image capture device is larger than the Fried parameter defining the quality of optical transmission through the atmosphere between said satellite and said planet.

10. The method of claims 1, 2, or 3, wherein the object is living biological tissue.

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