Reducing aberrations in an asymmetric imaging system

Abstract

An imaging system includes an optical stack that includes an image sensor, and at least one flat lens that is asymmetric relative to the image sensor or to at least one other optical element in the optical stack. In some implementations, the asymmetry is operable to reduce or eliminate keystone distortion.

Description

REDUCING ABERRATIONS IN AN ASYMMETRIC IMAGING SYSTEMFIELD OF THE DISCLOSURE

[0001] The present disclosure relates to reducing aberrations in an asymmetric imaging system.BACKGROUND

[0002] Meta-optical elements (MOEs) and diffractive optical elements (DOEs) are examples of optical elements that employ a flat optic technology. MOEs, for example, have a metasurface that includes distributed small subwavelength structures (e.g., nanostructures or other meta-atoms) arranged to interact with light in a particular manner. The meta-atoms can, individually and / or collectively, interact with light waves to change a local amplitude, a local phase, or both, of an incoming light wave. Likewise, DOEs have microstructure patterns that alter and control the phase of an incoming light wave. By altering the microstructures, it is possible for a DOE to produce a range of beam intensity profiles or beam shapes. MOEs or DOEs can be used, for example, in optical applications to take advantage of the internal properties given by a tailored phase function, compared to classic, curved refractive lenses.

[0003] In some applications of an optical imaging system incorporating flat optical elements, it may be desirable to provide a tilted object plane. Such tilt, however, can introduce aberrations and distortion. One example is keystone distortion, which refers to the apparent distortion of an image caused by projecting it onto an angled surface. For example, distortion of the image dimensions may cause a square-shaped feature in the object plane to appear on the image plane as a trapezoid. Further, the image plane may become tilted and resolution and / or focal distance may vary asymmetrically across the image plane.SUMMARY

[0004] In one aspect, the present disclosure describes an imaging system that includes an optical stack including an image sensor and at least one flat lens that is asymmetric relative to the image sensor or to at least one other optical element in the optical stack.

[0005] In some implementations, the asymmetry is operable to reduce or eliminate optical aberrations. In some implementations, the asymmetry is operable to reduce or eliminate optical off-axis aberrations. In some implementations, the asymmetry is operable to reduce or eliminate keystone distortion.

[0006] In some implementations, the asymmetry is provided at least in part by the image sensor being tilted relative to an optical axis of the imaging system. In some implementations, the asymmetry is provided at least in part by the at least one flat lens being decentered and / or tilted relative to an optical axis of the imaging system. In some implementations, the at least one flat lens has an asymmetric phase function.

[0007] In some implementations, the at least one flat lens is a MOE lens or a DOE lens. For example, in some cases, the at least one flat lens is a first MOE lens. Further, in some cases, the optical stack includes a second MOE lens. In some instances, an aperture stop is disposed between the first and second MOE lenses. In some instances, an aperture stop is disposed adjacent a front surface of the first MOE.

[0008] In some implementations, the imaging system includes an object plane, wherein the object plane is tilted relative to the optical axis of the lens stack or optical system. In some implementations, the imaging system has a field of view (FOV), wherein a chief ray of an on-axis field point OF in the FOV is tilted relative to an optical axis of the optical stack or the imaging system. In some implementations, the at least one flat lens includes a first metasurface having a mixture of symmetric and-asymmetric phase profile, and a second metasurface having a symmetric phase profile. In some cases, a symmetricor un-symmetric lens stop is disposed between the first and second metasurfaces and can help reduce distortion.

[0009] Various advantages may be obtained in some implementations. For example, in some implementations, the quality of the images can be enhanced or improved by reducing or eliminating the skewed output that would otherwise result due to the angled projection.

[0010] Other aspects, features and advantages will be readily apparent from the following detailed description, the accompanying drawing, and the claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 illustrates an example of a finite conjugate optical imaging system with a tilted object plane.

[0012] FIGS. 2A-2B illustrate an example in which an image sensor is tilted with respect to the optical axis.

[0013] FIG. 3 illustrates an example in which the flat lenses are decentered with respect to an optical axis of the imaging system.

[0014] FIGS. 4 A, 4B and 4C illustrate an example of footprint diagrams for rays appearing, respectively, at a first MOE, at a second MOE, and at an image sensor for the setup of FIG. 2A.

[0015] FIGS. 4D, 4E and 4F illustrate another example of footprint diagrams for rays appearing, respectively, at a first MOE, at a second MOE, and at an image sensor for a lens arrangement incorporating a lens having an asymmetric phase profile.

[0016] FIG. 5 illustrates an example of a finite conjugate imaging system having a tilted field of view (FOV).

[0017] FIG. 6 illustrates another example of an imaging system having a tilted FOV and differently tilted object plane.

[0018] FIG. 7 illustrates a further example of a finite conjugate imaging system having a tilted FOV, where the object plane is tilted towards the FOV.

[0019] FIG. 8 illustrates an example of a chief ray for a field point OF.DETAILED DESCRIPTION

[0020] As shown in the example of FIG. 1, an optical imaging system 10 includes a stack 12 of optical elements. The stack 12 includes an image sensor 14 having an image plane, and lenses 16, 18, where at least one of the lenses employs flat lens technology (e.g., a MOE lens, a DOE lens, or a Gradient Refractive Index (GRIN) lens). In the illustrated example, the stack 12 includes a first MOE lens 16 and a second MOE lens 18 positioned such that at least some incoming light rays pass through both MOEs 16, 18 before being incident on the image plane of the image sensor 14.

[0021] Each MOE lens 16, 18 includes at least one metasurface 30 on a respective substrate 32. In some implementations, the substrates 32 can be bonded together, for example, by index matching adhesive (e.g., glue). Each metasurface 30 has carefully arranged “unit cells” or “meta-atoms” with sub-wavelength structures (e.g., nanostructures). The term “sub wavelength” indicates that the nanostructures have at least one lateral dimension (parallel to the substrate on which they are disposed) that is less than a wavelength of light that is to be incident thereon. In general, the dimensions of the nanostructures scale with the shortest wavelength of interest. For example, in some implementations, the nanostructures can be in the form of nanoscale features having dimensions less than 1 micron. By adjusting the geometry of these unit cell elements,one can modify the phase above the elements in response to a plane wave. With the knowledge of the phase in terms of the geometry parameters, it is possible to create a metalens with an arbitrary phase profile by placing the meta-atoms at the necessary positions. In general, the derivative of the phase profile determines the ray bending.

[0022] The stack 12 also may include additional optical elements, including, for example, additional lenses (e.g., flat lenses or refractive lenses). In some implementations, an aperture stop 20 is present between the optical elements 16, 18. The chief ray for each field point is the ray that appears at the center of the stop 20, and the field point OF indicates the light rays that hit the center of the sensor 14 at the image plane. In some implementations, an aperture stop is present adjacent a front surface of the first optical element 16, which can be helpful to obtain a low F number (e.g., for time- of-flight (TOF) applications). The image sensor 14 can be implemented, for example, as a CMOS image sensor or a CCD image sensor. The full field point is defined as IF, which indicates the rays impinging on the corner(s) of the sensor 14.

[0023] As further shown in FIG. 1, an object plane 22 may be tilted with respect to the optical axis 24. The tilt, however, may introduce aberrations such as off-axis unfocused spots. In particular, the tilted object plane 22 may introduce keystone distortion, as well as lateral color resulting from variations in magnification.

[0024] To reduce aberrations, at least one of the lenses that employs flat lens technology has asymmetry relative to at least one of the other optical elements in the stack 12. In some cases, at least one of the lenses 16, 18 that employs flat lens technology has an asymmetric phase profile, which can help reduce aberrations. In particular, in some cases, keystone distortion and / or lateral resulting from a tilted object plane can be reduced.

[0025] In some cases, one or more of the optical elements in the stack 12 can be tilted to reduce aberrations. For example, as illustrated in FIG. 2A, the image sensor 14 is tilted with respect to the optical axis 24, which, as illustrated in FIG. 2B, refers to a line fromthe point at the center of the stop 20 to the field point OF at the center of the image plane at the sensor 14. In some implementations, the tilt of the sensor 14 is the same as the tilt of the object plane, which can help optimize reduction of aberrations. In particular, in some cases, keystone distortion and / or lateral color resulting from a tilted object plane can be reduced.

[0026] In some cases, one or more of the lenses 16, 18 that employs flat lens technology can be decentered and / or tilted with respect to the optical axis 24 to reduce aberrations such as keystone distortion and / or lateral color. An example is illustrated in FIG. 3, in which the MOE 16 is offset slightly downward in the Y direction, and the MOE 18 is offset slightly upward in the Y direction. In some cases, by decentering one or more of the lenses, the amount of tilt of the image sensor 14 needed to achieve a given reduction in aberrations (e.g., keystone distortion and / or lateral color aberration) can be reduced. Such an arrangement can, in some cases, facilitate manufacturability and / or reduce manufacturing complexity. In some instances, rather than decentering the lenses, the imaging system is arranged such that an incoming ray bundle from different field points uses different parts of the lenses 16, 18.

[0027] In some instances, tilting the sensor 14 may adversely impact image quality at the sensor because some field points may be out of focus and have blurred spots. To correct these aberrations, an asymmetric phase profile can be utilized. That is, in some implementations, one (or both) of the MOE lenses 16, 18 has an asymmetric phase profile, which can help focus or reduce aberrations for the different field points. In some instances, the asymmetric phase profile can have a variation of power.

[0028] In general, any one or more of the following techniques can be used to help reduce or eliminate aberrations (e.g., keystone distortion and / or lateral color) resulting either from either a tilted object plane and / or a tilted field-of-view (FOV): (i) providing one or more of the flat lenses with an asymmetric phase profile; (ii) tilting at least one optical element (e.g., the image sensor) in the stack relative to the optical axis; and / or (iii) decentering at least one of the flat lenses relative to the optical axis.

[0029] FIGS. 4 A, 4B and 4C illustrate an example of simulated footprint diagrams for rays (e.g., rays A, B, C . . .) appearing, respectively, at the first MOE 16 (FIG. 4A), at the second MOE 18 (FIG. 4B) and at the image sensor 14 (FIG. 4C) for the setup of FIG. 2A. Likewise, FIGS. 4D, 4E and 4F illustrate an example of simulated footprint diagrams for rays (e.g., rays A, B, C . . .) appearing, respectively, at the first MOE 16 (FIG. 4D), at the second MOE 18 (FIG. 4E) and at the image sensor 14 (FIG. 4F) for a lens arrangement for the setup of FIG. 3. In the example of FIGS. 4D, 4E, 4F, the ray beam passes through only a portion of the flat lens (e.g., the lower or upper part of the MOE).

[0030] In some applications, there is a tilted object plane, which may induce aberrations (e.g., keystone distortion). Other aberrations may occur for off-axis rays. Some applications have a tilted field of view (FOV), but not necessarily a tilted object plane.

[0031] The techniques described above can, in some instances, help reduce aberrations such as keystone distortion and can be used even for applications where the FOV is tilted relative to a surface that is normal to the lens stack (e.g., normal to one or more of the flat lenses 16, 18) or normal to the image plane (e.g., the surface of the image sensor 14). That is, a chief ray of an on-axis field point OF in the FOV is tilted relative to an optical axis of the optical stack or the imaging system. FIG. 5, for example, illustrates a situation in which the object plane 22 is tilted by the same amount (i.e., an angle a) as the FOV. Although the object plane 22 and FOV are not tilted relative to one another, they may be tilted, for example, relative to the image plane. FIG. 6 illustrates an example in which the object plane 22 and FOV are tilted by different amounts and in which one or more of the lenses 16, 18 that employs flat lens technology is decentered with respect to the optical axis 24 to reduce aberrations (e.g., keystone distortion). A similar optical system can be used for an infinite conjugate system or afocal system.

[0032] In some implementations, as shown in the example of FIG. 7, the object plane 22 is tilted relative to the FOV, but is not tilted relative to the image plane. That is, in the example of FIG. 7, the object plane 22 is parallel to the image plane of the sensor 14,whereas the chief ray for the field point OF (i.e., rays that impinges on the center of the of the sensor’s image plane) has a tilted FOV (see FIG. 8, where the angle depicted in the figure of the tilted FOV is defined as the angle between the chief ray of field point OF and the optical axis). In this case, there may be aberrations (e.g., keystone distortion) and the off-axis rays IF may be blurred (i.e., not focused) at the sensor 14. However, the off-axis rays can be corrected utilizing, for example, an asymmetric phase profile. A similar optical system can be used for an infinite conjugate system or afocal system.

[0033] Using the arrangements of FIGS. 5, 6 or 7, an optical imaging system can be provided with a tilted FOV using a meta optic lens design having a symmetric stop 20 disposed between two meta lens surfaces 30A, 30B. Such an arrangement can be helpful, for example, to reduce distortion or other aberrations. The first metasurface 30A can have, for example, a mixture of symmetric and-asymmetric phase profile. For example, the first metasurface 30A can have a phase profile that has radial symmetry to focus rays and Cartesian asymmetry to achieve a tilted FOV. The second metasurface 3 OB can have a symmetric phase profile. In some implementations, the second metasurface 3 OB also can have a phase profile that has radial symmetry and Cartesian asymmetry to correct for aberrations. Further, in some implementations, one or more of the lenses 16, 18 can be decentered or the image sensor 14 can be tilted to correct for aberrations such as chromatic aberrations.

[0034] In some implementations, high-angle tilt of the FOV can be enhanced by adding a refractive lens (e.g., a convex lens) on top of the first meta surface 30A (e.g., using a hybrid lens design). Further, a hybrid design can incorporate, for example, a refractive or GRIN lens anywhere at the lens stack in order to correct aberrations.

[0035] A tilted FOV can be advantageous, for example, in situations where space for the optical stack is limited, or where an array sensor is used as the image sensor 14 and it is desirable to use a relatively small image circle suitable for a small number of pixels.

[0036] Various modifications will be readily apparent from the foregoing detailed description. Accordingly, other implementations also are within the scope of the claims.

Claims

What is claimed is:

1. An imaging system comprising: an optical stack including: an image sensor; and at least one flat lens that is asymmetric relative to the image sensor or to at least one other optical element in the optical stack.

2. The imaging system of claim 1 wherein the asymmetry is operable to reduce or eliminate optical aberrations.

3. The imaging system of claim 1 wherein the asymmetry is operable to reduce or eliminate optical off-axis aberrations.

4. The imaging system of claim 1 wherein the asymmetry is operable to reduce or eliminate keystone distortion.

5. The imaging system of any one of claims 1-4 wherein the asymmetry is provided at least in part by the image sensor being tilted relative to an optical axis of the imaging system.

6. The imaging system of any one of claims 1-5 wherein the asymmetry is provided at least in part by the at least one flat lens being decentered and / or tilted relative to an optical axis of the imaging system.

7. The imaging system of any one of claims 1-6 wherein the at least one flat lens has an asymmetric phase function.

8. The imaging system of any one of claims 1-7 wherein the at least one flat lens is a MOE lens or a DOE lens.

9. The imaging system of any one of claims 1-8 wherein the at least one flat lens is a first MOE lens.

10. The imaging system of claim 9 wherein the optical stack includes a second MOE lens.

11. The imaging system of claim 10 further including an aperture stop disposed between the first and second MOE lenses.

12. The imaging system of any one of claims 1-11 including an object plane, wherein the object plane is tilted relative to an optical axis of the optical stack or the imaging system.

13. The imaging system of any one of claims 1-11 having a field of view (FOV), wherein a chief ray of an on-axis field point OF in the FOV is tilted relative to an optical axis of the optical stack or the imaging system.

14. The imaging system of claim 13 wherein the at least one flat lens includes: a first metasurface having a mixture of symmetric and-asymmetric phase profile; and a second metasurface having a symmetric phase profile.

15. The imaging system of claim 14 further including a symmetric stop disposed between the first and second metasurfaces.

16. The imaging system of claim 10 including an aperture stop adjacent a front surface of the first MOE.

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