# Patent

## Title
Variable imaging arrangements and methods therefor

## Abstract

Various approaches to imaging involve selecting directional and spatial resolution. According to an example embodiment, images are computed using an imaging arrangement to facilitate selective directional and spatial aspects of the detection and processing of light data. Light passed through a main lens is directed to photosensors via a plurality of microlenses. The separation between the microlenses and photosensors is set to facilitate directional and/or spatial resolution in recorded light data, and facilitating refocusing power and/or image resolution in images computed from the recorded light data. In one implementation, the separation is varied between zero and one focal length of the microlenses to respectively facilitate spatial and directional resolution (with increasing directional resolution, hence refocusing power, as the separation approaches one focal length).

## Background
RELATED PATENT DOCUMENTS
This patent document is the national stage filing under 35 U.S.C. §371 of International Application No. PCT/US2007/003346 filed on Feb. 6, 2007; which claims the benefit, under 35 U.S.C. §119(e), of U.S. Provisional Patent Application No. 60/765,903, entitled “Imaging Arrangements and Methods Therefor” and filed on Feb. 7, 2006; this patent document also claims the benefit, under 35 U.S.C. §119(e), of U.S. Provisional Patent Application No. 60/810,432, entitled “Digital Light Field Photography” and filed on Jun. 2, 2006; this patent document is also a continuation-in-part of PCT Patent Application No. PCT/US2005/035189 (publication number WO/2006/039486), entitled “Imaging Arrangements and Methods Therefor” and filed on Sep. 30, 2005, to which priority is claimed under 35 U.S.C. §120 for common subject matter.


FEDERALLY-SPONSORED RESEARCH OR DEVELOPMENT
This invention was made with Government support under contract 0085864 awarded by the National Science Foundation. The Government has certain rights in this invention.


FIELD OF THE INVENTION
The present invention relates generally to imaging applications, and more specifically to an arrangement and approach to facilitate light field imaging which accounts for spatial and directional resolution.
BACKGROUND
Imaging applications such as those involving cameras, video cameras, microscopes and telescopes have generally been limited in their ability to collect light and to process light to generate images having significant depth of field. For example, most imaging devices do not record most of the information about the distribution of light entering the device, and are limited in their ability to generate focused images with a significant depth of field. Conventional cameras such as digital still cameras and video cameras do not record most of the information about the light distribution entering from the world. In these devices, collected light is often not amenable to manipulation for a variety of approaches, such as for focusing at different depths (distances from the imaging device), correcting for lens aberrations or manipulating the viewing position.
The general inability to generate images with significant depth of field applies to both still and video imaging applications. For still-imaging applications, typical imaging devices capturing a particular scene generally focus upon a subject or object in the scene, with other parts of the scene left out of focus. Similar problems prevail for video-imaging applications, with a collection of images used in video applications failing to capture scenes in focus.
One general approach to solving this problem has been the development of integrated light field cameras, which detect the light traveling each ray flowing inside the body of a conventional camera (the “light field” flowing inside the camera body). For example, the plenoptic camera utilizes a microlens array in front of the sensor in order to capture not just how much light accumulates at each spatial location on the imaging plane, but how much light arrives from each direction. Processing the recorded light rays in different ways allows, for example, computational refocusing of final photographs, or computational extension of the depth of field.
As many imaging applications suffer from aberrations with equipment (e.g., lenses) used to collect light, correcting for these aberrations is desirable and often necessary to generate high quality images. Such aberrations may include, for example, spherical aberration, chromatic aberration, distortion, curvature of the light field, oblique astigmatism and coma. Correction for aberrations has historically involved the use of corrective optics that tend to add bulk, expense and weight to imaging devices. In some applications benefiting from small-scale optics, such as camera phones and security cameras, the physical limitations associated with the applications make it undesirable to include additional optics. Moreover, as the number of photosensors used to collect image data increases, and as the arrangement and processing of data from the same becomes increasingly important, aberration and other conditions that raise issue with the creation of images can significantly hinder the ability to create accurate images. Aberrations can particularly inhibit the ability to create accurate images focused at different depths.
Difficulties associated with the above have presented challenges to imaging applications, including those involving the collection and processing of digital light data for digital imaging.
SUMMARY
The present invention is directed to overcoming the above-mentioned challenges and others related to imaging devices and their implementations. The present invention is exemplified in a number of implementations and applications, some of which are summarized below.
According to an example embodiment of the present invention, the relative positioning of photosensors in an imaging arrangement is varied to facilitate a desired trade-off between the spatial and angular resolution of the recorded light data. The recorded light data is used in conjunction with information characterizing the imaging arrangement (including optical design and configuration of the photosensors) in order to generate a virtual image in which a portion of the image has been refocused and/or had its image quality corrected. For example, the image quality may have been corrected by computationally reducing the effect of optical aberrations due to the optics in the recording device.
According to an example embodiment of the present invention, the relative positioning of photosensors in an imaging arrangement is varied to facilitate the selective plenoptic collection of image data. Light is detected and used together with directional information characterizing the detected light, information characterizing the imaging arrangement and the relative positioning of the photosensors to generate a virtual image that corresponds to a refocused image and, in some instances, an image that is also corrected.
According to another example embodiment of the present invention, a digital imaging system processes data useful to synthesize an image of a scene. The system includes a photosensor array, and an optics arrangement including a main lens and a microlens array at a two-dimensional focal plane of the main lens. The optics arrangement directs light from the scene to the photosensor array via the main lens and microlens array to the photosensor array. A control arrangement sets a relative spatial relationship between the photosensor array and the microlens array, typically altering their separation while keeping them parallel. For each relative positioning of the photosensor array and the microlens, each photosensor senses a different set of rays flowing within the recording device. This set of rays is a function of the position of the photosensor array relative to the microlens array, and is used in order to interpret the photosensor values in subsequent processing. A processor provides image data characterizing a synthesized image as a function of the light sensed at different photosensors, the position of the photosensors relative to the microlens array, and the optical configuration of the microlenses and the rest of the optical train in the recording device.
According to another example embodiment of the present invention, a digital imaging system synthesizes an image from a set of light rays. The system includes a main lens, a photosensor array to collect a set of light rays and to output light data characterizing the set of light rays, and a microlens array, between the main lens and the photosensor array, to direct the set of light rays from the main lens to the photosensor array. A positioning arrangement sets a relative separation between the photosensor array and microlens array to facilitate selective detection of directional characteristics of the light ray data recorded by the photosensor array. In a more particular embodiment, this selectivity is achieved via a selectable trade-off between the spatial and directional resolution of the recorded light ray data. An image data processor computes a synthesized refocused image, using the light ray data, as a function of the optical properties, physical arrangement and relative position of the main lens, microlens array and photosensor array. The synthesized refocused image exhibits a virtual redirection of the set of light rays as collected by the photosensor array.
According to another example embodiment of the present invention, a camera having a main lens, a microlens array and a photosensor array is used for digital imaging. The photosensor array is arranged to sense light passed via the main lens to the microlens array and dispersed by the microlens array. The configuration of the camera is set by setting the separation between the planes in which the microlens array and photosensor array lie. The microlens array is used to direct light to the photosensor array, with the photosensor array including, for each microlens in the microlens array, a plurality of photosensors arranged to receive light from the microlens. The photosensor array is used to sample the light field flowing within the camera, and the light field is computationally resampled as a function of a ray-tracing approach for determining where the light rays passing into the camera will terminate on the photosensor array. The digitally re-sampled light field is used to compute an output image in which at least a portion of the image is refocused.
The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and detailed description that follow more particularly exemplify these embodiments.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention may be more completely understood in consideration of the detailed description of various embodiments of the invention that follows in connection with the accompanying drawings, in which:
FIG. 1A is an imaging arrangement for sampling a light field flowing past a lens, for computing images with refocusing and/or image correction, according to an example embodiment of the present invention;
FIG. 1B illustrates a different set of rays within the imaging arrangement of FIG. 1A, according to another example embodiment of the present invention;
FIGS. 1C and 1D show an approach to characterizing detected light rays to facilitate the computation of an image, such as with the arrangement in FIG. 1A, according to another example embodiment of the present invention;
FIGS. 2A-4B show an arrangement for sampling light fields for computing images, with varied microlens-to-photosensor distance in accordance with various embodiments of the present invention, wherein
FIGS. 2A-B show a relatively large microlens-to-photosensor distance,
FIGS. 3A-B show an intermediate microlens-to-photosensor distance, and
FIGS. 4A-B show a relatively small (e.g., near-zero) microlens-to-photosensor distance;
FIGS. 5A-5F show an approach to computing an image to facilitate different microlens-to-photosensor distances, according to another example embodiment of the present invention; and
FIGS. 6A and 6B respectively show perspective and cross-sectional views of a microlens array and circuit arrangement for computing images, with a microlens-to-photosensor distance actuator, according to another example embodiment of the present invention.



While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.
DETAILED DESCRIPTION
The present invention is believed to be useful for a variety of imaging applications, and the invention has been found to be particularly suited for electronic imaging devices and applications involving light field sampling and the correction of related images. In these contexts, a “light field” refers to the 4D function that defines the amount of light (e.g., radiance) traveling along each ray in some region of space. In the embodiments discussed below the region of space is typically the interior of the recording optical device, such as the space within the body of a camera, microscope, telescope, etc. In connection with various embodiments described below, primary interest is directed to the rays of light flowing into an imaging plane, such as the focal plane typically defined by the photosensor array in a conventional digital camera. With respect to this imaging plane, “spatial resolution” refers to the sampling density within the 2D imaging plane itself and “directional resolution” refers to the sampling density in the 2D angular domain of rays incident on the imaging plane. While the present invention is not necessarily limited to such applications, aspects of the invention may be appreciated through a discussion of various examples using these contexts.
According to an example embodiment, an imaging arrangement is adapted to select a desired spatial and directional resolution in recorded light field data to facilitate different performance characteristics in images computed from the detected light. In some applications, the imaging arrangement facilitates greater directional functionality, with a main lens passing light through microlenses that are focused on the main lens, to photosensors at or near the focal plane of the microlenses. In other applications, the aforesaid imaging arrangement controls the positioning of the photosensors to facilitate greater spatial resolution and less directional resolution by positioning the photosensors closer to the microlenses (physically and/or optically), relative to the microlenses' focal plane. In still other applications, the imaging arrangement positions the photosensors immediately adjacent to the microlenses to facilitate maximum spatial resolution. Characteristics of the optical arrangement, including the separation of the microlenses and photosensors, are used with light ray data from the photosensors to compute an image that is refocused and/or exhibits image enhancements such as aberration correction.
Turning now to the Figures, FIG. 1A shows an imaging system 100 that facilitates the collection of light and computation of an image using a selectable photosensor positioning approach to set spatial and directional resolution, according to another example embodiment of the present invention. The imaging system 100 includes an imaging arrangement 170 having a main lens 110, a microlens array 120, a photosensor array 130 and an actuator 180 for setting the distance between the microlens and photosensor arrays. In the context of examples herein, the microlens array 120 and photosensor array 130 may be referred to as implementing a light ray sensor. Although FIG. 1A illustrates a particular main lens 110 (single element) and particular microlens array 120, those skilled in the art will recognize that a variety of lenses and/or microlens arrays (currently available or developed in the future) are selectively implemented with a similar approach by, for example, replacing the shown main lens and/or microlens array. For instance, the main lens arrangement 210 in FIG. 2A may be implemented as main lens 110, in connection with various embodiments.
For illustrative purposes, rays of light from a point on a subject 105 in an imaged scene are brought to a convergence point on the microlens array 120, which is located at the optical conjugate distance of the subject. For this example, the photosensor array 130 is shown located at the focal plane 131, with the actuator 180 selectively moving the photosensor array 130 between the shown position and positions closer to the microlens array 120. However, the photosensor array 130 may be located at various separations from the microlens array to provide different sampling patterns of the light field. The typical separation range is between the shown location (at the focal length of the microlens array 120) and immediately adjacent the microlens array 120 (i.e., from zero to one focal length of separation). A microlens 122 at this convergence point separates these rays of light based on the direction of the light. In configurations where the separation between the microlens array and photosensor array is equal to the focal length of the microlenses, the microlens 122 creates a focused image of the aperture of the main lens 110 on the photosensors (positioned as shown) underneath the microlens. Where oriented laterally as shown, “underneath” refers to a position that is optically beyond the microlens, relative to the main lens. Other positions of the photosensor array 130 may involve a positioning similar, for example, to those shown in connection with FIG. 2A, FIG. 3A and FIG. 4A and discussed further below.
Each photosensor in the photosensor array 130 detects light incident upon it and generates an output that is processed using one or more of a variety of components. The output light ray data are passed to sensor data processing circuitry 140, which uses the data together with positional information indicating the location of the photosensor array 130 relative to the microlens array 120, as well as selective use of the optical configuration of the remainder of the imaging arrangement 170, to compute image data 150 of a scene (e.g., including subject 105).
The sensor data processing circuitry 140 is implemented, for example, with a computer or other processing circuit selectively implemented in a common component (e.g., a chip) or in different components. In various specific embodiments also according to the present invention, the input to the sensor data processing circuitry 140 includes the detected light ray values and the geometrical and optical configuration of the imaging arrangement 170. From these inputs, the sensor data processing circuitry 140 computes output imagery selectively exhibiting a refocused image and, where appropriate, a corrected image (where refocusing may include correcting). Various approaches to processing detected light data are described in detail herein, including those approaches described above, with and without reference to other figures. These approaches may be selectively implemented with an approach similar to that described with the sensor data processing circuitry 140.
FIGS. 1C and 1D show an approach to characterizing the light field sampling pattern to facilitate the computation of an image, such as with the arrangement in FIG. 1A, according to other example embodiments of the present invention. FIGS. 1C and 1D and the following description may also be applied in connection with other approaches described herein, including those described with FIGS. 2A-4B below. Generally, FIG. 1C shows an optical imaging arrangement 101, similar to that shown in FIG. 1A, with a main lens and photosensor arranged to detect light from a world focal plane and to pass light data to sensor processing circuitry 141 (similar to circuitry 140 in FIG. 1A) that is adapted to compute a refocused image using a Cartesian approach.
For illustration, a single ray 102 is characterized as passing through a point “u” at the main lens 104 to a point “x” on the photosensor (imaging plane) 106. Of course in general the ray exists in three dimensions and we would consider intersections (u, v) at the lens and (x, y) on the imaging plane. Let us refer to the value of the light field along the depicted ray as L(x, y, u, v), or L(x, u) if we are considering the two-dimensional simplification.
FIG. 1D shows a Cartesian ray-space plot 103 as an abstract representation of the two-dimensional light field, and is used in computing an image at the sensor processing circuitry 141. The ray depicted on the left is shown as a point (x, u) on the Cartesian ray-space in plot 103. In general, each possible ray in FIG. 1C corresponds to a different point on the ray-space diagram on FIG. 1D. This example approach to depicting the position related to light data is used in computing an image with various example embodiments below.
Returning to FIG. 1A, as discussed above, the position of the photosensor array 130 is set, relative to the microlens array 120, to facilitate certain imaging approaches. As the photosensor array 130 is moved from the focal plane 131 towards the microlens array 120, the images under each microlens (referred to as “microlens images”) become defocused. Each level of defocus provides a different light field sampling pattern, and these have different performance trade-offs (e.g., between spatial and directional resolution). As shown in FIG. 1A, the photosensor array 130 is located at approximately the focal plane of the microlens array 120, such that the microlens images are focused on the main lens. This configuration provides a relative maximal directional “u” resolution and relative minimal “x” spatial resolution on the ray-space of the recorded light field.
When the photosensor array 130 is moved to a position between the focal plane 131 and the microlens array 120, horizontal lines in the ray-space sampling pattern tilt, thereby becoming more concentrated vertically and less so horizontally. This effect can be intuitively understood in terms of the shearing of the light field when focusing at different depths. For illustrative purposes, the focus change can be considered to take place in a microscopic camera composed of each microlens and its patch of pixels, and the shearing takes place in the microscopic light field inside this microlens camera.
When the photosensor array 130 is located at close to zero separation (i.e., essentially against the microlens array), the imaging system 100 performs similarly to a conventional camera. The corresponding ray-space sampling converges to a pattern of vertical columns as the separation between microlenses and photosensor approaches zero. In this case the microlenses are almost completely defocused, with a relative minimal directional resolution and a relative maximal spatial resolution. That is, the values read off the photosensor array 130 for such a zero separation approach the values that would appear in a conventional camera in the absence of the microlens array 120.
In various embodiments, different portions of the imaging system 100 are selectively implemented in a common or separate physical arrangement, depending upon the particular application. For example, when implemented with certain applications, the microlens array 120, photosensor array 130 and actuator 180 are combined into a common image sensor arrangement 160. In some applications, the microlens array 120, photosensor array 130 and actuator 180 are coupled together on a common chip or other circuit arrangement. When implemented with a hand-held device such as a camera-like device, the main lens 110, microlens array 120, photosensor array 130 and actuator 180 are selectively combined into a common imaging arrangement 170 integrated with the hand-held device. Furthermore, certain applications involve the implementation of some or all of the sensor data processing circuitry 140 in a common circuit arrangement with the photosensor array 130 (e.g., on a common chip).
The microlens array 120 and photosensor array 130 are representatively shown from a two-dimensional perspective with relatively few microlenses and photosensors, but are readily implemented with varied arrangements of microlenses and photosensors. For instance, the microlens array 120 is generally implemented with a multitude (e.g., thousands or millions) of microlenses. The photosensor array 130 generally includes a relatively finer pitch than the microlens array 120, with a plurality of photosensors for each microlens in the microlens array 120. In addition, the f-numbers of the microlenses in the microlens array 120 and the f-number of the main lens 110 are generally set such that light passing via each microlens to the photosensor array does not significantly overlap light passed via adjacent microlenses. In some embodiments, this condition is achieved by setting the f-number of the lens to be equal or higher than the f-number of the microlenses.
The actuator 180 is represented generally, with a variety of actuation approaches readily implemented for moving the photosensor array 130. For example, actuators used in conventional cameras to position optics may be used in a similar manner to position the photosensor array 130. For example, the approaches shown in one or more of U.S. Pat. Nos. 7,167,203 and 7,164,446 may be implemented for positioning the photosensor array in connection with various example embodiments.
In various applications, the main lens 110 is translated along its optical axis (as shown in FIG. 1A, in a horizontal direction) to focus on a subject of interest at a desired depth “d” as exemplified between the main lens and an example imaging subject 105. By way of example, light rays from a point on the subject 105 are shown for purposes of this discussion. These light rays are brought to a convergence point at microlens 122 on the focal plane of the microlens array 120. The microlens 122 separates these rays of light based on direction, creating a focused image of the aperture of the main lens 110 on a set of pixels 132 in the array of pixels underneath the microlens.
FIG. 1B illustrates an example approach to separating light rays, such that all rays emanating from a point on a main lens 111 and arriving anywhere on the surface of the same microlens (e.g., 123) are directed by that microlens to converge at the same point on a photosensor (e.g., 133). This approach shown in FIG. 1B may, for example, be implemented in connection with FIG. lA (i.e., with the main lens 111 implemented for main lens 110, with microlens array 121 implemented for micro lens array 120, with photosensor array 135 implemented for photosensor array 130 and with the actuator 181 implemented for actuator 180).
The image that forms under a particular micro lens in the microlens array 121 dictates the directional resolution of the system for that location on the imaging plane. In some applications, directional resolution is enhanced by facilitating sharp micro lens images, with the microlenses focused on the principal plane of the main lens. In certain applications, the microlenses are at least two orders of magnitude smaller than the separation between the micro lens array and the main lens 111. In these applications, the main lens 111 is effectively at the microlenses' optical infinity; to focus the microlenses, the photosensor array 135 is located in a plane at the microlenses' focal plane. To facilitate increased spatial resolution, the photosensor array 135 is positioned closer to the microlens array 121.
The microlens array 120 is implemented using one or more of a variety of microlenses and arrangements thereof. In one example embodiment, a plane of microlenses with potentially spatially varying properties is implemented as the microlens array 120. For example, the microlens array may include lenses that are homogeneous and/or inhomogeneous, square in extent or non-square in extent, regularly distributed or non-regularly distributed, and in a pattern than is repeating or non-repeating, with portions that are optionally masked. The microlenses themselves may be convex, non-convex, or have an arbitrary profile to effect a desired physical direction of light, and may vary in profile from microlens to microlens on the plane. Various distributions and lens profiles are selectively combined. These various embodiments provide sampling patterns that are higher spatially (correspondingly lower angularly) in some regions of the array, and higher angularly (correspondingly lower spatially) in other regions. One use of such data facilitates interpolation to match desired spatial and angular resolution in the 4D space.
FIGS. 2A-4B show an arrangement for detecting light for computing images, each set of figures (i.e., FIGS. 2A&B; FIGS. 3A&B; and FIGS. 4A&B) depicting different microlens-to-photosensor distances, as implemented in accordance with various example embodiments. In each of FIGS. 2A, 3A and 4A, the imaging arrangement 200 may be implemented in a camera, microscope or other imaging device, such as that shown in FIG. 1A and discussed above. That is, the indicated main lenses, microlens arrays, photosensor arrays and actuators are arranged together and controlled to detect light ray data for processing locally and/or remotely. The shown rays are those that are integrated by the same photosensor pixel with different microlens focus for each position of the photosensor array. The following more particularly describes each figure relative to its indicated microlens-to-photosensor array separation.
Beginning with FIG. 2A, an imaging arrangement 200 includes a main lens arrangement 210 with aperture 212, a microlens array 220 and a photosensor array 230 that is positioned using an actuator 280 (i.e., to translate the photosensor array vertically, relative to the positioning shown in FIG. 2A). The actuator 280 has positioned the photosensor array 230 at a location relative to the microlens array 220 that is close to or at the focal length of the individual microlenses in the microlens array, with the microlenses focused on the main lens 210. This arrangement facilitates a recorded sampling of the light field with relatively high directional resolution. By way of example, a set of rays 205 is shown in FIG. 2A, and FIG. 2B shows a sampling pattern 240 with the set of rays integrated by a photosensor pixel 250.
FIG. 3A shows the imaging arrangement 200 with the photosensor array 230 positioned by the actuator 280 at an intermediate microlens-to-photosensor separation, less than the focal length of the microlenses in the microlens array 220. With this positioning and as relative to FIG. 2A, the light field is detected with greater spatial resolution and less directional resolution. A set of rays 305 is shown by way of example in FIG. 3A, and FIG. 3B shows a sampling pattern 340 with the set of rays integrated by a photosensor pixel 350. The horizontal lines in the sampling pattern 340 are slanted, relative to those shown in FIG. 2B, with more vertical and less horizontal concentration.
FIG. 4A shows the imaging arrangement 200 with the photosensor array 230 positioned by the actuator 280 at a relatively small (e.g., near-zero) microlens-to-photosensor distance, such that the microlenses in the microlens array 220 are almost completely defocused. This positioning facilitates a light field sampling with most if not all of the spatial resolution of the photosensor array 230; however, little to no directional information is obtained. A set of rays 405 is shown by example in FIG. 4A, with a corresponding sampling pattern 440 in FIG. 4B with the set of rays integrated by a photosensor pixel 450.
With the above discussion and corresponding figures in mind, the following describes various example embodiments directed to imaging with selective directional resolution as controlled via microlens-to-photosensor separation. In certain examples, reference is made to one or more of the above-discussed figures.
Considering the separation between the microlens array 120 and the photosensor array 130 in FIG. 1A, each separation of the microlens array and photosensor is effectively a different configuration of a generalized light field camera. By way of example, let us define β to be the separation as a fraction of the depth that causes the microlenses in the microlens array 120 to be focused on the main lens 110. For example, a plenoptic camera configuration corresponds to β=1 (e.g., with the microlens and photosensor arrays 120 and 130 separated by the focal length of the microlenses), and a configuration involving the microlens and photosensor array pressed up against one another corresponds to β=0. As described above, decreasing the β value defocuses the microlenses by focusing them beyond the aperture of the main lens 110.
Referring to FIGS. 2A, 3A and 4A, the respective sets of rays 205, 305 and 405 respectively reflect an increase in spatial resolution at the expense of directional resolution as the separation β approaches zero. Reducing the β value results in a shearing of the light field sampling pattern within each column, resulting in an increase in effective spatial resolution (reduction in x extent), and a corresponding decrease in directional resolution (increase in u extent).
In some applications, higher image resolution is obtained by reducing the separation between the microlenses and photosensors (by reducing the β value) respectively in the microlens and photosensor arrays 120 and 130, and by changing the optical focus of the main lens 110 and final image processing (e.g., via sensor data processing circuitry 140). For instance, at β=0, the microlenses are pressed up against the sensor and lose their optical power; thus the effective spatial resolution is that of the sensor. The effective resolution decreases linearly to zero as the β value increases, with the resolution of the microlens array 120 setting a lower bound. In equations, if the resolution of the photosensor array is Msensor×Msensor (where the photosensor array 130 has M×M photosensors) and the resolution of the microlens array is Mlenslets×Mlenslets (where the microlens array 120 has M×M microlenses) the output images have an effective resolution Meffective×Meffective, where

Meffective=max((1−β)Msensor, Mlenslets).   Equation 1

Changes in the sampling pattern of a generalized light field camera implemented in accordance with FIGS. 2A-4D are localized within columns defined by the microlens array. Each column represents the microscopic ray-space between one microlens and the patch of photosensors that it covers. Defocusing the microlens by reducing the β value shears the microscopic ray-space. The example derivation described below in connection with FIGS. 5A-5E describes an approach to calculating the sampling pattern of the light field as a function of β for use in connection with one or more example embodiments of the present invention. The derivation begins with the microscopic ray-spaces between each microlens and photosensor, where the sampling pattern is trivial, moving out into the ray-space of a full camera.
FIG. 5A is a schematic for a light field camera 500 having a main lens 510, microlens array 520 and photosensor array 530, showing a microlens labeled i whose microscopic light field is to be analyzed further. FIG. 5B illustrates a close-up of the microlens i, with its own local coordinate system, and a portion of the photosensor array 530 located under the microlens. The microscopic light field's ray-space is parameterized by the intersection of rays with the three illustrated planes: the microlens plane, xi, the sensor plane, si, and the focal plane of the microlens, wi. In order to map this microscopic ray-space neatly into a column of the macroscopic ray-space for the whole camera, the origins of the three planes are chosen to lie along the line 505 passing through the center of the main lens 510 and the center of microlens i, as indicated on FIGS. 5A and 5B. The focal length of the microlenses is f, and the separation between the microlens array 520 and the photosensor array 530 is βf, using the above characterization of β.
FIG. 5C illustrates the shape of an example sampling pattern on the ray-space parameterized by the microlens xi plane and the photosensor si plane. This choice of ray-space parameterization shows that the sampling is given by a rectilinear grid, since each photosensor pixel integrates all the rays passing through its extent on si, and the entire surface of the microlens on xi. Let us denote the microscopic light field as liβf(xi, wi), where the subscript βf refers to the separation between the parameterization planes. The sampling pattern in this ray-space is transformed into the macroscopic ray-space, using a change in coordinate systems.
FIG. 5D illustrates a first transformation: re-parameterizing liβf by changing the lower parameterization plane from the sensor plane si to the microlens focal plane wi. The microscopic light field parameterized by xi and wi is denoted as lif(xi,wi), where the f subscript reflects the increased separation of one microlens focal length. Re-parameterizing into this space introduces shear in the light field, with the following characterization:




  
    
      
        
          
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            )
          
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        3
      
    
  




Combining Equations 2 and 3 gives the complete transformation from liβf to the macroscopic space:





  
    
      
        
          
            l
            
              β
              ⁢
              
                
              
              ⁢
              f
            
            i
          
          ⁡
          
            (
            
              
                x
                i
              
              ,
              
                s
                i
              
            
            )
          
        
        =
        
          L
          ⁡
          
            (
            
              
                
                  Δ
                  ⁢
                  
                    
                  
                  ⁢
                  
                    x
                    i
                  
                
                +
                
                  x
                  i
                
              
              ,
              
                
                  
                    x
                    i
                  
                  ⁢
                  
                    F
                    f
                  
                  ⁢
                  
                    (
                    
                      
                        1
                        β
                      
                      -
                      1
                    
                    )
                  
                
                -
                
                  
                    F
                    f
                  
                  ⁢
                  
                    
                      s
                      i
                    
                    β
                  
                
              
            
            )
          
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        4
      
    
  




Equation 4 shows that the slope of the grid cells in FIG. 5E is





  
    
      
        
          
            F
            f
          
          ⁢
          
            (
            
              
                1
                β
              
              -
              1
            
            )
          
        
        ,
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        5
      
    
  




discussed further below.

The above discussion holds for the microscopic light fields under each microlens in the microlens array 520. FIG. 5E illustrates the transformation of all these microscopic light fields into the macroscopic ray space, showing how they pack together to populate the entire space. FIG. 5F illustrates that the main lens 510 truncates the sampling pattern vertically to fall within the range of u values passed by the lens aperture.
In the above description, the main lens is assumed ideal, and the f-numbers of the system are matched to prevent cross-talk between microlenses. In many applications, aberrations affect the above approach and various conditions are addressed via computational approaches similar to the above. For general information regarding imaging, and for specific information regarding lens aberration and correction therefor, reference may be made to International Patent Application Ser. No. PCT/US2002/003346 (now International Patent Publication WO 2007/092545) entitled “Correction of Optical Aberrations” and filed concurrently herewith.
In connection with various example embodiments, including those discussed in connection with the figures above, the focus of a main lens is set as a condition of the separation between a corresponding microlens array and photosensor array. For instance, referring to FIG. 1A, the main lens 110 is focused relative to the distance between the microlens array 120 and the photosensor array 130 (e.g., corresponding to the distance βf as referenced in FIG. 5B).
In one implementation involving intermediate β values, a high final image resolution is obtained by optically focusing slightly beyond the subject (e.g., 105 in FIG. 1A), and use digital refocusing to pull the virtual focal plane back onto the subject of interest. For example, referring to FIG. 3A, desirable (e.g., maximal) spatial resolution of resultant photographs is achieved by digitally refocusing slightly closer than the world focal plane, which is indicated by the upper horizontal line on the ray-trace diagram (at X). The refocus plane of greatest resolution corresponds to the plane passing through the point where the convergence of world rays is most concentrated (where the reference number 305 points in FIG. 3A). Optically focusing further facilitates a shift of this concentrated position of world rays onto the focal plane. As the β value increases, this concentrated position moves closer to the main lens.
Referring again to FIGS. 5A and 5B, some embodiments use the following approach to produce (about) maximum resolution in an output refocused image generated from the generalized light field camera. In the following equations, the assumptions is that the desired output photograph is focused as if a conventional photosensor were placed at a depth F from the main lens 510. In order to achieve this, the generalized light field camera's optical focus is set such that the separation between the main lens 510 and the microlens array is given by




  
    
      
        
          F
          opt
        
        =
        
          F
          +
          
            
              β
              
                (
                
                  β
                  -
                  1
                
                )
              
            
            ⁢
            
              f
              .
            
          
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        6
      
    
  




The maximum-resolution output image is computed by digital refocusing to compute a virtual image in which the virtual separation between the main lens 510 and imaging plane is F.

For the range of β values described with FIGS. 2A, 3A and 4A (0<β≦1), Fopt<F, meaning that the microlens plane is brought closer to the main lens to optically focus further in the world. The optical mis-focus is the difference between Fopt and F, given by




  
    
      
        
          β
          
            (
            
              β
              -
              1
            
            )
          
        
        ⁢
        
          f
          .
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        7
      
    
  




As β approaches 1, the optical mis-focus asymptotes to negative infinity, as the slope of the sampling grid cells becomes too horizontal, and the optimal resolution is dominated by the vertical columns of the sampling pattern set by the resolution of the microlens array, not by the slope of the cells within each column. When this occurs, the optical focal depth is set at the desired focal depth (i.e., Fopt=F), to provide the greatest latitude in refocusing about that center. In some applications, Equation 6 is no longer used once the effective resolution for the β configuration falls to less than twice the resolution of the microlens array.
As discussed above, the predicted effective resolution Meffective×Meffective of the output images is

Meffective=max((1−β)Msensor, Mlenslets). (repeating Equation 1)

As with Fopt, the predicted value for Meffective derives from an analysis of the ray-space sampling pattern. Refocusing aligns the imaging projection lines with the slope of the grid cells, facilitating the extraction of higher spatial resolution in the sheared sampling pattern. With this approach, the effective resolution of the computed image is equal to the number of grid cells that intersect the x axis. Within each microscopic light field, Equation 2 shows that the number of grid cells crossed is proportional to (1−β), because of the shearing of the microscopic light field sampling patterns. Hence, the overall resolution is proportional to (1−β). The maximum possible resolution is the resolution of the sensor, and the minimum is the resolution of the microlens array.

In some embodiments, when recording light fields with intermediate β, an auto-focus sensor is used to indicate the depth of the subject. If the auto-focus sensor indicates that the subject is at depth F, the sensor is positioned at Fopt as described above. Digital refocusing onto depth F after exposure is used to produce an image with a large (e.g., maximum) possible effective resolution of max((1−β)Msensor,Mlenslets).
In connection with various embodiments discussed above involving refocusing, output images are anti-aliased by combining samples from multiple spatial locations. In this regard, images with non-zero optical mis-focus are used to produce higher-quality images (e.g., relative to images with zero mis-focus), as facilitated by positioning photosensors relative to microlenses in a mis-focused position.
The relative positioning of microlenses and photosensors is chosen to facilitate the particular application, desirable spatial resolution and directional resolution. In one embodiment, the separation between microlenses and photosensors is set to about one half of the focal length of the microlenses to facilitate spatial resolution with all but about 2×2 of the full photosensor resolution. For intermediate separations, the spatial resolution varies continuously between the resolution of the microlens array and that of the photosensor. The refocusing power decreases roughly in proportion to the increase in spatial resolution, and in some instances, any loss in directional resolution as the separation between microlenses and photosensors is reduced is well contained within a factor of 2 of an ideal instance.
In one embodiment, a generalized light field camera is adapted for selective variation between a conventional camera with high spatial resolution, and a plenoptic camera with more moderate spatial resolution but greater directional resolution and hence refocusing power. A mechanism separating the microlenses and the photosensor is motorized and controlled via selection match the needs of a particular exposure. For example, a user could choose high spatial resolution and put the camera on a tripod for a landscape photograph, and later choose maximal directional resolution to maximize the chance of accurately focusing an action shot in low light.
FIGS. 6A and 6B respectively show perspective “exploded” and cross-sectional views of a microlens array and circuit arrangement 600 for computing images, with a microlens-to-photosensor distance actuator 652, according to another example embodiment of the present invention. The arrangement 600 may, for example, be implemented in a conventional camera, a specialized camera, a microscope arrangement or other video capture device.
Referring to the exploded view in FIG. 6A, the arrangement 600 includes a digital back 610 that supports a chip package 620 including a photosensor array 630. A base plate 640 mounts to the photosensor 630. A microlens array 660 held by a lens holder 650 is coupled to the base plate 640, with separation springs 642 separating the base plate and lens holder. An actuator arrangement 652 implemented in three locations, controls the relative positioning and separation of the microlens array 660 and the photosensor array 630.
In this example, the resolution of the photosensor array 630 is 4096×4096, and the resolution of the microlens array is 296×296 (providing about 14×14 sensor pixels under each microlens). The separation between an active surface 662 of the microlens array and the photosensor array 630 is shown at 0.5 mm by way of example, as positioned by the actuator arrangement 652.
The actuator arrangement 652 includes one or more of a variety of arrangements, and may be implemented with various arrangements (in position, type and otherwise) in replacement of that shown in FIGS. 6A and 6B. One application involves a screw-type fastener with threads that control the separation of the microlens array 660 and photosensor array 630, with the fastener adjusted manually and/or via mechanical actuation. Other arrangements include those similar to those used in other camera-type motors.
For general information regarding imaging approaches and for specific information regarding imaging approaches that may be selectively implemented in connection with one or more various example embodiments described herein, such as for focusing and/or correcting for lens aberration, reference may be made to PCT Patent Application No. PCT/US2005/035189, entitled “Imaging Arrangements and Methods Therefor,” naming inventors Yi-Ren Ng, Patrick Hanrahan, Marc Levoy, and Mark Horowitz and filed on Sep. 30, 2005, which is fully incorporated herein by reference.
The various embodiments described above are provided by way of illustration only and should not be construed to limit the invention. Based on the above discussion and illustrations, those skilled in the art will readily recognize that various modifications and changes may be made to the present invention without strictly following the exemplary embodiments and applications illustrated and described herein. For instance, such changes may include implementing the various optical imaging applications and devices in different types of applications, increasing or decreasing the number of rays collected per pixel (or other selected image area), or implementing different algorithms and/or equations than the examples described to assemble or otherwise process image data. Other changes may involve using coordinate representations other than or in addition to Cartesian coordinates, such as polar coordinates, and/or using various other weighting and other schemes to facilitate the reduction or elimination of aberrations. Such modifications and changes do not depart from the true spirit and scope of the present invention.

## Claims

1. A digital imaging system comprising:
an optics arrangement, including a main lens, and a microlens array at a two dimensional focal plane of the main lens;
a photosensor array comprising a plurality of photosensors, wherein the optic arrangement directs light rays from a scene to be sensed by the photosensor array, and wherein the photosensor array outputs light data characterizing the sensed light rays; and
a control arrangement configurable to set a distance between the photosensor array and the microlens array, wherein at a first position the distance is approximately zero, and a spatial resolution of an image of the scene captured by the digital imaging system approaches a maximum achievable value, and wherein at a second position the distance is approximately one focal length of the microlens array, and the image of the scene captured by the imaging arrangement approaches a maximum achievable directional resolution such that a focal point of the captured image may be changed after the image of the scene is captured.

2. The system of claim 1, wherein the control arrangement is further configurable to set the distance to be any distance between the distance of the first position and the distance of the second position, and wherein distances closer to the distance of the first position result in captured images with relatively greater spatial resolution, relatively less directional resolution and more defocusing of images formed by individual microlenses, while distances closer to the distance of the second position result in captured images with relatively greater directional resolution, relatively less spatial resolution, and more defocusing of the image formed by the optics arrangement.

3. The system of claim 1, wherein the control arrangement includes an actuator and a controller.

4. The system of claim 1, wherein the control arrangement moves the photosensor array to set the relative distance between the photosensor array and the microlens array.

5. The system of claim 1, wherein the control arrangement moves the microlens array to set the relative distance between the photosensor array and the microlens array.

6. The digital imaging system of claim 1, further comprising a processor programmed to generate image data characterizing a synthesized image of the scene as a function of the sensed light rays at different photosensors during image capture, the position of the photosensors relative to the microlens array, and an angle of incidence of the sensed light rays upon the two-dimensional focal plane, wherein the processor is further programmed to operate on the image data to generate an output image of the scene in which at least a portion of the image of the scene is refocused to have a focal point that is not on the two-dimensional focal plane.

7. The system of claim 6, wherein generating image data characterizing the synthesized image of the scene comprises determining for light rays sensed at each photosensor an expression L(x, u) where u is a position on the main lens through which a ray passes, and x is a photosensor location.

8. The system of claim 6, further including an auto-focus sensor to detect a depth of a subject in the scene being captured, wherein the control arrangement positions the photosensor array relative to the microlens array as a function of the depth of the subject, and wherein the processor is programmed to operate on the image data to generate an output image that is focused on the depth of the subject.

9. The system of claim 6, wherein generating the output image of the scene comprises anti-aliasing the output image, including combining light ray samples from multiple spatial locations.

10. The system of claim 9, wherein the control arrangement positions the photosensor array relative to the microlens array at a non-zero mis-focus position of the photosensor array when the light from the scene is sensed by the photosensor array, and wherein light ray samples combined are from multiple spatial locations at the mis-focus position of the photosensor array.

11. The system of claim 10, wherein combining light ray samples comprises computationally resampling the light rays as a function of a ray-tracing approach for determining where the light rays passing into the system terminate on the photosensor array.

12. The system of claim 1, wherein, the distance between the photosensor array and the microlens array is related to selective detection of directional characteristics of the light ray data recorded by the photosensor array corresponding to a selectable trade-off between degrees of spatial resolution and directional resolution in the generated output image.

13. A digital imaging system comprising:
a main lens;
a photosensor array configured to collect a set of light rays via the main lens during the capture of an image, and to output light data characterizing the set of light rays and corresponding to the captured image;
a microlens array, between the main lens and the photosensor array, the microlens array configurable to form a plurality of images on the photosensor array, each of the plurality of images corresponding to an individual microlens and including directional characteristics of light rays passing through the individual microlens; and
a positioning arrangement configurable to set a distance of the plane of the photosensor array relative to the plane of the microlens array, wherein a minimum distance corresponds to a maximum focus of an image formed by the main lens and a minimum focus of the plurality of images formed by the microlens array, while a maximum distance corresponds to a minimum focus of an image formed by the main lens and a maximum focus of the plurality of images formed by the microlens array.

14. The system of claim 13, wherein the minimum distance is about zero and the maximum distance is about equal to one focal length of a microlens in the microlens array.

15. The system of claim 13, wherein the minimum distance is-about zero and the maximum distance is greater than one focal length of a microlens in the microlens array.

16. The digital imaging system of claim 13, further comprising an image data processor to compute synthesized image data using the light data from the image capture, wherein the synthesized image data is computed as a function of the set distance.

17. An imaging method, comprising:
collecting image data, wherein collecting comprises,
setting a selectable distance between a photosensor array and a microlens array in an imaging device, wherein the photosensor array and the microlens array are on an opposite side of a main lens from a scene to be captured by the imaging device, wherein at a minimum selectable distance the image data has optimum spatial resolution, and as the selectable distance increases, the spatial resolution of the image data decreases while directional resolution of the image data increases; and
collecting light field data during a scene capture using the imaging device, the light field data comprising a plurality of light rays originating from the scene, wherein, for light rays passing through a particular portion of a two dimensional focal plane, a plurality of photosensors in the photosensor array are arranged to sense different ones of the light rays that concurrently arrive at the particular portion of the two-dimensional focal plane at different angles of incidence, and to output light field data characterizing the sensed light rays.

18. The method of claim 17, further comprising:
receiving the light field data in a processing circuit; and
processing the light field data for generating a synthesized image of the scene, wherein processing comprises changing a focus of the synthesized image from the focal point of the two-dimensional focal plane when the image data has sufficient directional resolution as determined by the selectable distance.

19. The method of claim 17, further comprising:
receiving the light field data in a processing circuit; and
processing the light field data for generating a synthesized image of the scene that is focused at the focal point of the two-dimensional plane.

20. The method of claim 17, wherein processing the light field data further comprises determining, for light rays sensed at each photosensor, an expression L(x, u) where u is a position on the main lens through which a light ray passes, and x is a photosensor location.

21. The method of claim 17, wherein processing the light field data further comprises anti-aliasing the output image, including combining light ray samples from multiple spatial locations.

22. The method of claim 21, further comprising setting the selectable distance to position the photosensor array relative to the microlens array at a non-zero mis-focus position of the photosensor array when the light rays from the scene are sensed by the photosensor array, and wherein the light ray samples combined are from multiple spatial locations at the mis-focus position of the photosensor.

23. The method of claim 22, wherein combining light ray samples comprises computationally resampling the light rays as a function of a ray-tracing approach for determining where the light rays passing into the imaging device terminate on the photosensor array.

24. The method of claim 17, wherein, setting the selectable distance between the photosensor array and the microlens array facilitates selective detection of directional characteristics of the light rays data recorded by the photosensor array corresponding to a selectable trade-off between degrees of spatial resolution and directional resolution in the synthesized image.