# Patent

## Title
System and methods for calibration of an array camera

## Abstract

Systems and methods for calibrating an array camera are disclosed. Systems and methods for calibrating an array camera in accordance with embodiments of this invention include the detecting of defects in the imaging components of the array camera and determining whether the detected defects may be tolerated by image processing algorithms. The calibration process also determines translation information between imaging components in the array camera for use in merging the image data from the various imaging components during image processing. Furthermore, the calibration process may include a process to improve photometric uniformity in the imaging components.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
The current application is a continuation of U.S. application Ser. No. 14/203,308 entitled “System and Methods for Calibration of an Array Camera”, filed Mar. 10, 2014, which application claims priority to U.S. Provisional Patent Application No. 61/780,748 entitled “System and Methods for Calibration of an Array Camera,” filed Mar. 13, 2013, the disclosures of which are incorporated herein by reference.


FIELD OF THE INVENTION
The present invention relates to calibration of an array camera. More particularly, this invention relates to systems and methods for calibrating an array camera to detect defects in individual imaging components of the array camera and to gather information that can be utilized to perform image processing with image data from the imaging components in an array camera.
BACKGROUND
An array camera is a camera that is made up of multiple imaging components. Each individual imaging component captures data for a two dimensional image of a view. For purposes of this discussion, an imaging component is an individual camera and/or circuitry including an array of pixels that capture data for a two dimensional image of the view. The data of the two-dimensional images can be used to produce super-resolution (SR) images and other types of images of the view using some or all of the data from the various two dimensional images captured by the individual imaging components.
During the manufacture of most digital cameras (including array cameras) and each imaging component of an array camera, a calibration process is typically performed. In a conventional camera or imaging component, the calibration process typically measures a number of characteristics of the imaging component that must be quantified to ensure effective image processing. The calibration of an array camera requires specialized calibration processes to quantify certain characteristics unique to an array camera and not particularly common in conventional digital cameras or imaging components, such as, the uniformity of various intrinsic characteristics among the elements of the array and the extrinsic geometric relationships between the elements of the array.
One aspect of an array camera that is different from conventional cameras is that some types of defects in pixels of an individual imaging component may be compensated for during image processing by using information collected from another imaging component in the array. Thus, an array camera is more tolerant of defects in the individual imaging components.
SUMMARY OF THE INVENTION
Systems and methods in accordance with embodiments of this invention enable calibration of array cameras. In accordance with embodiments of this invention, one or more of the imaging components of the array camera are designated as a reference imaging component and each of the remaining imaging components in the array camera is an associate imaging component. Each of the associate imaging components is associated with at least one of the reference imaging components. In accordance with embodiments of the invention, the calibration of an array camera includes performing a defect detection process and a geometric calibration process. The defect detection process identifies pixel defects in each of the imaging components in the array camera and determines whether the identified pixel defects in a particular imaging component may be corrected in image processing using image data from at least one other one of the plurality of imaging components. The geometric calibration process determines translation information that relates image data from particular pixels in a reference imaging component to image data from corresponding pixels in each associate imaging component associated with the reference imaging component.
In accordance with some embodiments of the invention, the calibration process also includes a Modulation Transfer Function (MTF) calibration process that is performed on each of the imaging components in the array camera to determine MTF data over the Field of View (FoV) or the Field Height (FH) for each of the imaging components. The MTF calibration process includes capturing image data of a target with each of the imaging components and analyzing the image data captured by each of imaging components to determine MTF data over one of a FoV and FH for each of the imaging components in accordance with a number of embodiments. The MTF data for each of the imaging components is determined from the MTF detected in the image data and is stored in memory.
In accordance with many embodiments of the invention, the calibration process also includes a photometric calibration process performed for each of the plurality of imaging components in the array camera. The photometric calibration process determining image data adjustments for each of the imaging components to provide image data from each of the imaging components with a uniform intensity profile. In accordance with a number of embodiments, the photometric calibration is performed in the following manner. Image data of a flat field target under controlled lighting conditions is captured with each of the imaging components in the array camera. The captured image data from each of the imaging components is analyzed to determine a per pixel adjustment needed to match cos ^4 fall-off response for an image and a grid of adjustments is generated for each of the imaging components. Each adjustment in the grid indicates the adjustment information for data from a set of pixels in a particular imaging component needed to match the cos ^4 fall-off response for an image.
In accordance with some embodiments of the invention, the defects in each of the imaging components are identified in the following manner. A first set of image data of a field target under high intensity lighting is captured with each of the imaging components. A second set of image data of a field target under low intensity lighting is also captured with each of the imaging components. For each imaging component, the first and second sets of image data are analyzed to identify each pixel having an unexpected response value. The identified pixels are flagged with a category indicating the category unexpected response received and the identified pixels and flagged categories of the pixels are stored. In accordance with a number of embodiments, the detecting of defects in each imaging component further determining the number of each category of defects identified in each imaging component, comparing the number of each category of defects in each imaging component to a threshold, and generating a disposition output for a particular imaging component in response to the number of defects in a category for the particular imaging component being greater than the threshold.
In accordance with some embodiments, the determining of whether a defect may be corrected during image processing is performed in the following manner. A defect a first imaging component in the array camera is selected and a parallax translation path for the defect over a range of supported distances is determined. A parallax translation path over the range of supported distances from each defect in other imaging components of the array camera is also determined. A determination is made as to whether a predetermined number of parallax translation paths of defects in other imaging components intersect the parallax translation path of the selected defect. A disposition output for the first imaging component is generated if a predetermined number of parallax translation paths for defects from other images components intersect the parallax translation path of the selected defect.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 conceptually illustrates an array camera in accordance with an embodiment of this invention.
FIG. 2 illustrates a flow diagram of a calibration process for an array camera in accordance with embodiments of this invention.
FIG. 3 illustrates a flow diagram of a defect detection process performed during a calibration process of an array camera in accordance with embodiments of this invention.
FIG. 4 illustrates a flow diagram of a defect tolerance determination process performed during a calibration of an array camera to determine whether certain defects in imaging components of the array camera may be handled during image processing in accordance with embodiments of this invention.
FIG. 5 illustrates a flow diagram of a photometric calibration process for imaging components performed during a calibration process for an array camera in accordance with embodiments of this invention.
FIG. 6 illustrates a flow diagram of a geometric calibration process performed during a calibration process to gather information relating pixel information of associate and reference imaging components in the array camera in accordance with embodiments of the invention.
FIG. 7 illustrates a flow diagram of an MTF calibration process performed during a calibration process to obtain MTF information for each imaging component of an array camera in accordance with embodiments of this invention.



DETAILED DESCRIPTION
Turning now to the drawings, systems and methods for performing a calibration process for an array camera to detect potential defects in the imaging components and to gather information that can be utilized during image processing in accordance with embodiments of the invention are illustrated. In accordance with many embodiments of the invention, at least one of the imaging components in the array camera is designated as a “master” or “reference” imaging component; and each of the remaining imaging components in the array is an associate imaging component. Each associate imaging component is associated with at least one of the reference imaging components. During use, the reference imaging component can be utilized to capture a reference image and associate imaging components can capture alternate view images that can be utilized in combination with the reference image to synthesize a higher resolution image from the viewpoint of the reference imaging component. In several embodiments, a calibration process is performed to determine relevant information about each imaging component including (but not limited to) colorimetric, MTF, and photometric information. A calibration process may also determine geometric information that relates image data collected by the imaging components to the image data collected by other imaging components to assist in integrating the image data during image processing. The calibration process may also identify defects in particular imaging arrays and gather information that may be used to correct the identified defects during image processing.
Capturing Light Field Image Data
A light field, which is often defined as a 4D function characterizing the light from all direction at all points in a scene, can be interpreted as a two-dimensional (2D) collection of data from 2D images of a scene. Array cameras, such as those described in U.S. patent application Ser. No. 12/935,504 entitled “Capturing and Processing of Images using Monolithic Array camera with Heterogeneous Imagers” to Venkataraman et al., can be utilized to capture light field images. In a number of embodiments, super-resolution processes such as those described in U.S. patent application Ser. No. 12/967,807 entitled “Systems and Methods for Synthesizing High Resolution Images Using Super-Resolution Processes” to Lelescu et al., are utilized to synthesize a higher resolution 2D image or a stereo pair of higher resolution 2D images from the lower resolution image data in the light field captured by the imaging components in an array camera. The terms high or higher resolution and low or lower resolution are used here in a relative sense and not to indicate the specific resolutions of the images captured by the array camera. The super-resolution processing techniques described in U.S. patent application Ser. No. 12/967,807 can utilize a variety of calibration information including scene independent geometric corrections related to the distortions introduced by the construction of the imaging components utilized to capture the lower resolution image data. Scene independent distortions introduced by the imaging components of an array camera can limit the fidelity of a higher resolution image generated by super-resolution imaging due to uncertainty concerning the relevant location within the scene of a piece of image data. By measuring the distortions during calibration, the uncertainty introduced by the distortions can be reduced, improving the performance of the super-resolution process. In a similar manner, colorimetric, MTF, and photometric variations between similar imaging components in a array camera can complicate image processing by increasing the difficulty of identifying similar points within a scene in the image data captured by the imaging components. The disclosures of U.S. patent application Ser. No. 12/935,504 and U.S. patent application Ser. No. 12/967,807 are hereby incorporated by reference in their entirety.
Each two-dimensional (2D) image in a captured light field is from the viewpoint of one of the imaging components in the array camera. A high resolution image synthesized using super-resolution processing is synthesized from a specific viewpoint that can be referred to as a reference viewpoint. The reference viewpoint can be from the viewpoint of one of the imaging components in an array camera. Alternatively, the reference viewpoint can be an arbitrary virtual viewpoint.
Due to the different viewpoint of each of the imaging components, parallax results in variations in the position of foreground objects within the image data of the scene from the various imaging components of the array camera. Processes for performing parallax detection are discussed in U.S. Provisional Patent Application Ser. No. 61/691,666 entitled “Systems and Methods for Parallax Detection and Correction in Images Captured Using Array Cameras” to Venkataraman et al., the disclosure of which is incorporated by reference herein in its entirety. As is disclosed in U.S. Provisional Patent Application Ser. No. 61/691,666, a depth map from a reference viewpoint can be generated by determining the disparity between the pixels in the images within a light field due to parallax. A depth map indicates the distance of the surfaces of scene objects from a reference viewpoint. In a number of embodiments, the computational complexity of generating depth maps is reduced by generating an initial low resolution depth map and then increasing the resolution of the depth map in regions where additional depth information is desirable such as (but not limited to) regions involving depth transitions and/or regions containing pixels that are occluded in one or more images within the light field.
During super-resolution processing, a depth map can be utilized in a variety of ways. U.S. patent application Ser. No. 12/967,807 describes how a depth map can be utilized during super-resolution processing to dynamically refocus a synthesized image to blur the synthesized image to make portions of the scene that do not lie on the focal plane to appear out of focus. U.S. patent application Ser. No. 12/967,807 also describes how a depth map can be utilized during super-resolution processing to generate a stereo pair of higher resolution images for use in 3D applications. A depth map can also be utilized to synthesize a high resolution image from one or more virtual viewpoints. In this way, a rendering device can simulate motion parallax and dolly zoom. In addition to utilizing a depth map during super-resolution processing, a depth map can be utilized in a variety of post processing processes to achieve effects including (but not limited to) dynamic refocusing, generation of stereo pairs, and generation of virtual viewpoints without performing super-resolution processing.
Array Camera Architecture
Array cameras in accordance with embodiments of the invention are configured so that the array camera software can control the capture of light field image data and can capture the light field image data into a file that can be used to render one or more images on any of a variety of appropriately configured rendering devices. An array camera including an array of imaging components in accordance with an embodiment of the invention is illustrated in FIG. 1. The array camera 100 includes an array camera module 102, a processor 108, a memory 110 and a display 112. The array camera module 102 includes an array of imaging components 104 formed by a sensor and a lens stack array and the array camera module 102 is configured to communicate with a processor 108. The processor 108 is also configured to communicate with one or more different types of memory 110 that can be utilized to store image data and/or contain machine readable instructions utilized to configure the processor to perform processes including (but not limited to) the various processes described below. The processor 108 may also be connected to a network via a network connection 106 to communicate with other processing systems connected to the network. The display 112 can be utilized by the processor 108 to present a user interface to a user and to display an image rendered using the light field image data. Although the processor is illustrated as a single processor, array cameras in accordance with embodiments of the invention can utilize a single processor or multiple processors including (but not limited to) a graphics processing unit (GPU).
In the illustrated embodiment, the processor 108 receives image data generated by the array camera module 102 and reconstructs the light field captured by the array camera module 102 from the image data. Sensors including multiple focal planes that can be utilized in the construction of array camera modules are discussed in U.S. patent application Ser. No. 13/106,797 entitled “Architectures for System on Chip Array Cameras”, to Pain et al., the disclosure of which is incorporated herein by reference in its entirety. The processor 108 can manipulate the light field in any of a variety of different ways including (but not limited to) determining the depth and visibility of the pixels in the light field and synthesizing higher resolution 2D images from the image data of the light field.
In the illustrated embodiment, the imaging components 104 are configured in a 5×5 array. Each imaging component 104 in the array camera module 102 is capable of capturing an image of the view. The sensor elements utilized in the imaging components 104 can be individual light sensing elements such as, but not limited to, traditional CIS (CMOS Image Sensor) pixels, CCD (charge-coupled device) pixels, high dynamic range sensor elements, multispectral sensor elements and/or any other structure configured to generate an electrical signal indicative of light incident on the structure. In many embodiments, the sensor elements of the imaging components 104 have similar physical properties and receive light via the same optical channel and color filter (where present). In other embodiments, the sensor elements of the imaging components 104 have different characteristics and, in many instances, the characteristics of the sensor elements are related to the color filter applied to each sensor element. In several embodiments, the sensor elements of an imaging component includes a plurality of rows of pixels that also form a plurality of columns of pixels and the pixels of each imaging component are contained within a region of the sensor that does not contain pixels from another imaging component.
In many embodiments, an array of images (i.e. a light field) is created using the image data captured by the imaging components 104. As noted above, the processor 108 in accordance with many embodiments of the invention is configured using appropriate software to take the image data within the light field and synthesize one or more high resolution images. In several embodiments, the high resolution image is synthesized from a reference viewpoint, typically the viewpoint of a reference imaging component 104 within the array camera module 102. In many embodiments, the processor is able to synthesize an image from a virtual viewpoint, which does not correspond to the viewpoints of any of the imaging components 104 in the array camera module 102. Unless all of the objects within a captured scene are a significant distance from the array camera, the images in the light field will include disparity due to the different fields of view of the focal planes used to capture the images. Processes for detecting and correcting for disparity when performing super-resolution processing in accordance with embodiments of the invention are discussed in U.S. Provisional Patent Application Ser. No. 61/691,666 (incorporated by reference above). The detected disparity can be utilized to generate a depth map. The high resolution image and depth map can be encoded and stored in memory 110 in a light field image file. The processor 108 can use the light field image file to render one or more high resolution images. The processor 108 can also coordinate the sharing of the light field image file with other devices (e.g. via a network connection), which can use the light field image file to render one or more high resolution images.
Although a specific array camera architecture is illustrated in FIG. 1, systems and methods for performing calibration of an array camera in accordance with embodiments of the invention may be performed on any array camera configuration appropriate to the requirements of a specific application. Systems and methods for performing a calibration process for an array camera in accordance with embodiments of the invention are discussed below.
Calibration Process for an Array Camera
An array camera has features beyond those in a conventional camera that are characterized during a calibration process to assist super-resolution processing algorithms to produce images of a desired resolution. During a manufacture process and/or periodically during the life of an array camera, a calibration process may be performed to generate or update information that can be utilized to perform the super-resolution processing algorithms. A calibration process to collect the calibration information in accordance with embodiments of this invention is illustrated in FIG. 2.
A Calibration process (200) can involve performing a defect detection process (205), a photometric calibration process (210), a geometric calibration process (215), and/or a MTF calibration process (220). One skilled in the art will recognize that a calibration process may include other processes for detecting other types of defects and/or to collect information about other aspects of the array camera and the underlying imaging components if other types of information are needed to perform the image processing algorithms without departing from this invention. The defect detection process (205) is performed to identify pixels in each of the imaging components that have defects and to determine how information from a defective pixel is to be handled during image processing. A defect detection process in accordance with some embodiments of this invention is described below with reference to FIGS. 3 and 4. The photometric calibration process (210) determines information used during image processing to adjust the data of a view captured by each of the imaging components so that the data of the view received from each of the imaging components has a uniform intensity profile. A photometric process performed in accordance with some embodiments of this invention is discussed below with reference to FIG. 5. The geometric calibration process (215) determines the information used to perform a translation to align image data from pixels in each associate imaging component with image data from corresponding pixels in a reference imaging component. A geometric calibration process performed in accordance with some embodiments of this invention is described below with reference to FIG. 6. The MTF calibration process (220) is performed to characterize the “sharpness” of the optics over the FoV of each imaging component in order to adjust the data from each of the imaging components to cause the image data from the various imaging components to have an essentially uniform MTF. A MTF calibration process performed in accordance with some embodiments of this invention is described below with reference to FIG. 7.
Furthermore, calibration process 200 and/or each of the individual calibration/error detection processes included in process 200 may be performed one or more times at each of several predetermined temperatures and the results may be aggregated into calibration information that is indexed by temperature in accordance with a number of embodiments of this invention. In accordance with these embodiments, the temperature of the environment and/or the array camera being tested at the time of an iteration of the process 200 is optionally determined (202). The temperature may be measured by a connected device such as, but not limited to, a dark sensor or may be input by an operator depending on the embodiment. The determined temperature is associated with the information generated during the iteration. Furthermore, process 200 may determine whether additional iterations need to be performed at other temperatures (225). If so, process 200 is repeated after the temperature of a test environment is adjusted. Otherwise process 200 ends.
Defect Detection
A defect detection process is a process that detects defects in individual pixel sensors of the imaging components in the array camera and then determines how data from the identified defective pixel sensors is to be handled. Like conventional digital imagers, the imaging components in an array camera often have typical pixel sensor defects including, but not limited to, cold pixels, cool pixels, warm pixels, hot pixels, column failures, row failures, non-uniformities, and random telegraph pixel failures. However, conventional digital cameras typically have set limits as to the number of defects in each class that can be tolerated in an imager. These limits are often zero or near zero when the failures are in a critical area of the pixel array (i.e. in the center of the pixel array). An array camera, on the other hand, may have a higher tolerance to these defects in individual imaging components because an image from an array camera is synthesized from image data received from one or more imaging components. Thus, image processing algorithms may be able to use image data from another imaging component to provide data about the image in the area covered by the image data from the defective pixel sensor in one of the imaging components. Because of this, a calibration process for an array camera should identify the defective pixels in each of the imaging components and determine if the identified defects can be tolerated during image processing. A process for identifying pixel defects in each of the imaging components of an array camera in accordance with an embodiment of the invention is illustrated in FIG. 3 and a process for determining whether certain types of defects in a particular imaging component may be tolerated during image processing in accordance with some embodiments of this invention is illustrated in FIG. 4.
To identify pixel defects, process (300) captures at least two images of a flat field target with the array camera under controlled lighting. The capturing of the image with the array camera captures image data for each of the imaging components in the array. A first one of the images is captured under high intensity lighting (305) and a second one of the images is captured under low intensity lighting (310). The detection of defective pixels may be performed in the following manner based on image data from the images captured. An imaging component is selected (315). The image data from the high intensity image (305) and low intensity image (310) captured by the selected imaging component is analyzed (320). The analyzing may include, but is not limited to, determining whether the value of a particular pixel is in agreement with the values of adjacent pixels or within an acceptable standard deviation from the values of the adjacent pixels; determining the value of the pixel is within a predetermined range or with an acceptable deviation from the range; and/or detecting patterns of values within an array of pixels that are indicative of a defect. From the analysis, pixels having an unexpected response value are identified (325). The following are examples of the categories of unexpected values that may be flagged in the identified pixels:











TYPE OF DEFECT
DEFECT VALUE







Cold Pixel
values below a Cold Pixel Threshold, such as 10


Hot Pixel
values above a Hot Pixel Threshold, such as 245


Cool Pixel
values below the Cool Pixel Threshold from a



nominal neighborhood value


Warm Pixel
values above a Warm Pixel Threshold from the



nominal neighborhood value


Cluster Fault
values outside the Cluster Fault threshold from


Threshold
the nominal neighborhood value


A2D fault
one or value bits stuck at “0” or “1”


Row fault
special type of cluster fault


Column Fault
special type of cluster fault









The identified pixels are stored and are flagged into categories (330) based on the type of unexpected response values received. The defective pixel location and indication of the category defect are stored for future use by the imaging processing of the array camera. The flagging and storing of the defects may be performed by generating a sorted list of identified defective pixels including a value indicating the type of defect of the pixel, such as the values shown above. The list is then searched for adjacent pixels having similar error values to identify clusters. In accordance with some embodiments, the search may be performed using two nested loops with two escape clauses. The defect values for the identified pixels in the detected cluster faults are then changed to reflect the appropriate cluster faults.
After all of the defects have been identified, the number of identified pixels having each particular type of defect is calculated and stored (335). The list of defective pixels may also be stored as defect data and any number of different data techniques may be used to reduce the size of the stored defect data. The number of each type of defect is then compared to a threshold. The threshold may be a failure limit for a particular type of error that may be determined by a manufacturer or designer. If the number of a particular type is greater than the threshold for the type (340), the camera is set for a particular disposition (345). For example, the camera may be set to “fail”, “fail with too many defects”, and/or “fail with logic faults”. The disposition may then be used in the manufacturing process to correct the defects in the particular imaging components or determine further processing steps in the manufacturing process for the array camera.
However, as discussed above, the image processing algorithms used in an array camera may be used to compensate for some types of defects in a particular imaging component. Thus, a tolerance test is performed for the identified defects in the selected imaging component (350). A tolerance test in accordance with some embodiments of this invention is discussed below with reference to FIG. 4. Process (300) is then repeated for each of the remaining imaging components until the images for all of the imaging components have been analyzed (355).
Unlike conventional digital camera, certain pixel defects in individual imaging components including, but not limited to, cluster defects may be tolerated in an array camera because other imaging components may provide the same information as the defective pixel and or cluster of pixels. A process for determining whether particular defects in an imaging component may be tolerated during the image processing in the array camera in accordance with embodiments of this invention is illustrated in FIG. 4. An identified defect is selected (405) from the imaging component being analyzed. Typically, the defects that are selected are limited to certain types of defects. For example, the defects that may be selected may be limited to cluster defects having a size less than or greater than a predetermined threshold.
Another imaging component is also selected (410). An identified defect from the selected imaging component is then chosen (415). The process then determines whether the selected defect (415) is within a programmable distance from the selected defect (405) from the imaging component under consideration. If the defects (405) and (415) from the different imaging components are within a programmable distance, the process determines a parallax translation path over a range of supported imaging distances for each of the defects (420). The paths are compared to determine whether the paths intersect (425). If the paths intersect, the process stores the intersection and distance (430). A defect intersection counter is then incremented and stored (435).
The defect intersection counter is then compared to a threshold (440). If the defect intersection counter is greater than the threshold, the camera is set for a particular disposition (445). This is to determine whether there is image data for the area of the image covered by the image data from the defect under consideration to still generate image information for that area. Alternatively, if there is not enough image data for the area, the camera is set to a particular disposition to handle the defect through conventional means. The process continues (450) until all the defects in the selected imaging component (410) are analyzed. After all defects in the selected imaging component (410) are compared, the process repeats (455) with respect to each of the other imaging components in the array camera. After the selected defect (405) has been compared to the defects in all of the other imaging components, the process is repeated (460) from (405) for each of the other selectable defects of the imaging component under consideration. Although specific processes for performing defect detection and for assessing the significance of defects are described above, any of a variety of processes can be utilized to detect defects and to determine whether patterns of defects are likely to impact images synthesized using image data captured by an array camera in accordance with embodiments of the invention.
Photometric Calibration
In accordance with many embodiments of this invention, a photometric calibration process is performed on the array camera. The photometric calibration is performed to adjust the image data captured by the different imaging components to have a uniform intensity profile so that corresponding pixels in the different imaging components have substantially equal values modulo the spectral band being imaged when imaging the same object in physical space. A photometric calibration in accordance with an embodiment of this invention is illustrated in FIG. 5.
Photometric calibration process (500) captures an image of a flat field target with the array camera (505). The capturing of the image with the array camera generates image data of the flat field target for each of the imaging components in the array camera. The process then does a photometric calibration process for each of the imaging components.
The photometric process for imaging components begins by retrieving the image data for a selected imaging component (510). The image data for the selected imaging component is analyzed (515). The analysis is performed by viewing the image data as an intensity surface. If the image data were ideal, the image data would have a cos ^4 fall-off response across the Field of View. The image data is analyzed to determine the per-pixel adjustment that is needed to change the intensity of each pixel to match a profile corresponding to the cos ^4 fall-off that would be associated with a pixel that varies from zero (0) at the optical center to some value, typically about 15 percent less, at a predetermined Field Height (520). This may be performed by detecting a horizontal line in the image data and performing line filtering on the detected horizontal line, and fitting a cos ^4 profile to the data. This is repeated with horizontal lines throughout the image data. In other embodiments, any of a variety of line fitting techniques can be utilized as appropriate to the requirements of specific applications.
As it is common that the variation across the Field of View of any imaging device is low frequency, the process may generate a grid representing adjustments of the data from the pixels in a step-size manner (525). The grid includes the multipliers that translate the fitted curves to the desired curves from line data in a particular area of the image data. The grid data is then stored in an appropriate file or data structure for the particular imaging component (530-535). The grid data in the file or data structure is then used in the processing pipeline to adjust the intensity profiles in the image data received from the particular imaging component. The process (500) is then repeated (540) for each of the remaining imaging components in the array camera. In accordance with a number of embodiments, a photometric calibration process may include steps that balance the response between imaging components in the same spectral bands; adjust a maximum, a minimum, average, and/or other parameter of a group; and other steps that may be needed to compensate for errors that affect the photometrics of an array camera.
Although specific processes for performing photometric calibration are described above with reference to FIG. 5, any of a variety of processes can be utilized to determine photometric calibration data for use in the photometric calibration of image data captured by array cameras in accordance with embodiments of the invention. Processes for performing geometric calibration of array cameras in accordance with embodiments of the invention are discussed further below.
Geometric Calibration
During the calibration process of an array camera, the translation information relating a position of a pixel in a reference imaging component to a position of a corresponding pixel in an associated imaging component can be determined in order to assist in fusing merge the image data captured by the various imaging components. This process is referred to as geometric calibration of the imaging components. A process for performing a geometric calibration of the reference and associate imaging components of an array camera in accordance with an embodiment of this invention is illustrated in FIG. 6.
In process (600), an image of a single target is captured with an array camera (605). The capturing of the image with the array camera involves capturing image data of the single target from each of the imaging components in the array camera. The single target image may include a set of easily identified control points. In accordance with some embodiments, the target may include a low contrast slanted edge pattern such as a checkerboard pattern. The target should change with a high enough frequency to provide a sufficient number of control points to identify an expected level of low-frequency non-uniformities in the optical systems of the imaging components.
For each reference imaging component, the following process may be used to determination the translation information that relates the associate imaging components associated with the reference imaging component. The image data from for a reference imaging component is retrieved (610). The location of the control points in the image data of the reference imaging component are identified (615). The locations of control points are identified to a reasonably fine sub-pixel level, typically to within 0.1 pixel accuracy.
In accordance with some embodiments, distortion information for the captured image optionally is measured (617). The distortion information is then used to modify control point information in the image data to compensate for the measured distortion. The modified control point information may then be used to correct geometric errors in the associate imaging components that are associated with the reference imaging component
The image data for an associate imaging component is then retrieved (620) and the locations of the control points in the image data of the associate imaging component are determined (625) in a similar manner as described above with respect to the reference imaging component (615). In accordance with several embodiments, the distortion in the image data of the associate imaging component is optionally measured (627) in a manner similar to the measurement of the distortion in the image data of the reference imaging component. After the location of control points are identified in the reference image data and the associate image data, the displacements between corresponding points in the reference and associate image data are determined (630). Adjustments may optionally be made to the image data of the reference imaging component to perform certain geometric adjustments to the reference imaging component that normally would be performed during the image processing if not corrected by the translation between the reference and associate imaging components. Such adjustments may be preferable where the basic prescription for a particular imaging device exhibits geometric aberrations such as pincushion and barrel distortion exceeding certain threshold such as 10%. However, this threshold may be as low as 2-3% or even lower in accordance with some embodiments. The displacement data is also adjusted to account for the parallax shift associated with the distance from the array camera to the single target (632). Furthermore, the displacement data may optionally be adjusted to correct for geometric distortions measured in order to increase alignment accuracy between the image data of the reference and associate imaging components in accordance with a number of embodiments.
The displacement information will typically be represented by irregular grid displacement values across the image for the associate imaging component. This may then be used to generate a regular grid of displacement information (640). The grid may be adjustable in a step-size manner to account for frequency variations in the translation information across the image. The translation information in the grid may then be used to generate a fusion data structure (645). The fusion data structure includes a list of offsets along an x axis and a y axis and may be used to merge the image data from all of the image components into a single image structure. The fusion structure essentially maps each pixel of an associate imaging component into an appropriate physical relation to the corresponding pixel in the reference imaging component.
The translation information in the grid may also be used to generate a sampling data structure (650). The sampling data structure includes information that allows the image data from an associate imaging component to be sampled to obtain information that corresponds precisely to the pixel sample points in the reference camera. In other embodiments, any of a variety of data structures and/or techniques can be utilized to store the translation information as appropriate to the requirements of specific applications.
The above described process (620-650) is then repeated for each associate imaging component that is associated with a particular reference component (655). The reference imaging component process (610-655) is then repeated for each reference imaging component in the array camera to obtain the translation information for each associate imaging component associated with the reference imaging component until all the grouping of reference and associate imaging components are processed.
Since the imaging components in the array camera observe a calibration target from different points in space, there is some parallax shift in the images from each of the imaging components. This parallax shift can be calculated from measurements associated with the array camera design and physical relationship between the calibration target and the array camera. The determined parallax shift is accounted for by systematic shifts of the translation data determined as described above in accordance with many embodiments.
Certain optical characteristics of the individual imaging components, principally those items known as camera intrinsics including but not limited to focal length, radial distortions, astigmatism and other geometric distortions, may lead to increased alignment error or a distorted final image. If known, these characteristics may be used to further modify the translation data for each imaging element in such a manner that residual alignment errors are reduced and/or the geometric distortion of the image from the reference imager and, thus, the final output image of the camera are minimized. In a number of embodiments, an appropriate calibration target, for instance, a checkerboard target as described above, is used, along with precise measurement information of the calibration set-up, to provide measurements of the relevant distortion characteristics.
Although specific processes for performing geometric calibration are described above with respect to FIG. 6, any of a variety of processes can be utilized to determine scene independent geometric corrections that can be applied to image data captured by an array camera in accordance with embodiments of the invention. Processes for performing MTF calibration in accordance with embodiments of the invention are discussed further below.
MTF Calibration
The individual imaging components in an array camera often exhibit non-uniformities due to the limits of the current manufacturing technologies and associated manufacturing processes; and variation in the manufacturing materials. Some of these non-uniformities exhibit themselves as variations in the resolving power of the optics over the Field of View (FoV). The generation of high resolution images from a set of low resolution imaging devices in an array camera can rely upon an accurate understanding of, adaptation to, and/or correction of these variations. As such, a MTF calibration process may be performed to characterize the “sharpness” of the optics over the FoV of each of the imaging components in the array camera. The “sharpness” information may then be used to adjust the image data received from each imaging component to provide data from the various imaging components that essentially has a uniform MTF. A process for performing MTF calibration in accordance with embodiments of this invention is show in FIG. 7.
Process 700 includes capturing images of a target at one or more distances with the array camera (705). The capturing of the images generates image data of the target from each of the imaging components for each of the images captured. The target should include one or more slanted edge structures. For purposes of this discussion, a slanted edge structure is a structure with a relatively light area and a relatively dark area with a boundary that is a straight line that may be approximately 6 degrees from vertical. Typically, the distances used to capture the images will range from approximately Hyperfocal (HF)/2 to HF*2. For example, in an array camera having an HF of 40 cm, 10 or more images may be captured between the distances of 10 cm to 2 m. In other embodiments, any of a variety of distances to targets can be utilized as appropriate to the requirements of a specific application.
After the image data is captured, the following is performed for each imaging component (730). The image data of the captured images from a selected imaging component is retrieved (710). The image data for each particular image of the selected imaging component is analyzed to generate MTF data over FoV or Field Height (FH) (715). The analysis may be completed by measuring the MTF at various FoV positions in the image data. The MTF is then plotted as a function of distance for each of the FoV positions. The position that yields a peak value for each FoV position estimates the best focus distance for the selected imaging component and can be included in the MTF information for the imaging component.
A data structure that stores the MTF information for the selected imaging component gathered from the analysis is then generated (720) and stored (725) for use in the image processing pipeline. In other embodiments, any of a variety of data structures and/or techniques can be utilized to store MTF information obtained during calibration as appropriate to the requirements of specific applications.
While the above description contains many specific embodiments of the invention, these should not be construed as limitations on the scope of the invention, but rather as an example of embodiments thereof. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their equivalents.

## Claims

1. A method for calibrating an array camera using a processing system wherein the array camera includes a plurality of imaging components where the plurality of imaging components includes a reference imaging component and a plurality of associate imaging components associated with the reference imaging component, the method comprising:
performing a defect detection process that includes:
identifying pixel defects in each of the plurality imaging components, and
determining whether pixel defects in a particular imaging component may be corrected in image processing using image data from at least one other one of the plurality of imaging components by:
selecting a defect in a first one of the plurality of imaging components in the array camera and determining a parallax translation path of the defect over a range of supported distances,
determining a parallax translation path over the range of supported distances from defects in other ones of the plurality of imaging components of the array camera,
determining whether a predetermined number of parallax translation paths of defects in other ones of the plurality of imaging components intersect the parallax translation path of the selected defect, and
generating a disposition output for the first one of the imaging components having the selected defect if a predetermined number of parallax translation paths for defects from other images components intersect the parallax translation path of the selected defect; and


performing a geometric calibration process that includes determining translation information that relates image data from particular pixels in a reference imaging component to image data from corresponding pixels in each associate imaging component associated with the reference imaging component.

2. The method of claim 1 further comprising performing a Modulation Transfer Function (MTF) calibration process on each of the plurality of imaging components in the array camera that includes determining MTF data over the Field of View (FoV) or the Field Height (FH) for each of the imaging components.

3. The method of claim 2 wherein the MTF calibration process further comprises:
capturing image data of a target with each of the plurality imaging components;
analyzing the image data captured by the plurality of imaging components to determine MTF data over one of a FoV and FH for each of the imaging components wherein the MTF data for each of the imaging components is generated from the MTF detected in the image data; and
storing the MTF data for each of the imaging components.

4. The method of claim 1 further comprising performing a photometric calibration process of each of the plurality of imaging components in the array camera that includes determining image data adjustments for each of the imaging components that provide image data from each of the imaging components with a uniform intensity profile.

5. The method of claim 4 wherein the performing the photometric calibration comprises:
capturing image data of a flat field target under controlled lighting conditions with each of the plurality of imaging components in the array camera;
analyzing the image data of each of the imaging components to determine a per pixel adjustment needed to match cos ^4 fall-off response for an image; and
generating a grid of adjustments for each of the plurality of imaging components wherein each adjustment in the grid indicates the adjustment information for data from a set of pixels in the particular imaging component needed to match the cos ^4 fall-off response for an image.

6. The method of claim 1 wherein the identifying of defects in each of the imaging components comprises:
capturing a first set of image data of a field target under high intensity lighting with each of the plurality of imaging components;
capturing a second set of image data of a field target under low intensity lighting;
analyzing the first and second sets of image data to identify each pixel having an unexpected response value in each imaging component;
flagging the identified pixels with a category indicating the category unexpected response received for each imaging component; and
storing the identified pixels with the flagged categories.

7. The method of claim 6 wherein the detecting of defects in each imaging component further comprises:
determining the number of each category of defects identified in each imaging component;
comparing the number of each category of defects to a threshold; and
generating a disposition output for a particular imaging component in response to the number of defects in a category for the particular imaging component being greater than the threshold.

8. A device for calibrating an array camera including a plurality of imaging components where the plurality of imaging components include a reference imaging component and a plurality of associate imaging components associated with the reference imaging component comprising:
a memory; and
a processor configured via one or more applications stored in the memory to:
perform a defect detection process that includes:
identify pixel defects in each of the plurality imaging components, and
determine whether pixel defects in a particular imaging component may be corrected in image processing by:
selecting a defect in a first one of the plurality of imaging components in the array camera and determining a parallax translation path of the defect over a range of supported distances,
determining a parallax translation path over the range of supported distances from defects in other ones of the plurality of imaging components of the array camera,
determining whether a predetermined number of parallax translation paths of defects in other ones of the plurality of imaging components intersect the parallax translation path of the selected defect, and

generating a disposition output for the first one of the imaging components having the selected defect if a predetermined number of parallax translation paths for defects from other images components intersect the parallax translation path of the selected defect; and

perform a geometric calibration process that includes determining translation information that relates image data from particular pixels in a reference imaging component to image data from corresponding pixels in each associate imaging component associated with the reference imaging component.

9. A non-transitory machine readable medium containing processor instructions, where execution of the instructions by a processor causes the processor to perform a process for calibrating an array camera including a plurality of imaging components where the plurality of imaging components include a reference imaging component and a plurality of associate imaging components associated with the reference imaging component, the process comprising:
performing a defect detection process that includes:
identifying pixel defects in each of the plurality imaging components, and
determining whether pixel defects in a particular imaging component may be corrected in image processing by:
selecting a defect in a first one of the plurality of imaging components in the array camera and determining a parallax translation path of the defect over a range of supported distances,
determining a parallax translation path over the range of supported distances from defects in other ones of the plurality of imaging components of the array camera,
determining whether a predetermined number of parallax translation paths of defects in other ones of the plurality of imaging components intersect the parallax translation path of the selected defect, and

generating a disposition output for the first one of the imaging components having the selected defect if a predetermined number of parallax translation paths for defects from other images components intersect the parallax translation path of the selected defect; and
performing a geometric calibration process that includes determining translation information that relates image data from particular pixels in a reference imaging component to image data from corresponding pixels in each associate imaging component associated with the reference imaging component.