Patent Publication Number: US-9888194-B2

Title: Array camera architecture implementing quantum film image sensors

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to array cameras and more specifically to systems and methods for implementing quantum film image sensors in array cameras. 
     BACKGROUND 
     In capturing an image in a typical camera, light enters through an opening (aperture) at one end of the camera and is directed to a focal plane by an optical array. In most cameras a lens stack including one or more layers of optical elements are placed between the aperture and the focal plane to focus light onto the focal plane. The focal plane consists of light sensitive pixels that generate signals upon receiving light via the optic array. Commonly used light sensitive sensors for use as light sensitive pixels include CCD (charge-coupled device) and CMOS (complementary metal-oxide-semiconductor) sensors. 
     Filters are often employed in the camera to selectively transmit lights of certain wavelengths onto the light sensitive pixels. In conventional cameras a Bayer filter mosaic is often formed on the light sensitive pixels. The Bayer filter is a color filter array that arranges one of the RGB color filters on each of the color pixels. The Bayer filter pattern includes 50% green filters, 25% red filters and 25% blue filters. Since each pixel generates a signal representing strength of a color component in the light and not the full range of colors, demosaicing is performed to interpolate a set of red, green and blue values for each pixel. 
     Cameras are subject to various performance constraints. The performance constraints for cameras include, among others, dynamic range, signal to noise (SNR) ratio and low light sensitivity. The dynamic range is defined as the ratio of the maximum possible signal that can be captured by a pixel to the total noise signal. The maximum possible signal in turn is dependent on the strength of the incident illumination and the duration of exposure (e.g., integration time, and shutter width). The signal to noise ratio (SNR) of a captured image is, to a great extent, a measure of image quality. In general, as more light is captured by the pixel, the higher the SNR. Accordingly, the SNR of a captured image is usually related to the light gathering capability of the pixel. 
     Generally, Bayer filter sensors have low light sensitivity. At low light levels, each pixel&#39;s light gathering capability is constrained by the low signal levels incident upon each pixel. In addition, the color filters over the pixel and the necessity to confine the chief ray angle incident on the pixel to avoid cross-talk further constrain the signal reaching the pixel. IR (Infrared) filters also reduce the photo-response from near-IR signals, which can carry valuable information. These performance constraints are greatly magnified in cameras designed for mobile systems due to the nature of design constraints. Pixels for mobile cameras are typically much smaller than the pixels of digital still cameras (DSC). Due to limits in light gathering ability, reduced SNR, limits in the dynamic range, and reduced sensitivity to low light scenes, the cameras in mobile cameras show poor performance. 
     Quantum dots are semiconductor particles that can take any number of shapes including cubes, spheres, pyramids, etc., and have a size that is comparable to the Bohr radius of the separation of electron and hole (exciton). The electronic characteristics of quantum dots are closely related to the size and shape of the individual crystal. In particular, when the size of the quantum dot is smaller than the exciton Bohr radius, the electrons crowd together leading to the splitting of the original energy levels into smaller ones with smaller gaps between each successive level. Thus, if the size of the quantum dot is small enough that the quantum confinement effects dominate (typically less than 10 nm), the electronic and optical properties change, and the fluorescent wavelength is determined by the particle size. In general, the smaller the size of the quantum dot particle, the larger the band gap, the greater becomes the difference in energy between the highest valence band and the lowest conduction band, therefore more energy is needed to excite the dot, and concurrently, more energy is released when the crystal returns to its resting state. For example, in fluorescent dye applications, this equates to higher frequencies of light emitted after excitation of the dot as the crystal size grows smaller, resulting in a color shift from red to blue in the light emitted. Beneficial to this tuning is that a high level of control over the size of the quantum dot particles produced is possible. As a result, it is possible to have very precise control over the fluorescent properties of quantum dot materials. 
     Quantum dots can be assembled into light sensitive quantum films that can be integrated onto a silicon wafer at the pixel level to replace the silicon photodiode-parts of a CCD or CMOS sensor. These quantum films are significantly more sensitive to light than are standard CMOS sensors that utilize a silicon photodiode. In addition, because the photosensitive volumes of the quantum film is not deep in the structure, as it is in a CMOS image sensor, but at the light sensitive quantum film surface, there is reduced cross talk between neighboring pixels, which relaxes lens CRA requirements. 
     SUMMARY OF THE INVENTION 
     Systems and methods in accordance with embodiments of the invention utilize quantum film imagers in the construction of array cameras. One embodiment of the invention includes: a plurality of focal planes, where each focal plane comprises a plurality of rows of pixels that also form a plurality of columns of pixels and each focal plane is contained within a region of the imager array that does not contain pixels from another focal plane; at least one quantum film located on the surface of the imager array, where each quantum film comprises a plurality of quantum dots; control circuitry configured to control the capture of image information by the pixels within the focal planes, where the control circuitry is configured so that the capture of image information by the pixels in at least two of the focal planes is separately controllable; and sampling circuitry configured to convert pixel outputs into digital pixel data. 
     Another embodiment of the invention includes an imager array including: a plurality of focal planes, where each focal plane comprises a plurality of rows of pixels that also form a plurality of columns of pixels and each focal plane is contained within a region of the imager array that does not contain pixels from another focal plane; at least one quantum film located on the surface of the imager array, where each quantum film comprises a plurality of quantum dots; control circuitry configured to control the capture of image information by the pixels within the focal planes, where the control circuitry is configured so that the capture of image information by the pixels in at least two of the focal planes is separately controllable; and sampling circuitry configured to convert pixel outputs into digital pixel data. In addition, the array camera includes an optic array of lens stacks aligned with respect to the imager array so that an image is formed on each focal plane by a separate lens stack in said optic array of lens stacks. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an array camera in accordance with an embodiment of the invention. 
         FIG. 2  conceptually illustrates an optic array and an imager array in an array camera module in accordance with an embodiment of the invention. 
         FIG. 3A  is a block diagram of an imager array in accordance with an embodiment of the invention. 
         FIG. 3B  is a cross-sectional illustration of a camera channel incorporating a light sensitive quantum film sensor in accordance with an embodiment of the invention. 
         FIG. 4  conceptually illustrates crosstalk between pixels in cameras having: (A) the photodiodes in the Silicon (as in a conventional CMOS), and (B) having the photon to electron conversion in the light sensitive quantum film. 
         FIG. 5  conceptually illustrates chief ray angles for cameras having: (A) a conventional CMOS, and (B) a CMOS where the photodiode is implemented with a light sensitive quantum film. 
         FIG. 6A  is a cross-sectional illustration of a camera channel incorporating vertical optical isolation arrangements in accordance with an embodiment of the invention. 
         FIG. 6B  is a cross-sectional illustration of a camera channel incorporating horizontal optical isolation arrangements in accordance with an embodiment of the invention. 
         FIG. 7  conceptually illustrates a Bayer filter applied at the pixel level of a light sensitive element. 
         FIG. 8  is a plan view of a camera array having spectral filtration applied at the camera level in accordance with an embodiment of the invention. 
         FIG. 9  conceptually illustrates the application of a negative lens element to a focal plane in accordance with an embodiment of the invention. 
         FIG. 10  is a block diagram of an imager array in accordance with an embodiment of the invention. 
         FIG. 11  is a high level circuit diagram of pixel control and readout circuitry for a plurality of focal planes in an imager array in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DISCLOSURE OF THE INVENTION 
     Turning now to the drawings, systems and methods for implementing quantum film imagers on array cameras in accordance with embodiments of the invention are illustrated. Many embodiments relate to using quantum film imagers as image sensors in camera modules of array cameras. 
     Array cameras including camera modules that can be utilized to capture image data from different viewpoints (i.e. light field images) are disclosed in U.S. patent application Ser. No. 12/935,504 entitled “Capturing and Processing of Images using Monolithic Camera Array with Heterogeneous Imagers” to Venkataraman et al. In many instances, fusion and 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., can be utilized to synthesize a higher resolution 2D image or a stereo pair of higher resolution 2D images from the lower resolution images in the light field captured by 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 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 cameras in the array camera. Due to the different viewpoint of each of the cameras, parallax results in variations in the position of foreground objects within the images of the scene. Processes such as those disclosed in U.S. Provisional Patent Application No. 61/691,666 entitled “Systems and Methods for Parallax Detection and Correction in Imaged Captured Using Array Cameras” to Venkataraman et al. can be utilized to provide an accurate account of the pixel disparity as a result of parallax between the different cameras in an array. The disclosure of U.S. Patent Application Ser. No. 61/691,666 is hereby incorporated by reference in its entirety. Array cameras can use disparity between pixels in images within a light field to generate a depth map from a reference viewpoint. A depth map indicates the distance of the surfaces of scene objects from the reference viewpoint and can be utilized to determine scene dependent geometric corrections to apply to the pixels from each of the images within a captured light field to eliminate disparity when performing fusion and/or super-resolution processing. 
     In further embodiments, each camera in an array camera may include separate optics with different filters and operate with different operating parameters (e.g., exposure time). In many embodiments, the separate optics incorporated into each imager are implemented using a lens stack array. The lens stack array can include one or more optical elements, including color filters, that can be fabricated using wafer level optics (WLO) technology and/or any other technology appropriate for manufacturing lens stack arrays. In many embodiments, each of the cameras includes a quantum film configured with special color filters to receive certain wavelengths of light. Systems and methods for using quantum films to capture image data within an array camera in accordance with embodiments of the invention are discussed further below. 
     Array Cameras 
     Array cameras in accordance with embodiments of the invention can include a camera module and a processor. An array camera in accordance with an embodiment of the invention is illustrated in  FIG. 1 . The array camera  100  includes a camera module  102  with an array of individual cameras  104  where an array of individual cameras refers to a plurality of cameras in a particular arrangement, such as (but not limited to) the square arrangement utilized in the illustrated embodiment. The camera module  102  is connected to the processor  106  and the processor  106  is connected to a memory  108 . Although a specific array camera is illustrated in  FIG. 1 , any of a variety of different array camera configurations can be utilized in accordance with many different embodiments of the invention. Multiple camera arrays may operate in conjunction to provide extended functionality over a single camera array, such as, for example, stereo resolution. 
     Array Camera Modules 
     Camera modules in accordance with embodiments of the invention can be constructed from an imager array and an optic array. A camera module in accordance with an embodiment of the invention is illustrated in  FIG. 2 . The camera module  200  includes an imager array  230  including an array of focal planes  240  along with a corresponding optic array  210  including an array of lens stacks  220 . Within the array of lens stacks, each lens stack  220  creates an optical channel that forms an image of the scene on an array of light sensitive pixels  242  within a corresponding focal plane  240 . As is described further below, the light sensitive pixels  242  can be formed from quantum films. Each pairing of a lens stack  220  and focal plane  240  forms a single camera  104  within the camera module. Each pixel within a focal plane  240  of a camera  104  generates image data that can be sent from the camera  104  to the processor  108 . In many embodiments, the lens stack within each optical channel is configured so that pixels of each focal plane  240  sample the same object space or region within the scene. In several embodiments, the lens stacks are configured so that the pixels that sample the same object space do so with sub-pixel offsets to provide sampling diversity that can be utilized to recover increased resolution through the use of super-resolution processes. The camera module may be fabricated on a single chip for mounting or installing in various devices. 
     In several embodiments, an array camera generates image data from multiple focal planes and uses a processor to synthesize one or more images of a scene. In certain embodiments, the image data captured by a single focal plane in the sensor array can constitute a low resolution image (the term low resolution here is used only to contrast with higher resolution images), which the processor can use in combination with other low resolution image data captured by the camera module to construct a higher resolution image through Super Resolution processing. 
     Quantum Film Focal Plane Imager Arrays 
     Imager arrays in accordance with embodiments of the invention can be constructed from an array of focal planes formed of arrays of light sensitive pixels. As discussed above in relation to  FIG. 2 , in many embodiments the imager array  230  is composed of multiple focal planes  240 , each of which have a corresponding lens stack  220  that directs light from the scene through optical channel and onto a plurality of light sensing elements formed on the focal plane  240 . Typically the light sensing elements are formed on a CMOS device using photodiodes formed in the silicon where the depleted areas used for photon to electron conversion are disposed at specific depths within the bulk of the silicon. In many embodiments, a focal plane of an array of light sensitive pixels formed from a quantum film sensor may be implemented. A quantum film focal plane in accordance with an embodiment of the invention is illustrated in  FIGS. 3A and 3B . The focal plane  300  includes a focal plane array core  310  that includes an array of light sensitive pixels  320  formed from a light sensing quantum film  330  of quantum dots disposed at the focal plane of the lens stack  350  of a camera  360  on a semiconducting integrated circuit substrate  340 , such as a CMOS or CCD. The focal plane can also include all analog signal processing, pixel level control logic, signaling, and analog-to-digital conversion (ADC) circuitry used in the readout of pixels. The lens stack  350  of the camera  360  directs light from the scene and onto the light sensing quantum film  330 . In many embodiments the light sensitive quantum film may be coated or otherwise disposed directly or indirectly onto crystallized silicon, such as, for example, an integrated circuit configured to provide the necessary signaling, etc. The formation, architecture and operation of imager arrays and light sensitive pixel arrays, and their use in optical detection in association with array cameras are described in U.S. patent application Ser. No. 13/106,797, entitled “Architectures for Imager Arrays and Array Cameras”, filed May 12, 2011, the disclosure of which is incorporated by reference herein in its entirety. 
     A light sensitive quantum film utilized in accordance with embodiments of the invention may employ colloidal quantum dots that are quantum size-effect tunable. This tunability means that quantum films may be made that are either homogeneous (meaning that incorporate a mixture of quantum dots with different spectral absorption characteristics) such that the film is sensitive to light across a wide spectral range (from UV-VIS to IR), or structured such that only quantum dots that absorb over a specific spectral band are included in the film. It should be understood that spectral bands for which structured films can be formed are not limited to any spectral region and may include X-ray, UV, visible (including specific spectral bands within the visible spectrum such as red, green, and blue), near-infrared, and short-wavelength infrared. 
     A light sensitive quantum film used in the construction of pixels within the focal plane of an array camera may also provide built-in bias-dependent and/or intensity-dependent gain which may simplify read-out circuit design and enable the use of an enhanced dynamic range. The formation, composition, performance and function of various quantum films, and their use in optical detection in association with semiconductor integrated circuits are described in U.S. Patent Publication U.S./2009/0152664, entitled “Materials, Systems and Methods for Optoelectronic Devices”, published Jun. 18, 2009, the disclosure of which is incorporated by reference herein in its entirety. 
     Optical Crosstalk 
     As illustrated in  FIG. 4 , an issue with silicon-based light-sensitive elements, such as are used in conventional CMOS devices (A), is that the light sensitive elements  400  are disposed within the volume of the silicon crystal  410 , that is, that photon absorption takes place over the full length of the silicon crystal  410  at different depths within the crystal (typically first blue, then green, then red). The result is that, when color channels ((R)ed and (B)lue for example) are mapped on adjacent pixels, such as with a Bayer filter disposed atop the focal plane, the risk of optical crosstalk between pixels is increased. Any crosstalk between optical channels can introduce artifacts in the image data. In particular, crosstalk between optical channels means that an imager will sense the flux from a source on the pixel or focal plane that is inconsistent with the reconstructed position of the image of that pixel and the position of the image. This results in both a loss of image data, and the introduction of overlapping noise that cannot be distinguished from real image data. Crosstalk between pixels in the same color channel can also lead to an increase in blurriness and/or other artifacts. To address this, it is typically necessary to dispose microlenses above the pixels to direct the incident light through the bulk of the silicon crystal substrate and avoid crosstalk. In contrast, having the light sensitive material ( 420 ) at the surface of the focal plane as when using light sensitive quantum film (B), rather than within the silicon crystal bulk, limits the potential of optical crosstalk, which in turn provides a number of benefits. In particular, direct incidence of light through the lens stack onto the light sensitive pixel (e.g., quantum film) can be achieved without requiring optical manipulation, such as through microlenses. As illustrated in  FIG. 5 , the use of microlenses and the requirement to navigate the bulk of the silicon crystal (A) to reach the light-sensitive element in the conventional CMOS systems limits the acceptable chief ray angle (CRA 1) and also requires shifting of microlenses across the image sensor, rendering the image sensor design tightly coupled to micoroptics design. Reducing the thickness of the sensor allows for the use of a larger CRA for the cameras in an array camera. Accordingly, using a quantum film imager can achieve the maximal conveyance of light to the light-sensitive device, providing for increased sensitivity, and reduced optical crosstalk. Moreover, while embodiments of quantum film sensor cameras may use lenses or microlenses, in many embodiment microlenses are eliminated entirely. All of these factors improve the quality of the system by relaxing limitations (to the upper side) with respect to the CRA (CRA 2) which, in turn, results in a lower F# and improved Modulation Transfer Function (MTF). 
     In addition to implementing a light sensitive quantum film, embodiments may also include optical crosstalk suppressors for ensuring that light impinging on any particular focal plane comes only from its designated optical pathway or channel. Optical crosstalk suppressors used in accordance with embodiments of the invention are shown in  FIGS. 6A and 6B . Optical crosstalk can be considered to occur when light that is supposed to be incident on the top of one focal plane is instead also received by light sensing elements of another focal plane within the array. Accordingly, in many embodiments optical channels of the camera array are optically isolated so that a ray of light from one optical channel cannot cross to another optical channel. In some embodiments, illustrated in  FIG. 6A , lens stacks  600  are formed with opaque spacers  610  or vertical opaque walls  620 , which may be disposed between the optical channels  630 . Although opaque spacers do provide a level of optical crosstalk suppression, vertical opaque walls are preferable because in such an embodiment both the space between substrates and the relevant sections of the substrates themselves are rendered non-transparent. In other embodiments, shown schematically in  FIG. 6B , optical crosstalk suppression is achieved by creating a virtual opaque wall formed by a series of stacked apertures. In this embodiment, a series of aperture stops are formed on the various substrate levels  640  of the optical channel  600  by coating the substrates with opaque layers  650  provided with a narrow opening or aperture  660 . If enough of these apertures are formed, it is possible to mimic the optical isolation provided by a vertical opaque wall. In such a system, a vertical wall would be the mathematical limit of stacking apertures one on top of each other. Preferably, as many apertures as possible, separated from each other by sufficient space, are provided so that such a virtual opaque wall is created. For any lens stack, the number and placement of opaque layers needed to form such a virtual vertical opaque wall can be determined through a ray tracing analysis. 
     Although specific structure and components for preventing optical crosstalk are described above, any of a variety of arrangement for reducing crosstalk can be constructed in accordance with embodiments of the invention. 
     Color Information 
     In order to obtain information about the color of an image, light can be separated into spectral bands and then directed to different color channels. In conventional monolithic cameras this typically involves applying color filters to the individual pixels of the focal plane of the camera. As shown in  FIG. 7 , when using a light sensitive quantum film sensor this is accomplished by using a homogeneous mixture of quantum dots within the film that absorb across a broad range of spectral bands (UV-VIS) and then forming over this focal plane quantum film ( 700 ) an array of color filters ( 710 ) indexed to the underlying pixel geometry ( 720 ). However, even in a focal plane incorporating a light sensitive quantum film, because the pixels ( 710 ) of the focal plane are tightly packed, light rays traversing adjacent color filters can impact the light-sensitive element of an adjacent pixel, producing crosstalk between neighboring light-sensitive pixels that are associated with different color filters. To reduce pixel crosstalk in these conventional monolithic cameras optical elements such as, for example, optical crosstalk suppressors that block stray radiation, microlenses that focus light onto the individual pixels, or image data processing to resolve crosstalk among pixels can be utilized. Thus, the requirement that color filters be implemented on a pixel level and the concomitant need to resolve spatial differences between a filter or other optical elements (such as microlenses) and the light sensing pixel obviates many of the advantages obtained by incorporating a light-sensitive quantum film in the imager array. In conventional monolithic cameras, the alternative to using a homogeneous light sensitive quantum film and a pixel-level color filter is to form color-selective stacked pixels, in which a plurality of optically-sensitive layers are superimposed in the vertical direction. In these systems the quantum film layers are selected to have a decreasing bandgap such that the upper layer or layers provide sensing of shorter-wavelength light and the lower layer or layers primarily sense longer-wavelength light. Then contacts are made to each of the different layers and the various signals processed on a pixel by pixel basis to separate out the various spectral bands. Although such layered quantum film structures obviate the need for pixel-level color filters, it increases the complexity of the light-sensitive element and related signal processing. 
     The above discussion of separating light into different color channels has focused on the case where optically different camera channels (e.g., color channels or the like) are implemented on a pixel-by-pixel basis. As illustrated in  FIG. 8 , in many embodiments of the array camera ( 800 ), color filters ( 810 ) differentiated at the camera level ( 820 ) can be used to pattern the camera module. Example filters include a traditional filter used in the Bayer pattern (R, G, B or their complements C, M, Y), an IR-cut filter, a near-IR filter, a polarizing filter, and a custom filter to suit the needs of hyper-spectral imaging, and/or a polychromatic (P) filter. The number of distinct filters may be as large as the number of cameras in the camera array. Embodiments where π filter groups are formed are further discussed in U.S. Provisional Patent Application No. 61/641,165 entitled “Camera Modules Patterned with pi Filter Groups” filed May 1, 2012, the disclosure of which is incorporated by reference herein in its entirety. These cameras can be used to capture data with respect to different colors, or a specific portion of the spectrum. Accordingly, instead of applying color filters at the pixel level of the camera, color filters in many embodiments of the invention are included in the lens stack of the camera. For example, a green color camera can include a lens stack with a green light filter that allows green light to pass through the optical channel. In many embodiments, the pixels in each focal plane are the same and the light information captured by the pixels is differentiated by the color filters in the corresponding lens stack for each filter 
     The ability to apply color filters at a camera level and not at a pixel level means that the color filter may be applied either directly at the focal plane or in the lens stack. Accordingly, in some embodiments the color filter is applied at the focal plane directly on the light sensitive quantum film across the entire camera and not at the pixel level. Such an arrangement relaxes the CRA limitations for the microlenses to focus the incoming light through the color filter onto the focal plane, because there is no need to separate the incoming light between the individual pixels of the pixel array to avoid crosstalk. Alternatively, in other embodiments the color filter is applied within the lens stack of the camera and not at the focal plane. Placing the color filter within the lens stack can remove the need for microlenses entirely thereby significantly reducing the overall sensor cost. 
     Although a specific construction of camera modules with an optic array including color filters in the lens stacks is described above, camera modules including color filters and π filter groups can be implemented in a variety of ways. In several embodiments, at least one of the cameras in the camera module can include uniform color filters applied to the pixels in its focal plane. In many embodiments, a Bayer filter pattern is applied to the pixels of one of the cameras in a camera module either as the sole color filter or in combination with a camera level filter. In some number of embodiments, camera modules are constructed in which color filters are utilized in both the lens stacks and on the pixels of the imager array. 
     Furthermore, in many embodiments the structured color filter and (VIS-sensitive) homogeneous quantum film light sensitive sensor are replaced by a structured quantum film light sensitive sensor. In such embodiments, a structured light sensitive quantum film sensor is configured to include quantum dots that are sensitive only to a narrow spectral band. Tunability of the spectral band at the sensor level allows for potentially narrower spectral bands at more desired spectral positions. In many embodiments such a structured quantum film light sensitive sensor would be applied at camera level. This would allow for the formation of the quantum film on the lateral scale of several hundreds of microns to a few millimeters. Such a structure would be much simplified compared to applying a number of different structured quantum film sensors, each tuned to a different spectral band (such as in a Bayer pattern) at the pixel level. 
     Accordingly, in many embodiments of array cameras, color separation is implemented through a homogeneous quantum film an associated homogeneous color filter paired on each focal plane. In such embodiments, different focal planes could then be associated with different color filters, but could, in some instances use identical homogeneous quantum films. In other embodiments of array cameras, color separation is implemented by associating differently spectrally tuned quantum films with each focal plan, thereby rendering the need for color filters unnecessary. 
     Lens Stack Arrangements 
     Another issue raised in camera design is how to correct for field curvature. An image projected through a lens is not planar, but has an inherently curved surface. As illustrated in  FIG. 9  one way to correct this field curvature is to position a thick negative lens element  900  close to or directly on the focal plane  910 . The negative lens element planarizes the various angled beams of light  920  from the image, thereby addressing the field curvature problem. Such field flattened images provide superior image performance, allow for the manufacture of array cameras with relaxed through-the-lens (TTL) requirements, and deliver very homogeneous MTF. However, one problem with this approach is that this field flattening approach intrinsically requires a high CRA. This makes the technique unsuitable for conventional camera light-sensitive sensors; however, camera arrays using a light sensitive quantum film in accordance with embodiments of the invention can be constructed with a relaxed CRA angle requirement, thereby allowing for the use of the negative lens element field flattening approach shown in  FIG. 9 . Accordingly, in some embodiments of a camera incorporating a light sensitive quantum film a negative lens element is included in the lens stack above the focal plane. 
     Although an embodiment of an optical arrangement for use in an array camera that captures images using a distributed approach using a quantum film sensor is described above, it should be understood that other optical elements and arrangements may be fabricated for use with embodiments of the array camera. Embodiments where various lens stacks are incorporated into array cameras is further discussed in U.S. patent application Ser. No. 13/536,520 entitled “Optical Arrangements for Use With An Array Camera” filed Jun. 28, 2012, the disclosure of which is incorporated by reference herein in its entirety. 
     Imager Array Control 
     An imager array in which the image capture settings of a plurality of focal planes can be independently configured in accordance with an embodiment of the invention is illustrated in  FIG. 10 . The imager array  1000  includes a focal plane array core  1002  that includes an array of focal planes  1004  and all analog signal processing, pixel level control logic, signaling, and analog-to-digital conversion (ADC) circuitry. The imager array also includes focal plane timing and control circuitry  1006  that is responsible for controlling the capture of image information using the pixels. In a number of embodiments, the focal plane timing and control circuitry utilizes reset and read-out signals to control the integration time of the pixels. In other embodiments, any of a variety of techniques can be utilized to control integration time of pixels and/or to capture image information using pixels. In many embodiments, the focal plane timing and control circuitry  1006  provides flexibility of image information capture control, which enables features including (but not limited to) high dynamic range imaging, high speed video, and electronic image stabilization. In various embodiments, the imager array includes power management and bias generation circuitry  1008 . The power management and bias generation circuitry  1008  provides current and voltage references to analog circuitry such as the reference voltages against which an ADC would measure the signal to be converted against. In many embodiments, the power management and bias circuitry also includes logic that turns off the current/voltage references to certain circuits when they are not in use for power saving reasons. In several embodiments, the imager array includes dark current and fixed pattern (FPN) correction circuitry  1010  that increases the consistency of the black level of the image data captured by the imager array and can reduce the appearance of row temporal noise and column fixed pattern noise. In several embodiments, each focal plane includes reference pixels for the purpose of calibrating the dark current and FPN of the focal plane and the control circuitry can keep the reference pixels active when the rest of the pixels of the focal plane are powered down in order to increase the speed with which the imager array can be powered up by reducing the need for calibration of dark current and FPN. 
     In many embodiments, a single self-contained chip imager includes focal plane framing circuitry  1012  that packages the data captured from the focal planes into a container file and can prepare the captured image data for transmission. In several embodiments, the focal plane framing circuitry includes information identifying the focal plane and/or group of pixels from which the captured image data originated. In a number of embodiments, the imager array also includes an interface for transmission of captured image data to external devices. In the illustrated embodiment, the interface is a MIPI CSI 2 output interface (as specified by the non-profit MIPI Alliance, Inc.) supporting four lanes that can support read-out of video at 30 fps from the imager array and incorporating data output interface circuitry  1016 , interface control circuitry  1016  and interface input circuitry  1018 . Typically, the bandwidth of each lane is optimized for the total number of pixels in the imager array and the desired frame rate. The use of various interfaces including the MIPI CSI 2 interface to transmit image data captured by an array of imagers within an imager array to an external device in accordance with embodiments of the invention is described in U.S. Pat. No. 8,305,456, entitled “Systems and Methods for Transmitting Array Camera Data”, issued Nov. 6, 2012, the disclosure of which is incorporated by reference herein in its entirety. 
     Although specific components of an imager array architecture are discussed above with respect to  FIG. 10 , any of a variety of imager arrays can be constructed in accordance with embodiments of the invention that enable the capture of images of a scene at a plurality of focal planes in accordance with embodiments of the invention. Independent focal plane control that can be included in imager arrays in accordance with embodiments of the invention are discussed further below. 
     Independent Focal Plane Control 
     Imager arrays in accordance with embodiments of the invention can include an array of focal planes that can independently be controlled. In this way, the image capture settings for each focal plane in an imager array can be configured differently. The ability to configure active focal planes using difference image capture settings can enable different cameras within an array camera to make independent measurements of scene information that can be combined for use in determining image capture settings for use more generally within the camera array. 
     An imager array including independent control of image capture settings and independent control of pixel readout in an array of focal planes in accordance with an embodiment of the invention is illustrated in  FIG. 11 . The imager array  1100  includes a plurality of focal planes or pixel sub-arrays  1102 . Control circuitry  1103 ,  1104  provides independent control of the exposure timing and amplification gain applied to the individual pixels within each focal plane. Each focal plane  1102  includes independent row timing circuitry  1106 ,  1108 , and independent column readout circuitry  1110 ,  1112 . In operation, the control circuitry  1103 ,  1104  determines the image capture settings of the pixels in each of the active focal planes  1102 . The row timing circuitry  1106 ,  1108  and the column readout circuitry  1110 ,  1112  are responsible for reading out image data from each of the pixels in the active focal planes. The image data read from the focal planes is then formatted for output using an output and control interface  1116 . 
     Although specific imager array configurations are discussed above with reference to  FIG. 11 , any of a variety of imager array configurations including independent and/or related focal plane control can be utilized in accordance with embodiments of the invention including those outlined in U.S. patent application Ser. No. 13/106,797, entitled “Architectures for Imager Arrays and Array Cameras”, filed May 12, 2011, the disclosure of which is incorporated by reference herein in its entirety. The use of independent focal plane control to capture image data using array cameras is discussed further below. 
     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 one embodiment thereof. It is therefore to be understood that the present invention may be practiced otherwise than specifically described, without departing from the scope and spirit of the present invention. Thus, embodiments of the present invention should be considered in all respects as illustrative and not restrictive.