Patent Application
20080259194
Generally, embodiments disclosed herein relate to adjusting the outputted signals of imaging pixels according to their geographic location in a pixel array to compensate for various optical problems.
A CMOS imager circuit includes a focal plane array of pixels, each one of the pixels including a photosensor, for example, a photogate, photoconductor or a photodiode overlying a substrate for accumulating photo-generated charge in the underlying portion of the substrate. Each pixel has a readout circuit that includes at least an output field effect transistor formed in the substrate and a charge storage region formed on the substrate connected to the gate of an output transistor. The charge storage region may be constructed as a floating diffusion region. Each pixel may include at least one electronic device such as a transistor for transferring charge from the photosensor to the storage region and one device, also typically a transistor, for resetting the storage region to a predetermined charge level prior to charge transference.
In a CMOS imager, the active elements of a pixel perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) resetting the storage region to a known state before the transfer of charge to it; (4) transfer of charge to the storage region accompanied by charge amplification; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge. Photo charge may be amplified when it moves from the initial charge accumulation region to the storage region. The charge at the storage region is typically converted to a pixel output voltage by a source follower output transistor.
CMOS imagers of the type discussed above are generally known as discussed, for example, in U.S. Pat. No. 6,140,630, U.S. Pat. No. 6,376,868, U.S. Pat. No. 6,310,366, U.S. Pat. No. 6,326,652, U.S. Pat. No. 6,204,524 and U.S. Pat. No. 6,333,205, assigned to Micron Technology, Inc., which are hereby incorporated by reference in their entirety.
Conventional CMOS imaging devices have attempted to correct lens shading during post-processing of already-acquired image data or during image acquisition (i.e., as the image is read out from the imaging device). There is a need and desire in the art for additional image correction methods that do not require post-readout image processing or special image acquisition techniques to compensate for lens shading.
In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which are shown by way of illustration of specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those of ordinary skill in the art to make and use them, and it is to be understood that structural, logical or procedural changes may be made to the specific embodiments disclosed herein.
Various embodiments described herein modify the size of pixel photosensors according to their spatial location in a pixel array to compensate for different spatial output signal levels. The embodiments described can thus correct for spatial optical effects such as lens shading, as well as other spatial optical effects.
As discussed above, pixels located farther away from the optical axis of a lens in an imaging device may experience decreased light intensity, for example, caused by lens shading. One embodiment herein mitigates against this by increasing the size of the pixel, and thus the size of the photosensor, according to the pixel's geographical distance form the optical axis in the pixel array.
Although this disclosure refers to modifying pixel size, it is to be understood that what is most important is the size of the photosensor in accordance with spatial location of a pixel in the array. Accordingly, when the discussion herein refers to changes in the size of the pixel, it is understood that the size of the photosensor changes proportionately to the change in the pixel size.
In one embodiment, the pixels may be arranged in the pixel array in a rectilinear grid. The pixels may be generally rectangular in shape or may comprise various other shapes. The size of the pixels and thus the size of the photosensor may be increased according to the distance of the pixels from the optical axis in the x-direction and/or the y-direction. This allows for differently sized pixels according to their geographic location, while maintaining a rectilinear pixel array. Furthermore, lines connecting the pixels to appropriate circuitry may be formed in straight lines, which may be parallel to the x-axis or the y-axis, and in which simplifies the construction of the pixel array.
The relationship between the length of the sides of the pixels and the distance from the x-axis and/or y-axis of the pixel array may be determined according to the spatial optical problem to be corrected. As shown in the
As shown in
For rectilinear pixel layout grids such as those shown in
Sampling with a non-uniform pixel location grid may create spatial distortion when the signals from the pixels are assembled to form a digital image. For example, the image may appear stretched near the center of the image because the pixels near the center of the image are smaller than the pixels near the edges of the image. One method that may be used to correct the spatial distortion is digital resampling. The spatial distortion may also be corrected using an optical solution, such as using a lens to correct the distortion. Alternatively, if the spatial distortion is small enough, it may be ignored.
As noted, in addition to changing the size of the pixels in order to change the size of the associated photosensors, an embodiment also keeps the pixel size the same throughout the array, but changes the size of the photosensors within the pixels in accordance with a spatial location of the pixels in an array.
The CMOS imaging device 500 is operated by a control circuit 530, which controls address decoders 515, 525 for selecting the appropriate row and column select lines for pixel readout. Control circuit 530 also controls the row and column driver circuitry 510, 520 so that they apply driving voltages to the drive transistors of the selected row and column select lines. The pixel output signals typically include a pixel reset signal Vrst read out of the pixel storage region after it is reset by the reset transistor and a pixel image signal Vsig, which is read out of the pixel storage region after photo-generated charges are transferred to the region. The Vrst and Vsig signals are sampled by a sample and hold circuit 535 and are subtracted by a differential amplifier 540, to produce a differential signal Vrst-Vsig for each readout pixel. Vrst-Vsig represents the amount of light impinging on the pixels. This difference signal is digitized by an analog-to-digital converter 545. The digitized pixel signals are fed to an image processor 550 to form a digital image output. The digitizing and image processing can be located on or off the chip containing the pixel array 505. In some arrangements the differential signal Vrst-Vsig can be amplified as a differential signal and directly digitized by a differential analog to digital converter and then fed to image processor 550, which assembles an image for output. Resampling of the image to correct image distortion caused by pixel arrays in accordance with embodiments described herein may also be performed by the image processor 550.
System 600, for example a camera system, generally comprises a central processing unit (CPU) 610, such as a microprocessor, that communicates with an input/output (I/O) device 640 over a bus 660. Image sensor 630 also communicates with the CPU 610 over the bus 660. The system 600 also includes random access memory (RAM) 620, and can include removable memory 650, such as flash memory, which also communicate with the CPU 610 over the bus 660. Image sensor 630 may be combined with a processor, such as a CPU 610, digital signal processor, or microprocessor, in a single integrated circuit. In a camera, a shutter release button 670 is used to operate a mechanical or electronic shutter to allow light to pass through a lens 675 to the pixel array 505 of imaging device 200.
While embodiments of the invention have been described in detail in connection with embodiments known at the time, it should be readily understood that they are not limited to the disclosed embodiments. Rather, they can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described. For example, while embodiments are described in connection with a CMOS pixel imaging device, they can be practiced with any other type of pixel imaging device (e.g., CCD, etc.) or image source. Furthermore, the pixel sizing of the various embodiments could be used in other applications that require modified pixel sizes, such as in a display projector in which bigger pixels located away from the center of the pixel array could be used to correct spatial distortion caused by the projector lens.
In addition, although various embodiments have been described as being useful to correct lens shading, any spatially varying image problem that can be corrected by changing pixel size in relation to pixel distance from a predefined area can be corrected, such as barrel distortion or fish-eye lens. In one embodiment, each row and each column may have an arbitrary scale factor. Furthermore, if certain rows and/or columns of pixels have different characteristics, for example, due to the layout of the lines, transistors, or color filters, the row height or column width may be adjusted accordingly. By modifying the width of the columns and the height of the rows, various sizes of the pixels in the pixel array can be achieved, and the pixel array can include larger or smaller pixels in various places to form a symmetric or asymmetric pattern.
