Abstract:
A method of increasing the dynamic range of a captured image using a pixel array having a plurality of rows includes reading first pixel information corresponding to a long integration period from each pixel of a first row, reading second pixel information corresponding to a short integration period from each pixel of the first row, and merging the first pixel information and the second pixel information to thereby produce wide dynamic range pixel information for each pixel of the first row. Reading first pixel information takes place during a first interval, reading second pixel information takes place during a second interval, and at least a portion of the second interval takes place during a long integration period corresponding to a second row of the pixel array.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a non-provisional, and claims the benefit, of commonly assigned U.S. Provisional Application No. 60/871,388, filed Dec. 21, 2006, entitled “CMOS Image Sensor With Increased Dynamic Range,” and U.S. Provisional Application No. 60/805,942, filed Jun. 27, 2006, entitled “CMOS Image Sensor With Increased Dynamic Range,” the entirety of each of which is herein incorporated by reference for all purposes. 
     This application is related to co-pending, commonly-assigned U.S. patent application Ser. No. 11/345,642, filed Jan. 31, 2006, entitled “Dual Exposure For Image Sensors,” which is hereby expressly incorporated by reference in its entirety for all purposes. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the field of CMOS Image Sensors (CIS), and, in particular to methods for increasing the dynamic range of CIS, using dual exposure techniques. 
     BACKGROUND OF THE INVENTION 
     It is known (e.g., U.S. Pat. No. 5,144,442) that the dynamic range of captured images (both still and video) may be increased by acquiring multiple images of the same scene then merging the multiple images into a single wide dynamic range image. This may be accomplished using multiple image sensors and/or by using sequential image acquisitions, with different exposure settings. The former is expensive, not only because of the need for multiple image sensors, but also because the two image sensors need to be optically aligned with great precision so that the image of any object in front of the lens will be projected on the same pixel row and column in both image sensors. The latter is cheaper; however, because the two acquisitions do not take place at the same time, this approach is highly susceptible to motion artifacts. 
     Previously-incorporated U.S. patent application Ser. No. 11/345,642 discloses methods to generate short and long exposures of an image for the purpose of expanding its dynamic range, without the need to duplicate the number of image sensors, or to acquire two images at two different timing instances. However, further improvements are desirable. 
     BRIEF SUMMARY OF THE INVENTION 
     Embodiments of the invention provide a method of increasing the dynamic range of a captured image using a pixel array having a plurality of rows. The method includes reading first pixel information corresponding to a long integration period from each pixel of a first row, reading second pixel information corresponding to a short integration period from each pixel of the first row, and merging the first pixel information and the second pixel information to thereby produce wide dynamic range pixel information for each pixel of the first row. Reading first pixel information takes place during a first interval, reading second pixel information takes place during a second interval, and at least a portion of the second interval takes place during a long integration period corresponding to a second row of the pixel array. 
     In some embodiments at least a portion of the first interval may take place during the long integration period corresponding to the second row of the pixel array. Merging the first pixel information and the second pixel information may include, for each pixel, comparing the first pixel information to a predetermined threshold value, setting the wide dynamic range pixel information to be the first pixel information if the first pixel information is less than or equal to the threshold value, and setting the wide dynamic range of the pixel information to be a multiple of the second pixel information if the first pixel information is greater than the threshold value. The multiple may be equal to a ratio of the long integration period to the short integration period. The pixel array may be a CMOS array. The method may include reading third pixel information corresponding to a medium integration period for each pixel of the first row. The medium integration period may take place during a single line scan of the first row. 
     Other embodiments provide an image sensor having a pixel array having a plurality of rows, each row having a plurality of pixels, and a controller configured to cause each pixel of each row to integrate first pixel information over a long integration period and second pixel information over a short integration period. The controller is further configured to cause at least a portion of the short integration period for a row to take place during a long integration period of a second row. The sensor also includes an arrangement configured to determine wide dynamic range pixel information for each pixel based on the first pixel information and the second pixel information. 
     In some embodiments the controller is further configured to cause at least a portion of the long integration period for the row to take place during a long integration period of a second row. The arrangement configured to determine wide dynamic range pixel information for each pixel based on the first pixel information and the second pixel information may be configured to determine wide dynamic range pixel information for each pixel based on the first pixel information and the second pixel information by comparing the first pixel information to a predetermined threshold value, setting the wide dynamic range pixel information to be the first pixel information if the first pixel information is less than or equal to the threshold value, and setting the wide dynamic range of the pixel information to be a multiple of the second pixel information if the first pixel information is greater than the threshold value. The multiple may be equal to a ratio of the long integration period to the short integration period. The pixel array may be a CMOS array. The controller may be further configured to cause each pixel of each row to integrate third pixel information over a medium integration period wherein the medium integration period takes place during a single line scan of the first row. 
     Still other embodiments provide an image sensor having means for reading first pixel information corresponding to a long integration period from each pixel of a first row, means for reading second pixel information corresponding to a short integration period from each pixel of the first row, and means for merging the first pixel information and the second pixel information to thereby produce wide dynamic range pixel information for each pixel of the first row. 
     In some embodiments, the step of reading first pixel information takes place during a first interval, the step of reading second pixel information takes place during a second interval, and at least a portion of the second interval takes place during a long integration period corresponding to a second row of the pixel array. At least a portion of the first interval may take place during the long integration period corresponding to the second row of the pixel array. The means for merging the first pixel information and the second pixel information may include means for comparing the first pixel information to a predetermined threshold value for each pixel, means for setting the wide dynamic range pixel information to be the first pixel information if the first pixel information is less than or equal to the threshold value, and means for setting the wide dynamic range of the pixel information to be a multiple of the second pixel information if the first pixel information is greater than the threshold value. The multiple is equal to a ratio of the long integration period to the short integration period. The pixel array may be a CMOS array. The image sensor may include means for reading third pixel information corresponding to a medium integration period for each pixel of the first row, wherein the medium integration period takes place during a single line scan of the first row. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A further understanding of the nature and advantages of the present invention may be realized by reference to the following drawings. In the appended figures, similar components or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label by a dash and a second label that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label. 
         FIG. 1  depicts basic timing waveforms for four rows of a dual exposure image sensing technique according to embodiments of the present invention. 
         FIG. 2  depicts timing waveforms for a first specific example. 
         FIG. 3  depicts a schematic block diagram for implementing the first specific example. 
         FIG. 4  depicts timing waveforms for a second specific example. 
         FIG. 5  depicts a schematic block diagram for implementing the second specific embodiment. 
         FIG. 6  depicts timing waveforms for a third specific example. 
         FIG. 7  depicts a schematic block diagram for implementing the third specific embodiment. 
         FIG. 8  depicts signal-to-noise ratio for a three-exposure embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention relate to image sensors. In order to provide a context for describing embodiments of the present invention, embodiments of the invention will be described herein with reference to CMOS Image Sensors (CIS). Those skilled in the art will appreciate, however, that other embodiments are possible. 
     The ensuing description provides preferred exemplary embodiment(s) only, and is not intended to limit the scope, applicability or configuration of the invention. Rather, the ensuing description of the preferred exemplary embodiment(s) will provide those skilled in the art with an enabling description for implementing a preferred exemplary embodiment of the invention. It is to be understood that various changes may be made in the function and arrangement of elements without departing from the spirit and scope of the invention as set forth in the appended claims. 
     Specific details are given in the following description to provide a thorough understanding of the embodiments. However, it will be understood by one of ordinary skill in the art that the embodiments may be practiced without these specific details. For example, systems may be shown in block diagrams in order not to obscure the embodiments in unnecessary detail. In other instances, well-known processes, structures and techniques may be shown without unnecessary detail in order to avoid obscuring the embodiments. 
     Also, it is noted that the embodiments may be described as a process which is depicted as a flowchart, a flow diagram, a data flow diagram, a structure diagram, or a block diagram. Although a flowchart may describe the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be re-arranged. A process is terminated when its operations are completed, but could have additional steps not included in the figure. A process may correspond to a method, a function, a procedure, a subroutine, a subprogram, etc. When a process corresponds to a function, its termination corresponds to a return of the function to the calling function or the main function. 
     Moreover, as disclosed herein, the term “storage medium” may represent one or more devices for storing data, including read only memory (ROM), random access memory (RAM), magnetic RAM, core memory, magnetic disk storage mediums, optical storage mediums, flash memory devices and/or other machine readable mediums for storing information. The term “computer-readable medium” includes, but is not limited to portable or fixed storage devices, optical storage devices, wireless channels and various other mediums capable of storing, containing or carrying instruction(s) and/or data. 
     Furthermore, embodiments may be implemented by hardware, software, firmware, middleware, microcode, hardware description languages, or any combination thereof. When implemented in software, firmware, middleware or microcode, the program code or code segments to perform the necessary tasks may be stored in a machine readable medium such as storage medium. A processor(s) may perform the necessary tasks. A code segment may represent a procedure, a function, a subprogram, a program, a routine, a subroutine, a module, a software package, a class, or any combination of instructions, data structures, or program statements. A code segment may be coupled to another code segment or a hardware circuit by passing and/or receiving information, data, arguments, parameters, or memory contents. Information, arguments, parameters, data, etc. may be passed, forwarded, or transmitted via any suitable means including memory sharing, message passing, token passing, network transmission, etc. 
     A CIS-based camera typically adjusts the exposure level based on the brightness of the image to be captured. If the exposure is too long, some of the pixels—in particular those in the brighter areas of the image—tend to reach saturation—a point where they can no longer integrate light energy. Image regions with such over-exposed pixels are referred to as saturated regions, and pictures with large saturated regions are considered to be of low quality. On the other hand, when the exposure time is too short, the energy accumulated in some of the pixels—in particular those in the darker areas of the image—will be low relative to the energy of the inherent noise, resulting in poor SNR and, again, poor image quality. 
     According to embodiments of the invention, the dynamic range of images captured using CMOS image sensors is improved using dual exposure techniques. According to embodiments of the present invention, a shorter exposure period for a line (row) is accomplished immediately following an integration and readout of the line. This minimizes the buffer needed as compared to other dual exposure techniques. According to embodiments of the present invention, a buffer need only puffer the duration of the shorter exposures, which may be several lines in some embodiments, one line in other embodiments, and no buffer in yet other embodiments. 
     An apparatus built in accordance with the present invention follows the long exposure period of each row by a short exposure period of the same row, with duration of one line or less. Then, the short exposure row readout is merged with the long exposure readout shortly after the long exposure row has been registered, with a minimal or with no extra buffering. Further embodiments of the current invention allow more than two exposures of each row, for example, a Long, a Short and a Medium exposure. 
     In a first embodiment of the current invention, row conversions are done at twice the rate of row readout. The Short exposure proceeds until the analog to digital conversion of the long exposure is completed. Thereafter, the short exposure information is latched and converted to digital. The conversion completes before the next row is to be latched. The results of the long and short exposures of the current row are merged and output. 
     As the conversion of the long and short exposures is done serially in this first embodiment, the conversion circuits may be shared by the long and short exposure, and no extra buffer memory or ADC circuits are needed. 
     In a second embodiment, conversion of the long exposure takes the time of a full row scan, but the conversion of the short exposure takes half the time period of a row. This embodiment improves picture noise, but may require duplication of analog latching and conversion circuits. The buffer memory does not need to be duplicated, as the information on both exposures is ready at the same time. 
     In a third embodiment, both conversions take the time duration of a full row. This case yields the best image quality, but may require duplication of the conversion circuit as well as a full row buffer. 
     In other embodiments, each row is subject to three or more exposures of varying durations, for wider dynamic range and better SNR. 
     Having described embodiments of the present invention generally, attention is directed to  FIG. 1 , which depicts a basic timing waveforms of the present invention for four consecutive rows (“lines”). As can be seen, a first reset pulse (RST) for each row designates the start of a long integration period. Following this period, the row is read (RD), reset again for a short exposure period, and read. This process, shifted in time, repeats for subsequent rows, employing a “rolling shutter” technique. 
     For double sampling CMOS image sensors, a read process includes a series of events: resetting a floating diffusion; a first read; a transfer of a photodiode charge to the floating diffusion; and a further read. For the sake of clarity, this is not shown herein. It should be assumed that for those cases, the reset pulse shown includes a series of pulses similar in nature to the one described above. 
       FIG. 2  depicts timing waveforms  10  for a first specific example of a dual exposure embodiment, including Read (Rd) and Reset (Rst) pulses for a single line and the corresponding integration and analog-to-digital conversion (ADC) periods. A line scan period  180  is depicted for reference. A long integration period  100  is accomplished mostly outside the time frame covered by waveforms  10 , and starts before the current line is read. A read pulse  120  initiates reading of the long integration  100  into sample and hold (S&amp;H) circuits. Shortly after long integration values are read, analog signals are converted to digital values during an ADC conversion period  160 . A read pulse  120  is followed by a reset pulse  130 , which starts the Short integration period  110 . A read pulse  140  samples the result of the short integration period, which is then converted to digital ( 170 ). 
       FIG. 3  depicts a schematic block diagram  20  in which the first specific example of the present invention may be embodied. A timing unit  210  controls an X decoder  220  through control lines  215 , causing it to generate Read and Reset pulses on a multitude of Read and Reset lines  225 , each corresponding to a row or to groups of adjacent rows in Image Sensor Array  230 , according to the timing diagram described above. Outputs  235  from Image Sensor Array  230 , each corresponding to the signal read on a specific column, are Sampled and converted to digital in S&amp;H and ADC block  240 . This process is typically done in parallel for all columns. 
     For the Long exposure, digital value from block  240  is output, through parallel bus  245 , to register  250 , where it is stored until the end of the line, and output on parallel bus  255 . When conversion of the short exposure data is completed, it is output from  240  via parallel bus  246 . A Select Merge block  260  generates a high dynamic range image  265  from the Long exposure information asserted on bus  255  and the short exposure image asserted on bus  246 . 
     The Select/Merge function may be, in some embodiments, a simple algorithm where the Long exposure image is transferred if its value is less than a pre-defined or a programmable threshold. Otherwise, the output value may be a multiple (G) of the Short exposure value, where G may be the exposure ratio between the Long and the Sort exposures. 
     Waveforms for a second specific example are depicted in  FIG. 4 . In this example, a Long Integration period  100  take place mostly outside the time frame covered by 10, and starts before the current line is read. A read pulse  120  initiates reading of the Long integration  100  into S&amp;H circuitry. Shortly after long integration values are read, a Long Exposure ADC conversion  200  starts, converting the read value to digital. A Read pulse  120  is followed by a Reset pulse  130 , which starts the Short integration period  110 . A Read pulse  140  samples the result of the short integration period, which is next converted to digital in a different circuit, during period  210 . The results of the two conversions are ready to be merged before the end of the line. 
       FIG. 5  depicts a schematic block diagram  20  implementing the second specific example. Similar to the first specific example, a Timing Unit  210  controls an X Decoder  220  through control lines  215 , causing the X decoder to generate Read and Reset pulses on a multitude of Read and Reset lines  225 , each corresponding to a row or to groups of rows in Image Sensor Array  230 , according to the timing diagram described above. The outputs of Image Sensor Array  230 , each corresponding to the signal read on a specific column, are Sampled and converted to digital, first in S&amp;H and ADC block  300 , and half a line later, by S&amp;H and ADC block  310 . The former is done for the Long Exposure and the latter for the Short Exposure. As illustrated in  FIG. 4 , the conversion done by block  300  for the Long Exposure and that done by block  310  for the Shot Exposure finish around the same time. A Select/Merge block  320  combines the two outputs to wide dynamic range (WDR) columns  265  in a process which may be similar to that described above for the first specific example. 
       FIG. 6  depicts timing waveforms  10  for a third specific example. Unlike the previous examples, conversions for both Long and Short integrations take a full row scan time  180 , and  FIG. 6  depicts a period of two lines rather than one. The Long Integration Time  100  for Line n ends with a Read pulse  120 , causing the signal on the column lines to start ADC conversion  300 . Soon thereafter, Reset pulse  130  is applied, initiating short integration period  110  for Line n. At the end of the current line, Line n+1 finishes its long integration with Read pulse  220 , and starts ADC conversion  310 . At approximately the same time, short integration period  110  for line n ends. The short exposure value is read by pulse  140  and starts ADC conversion  320 . Reset pulse  230  is applied to line n+1 shortly thereafter, to initiate short exposure  210 . 
     The two steps described above repeat for all rows so that one line finishes long integration and starts short integration while another line finishes short integration and begins long integration. In a similar manner, there will always be a Long integration ADC conversion of consecutive lines, and Short integration ADC conversions of same consecutive lines, lagging the first conversion by one line. 
       FIG. 7  depicts a conceptual block diagram of the third specific example.  FIG. 7  is identical to  FIG. 5 , with an extra line buffer  315 . Line Buffer  315  is needed because the Short exposure conversion lags after the Long exposure conversion by a full line. 
     In still other embodiments, each row may be subject to more than two exposures. In one such embodiment, an image sensor has 1024 video rows. There are three exposure times: a short exposure time of one row, a medium exposure time of 32 rows, and a long exposure time of 1024 rows, or full frame. Each row is subject to the three exposure times sequentially—first the Long exposure, then the Medium and lastly the Short. 
     A buffer of 33 video lines may be required to store the results of the Long exposure, and a buffer of one video line may be required to store the results of the medium exposure. The results of the three exposures are merged when the results of the Short exposure are obtained from an analog to digital conversion. The results from the Long and Medium exposures are read from the 33 and 1 line buffers, respectively. 
     As would be appreciated by those skilled in the art, such an embodiment will yield a 20*log(1024)=˜60 dB increase in dynamic range. The SNR benefits from the fact that there are three exposure times. If the merge/select function of the three exposures is degenerated to select only, the SNR as a function of the relative irradiance will appear as shown in  FIG. 8 . The troughs occur when a switch from one exposure to another takes place, and their value is 20*log(32)=˜15 dB. If a smooth merge function is applied, the dips may rounded, which may be desirable because abrupt changes in SNR may be discernible to the eye. 
     Having described several embodiments, it will be recognized by those of skill in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit and scope of the invention. Additionally, a number of well known processes and elements have not been described in order to avoid unnecessarily obscuring the present invention. Accordingly, the above description should not be taken as limiting the scope of the invention, which is defined in the following claims.