Abstract:
A self-calibration system for image sensors is disclosed. The system compensates signals from the image sensors for any distortions caused by inherent deficiencies in the image sensors. Image sensing modules employing the present invention will produce self-calibrated image signals without requiring the commonly used follow-up signal processing schemes. To achieve the self-calibration, an analyzing circuit is used to determine gains and offset values for each of the photodetectors in the image sensor based on a set of test signals. The gains and offset values are then stored in a persistent memory for access by an amplifier to adjust scanning signals representing a scanning object.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to image sensing systems and more particularly relates to a self-calibration method and circuit architecture of linear image sensors that can be used in scanners, facsimile, photocopy machines and other image reproduction systems. 
     2. Description of the Related Art 
     There are many applications that need an imaging system to convert a target to an electronic format that can be subsequently analyzed, printed, distributed or archived. The electronic format is generally a digital image of the target. A typical example of the imaging system is a scanner and the target is a sheet of paper from a book or an article. Through the scanner, an electronic or digital image of the paper is generated and subsequently may be analyzed, computed, or transmitted through the Internet. 
     An imaging system generally includes an image sensing module that converts a target optically into an image. The key element in the sensing module that converts the target optically to the image is an image sensor comprising an array of photodetectors responsive to light impinged upon the image sensor. Each of the photodetectors produces an electronic signal representing the intensity of light reflected from the target. The electronic signals from all the photodetectors are readout as a video signal that is then digitized through an analog-to-digital converter to produce a digital signal or an image of the target. 
     FIG. 1A illustrates a configuration system  100  that has been used for the past tens of years. A scanning document  110  that can be a page from an article or book is scanned in by an image sensing system  111  that can be a scanner, such as SCANJET 4100CSE Color Scanner from Hewlett Packard. The output of the scanner is typically a digital image  114  of scanning document  110 . Scanner  111  includes an image sensor  112  and an analog-to-digital converter  115 . Image sensor  112  generates images  117  that are typically digitized to gray scale or color images of 8-bit precision. Binalization process  116  receives and converts image  114  to binary image  118  that is a preferred form for data analysis and understanding in data process  120 . Binalization process  116  is typically implemented in a separate circuit or a software application. The separate circuit may be implemented in a post-processing circuit coupled to A/D converter  115  and the software application may be embedded in a scanner driver or provided in a commercial image editing software, such as Adobe PhotoShop, running in a host computer  119 . 
     FIG. 1B depicts a contact image sensor (CIS) system that can be used in image sensor  112  of FIG.  1 A. Scanning document  110  is illuminated by an illumination source  121 . Reflected light from scanning document  110  is collected and focused by a full-width rod-lens system  122 . The CIS system allows one-to-one scanning of the document because rod lens  122  and an image sensor chip  124  are of the same width as (or greater width than) scanning document  110 . 
     FIG. 1C is a functional block diagram of image sensor  112 , along with FIG. 1D showing some detail of the construction of image sensor array  126 . To be specific, a plurality of individual sensor chips  130  are butted end-to-end on a single substrate. Each of the individual sensor chips comprises a plurality of photodetectors  128  arranged in a row. In operation, image sensor array  126  is triggered by a start pulse to the first-in-sequence individual sensor chip  130  which serially activates the photodetectors on the first individual sensor chip  130 . After the signal from the last photodetector element of the first individual sensor chip  130  is read, an end-of-scan pulse is generated so that the next sensor chip in sequence is triggered. 
     The number of individual sensor chips chosen is dependent upon the desired width of scanning. Sensor array  126  also comprises necessary circuits to serially activate the individual chips and to readout signals generated from photodetectors. The strength of the signals is directly proportionate to the reflected light from the scanning document. To preserve the contents in the scanning document, most CIS systems produce signals that are subsequently digitized to 8 or 12 bit data by a following analog-to digital (A/D) converter. 
     In many imaging applications, such as check verification at checkout counters in a retail store and document archival, the primary interest is to extract texture information from captured images, for example, for optical character recognition (OCR). To be applicable for such process, the images are preferably in binary format, namely the texture information in black and the background in white or vice versa. In other words, the digitized signals from the A/D converter must be binalized. 
     The photodetectors in sensor array  126  are, however, subject to several inherent shortcomings that may cause sensor array  126  to produce errors that are hardly correctable in binary data. One of the shortcomings is that the gain from the photodetectors is not uniform from one photodetector to another within a chip or within the array. For example, the base of an NPN photodetector is formed by ion implantation. There is typically a ±5% non-uniformity across a wafer subject to ion implantation. This non-uniformity results in a current gain variation of as much as ±30% across the wafer. The non-uniformity of the gain yields a non-uniformity of the photo response of the same magnitude. Another shortcoming that may adversely affect the performance of sensor array  126  is inherent noises from some or all of the photodetectors or the non-uniformity of the wafer. Signals generated by the photodetectors could be distorted by the noises, which could cause misrepresentation of the contents in scanning objects. There is therefore a great need for an image sensor that produces signals, ideally, independent from the noises or at least with minimized noise effects. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in consideration of the above described problems and needs and has particular applications to image sensors that are widely used in image systems, such as scanners, digital cameras and computer vision systems. The present invention discloses a self-calibration method and circuit architecture for image sensors and compensates signals from the image sensors for any distortions caused by inherent deficiencies in the image sensors. Image sensing modules employing the present invention will produce self-calibrated image signals without requiring the commonly used follow-up signal processing schemes. 
     According to one embodiment, the present invention is a method for self-calibration of an image sensor comprising a plurality of photodetectors, each producing an electronic signal when the image sensor is activated, the method comprising 
     generating respectively first test electronic signals and second test electronic signals; said first test electronic signals and said second test electronic signals representing respectively a first test target and a second test target; 
     determining pairs of gain and offset from said first test electronic signals and said second test electronic signals; wherein each of said pairs of gain and offset corresponds to one of said photodetectors; 
     storing said pairs of gain and offset in a memory; and 
     adjusting, in a circuit, electronic signals from said photodetectors in accordance to said pairs of gain and offset in said memory, wherein said circuit is directly connected to said memory. 
     According to another embodiment, the present invention is a circuit architecture for self-calibration of an image sensor, said circuit architecture comprising 
     an amplifier coupled to said image sensor, said image sensor producing first electronic test signals and second electronic test signals when said image sensor is activated to image a first test target and a second test target; 
     an analyzing circuit coupled to said amplifier and receiving said first and second electronic test signals, said analyzing circuit determining gain values and offset values from said first and second electronic signals with respect to a first benchmark signal and a second benchmark signal; wherein said first benchmark signal and said second benchmark signal are respectively corresponding to said first test target and said second test target in an ideal condition; and 
     an electrically erasable programmable read-only memory (EEPROM) coupled to said analyzing circuit and receiving said gain values and offset values in response to a special signal. 
     Accordingly, one of the objects in the present invention is to provide a solution to self-calibrate signals from image sensors so that the signals can be produced without requiring post signal processing. 
     Objects, together with the foregoing are attained in the exercise of the invention in the following description and resulting in the embodiment illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
     FIG. 1A depicts a configuration system showing an imaging system using an image sensor and an analog-to-digital which has been used for the past tens of years; 
     FIG. 1B depicts a contact image sensor (CIS) system that can be used in the image sensor in FIG. 1A; 
     FIG. 1C depicts a functional block diagram of the image sensor in FIG. 1A; 
     FIG.1D shows some detail of the construction of the image sensor in FIG. 1A; 
     FIG. 2 shows a systematic configuration in which the present invention may be applied; 
     FIG. 3 shows an architecture of a modified CIS employing the present invention; 
     FIG. 4A shows a functional block diagram of an image acquisition system (CIS) according to one embodiment of the present invention; 
     FIG. 4B shows a check as an example of a scanning object; 
     FIG. 4C shows a corresponding (reversed) video signal obtained from a scanning line in the check; 
     FIG. 5 demonstrates a pair of test signals from an image sensor and a pair of resulting self-calibrated test signals; 
     FIG. 6; illustrates a self-calibration process for an image sensor according to one embodiment of the present invention and should be understood in conjunction with FIG. 4A; 
     FIG. 7 illustrates resultant bi-level signal from a comparator, the bi-level signal is generated from an adjusted signal in accordance with a dynamic threshold; and 
     FIG. 8 shows a process flowchart of the present invention according to one embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will become obvious to those skilled in the art that the present invention may be practiced without these specific details. The description and representation herein are the common means used by those experienced or skilled in the art to most effectively convey the substance of their work to others skilled in the art. In other instances, well known methods, procedures, components, and circuitry have not been described in detail to avoid unnecessarily obscuring aspects of the present invention. 
     Referring now to the drawings, in which like numerals refer to like parts throughout the several views. FIG. 2 shows a systematic configuration  200  in which the present invention may be applied thereto. The graphic difference between system  200  hereof and system  100  of FIG. 1A is the elimination of the A/D converter and binalization process, which means that image sensing system  202  now outputs directly binary images that can be directly processed in computing device  204 . It appears that the A/D converter and binalization process in FIG. 1A have been absorbed into image sensing system  202 , which indeed has been one of the efforts in the past. As described in detail below, the principle and implementation of the present invention is fundamentally different from the efforts to just integrate the A/D converter and binalization process together with an image sensor within a CIS in an imaging system. 
     As used herein, a gray-scale or gray image means an image presented in multiple intensity levels, for example, an 8-bit gray image means that there are 256 levels to represent varieties of the intensity. Similarly, a binary image means an image having only two intensity levels either in high or low and may be alternatively referred to as a bi-level, black-and-white or silhouette image. Binary images have several advantages: the most noticeable one is low storage requirement: no more than 1 bit/pixel, often this can be reduced as such images are very amenable to compression (e.g. run-length coding). The next one may be simple processing, namely the algorithms in most cases much simpler and faster than those applied to gray images. Therefore for applications like those check verifications over a teller/retail counter, the binary images may be the most appropriate format. 
     Referring now to FIG. 3, there is shown a circuit architecture of a modified CIS  300  (e.g.  202  of FIG. 2) employing the present invention. CIS  300  comprises an illumination source that comprises a light guide  304  and a light source  306 . Light source  306  may be one or more green LEDs and driven and controlled by an “ON” signal at respective connector  305 . 
     Rod lens array  310  collects reflected light from a scanning object illuminated by the light source and focuses the light onto image sensor  312 . Image sensor  312  comprises, for example, N photodetectors. Each of the photodetectors collects light cast thereon during each light integration process and generates an electronic signal. Upon the completion of the integration process, the electronic signals, each respectively generated by one of the photodetectors, are sequentially readout to the video bus  314  as a scanning or video signal via readout switch array  316 . It should be noted that image sensor  312  is assumed a CMOS (Complementary Metal-Oxide Semiconductor) type sensor and those skilled in the art will understand that the description works in the same way for other types of sensors. 
     Switch array  316  comprises the same number of the readout switches as the number of the photodetectors in image array  312 . It is understood to those skilled in the art that each of the readout switches may be implemented by a diode that becomes “On” or “passing through” when a proper voltage is applied across. As shown in the figure, the video signal is sent to gain &amp; offset control circuit  318 . The video signal is sequentially processed, including amplified and offset, in gain &amp; offset control circuit  318  with respect to a desired adjustment and then passed to binalization circuit  320 . The adjusted signal is processed in binalization circuit  320  and output as a binary signal at Vout. 
     It should be further pointed out binalization circuit  320  is preferably in the same substrate as those photodetectors  312 . One of the advantages of this single chip approach is reduced cost and possible high throughput rate comparing to separate circuits to accomplish the same. What makes this approach possible, however, is one of the key features in the present invention. As described below, there is no A/D converter and no process involved to convert from n-bit data to a binary representation. 
     Referring now to FIG. 4A, there is shown a functional block diagram  400  that may correspond to image acquisition  202  of FIG. 2 or modified CIS  300  of FIG.  3 . Electronic signals from image sensor  402  are successively amplified in amplifier  404  that is synchronized and controlled by pairs of parameters (gain and offset) provided by electrically erasable programmable read-only memory (EEPROM)  406 . Specifically, amplifier  404  corresponds to gain and offset control  318  of FIG.  3  and adjusts signals respectively for each provided pair of gain and offset. 
     EEPROM is a user-modifiable read-only memory (ROM) that can be erased and reprogrammed (written to) repeatedly through an application of higher than normal electrical voltage. EEPROM  406  provides successively a pair of appropriate gain and offset to adjust an electronic signal specifically coming from an identified photodetector in image sensor  402 . In other words, each pair of gain and offset correspond to one of the photodetectors and respectively derived from a set of test signals from the one of the photodetectors. 
     As seen in the figure, EEPROM  406  is synchronized by a counter  408  that is clocked by a clock source CLK. On the other hand, the clock source CLK also synchronizes image sensor  402 . In one embodiment, for example, when the clock source CLK starts to clock (or output a pulse) after both sensor  402  and counter  408  are initialized by a start pulse (SP), sensor  402  outputs an electronic signal generated from n-th photodetector, counter  408  causes EEPROM  406  to output a pair of gain and offset for the n-th photodetector. The electronic signal is then adjusted in amplifier  404  in accordance with the pair of gain and offset from EEPROM  406 . When the clock source CLK starts a next pulse, sensor  402  outputs another electronic signal generated from (n+1)-th photodetector, counter  408  causes EEPROM  406  to output a new pair of gain and offset for the (n+1)-th photodetector. The electronic signal is adjusted in amplifier  404  in accordance with the new pair of gain and offset from EEPROM  406 . Successively, all the electronic signals from sensor  402  are respectively adjusted according to respective pairs of gain and offset provided from EEPROM  406 . Subsequently, output signals from amplifier  404  are the adjusted electronic signals with minimized errors. A detailed description of deriving gains and offsets for an image sensor to compensate the electronic signals for inherent errors from the sensor itself is provided below. 
     Amplifier  404  provides adjusted signals to both analyzing circuit  410  and comparing circuit (comparator)  412 . Analyzing circuit  410  is designed to trace how the signals from amplifier  404  moves in order to provide an appropriate (dynamic) threshold to comparator  412  to generate a binary data signal. 
     To fully understand the functional block diagram  400 , FIG. 4B shows a check  420  as a scanning object. Line  422  indicates that a particular line on check  420  is being scanned and shows a number of distinctions along the line. As line  422  goes from left to right, it first experiences non-uniform background  424  which typically colorful. Then it hits printed character  426  and is followed by a mark trace  428  and further continues with background  424  and character  430 . The corresponding (reversed) video signal  432  (all electronic signals from the photodetectors) is shown in FIG.  4 C and from which a binary image needs to be derived. 
     Ideally, a binary image could be derived from a gray scale image with high contrast, which means signals representing a black area (typical texture information) are distinct from those representing white area (typically background information). With such signal, those skilled in the art understand that a constant threshold (e.g. a voltage) can be readily applied to binalize the signal. However, besides added noises inherently from the photodetectors, video signal  432  presents a challenge that signals representing the black areas are not distinctly separated from those representing the white areas. 
     Binalization of images has been a long researched topic in the area of image processing and led to many solutions (algorithms) that are typically executed in a computing device. In the present embodiment, an approach of edge-to-edge detection is used. Specifically, analyzing circuit  410  detects a pair of adjacent low and high magnitude points and derives a median or middle value from the low and high magnitude points. The middle value is then supplied to comparator  412  to binalize the set of signals from which the pair of the adjacent low and high magnitude points are obtained. 
     To be more specific, analyzing circuit  410  first detects that the video signal from the image sensor tends to rise up and marks a maximum magnitude or a nearby point. Analyzing circuit  410  then traces the video signal and subsequently detects that the video signal starts going downward and marks a minimum magnitude or a nearby point. It should be understood that the pair of minimum and maximum magnitudes are a local comparison within that segment of the signal. Therefore a median or middle value between the pair of minimum and maximum magnitudes is only valid for and can be used to binalize this segment of the signal. Those skilled in the art understand that various criteria may be introduced in determining the pair of minimum and maximum magnitudes. Curve  432  is a resultant threshold signal resembling all the middle values (a dynamic threshold) derived from signal  430  and supplied to comparator  412 . 
     With curve  432  as the dynamic threshold, video signal  430  is now readily to be binalized. The binalization means that anything in signal  430  greater than the corresponding value in curve  432  is output as one level and anything in signal  430  less than the corresponding value in curve  432  is output as another level, hence a bi-level signal is generated. 
     It is understood that analyzing circuit  410  is necessary to determine a dynamic threshold for the video signal from amplifier  404  although there are many ways to derive the dynamic threshold. What is important in the present embodiment is that the process of deriving the dynamic threshold is an on-chip process and capable of supporting real-time performance. Unlike the processes with the similar purposes in prior arts, analyzing circuit  410  hereof provides a dynamically changed threshold value in real time to comparator  412  that meanwhile receives streaming signal  415  from amplifier  404 . With the incoming streaming signal  415  and the synchronized threshold value from analyzing circuit  410 , comparator  412  produces in real time a streaming bi-level signal. Those skilled in the art are familiar with the implementation and operation of comparator  412 , therefore no further details thereof are provided to avoid obscuring other aspects of the present invention. 
     The forgoing description is based on the assumption that signals generated from the photodetectors in the image sensor faithfully represent the scanning object so that a dynamic threshold can be derived with minimum errors to binalize the signals. In reality, the photodetectors, however, are subject to many deficiencies some of that are described above and could adversely affect the signals and produce errors that are hardly correctable in a bi-level signal representing a scanning line. 
     Referring now to FIG. 5, there are shown a first pair of test signals  502 ,  504  and a second pair of resultant corresponding adjusted test signals  506  and  508 , all from the same image sensor. Signal  502  is produced when the image sensor is activated to image a complete dark target that is typically from a test sheet. The purpose is to determine the inherent noises from the sensor itself. It can be observed that signal  502  is not what one would have expected, a straight line representing uniformly the dark target, but comprises all sorts of noises, including misaligned sensor noise. As each image sensor chip is made to a certain size, for example, a sensor chip comprises  256  photodetectors, several such sensor chips have to be used and concatenated in series to accommodate a scanning width in a range of several inches to conform to a certain resolution requirement. The misaligned sensor noises, noticeably at  510  can be observed as periodical noises in signal  502 . It is not difficult to see that the misaligned sensor noises alone could corrupt a signal so as to misrepresent a scanning line. 
     Regardless where the noises come from, the noises can, however, cause errors in binalizing signal  502 . For example, electronic signals representing a light colored background could be segmented to a level that represents texture information because the noises are inherently added to the electronic signals and thus cause magnitudes in the electronic signals to shrink. Therefore, there must be a process that obtains the characteristics of the noises in each of the photodetectors and calibrates signals from the image sensors to compensate for the errors caused by the noises. 
     One of the features in the present invention is what is called self-calibration of image sensors. Traditional approaches rely largely on a process embedded in a driver that comes along with a sensing device, for example, a scanner driver loaded into a host computer that operates and synchronizes a scanner. After images data are acquired, the process is activated to calibrate the data to minimize the errors with respect to predefined parameters in the process. The approaches are difficult to support the real-time performance. 
     Referring now to FIG. 6, there is shown a self-calibration process  600  of an image sensor according to one embodiment of the present invention and should be understood in conjunction with FIG.  4 A. Certain procedures in process  600  are typically repeated twice, one for a black (dark) test sheet and one for a white test sheet. The black test sheet and white test sheet provide two extreme situations from which two basic parameters, gain and offset, can be obtained. 
     For various reasons, some of the photodetectors in the image sensor may not produce valid signals. To avoid undesirable signals from those photodetectors, at  604 , one or more lines of scanning signals are acquired to detect if there are any undesirable magnitudes in the acquired scanning signals. For example, given a dark sheet, amplifier  404  outputs an amplified signal that is coupled to analyzing circuit  410 . Ideally, the signal should be a constant (straight line). Because of the noises from the photodetectors, the signal possesses variations. Analyzing circuit  410  determines if there are any individual signals with magnitudes off a certain percentage, for example ±50%. If there are such signals, those corresponding photodetectors that produce the signals are respectively labeled, which means that signals from these photodetectors are either discarded or need special processing when real scanning signals come. With known benchmarks (for both test sheets, but mainly for the black sheet), an array of offsets or offset(i) corresponding respectively to the photodetectors are collected. 
     At  606 , process  600  is moved to get compensation data for both test sheets, i.e. black and white colors. It is known that the sensitivity of the image sensor to the colors of a scanning document, the reflected light thereof, and illumination from an elongated illumination source are not necessarily uniform. To avoid any biased signals result from the non-uniformity, an array of pairs of magnitudes, each corresponding to one of the photodetectors, must be collected with respect to the black and white test sheets. Typically, sensor  402  takes a couple of hundreds of scanning on a given test sheet, signals are respectively amplified in amplifier  404  and an average signal is determined in analyzing circuit  410  from the signals. To be specific, the average signal for the white sheet is kept in W(i) and for the black sheet is kept in B(i), where i=1, 2 . . . N. 
     At  610 , an array of pairs of gain and offset are determined in analyzing circuit  410  for each of the photodetectors. It is known to those skilled in the art that there are many ways to determine the gains. Given W(i), B(i), and offset(i) as well as benchmarks (signals) for the test sheets, gain(i) can be uniquely determined. 
     Each of the photodetectors now has its own gain and offset (a pair). The array of pairs of gain and offset are stored in EEPROM  406  that causes signals from sensors  402  to be adjusted in amplifier  404 . When a signal is generated from a particular photodetector, the signal is adjusted in amplifier  404  with its own registered gain and offset provided from EEPROM  406 . As an example, signals  506  and  508  are a pair of resultant signals, respectively corresponding to signals  502  and  504 , adjusted with gains and offsets from EEPROM  406 . It can be observed that the noises present in the original signals have been much reduced. Statistically, the non-uniformity has been reduced from 52% to 9%, a significant reduction in this example. 
     As a practical example, FIG. 7 illustrates resultant (binary) signal  702  from comparator  412 , the low and high magnitudes represent respectively two distinct groups of information. Signal  702  is generated from input adjusted signal  704  with respect to dynamic threshold signal  706  provided by EEPROM  406 . 
     FIG. 8 shows a process flowchart  800  according to one embodiment of the present invention. At  802 , an array of pairs of gain and offset are obtained. FIG. 6 is one of the implementations that can be used at  802 . Typically, if there are N photodetectors in an image sensor, there are N pairs of gain and offsets stored in a memory. At  804 , the image sensor is in normal operation, generating electronic signals (a video signal) to represent a scanning line of a scanning object. The electronic signals are then adjusted or amplified at  806  in accordance with the gains and offsets in the memory. 
     At  808 , the adjusted electronic signals are analyzed to determine a threshold value for each of the electronic signals. The threshold value is meanwhile sent to  810  in which a comparison process takes place. To be specific, if an incoming adjusted electronic signal has a magnitude larger than the derived threshold value, a first level signal is generated at  812 . Conversely, if the incoming adjusted electronic signal has a magnitude smaller than the derived threshold value, a second level signal is generated at  814 . The first or second level signal can be adjusted according to a specific application and has no impact on the operation of the present invention. Outputs from both  812  and  816  are combined to produce a bi-level signal that may be used for a host computer to perform analysis, compression, or transmission. 
     The present invention has been described in sufficient detail with a certain degree of particularity. It is understood to those skilled in the art that the present disclosure of embodiments has been made by way of examples only and that numerous changes in the arrangement and combination of parts may be resorted without departing from the spirit and scope of the invention as claimed. For example, not just gain and offset, there can be more parameters used to adjust signals from an image sensor. Accordingly, the scope of the present invention is defined by the appended claims rather than the forgoing description of embodiments.