Abstract:
In a photosensitive device wherein voltages are read sequentially from a dark, or dummy, photosensor and a plurality of active photosensors with each of a series of scans, a circuit downstream of the photosensors resets the offset value of the voltage signals, based on successive voltage readings from the dark photosensor. An RC averaging circuit maintains a running average of readings from the dark photosensor over a large number of scans. Signals from the dark photosensors are read a first time into the averaging circuit, and then signals from the dark photosensors are read directly to downstream video circuitry. This double readout of dark-photosensor signals enables precise calibration of both on-chip circuitry and downstream video circuitry.

Description:
INCORPORATION BY REFERENCE 
     The present application incorporates by reference U.S. Pat. No. 5,654,755, assigned to the assignee hereof. 
     FIELD OF THE INVENTION 
     The present invention relates to image sensor arrays used in raster input scanners. In particular, the invention relates to photosensitive chips wherein each photosensor outputs signals onto a common video line, and where there are provided dark photosensors for setting an offset level on the common video line. 
     BACKGROUND OF THE INVENTION 
     Image sensor arrays typically comprise a linear array of photosensors which raster scan an image bearing document and convert the microscopic image areas viewed by each photosensor to image signal charges. Following an integration period, the image signal charges are amplified and transferred as an analog video signal to a common output line or bus through successively actuated multiplexing transistors. 
     For high-performance image sensor arrays, a preferred design includes an array of photosensors of a width comparable to the width of a page being scanned, to permit one-to-one imaging without reductive optics. In order to provide such a “full-width” array, relatively large silicon structures must be used to define the large number of photosensors. A preferred technique to create such a large array is to make the array out of several butted silicon chips. In one proposed design, an array is intended to be made of 20 silicon chips, butted end-to-end, each chip having 248 active photosensors spaced at 400 photosensors per inch. 
     Although most scanning systems currently in use are ultimately digital systems, the “raw signal” coming out of the photosensors during the scanning process is an analog video signal, with the voltage magnitude corresponding to the intensity of light impinging on the photosensor at a given time. Thus, when signals are read out from the photosensors on a chip to be converted to digital data, different video levels, corresponding to the brightness of the reflected area being scanned by a particular photosensor at a particular moment, are output as a series of analog voltage levels. 
     Photosensitive devices may be one-dimensional or two-dimensional, and can be either of the “active” variety, wherein the photosensors output voltage signals, or in the form of a charge-coupled device, or CCD, which outputs a sequence of charges from a series of individual photosensors. In all of these various types of photosensitive devices, a common design feature is the use of “dark” photosensors, which are used to periodically reset the offset voltage for the photosensors being read out. These dark photosensors are of the same semiconductor structure as the other “active” photosensors on each chip, but the dark photosensors are not exposed to light. In most designs, the dark photosensors are provided with an opaque shield, such as of aluminum or silicon, to prevent the influence of light thereon. In the scanning process, with each readout cycle of active photosensors on each chip, the readout of the first photosensor is proceeded by readouts of one or more dark photosensors, which are used to reset the voltage offset associated with the whole chip, and thereby correct signal drift when the active photosensors are reading out their signals. In other words, the readout of a dark photosensor with each scan can serve as a reference offset or “zero point” so that the absolute values of light intensity on the active photosensors may be determined. The use of a dark photosensor output when reading out signals from active photosensors can significantly compensate for performance variations of multiple chips in a single apparatus, and also for changes in the performance of a photosensitive device over time. 
     DESCRIPTION OF THE PRIOR ART 
     U.S. Pat. No. 5,654,755 describes a circuit for correcting the offset of the video output of a set of active photosensors, based on the output of dark photosensors. An averaging RC circuit in parallel with the video line accumulates an average signal based on a large number of readings from the dark photosensors. The average signal is periodically clamped to a correction capacitor in series on the video line. The charged correction capacitor adjusts the offset on the active-photosensor signals which subsequently pass through the video line. In this context, the correction of the offset on active-photosensor signals is known as “DC restore.” 
     While the system of the &#39;755 patent works well from the perspective of correcting offset on an integrated photosensor chip, certain subtleties of operation must be addressed when such a chip is incorporated into a larger system. One problem is that the signals from the dark photosensors add to the fixed-pattern noise, or dark non-uniformity, of the video signals that must be processed. Dark photosensors should have the same drift characteristics as the active photosensors. In the case where dark photosensor signals are flushed straight through the video circuitry, the on-chip drift characteristic follows that of the drift of the photosensor circuitry in addition to the drift of the video amplifiers. However, the drift of the active photosensors does not reflect the drift of the circuitry since this is subtracted out during the DC restore operation. During the DC restore operation the video signal is restored to a dark reference level plus the active pixel level minus the averaged dark photosensor level. Since both the active photosensor level and the dark photosensor level have the same drift, this drift is cancelled out in the video signal. Therefore there exists a need to provide an offset-correction system for dark and active photosensors, which provides the offset correction from both the perspective of the on-chip photosensor circuitry, and also from the perspective of any downstream image circuitry. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a photosensitive device and a method of operating thereof. The photosensitive device comprises a set of photosensors, each photosensor outputting a voltage signal representative of light intensity thereon. A video line is adapted to receive voltage signals from the set of photosensors. A correction capacitor is associated with the video line, the correction capacitor adapted to retain a correction charge thereon to influence the voltage signals from the photosensors. A bypass switch selectably causes the signal on the video line to bypass the correction capacitor. A signal is read from a photosensor a first time with the correction capacitor bypassed by the bypass switch and then a signal is read from the photosensor a second time through the correction capacitor. 
     According to another aspect of the present invention, there is provided a photosensitive device and method of operating thereof. The photosensitive device comprises a set of photosensors, each photosensor outputting a voltage signal representative of light intensity thereon. A video line is adapted to receive voltage signals from the set of photosensors. An averaging circuit is in parallel with the video line. Within a cycle of operation, a signal is read from a photosensor a first time, with reading a signal to the averaging circuit, and then a signal is read from the photosensor a second time, but the signal is not read to the averaging circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of the relevant portions of an active photosensitive device having dark photosensors and active photosensors reading to a common video line; 
     FIG. 2 is a schematic of a simple circuit for causing dark photosensor signals to determine an offset for subsequent active photosensor video signals; 
     FIG. 3 is a schematic diagram of a circuit, according to a preferred embodiment of the present invention, showing a circuit for determining an offset for a video line from dark photosensor signals; 
     FIG. 4 is a comparative timing diagram of different waveforms for operating the transistors in the circuit of FIG. 3; and 
     FIGS. 5 and 6 are schematic diagrams showing the basic elements of two different possible embodiments of circuits for enabling the “double readout” of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a simplified plan view showing a configuration of photosensors on a single active photosensor chip  10 . Such a chip would be found, for example, in a raster input scanner (RIS) having a linear array of photosensors, as would be used, for example, in a digital copier or scanner. As is well known, such a chip can be exposed with a thin line of dark and light areas from an original hard-copy image; over time, the original hard-copy image is moved relative to the chip, so that each individual photosensor on the chip is exposed to a sequence of small areas on the original image. A typical raster input scanner may include a single chip which, in combination with reduction optics, is exposed to an entire width of a page being scanned; or alternately could include a plurality of such chips, butted end-to-end, which together form a single page-width linear array of photosensors. 
     In the plan view of FIG. 1, there is shown a long array of active (i.e., photosensitive) photosensors  102  which share a common video line  108  with a plurality of “dark photosensor” photosensors  110 , which are individually indicated as D 1 -D 4 . Also associated with the photosensors  102  and  110  is a shift register line  112  which connects a series of shift register select switches  114 . The shift register select switches  114  have associated therewith a series of transistor switches  116 . When each transistor switch  116  is activated, the transistor switch permits the charge from its associated individual photosensor  102  or  110  to be readout through the switch  116  onto the common video line  108 . There is thus run through the line of shift register switches  114  on line  112  digital information in the form of a string of 0&#39;s with a single “1” therein. As the string of digits moves through the shift register switches  114  along line  112 , the single “1” activates the photosensors in a sequence as it moves down the input line  112 , thereby causing the photosensors  108  and  102  to output the signals thereon, in order, onto video line  108 . 
     In the particular design of FIG. 1, the first photosensors to be activated with each scan are the dark photosensors  110 , followed by the active photosensors  102 . As mentioned above, the dark photosensors  110  are typically of the same general structure as the active photosensors  102 , except that they are not exposed to light in any way. Typically, the dark photosensors  110  are shielded from light, such as with a layer of aluminum. Because both the dark photosensor  110  and active photosensors  102  are created in the same chip, they will be as physically similar as possible, so that any systemic process variation or parasitic effect of the active photosensors  102  will be equally apparent in the dark photosensors  110 . In this way, with each scan of video signals the dark photosensors in effect recalibrate the chip by establishing a video signal consistent with no light impinging on a photosensor. Once this offset is determined, systemic errors in the outputs of the active photosensors can be compensated for. 
     A simple apparatus for carrying out the operation of causing the dark photosensors to determine the offset for the active photosensor which are subsequently read out on video line  108  is shown in FIG.  2 . The circuit shown in FIG. 2 is intended to be immediately downstream of the active photosensors on video line  108 . Following passage through a unity-gain amplifier  128 , there is provided on video line  108  a capacitor  130 , which will specifically be referred to as “correction capacitor”  130 . Correction capacitor  130  retains a charge thereon which influences the magnitude of voltage signals from active photosensors which are read out through video line  108 . Because of the relatively high speeds of reading out active photosensors with each. scan, the charge on correction capacitor  130  will remain reasonably constant through every readout sequence. In order to place an original charge on correction capacitor  130 , that is, a charge on correction capacitor  130  which will influence the magnitude of voltage signals in a desirable manner, the correction charge on correction capacitor  130  is fixed with every scan when the dark photosensors  116  are read through video line  108 . 
     In a typical embodiment of a chip such as  100  with four dark photosensors  110 , the selection of four dark photosensors is mandated mainly by standard engineering practice; typically, only one such dark photosensor, such as dark photosensor D 3 , is used to determine the offset for the subsequent readout of active photosensors. At the beginning of each readout, when it is the turn of dark photosensor D 3  to output its dark photosensor signal onto video line  108 , a reference voltage, from a source  132  in parallel to the video line  108 , is activated, such as through a switch  134 . The output of the dark photosensor D 3  of dark photosensors  110 , simultaneous with the application of reference voltage V REF  on the other side of correction capacitor  130 , has the effect of placing on correction capacitor  130  a charge, referred to as the “correction charge,” representative of both the dark photosensor signal and V REF . There may also be other circuits along video line  108 , which are here summarized as the influence of an extra unity gain amplifier, indicated as  136 . 
     As soon as dark photosensor D 3  of dark photosensors  110  has output its dark signal onto video line  108  the transistor  134  goes low, shutting off V REF  on video line  108 , and leaving a residual correction charge on correction capacitor  130 . Henceforth, for all subsequent signals on video line  108  until the next operation on dark photosensor D 3 , the output of V OUT  on video line  108  is: 
     
       
           V   OUT =( V   n   −V   D3   +v   na   +v   nd )+ V   REF   +V   OS   
       
     
     where V n =the output of an active photosensor n; v na =active photosensor noise; v nd =dark photosensor noise; V D3 =the voltage signal from dark photosensor  3 ; and V OS =the offset contributed by other circuitry, symbolized by unity gain amplifier  136 . The signal from dark photosensor  3  will, through correction capacitor  130 , influence the magnitude of all voltage signals from the active photosensors  102  through the whole scan. In this FIG. 2 embodiment, the particular influence of V D3  will last only until the next scan when dark photosensor D 3  is again caused to set the charge on correction capacitor  130 . 
     One practical problem with the simple implementation of FIG. 2, which has been identified as significant in high-precision scanners, is the influence of thermal noise on dark photosensor D 3 , which is symbolized in the above equation by v nd . This thermal noise v nd  may significantly change with each scan. The thermal noise on dark photosensor D 3  has a pernicious influence, because the noise on dark photosensor D 3  will ultimately influence the magnitude of the voltage signal from every single active photosensor in the scan, and may vary significantly from scan to scan. It is a purpose of the present invention to provide a system by which the influence of thermal or other noise on dark photosensors, which influence the offset voltage of the entire chip, is minimized. 
     FIG. 3 is a schematic diagram of a simple embodiment of a dark photosensor offset circuit according to the present invention. It will be noted that the circuit of 
     FIG. 3 shares key similarities with the simple circuit of FIG.  2 : the circuit of FIG. 3 is disposed at the end of the video line  108 , so as to receive signals from both the dark photosensors  110  and the active photosensors  102 . There is also a unity gain amplifier  128 , and correction capacitor  130  on the video line. As in the FIG. 2 circuit, a reference voltage V REF  is available to the video line  108  through transistor  134 . 
     A significant feature of the embodiment of FIG. 3 is an RC circuit, indicated as  140 , in parallel with the video line  108 . This RC, in turn, is connected through an average voltage line  142  (which may include a unity-gain amplifier as shown), to selectably apply an average voltage V AVE  to the video line  108  when activated by switching means  144 . There is also provided in the circuit of FIG. 3 a bypass switch  148 , which, as shown, causes correction capacitor  130  to be bypassed on video line  108  when a voltage is applied thereto. 
     FIG. 4 is a set of comparative, simultaneous waveforms indicating the operation of the different switches in the circuit of FIG. 3, during the time in which dark photosensors, such as dark photosensor D 2  and dark photosensor D 3  of dark photosensors  110 , are used to set the offset for the readout of active photosensor  102  for each scan. Waveform Φ s , at the top of FIG. 4, shows the clock pulses by which video signals from dark photosensors  110  or active photosensors  102  are read out on video line  108 . The video line shown in FIG. 4 gives an example of the typical behavior of voltages on video line  108  with each clock cycle Φ s : as can be seen, the magnitude of the voltage on video line  108  starts on a new tendency (i.e., the voltage moves toward a new plateau, depending on the light intensity on the particular photosensor) with every complete cycle of Φ s . It will also be noted that even though dark photosensors are shielded from light and are technically supposed to output no voltage, there will inevitably be some sort of DC-level variations from dark photosensors  110 . 
     The waveforms Φ DCR1  and Φ DCR2  in FIG. 4 illustrate the operation of the corresponding switches in the circuit of FIG. 3, such as the switch in RC circuit  140  associated with switching means  144  and bypass switch  148 . (“DCR” stands for “DC restore.”) When dark photosensor D 2  and dark photosensor D 3  of dark photosensors  110  are reading out their signals onto video line  108 , it can be seen that Φ DCR1  goes high, which connects RC circuit  140  to video line  108 , and activates bypass switch  148 , which causes the signal on video line  108  to bypass correction capacitor  130 . Thus, when dark photosensor D 2  and dark photosensor D 3  are readout on video line  108 , their signals pass through video line  108 , through bypass switch  148 , and contribute charge to the RC circuit  140 . 
     The RC circuit  140  functions as an averaging circuit which samples the video from both the dark photosensor D 2  and dark photosensor D 3  with every scan on video line  108 . The values of R and C of the RC circuit  140  should be set so that samples of the video signals from dark photosensor D 2  and dark photosensor D 3  over a relatively large number of scans are accumulated. In other words, R and C should provide a time constant by which the effect of numerous scans of dark photosensors D 2  and D 3  loaded onto the RC circuit  140  cause the RC circuit  140  to maintain a running average of the outputs of dark photosensors D 2  and D 3  over a large number of scanlines. For example, if it is desired to maintain a running average of 100 scanlines, then the value of RC should be set equal to 100 times the duration of the sample clock with each scan. Generally, for a practical embodiment of the present invention, the RC should be chosen to sample at least 10 scanlines, and preferably about 100 scanlines, in order to obtain its average dark photosensor signal. 
     RC circuit  140  thus has the effect, by its accumulation of sample charges over as many as 100 scanlines, of averaging put the random thermal noise of the individual dark photosensors. The noise on the dark photosensors will be reduced by a factor of (n×m) 0.5 , where n is the number of scanlines averaged, and m is the number of dark photosensors averaged with each scanline. 
     Returning to FIG. 4, it can be seen, that after the video signals from dark photosensor D 2  and dark photosensor D 3  are read out on video line  108  to RC circuit  140 , Φ DCR1  goes low, thus disconnecting bypass switch  148  and again isolating RC  140  from video line  108 . However, simultaneous with Φ DCR1  going low, another clock signal, Φ DCR2 , goes high. As can be seen in FIG. 3, the effect of Φ DCR2  going high is to activate switching means  144  and cause switch  134  to apply V REF  from source  132  onto video line  108 . In effect, the charge from RC circuit  140  is connected, through line  142 , to video line  108 , so that the charge on RC circuit  140  can be used to affect correction capacitor  130  on one side while V REF  is applied to correction capacitor  130  on the other side. The charge on RC circuit  140 , which as mentioned above is representative of an average of a large number of samples from dark photosensors D 2  and D 3  over many scanlines, is used to set correction capacitor  130  prefatory to the readout of the active photosensors on video line  108 . 
     The setting of correction capacitor  130  in the FIG. 3 embodiment is the same as with the FIG. 2 embodiment described above, with the significant difference that, whereas the FIG. 2 embodiment merely used a single reading of a single dark photosensor to set the charge on correction capacitor  130 , the circuit of FIG. 3 uses an average reading of two of dark photosensors sampled many times, this average reading being maintained by RC circuit  140 . 
     As mentioned above, one practical problem with the above-described system for using an averaging circuit and a correction capacitor to remove noise from the offset value is that offset correction from the perspective of the photosensor circuitry is to some extent at cross-purposes with offset correction with regard to the downstream image-processing circuitry. According to the present invention, this problem is addressed by providing, with each scanline of reading out dark photosensor signals and active photosensor signals, two readings from the dark photosensors: in the first reading of signals from the dark photosensors, the correction capacitor  130  is bypassed by bypass switch  148  and the dark signals are thus transferred to the averaging circuit  140 ; in the second reading of the dark pixels, the same dark photosensor signals are read out again, with this time the correction capacitor  130  not bypassed by bypass switch  148 ; instead of the dark photosensor signals being sent to averaging circuit  140 , an “output enable,” or OE, transistor switch  150  is activated so that the dark photosensor signals are sent downstream to video circuitry. 
     In the particular embodiment of FIG. 4, the averaged dark photosensor signals come from dark photosensors D 2  and D 3  (with reference to FIG. 1 above). As can be seen in the output signals at the bottom of the clocking diagram, with every scanline, the dark photosensors are read out twice before the active pixels (such as  102  in FIG. 1) read out to the video line  108 . 
     Looking at the first set of dark pixel read outs D 1 -D 4  in FIG. 4, it can be seen that for the first readout of dark photosensors, the switch DCR 1  is made high for the duration of readouts of: dark photosensors D 2  and D 3 : With reference to the circuit diagram of FIG. 3, this DCR 1  going high causes both a bypass of the video signals around correction capacitor  130 , and a connection of the dark photosensor signals to averaging circuit  140 . Immediately following DCR 1  going low, DCR 2  goes high: Once again, with reference to FIG. 3, DCR 2  going high connects the potential on the capacitor of averaging circuit  140  to the video line  108 , and also causes a clamping of a reference voltage V REF  from source  132  onto correction capacitor  130 . As described above, this combination of readout of the potential from the averaging circuit  140  with the reference voltage  132  creates a correction potential on correction capacitor  130 , which in turn is used to correct signals which will be subsequently output from active photosensors on video line  108 . 
     With continuing reference to FIG. 4, it will be noted that, during the first readout of dark photosensors, the line OE INT  is low. With reference to FIG. 3, this means that, during the first readout of dark photosensors, transistor  150  effectively cuts off the circuit of FIG. 3 from downstream circuitry, so that the dark photosensor signals on video line  108  are sent only to the averaging circuit  140 . 
     Following the first readout of dark photosensor signals, shown in FIG. 4 bye the second cycle of output signals D 1 -D 4 , both DCR 1  and DCR 2  are low, while the signal on OE INT  (output enable transistor  150 ) is high. With reference to the circuit in FIG. 3, this condition means that the dark photosensor signals read out on video line  108  pass through correction capacitor  130 , and are not sent to averaging circuit  140  but through output enable transistor  150  to downstream video circuitry. This particular cycle of dark-photosensor readouts is used for the benefit of calibrating downstream video circuitry. 
     This is very important, because as can be seen in FIG. 4, the uncorrected “Video” may not be close in value to the DCR reference voltage, “Vref”. This means that when D 2  and D 3  are being flushed through during the average operation, these pixels will not match the DC level of all the other pixels, which are restored (on average) to the reference level, “Vref”. FIG. 4 shows the first D 2  and D 3  readout in “Vout” is not DCR restored. If the first D 2  &amp; D 3  levels were included in the video sent downstream it could significantly add to the dark nonuniformity of the video and the range needed for pixel to pixel dark level correction. 
     FIGS. 5 and 6 are simplified diagrams showing various embodiments of circuits which carryout the double readout of the dark photosensors. In FIGS.  5  and  6 , like numbers such as shown in FIG. 1 above represent like elements. The arrangement of FIG. 5 shows a modification of the shift register relationship shown in FIG. 1, where there is provided, along shift register line  112 , two sets of shift register stages for each of the dark photosensors  110 . As can be seen in the FIGURE, the shift register input to each dark photosensor  110  is connected through an OR gate  200  to two separate shift register stages  114 . As a digital “1” passes along shift register line  112  to the various shift register stages  114 , it can be seen that the double set of shift register stages  114  will, in combination with the OR gate for each dark pixel  110 , cause a readout of a signal from the dark photosensor  110  twice, whenever a shift register stage interacts with the OR gate associated with a particular dark photosensor  110 . After each dark photosensor is read out twice in the manner shown, then each active photosensor is read out once. 
     FIG. 6 shows another possible implementation to enable the double readout system of the present invention. Here, the dark photodiodes D 1 -D 4  share the video line  108  with the active photosensors  102 , but the dark photosensors  110  are controlled by a separate, small shift register, forming a shift register line  113 , which is distinct from the shift register line  112  used for the active photosensors  102 . By controlling the shift register stages  114  on shift register line  113 , the dark photosensors  110  can be read out essentially independently of the readout of the active photosensors  102 . 
     It will be noted that certain basic principles relating to the present invention, in particular using two readouts of a set of photosensors, with one readout going to an averaging circuit and the other readout going to downstream circuitry, can be applied to other basic designs of an active photosensor array. For example, it is known in the art to provide a calibration system which does not use dark photosensors at all, but which rather takes an average of all active photosensors as a basis for offset correction. In such a case, the active photosensors could be read out first to an averaging circuit and then to downstream circuitry, such as in the present invention. If a design is chosen in which offset correction, regardless of the signal source, is performed digitally by downstream circuitry, the use of an averaging circuit such as  140  may not be necessary; similarly, the use of a series capacitor for correction, such as correction capacitor  130 , could be replaced by other means for correcting dark or active photosensor signals being read out on the video line. Also, although the below claim language occasionally refers to reading out dark photosensor signals “a first time” and “a second time”, this language should not be construed necessarily to imply a sequence of operations, or that one readout should immediately follow the other. 
     While this invention has been described in conjunction with various embodiments, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims.