Abstract:
A device includes an electron emission source array including a plurality of electron emission sources, a photoelectric conversion film disposed to face the electron emission source array, and an intermediate electrode provided between the electron emission source array and the photoelectric conversion film, and an intermediate electrode current detector that applies a positive potential, relative to a potential of the electron emission source array, to the intermediate electrode and detects a current flowing in the intermediate electrode.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an image sensing device that includes an image sensor having both an array of electron emission sources and a photoelectric conversion film and a drive circuit that drives the image sensor. 
         [0003]    2. Description of the Related Art 
         [0004]    An image sensing device, which includes an image sensor having an array of electron emission sources arranged in a matrix, each outputting electrons through application of an electric field, and a photoelectric conversion film, has been proposed. For example, a High-efficiency Electron Emission Device (HEED) has been proposed as an electron emission source (specifically, as a cold cathode electron source) (for example, see Pioneer R&amp;D, Vol. 17, No. 2, 2007, pp. 61-69). 
         [0005]    The HEED enables low-voltage driving and features a simple structure, and studies are underway on application of HEEDs to image sensing devices. An example of the photoelectric conversion film is a High-gain Avalanche Rushing amorphous Photoconductor (HARP). 
         [0006]    However, if a very bright image is incident on the photoelectric conversion film, an excessively large amount of hole charge is accumulated in one pixel area of the photoelectric conversion film, and the amount of electrons emitted by the electron emission source is insufficient to neutralize the accumulated holes, causing a problem of detected signal saturation when image information is read. 
         [0007]    In addition, due to defective pixels of the cold cathode array, there is a problem, for example in that there is a pixel which cannot detect (or reproduce) an image signal since no or almost no electrons are emitted from the pixel or there is a pixel which causes signal saturation even though the brightness of incident light is normal since the amount of emitted electrons is less than a criterion value or a required value. 
         [0008]    Such a defective pixel always becomes a black dot, which is obtrusive noise in terms of image quality. Thus, in the related art, a processor provided downstream for processing the reproduced signal performs so-called signal interpolation. That is, the processor calculates an average of the reproduced signals of left and right pixels or top, bottom, left, and right pixels and replaces the reproduced signal of the black-dot pixel with the average. However, in the related art, a pixel for interpolation is specified from the reproduced image and thus there is a problem in that it is difficult to determine whether the reason why the pixel is black is that the sensed image is black or that the pixel is defective. In addition, there is a problem in that correction is not performed on a pixel that suddenly becomes defective during use. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention has been made in view of the above problems, and it is an object of the present invention to provide an image sensing device that can correctly detect a defective pixel such as an irregular electron emission pixel of a cold cathode array or a residue charge pixel of a photoelectric conversion film. For example, the present invention provides a high-image-quality, high-performance, high-reliability image sensing device which can correctly perform electron emission control and sensed image data interpolation processing based on the information of the detected pixel. 
         [0010]    In accordance with the present invention, the above and other objects can be accomplished by the provision of an image sensing device which includes an electron emission source array including a plurality of electron emission sources arranged in a matrix, a photoelectric conversion film disposed to face the electron emission source array, and an intermediate electrode provided between the electron emission source array and the photoelectric conversion film, wherein the image sensing device outputs, as an image signal, a photoelectric conversion current generated by combination of holes in the photoelectric conversion film through incidence of light thereon and part of electrons emitted from the electron emission source array toward the photoelectric conversion film, the image sensing device further including an intermediate electrode current detector that applies a positive potential, relative to a potential of the electron emission source array, to the intermediate electrode and detects a current flowing in the intermediate electrode. 
         [0011]    According to one aspect of the invention, the intermediate electrode current detector detects a current flowing in the intermediate electrode corresponding to each of the electron emission sources in synchronization with a clock signal that is provided to perform dot sequential scanning of each of the electron emission sources. 
         [0012]    According to another aspect of the invention, the image sensing device further includes a residue hole identifier that identifies an electron emission source that emits a smaller amount of electrons than an amount of holes generated in the photoelectric conversion film based on both the current flowing in the intermediate electrode and the current flowing in the photoelectric conversion film. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0013]    The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
           [0014]      FIG. 1  is a cross-sectional view schematically illustrating a configuration of an HEED cold cathode HARP image sensor; 
           [0015]      FIG. 2  is a block diagram illustrating a configuration of an HEED cold cathode array, a Y-scan driver and an X-scan driver that drive the HEED cold cathode array, and a controller that controls all components of the device; 
           [0016]      FIG. 3  is a partial cross-sectional view schematically illustrating an enlarged pixel portion of the active-matrix HEED cold cathode array to explain the structure of the active-matrix HEED cold cathode array; 
           [0017]      FIG. 4  schematically illustrates a configuration of an image sensing device of the embodiment; 
           [0018]      FIG. 5  schematically illustrates an HARP signal waveform and a mesh signal waveform, i.e., changes of currents of the HARP photoelectric conversion film and the mesh electrode; 
           [0019]      FIG. 6  is a flow chart illustrating how an additional electron emission control operation is performed under control of the controller; 
           [0020]      FIG. 7  schematically illustrates the operation of the controller and the additional electron emission operation in the case where dot sequential scanning is performed on pixels  1  to m of a scan line Yj in the X direction (i.e., the horizontal direction); and 
           [0021]      FIG. 8  is a flow chart illustrating a procedure for performing data interpolation processing under control of the controller. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    The embodiments of the present invention will now be described with reference to the drawings. 
         [0023]      FIG. 1  is a cross-sectional view schematically illustrating a configuration of an HEED cold cathode HARP image sensor  10 . The HEED cold cathode HARP image sensor  10 , which will hereinafter also be referred to as a “cold cathode image sensor” for short, is an image sensor which combines a High-efficiency Electron Emission Device (HEED) with a High-gain Avalanche Rushing amorphous Photoconductor (HARP). In more detail, the cold cathode image sensor  10  includes an HARP photoelectric conversion film  11 , an HEED cold cathode array chip  24 , and a mesh electrode (intermediate electrode)  15  which is disposed between the HARP photoelectric conversion film  11  and the HEED cold cathode array  20 . As described later, the HEED cold cathode array chip  24  integrally includes an active-matrix HEED cold cathode array  20 , which will hereinafter be referred to as an “HEED cold cathode array” for short, and a Y-scan driver  22  and an X-scan driver  23  which are not shown in  FIG. 1 . Although the embodiments are described with reference to the case where a photoelectric conversion film having an HARP structure and a cold cathode array having an HEED structure are used, these are only illustrative and a photoelectric conversion film having a different structure and a cold cathode having a different structure may also be used. 
         [0024]    As shown in  FIG. 1 , the HARP photoelectric conversion film  11  is formed on a transparent conductive layer  12  and the transparent conductive layer  12  is formed on the transparent substrate  13 . The HARP photoelectric conversion film  11  includes amorphous selenium (Se) as a main component and may also include another substance or material, for example, silicon (Si), a compound semiconductor such as lead oxide (PbO), cadmium selenium (CdSe), or gallium arsenide (GaAs). The transparent conductive layer  12  may be formed of a tin oxide (SnO 2 ) layer or an Indium Tin Oxide (ITO) layer. As described later, a predetermined positive voltage, which will also be referred to as an HARP potential or an HARP voltage, is applied to the transparent conductive layer  12  via a connection terminal (input/output terminal) T 1  provided on a glass housing  10 A. 
         [0025]    The transparent substrate  13  may be formed of a substance that transmits light having wavelengths that are sensed by the cold cathode image sensor  10 . For example, the transparent substrate  13  is formed of a substance such as glass that transmits visible light when the cold cathode image sensor  10  performs image sensing using visible light, and is formed of a substance such as sapphire or quartz glass that transmits ultraviolet light when the cold cathode image sensor  10  performs image sensing using ultraviolet light. In addition, the transparent substrate  13  is formed of a substance that transmits x-rays, such as beryllium (Be), silicon (Si), boron nitride (BN), or aluminum oxide (Al 2 O 3 ) when the cold cathode image sensor  10  performs image sensing using x-rays. 
         [0026]    The mesh electrode  15  has a plurality of openings and is formed of a substance such as a known metal substance, an alloy, or a semiconductor substance. A predetermined positive voltage, which will also be referred to as a mesh voltage or a mesh potential, is applied to the mesh electrode  15  via a connection terminal T 5 . The mesh electrode is an intermediate electrode provided for accelerating electrons and collecting surplus electrons. 
         [0027]    As described later, gate electrodes of Metal Oxide Semiconductor (MOS) transistors, which drive the HEED, in the US 01 - 09054  HEED cold cathode array  20  are connected to the X-scan driver  23 , which is a horizontal scanning circuit, and source electrodes (S) thereof are connected to the Y-scan driver  22 , which is a vertical scanning circuit, such that dot sequential scanning is performed through the X-scan driver  23  and the Y-scan driver  22 . The Y-scan driver  22  and the X-scan driver  23  are constructed as a single chip integrally with the HEED cold cathode array  20  on the HEED cold cathode array chip  24  and are provided inside the glass housing  10 A although not illustrated. Signals or voltages required to drive the HEED cold cathode array chip  24  are provided to the HEED cold cathode array chip  24  through connection terminals (input/output terminals) T 2 , T 3 , and T 4  provided on the glass housing  10 A. 
         [0028]    All of these components are vacuum-encapsulated within the glass housing  10 A sealed in frit glass or indium metal. 
         [0029]      FIG. 2  is a block diagram illustrating configurations of the HEED cold cathode array  20 , the Y-scan driver  22  and the X-scan driver  23  that drive the HEED cold cathode array  20 , and the controller  25  that controls all components of the device. The Y-scan driver  22  and the X-scan driver  23  are integrally constructed as the single HEED cold cathode array chip  24 . The controller  25  or other circuits described later may also be mounted on the chip. 
         [0030]    As schematically shown in  FIG. 2 , the HEED cold cathode array  20  is constructed as an active-matrix Field Emitter Array (FEA) by integrally mounting an HEED cold cathode array directly on a drive circuit LSI that is formed on an Si wafer, and allows high-speed driving for image sensing operations (for example, operations with a drive pulse width of 10 ns per pixel) in which dot sequential scanning is performed. The HEED cold cathode array  20  includes a plurality of pixels (n×m pixels) arranged in a matrix having n rows and m columns that are connected to scan drive lines including n lines and m lines that are arranged respectively in the-Y direction (i.e., the vertical direction) and the X direction (i.e., the horizontal direction). For example, the HEED cold cathode array  20  is constructed as a high-precision HEED cold cathode array of 640×480 pixels (i.e., VGA). 
         [0031]    The Y-scan driver  22  and the X-scan driver  23  perform dot sequential scanning and pixel driving based on control signals such as a vertical synchronization signal (V-Sync), a horizontal synchronization signal (H-Sync), and a clock signal (CLK) from the controller  25 . That is, the Y-scan driver  22  and the X-scan driver  23  perform dot sequential scanning such that the Y-scan driver  22  sequentially scans the scan lines (Yj, j=1, 2, . . . , n) in the Y direction and, when one scan line (Yk) is selected, the X-scan driver  23  sequentially scans the line (Xi, i=1, 2, . . . , m) in the X direction to select and drive each pixel on the scan line (Yk). 
         [0032]      FIG. 3  is a partial cross-sectional view schematically illustrating an enlarged pixel portion of the active-matrix HEED cold cathode array  20  to explain the structure of the active-matrix HEED cold cathode array. The HEED cold cathode array  20  includes a drive circuit  40  and an HEED portion  31  which is formed on the top of the drive circuit  40  including a MOS transistor array after the drive circuit  40  and Y-scan and X-scan drivers  22  and  23  that drive and control the drive circuit  40  are formed. 
         [0033]    As shown in  FIG. 3 , the HEED portion  31  is a Metal Insulator Semiconductor (MIS) cold cathode electron emission source that has a layered structure including a lower electrode  33 , a silicon (Si) layer  34 , a silicon oxide (SiO x ) layer  35 , an upper electrode  36  formed of, for example, tungsten (W), and a carbon (C) layer  37 . The upper electrode  36  of the HEED cold cathode array  20  is common to all pixels and the lower electrode  33  and the Si layer  34  are divided to electrically separate the pixels from each other. 
         [0034]    The lower electrode  33  of the HEED portion  31  is connected to a drain electrode D of a MOS transistor of the drive circuit  40  through a via hole. As described above, a gate electrode G and a source electrode S of the MOS transistor are connected to the X-scan driver  23  and the Y-scan driver  22 . Switching of each pixel that emits electrons is performed by controlling the drain potential of the MOS transistor, i.e., the potential of the lower electrode  33  of each pixel of the HEED portion  31 . 
         [0035]    The number of pixels of the HEED cold cathode array  20  is, for example, 640×480 (VGA) and the size of one pixel is 20×20 μm 2 . Emission sites ES, which are openings for electron emission, are formed in the surface portion of one pixel. For example, 3×3 emission sites ES having a diameter DE of about 1 μm (i.e., 3×3 1 μm φ emission sites ES) are formed in an 8×8 μm 2  area of one pixel. For example, an electron current of several microamperes (μA) is emitted (in an electron density of about 4 A/cm 2 ) through one emission site ES. Such numerical values described in this embodiment are only illustrative and may be appropriately changed according to the type of an apparatus that uses the image sensor, the resolution or sensitivity of the image sensor, or the like. 
       Configuration and Operation of Image Sensing Device  
       [0036]      FIG. 4  schematically illustrates a configuration of an image sensing device  50  of the embodiment. The image sensing device  50  includes an HARP signal detector  51 , which is a photoelectric conversion signal detector, and a mesh electrode signal detector (hereinafter referred to as a “mesh signal detector”)  52 . The image sensing device  50  also includes a signal interpolation processor  53  and a cold cathode array controller  54 . Although it is schematically shown in  FIG. 4  that the signal interpolation processor  53  and the cold cathode array controller  54  are provided as a part of a controller  25 , the signal interpolation processor  53  and the cold cathode array controller  54  may, of course, be provided separately from the controller  25 . An HARP. signal and a mesh electrode signal detected by the HARP signal detector  51  and the mesh signal detector  52  are provided to the controller  25 . 
         [0037]    As shown in  FIG. 4 , an external power supply circuit  56  is connected to a transparent conductive layer  12  and is constructed such that a predetermined positive voltage (HARP voltage) Vharp is applied to an HARP photoelectric conversion film  11  and an HARP signal is provided to the HARP signal detector  51  through a capacitor C 1 . The external power supply circuit  56  is also constructed such that a predetermined positive voltage (mesh voltage) Vmesh is applied to a mesh electrode  15  and a mesh electrode signal, which will also be referred to as a “mesh signal”, is provided to the mesh signal detector  52  through a capacitor C 2 . The external power supply circuit  56  is also constructed such that a predetermined positive voltage (HEED drive voltage) Vd is applied to an upper electrode  36  of an HEED portion  31 . For example, the voltage values are such that Vharp=1.5 kV, Vmesh=470V, and Vd=23V. However, the present invention is not limited to these voltage values. 
         [0038]    Next, a description is given of the operation of the image sensing device  50 . When external light is incident on the HARP photoelectric conversion film  11  through the transparent conductive layer  12 , pairs of electrons and holes are generated according to the amount of the incident light at an inner portion of the HARP photoelectric conversion film  11  adjacent to the transparent conductive layer  12 . Among the pairs of electrons and holes, the holes are accelerated by a high electric field, which is applied to the HARP photoelectric conversion film  11  through the transparent conductive layer  12 , and then successively collide with atoms included in the HARP photoelectric conversion film  11  to generate new pairs of electrons and holes. The holes generated through avalanche multiplication are accumulated on one side of the HARP photoelectric conversion film  11  which faces the HEED cold cathode array  20  (i.e., the side of the HARP photoelectric conversion film  11  opposite to the transparent conductive layer  12 ), thereby forming a hole pattern corresponding to the incident optical image. A current generated when the holes combine with electrons emitted by the HEED cold cathode array  20  is detected as an image signal corresponding to the incident optical image. 
         [0039]    The controller  25  performs cold cathode array control and signal processing based on an HARP signal Sh and a mesh signal Sm detected by the HARP signal detector  51  and the mesh signal detector  52 . The components of the image sensing device  50 , including a Y-scan driver  22 , an X-scan driver  23 , the HARP signal detector  51 , the mesh signal detector  52 , and the controller  25 , operate based on (i.e., in synchronization with) a clock signal (CLK) and perform the variety of operations described herein such as detection of a variety of signals, control of the drivers, signal processing. 
         [0040]    Based on the HARP signal Sh and the mesh signal Sm from the HARP signal detector  51  and the mesh signal detector  52 , the controller  25  identifies (or determines) an area of pixels of the HARP photoelectric conversion film  11  in which holes are remaining (hereinafter referred to as “residue charge pixels”) and also identifies pixel(s) of the HEED cold cathode array  20  which do not emit electrons or which emit a smaller amount of electrons than an amount corresponding to a criterion value or a required value (ε) (hereinafter referred to as “non-electron-emission pixels”). 
         [0041]      FIG. 5  schematically illustrates an HARP signal waveform and a mesh signal waveform, i.e., changes of currents of the HARP photoelectric conversion film  11  and the mesh electrode  15 . In the following description, it is assumed that (the absolute values of) the currents of the HARP photoelectric conversion film  11  and the mesh electrode  15  are an HARP current Ih and a mesh current Im, respectively. It is preferable that the following method be used to detect and identify irregular or defective pixels such as residue charge pixels or non-electron-emission pixels although various other methods may also be used. 
         [0042]    The HARP current Ih varies according to the amount (i.e., brightness) of light incident on each pixel and the mesh current Im varies according to the amount of electrons emitted from each pixel of the HEED cold cathode array  20 . The HARP current Ih and the mesh current Im also vary according to the electron transmittance rate (κ) of the mesh electrode  15 . The electron transmittance rate (κ) is a value that defines the rate of transmittance of electrons of the mesh electrode. A known value that is almost constant in the surface as a mesh opening area rate is used as the electron transmittance rate (κ). Alternatively, a distribution may be obtained for each location through experiments or the like and then may be used as the electron transmittance rate (κ), instead of the known value. Here, a relation of Ih=Ih(MAX) or Im=Im(MIN) is satisfied in the residue charge pixels. However, in reality, it is difficult to detect only the state that satisfies such relations due to problems such as noise. Accordingly, practically, it is preferable to set appropriate ranges for identification of such pixels (specifically, a range greater than a threshold value of Ih(th) obtained by multiplying Ih(MAX) by a predetermined factor and a range smaller than a threshold value of Im(th) obtained by multiplying Ih(MIN) by a predetermined factor) as shown in the following expressions. Accordingly, it is preferable to determine the criterion values Ih(th) and Im(th) based on the following expressions and then to identify a residue charge pixel and a non-electron-emission pixel based on the determined criterion values. 
         [0000]        Ih+Im=Ie    (1) 
         [0000]        Ie×κ=Ih (MAX),  Ie ×(1−κ)= Im (MIN)   (2) 
         [0000]      0.9× Ih (MAX)= Ih (th), 1.1× Im (MIN)= Im (th)   (3) 
         [0043]    The following are determination equations for identification. 
         [0000]        Ih&gt;Ih (th) or  Im&lt;Im (th)   (4) 
         [0000]      Ie&lt;ε  (5) 
         [0044]    In addition, in this embodiment, a lower limit of a readable current value is defined as “ε”. 
         [0045]    For example, it is possible to identify a pixel that satisfies the determination equation (4) as a residue charge pixel (which will hereinafter be referred to as an “irregular pixel type A” or a “pixel A”). It is also possible to identify a pixel that satisfies the determination equation (5) as a non-electron-emission pixel which emits a smaller amount of electrons than an amount corresponding to the criterion value (ε) (which will hereinafter be referred to as an “irregular pixel type B” or a “pixel B”). This pixel also satisfies the determination equation (4). The factors used in Equation (3) are not limited to 0.9 and 1.1. Although pixels that satisfy the above reference expressions for identification may include normal pixels, the expressions have been determined taking into consideration balance between circuitry stability and exclusion of influence of noise. 
         [0046]    Although it is preferable that the process for detecting whether or not a residue charge pixel and a non-electron-emission pixel are present and performing correction when they are present be performed every image frame in terms of image quality, the frame interval at which the process is performed is not limited to one frame. For example, the process may be performed once per a number of frames determined according to circuitry size or processing capabilities. 
         [0047]    Since such detection is performed every pixel, it is possible to perform the correction operation during a horizontal blanking period immediately after detection. It is also possible to perform the correction operation over a plurality of horizontal blanking periods after detection to improve the effect of removal of residue charge. 
         [0048]    The following is a more detailed description of the method for identifying a residue charge pixel and a non-electron-emission pixel with reference to  FIG. 5 . As shown in  FIG. 5 , in the case of a normal pixel (N 1 ), the amount (brightness) of incident light is large and a large amount of charge is produced. In this case, the current value Ie (=Ih+Im) is relatively large. That is, although the criterion value Im(th) is large, the amount of emitted electrons is sufficient and the current value Ih is equal to or less than the criterion value Ih(th) (i.e., Ih&lt;Ih(th)), and thus it is possible to determine that the pixel is normal while no residual charge is produced. 
         [0049]    In the case of a normal pixel (N 2 ), the amount of generated charge is smaller and the amount of emitted electrons is smaller, compared to the pixel (N 1 ). In this case, the current value Ie (=Ih+Im) is small compared to the case of the pixel (N 1 ). That is, the criterion value Im(th) is small, the current value Im is equal to or greater than the criterion value Im(th) (i.e., Im≧Im(th)), and the amount of emitted electrons is sufficient. In addition, the criterion value Ih(th) is small, compared to the case of the pixel (N 1 ). However, the current value Ih is equal to or less than the criterion value Ih(th) (i.e., Ih≦Ih(th)), and thus it is possible to determine that the pixel is normal while no residue charge is produced. 
         [0050]    As shown in  FIG. 5 , in the case of an irregular pixel A, the amount (brightness) of incident light is large and the amount of emitted electrons is large, but the amount of emitted electrons is not sufficient so that residue charge is produced (i.e., Ih&gt;Ih(th) and Im&lt;Im(th)). In the case of an irregular pixel B, the amount of emitted electrons is small (Ie&lt;ε) and thus residue charge is produced. These pixels can be determined to be irregular. 
         [0051]    The method for detecting and identifying a residue charge pixel and/or a non-electron-emission pixel is not limited to that described above. For example, the current value Ie (=Ih+Im) may be calculated and then presence or absence of residue charge may be determined and non-electron-emission pixel may be identified based on whether or not the current value exceeds a predetermined criterion value. 
         [0052]      FIG. 6  is a flow chart illustrating how an additional electron emission control operation is performed under control of the controller  25 .  FIG. 7  schematically illustrates the operation of the controller  25  and the additional electron emission operation in the case where dot sequential scanning is performed on pixels  1  to m (m=640 in this example) of a scan line Yj in the X direction (i.e., the horizontal direction). 
         [0053]    First, when dot sequential scanning is performed on a scan line Yj (step S 11 ) (j=1 when an image frame starts), the controller  25  determines whether or not each of the pixels  1  to  640  of the scan line Yj arranged along the horizontal direction is a residue charge pixel in an effective horizontal image period (step S 12 ) and stores the location of a pixel that has been determined to be a residue charge pixel (i.e., the address of the pixel, specifically, the row and column numbers thereof). The controller  25  returns to step S 11  to perform the above procedure on a next scan line when it has been determined at step S 12  that no residue charge pixel is present among the pixels of the scan line. 
         [0054]    When it has been determined that a residue charge pixel is present, the controller  25  controls the cold cathode array controller  54  to perform additional electron emission of a pixel of the HEED cold cathode array  20  corresponding to the pixel which has been determined to be a residue charge pixel in the image blanking period after scanning of the scan line (Yj) (step S 13 ). For example, when a kth pixel of the scan line has been determined to be a residue charge pixel, the cold cathode array controller  54  performs additional electron emission of the k-th pixel of the scan line (Yj) of the HEED cold cathode array  20  in the blanking period. 
         [0055]    Although additional electron emission is performed on the residue charge pixel in the example described above, additional electron emission may also be performed on a pixel located near the residue charge pixel, for example, on a pixel vertically or horizontally adjacent to the residue charge pixel. 
         [0056]    The controller  25  then determines whether or not to terminate the additional electron emission control (step S 14 ). When the controller  25  has determined to continue the additional electron emission control, it returns to step S 11  to repeat the same residue charge pixel identification and additional electron emission control as described above on the next scan line Yj (j=j+1). The controller  25  terminates this routine when it has determined to terminate the additional electron emission control at step S 14 . 
         [0057]      FIG. 8  is a flow chart illustrating a procedure for performing data interpolation processing under control of the controller  25 . First, the controller  25  performs dot sequential scanning of a scan line Yj (j=1 when an image frame starts) (step S 21 ). The controller  25  then determines whether or not each of the pixels  1  to  640  of the scan line Yj arranged along the horizontal direction is a non-electron-emission pixel and stores the location (i.e., address) of a pixel that has been determined to be a non-electron-emission pixel (step S 22 ). 
         [0058]    When a non-electron-emission pixel is detected, the controller  25  controls the signal interpolation processor  53  to perform signal interpolation processing. The signal interpolation processor  53  performs interpolation processing on brightness data of the non-electron-emission pixel using data regarding brightness (the amount of light) of the pixels adjacent to the non-electron-emission pixel in the vertical and/or horizontal direction of the non-electron-emission pixel. The controller  25  also includes a memory (not shown) that stores sensed image data. The controller  25  outputs data interpolated by the signal interpolation processor  53  as a sensed image data signal which will also be referred to as an “image signal” (step S 23 ). 
         [0059]    The controller  25  then counts the number of non-electron-emission pixels (irregular pixels) (step S 24 ) and determines whether or not the counted number exceeds a prescribed value (step S 25 ). When the counted number exceeds the prescribed value, the controller  25  performs fault notification, for example by displaying information indicating fault notification on a display (step S 26 ). 
         [0060]    When it has been determined at step S 25  that the counted number does not exceed the prescribed value, the controller  25  determines whether or not to terminate the interpolation processing control (step S 27 ). When the controller  25  has determined to continue the interpolation processing control, it returns to step S 21  to repeat the same procedure as described above on the next scan line. The controller  25  terminates this routine when it has determined to terminate the interpolation processing control at step S 27 . 
         [0061]    Although the procedure for additional electron emission control and the procedure for data interpolation processing control have been individually described, the two control procedures may, of course, be combined for application. 
         [0062]    Although the HEED cold cathode array has been described as an example of the cold cathode array and the HARP photoelectric conversion film has been described as an example of the photoelectric conversion film in the above embodiment, the present invention may also be applied to an image sensing device using any of various other types of cold cathode arrays and photoelectric conversion films. Substances or materials and numerical values illustrated in the above embodiment are only illustrative. 
         [0063]    As described above, the present invention uses a signal of a mesh electrode provided between an image sensing surface formed of a photoelectric conversion film such as an HARP photoelectric conversion film and a cold cathode array that is disposed facing the photoelectric conversion film with a vacuum space formed therebetween. That is, in the related art, the mesh electrode is merely applied with a constant voltage and is merely used to accelerate emitted electrons and to collect surplus electrons. In the present invention, a current (i.e., a mesh signal) accompanied by the collection of surplus electrons is detected, and, with reference to the detected mesh signal together with a photoelectric conversion signal obtained at the photoelectric conversion film, it is possible to correctly identify residue charge pixels of the photoelectric conversion film and defective pixels of the cold cathode array and to correctly determine the amount of emitted electrons, a variation thereof, the degree of degradation, etc., of each pixel. 
         [0064]    As described above in detail, it is possible to correctly detect irregular pixels of the cold cathode array and residue charge pixels of the photoelectric conversion film using a signal of the mesh electrode. It is also possible to correctly perform additional electrode emission control and/or interpolation processing of sensed image data (i.e., an image signal) based on information of the detected pixels. That is, it is possible to perform additional electron emission only on pixels, which emit a smaller amount of electrons than a prescribed value or criterion value and thus require additional electron emission, thereby achieving advantages such as an increase in the life of the cold cathode and reduction of surplus electrons. It is also possible to realize a high-image-quality, high-performance image sensing device which eliminates or reduces white defects or gray-scale saturation. Accordingly, it is possible to achieve improvement of image quality and resolution and improvement of the reliability and life of the image sensing device. 
         [0065]    Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. 
         [0066]    This application is based on Japanese Patent Application No. 2008-184596 which is hereby incorporated by reference.