Patent Publication Number: US-7898699-B2

Title: Electronic endoscope apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electronic endoscope apparatus for obtaining a color image signal by obtaining an image of an object to be observed by a scope unit which includes an imaging element for obtaining the image of the object to be observed by receiving light transmitted through a color filter. 
     2. Description of the Related Art 
     Conventionally, various kinds of electronic endoscope apparatuses for displaying color images based on color image signals have been proposed. In the electronic endoscope apparatuses, the color image signal is obtained by obtaining an image of an object to be observed by a scope unit which includes an imaging element on which a color filter is provided. 
     As an example of the electronic endoscope apparatus as described above, an electronic endoscope apparatus which obtains a color image signal including an R component, G component and B component, for example, using a single CCD (imaging element) has been proposed. The electronic endoscope apparatus is a so-called single-chip electronic endoscope apparatus. 
     In the single-chip electronic endoscope apparatus as described above, a color filter such as a primary color filter and a complementary color filter is used. The primary color filter includes a filter of an R component, a filter of a G component and a filter of a B component. The complementary color filter includes a filter of a Cy component, a filter of a Ye component, a filter of an Mg component and a filter of a G component. 
     When a color image is displayed on a monitor or the like based on a color image signal obtained by a CCD (charge coupled device) on which the color filter as described above is provided, video signal conversion processing is performed on the color image signal. However, the method for performing the video signal conversion processing is different according to the kind of the color filter. For example, if a color image signal including R, G and B components is obtained by a CCD on which a primary color filter is provided, a luminance signal Y and chrominance signals R-Y and B-Y are calculated using the color image signal including the R, G and B components without processing. The luminance signal Y and the chrominance signals R-Y and B-Y are used as video signals. 
     Further, generally, if a color image signal is obtained by a CCD on which, for example, a complementary color filter including Cy, Mg, Ye and G components is provided, the color image signal of these components is used to calculate a luminance signal Y and chrominance signals Cr and Cb. Further, the luminance signal Y and the chrominance signals Cr and Cb are used to calculate R, G and B signals. The calculated R, G and B signals are used to calculate a luminance signal Y and chrominance signals R-Y and B-Y. These signals are used as video signals. 
     Here, when a color image signal is obtained by a CCD on which a complementary color filter is provided, there is a well-known technique in readout of signals. In this technique, when the signals are read out, two adjacent pixels are mixed with each other. 
     However, when R, G and B components are calculated based on signal components obtained by mixing pixels as described above, the R, G and B components are not calculated based on pixel information but a mixed signal. Therefore, the R, G and B components of the adjacent pixels are mixed. Further, when the video signal is calculated as described above, the video signal is calculated within a limited range according to the hardware configuration of an endoscope apparatus. In other words, the video signal is calculated using limited numerical values. Therefore, the numerical values are rounded to an integer. Hence, a color which is reproduced from a video signal based on a color image signal obtained by a CCD on which a primary color filter is provided and a color which is reproduced from a video signal based on a color image signal obtained by a CCD on which a complementary color filter is provided are different from each other. 
     Therefore, even if the same color is photographed, a color which represents a different point in color space is reproduced, for example, according to a difference in the filter. 
     Therefore, when an image is displayed using color space that can represent each of RGB colors in 256 values, if each scope unit that has a different kind of color filter, as described above, is connected and used, the color of the displayed image is different according to the scope unit which is used. 
     The color image which is displayed by the electronic endoscope apparatus is provided to perform image diagnoses for observing the color of a mucous membrane, the color of a dyed mucous membrane, or the like. However, if a color representing a different point in color space is reproduced according to the kind of a color filter provided on the CCD, it is difficult to always perform accurate image diagnoses. 
     A technique for adjusting white balance based on the sensitivity of a CCD provided in the scope unit is disclosed in Japanese Unexamined Patent Publication No. 61 (1986)-179129. However, a difference in the kind of the color filter is not considered in the technique. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing circumstances, it is an object of the present invention to provide an electronic endoscope apparatus which includes a scope unit in which a color filter as described above is provided, and which can produce a signal for displaying a color image so that when the same color is photographed, a color represented by the same point in color space is always displayed regardless of a difference in the kind of the color filter as described above. 
     An electronic endoscope apparatus according to the present invention is an electronic endoscope apparatus comprising:
         a scope unit including a color filter which transmits light reflected by an object to be observed when the object is illuminated with light and an imaging element for obtaining an image of the object to be observed by receiving the reflected light transmitted through the color filter, which outputs a color image signal by obtaining the image of the object to be observed by the imaging element; and   a signal processing unit for producing a signal for display, which can display an image including predetermined color components by performing signal processing, based on the kind of the color filter, on the color image signal output from the imaging element of the scope unit, wherein the scope unit further includes a filter information storage means for storing filter information showing the kind of the color filter provided in the scope unit, and wherein the signal processing unit includes a connection unit for selectively connecting a plurality of kinds of scope units, each of which has the filter information showing a different kind of color filter from each other, a filter information obtainment means for obtaining the filter information stored in the filter information storage means of the scope unit connected to the connection unit, and a color space correction processing unit for performing color space correction processing, based on the filter information obtained by the filter information obtainment means, on the color image signal so that each of the signals for display, produced from each of the color image signals which are output from the plurality of kinds of scope units represents the same point in color space.       

     The electronic endoscope apparatus as described above may be configured so that the color space correction processing unit does not perform the color space correction processing on the color image signal if the kind of the color filter in the filter information is a primary color filter and the color space correction processing unit performs the color space correction processing on the color image signal if the kind of the color filter in the filter information is a complementary color filter 
     Further, the scope unit may further include an image formation optical system for forming the image of the object to be observed on the imaging element and an optical system information storage means for storing optical system information showing the kind of the image formation optical system. Further, the signal processing unit may further include an optical system information obtainment means for obtaining the optical system information stored in the optical system information storage means of the scope unit connected to the connection unit and a luminance correction processing unit for performing luminance correction processing on the color image signal based on the optical system information obtained by the optical system information obtainment means. 
     Further, the scope unit may further include an image formation optical system for forming the image of the object to be observed on the imaging element and a mask information storage means for storing mask information showing a relationship between the size of an imaging plane of the imaging element provided in the scope unit and the size of an image formed on the imaging plane by the image formation optical system. Further, the signal processing unit may further include a mask information obtainment means for obtaining the mask information stored in the mask information storage means of the scope unit connected to the connection unit and a mask processing unit for performing mask processing on the color image signal based on the mask information obtained by the mask information obtainment means. 
     Here, the “filter information” includes information which indirectly shows the kind of a color filter as well as information which directly shows the kind of the color filter. 
     Further, the expression “so that each of the signals for display, produced from each of the color image signals which are output from the plurality of kinds or scope units, represents the same point in color space” means that it is not necessary that each of the signals for display represents exactly the same point in the color space. Each of the signals for display may represent approximately the same point in the color space. 
     The phrase “optical system information” includes information which indirectly shows the kind of an image formation optical system as well as information which directly shows the kind of the image formation optical system. 
     Further, the “mask information” includes information which indirectly shows a relationship between the size of an imaging plane of an imaging element and that of an image formed on the imaging plane by the image formation optical system as well as information which directly shows the relationship. 
     Further, the “mask processing” is processing for masking or covering the peripheral portion of the display image of the object to be observed. The peripheral portion of the display image is masked, for example, by converting the color image signal corresponding to the peripheral portion of the image formed on the imaging plane of the imaging element into a signal representing black, or the like. 
     Further, the “mask information” indirectly shows an area on which the “mask processing” is performed. 
     According to the electronic endoscope according to the present invention, the scope unit includes a filter information storage means for storing filter information showing the kind of a color filter provided in the scope unit. Further, the signal processing unit obtains the filter information stored in the filter information storage means. Then, color space correction processing is performed based on the obtained filter information. Therefore, when the same color is photographed, a color image can be displayed so that a color represented by the same point in color space is always displayed regardless of a difference in the kind of the scope unit, namely a difference in the kind of the color filter. 
     Further, generally, when an image of an object to be observed is formed on an imaging plane of an imaging element by an image formation optical system, as the angle of view of the image formation optical system is wider, the peripheral portion of the image becomes darker. Therefore, in the electronic endoscope apparatus, an optical system information storage means for storing optical system information showing the kind of the image formation optical system is provided in the scope unit. Further, the signal processing unit obtains the optical system information and performs luminance correction processing on the color image signal based on the obtained optical system information. If the luminance correction processing is performed so that the peripheral portion of the display image of the object to be observed becomes lighter as the angle of view is wider, an image which has the same distribution of luminance can be always displayed regardless of the kind of the scope unit, namely the kind of the image formation optical system. 
     Further, when mask processing as described above is performed in the electronic endoscope apparatus, the area of the image, on which the mask processing is performed, is different according to a relationship between the size of the imaging plane of the imaging element and that of an image formed on the imaging plane by the image formation optical system. Therefore, in the electronic endoscope apparatus as described above, a mask information storage means for storing mask information showing the relationship between the size of the imaging plane of the imaging element and that of the image formed on the imaging plane by the image formation optical system is provided in the scope unit. Further, the image processing unit obtains the mask information and performs mask processing on the color image signal based on the obtained mask information. Accordingly, the mask processing can be always performed in an appropriate area of the image regardless of the kind of the scope unit, namely the mask information as described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating the schematic configuration of an electronic endoscope apparatus according to an embodiment of the present invention; 
         FIG. 2  is a diagram for explaining mask information; 
         FIG. 3  is a diagram for explaining color space correction processing in the electronic endoscope apparatus illustrated in  FIG. 1 ; 
         FIG. 4  is a diagram for explaining the effect of color space correction processing; 
         FIG. 5  is a diagram for explaining luminance correction processing in the electronic endoscope apparatus illustrated in  FIG. 1 ; 
         FIG. 6A  is a diagram for explaining mask processing in the electronic endoscope apparatus illustrated in  FIG. 1 ; 
         FIG. 6B  is a diagram for explaining mask processing in the electronic endoscope apparatus illustrated in  FIG. 1 ; and 
         FIG. 6C  is a diagram for explaining mask processing in the electronic endoscope apparatus illustrated in  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an embodiment of the present invention will be described with reference drawings.  FIG. 1  is a block diagram illustrating the schematic configuration of an electronic endoscope apparatus according to the present invention. 
     As illustrated in  FIG. 1 , an electronic endoscope apparatus  1  according to the present invention includes a scope unit  2  for obtaining an image of an object  10  to be observed, such as living tissue in a body cavity. The scope unit  2  can be inserted into the body cavity or the like. The electronic endoscope apparatus  1  also includes a signal processing unit  3  for producing a digital video signal for displaying an image on a monitor by performing predetermined signal processing on a signal obtained by the scope unit  2 . The electronic endoscope apparatus  1  also includes a light source apparatus  4  for outputting white light for illuminating the object  10  to be observed to the scope unit  2 . 
     As illustrated in  FIG. 1 , the scope unit  2  includes a CCD  21  for obtaining an image of the object  10  to be observed. The scope unit  2  also includes a first signal processing circuit  22  for performing predetermined processing on a signal obtained by the CCD  21  and a first microcomputer  23  for performing various kinds of control processing. The scope unit  2  also includes a first memory  24  for storing scope machine type information about the scope unit  2 . The scope unit  2  also includes a light guide means  25  for guiding the white light output from the light source apparatus  4  to the leading edge of the scope unit  2 . The scope unit  2  also includes an optical filter  26  for obtaining illumination light in a desired wavelength range by removing light in the other range from the white light output from the light source apparatus  4 . The scope unit  2  also includes an illumination lens  27  for illuminating the object  10  to be observed with the illumination light L which is guided by the light guide means  25 . Further, an objective optical system  21   b  for forming an image of the object  10  to be observed on the CCD  21  is provided at the leading edge of the scope unit  2 . The objective optical system  21   b  includes two concave lenses and a single convex lens, as illustrated in  FIG. 1 . Further, the scope unit  2  includes a connector unit  28  for connecting to a signal processing unit  3 . 
     The CCD  21  is attached to the leading edge of the scope unit  2 . The CCD  21  obtains an image of the object  10  to be observed by performing photoelectric conversion on light reflected at the object  10  to be observed when the object is illuminated with light. Further, a color filter  21   a  is provided on the CCD  21 . The CCD  21  outputs a color image signal by performing photoelectric conversion on the light transmitted through the color filter  21   a . The color filter  21   a  may be a primary color filter, complementary color filter, or the like. The primary color filter includes three color components, namely an R (red) component, G (green) component and B (blue) component. The complementary color filter includes four color components, namely a Cy (cyan) component, Mg (magenta) component, Ye (yellow) component and G (green) component. One of these kinds of color filters is provided on the CCD  21 . In this embodiment, the CCD  21  and the color filter  21   a  are integrated. However, the CCD  21  and the color filter  21   a  may be provided separately. 
     The first signal processing circuit  22  performs signal processing such as correlated double sampling processing and automatic gain control and A/D conversion processing on the signal output from the CCD  21 . The operation of the first signal processing circuit is controlled by the first microcomputer  23 . 
     The first memory  24  stores the scope machine type information about the scope unit  2 . The scope machine type information may be any kind of information as far as the information indirectly shows the kind of a color filter  21   a  provided on the CCD  21 . The name of the machine type of the scope, ID information, or the like may be used as the scope machine type information. In the present embodiment, it is assumed that the name of the machine type, such as scope A, scope B, or scope C, is stored in the first memory  24 . The scope machine type information is read out by the first microcomputer  23  and output to a second microcomputer  36  in the signal processing unit  3 . 
     As the optical filter  26 , an optical filter for removing light in a wavelength range of red, an optical filter for removing light in a wavelength range of yellow, or the like is used based on the kind of the object to be observed. 
     The signal processing unit  3  includes a second signal processing circuit  31  for producing a digital video signal based on the signal output from the first signal processing circuit  22  of the scope unit  2 . The signal processing unit  2  includes a color space correction processing unit  32  for performing color space correction processing on the digital video signal output from the second signal processing circuit  31 . The signal processing unit  3  also includes a luminance correction processing unit  33  for performing luminance correction processing and a mask processing unit  34  for performing mask processing. The signal processing unit  3  also includes a D/A conversion circuit  35  for performing D/A conversion and a second microcomputer  36  for controlling various kinds of signal processing as described above. The signal processing unit  3  also includes a second memory  37  for storing a correspondence table of a plurality of kinds of scope machine type information, filter information, optical system information and mask information. Further, a connector unit  38  for connecting the scope unit  2  to the signal processing unit  3  is provided in the image processing unit  3 . The connector unit  38  is structured so that a plurality of kinds of scope units  2  is attached to or detached from the connector unit  38 . A scope unit  2  is selected from the plurality of kinds of scope units  2 , and the selected scope unit  2  is connected to the connector unit  38  of the image processing unit  3 . 
     The second memory  37  stores a correspondence table of a plurality of kinds of scope machine type information, filter information, optical system information and mask information, for example, as illustrated below. 
     Here, the optical system information in the present embodiment is information showing the angle of view of the objective optical system  21   b  of the scope unit  2 . Further, the mask information in the present embodiment is information showing a relationship between the size of an imaging plane  21   c  of the CCD  21  in the scope unit  2  and that of an image  21   d  formed on the imaging plane  21   c  of the CCD  21  by the objective optical system  21   b . For example, a ratio of the diameter L 2  of the image  21   d  formed on the imaging plane  21   c  of the CCD  21  with respect to the length L 1  of the shorter side of the imaging plane  21   c  of the CCD  21  may be used as the information (please refer to  FIG. 2 ). In table 1, the value of 130% in the mask information indicates, for example, that the ratio of L 4  with respect to L 1  in  FIG. 2  and the ratio of L 3  with respect to L 1  in  FIG. 2  are 15%, respectively. Meanwhile, the value of 90% in the mask information indicates that the size of the image  21   d  formed on the imaging plane  21   c  is smaller than that of the imaging plane  21   c  of the CCD  21 . 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Optical 
                   
               
               
                 Scope Machine 
                 Filter 
                 System 
                 Mask 
               
               
                 Type Information 
                 Information 
                 Information 
                 Information 
               
               
                   
               
             
            
               
                 Scope A 
                 Primary Color 
                 140 degrees 
                 130% 
               
               
                 Scope B 
                 Complementary 
                 120 degrees 
                 110% 
               
               
                   
                 Color 
               
               
                 Scope C 
                 Primary Color 
                  90 degrees 
                  90% 
               
               
                   
               
            
           
         
       
     
     The second microcomputer  36  receives the scope machine type information from the first microcomputer  23  of the scope unit  2 , and refers to the above table based on the scope machine type information received from the first microcomputer  23 . Accordingly, the second microcomputer  36  obtains filter information, optical system information and mask information corresponding to the scope machine type information. Then, the second microcomputer  36  outputs various kinds of control signals, based on the content of the obtained filter information, optical system information and mask information, to the second signal processing circuit  31 , space correction processing unit  32 , luminance correction processing unit  33  and the mask processing unit  34 . 
     It the signal output from the first signal processing circuit  22  of the scope unit  2  is a signal of R, G and B, in other words, if the color filter  21   a  provided on the CCD  21  is a primary color filter, the second signal processing circuit  31  uses the signal of R, G and B to produce a luminance signal Y and chrominance signals R-Y and B-Y. Then, the luminance signal Y and the chrominance signals R-Y and B-Y are output to the color space correction processing unit  32 . If the signal output from the first signal processing circuit  22  of the scope unit  2  is a signal of Cy, Mg, Ye and G, in other words, if the color filter  21   a  provided on the CCD  21  is a complementary color filter, the signal of Cy, Mg, Ye and G is converted into a luminance signal Y and chrominance signal Cr and Cb. Then, a signal of R, G and B is further calculated from the luminance signal Y and the chrominance signals Cr and Cb, and the calculated signal of R, G and B is output to the color space correction processing unit  32 . 
     If the color filter  21   a  provided on the CCD  21  of the scope unit  2  is a primary color filter, the second microcomputer  36  outputs a control signal to the second signal processing circuit  31  so that the second signal processing circuit  31  produces and outputs the luminance signal Y and the chrominance signals R-Y and B-Y, as described above. If the color filter  21   a  provided on the CCD  21  of the scope unit  2  is a complementary color filter, the second microcomputer  36  outputs a control signal to the second signal processing circuit  31  so that the second signal processing circuit  31  produces and outputs the signal of R, G and B, as described above. 
     The color space correction processing unit  32  performs processing on the signal output from the scope unit  2  so that a difference between the point in color space, represented by the signal output from the scope unit  2  in which a primary color filter is provided as the color filter  21   a , and the point in color space, represented by the signal output from the scope unit  2  in which a complementary color filter is provided as the color filter  21   a , becomes small. If the color space correction processing as described above is performed, when an image of the same color is photographed, a signal representing an equivalent point in color space can be always obtained regardless of a difference in the machine type of the scope unit  2  connected to the signal processing unit  3 . Specifically, if the signal output from the scope unit  2  is a signal obtained by a CCD  21  on which a primary color filter is provided, in other words, if the signal is a signal of R, G and B, the color space correction processing unit  32  does not perform any processing. The luminance signal Y and the chrominance signals R-Y and B-Y which are output from the second signal processing unit  22  are output to the luminance correction processing unit  33  without processing. 
     Meanwhile, if the signal output from the scope unit  2  is a signal obtained by a CCD  21  on which a complementary color filter is provided, in other words, if the signal is a signal of Cy, Mg, Ye and G, the color space correction processing unit  32  performs color space correction processing on the signal of R, G and B output from the first signal processing circuit  22 . In the color space correction processing, if the signal of R, G and B output from the first signal processing circuit  22  is represented, for example, by eight bits, the signal represented by eight bits is shifted by two bits in the direction of higher digits, as illustrated in  FIG. 3 . Accordingly, the signal of R, G and B of eight bits is converted into a signal of R, G and B of ten bits. Then, a signal of R, G and B of eight bits corresponding to the signal of R, G and B of ten bits is calculated, and a luminance signal Y and chrominance signals R-Y and B-Y are calculated by using the calculated signal of R, G and B of eight bits. Then, the luminance signal Y and the chrominance signals R-Y and B-Y are output to the luminance correction processing unit  33 . 
     As a method for calculating the signal of R, G and B of eight bits corresponding to the signal of R, G and B of ten bits, the value of a signal of R, G and B of eight bits corresponding to the value of a signal of R, G and B of ten bits, as described above, may be stored in a table in advance, for example. Then, the value of the signal of R, G and B of eight bits may be obtained by referring to the table. Alternatively, a function which has been obtained in advance may be set, and the value of the signal of R, G and B of eight bits corresponding to the signal of R, G and B of ten bits may be obtained by using the function. 
     Here, color space in a CIE color chart is illustrated in  FIG. 4 . If attention is paid to a point (for example, a point representing red) in the color space, the signal output from the scope unit  2  in which the complementary color filter is provided represented by the point of Red 0  and the signal output from the scope unit  2  in which the primary color filter is provided is represented by the point of Red 1 . Therefore, there is a difference in the position (coordinate point) of the color in the color space. Hence, in the present invention, color space correction processing is performed so that the coordinate point of red (Red 0 ) in the color space, which is obtained using the complementary color filter, is moved to the coordinate point of red (Red 1 ) in the color space, which is obtained using the primary color filter. In other words, the color space correction processing is performed so that the coordinate point (X 0 , Y 0 ) in the color space, which is obtained using the complementary color filter, becomes the same as the coordinate point (X 1 , Y 1 ) in the color space, which is obtained using the complementary color filter. 
     If the color filter  21   a  provided on the CCD  21  of the scope unit  2  is a primary color filter, the second microcomputer  36  outputs a control signal to the color space processing unit  32  so that the color space processing unit  32  outputs the luminance signal Y and the chrominance signals R-Y and B-Y, which are output from the second signal processing circuit  22 , to the luminance correction processing unit  33  without performing the color space correction processing as described above. If the color filter  21  provided on the CCD  21  of the scope unit  2  is a complementary color filter, the second microcomputer  36  outputs a control signal to the color space correction processing unit  32  so that the color space correction processing unit  32  performs the color space correction processing as described above to obtain the luminance signal Y and chrominance signals R-Y and B-Y and outputs the obtained luminance signal Y and chrominance signals R-Y and B-Y to the luminance correction processing unit  33 . 
     The luminance correction processing unit  33  performs luminance correction processing, based on the content of the optical system information, on the signal output from the color space correction processing unit  32 . 
     Here, the distribution of the luminance of the image formed on the CCD  21  in the case that the optical system information (angle of view of the objective optical system  21   b ) shows 140 degrees and in the case that the optical system information shows 90 degrees is illustrated in  FIG. 5 , for example. As illustrated in  FIG. 5 , the peripheral portion of the image is darker when the optical system information shows 140 degrees. The luminance correction processing unit  33  performs luminance correction processing so as to reduce the difference in luminance at the peripheral portion of the image due to a difference in the angle of view among a plurality of kinds of scope units  2 , as described above. Specifically, the luminance correction processing unit  33  performs processing so that the luminance represented by the signal corresponding to the peripheral portion of the image formed on the CCD  21  becomes higher as the value of the optical system information is larger. More specifically, a table showing a correspondence between the content of the optical system information and the range and degree of changing the luminance is stored in the second microcomputer  36  in advance. The second microcomputer  36  obtains optical system information based on the scope machine type information. Then, the second microcomputer  36  obtains the range and degree of changing the luminance, which corresponds to the optical system information, by referring to the table. The obtained range and degree of changing the luminance is output to the luminance correction processing unit  33 . Then, the luminance correction processing unit  33  performs luminance correction processing based on the obtained information. Any well-known operation method may be adopted as an actual operation method in the luminance correction processing. For example, an operation method using a blurred image may be adopted. 
     The mask processing unit  34  performs mask processing on the signal obtained by the CCD  21 . The mask processing in the present embodiment is processing for converting the signal corresponding to the peripheral portion of the image obtained by the CCD  21  and the signal corresponding to the area on the outside of the peripheral portion of the image into signals representing black. The signal corresponding to the peripheral portion of the image obtained by the CCD  21  is converted into a signal representing black because an image of the lens barrel of the objective optical system or the like instead of an image of living tissue is present at the peripheral portion of the image obtained by the CCD  21 . The signal corresponding to the peripheral portion is converted into a signal representing black to cover the image of the lens barrel or the like. The signal corresponding to the area on the outside of the peripheral portion of the image obtained by the CCD  21  is converted into a signal representing black so that the image of the living tissue is displayed more clearly. 
     When the mask processing is performed as described above, it is necessary to obtain information about the range or area on which the mask processing is performed. However, the range is different according to the relationship between the size of the imaging plane of the CCD  21  and that of the image formed on the imaging plane. The mask processing unit  34  performs mask processing while considering the relationship as described above. Specifically, a table showing a correspondence between the content of the mask information and the range on which mask processing is performed is set in the second microcomputer  36  in advance. The second microcomputer  36  obtains mask information based on the scope machine type information, and obtains the range of mask processing corresponding to the mask information by referring to the table. Then, the second microcomputer  36  outputs the information to the mask processing unit  34 . The mask processing unit  34  performs mask processing based on the information.  FIGS. 6A through 6C  illustrate the relationship between the size of the imaging plane  21   c  of the CCD  21  and that of the image  21   d  formed on the imaging plane  21   c  and the range of mask processing, which is determined based on the relationship. In each of  FIGS. 6A through 6C , a round area surrounded by a solid line is an area of an image formed on the imaging plane  21   c . A shaded area represents a range on which mask processing is performed. As illustrated in  FIGS. 6A through 6C , the range on which mask processing is performed becomes larger as the value of the mask information is larger. 
     Next, the operation of the electronic endoscope apparatus according to the present embodiment will be described. 
     First, a scope unit  2  is selected from a plurality of kinds of scope units  2  based on the kind of examination or the like. When a connector unit  28  of the selected scope unit  2  is connected to a connector unit  38  of the signal processing unit  3 , the first microcomputer  23  of the scope unit  2  reads out the scope machine type information stored in the first memory  24 . Then, the first microcomputer  23  outputs the readout scope machine type information to the second microcomputer  36  of the signal processing unit  3 . The second microcomputer  36  of the signal processing unit  3  refers to Table 1, illustrated above, and obtains the filter information, optical system information and mask information corresponding to the scope machine type information, input by the first microcomputer  23 . Then, the second microcomputer  36  outputs a control signal as described above, based on the obtained filter information, to the second signal processing circuit  31  and the color space correction processing unit  32 . The second microcomputer  36  also obtains the range and degree of changing the luminance, which corresponds to the optical system information, based on the obtained optical system information, and outputs the range and degree of changing the luminance to the luminance correction processing unit  33 . Further, the second microcomputer  36  obtains the range of performing mask processing based on the obtained mask information, and outputs the range of performing mask processing to the mask processing unit  34 . 
     Meanwhile, after the scope unit  2  is connected to the signal processing unit  3 , as described above, the leading edge of the scope unit  2  is inserted into a body cavity. Then, white light emitted from the light source apparatus  4  is guided to the optical filter  26  by the light guide means  25 . Then, illumination light in a desired wavelength range is obtained by the optical filter  26  by removing light in the other range from the white light, and the illumination light is emitted from the illumination lens  27  to illuminate the object to be observed  10 . 
     Then, the illumination light is reflected by the object  10  to be observed, and an image is formed on the imaging surface of the CCD  21  by the objective optical system  21   b  with the reflected light. At this time, the light which is transmitted through the color filter  21   a  in the CCD  21  is formed on the imaging plane of the CCD  21 . Then, photoelectric conversion is performed on the image formed on the imaging plane of the CCD  21 , and the signal which is produced by performing photoelectric conversion is output to the first signal processing circuit  22 . 
     Then, processing such as correlated double sampling processing and automatic gain control and A/D conversion processing is performed on the input signal at the first signal processing circuit  22 , and a digital image signal is output. The digital image signal output from the first signal processing circuit  22  is input to the second signal processing circuit  31  of the signal processing unit  3  through the connector units  28  and  38 . 
     If the filter information obtained by the second microcomputer  36  shows a primary filter, the luminance signal Y and chrominance signals R-Y and B-Y are calculated in the second signal processing circuit  31 , as described above. Then, the luminance signal Y and the chrominance signals R-Y and B-Y are output to the color space correction processing unit  32 . The color space correction processing unit  32  outputs the luminance signal Y and the chrominance signals R-Y and B-Y to the luminance correction processing unit  33  without processing. 
     Meanwhile, if the filter information obtained by the second microcomputer  36  shows a complimentary filter, the signal of R, G and B is calculated at the second signal processing circuit  31 , as described above. Then, the signal of R, G and B is output to the color space correction processing unit  32 . Then, the color space correction processing unit  32  performs color space correction processing on the signal of R, G and B, as described above. Then, the luminance signal Y and the chrominance signals R-Y and B-Y are calculated based on the signal of R, G and B on which the color space correction processing has been performed. The luminance signal Y and the chrominance signals R-Y and B-Y are output to the luminance correction processing unit  33 . 
     Then, the luminance correction processing unit  33  performs luminance correction processing on the luminance signal Y and the chrominance signals R-Y and B-Y which are input by the color space correction processing unit  32 . The luminance correction processing unit  33  performs luminance correction processing based on the range and degree of changing the luminance, which is output from the second microcomputer  36 . 
     Then, the luminance signal Y and the chrominance signals R-Y and B-Y on which the luminance correction processing has been performed is output to the mask processing unit  34 . Then, the mask processing unit  34  performs mask processing on the luminance signal Y and chrominance signals R-Y and B-Y based on the range on which mask processing is performed. The range on which mask processing is performed is the range output from the second microcomputer  36 . 
     Then, the luminance signal Y and the chrominance signals R-Y and B-Y on which mask processing has been performed is output to the D/A conversion circuit  35 , and converted into analog signals. The analog signals are output to a monitor connected to the signal processing unit  3 . Then, a color image is displayed on the monitor based on the analog signal. 
     In the electronic endoscope according to the embodiment as described above, the scope machine type information is output to the signal processing unit  3  as information which indirectly shows the kind of the color filter  21   a  of the scope unit  2 , the angle of view of the objective optical system  21   b  and the relationship between the size of the imaging plane of the CCD  21  and that of the image formed on the imaging plane. However, the information is not limited to the information as described above. Information which directly shows the kind of the color filter  21   a  of the scope unit  2 , the angle of view of the objective optical system  21   b  and the relationship between the size of the imaging plane of the CCD  21  and that of the image formed on the imaging plant may be output from the scope unit  2  to the signal processing unit  3 . 
     In the electronic endoscope apparatus according the present embodiment, information showing the kind of the optical filter  26  of the scope unit  2  may be output to the color space correction processing unit  32 , and the color space correction processing unit  32  may perform color space correction processing based on the kind of the optical filter  26 . Specifically, if the optical filter  26  is a filter for removing the wavelength range of red, color correction processing should be performed so as to add red color components. If the optical filter  26  is a filter for removing the wavelength range of yellow, color correction processing should be performed so as to add yellow color components. As an actual operation method in the color correction processing, any well-know operation method can be adopted.