Patent Publication Number: US-8125514-B2

Title: Electronic endoscope apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electronic endoscope apparatuses, and more particularly to an electronic endoscope apparatus capable of forming and displaying a spectral image (video image) at a particular wavelength band by operation-processing an image signal carrying a subject color image. 
     2. Description of Related Art 
     In the field of an electronic endoscope using a solid-state imaging device, attentions are recently drawn to the apparatus for performing a spectral imaging combined with a narrow-band pass filter depending upon the spectral reflectance at a digestive organ, such as gastric mucous membrane, i.e. the electronic endoscope incorporating a narrow-band filter (narrow band imaging NBI). This apparatus has three narrow (wavelength) band-pass filters in place of the field-sequential rotary filters of R (red), G (green) and B (blue). By sequentially outputting illumination light through the narrow band-pass filters, three signals obtained based on the illumination light are processed similarly to the RGB signals while changing weighting, thereby forming a spectral image. With such a spectral image, a fine tissue, etc. unobtainable in the background art can be extracted out of a digestive organ, such as stomach or large intestine. 
     Meanwhile, in a synchronous type arranging fine mosaic filters on a solid-state imaging device as disclosed in JP-B-7-96005, JP-A-2003-93336 and Y. Miyake, “Analysis/Evaluation of a Digital Color Image”, University of Tokyo Press, 2000, p.148-153, instead of the field-sequential type using narrow band-pass filters like the above, there is a proposal to form a spectral image by operation-processing the image signal obtained by taking an image of a subject illuminated with white light. This is to determine, as matrix data (coefficient sets), the relationship between digitized data of RGB color sensitivity characteristics and digitized data of spectral characteristics at a particular narrow band, then obtaining a spectral-image signal presuming a spectral image obtained by operating the matrix data with RGB signals through a narrow band-pass filter. Where forming a spectral image by such operation, there is no need to prepare a plurality of filters corresponding to a desired wavelength band. In addition, because of no need of exchanging those, the apparatus can be avoided from increasing its size thus reducing the cost thereof. 
     However, in the electronic endoscope for obtaining a spectral image through operation-processing as above, a spectral image is configured to be displayed as a monochromatic image or as a color image based on particular three colors previously designated. Despite distinction is clear between a normal region and a focal site, the region on display is possibly in a color different from the real one. In this respect, annoyance is to be felt by a physician not accustomed to using the apparatus. 
     SUMMARY OF THE INVENTION 
     An object of an illustrative, non-limiting embodiment of the invention is to an electronic endoscope apparatus arranged to obtain a spectral image by an operation, in which a spectral image in a region can be displayed similar in hue to the real one. 
     According to an aspect of the invention, there is provided an electronic endoscope apparatus comprising: a light source that illuminates a subject with white light; a color imaging device that takes an image of the subject illuminated with white light; and a spectral-image forming circuit that operates RGB three color image signals based on an output from the color imaging device and a matrix data to generate an spectral image signal representative of a spectral image in a color at a designated wavelength, wherein the spectral-image forming circuit is adapted to generate spectral image signals representative of spectral images in respective colors at least three wavelengths different from one another. 
     Incidentally, the three wavelengths are preferably wavelengths at red, green and blue portions of light. 
     Meanwhile, the matrix data for generating the spectral-image signal representative of the spectral image in the color at the designated wavelength can be prepared and stored in a storage section in advance. 
     Meanwhile, the spectral-image forming circuit may be adapted to be capable of generating a spectral-image signal representative of a spectral image in a fixed colors so as to selectively output the signal representative of the spectral image in the fixed color and the spectral image signal representative of the spectral image in the color of the designated wavelength. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features of the invention will appear more fully upon consideration of the exemplary embodiment of the invention, which are schematically set forth in the drawings, in which: 
         FIG. 1  is a block diagram showing a configuration of an electronic endoscope apparatus according to an exemplary embodiment of the present invention; 
         FIG. 2  is a figure showing an operation panel arrangement of a processor unit constituting the electronic endoscope apparatus in  FIG. 1 , with an example of wavelength set; 
         FIGS. 3A to 3C  are figures showing a wavelength-information indication area on the monitor of the electronic endoscope apparatus in  FIG. 1 , with an indication example; 
         FIG. 4  is a graph showing an example of a spectral-image wavelength bands, together with a spectral sensitivity characteristic of a primary-color type CCD; 
         FIG. 5  is a graph showing an example of a spectral-image wavelength bands, together with a reflection spectrum of from a living body; 
         FIG. 6  is a figure showing a wavelength change made with a wavelength change switch of the electronic endoscope apparatus in  FIG. 1 ; and 
         FIG. 7  is a figure showing a wavelength set to be selected in single-color mode for the electronic endoscope apparatus in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     Although the invention will be described below with reference to exemplary embodiments thereof, the following exemplary embodiments and modifications do not restrict the invention. 
     An electronic endoscope apparatus according to an exemplary embodiment of the invention is configured to generate spectral-image signals as to at least three wavelengths different one from another. Accordingly, based on the spectral-image signals, a color spectral image can be formed. Those spectral-image signals are given as signals representing spectral images in colors at designated wavelengths. Accordingly, a spectral image, displayed on a display or to be recorded at an image recorder depending upon the spectral-image signals, is to represent the region taken an image of by a color imaging device basically identically in hue to the real one. 
     Meanwhile, in the electronic endoscope apparatus, particularly where matrix data as in the above is previously prepared and stored in a storage section, operation processing can be performed swiftly to display or record a spectral image. 
     Meanwhile, in the electronic endoscope apparatus, particularly where the spectral-image forming circuit is configured to generate also a signal representative of a spectral image in a fixed color and to selectively output the signal representative of the spectral image in the fixed colors and the signal representative of the spectral image in the color at the designated wavelength, a spectral image can be desirably selectably displayed or recorded identically in hue to the real one or displayed or recorded in fixed hue in accordance with the requirement of a physician or the like. 
     With reference to the drawings, explanation will be now made on an exemplary embodiment according to the present invention. 
       FIG. 1  shows a basic configuration of an electronic endoscope apparatus according to an exemplary embodiment of the invention. The electronic endoscope apparatus of the embodiment is to be set in any of a mode to display a subject spectral image formed as referred later, and a mode to display a subject ordinary image. In the spectral-image display mode, setting is selectively available in any one of a mode to display a spectral image as a color image based on fixed three color-image signals and a mode to display a color image showing a subject in a hue as per (approximate to) the actual. In the outset, explanation is made on the spectral-image display mode and the configuration for realizing same. 
     As shown in  FIG. 1 , the electronic endoscope apparatus is constructed with a scope  10 , i.e. an endoscope main body, and a processor unit  12  detachably connected with the scope  10 . The processor unit  12  is arranged therein with a light source  14  that emits white light. An illumination window  23  is provided at a tip of the scope  10 . The illumination window  23  is opposed with one end of a light guide  24  the other end of which is connected to the light source  14 . Note that the light source  14  may be arranged separately from the processor unit  12 . 
     A CCD, or solid-state imaging device,  15  is provided at a tip of the scope  10 . The CCD  15  uses a complementary-color type having Mg (magenta), Ye (yellow) and Cy (cyan) color filters or a primary-color type having RGB color filters at the image surface. 
     The CCD  15  is connected with a CCD drive circuit  16  that forms a drive pulse depending upon a synchronization signal, and a CDS/AGC (correlated double sampling/auto gain control) circuit  17  that samples and amplifies the image (video) signal outputted from the CCD  15 . The CDS/AGC circuit  17  is connected with an A/D converter  18  that digitizes the analog output thereof. Furthermore, within the scope  10 , a microcomputer  20  is provided to control the various circuits provided therein and take communication control with the processor unit  12 . 
     Meanwhile, in the processor unit  12 , a DSP (digital signal processor)  25  is provided to perform various image processes on the image signal digitized at the A/D converter  18 . From the image signal, the DSP  25  is to generate and output a Y/C signal constituted by luminance (Y) and color difference [C(R−Y, B−Y)] signals. The DSP  25  is connected with a first conversion circuit  28 . The first color-conversion circuit  28  is to convert the Y/C signal outputted from the DSP  25  into R, G and B three color-image signals. Note that the DSP  25  may be arranged on the scope  10  side. 
     In the rear stage to the first color-conversion circuit  28 , there are connected in order a color-space transforming circuit  29  that performs matrix operation for forming a spectral image and outputs image signals representative of spectral images at selected wavelength bands λ 1 , λ 2 , λ 3 , a mode selector  30  that selects any of a single-color mode to form a spectral image at one narrow wavelength band and a three-color mode to form a spectral image at three wavelength bands, a second color-conversion circuit  31  that inputs image signals λ 1 s, λ 2 s, λ 3 s at one or three wavelength bands as signals Rs, Gs, Bs in order to perform a process corresponding to RGB signals and converts the signals Rs, Gs, Bs into a Y/C signal, a signal processing circuit  32  that performs signal processing including mirror image processing, mask generation and character generation, and a D/A converter  33 . The D/A converter  33 , in the last stage, is connected to the outside of the processor unit  12 , e.g. a monitor  34  made by a liquid-crystal display, a CRT or the like and to an image recorder  45  made by a light-scanning recorder. Incidentally, in place of the three-color mode to be selected by the mode selector  30 , two-color mode may be established to form a spectral image at two wavelength bands. 
     Meanwhile, in the processor unit  12 , a microcomputer  35  is provided having a function to have communications with the scope  10 , to control the circuits of the processor unit  12  and to input the matrix (coefficient) data stored in a memory  36  to the color-space transforming circuit  29  or so. The memory  36  is stored with matrix data in a table form to form a spectral image based upon RGB signals. In the embodiment, the matrix data stored in the memory  36  is exemplified as in Table 1. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Parameter 
                 K pr   
                 K pg   
                 K pb   
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 p1 
                 0.000083 
                 −0.00188 
                 0.003592 
               
               
                   
                 . 
                 . 
                 . 
                 . 
               
               
                   
                 . 
                 . 
                 . 
                 . 
               
               
                   
                 . 
                 . 
                 . 
                 . 
               
               
                   
                 p18 
                 −0.00115 
                 0.000569 
                 0.003325 
               
               
                   
                 p19 
                 −0.00118 
                 0.001149 
                 0.002771 
               
               
                   
                 p20 
                 −0.00118 
                 0.001731 
                 0.0022 
               
               
                   
                 p21 
                 −0.00119 
                 0.002346 
                 0.0016 
               
               
                   
                 p22 
                 −0.00119 
                 0.00298 
                 0.000983 
               
               
                   
                 p23 
                 −0.00119 
                 0.003633 
                 0.000352 
               
               
                   
                 . 
                 . 
                 . 
                 . 
               
               
                   
                 . 
                 . 
                 . 
                 . 
               
               
                   
                 . 
                 . 
                 . 
                 . 
               
               
                   
                 p43 
                 0.003236 
                 0.001377 
                 −0.00159 
               
               
                   
                 p44 
                 0.003656 
                 0.000671 
                 −0.00126 
               
               
                   
                 p45 
                 0.004022 
                 0.000068 
                 −0.00097 
               
               
                   
                 p46 
                 0.004342 
                 −0.00046 
                 −0.00073 
               
               
                   
                 p47 
                 0.00459 
                 −0.00088 
                 −0.00051 
               
               
                   
                 p48 
                 0.004779 
                 −0.00121 
                 −0.00034 
               
               
                   
                 p49 
                 0.004922 
                 −0.00148 
                 −0.00018 
               
               
                   
                 p50 
                 0.005048 
                 −0.00172 
                 −0.000036 
               
               
                   
                 p51 
                 0.005152 
                 −0.00192 
                 0.000088 
               
               
                   
                 p52 
                 0.005215 
                 −0.00207 
                 0.000217 
               
               
                   
                 . 
                 . 
                 . 
                 . 
               
               
                   
                 . 
                 . 
                 . 
                 . 
               
               
                   
                 . 
                 . 
                 . 
                 . 
               
               
                   
                 p61 
                 0.00548 
                 −0.00229 
                 0.00453 
               
               
                   
                 p1 
                 1.00000 
                 0.00000 
                 0.00000 
               
               
                   
                 p2 
                 0.00000 
                 1.00000 
                 0.00000 
               
               
                   
                 p3 
                 0.00000 
                 0.00000 
                 1.00000 
               
               
                   
                   
               
            
           
         
       
     
     The matrix data in Table 1 is constituted with sixty-one wavelength parameters (coefficient sets) p 1 -p 61  obtained by segmenting the wavelength range of 400 to 700 nm at an interval of 5 nm and parameters P 1 -P 3  for forming an ordinary image. The parameters p 1 -p 61  are each constituted with coefficients k pr , k pg  and k pb  (p- 1 - 61 ) while the parameter P 1  is with coefficients (1.00000, 0.00000, 0.00000), the parameter P 2  is with coefficients (0.00000, 1.00000, 0.00000) and the parameter P 3  is with coefficients (0.00000, 0.00000, 1.00000). 
     In the color-space transforming circuit  29 , matrix operation, shown in the following equation 1, is performed by use of the coefficients k pr , k pg , k pb  and the RGB signals outputted from the first color-conversion circuit  28 , to thereby form spectral image signals λ 1 s, λ 2 s, λ 3 s. 
     
       
         
           
             
               
                 
                   
                     [ 
                     
                       
                         
                           
                             λ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                         
                       
                       
                         
                           
                             λ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                           
                         
                       
                       
                         
                           
                             λ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             3 
                           
                         
                       
                     
                     ] 
                   
                   = 
                   
                     
                       [ 
                       
                         
                           
                             
                               k 
                               
                                 1 
                                 ⁢ 
                                 r 
                               
                             
                           
                           
                             
                               k 
                               
                                 1 
                                 ⁢ 
                                 g 
                               
                             
                           
                           
                             
                               k 
                               
                                 1 
                                 ⁢ 
                                 b 
                               
                             
                           
                         
                         
                           
                             
                               k 
                               
                                 2 
                                 ⁢ 
                                 r 
                               
                             
                           
                           
                             
                               k 
                               
                                 2 
                                 ⁢ 
                                 g 
                               
                             
                           
                           
                             
                               k 
                               
                                 2 
                                 ⁢ 
                                 b 
                               
                             
                           
                         
                         
                           
                             
                               k 
                               
                                 3 
                                 ⁢ 
                                 r 
                               
                             
                           
                           
                             
                               k 
                               
                                 3 
                                 ⁢ 
                                 g 
                               
                             
                           
                           
                             
                               k 
                               
                                 3 
                                 ⁢ 
                                 b 
                               
                             
                           
                         
                       
                       ] 
                     
                     × 
                     
                       [ 
                       
                         
                           
                             R 
                           
                         
                         
                           
                             G 
                           
                         
                         
                           
                             B 
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
             
           
         
       
     
     Namely, in the case that 500 nm, 620 nm and 650 nm, for example, are selected respectively as the wavelength bands λ 1 , λ 2 , λ 3  for constituting a spectral image, matrix operation is performed by using, as coefficients (k pr , k pg , k pb ), the coefficients (−0.00119, 0.002346, 0.0016) of the parameter p 21  corresponding to the center wavelength 500 nm, the coefficients (0.004022, 0.000068, −0.00097) of the parameter p 45  corresponding to the center wavelength 620 nm and the coefficients (0.005152, −0.00192, −0.000088) of the parameter p 51  corresponding to the center wavelength 650 nm, of among the sixty-one parameters in Table 1. 
     Incidentally, the color-space transforming circuit  29  performs matrix operation by use of the coefficients of parameters P 1 -P 3  when instructed to display or record an ordinary image. In such a case, the RUB signals outputted from the first color-conversion circuit  28  are outputted, as they are, from the color-space transforming circuit  29 . 
     The microcomputer  35  is connected with an operation panel  41 , an image record controller  42 , and an input potion  43  such as a keyboard, in addition to the memory  36 .  FIG. 2  illustrates in detail the operation panel  41 . The operation panel  41  is provided with a set select switch  41   a  for selecting any of wavelength sets a-h schematically shown together therewith, a wavelength select switch  41   b  for selecting respective center wavelengths of wavelength bands λ 1 , π 2 , λ 3 , a change-interval setting switch  41   c  for setting an interval of wavelength to be changed by the wavelength select switch  41   b , a single-three color mode change switch for switching between single-color mode and three-color mode as mentioned before, and a spectral-image forming switch  41   j  for giving an instruction to form a spectral image. 
     Incidentally, the spectral-image forming switch  41   j  can be provided on the scope  10  side. Meanwhile, although a fixed-color mode switch  41   e  and a real-color mode switch  41   f  are provided in a position lower than the spectral-image forming switch  41   j , those will be referred later. 
     Now explanation is made on the operation of the electronic endoscope apparatus according to the embodiment configured as above. In the outset, explanation is made for the case the spectral-image forming switch  41   j  is pressed, i.e. forming a spectral image. 
     In forming a spectral image, the light source  14  shown in  FIG. 1  is driven to emit white light into the light guide  24 . The white light, exiting at the light guide  24  arranged within the scope  10 , is illuminated to a subject. The CCD  15 , driven by the CCD drive circuit  16 , takes an image of the subject and outputs an image signal. The image signal is subjected to correlated double sampling and auto gain control at the CDS/AGC circuit  17 , and then A/D-converted at the A/D converter, thus being inputted as a digital signal to the DSP  25  of the processor unit  12 . 
     At the DSP  25 , gamma processing is made on the output signal of from the scope  10  while color conversion is made on the signals obtained through the Mg, Ye, Cy and g color filters, to form a Y/C signal as noted before. The Y/C signal, outputted from the DSP  25 , is inputted to the first color-conversion circuit  28  where it is converted into an RGB signals. The RGB signals are inputted to the color-space transforming circuit  29 . In the color-space transforming circuit  29 , matrix operation is performed based on the RGB signals and the matrix data, in order to form a spectral image. 
     Now explanation is made in detail on the operation. When pressing the spectral-image forming switch  41   j  of the  FIG. 2  operation panel  41 , the color-space transforming circuit  29  performs matrix operation, for forming a spectral image, according to the equation 1 by use of the matrix data stored in the memory  36  together with the pixel-based RGB signals. Namely, in this case, three wavelength bands λ 1 , λ 2 , λ 3  are established by the operation on the operation panel  41  so that the microcomputer  35  can read the matrix data corresponding to the selected three wavelength bands out of the memory  36  and then input those to the color-space transforming circuit  29 . 
     For example, in the case that 500 nm, 620 nm and 650 nm are selected respectively as the three wavelength bands λ 1 , λ 2 , λ 3 , used are the coefficients of parameter p 21 , p 45  and p 51  in Table 1 corresponding to the respective wavelengths. From the pixel-based RGB signals, spectral image signals λ 1 s, λ 2 s, λs are formed according to matrix operation with the following equation 2. 
     
       
         
           
             
               
                 
                   
                     [ 
                     
                       
                         
                           
                             λ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                             ⁢ 
                             s 
                           
                         
                       
                       
                         
                           
                             λ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                             ⁢ 
                             s 
                           
                         
                       
                       
                         
                           
                             λ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             3 
                             ⁢ 
                             s 
                           
                         
                       
                     
                     ] 
                   
                   = 
                   
                     
                       [ 
                       
                         
                           
                             
                               - 
                               0.00119 
                             
                           
                           
                             0.002346 
                           
                           
                             0.0016 
                           
                         
                         
                           
                             0.004022 
                           
                           
                             0.000068 
                           
                           
                             
                               - 
                               0.00097 
                             
                           
                         
                         
                           
                             0.005152 
                           
                           
                             
                               - 
                               0.00192 
                             
                           
                           
                             0.000088 
                           
                         
                       
                       ] 
                     
                     × 
                     
                       [ 
                       
                         
                           
                             R 
                           
                         
                         
                           
                             G 
                           
                         
                         
                           
                             B 
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
               
             
           
         
       
     
     In the case that three-color mode is selected at the mode selector  30  connected to the  FIG. 2  single-three color mode change switch  41   d , the spectral image signals λ 1 s, λ 2 s, λs are respectively inputted as three color image signals Rs, Gs, Bs to the second color-conversion circuit  31 . Meanwhile, in the case that single-color mode is selected, any one of the spectral image signals λ 1 s, λ 2 s, λs is inputted as a signal Rs, Gs, Bs to the second color-conversion circuit  31 . From now on, explanation is made in detail on the case the three-color mode is selected. 
     In the second color-conversion circuit  31 , the three-color image signals Rs, Gs, Bs are converted into a Y/C signal (Y, Rs−Y, Bs−Y). The Y/C signal is inputted to the monitor  34  and image recorder  45  through the signal processing circuit  32  and D/A converter  33 . 
     Based on the Y/C signal, the monitor  34  displays a spectral image given as a color image constituted by color components at wavelength bands as shown in  FIGS. 4 and 5 . Namely,  FIG. 4  is a concept figure showing a superposition of the three wavelength bands λ 1 , λ 2 , λ 3 , for forming a spectral image, over color-filter spectral sensitivity characteristics R, G, B of the primary-color CCD  15 .  FIG. 5  is a concept figure showing a superposition of the three wavelength bands λ 1 , λ 2 , λ 3  over a reflection spectrum of from a living body. The spectral image signals λ 1 s, λ 2 s, λ 3 s, based on the exemplified parameters p 21 , p 45 , p 51 , represent a spectral image having a wavelength band in a range of within ±10 nm respectively about 500 nm, 620 nm and 650 nm, as shown in  FIG. 5 . The three signals, in a combination, constitute a color spectral image (moving or still image) that is to be displayed or recorded. 
     In the case the fixed-color mode switch  41   e  shown in  FIG. 2  is being pressed, the color spectral image is given as a color image based on fixed three color-image signals. Meanwhile, in the case the real-color mode switch  41   f  shown in  FIG. 2  is pressed, a color image of a subject is displayed or to be recorded in a real color (in a color approximate to the actual), explanation will be made in detail later in respect of this point. 
     Now explanation is made on the selection of wavelength bands λ 1 , λ 2 , λ 3 . In this embodiment, the  FIG. 1  memory  36  in its part is stored with eight wavelength sets λ 1 , λ 2 , λ 3  as default, i.e. a standard set “e” at 400, 500, 600 (nm, true for the following), a blood vessel B 1  set “b” at 470, 500, 670 for rendering a blood vessel, a blood vessel B 2  set “c” at 475, 510, 685 also for rendering a blood vessel, a tissue E 1  set “d” at 440, 480, 520 for rendering a particular tissue, a tissue E 2  set “e” at 480, 510, 580 also for rendering a particular tissue, a hemoglobin set “f” at 400, 430, 475 for rendering a difference between oxy-hemoglobin and deoxy-hemoglobin, a blood-carotene set “g” at 415, 450, 500 for rendering a difference between blood and carotene, and a blood-cytoplasm set “h” at 420, 550, 600 for rendering a difference between blood and carotene. 
     In case the electronic endoscope apparatus after shop shipment is first booted up by powering on, the default wavelength set is selected by the microcomputer  35 . When the spectral-image forming switch  41   j  is pressed on the operation panel  41  shown in  FIG. 2 , the monitor  34  in  FIG. 3  displays a standard set “a” of the selected wavelength set at its wavelength-information indication area  34 s. At this time, in case three-color mode is being selected by pressing the mode change switch  41   d , the parameters corresponding to λ 1 =400 nm, λ 2 =500 nm and λ 3 =600 nm of the standard set “a” are read out of the memory  36  and inputted to the color-space transforming circuit  29 . The color-space transforming circuit  29  performs matrix operation by use of the parameters thus inputted. 
     Meanwhile, the operator, e.g. clinical physician, is allowed to desirably select any of the other wavelength sets “b”-“h” of the default wavelength set by operating the select switch  41   a  of the operation panel  41  in  FIG. 2 . The microcomputer  35  causes the monitor  34  in  FIG. 3  to display the wavelength set thus selected at its wavelength-information indication area  34 s. Simultaneously, the microcomputer  35  reads the parameters corresponding to the wavelength bands λ 1 , λ 2 , λ 3  of the selected wavelength set out of the memory  36  and inputs those to the color-space transforming circuit  29 . The color-space transforming circuit  29  performs matrix operation by use of the parameters thus inputted. 
     The set select switch  41   a  is made up by an upper switch in which an operating portion has an upward triangular shape and a lower switch in which an operating portion has an downward triangular shape. Each time the former is pressed, wavelength set is selected in the order of “a”→“h”→“g” . . . whereas, each time the latter is pressed, wavelength set is selected in the order of “a”→“b”→“c” . . . 
     Meanwhile, where one of the wavelength sets “a”-“h” is being selected, the wavelength bands λ 1 , λ 2 , λ 3  of the selected wavelength set can be respectively changed to desired values by operator&#39;s operation of the wavelength select switch  41   b . In changing the wavelength bands, the interval of wavelength change can be varied by means of the change-interval setting switch  41   c . Namely, by rotating the knob of the change-interval setting switch  41   c , continuous or stepwise switchover is available e.g. at an interval of 1 nm approximate to continuous change, or at an interval of 5 nm, 10 nm or 20 nm as stepwise change. Incidentally, where switchover is at an interval of 1 nm for example, 301 wavelength bands are set up in a range of 400-700 nm, to prepare matrix data (p′ 1 -p′ 301 ) correspondingly to the 301 wavelength bands. 
       FIG. 6  shows a selection of a wavelength band. At a setting of 5 nm interval, switchover is available as 400→405→410 as shown in changing λ 1 . At a setting of 20 nm interval, switchover is as 600→620→640 as shown in changing λ 3 , which value is shown on the monitor  34  at its wavelength-information indication area  34 s. 
       FIGS. 3A to 3C  shows in detail a display state in the wavelength-information indication area  34 s. In the embodiment, waveform information is to be displayed in the wavelength-information indication area  34 s set up on the monitor  34  at its lower right area, as shown in  FIG. 3A . Namely, in the wavelength-information indication area  34 s, selected wavelength values (nm) are indicated underneath the characters λ 1 , λ 2 , λ 3 , etc., as shown in  FIG. 3B . Alternatively, as shown in  FIG. 3C , wavelength division is taken on the abscissa while sensitivity is on the ordinate so that a selected wavelength band can be visually displayed on a movable graph (correspondingly to  FIG. 4 ). 
     The mode change switch  41   d  shown in  FIG. 2  is to switch between single-color mode and three-color mode. In the three-color mode operation, pressing the mode change switch  41   d  provides a switchover into single-color mode so that the microcomputer  35  can set all the wavelength bands λ 1 , λ 2 , λ 3  at the same value, e.g. 470, 470, 470. On the monitor  34 , wavelength band common in value are displayed as shown in  FIG. 7 . As for such a common wavelength band, a desired value can be selected by the wavelength select switch  41   b.    
     Here, besides the default wavelength sets mentioned before, other sets may be prepared as the foregoing eight wavelength sets in accordance with the desire, etc. of the physician who is an apparatus user so that those can be stored in the memory and selected for proper use. Meanwhile, the switches on the operation panel  41  in a part or whole function thereof may be replaced with a key function of a keyboard. 
     Now explanation is made on the mode to make a color display of a spectral image based on fixed three colors and the mode to make a color display of same in a real color of the subject (in a color approximate to the real one). First, the former mode is explained, i.e. the case the fixed-color mode switch  41   e  in  FIG. 2  is being pressed. In this case, the color-space transforming circuit  29  inputs spectral-image signals λ 1 s, λ 2 s, λ 3 s respectively as signals Rs, Gs, Bs, corresponding to fixed colors (colors, say, at wavelengths of 400 nm, 500 nm, 600 nm), to the second color-conversion circuit  31  regardless of the wavelengths λ 1 , λ 2 , λ 3 . Due to this, the spectral image, displayed on the monitor  34  or to be recorded at the image recorder  45 , is given as a color image configured based on the fixed three colors. Incidentally, where single-color mode is selected at the mode selector  30 , a spectral-image signal λ 1 s, λ 2 s or λ 3 s, representative of a spectral image based on one of the fixed three colors, is inputted as an Rs, Gs or Bs signal to the second color-conversion circuit  31 . 
     Now explanation is made on the case the 2 real-color mode switch  41   f  in  FIG. 2  is being pressed. In this case, the color-space transforming circuit  29  inputs spectral-image signals λ 1 s, λ 2 s, λ 3 s respectively as signals Rs, Gs, Bs, representative of a spectral image in colors at the wavelengths λ 1 , λ 2 , λ 3 , to the second color-conversion circuit  31 . Due to this, the spectral image, displayed on the monitor  34  or to be recorded at the image recorder  45 , is indicated basically identical in hue to the real one, as to the region of the subject taken an image of by the CCD  15 . 
     Now explanation is made in detail on the generation of spectral-image signals λ 1 s, λ 2 s, λ 3 s for realizing such real-color mode. In the foregoing fixed-color mode, the color-space transforming circuit  29  outputs the spectral-image signals λ 1 s, λ 2 s, λ 3 s, formed by matrix operation according to equation 2, respectively as three-color image signals Rs, Gs, Bs carried with light intensities in colors at wavelengths of 400 nm, 500 nm and 600 nm (in a image display case). On the contrary, in the real-color mode, provided that Rs, Gs and Bs are also carried with light intensities in colors at wavelengths of 400 nm, 500 nm and 600 nm for example, the color-space transforming circuit  29  outputs spectral-image signals λ 1 s, λ 2 s, λ 3 s respectively formed by the following operations.
 
λ1 s=aRs+bGs+cBs  
 
λ2 s=dRs+eGs+fBs  
 
λ3 s=gRs+hGs+iBs  
 
     The above nine coefficients “a”-“i” are defined as per the following and stored in a table form based on each combination of wavelengths λ 1 , λ 2 , λ 3 , say, in the memory  36 . In operation, those are read out by the microcomputer  35  and inputted to the color-space transforming circuit  29 . Provided that the coefficients “a”-“i” are properly defined based on each combination of wavelengths λ 1 , λ 2 , λ 3 , then the spectral image, displayed on the monitor  34  or to be recorded at the image recorder  45 , is indicated basically identical in hue to the real one as to the region of the subject taken an image of by the CCD  15 . 
     Now explanation is made on the method to determine the coefficients “a”-“i”. It is herein considered that, provided that the tristimulus values (X, Y, Z) are identical with using color-matching functions, the image in a portion to display on the monitor  34  is displayed identical in hue to the real one. From now on, vectors are assumed shown in lower cases while matrixes in upper case. The color-matching functions at a wavelength λ is assumed given as:
 
  x (λ),  y (λ),  z (λ), and   Equation 3:
 
the colors on the monitor  34  is as:
 
  r (λ),  g (λ),  b (λ).   Equation 4:
 
     Furthermore, the chromaticities at the three wavelengths λ 1 , λ 2 , λ 3  are assumed respectively as (x 1 , y 1 , z 1 ), (x 2 , y 2 , z 2 ) and (x 3 , y 3 , z 3 ). 
     At the equal chromaticity value, human eye is to visually perceive the color equivalent. Accordingly, in case the following expression is held with the nine coefficients “a”-“i”, the monitor  34  displays an image at its portion identical in hue to the real one. 
     
       
         
           
             
               
                 
                   
                     
                       ( 
                       
                         
                           
                             
                               
                                 x 
                                 _ 
                               
                               ⁡ 
                               
                                 ( 
                                 λ 
                                 ) 
                               
                             
                           
                         
                         
                           
                             
                               
                                 y 
                                 _ 
                               
                               ⁡ 
                               
                                 ( 
                                 λ 
                                 ) 
                               
                             
                           
                         
                         
                           
                             
                               
                                 z 
                                 _ 
                               
                               ⁡ 
                               
                                 ( 
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                                 ) 
                               
                             
                           
                         
                       
                       ) 
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           
                             r 
                             _ 
                           
                           ⁡ 
                           
                             ( 
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                         ⁢ 
                         
                           
                             g 
                             _ 
                           
                           ⁡ 
                           
                             ( 
                             λ 
                             ) 
                           
                         
                         ⁢ 
                         
                           
                             b 
                             _ 
                           
                           ⁡ 
                           
                             ( 
                             λ 
                             ) 
                           
                         
                       
                       ) 
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           
                             a 
                           
                           
                             d 
                           
                           
                             g 
                           
                         
                         
                           
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                             e 
                           
                           
                             h 
                           
                         
                         
                           
                             c 
                           
                           
                             f 
                           
                           
                             i 
                           
                         
                       
                       ) 
                     
                   
                   = 
                   
                     ( 
                     
                       
                         
                           
                             x 
                             1 
                           
                         
                         
                           
                             x 
                             2 
                           
                         
                         
                           
                             x 
                             3 
                           
                         
                       
                       
                         
                           
                             y 
                             1 
                           
                         
                         
                           
                             y 
                             2 
                           
                         
                         
                           
                             y 
                             3 
                           
                         
                       
                       
                         
                           
                             z 
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                             z 
                             2 
                           
                         
                         
                           
                             z 
                             3 
                           
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   5 
                 
               
             
           
         
       
     
     Here, 
     
       
         
           
             
               
                 
                   
                     
                       ( 
                       
                         
                           
                             x 
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                           ⁡ 
                           
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                             ( 
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                       ) 
                     
                     = 
                     A 
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       r 
                       ⁡ 
                       
                         ( 
                         
                           
                             
                               r 
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                             ⁡ 
                             
                               ( 
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                           ⁢ 
                           
                             
                               g 
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                               ( 
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                               b 
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                         ) 
                       
                     
                     = 
                     
                       
                         B 
                         ⁢ 
                         
                           
 
                         
                         ( 
                         
                           
                             
                               a 
                             
                             
                               d 
                             
                             
                               g 
                             
                           
                           
                             
                               b 
                             
                             
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                       = 
                       
                         
                           C 
                           ⁢ 
                           
                             
 
                           
                           ( 
                           
                             
                               
                                 
                                   x 
                                   1 
                                 
                               
                               
                                 
                                   x 
                                   2 
                                 
                               
                               
                                 
                                   x 
                                   3 
                                 
                               
                             
                             
                               
                                 
                                   y 
                                   1 
                                 
                               
                               
                                 
                                   y 
                                   2 
                                 
                               
                               
                                 
                                   y 
                                   3 
                                 
                               
                             
                             
                               
                                 
                                   z 
                                   1 
                                 
                               
                               
                                 
                                   z 
                                   2 
                                 
                               
                               
                                 
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                                   3 
                                 
                               
                             
                           
                           ) 
                         
                         = 
                         D 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   6 
                 
               
             
           
         
       
     
     If representing transposition with “˜”,
 
 ÃB C=D∴C =( Ã B )− 1    D , and
 
Ã B   Equation 8:
 
is given as 3×3 thus resulting in the existence of an inverse matrix.
 
Accordingly, based on the known A, B and D, coefficient matrix C (namely, coefficients “a”-“i”) can be determined as:
 
 C =( Ã B ) −1    D.    Equation 9:
 
     However, Equation 10: C does not include a negative element. Hence, coefficients “a”-“i” are determined by forcibly taking as 0 (zero) the negative element resulting from calculation. 
     Incidentally, the explanation so far was made on the case to display or record a color image based on three-color image signals λ 1 s, λ 2 s, λ 3 s. In the case of displaying or recording a color image based on four or more colors of image signals, it is possible to form signals representing a spectral image identical in hue to the real one basically on the basis of the similar way of thinking. 
     Now explanation is made on the mode to display an ordinary image of a subject. In the case the spectral image forming switch  41   j  on the operation panel  41  in  FIG. 2  is pressed again when displaying a spectral image formed in the foregoing manner or the spectral image forming switch  41   j  is not pressed from the beginning, the parameter P 1 -P 3  coefficients are selected for matrix operation at the color-space transforming circuit  29 . Due to this, the color-space transforming circuit  29  outputs the RGB signal, which the first color-conversion circuit  28  outputted, without change. The RGB signal is converted into a Y/C signal at the second color-conversion circuit  31 . The Y/C signal is inputted through the signal processing circuit  32  and D/A converter  33  onto the monitor  34 . The monitor  34  displays thereon an ordinary color image (moving or still image) of the subject. 
     Incidentally, in the embodiment, the output of the D/A converter  33  is inputted to the image recorder  45  besides to the monitor  34 . When the image-record controller  42  under control of the microcomputer  35  instructs the image recorder  45  to record an image, the image recorder  45  outputs an ordinary color image of or a spectral-image hard copy of a scene as designated by the instruction. 
     Incidentally, with the conventional endoscope, it is a practice to disperse a coloring agent, such as indigo or pyoktanin to the subject and take an image of a tissue colored by the dispersion. On the contrary, by selecting, as a set of wavelengths λ 1 , λ 2 , λ 3 , wavelength bands at which to be rendered is a tissue colored by dispersing a coloring agent, it is possible to obtain a spectral image equivalent to that due to coloring-agent dispersion without dispersing a coloring agent. 
     Although description has been given heretofore of the invention with reference to the above-mentioned embodiments and examples, the invention is not limited to such embodiments and examples but various modifications are also possible. For example, the values of the radii of curvature, surface intervals and refractive indexes of the respective lens components are not limited to the values that are shown in the above-mentioned numerical examples, but other values can also be used. Also, in the above-mentioned embodiments and examples, the both surfaces of the first to fourth lenses are all formed as aspherical surfaces; however, the invention is not limited to this. 
     This application claims foreign priority from Japanese Patent Application No. 2006-267104, filed Sep. 29, 2006, the entire disclosure of which is herein incorporated by reference.