Patent Publication Number: US-2022211259-A1

Title: Endoscope system and method of operating the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of PCT International Application No. PCT/JP2020/035328 filed on 17 Sep. 2020, which claims priority under 35 U.S.C § 119(a) to Japanese Patent Application No. 2019-172952 filed on 24 Sep. 2019. The above application is hereby expressly incorporated by reference, in its entirety, into the present application. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an endoscope system that is used to observe chemical fluorescence emitted from a drug given to a patient, and a method of operating the endoscope system. 
     2. Description of the Related Art 
     An endoscope system comprising a light source device, an endoscope, and a processor device has been widely used in a medical field in recent years. In the endoscope system, an object to be observed is irradiated with illumination light from the endoscope, and the image of the object to be observed is displayed on a monitor on the basis of RGB image signals that are obtained in a case where the image of the object to be observed, which is being illuminated with the illumination light, is picked up by an image pickup element of the endoscope. 
     Further, as disclosed in Photodynamic diagnosis (PDD), etc., [online], [Search on Jul. 2, 2019], Internet &lt;URL:https://home.hiroshima-u.ac.jp/urology/whatsnew-PDD.html&gt;, in a diagnosis using an endoscope, in a state where a drug having an affinity for a lesion area is given to a patient and is selectively accumulated, an object to be observed of the patient is irradiated with excitation light so that chemical fluorescence is excited and emitted from the drug contained in the object to be observed. However, in a case where the object to be observed is irradiated with only blue excitation light, the object to be observed is displayed on a monitor as a bluish image that is difficult to be visually recognized by a user (see Photodynamic diagnosis (PDD), etc., [online], [Search on Jul. 2, 2019], Internet &lt;URL:https://home.hiroshima-u.ac.jp/urology/whatsnew-PDD.html&gt;). Accordingly, a user needs to be accustomed to the bluish image to some extent for a screening purpose for detecting a lesion area. Therefore, in JP2006-122234A, a normal image is combined with a fluorescence image and a composite image is displayed on a monitor. As a result, even in a case where a user is not accustomed to fluorescence observation, an image is displayed with a color to be likely to be visually recognized by a user so that the user easily performs fluorescence observation. However, there is a problem that a frame rate drops since the images are always combined and displayed. 
     SUMMARY OF THE INVENTION 
     It may be difficult to see a difference between a fluorescence region, which includes chemical fluorescence, and a region in which the drug is not accumulated, such as a normal mucous membrane, in a fluorescence image. For example, in a case where the object to be observed is irradiated with blue excitation light to emit red chemical fluorescence and the normal mucous membrane is reddish as a whole, it is difficult to see a difference in color between the normal mucous membrane and the fluorescence region. 
     An object of the present invention is to provide an endoscope system that can display an image with a color to be likely to be visually recognized by a user and improve the visibility of a fluorescence region by making a difference in color between a normal mucous membrane and the fluorescence region, and a method of operating the endoscope system. 
     An endoscope system according to an aspect of the present invention comprises: a light source unit that emits excitation light, which causes a drug contained in an object to be observed to be excited to emit chemical fluorescence, and reference light which has at least a wavelength range from a blue-light wavelength range to a red-light wavelength range; and an image control processor. Fluorescence and reference image signals, which are obtained from image pickup of the object to be observed illuminated with the excitation light and the reference light, are input to the image control processor; and the image control processor acquires a plurality of pieces of color information from the fluorescence and reference image signals and expands a color difference between a normal mucous membrane and a fluorescence region, which includes the chemical fluorescence, in a feature space formed by the plurality of pieces of color information. 
     It is preferable that the color difference between the normal mucous membrane and the fluorescence region is increased by being expanded around an expansion center determined in the feature space. It is preferable that the image control processor changes the expansion center on the basis of the fluorescence and reference image signals and fluorescence image signals obtained from image pickup of the object to be observed illuminated with only the excitation light. It is preferable that the image control processor changes the expansion center on the basis of reference image signals obtained from a difference between the fluorescence and reference image signals and the fluorescence image signals. It is preferable that the image control processor calculates an amount of change of the expansion center on the basis of components of fluorescence and reference light included in the fluorescence and reference image signals and components of fluorescence included in the fluorescence image signals. 
     It is preferable that the image control processor generates a first lesion image in which a lesion area including components of the fluorescence and reference light is displayed and a lesion-excluding image in which portions other than the lesion area including the components of the fluorescence and the reference light are displayed, from the fluorescence and reference image signals and the fluorescence image signals, and calculates an amount of change of the expansion center on the basis of the first lesion image and the lesion-excluding image. 
     It is preferable that the endoscope system further comprises a light source processor switching a reference frame where the excitation light and the reference light are emitted and a fluorescence frame where only the excitation light is emitted at a specific number of frames. It is preferable that the image control processor calculates an amount of the fluorescence and an amount of the reference light on the basis of the fluorescence and reference image signals and fluorescence image signals obtained at the fluorescence frame. 
     It is preferable that the image control processor acquires a first lesion image in which a lesion area including components of the fluorescence and reference light is displayed and a second lesion image in which a lesion area including components of the fluorescence is displayed, on the basis of the fluorescence and reference image signals and the fluorescence image signals, and calculates an amount of the fluorescence and an amount of the reference light on the basis of the first lesion image and the second lesion image. 
     It is preferable that the image control processor changes contents of processing to be performed on the fluorescence and reference image signals on the basis of the amount of the fluorescence and the amount of the reference light. It is preferable that the contents of the processing are gain processing or matrix processing. It is preferable that the image control processor corrects the amount of the reference light on the basis of the contents of the processing before and after the change. 
     A method of operating an endoscope system according to another aspect of the present invention comprises: a step of emitting excitation light, which causes a drug contained in an object to be observed to be excited to emit chemical fluorescence, and reference light which has a wavelength range from a blue-light wavelength range to a red-light wavelength range; a step of inputting fluorescence and reference image signals that are obtained from image pickup of the object to be observed illuminated with the excitation light and the reference light; a step of acquiring a plurality of pieces of color information from the fluorescence and reference image signals; and a step of expanding a color difference between a normal mucous membrane and a fluorescence region, which includes the chemical fluorescence, in a feature space formed by the plurality of pieces of color information. 
     According to the present invention, it is possible to display an image with a color to be likely to be visually recognized by a user and to improve the visibility of a fluorescence region by making a difference in color between a normal mucous membrane and the fluorescence region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing the appearance of an endoscope system according to a first embodiment. 
         FIG. 2  is a block diagram showing the functions of the endoscope system according to the first embodiment. 
         FIG. 3  is a graph showing the emission spectra of violet light V, blue light B, green light G, and red light R. 
         FIG. 4  is a diagram showing a reference frame and a fluorescence frame. 
         FIG. 5  is a diagram illustrating a specific number of frames. 
         FIG. 6  is a block diagram showing the functions of a fluorescence image processing unit that is used in a case where a feature space is a signal ratio space. 
         FIG. 7  is a diagram illustrating a normal mucous membrane and a fluorescence region in a signal ratio space. 
         FIG. 8  is a diagram illustrating a radius vector change range Rm. 
         FIG. 9  is a graph showing a relationship between a radius vector r and a radius vector Rx(r) that is obtained after chroma saturation enhancement processing. 
         FIG. 10  is a diagram illustrating a positional relationship between fluorescence regions before and after chroma saturation enhancement processing in the signal ratio space. 
         FIG. 11  is a diagram illustrating an angle change range Rn. 
         FIG. 12  is a graph showing a relationship between an angle θ and an angle Fx(θ) that is obtained after hue enhancement processing. 
         FIG. 13  is a diagram illustrating a positional relationship between fluorescence regions before and after hue enhancement processing in the signal ratio space. 
         FIG. 14  is a diagram illustrating a positional relationship between fluorescence regions before and after chroma saturation enhancement processing in an ab space. 
         FIG. 15  is a diagram illustrating a positional relationship between fluorescence regions before and after hue enhancement processing in the ab space. 
         FIG. 16  is a diagram illustrating a method of calculating the amount of change of an expansion center from the frequency distribution of B/G ratios and the frequency distributions of G/R ratios based on reference image signals. 
         FIGS. 17A and 17B  are diagrams illustrating the positions of a normal mucous membrane, a fluorescence region, and an expansion center line SLs for chroma saturation or an expansion center line SLh for hue. 
         FIG. 18  is a diagram illustrating a method of generating a binarized fluorescence image. 
         FIG. 19  is a diagram illustrating a method of calculating the amount A of change of an expansion center. 
         FIG. 20  is a diagram illustrating a method of generating a first lesion image and a lesion-excluding image. 
         FIG. 21  is a diagram illustrating a method of calculating the amounts Mx and My of change of an expansion center. 
         FIG. 22  is a diagram illustrating a method of generating a first lesion image and a second lesion image. 
         FIG. 23  is a diagram illustrating a method of calculating a first R-pixel value and a second R-pixel value. 
         FIG. 24  is a table showing the pixel values of fluorescence, the pixel values of fluorescence and reference light, the pixel values of reference light, R-gain coefficients for fluorescence observation, the amounts of fluorescence, and the amounts of reference light before and after correction. 
         FIG. 25  is a flowchart showing a series of flows of a fluorescence observation mode. 
         FIG. 26  is a block diagram showing the functions of a fluorescence image processing unit that is used in a case where a feature space is a CrCb space. 
         FIG. 27  is a diagram illustrating a positional relationship between fluorescence regions before and after chroma saturation enhancement processing in the CrCb space. 
         FIG. 28  is a diagram illustrating a positional relationship between fluorescence regions before and after hue enhancement processing in the CrCb space. 
         FIG. 29  is a block diagram showing the functions of a fluorescence image processing unit that is used in a case where a feature space is a HS space. 
         FIG. 30  is a diagram illustrating a positional relationship between fluorescence regions before and after chroma saturation enhancement processing in the HS space. 
         FIG. 31  is a diagram illustrating a positional relationship between fluorescence regions before and after hue enhancement processing in the HS space. 
         FIG. 32  is a block diagram showing the functions of an endoscope system according to a second embodiment. 
         FIG. 33  is a graph showing the emission spectrum of normal light. 
         FIG. 34  is a graph showing the emission spectrum of reference light. 
         FIG. 35  is a graph showing the emission spectrum of excitation light. 
         FIG. 36  is a block diagram showing the functions of an endoscope system according to a third embodiment. 
         FIG. 37  is a plan view of a rotary filter. 
         FIG. 38  is a graph showing the emission spectra of violet light V, blue light B, green light G, and red light R different from  FIG. 3 . 
         FIG. 39  is a block diagram showing the functions of a fluorescence image processing unit that is used in a case where a two-dimensional LUT is used. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     As shown in  FIG. 1 , an endoscope system  10  according to a first embodiment includes an endoscope  12 , a light source device  14 , a processor device  16 , a monitor  18 , and a user interface  19 . The endoscope  12  is optically connected to the light source device  14 , and is electrically connected to the processor device  16 . The endoscope  12  includes an insertion part  12   a  that is to be inserted into an object to be examined, an operation part  12   b  that is provided at the proximal end portion of the insertion part  12   a , and a bendable part  12   c  and a distal end part  12   d  that are provided on the distal end side of the insertion part  12   a . In a case where angle knobs  12   e  of the operation part  12   b  are operated, the bendable part  12   c  is operated to be bent. As the bendable part  12   c  is operated to be bent, the distal end part  12   d  is made to face in a desired direction. 
     Further, the operation part  12   b  is provided with a mode changeover SW  13   a  in addition to the angle knobs  12   e . The mode changeover SW  13   a  is used for an operation for switching a normal observation mode and a fluorescence observation mode. The normal observation mode is a mode where a normal image is displayed on the monitor  18 . The fluorescence observation mode is a mode where a fluorescence image is displayed on the monitor  18 . A foot switch (not shown) may be used as a mode switching unit, which is used to switch a mode, other than the mode changeover SW  13   a.    
     The processor device  16  is electrically connected to the monitor  18  and the user interface  19 . The monitor  18  outputs and displays image information and the like. The user interface  19  includes a keyboard, a mouse, and the like, and has a function to receive an input operation, such as function settings. An external recording unit (not shown), which records image information and the like, may be connected to the processor device  16 . 
     As shown in  FIG. 2 , the light source device  14  comprises: a light source unit  20  that includes a violet light emitting diode (V-LED)  20   a , a blue light emitting diode (B-LED)  20   b , a green light emitting diode (G-LED)  20   c , and a red light emitting diode (R-LED)  20   d ; a light source processor  21  that controls the drive of the four color LEDs  20   a  to  20   d ; and an optical path-combination unit  23  that combines the optical paths of four types of color light that are emitted from the four color LEDs  20   a  to  20   d . The inside of an object to be examined is irradiated with the pieces of light, which are combined by the optical path-combination unit  23 , through a light guide  41  inserted into the insertion part  12   a  and an illumination lens  45 . A laser diode (LD) may be used instead of the LED. 
     As shown in  FIG. 3 , the V-LED  20   a  generates violet light V of which the central wavelength is in the range of 405±10 nm and the wavelength range is in the range of 380 to 420 nm. The B-LED  20   b  generates blue light B of which the central wavelength is in the range of 460±10 nm and the wavelength range is in the range of 420 to 500 nm. The G-LED  20   c  generates green light G of which the wavelength range is in the range of 480 to 600 nm. The R-LED  20   d  generates red light R of which the central wavelength is in the range of 620 to 630 nm and the wavelength range is in the range of 600 to 650 nm. 
     In the fluorescence observation mode, blue light B, green light G, and red light R are used as reference light that has at least a wavelength range from a blue-light wavelength range to a green-light wavelength range and violet light V is used as excitation light that excites a drug contained in an object to be observed to cause chemical fluorescence to be emitted. A drug having an affinity for a lesion area, such as cancer, is used as the drug. In this case, it is preferable that 5-ALA is used as the drug in a case where violet light V is used as excitation light. It is preferable that excitation light in a red-light wavelength range is used in a case where Laserphyrin is used as the drug. Further, it is preferable that excitation light in a blue-light wavelength range is used in a case where fluorescein or Rhodamine Green is used as the drug. Furthermore, it is preferable that excitation light in a green-light wavelength range is used in a case where SYPRO RED is used as the drug. 
     The light source processor  21  turns on the V-LED  20   a , the B-LED  20   b , the G-LED  20   c , and the R-LED  20   d  in the normal observation mode. Accordingly, the object to be observed is irradiated with light in which four types of color light, that is, violet light V, blue light B, green light G, and red light R are mixed, as normal light. Further, the light source processor  21  controls the respective LEDs  20   a  to  20   d  so that the light amount ratios of violet light V, blue light B, green light G, and red light R are Vc:Bc:Gc:Rc in the normal observation mode. On the other hand, the light source processor  21  controls the respective LEDs  20   a  to  20   d  so that the light amount ratios of violet light V, blue light B, green light G, and red light R are Vs:Bs:Gs:Rs in the fluorescence observation mode. 
     The light source processor  21  performs a control to switch a reference frame where excitation light and reference light are emitted and a fluorescence frame where only excitation light is emitted at a specific number of frames in the fluorescence observation mode. Specifically, the light source processor  21  emits violet light V, blue light B, green light G, and red light R as excitation light and reference light by turning on the V-LED  20   a , the B-LED  20   b , the G-LED  20   c , and the R-LED  20   d  at the reference frame as shown in  FIG. 4 . Further, the light source processor  21  emits violet light V as excitation light by turning on only the V-LED  20   a  at the fluorescence frame. In the fluorescence observation mode, the V-LED  20   a , the B-LED  20   b , the G-LED  20   c , and the R-LED  20   d  may be constantly turned on without the division of the reference frame and the fluorescence frame. 
     With regard to the specific number of frames, it is preferable that the number of reference frames is set to be equal to or larger than the number of fluorescence frames. For example, it is preferable that the number of reference frames is set to three and the number of fluorescence frames is set to one as the specific number of frames as shown in  FIG. 5 . Further, it is preferable that the minimum number of fluorescence frames is one in a case where an image pickup sensor  48  uses a global shutter system (CCD), and it is preferable that the minimum number of fluorescence frames is two in a case where the image pickup sensor  48  uses a rolling shutter system (CMOS). The reference frame and the fluorescence frame are automatically switched in this embodiment. However, only the reference frame may be continuously displayed as long as a user does not perform a switching operation, and the reference frame may be switched to the fluorescence frame in a case where a user performs a switching operation. 
     In this specification, the light amount ratios include a case where the ratio of at least one semiconductor light source is 0 (zero). Accordingly, the light amount ratios include a case where any one or two or more of the respective semiconductor light sources are not turned on. For example, even though only one semiconductor light source is turned on and the other three semiconductor light sources are not turned on as in a case where the light amount ratios of violet light V, blue light B, green light G, and red light R are 1:0:0:0, it is regarded that the light source unit  20  has light amount ratios. 
     As shown in  FIG. 2 , the light guide  41  is built in the endoscope  12  and a universal cord (a cord connecting the endoscope  12  to the light source device  14  and the processor device  16 ), and transmits the pieces of light, which are combined by the optical path-combination unit  23 , to the distal end part  12   d  of the endoscope  12 . A multimode fiber can be used as the light guide  41 . For example, a thin fiber cable of which a total diameter of a core diameter of 105 μm, a cladding diameter of 125 μm, and a protective layer forming a covering is in the range of φ 0.3 to 0.5 mm can be used. 
     The distal end part  12   d  of the endoscope  12  is provided with an illumination optical system  30   a  and an image pickup optical system  30   b . The illumination optical system  30   a  includes an illumination lens  45 , and an object to be observed is irradiated with light transmitted from the light guide  41  through the illumination lens  45 . The image pickup optical system  30   b  includes an objective lens  46  and the image pickup sensor  48 . Light reflected from the object to be observed is incident on the image pickup sensor  48  through the objective lens  46 . Accordingly, the reflected image of the object to be observed is formed on the image pickup sensor  48 . 
     The image pickup sensor  48  is a color image pickup sensor, and picks up the reflected image of an object to be examined and outputs image signals. It is preferable that the image pickup sensor  48  is a charge coupled device (CCD) image pickup sensor, a complementary metal-oxide semiconductor (CMOS) image pickup sensor, or the like. The image pickup sensor  48  used in the embodiment of the present invention is a color image pickup sensor that is used to obtain RGB image signals corresponding to three colors of R (red), G (green), and B (blue), that is, a so-called RGB image pickup sensor that comprises R-pixels provided with R-filters, G-pixels provided with G-filters, and B-pixels provided with B-filters. 
     In the case of the normal observation mode, the image pickup sensor  48  outputs normal image signals by picking up the image of an object to be observed that is illuminated with normal light. Further, in the case of the fluorescence observation mode, the image pickup sensor  48  outputs fluorescence and reference image signals by picking up the image of an object to be observed that is illuminated with excitation light and reference light at the reference frame. Furthermore, the image pickup sensor  48  outputs fluorescence image signals, which include the components of chemical fluorescence excited and emitted from an object to be observed, by picking up the image of the object to be observed that is illuminated with excitation light at the fluorescence frame. 
     The image pickup sensor  48  may be a so-called complementary color image pickup sensor, which comprises complementary color filters corresponding to C (cyan), M (magenta), Y (yellow), and G (green), instead of an RGB color image pickup sensor. In a case where a complementary color image pickup sensor is used, image signals corresponding to four colors of C, M, Y, and G are output. Accordingly, the image signals corresponding to four colors of C, M, Y, and G need to be converted into image signals corresponding to three colors of R, G, and B by complementary color-primary color conversion. Further, the image pickup sensor  48  may be a monochrome image pickup sensor that includes no color filter. In this case, since the light source processor  21  causes blue light B, green light G, and red light R to be emitted in a time-sharing manner, demosaicing needs to be added to the processing of image pickup signals. 
     The image signals output from the image pickup sensor  48  are transmitted to a CDS/AGC circuit  50 . The CDS/AGC circuit  50  performs correlated double sampling (CDS) or auto gain control (AGC) on the image signals that are analog signals. The image signals having passed through the CDS/AGC circuit  50  are converted into digital image signals by an analog/digital converter (A/D converter)  52 . The digital image signals, which have been subjected to A/D conversion, are input to the processor device  16 . 
     In the processor device  16 , programs related to various types of processing are incorporated into a program memory. The processor device  16  is provided with a central controller that is formed of an image control processor. The programs incorporated into the program memory are executed by the central controller, so that the functions of an image signal input unit  53 , a digital signal processor (DSP)  56 , a noise removing unit  58 , a signal switching unit  60 , a normal image processing unit  62 , a fluorescence image processing unit  64 , and a video signal generation unit  66  are realized. Digital image signals obtained from the endoscope  12  are input to the image signal input unit  53 . 
     The DSP  56  performs various types of signal processing, such as defect correction processing, offset processing, gain processing, matrix processing, gamma transformation processing, and demosaicing processing, on the received image signals. Signals of defective pixels of the image pickup sensor  48  are corrected in the defect correction processing. Dark current components are removed from the image signals subjected to the defect correction processing in the offset processing, so that an accurate zero level is set. The image signals subjected to the offset processing are multiplied by a specific gain in the gain processing, so that signal levels are adjusted. The gain processing varies depending on the normal observation mode and the fluorescence observation mode. With regard to the gain processing in the normal observation mode, R-image signals, G-image signals, and B-image signals of the normal image signals are multiplied by an R-gain coefficient, a G-gain coefficient, and a B-gain coefficient for normal observation, respectively. With regard to the gain processing in the fluorescence observation mode, R-image signals, G-image signals, and B-image signals of the fluorescence and reference image signals are multiplied by an R-gain coefficient, a G-gain coefficient, and a B-gain coefficient for fluorescence observation, respectively. 
     The matrix processing for improving color reproducibility is performed on the image signals subjected to the gain processing. The matrix processing varies depending on the normal observation mode and the fluorescence observation mode. With regard to the matrix processing in the normal observation mode, matrix processing for normal observation is performed on the normal image signals. With regard to the matrix processing in the fluorescence observation mode, matrix processing for fluorescence observation is performed on the fluorescence and reference image signals. 
     After that, brightness or chroma saturation is adjusted by the gamma transformation processing. The demosaicing processing (also referred to as equalization processing or demosaicing) is performed on the image signals subjected to the matrix processing, so that signals of colors deficient in each pixel are generated by interpolation. All the pixels are made to have the signals corresponding to the respective colors of R, G, and B by this demosaicing processing. 
     The noise removing unit  58  performs noise removal processing (for example, a moving-average method, a median filtering method, or the like) on the image signals, which have been subjected to gamma correction and the like by the DSP  56 , to remove noise from the image signals. The image signals from which noise has been removed are transmitted to the signal switching unit  60 . 
     In a case where a mode is set to the normal observation mode by the mode changeover SW  13   a , the signal switching unit  60  transmits the normal image signals to the normal image processing unit  62  as image signals. In a case where a mode is set to the fluorescence observation mode by the mode changeover SW  13   a , the signal switching unit  60  transmits the fluorescence and reference image signals and the fluorescence image signals to the fluorescence image processing unit  64  as image signals. 
     The normal image processing unit  62  performs image processing for a normal image on the normal image signals. The image processing for a normal image includes structure enhancement processing for a normal image, and the like. The normal image signals, which have been subjected to the image processing for a normal image, are input to the video signal generation unit  66  from the normal image processing unit  62  as a normal image. 
     The fluorescence image processing unit  64  generates a fluorescence image on the basis of the fluorescence and reference image signals or the fluorescence image signals. The details of the fluorescence image processing unit  64  will be described later. The fluorescence image generated by the fluorescence image processing unit  64  is input to the video signal generation unit  66 . 
     The video signal generation unit  66  converts the normal image or the special image, which is input from the normal image processing unit  62  or the fluorescence image processing unit  64 , into video signals for displaying the image as an image that can be displayed on the monitor  18 . The monitor  18  displays the normal image on the basis of the video signals. 
     As shown in  FIG. 6 , the fluorescence image processing unit  64  comprises an inverse gamma transformation section  70 , a Log transformation section  71 , a signal ratio calculation section  72 , a polar coordinate transformation section  73 , a color difference expansion section  74 , a Cartesian coordinate transformation section  78 , an RGB conversion section  79 , a brightness adjustment section  81 , a structure enhancement section  82 , an inverse Log transformation section  83 , a gamma transformation section  84 , an expansion center change section  85 , and a fluorescence/reference light amount-calculation section  88 . 
     The fluorescence and reference image signals are input to the inverse gamma transformation section  70 . Further, the fluorescence and reference image signals and the fluorescence image signals, which are required to change an expansion center, are input to the expansion center change section  85 . Furthermore, the fluorescence and reference image signals and the fluorescence image signals, which are required to calculate the amount of fluorescence and reference light, are input to the fluorescence/reference light amount-calculation section  88 . It is preferable that the fluorescence and reference image signals are RGB image signals corresponding to three colors and consisting of B-image signals output from the B-pixels of the image pickup sensor  48 , G-image signals output from the G-pixels of the image pickup sensor  48 , and R-image signals output from the R-pixels of the image pickup sensor  48 . Further, it is preferable that the fluorescence image signals are three color image signals of RGB image signals, but the fluorescence image signals may be only image signals including the components of chemical fluorescence, for example, R-image signals in a case where the components of chemical fluorescence have a red-light wavelength range. 
     The inverse gamma transformation section  70  performs inverse gamma transformation on the input RGB three-channel digital image signals. Since the RGB image signals subjected to this inverse gamma transformation are linear reflectance-RGB signals that are linear in a reflectance from a sample, a ratio of signals related to a variety of biological information of the sample among the RGB image signals is high. A linear reflectance-R-image signal is referred to as a first R-image signal, a linear reflectance-G-image signal is referred to as a first G-image signal, and a linear reflectance-B-image signal is referred to as a first B-image signal. The first R-image signal, the first G-image signal, and the first B-image signal are collectively referred to as first RGB image signals. 
     The Log transformation section  71  performs Log transformation on each of the linear reflectance-RGB image signals. Accordingly, an R-image signal (logR) subjected to Log transformation, a G-image signal (logG) subjected to Log transformation, and a B-image signal (logB) subjected to Log transformation are obtained. The signal ratio calculation section  72  (corresponding to “color information acquisition section” of the present invention) calculates a B/G ratio (a value obtained after “−log” is omitted from −log(B/G) is written as “B/G ratio”) by performing differential processing (logG−logB=logG/B=−log(B/G)) on the basis of the G-image signal and the B-image signal subjected to Log transformation. Further, the signal ratio calculation section  72  calculates a G/R ratio by performing differential processing (logR−logG=logR/G=−log(G/R)) on the basis of the R-image signal and the G-image signal subjected to Log transformation. Like the B/G ratio, a value obtained after “−log” is omitted from −log(G/R) is referred to as “G/R ratio”. 
     The B/G ratio and the G/R ratio are obtained for each pixel from the pixel values of pixels that are present at the same positions in the B-image signals, the G-image signals, and the R-image signals. Further, the B/G ratio and the G/R ratio are obtained for each pixel. Furthermore, the B/G ratio correlates with a blood vessel depth (a distance between the surface of a mucous membrane and the position of a specific blood vessel). Accordingly, in a case where a blood vessel depth varies, the B/G ratio is also changed with a variation in blood vessel depth. Moreover, the G/R ratio correlates with the amount of blood (hemoglobin index). Accordingly, in a case where the amount of blood is changed, the G/R ratio is also changed with a variation in the amount of blood. 
     The polar coordinate transformation section  73  transforms the B/G ratio and the G/R ratio, which are obtained from the signal ratio calculation section  72 , into a radius vector r and an angle θ. In the polar coordinate transformation section  73 , the transformation of the B/G ratio and the G/R ratio into the radius vector r and the angle θ are performed for all the pixels. The color difference expansion section  74  expands a color difference between a normal mucous membrane, which is included in an object to be observed, and a fluorescence region, which includes chemical fluorescence excited and emitted from a drug contained in the object to be observed, in a signal ratio space (feature space) formed by the B/G ratio and the G/R ratio that are one of a plurality of pieces of color information. The expansion of a chroma saturation difference between the normal mucous membrane and the fluorescence region or the expansion of a hue difference between the normal mucous membrane and the fluorescence region is performed in this embodiment as the expansion of a color difference. For this purpose, the color difference expansion section  74  includes a chroma saturation enhancement processing section  76  and a hue enhancement processing section  77 . 
     The chroma saturation enhancement processing section  76  performs chroma saturation enhancement processing for expanding a chroma saturation difference between the normal mucous membrane and the fluorescence region in the signal ratio space. Specifically, the chroma saturation enhancement processing is performed by the expansion or compression of the radius vector r in the signal ratio space. The hue enhancement processing section  77  performs hue enhancement processing for expanding a hue difference between the normal mucous membrane and the fluorescence region in the signal ratio space. Specifically, the hue enhancement processing is performed by the expansion or compression of the angle θ in the signal ratio space. The details of the chroma saturation enhancement processing section  76  and the hue enhancement processing section  77  having been described above will be described later. 
     The Cartesian coordinate transformation section  78  transforms the radius vector r and the angle θ, which have been subjected to the chroma saturation enhancement processing and the hue enhancement processing, into Cartesian coordinates. Accordingly, the radius vector r and the angle θ are transformed into the B/G ratio and the G/R ratio subjected to the expansion/compression of the angle. The RGB conversion section  79  converts the B/G ratio and the G/R ratio, which have been subjected to the chroma saturation enhancement processing and the hue enhancement processing, into second RGB image signals using at least one image signal of the first RGB image signals. For example, the RGB conversion section  79  converts the B/G ratio into a second B-image signal by performing an arithmetic operation that is based on the first G-image signal of the first RGB image signals and the B/G ratio. Further, the RGB conversion section  79  converts the G/R ratio into a second R-image signal by performing an arithmetic operation that is based on the first G-image signal of the first RGB image signals and the G/R ratio. Furthermore, the RGB conversion section  79  outputs the first G-image signal as a second G-image signal without performing special conversion. The second R-image signal, the second G-image signal, and the second B-image signal are collectively referred to as the second RGB image signals. 
     The brightness adjustment section  81  adjusts the pixel values of the second RGB image signals using the first RGB image signals and the second RGB image signals. The reason why the brightness adjustment section  81  adjusts the pixel values of the second RGB image signals is as follows. The brightness of the second RGB image signals, which are obtained from processing for expanding or compressing a color region by the chroma saturation enhancement processing section  76  and the hue enhancement processing section  77 , may be significantly different from that of the first RGB image signals. Accordingly, the pixel values of the second RGB image signals are adjusted by the brightness adjustment section  81  so that the second RGB image signals subjected to brightness adjustment have the same brightness as the first RGB image signals. 
     The brightness adjustment section  81  comprises a first brightness information-calculation section  81   a  that obtains first brightness information Yin on the basis of the first RGB image signals, and a second brightness information-calculation section  81   b  that obtains second brightness information Yout on the basis of the second RGB image signals. The first brightness information-calculation section  81   a  calculates the first brightness information Yin according to an arithmetic expression of “kr×pixel value of first R-image signal+kg×pixel value of first G-image signal+kb×pixel value of first B-image signal”. Like the first brightness information-calculation section  81   a , the second brightness information-calculation section  81   b  also calculates the second brightness information Yout according to the same arithmetic expression as described above. In a case where the first brightness information Yin and the second brightness information Yout are obtained, the brightness adjustment section  81  adjusts the pixel values of the second RGB image signals by performing arithmetic operations that are based on the following equations (E1) to (E3). 
         R *=pixel value of second  R -image signal× Y in/ Y out  (E1):
 
         G *=pixel value of second  G -image signal× Y in/ Y out  (E2):
 
         B *=pixel value of second  B -image signal× Y in/ Y out  (E3):
 
     “R*” denotes the second R-image signal subjected to brightness adjustment, “G*” denotes the second G-image signal subjected to brightness adjustment, and “B*” denotes the second B-image signal subjected to brightness adjustment. Further, “kr”, “kg”, and “kb” are arbitrary constants that are in the range of “0” to “1”. 
     The structure enhancement section  82  performs structure enhancement processing on the second RGB image signals having passed through the RGB conversion section  79 . Frequency filtering or the like is used as the structure enhancement processing. The inverse Log transformation section  83  performs inverse Log transformation on the second RGB image signals having passed through the structure enhancement section  82 . Accordingly, second RGB image signals having anti-logarithmic pixel values are obtained. The gamma transformation section  84  performs gamma transformation on the RGB image signals having passed through the inverse Log transformation section  83 . Accordingly, second RGB image signals having gradations suitable for an output device, such as the monitor  18 , are obtained. The second RGB image signals having passed through the gamma transformation section  84  are transmitted to the video signal generation unit  66 . 
     The chroma saturation enhancement processing section  76  and the hue enhancement processing section  77  increase a chroma saturation difference or a hue difference between a normal mucous membrane and a fluorescence region that are distributed in a first quadrant of the signal ratio space (feature space) formed by the B/G ratio and the G/R ratio as shown in  FIG. 7 . Since the fluorescence region is reddish, the fluorescence region is positioned on the right side of the normal mucous membrane in the first quadrant of the signal ratio space. In a situation where the normal mucous membrane and the fluorescence region are distributed in the signal ratio space as described above, the color difference expansion section  74  determines an expansion center in the signal ratio space so that a color difference between the normal mucous membrane and the fluorescence region expands. Specifically, the chroma saturation enhancement processing section  76  determines an expansion center for chroma saturation that is used to expand a chroma saturation difference between the normal mucous membrane and the fluorescence region. Further, the hue enhancement processing section  77  determines an expansion center for hue that is used to expand a hue difference between the normal mucous membrane and the fluorescence region. 
     As shown in  FIG. 8 , the chroma saturation enhancement processing section  76  changes a radius vector r, which is represented by coordinates positioned inside a radius vector change range Rm, in the signal ratio space but does not change a radius vector r that is represented by coordinates positioned outside the radius vector change range Rm. In the radius vector change range Rm, the radius vector r is in the range of “r1” to “r2” (r1&lt;r2). Further, an expansion center line SLs for chroma saturation is set on a radius vector rc positioned between the radius vector r1 and the radius vector r2 in the radius vector change range Rm. The expansion center line SLs for chroma saturation is provided outside a region of the normal mucous membrane (see  FIG. 7 ). Furthermore, an expansion center CES for chroma saturation is included in the expansion center line SLs for chroma saturation. 
     Here, as the radius vector r is larger, chroma saturation is higher. Accordingly, a range rcr1 (r1&lt;r&lt;rc) in which the radius vector r is smaller than the radius vector rc represented by the expansion center line SLs for chroma saturation is defined as a low chroma saturation range. On the other hand, a range rcr2 (rc&lt;r&lt;r2) in which the radius vector r is larger than the radius vector rc represented by the expansion center line SLs for chroma saturation is defined as a high chroma saturation range. 
     As shown in  FIG. 9 , the chroma saturation enhancement processing outputs a radius vector Rx(r) in response to the input of the radius vector r of coordinates included in the radius vector change range Rm. A relationship between the input and output of the chroma saturation enhancement processing is shown by a solid line. In the chroma saturation enhancement processing, an S-shaped conversion curve is used and an output value Rx(r) is made smaller than an input value r in a low chroma saturation range rcr1 but an output value Rx(r) is made larger than an input value r in a high chroma saturation range rcr2. Further, an inclination Kx of Rx(rc) is set to “1” or more. Accordingly, the chroma saturation of an object to be observed included in the low chroma saturation range can be made lower, but the chroma saturation of an object to be observed included in the high chroma saturation range can be made higher. A chroma saturation difference between a plurality of ranges to be observed can be increased by such chroma saturation enhancement processing. 
     In a case where the chroma saturation enhancement processing is performed as described above, a fluorescence region (solid line) subjected to the chroma saturation enhancement processing is moved to be farther from the expansion center line SLs for chroma saturation than a fluorescence region (dotted line) not yet subjected to the chroma saturation enhancement processing as shown in  FIG. 10 . Since the direction of a radius vector in the feature space represents the magnitude of chroma saturation, a chroma saturation difference between the fluorescence region (solid line) subjected to the chroma saturation enhancement processing and the normal mucous membrane is larger than a chroma saturation difference between the fluorescence region (dotted line) not yet subjected to the chroma saturation enhancement processing and the normal mucous membrane. 
     As shown in  FIG. 11 , the hue enhancement processing section  77  changes an angle θ, which is represented by coordinates positioned inside an angle change range Rn, in the signal ratio space but does not change an angle θ that is represented by coordinates positioned outside the angle change range Rn. The angle change range Rn is formed of the range of an angle θ1 in a counterclockwise direction (first hue direction) from an expansion center line SLh for hue and the range of an angle θ2 in a clockwise direction (second hue direction) from the expansion center line SLh for hue. The expansion center line SLh for hue is provided to intersect with the region of the normal mucous membrane (see  FIG. 7 ). Further, an expansion center CEH for hue is included in the expansion center line SLh for hue. 
     The angle θ of coordinates included in the angle change range Rn is redefined as an angle θ from the expansion center line SLh for hue, the side of the expansion center line SLh for hue corresponding to the counterclockwise direction is defined as a positive side, and the side of the expansion center line SLh for hue corresponding to the clockwise direction is defined as a negative side. In a case where the angle θ is changed, hue is also changed. Accordingly, the range of the angle θ1 of the angle change range Rn is defined as a positive hue range θ1, and the range of the angle θ2 thereof is defined as a negative hue range θ2. It is preferable that the expansion center line SLh for hue is also a line intersecting with the range of the normal mucous membrane in the feature space like the expansion center line SLs for chroma saturation. 
     As shown in  FIG. 12 , the hue enhancement processing outputs an angle Fx(0) in response to the input of the angle θ of coordinates included in the angle change range Rn. A relationship between the input and output of the hue enhancement processing is shown by a solid line. In the hue enhancement processing, an output Fx(0) is made smaller than an input θ in the negative hue range θ2 but an output Fx(0) is made larger than an input θ in the positive hue range θ1. Accordingly, a difference in hue between an object to be observed included in the negative hue range and an object to be observed included in the positive hue range can be increased. 
     In a case where the hue enhancement processing is performed as described above, a fluorescence region (solid line) subjected to the hue enhancement processing is moved to be farther from the expansion center line SLh for hue than a fluorescence region (dotted line) not yet subjected to the hue enhancement processing as shown in  FIG. 13 . Since the direction of an angle in the feature space represents a difference in hue, a hue difference between the fluorescence region (solid line) subjected to the hue enhancement processing and the normal mucous membrane is larger than a hue difference between the fluorescence region (dotted line) not yet subjected to the hue enhancement processing and the normal mucous membrane. 
     Even in the case of a feature space (ab space) formed by a* and b* (indicating the tint elements a* and b* of a CIE Lab space that are color information. The same applies hereinafter) obtained from the Lab conversion of the first RGB image signals that is performed by a Lab conversion unit, in a case where the chroma saturation enhancement processing is performed, a fluorescence region (solid line) subjected to the chroma saturation enhancement processing is moved to be farther from the expansion center line SLs for chroma saturation than a fluorescence region (dotted line) not yet subjected to the chroma saturation enhancement processing as shown in  FIG. 14 . Further, in a case where the hue enhancement processing is performed, a fluorescence region (solid line) subjected to the hue enhancement processing is moved to be farther from the expansion center line SLh for hue than a fluorescence region (dotted line) not yet subjected to the hue enhancement processing as shown in  FIG. 15 . 
     The expansion center change section  85  changes the expansion center, which is determined in the signal ratio space, on the basis of the fluorescence and reference image signals and the fluorescence image signals. Specifically, the expansion center change section  85  calculates reference image signals from differences between the fluorescence and reference image signals and the fluorescence image signals, and changes the expansion center on the basis of the calculated reference image signals. For example, the frequency distribution of B/G ratios obtained on the basis of the B-image signals and the G-image signals of the reference image signals and the frequency distribution of G/R ratios obtained on the basis of the G-image signals and the R-image signals of the reference image signals are obtained as shown in  FIG. 16 . Then, the amount of change of the expansion center in a vertical direction is obtained from a pixel value (specific pixel value) corresponding to a specific ratio (for example, 80%) of the maximum pixel value included in the frequency distribution of the B/G ratios. Further, the amount of change of the expansion center in a horizontal direction is obtained from a pixel value (specific pixel value) corresponding to a specific ratio (for example, 80%) of the maximum pixel value included in the frequency distribution of the G/R ratios. 
     For example, the expansion center change section  85  does not change the expansion center for chroma saturation or the expansion center for hue in a case where the specific pixel value is in a certain range. On the other hand, in a case where the specific pixel value is out of a certain range, the expansion center change section  85  changes the expansion center CES for chroma saturation or the expansion center CEH for hue in the vertical direction or the horizontal direction according to the amounts of change of the expansion center in the vertical direction and the horizontal direction. As the expansion center CES for chroma saturation or the expansion center CEH for hue is changed, the position of the expansion center line SLs for chroma saturation or the expansion center line SLh for hue is also changed. 
     In a case where the expansion center is set to the maximum pixel values included in the frequency distributions of the B/G ratios and the G/R ratios, the positions of the normal mucous membrane and the fluorescence region and the expansion center line SLs for chroma saturation or the expansion center line SLh for hue are as shown in  FIG. 17A . On the other hand, in a case where the expansion center is set to pixel values (specific pixel values) corresponding to 80% of the maximum pixel values included in the frequency distributions of the B/G ratios and the G/R ratios, the positions of the normal mucous membrane and the fluorescence region and the expansion center line SLs for chroma saturation or the expansion center line SLh for hue are as shown in  FIG. 17B . 
     Further, the expansion center change section  85  may calculate the amount of change of the expansion center on the basis of the components of fluorescence and reference light included in the fluorescence and reference image signals and the components of fluorescence included in the fluorescence image signals. In this case, the expansion center change section  85  generates a binarized fluorescence image signal in which a fluorescence component FL and other components of the fluorescence image signal are binarized as shown in  FIG. 18 . In a case where the fluorescence component FL included in the binarized fluorescence image signal is equal to or lower than a threshold value for fluorescence, the expansion center change section  85  does not change the expansion center. 
     On the other hand, in a case where the fluorescence component FL exceeds the threshold value for fluorescence, the expansion center change section  85  changes the expansion center. Here, the expansion center is changed by the subtraction of the maximum amount of fluorescence to low chroma saturation side of a lesion area, or the like. As shown in  FIG. 19 , the expansion center change section  85  acquires a first lesion image in which a lesion area including the reference light RL and the fluorescence component FL is displayed by taking a logical product (AND) of the fluorescence and reference image signals and a binarized fluorescence image. Then, the expansion center change section  85  calculates the minimum pixel values Min_B, Min_G, and Min_R from the B-image signals, the G-image signals, and the R-image signals of the first lesion image, respectively. Further, the expansion center change section  85  calculates the maximum pixel value Max_R from the fluorescence image signals. Then, the expansion center change section  85  calculates the amount A of change of the expansion center by the following equation 1), and shifts the expansion center CES for chroma saturation or the expansion center CEH for hue in the signal ratio space in the horizontal direction by the amount A of change. 
     
       
         
           
             
               
                 
                   A 
                   = 
                   
                     Min_G 
                     / 
                     
                       ( 
                       
                         Min_R 
                         - 
                         Max_R 
                       
                       ) 
                     
                   
                 
               
               
                 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
     In a case where the fluorescence component FL included in the binarized fluorescence image signal exceeds the threshold value for fluorescence, the expansion center change section  85  may change the expansion center in the vertical direction and the horizontal direction of the signal ratio space as described below instead of a method of changing the expansion center in the horizontal direction of the signal ratio space. For example, the expansion center change section  85  changes the expansion center so that the expansion center is positioned on a boundary between the normal mucous membrane and a lesion area, such as the fluorescence region. Specifically, the expansion center change section  85  generates the first lesion image in which a lesion area including the components of the fluorescence and the reference light is displayed and a lesion-excluding image in which portions other than the lesion area including the components of the fluorescence and the reference light are displayed, from the fluorescence and reference image signals and the fluorescence image signals; and calculates the amount of change of the expansion center on the basis of the first lesion image and the lesion-excluding image. 
     In this case, as shown in  FIG. 20 , the expansion center change section  85  acquires the first lesion image in which the lesion area including the reference light RL and the fluorescence component FL is displayed by taking a logical product (AND) of the fluorescence and reference image signals and the binarized fluorescence image as described above. In addition to the first lesion image, the expansion center change section  85  acquires the lesion-excluding image in which portions other than the lesion area including the reference light RL and the fluorescence component FL are displayed by taking a logical product (AND) of the fluorescence and reference image signals and an inverted binarized fluorescence image in which the pixel values of the binarized fluorescence image are inverted. 
     Then, as shown in  FIG. 21 , the expansion center change section  85  calculates the B/G ratios and the G/R ratios of the first lesion image from the B-image signals, the G-image signals, and the R-image signals of the first lesion image and acquires Min_B/G and Min_G/R that are the minimum B/G ratio and the minimum G/R ratio among the calculated positive B/G ratios (&gt;0) and the calculated positive G/R ratios (&gt;0). On the other hand, the expansion center change section  85  calculates the B/G ratios and the G/R ratios of the first lesion image from the B-image signals, the G-image signals, and the R-image signals of the lesion-excluding image and acquires Max_B/G and Min_G/R that are the maximum B/G ratio and the maximum G/R ratio among the calculated B/G ratios and the calculated G/R ratios. Then, the expansion center change section  85  calculates the amount Mx of change of the expansion center in the vertical direction and the amount My of change of the expansion center in the horizontal direction by the following equation 2). 
     
       
         
           
             
               
                 
                   
                     Mx 
                     = 
                     
                       
                         ( 
                         
                           
                             Max_B 
                             / 
                             G 
                           
                           + 
                           
                             Min_B 
                             / 
                             G 
                           
                         
                         ) 
                       
                       / 
                       2 
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     My 
                     = 
                     
                       
                         ( 
                         
                           
                             Max_G 
                             / 
                             R 
                           
                           + 
                           
                             Min_G 
                             / 
                             R 
                           
                         
                         ) 
                       
                       / 
                       2 
                     
                   
                 
               
               
                 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   ) 
                 
               
             
           
         
       
     
     After that, the expansion center change section  85  shifts the expansion center CES for chroma saturation or the expansion center CEH for hue in the signal ratio space in the vertical direction by the amount Mx of change and in the horizontal direction by the amount My of change. 
     The fluorescence/reference light amount-calculation section  88  changes the emission ratio of the reference light or obtains the amount of fluorescence and the amount of reference light, which are required for the contents of processing to be performed on the fluorescence and reference image signals, on the basis of the fluorescence and reference image signals and the fluorescence image signals. In this case, the fluorescence/reference light amount-calculation section  88  generates a binarized fluorescence image signal in which a fluorescence component FL and other components of the fluorescence image signal are binarized as shown in  FIG. 18 . In a case where the fluorescence component FL included in the binarized fluorescence image signal is equal to or lower than the threshold value for fluorescence, the fluorescence/reference light amount-calculation section  88  determines that the fluorescence component FL is not included in the fluorescence image signal and does not calculate the amount of chemical fluorescence. That is, the emission ratio of the reference light is not changed or the contents of processing to be performed on the fluorescence and reference image signals are not changed. 
     On the other hand, in a case where the fluorescence component FL exceeds the threshold value for fluorescence, the amount of chemical fluorescence is calculated as shown in  FIG. 22 . The fluorescence/reference light amount-calculation section  88  acquires a first lesion image in which a lesion area including the reference light RL and the fluorescence component FL is displayed by taking a logical product (AND) of the fluorescence and reference image signals and a binarized fluorescence image. In addition to the first lesion image, the fluorescence/reference light amount-calculation section  88  acquires a second lesion image in which a lesion area including the fluorescence component FL is displayed by taking a logical product (AND) of the fluorescence image signals and the binarized fluorescence image. 
     Then, as shown in  FIG. 23 , the fluorescence/reference light amount-calculation section  88  calculates a pixel value, which corresponds to a specific ratio (for example, 80%) of the maximum pixel value included in the frequency distribution (histogram) of the pixel values of the R-image signals of the first lesion image, as a first R-pixel value. The first R-pixel value corresponds to the pixel value of fluorescence and reference light, and a value, which is obtained in a case where the first R-pixel value is divided by an R-gain coefficient for fluorescence observation, corresponds to the amount of fluorescence and reference light. In the same way, the fluorescence/reference light amount-calculation section  88  calculates a pixel value, which corresponds to a specific ratio (for example, 80%) of the maximum pixel value included in the frequency distribution (histogram) of the pixel values of the R-image signals of the second lesion image, as a second R-pixel value. The second R-pixel value corresponds to the pixel value of fluorescence, and a value, which is obtained in a case where the second R-pixel value is divided by the R-gain coefficient for fluorescence observation, corresponds to the amount of fluorescence. Further, the fluorescence/reference light amount-calculation section  88  calculates the pixel value of the reference light by subtracting the second R-pixel value from the first R-pixel value. It is preferable that the first R-pixel value is converted into a linear luminance-signal of which the pixel value changes linearly with respect to the input signal of the image pickup sensor  48 . It is preferable that the second R-pixel value is also converted into a linear luminance-signal like the first R-pixel value. 
     The amount of fluorescence and the amount of reference light, which are obtained by the fluorescence/reference light amount-calculation section  88 , are used to change the contents of processing to be performed on the fluorescence and reference image signals, for example, the contents of gain processing or matrix processing and to correct the emission ratio of reference light. A processing change section  91  changes the contents of processing to be performed on the fluorescence and reference image signals on the basis of the amount of fluorescence and the amount of reference light. In a case where, for example, the change of an R-gain coefficient Gn_R for fluorescence observation used for gain processing is to be performed as the change of the contents of the processing, AGn_R is obtained as a changed R-gain coefficient for fluorescence observation by the following equation 3). 
     
       
         
           
             
               
                 
                   
                     
                       AGn_R 
                       = 
                       
                         pixel 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         value 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         of 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         Max 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           fluorescence 
                           / 
                         
                       
                     
                       
                   
                   ⁢ 
                   pixel 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   value 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   of 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   fluorescence 
                   × 
                   Gn_R 
                 
               
               
                 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ) 
                 
               
             
           
         
       
     
     Here, the pixel value of Max fluorescence means a value that is obtained in a case where the pixel value of the reference light is subtracted from the pixel value of the maximum fluorescence at which the image pickup sensor  48  is saturated. 
     A reference light amount-correction section  93  corrects the amount of reference light on the basis of the contents of processing that is to be performed on the fluorescence and reference image signals before and after the change. In a case where the change of the R-gain coefficient Gn_R for fluorescence observation is performed as the change of the contents of the processing as described above, the reference light amount-correction section  93  calculates the corrected amount of reference light, which is obtained in a case where the amount of reference light is corrected, by the following equation 4. 
     
       
         
           
             
               
                 
                   
                     Corrected 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     amount 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     of 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     reference 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     light 
                   
                   = 
                   
                     amount 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     of 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     reference 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     light 
                     × 
                     
                       Gn_R 
                       / 
                       AGn_R 
                     
                   
                 
               
               
                 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     4 
                   
                   ) 
                 
               
             
           
         
       
     
     Then, the light source processor  21  corrects the emission ratio of the reference light on the basis of the corrected amount of reference light. For example, in a case where the emission ratio of light in a fluorescence wavelength range including the components of chemical fluorescence of the reference light, that is, the emission ratio of red light R (light in a fluorescence wavelength range) is to be corrected, the reference light amount-correction section  93  calculates the amount of corrected red light R, which is obtained in a case where the emission ratio of red light R is corrected, by the following equation 5). 
     
       
         
           
             
               
                 
                   
                     Amount 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     of 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     corrected 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     red 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     light 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     R 
                   
                   = 
                   
                     amount 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     of 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     red 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     light 
                     × 
                     corrected 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     amount 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     of 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     reference 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       light 
                       / 
                       amount 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     of 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     reference 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     light 
                   
                 
               
               
                 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     5 
                   
                   ) 
                 
               
             
           
         
       
     
     For example, in a case where chemical fluorescence is detected from an object to be observed so that the amount of chemical fluorescence is equal to or larger than a certain level, the emission ratio of red light R is reduced according to the change of the contents of the processing and the correction of the amount of reference light as the amount of chemical fluorescence is increased. Accordingly, the visibility of chemical fluorescence having the same wavelength range as red light R is improved. On the other hand, in a case where chemical fluorescence is not detected or hardly detected from an object to be observed so that the amount of chemical fluorescence is less than a certain level, the emission ratio of red light R is increased according to the change of the contents of the processing and the correction of the amount of reference light. Accordingly, it is possible to prevent the brightness the red color of an object to be observed from being reduced even though chemical fluorescence is not emitted. 
     Methods of calculating the amount of fluorescence, the amount of reference light, the corrected R-gain coefficient for fluorescence observation, and the corrected amount of reference light in a case where specific numerical values are used will be described below. As shown in  FIG. 24 , in a case where the pixel value of fluorescence is “15” and the pixel value of fluorescence and reference light is “165”, the pixel value of fluorescence is subtracted from the pixel value of fluorescence and reference light, so that the pixel value of reference light is “150”. Further, in a case where an R-gain coefficient for fluorescence observation is set to “1.5”, “15”, which is the pixel value of fluorescence, is divided by “1.5”, which is the R-gain coefficient for fluorescence observation, so that the amount of fluorescence is “10”. Furthermore, “150”, which is the pixel value of reference light, is divided by “1.5”, which is the R-gain coefficient for fluorescence observation, so that the amount of reference light is “100”. 
     Next, the corrected R-gain coefficient AGn_R for fluorescence observation is obtained in a case where the pixel value of Max fluorescence/the pixel value of fluorescence is multiplied by the R-gain coefficient Gn_R for fluorescence observation as shown in Equation 3). “150”, which is the pixel value of reference light is subtracted from “230”, which is the pixel value of the maximum fluorescence at which the image pickup sensor  48  is saturated, so that the pixel value of Max fluorescence is “80”. Accordingly, “80”, which is the pixel value of Max fluorescence,/“15”, which is the pixel value of fluorescence is multiplied by “1.5”, which is the R-gain coefficient Gn_R for fluorescence observation, so that AGn_R is “8”. 
     Next, the amount of reference light is multiplied by Gn_R/AGn_R as shown in Equation 4), so that the corrected amount of reference light is obtained. Specifically, “100”, which is the amount of reference light, is multiplied by “1.5”, which is Gn_R,/“8”, which is AGn_R, so that the corrected amount of reference light is “18.75”. With regard to the pixel value of corrected fluorescence, “10”, which is the amount of fluorescence substantially constant regardless of the magnitude of the amount of reference light, is multiplied by “8”, which is the corrected R-gain coefficient AGn_R for fluorescence observation, so that the pixel value of corrected fluorescence is “80”. On the other hand, with regard to the pixel value of corrected reference light, “18.75”, which is the corrected amount of reference light, is multiplied by “8”, which is the corrected R-gain coefficient for fluorescence observation, so that the pixel value of corrected reference light is “150”. With regard to the pixel value of corrected fluorescence and corrected reference light, “80”, which is the pixel value of corrected fluorescence, and “150”, which is the pixel value of corrected reference light, are added to each other, so that the pixel value of corrected fluorescence and corrected reference light is “230”. 
     Next, the fluorescence observation mode will be described with reference to a flowchart of  FIG. 25 . First, a drug, which causes chemical fluorescence to be excited and emitted from an object to be observed, is given to a patient. The mode changeover SW  13   a  is operated to switch a mode to the fluorescence observation mode. In a case where a mode is switched to the fluorescence observation mode, the light source unit  20  emits excitation light, which causes the drug contained in the object to be observed to be excited to emit chemical fluorescence, and reference light that has a wavelength range from a blue-light wavelength range to a red-light wavelength range. The image pickup sensor  48  picks up the image of the object to be observed, which is illuminated with the excitation light and the reference light, and outputs fluorescence and reference image signals. 
     The fluorescence and reference image signals, which are output from the image pickup sensor  48 , are input to the image signal input unit  53 . The signal ratio calculation section  72 , which is the color information acquisition section, calculates a B/G ratio and a G/R ratio, which are a plurality of pieces of color information, on the basis of the fluorescence and reference image signals. The color difference expansion section  74  expands a color difference between a normal mucous membrane, which is included in the object to be observed, and a fluorescence region, which includes chemical fluorescence excited and emitted from the drug contained in the object to be observed, in a signal ratio space (feature space) that is formed by the B/G ratio and the G/R ratio. A fluorescence image is obtained on the basis of a B/G ratio and a G/R ratio that are obtained after the expansion of a color difference between the normal mucous membrane and the fluorescence region. The fluorescence image is displayed on the monitor  18 . Since the fluorescence image is not a monochrome image including only fluorescence components and includes the components of visible light, which has a wavelength range from a blue-light wavelength range to a red-light wavelength range, the fluorescence image is displayed as a color image. Accordingly, a fluorescence region can be grasped in a situation where the fluorescence image is likely to be visually recognized by a user. 
     In the above-mentioned embodiment, the B/G ratio and the G/R ratio are obtained from the first RGB image signals by the signal ratio calculation section  72  and the chroma saturation enhancement processing and the hue enhancement processing are performed in the signal ratio space formed by the B/G ratio and the G/R ratio. However, color information different from the B/G ratio and the G/R ratio may be obtained and the chroma saturation enhancement processing and the hue enhancement processing may be performed in a feature space formed by the color information. 
     For example, color difference signals Cr and Cb may be obtained as the color information, and the chroma saturation enhancement processing and the hue enhancement processing may be performed in a feature space formed by the color difference signals Cr and Cb. In this case, a fluorescence image processing unit  92  shown in  FIG. 26  is used. Unlike the fluorescence image processing unit  64 , the fluorescence image processing unit  92  does not comprise the Log transformation section  71 , the signal ratio calculation section  72 , and the inverse Log transformation section  83 . The fluorescence image processing unit  92  comprises a luminance-color difference signal conversion section  86  instead. Other components of the fluorescence image processing unit  92  are the same as those of the fluorescence image processing unit  64 . 
     The luminance-color difference signal conversion section  86  (corresponding to “color information acquisition section” of the present invention) converts the first RGB image signals into a luminance signal Y and color difference signals Cr and Cb. A well-known conversion equation is used for the conversion of the first RGB image signals into the color difference signals Cr and Cb. The color difference signals Cr and Cb are transmitted to the polar coordinate transformation section  73 . The luminance signal Y is transmitted to the RGB conversion section  79  and the brightness adjustment section  81 . The RGB conversion section  79  converts the color difference signals Cr and Cb, which have passed through the Cartesian coordinate transformation section  78 , and the luminance signal Y into second RGB image signals. The brightness adjustment section  81  uses the luminance signal Y as the first brightness information Yin, and uses the second brightness information, which is obtained by the second brightness information-calculation section  81   b , as the second brightness information Yout to adjust the pixel values of the second RGB image signals. A method of calculating the second brightness information Yout and a method of adjusting the pixel values of the second RGB image signals are the same as those in the case of the fluorescence image processing unit  64 . 
     Further, as in the case of the signal ratio space, a radius vector r and an angle θ in a CrCb space that is a feature space consisting of the color difference signals Cr and Cb are changed to perform the chroma saturation enhancement processing or the hue enhancement processing. Accordingly, in a case where the chroma saturation enhancement processing is performed in the CrCb space, a fluorescence region (solid line) subjected to the chroma saturation enhancement processing is moved to be farther from an expansion center line SLs for chroma saturation than a fluorescence region (dotted line) not yet subjected to the chroma saturation enhancement processing as shown in  FIG. 27 . Further, in a case where the hue enhancement processing, a fluorescence region (solid line) subjected to the hue enhancement processing is moved to be farther from an expansion center line SLh for hue than a fluorescence region (dotted line) not yet subjected to the hue enhancement processing as shown in  FIG. 28 . 
     Further, hue H(Hue) and chroma saturation S(Saturation) may be obtained as the color information, and the chroma saturation enhancement processing and the hue enhancement processing may be performed in a HS space formed by the hue H and the chroma saturation S. In a case where the hue H and the chroma saturation S are used, a fluorescence image processing unit  96  shown in  FIG. 29  is used. Unlike the fluorescence image processing unit  64 , the fluorescence image processing unit  96  does not comprise the Log transformation section  71 , the signal ratio calculation section  72 , the polar coordinate transformation section  73 , the Cartesian coordinate transformation section  78 , and the inverse Log transformation section  83 . The fluorescence image processing unit  96  comprises a HSV conversion section  87  instead. Other components of the fluorescence image processing unit  96  are the same as those of the fluorescence image processing unit  64 . 
     The HSV conversion section  87  (corresponding to “color information acquisition section” of the present invention) converts the first RGB image signals into hue H, chroma saturation S, and value V. A well-known conversion equation is used for the conversion of the first RGB image signals into the hue H, the chroma saturation S, and the value V. The hue H and the chroma saturation S are transmitted to a translation section  90 . The value V is transmitted to the RGB conversion section  79 . The RGB conversion section  79  converts the hue H and the chroma saturation S, which have passed through the translation section  90 , and the value V into second RGB image signals. The brightness adjustment section  81  uses the first brightness information Yin, which is obtained by the first brightness information-calculation section, and the second brightness information Yout, which is obtained by the second brightness information-calculation section  81   b , to adjust the pixel values of the second RGB image signals. Methods of calculating the first brightness information Yin and the second brightness information Yout and a method of adjusting the pixel values of the second RGB image signals are the same as those in the case of the fluorescence image processing unit  64 . 
     The chroma saturation enhancement processing and the hue enhancement processing in a HS space formed by the hue H and the chroma saturation S do not expand or compress the radius vector r and the angle θ as in the signal ratio space and the CrCb space, and is performed as processing for translating each pixel. In a case where chroma saturation enhancement processing is performed in the HS space rather than normal chroma saturation enhancement processing and normal hue enhancement processing to be performed in the HS space, a fluorescence region (solid line) subjected to the chroma saturation enhancement processing is moved to be farther from an expansion center line SLs for chroma saturation than a fluorescence region (dotted line) not yet subjected to the chroma saturation enhancement processing as shown in  FIG. 30 . Further, in a case where hue enhancement processing is performed, a fluorescence region (solid line) subjected to the hue enhancement processing is moved to be farther from an expansion center line SLh for hue than a fluorescence region (dotted line) not yet subjected to the hue enhancement processing as shown in  FIG. 31 . 
     Second Embodiment 
     In a second embodiment, an object to be observed is illuminated using a laser light source and a phosphor instead of the four color LEDs  20   a  to  20   d  described in the first embodiment. Others are the same as those of the first embodiment. 
     As shown in  FIG. 32 , in an endoscope system  100  according to a second embodiment, a light source device  14  is provided with a blue laser light source (written in  FIG. 32  as “ 445 LD”)  104  emitting blue laser light of which the central wavelength is in the range of 445±10 nm and a blue-violet laser light source (written in  FIG. 32  as “ 405 LD”)  106  emitting blue-violet laser light of which the central wavelength is in the range of 405±10 nm, instead of the four color LEDs  20   a  to  20   d . Since the emission of light from semiconductor light-emitting elements of the respective light sources  104  and  106  is individually controlled by a light source processor  108 , a ratio of the amount of light emitted from the blue laser light source  104  to the amount of light emitted from the blue-violet laser light source  106  can be freely changed. 
     The light source processor  108  drives the blue laser light source  104  in the case of the normal observation mode. On the other hand, in the case of the fluorescence observation mode, the light source processor  108  drives both the blue laser light source  104  and the blue-violet laser light source  106  at the reference frame and drives only the blue-violet laser light source  106  at the fluorescence frame. Laser light emitted from each of the above-mentioned light sources  104  and  106  is incident on the light guide  41  through optical members (all of the optical members are not shown), such as a condenser lens, optical fibers, or a multiplexer. 
     It is preferable that the half-width of blue laser light or blue-violet laser light is set to about ±10 nm. Further, broad area-type InGaN-based laser diodes can be used as the blue laser light source  104  and the blue-violet laser light source  106 , and InGaNAs-based laser diodes or GaNAs-based laser diodes can also be used. Furthermore, a light emitter, such as a light emitting diode, may be used as the light source. 
     The illumination optical system  30   a  is provided with a phosphor  110  on which blue laser light or blue-violet laser light transmitted from the light guide  41  is to be incident in addition to the illumination lens  45 . In a case where the phosphor  110  is irradiated with blue laser light, fluorescence for a phosphor is emitted from the phosphor  110 . Further, a part of blue laser light is transmitted through the phosphor  110  as it is. Blue-violet laser light is transmitted through the phosphor  110  without exciting the phosphor  110 . The inside of a sample is irradiated with light, which is emitted from the phosphor  110 , through the illumination lens  45 . 
     Here, since blue laser light is mainly incident on the phosphor  110  in the normal observation mode, an object to be observed is irradiated with normal light shown in  FIG. 33  in which blue laser light and fluorescence for a phosphor excited and emitted from the phosphor  110  due to blue laser light are multiplexed. On the other hand, since both blue-violet laser light and blue laser light are incident on the phosphor  110  at the reference frame in the fluorescence observation mode, the inside of the sample is irradiated with reference light shown in  FIG. 34  in which excitation light, which is blue-violet laser light, blue laser light, and fluorescence for a phosphor excited and emitted from the phosphor  110  due to blue laser light are multiplexed. Further, since only blue-violet laser light is incident on the phosphor  110  at the fluorescence frame, the inside of the sample is irradiated with excitation light shown in  FIG. 35  that is blue-violet laser light transmitted through the phosphor as it is. 
     It is preferable that a phosphor including plural kinds of phosphors absorbing a part of blue laser light and excited by green to yellow light to emit light (for example, YAG-based phosphors or phosphors, such as BAM (BaMgAl 10 O 17 )) is used as the phosphor  110 . In a case where the semiconductor light-emitting elements are used as the excitation light source of the phosphor  110  as in this example of configuration, high-intensity white light is obtained with high luminous efficacy and not only the intensity of white light can be easily adjusted but also a change in the color temperature and chromaticity of white light can be suppressed to be small. 
     Third Embodiment 
     In a third embodiment, an object to be observed is illuminated using a broadband light source, such as a xenon lamp, and a rotary filter instead of the four color LEDs  20   a  to  20   d  described in the first embodiment. Further, the image of the object to be observed is picked up by a monochrome image pickup sensor instead of the color image pickup sensor  48 . Others are the same as those of the first embodiment. 
     As shown in  FIG. 36 , in an endoscope system  200  according to a third embodiment, a light source device  14  is provided with a broadband light source  202 , a rotary filter  204 , and a filter switching unit  205  instead of the four color LEDs  20   a  to  20   d . Further, an image pickup optical system  30   b  is provided with a monochrome image pickup sensor  206 , which is not provided with a color filter, instead of the color image pickup sensor  48 . 
     The broadband light source  202  is a xenon lamp, a white LED, or the like, and emits white light of which the wavelength range reaches the wavelength range of red light from the wavelength range of blue light. The rotary filter  204  comprises a filter  208  for a normal observation mode provided on the inside and a filter  209  for a fluorescence observation mode provided on the outside (see  FIG. 37 ). The filter switching unit  205  is to move the rotary filter  204  in a radial direction, inserts the filter  208  for a normal observation mode of the rotary filter  204  into the optical path of white light in a case where a mode is set to the normal observation mode by the mode changeover SW  13   a , and inserts the filter  209  for a fluorescence observation mode of the rotary filter  204  into the optical path of white light in a case where a mode is set to the fluorescence observation mode. 
     As shown in  FIG. 37 , the filter  208  for a normal observation mode is provided with a B-filter  208   a , a G-filter  208   b , and an R-filter  208   c  that are arranged in a circumferential direction. The B-filter  208   a  transmits blue light of white light, the G-filter  208   b  transmits green light of white light, and the R-filter  208   c  transmits red light of white light. Accordingly, in a case where the rotary filter  204  is rotated in the normal observation mode, the object to be observed is alternately irradiated with blue light, green light, and red light. 
     The filter  209  for a fluorescence observation mode is provided with a Bn-filter  209   a , a B-filter  209   b , a G-filter  209   c , and an R-filter  209   d  that are arranged in the circumferential direction. The Bn-filter  209   a  transmits narrow-band blue light, which has a specific wavelength, of white light, the B-filter  209   b  transmits blue light of white light, the G-filter  209   c  transmits green light of white light, and the R-filter  209   d  transmits red light of white light. Accordingly, in a case where the rotary filter  204  is rotated in the fluorescence observation mode, the object to be observed is alternately irradiated with narrow-band blue light, blue light, green light, and red light. Image signals based on narrow-band blue light are combined with image signals based on blue light, and are used as the B-image signals of fluorescence and reference image signals. Image signals based on green light are used as the G-image signals of fluorescence and reference image signals, and image signals based on red light are used as the R-image signals of fluorescence and reference image signals. Image signals based on narrow-band blue light are used as fluorescence image signals. 
     In the endoscope system  200 , in the normal observation mode, the image of the inside of the sample is picked up by the monochrome image pickup sensor  206  whenever the object to be observed is irradiated with blue light, green light, and red light. Accordingly, image signals corresponding to three colors of R, G, and B are obtained. Then, a normal image is generated by the same method as the first embodiment on the basis of the image signals corresponding to R, G, and B. 
     On the other hand, in the fluorescence observation mode, the image of the inside of the sample is picked up by the monochrome image pickup sensor  206  whenever the object to be observed is irradiated with narrow-band blue light, green light, and red light. Accordingly, Bn-image signals, G-image signals, and R-image signals are obtained. A special image is generated on the basis of the Bn-image signals, the G-image signals, and the R-image signals. The Bn-image signals are used instead of the B-image signals to generate the special image. Except for that, the special image is generated by the same method as the first embodiment. 
     Four types of color light having the emission spectra shown in  FIG. 3  are used in the embodiment, but the emission spectra are not limited thereto. For example, as shown in  FIG. 38 , green light G and red light R have the same spectra as those shown in  FIG. 3  but light of which the central wavelength is in the range of 410 to 420 nm and the wavelength range is slightly closer to a long wavelength side than that of violet light V shown in  FIG. 3  may be used as violet light Vs. Further, light of which the central wavelength is in the range of 445 to 460 nm and the wavelength range is slightly closer to a short wavelength side than that of blue light B shown in  FIG. 3  may be used as blue light Bs. 
     In the first embodiment, a B/G ratio and a G/R ratio have been transformed into a radius vector r and an argument θ by polar coordinate transformation, the chroma saturation enhancement processing and the hue enhancement processing for expanding or compressing an angle on the basis of the radius vector r and the argument θ having been transformed have been performed, and the radius vector r and the argument θ then have been returned to a B/G ratio and a G/R ratio again. However, as shown in  FIG. 39 , a B/G ratio and a G/R ratio may be directly converted into a B/G ratio and a G/R ratio, which have been subjected to first or second processing, using a two-dimensional look up table (LUT)  400  without polar coordinate transformation or the like from a B/G ratio and a G/R ratio. 
     A B/G ratio and a G/R ratio and a B/G ratio and a G/R ratio having been subjected to chroma saturation enhancement processing and hue enhancement processing, which are obtained from the chroma saturation enhancement processing and the hue enhancement processing based on the B/G ratio and the G/R ratio, are stored in the two-dimensional LUT  400  in association with each other. Here, in a case where a mode is set to a normal chroma saturation enhancement mode, a correspondence relationship between a B/G ratio and a G/R ratio and a B/G ratio and a G/R ratio having been subjected to normal chroma saturation enhancement processing and normal hue enhancement processing is used. In a case where a mode is set to a specific chroma saturation enhancement mode, a correspondence relationship between a B/G ratio and a G/R ratio and a B/G ratio and a G/R ratio having been subjected to normal chroma saturation enhancement processing and specific hue enhancement processing is used. Further, first RGB image signals output from the inverse gamma transformation section  70  are input to the two-dimensional LUT  400  and the RGB conversion section  79 . 
     The hardware structures of the processing units, which are included in the processor device  16  in the embodiments, such as the image signal input unit  53 , the noise removing unit  58 , the signal switching unit  60 , the normal image processing unit  62 , the fluorescence image processing unit  64 , the fluorescence image processing unit  92 , the fluorescence image processing unit  96 , the video signal generation unit  66 , the inverse gamma transformation section  70 , the Log transformation section  71 , the signal ratio calculation section  72 , the polar coordinate transformation section  73 , the color difference expansion section  74 , the chroma saturation enhancement processing section  76 , the hue enhancement processing section  77 , the Cartesian coordinate transformation section  78 , the RGB conversion section  79 , the brightness adjustment section  81 , the structure enhancement section  82 , the inverse Log transformation section  83 , the gamma transformation section  84 , the expansion center change section  85 , the fluorescence/reference light amount-calculation section  88 , the reference light amount-correction section  93 , the processing change section  91 , the luminance-color difference signal conversion section  86 , and the HSV conversion section  87 , are various processors to be described below. The various processors include: a central processing unit (CPU) that is a general-purpose processor functioning as various processing units by executing software (program); a programmable logic device (PLD) that is a processor of which the circuit configuration can be changed after manufacture, such as a field programmable gate array (FPGA); a dedicated electrical circuit that is a processor having circuit configuration designed exclusively to perform various types of processing; and the like. 
     One processing unit may be formed of one of these various processors, or may be formed of a combination of two or more same type or different types of processors (for example, a plurality of FPGAs, or a combination of a CPU and an FPGA). Further, a plurality of processing units may be formed of one processor. As an example where a plurality of processing units are formed of one processor, first, there is an aspect where one processor is formed of a combination of one or more CPUs and software as typified by a computer, such as a client or a server, and functions as a plurality of processing units. Second, there is an aspect where a processor fulfilling the functions of the entire system, which includes a plurality of processing units, by one integrated circuit (IC) chip as typified by a system-on-chip (SoC) or the like is used. In this way, various processing units are formed using one or more of the above-mentioned various processors as hardware structures. 
     In addition, the hardware structures of these various processors are more specifically electrical circuitry where circuit elements, such as semiconductor elements, are combined. Further, the hardware structure of a storage unit is a storage device, such as a hard disc drive (HDD) or a solid state drive (SSD). 
     EXPLANATION OF REFERENCES 
     
         
         
           
               10 : endoscope system 
               12 : endoscope 
               12   a : insertion part 
               12   b : operation part 
               12   c : bendable part 
               12   d : distal end part 
               12   e : angle knob 
               13   a : mode changeover SW 
               14 : light source device 
               16 : processor device 
               18 : monitor 
               19 : user interface 
               20 : light source unit 
               20   a : V-LED 
               20   b : B-LED 
               20   c : G-LED 
               20   d : R-LED 
               21 : light source processor 
               23 : optical path-combination unit 
               30   a : illumination optical system 
               30   b : image pickup optical system 
               41 : light guide 
               45 : illumination lens 
               46 : objective lens 
               48 : image pickup sensor 
               50 : CDS/AGC circuit 
               52 : A/D converter 
               53 : image signal input unit 
               56 : DSP 
               58 : noise removing unit 
               60 : signal switching unit 
               62 : normal image processing unit 
               64 : fluorescence image processing unit 
               66 : video signal generation unit 
               70 : inverse gamma transformation section 
               71 : Log transformation section 
               72 : signal ratio calculation section 
               73 : polar coordinate transformation section 
               74 : color difference expansion section 
               76 : chroma saturation enhancement processing section 
               77 : hue enhancement processing section 
               78 : Cartesian coordinate transformation section 
               79 : RGB conversion section 
               81 : brightness adjustment section 
               81   a : first brightness information-calculation section 
               81   b : second brightness information-calculation section 
               82 : structure enhancement section 
               83 : inverse Log transformation section 
               84 : gamma transformation section 
               85 : expansion center change section 
               86 : luminance-color difference signal conversion section 
               87 : HSV conversion section 
               88 : fluorescence/reference light amount-calculation section 
               91 : processing change section 
               92 : fluorescence image processing unit 
               93 : reference light amount-correction section 
               96 : fluorescence image processing unit 
               100 : endoscope system 
               104 : blue laser light source 
               106 : blue-violet laser light source 
               108 : light source processor 
               110 : phosphor 
               200 : endoscope system 
               202 : broadband light source 
               204 : rotary filter 
               205 : filter switching unit 
               206 : image pickup sensor 
               208 : filter for normal observation mode 
               208   a : B-filter 
               208   b : G-filter 
               208   c : R-filter 
               209 : filter for fluorescence observation mode 
               209   a : Bn-filter 
               209   b : B-filter 
               209   c : G-filter 
               209   d : R-filter 
               400 : two-dimensional LUT 
             SLs: expansion center line for chroma saturation 
             SLh: expansion center line for hue 
             CES: expansion center for chroma saturation 
             CEH: expansion center for hue