Abstract:
An endoscope processor including a light source controller, an imaging device driver, and a gamma-correction block, is provided. The light source controller can order first and second light sources to alternately emit first and second lights, respectively. The imaging device driver orders an imaging device to generate first and second image signals by capturing an optical image of a subject while the firs and the second light are shone on the subject, respectively. The gamma-correction block carries out gamma correction on the first and second image signals using first and second color gamma coefficients, respectively. The first and second color gamma coefficients are predetermined according to the wavelength band of the first and second lights.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an endoscope processor which carries out signal processing on an image signal generated on the basis of a captured optical image of a subject illuminated by one type of light selected from among different kinds of light and which carries out the signal processing according to the specific kind of light. 
     2. Description of the Related Art 
     A fluorescence endoscope system which shines exciting light on tissue and captures an optical autofluorescence image from the tissue is known. In addition, Japanese Unexamined Patent Publication No. 2006-187426 discloses that a white-light image which is an optical image of a subject illuminated by white light and a fluorescence image of a subject illuminated by exciting light are simultaneously displayed on a monitor in such a fluorescence endoscope system. 
     In prior fluorescence endoscope systems, color gamma correction is carried out on a white-light image signal and a fluorescence image signal, which are generated by photographing either white light or exciting light, respectively are shone on a subject. However, in prior art, because color gamma correction is carried out on both the white-light image signal and the fluorescence image signal using the same color gamma coefficient, there is a troublesome increase in color noise and/or color fading in the displayed images. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide an endoscope processor that carries out signal processing on image signals of a subject illuminated by different kinds of light so that color noise can be reduced and color fading can be prevented. 
     According to the present invention, an endoscope processor comprising a light source controller, an imaging device driver, and a gamma-correction block, is provided. The light source controller can order first and second light sources to alternately emit first and second lights, respectively. The wavelength band of the first light differs from that of the second light. The imaging device driver orders an imaging device to generate first and second image signals by capturing an optical image of a subject while the first and the second light are shone on the subject, respectively. The gamma-correction block carries out gamma correction on the first and the second image signals using first and second color gamma coefficients, respectively. The first and the second color gamma coefficients are predetermined according to the wavelength band of the first and the second light. 
     Further, the gamma-correction block comprises first and second correction blocks. The first and the second correction blocks carry out gamma correction on the first and the second image signals using the first and the second color gamma coefficients, respectively. 
     Further, the endoscope processor comprises a coefficient memory. The coefficient memory stores the first and the second color gamma coefficients. The gamma-correction block carries out gamma correction on the first image signal using the first color gamma coefficient stored in the coefficient memory when the gamma-correction block receives the first image signal. The gamma-correction block carries out gamma correction on the second image signal using the second color gamma coefficient stored in the coefficient memory when the gamma-correction block receives the second image signal. 
     According to the present invention, an endoscope system comprising first and second light sources, a light source controller, a light guide, an endoscope, and a gamma-correction block, is provided. The first and the second light sources emit first and second lights, respectively. The wavelength band of the first light differs from that of the second light. The light source controller can order the first and the second light sources to alternately emit the first and the second lights, respectively. The light guide transmits the first and the second lights emitted by the first and the second light sources to a subject, respectively. The endoscope generates first and second image signals by capturing an optical image of the subject while the first and the second light are shone on the subject, respectively. The gamma-correction block carries out gamma correction on the first and the second image signals using first and second color gamma coefficients, respectively. The first and the second color gamma coefficients are predetermined according to a wavelength band of the first and the second lights. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects and advantages of the present invention will be better understood from the following description, with reference to the accompanying drawings in which: 
         FIG. 1  is a block diagram showing the internal structure of an endoscope system having an endoscope processor of an embodiment of the present invention; 
         FIG. 2  is a block diagram showing the internal structure of a light-source unit; 
         FIG. 3  is a block diagram showing the internal structure of an image-processing unit; 
         FIG. 4  is a graph showing the first and the second color gamma coefficients according to signal level; 
         FIG. 5  is a timing chart illustrating the timing used to drive some components in the endoscope system in the white-light image observation mode; 
         FIG. 6  is a timing chart illustrating the timing used to drive some components in the endoscope system in the fluorescence image observation mode; and 
         FIG. 7  is a timing chart illustrating the timing used to drive some components in the endoscope system in the two-image observation mode. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is described below with reference to the embodiment shown in the drawings. 
     In  FIG. 1 , an endoscope system  10  comprises an endoscope processor  20 , an electronic endoscope  30 , and a monitor  11 . The endoscope processor  20  is connected to the electronic endoscope  30  and the monitor  11 . 
     The endoscope processor  20  emits light to illuminate a required subject. An optical image of the illuminated subject is captured by the electronic endoscope  30 , and then the electronic endoscope  30  generates an image signal. The image signal is sent to the endoscope processor  20 . 
     The endoscope processor  20  carries out predetermined signal processing on the received image signal, and then a video signal is generated. The video signal is sent to the monitor  11 , where an image corresponding to the video signal is displayed. 
     The endoscope processor  20  comprises a light-source unit  40 , an image-processing unit  50  (gamma-correction block), a system controller  21 , a timing controller  22  (light source controller), and other components. As described below, the light-source unit  40  emits white light for illuminating a desired subject, and/or exciting light, which makes tissue autofluoresce. In addition, as described in detail below, the image-processing unit  50  carries out predetermined signal processing on the image signal. 
     The system controller  21  controls the operations of all components of the endoscope processor  20 , including the light-source unit  40  and the image-processing unit  50 . The timing controller  22  times some operations of the components of the endoscope processor  20 . 
     By connecting the endoscope processor  20  to the electronic endoscope  30 , the light-source unit  40  and a light-guide  31  mounted in the electronic endoscope  30  are optically connected. In addition, by connecting the endoscope processor  20  to the electronic endoscope  30 , electrical connections are made between the image-processing unit  50  and the imaging device  32  mounted in the electronic endoscope  30 , and between the timing controller  22  and the imaging device  32  via the AFE (Analog Front End)  33 . 
     As shown in  FIG. 2 , the light-source unit  40  comprises a white-light source  41   w , an exciting-light source  41   e , a diaphragm  42 , a shutter  43 , a condenser lens  44 , a white-light power circuit  45 , an exciting-light control circuit  46 , a shutter driver  47   s , a diaphragm driver  47   d , and other components. 
     The white-light source  41  emits white light. The exciting-light source  42  emits exciting light of specified wavelength, such as in the ultraviolet range. 
     The diaphragm  42 , the shutter  43 , a dichroic mirror  48 , and the condenser lens  44  are mounted between the white-light source  41   w  and the light guide  31 . The white light emitted by the white-light source  41   w  passes the dichroic mirror  48 , is condensed by the condenser lens  44 , and is directed to the incident end of the light guide  31 . The white-light power circuit  45  supplies the white-light source  41   w  with power. 
     The intensity of the white light used to illuminate a subject, is controlled by adjusting the aperture ratio of the diaphragm  42 . The aperture ratio of the diaphragm  42  is adjusted by a first motor  49   a . The movement of the first motor  49   a  is controlled by the diaphragm driver  47   d . The diaphragm driver  47   d  is connected to an image-processing unit  50  via a system controller  21 . 
     As described below, the image-processing unit  50  detects the intensity of light received in the captured image of a subject based on the image signal generated by the imaging device. The detected intensity of light is communicated to the diaphragm driver  47   d  via the system controller  21 . The diaphragm driver  47   d  calculates the necessary degree of adjustment for the first motor  49   a  based on the intensity of light received. 
     The shutter  43  is a rotary shutter having an aperture area and a blocking area. The shutter  43  controls the passage of, or blocks the white light. When white light should be allowed to pass, the aperture area is inserted into the optical path of the white light. When white light should be blocked, the blocking area is inserted into the optical path of the white light. The shutter  43  is driven by a second motor  49   b . The movement of the second motor  49   b  is controlled by the shutter driver  47   s.    
     Exciting light emitted by the exciting-light source  41   e  is reflected by the dichroic mirror  48 , condensed by the condenser lens  44 , and directed to the incident end of the light guide  31 . The exciting-light control circuit  46  switches the exciting-light source  41   e  between lighting on and off. 
     The shutter driver  47   s  and the exciting-light control circuit  46  are connected to the timing controller  22 . The white-light control signal, for controlling the aperture time and blockage time of white light by shutter  43  is output from the timing controller  22  to the shutter driver  47   s . In addition, the exciting-light control signal for controlling times to switch the exciting-light source  41   e  between lighting on and off is output from the timing controller  22  to the exciting-light control circuit  46 . Both the white-light control signal and the exciting-light control signal are oscillating signals. 
     When the white-light control signal is in the high state, the shutter driver  47   s  drives shutter  43  so as to pass the white light. On the other hand, when the white-light control signal is in the low state, the shutter driver  47   s  drives the shutter  43  so as to block the white light. 
     When the exciting-light control signal is in the high state, the exciting-light control circuit  46  switches the exciting-light source  41   e  on. On the other hand, when the exciting-light control signal is in the low state, the exciting-light control circuit  46  switches the exciting-light source  41   e  off. 
     The timing controller  22  controls the high and low states of the white-light control signal and the exciting-light control signal so that the high and low states of the white-light control signal inverted with respect to the exciting-light control signal. Accordingly, when the white-light control signal and the exciting-light control signal are in high and low states, respectively, the white light is supplied to the incident end by the light source unit  40 . On the other hand, when the white-light control signal and the exciting-light control signal are in low and high states, respectively, the exciting light is supplied to the incident end by the light source unit  40 . 
     The white-light power circuit  45  and the exciting-light control circuit  46  are connected to the system controller  21 . The system controller  21  switches the white-light power circuit  45  and the exciting-light control circuit  46  between on and off. 
     Next, the structure of the electronic endoscope  30  is explained in detail. As shown in  FIG. 1 , the electronic endoscope  30  comprises the light guide  31 , the imaging device  32 , the AFE (analog front end)  33 , an exciting-light cut-off filter  34 , and other components. 
     The incident end of the light guide  31  is mounted in a connector (not depicted) which connects the electronic endoscope  30  to the endoscope processor  20 . And the other end, hereinafter referred to as the exit end, is mounted at the head end of the insertion tube  37  of the electronic endoscope  30 . As described above, the white light or the exciting light emitted by the light-source unit  40  arrives at the incident end of the light guide  31 . The light is then transmitted to the exit end. The light transmitted to the exit end illuminates a peripheral area near the head end of the insertion tube  37  through a diffuser lens  35 . 
     At the head end of the insertion tube  37 , an object lens  36 , the exciting-light cut-off filter  34 , and the imaging device  32  are also mounted. The exciting-light cut-off filter  34  is arranged between the object lens  36  and the imaging device  32 . 
     An optical image of the subject illuminated by the white light or the exciting light is formed on the light-receiving surface of the imaging device  32  through the object lens  36  and the exciting-light cut-off filter  34 . 
     The exciting-light cut-off filter  34  cuts off the whole band of the exciting light emitted by the exciting-light source  41   e . Accordingly, the same light component as the exciting light emitted by the exciting-light source  41   e  from an optical image of the subject illuminated by the white light or the exciting light is attenuated by the exciting-light cut-off filter  34 . The optical image passing through the exciting-light cut-off filter  34  reaches the light-receiving surface of the imaging device  32 . 
     The AFE  33  comprises an imaging device driver (not depicted). The imaging device driver drives the imaging device  32  so that the imaging device  32  can capture an optical image incident on the light-receiving surface during each field period. Field periods are usually 1/60 second in duration. The timing of various operations for driving the imaging device  32  by the imaging device driver is controlled by the timing controller  22 . 
     The imaging device  32  generates an image signal based on the optical image captured by the light-receiving surface. The generated analog image signal is digitized by the AFE  33 . The digital image signal is sent to the image-processing unit  50  every field period. 
     Next, the structure of the image-processing unit  50  is explained using  FIG. 3 . The image-processing unit  50  comprises a first-signal processing circuit  51 , a white-light image processing circuit  52   a , a fluorescence-image processing circuit  52   b , first and second image memories  53   a  and  53   b , first and second color gamma-correction circuits  54   a  and  54   b  (first and second correction blocks), a second-signal processing circuit  55  (image generation block), an encoder  56 , and other components. 
     The image-processing unit  50  is connected to the timing controller  22 . The timing controller  22  sends the white light control signal and the exciting-light control signal to the image-processing unit  50 . When the white-light control signal is in the high state, the image-processing unit  50  determines that a received image signal is a white-light image signal. On the other hand, when the exciting-light control signal is in the high state, the image-processing unit  50  determines that the received image signal is instead a fluorescence image signal. 
     The white-light image signal and the fluorescence image signal received by the image-processing unit  50  are input to the first-signal processing circuit  51 . The first-signal processing circuit  51  carries out gain-control processing on the white-light image signal and the fluorescence image signal. Through gain-control processing, median luminance value among the whole pixel signals which compose a white-light image signal and a fluorescence image signal is set according to the median value of the digital signal level of the first-signal processing circuit  51 . In addition, the first-signal processing circuit  51  carries out predetermined signal processing, such as color interpolation, on the white-light image signal and the fluorescence image signal. 
     As described above, the average intensity of light of a white-light image is calculated on the basis of the white-light image signal prior to gain-control processing. The calculated average intensity of light is communicated to the diaphragm driver  47   d  via the system controller  21 , and used for calculating the degree of adjustment required for the diaphragm  42 . 
     Once undergoing predetermined signal processing, the white-light image signal and the fluorescence-image signal are sent to the white-light image processing circuit  52   a  and the fluorescence-image processing circuit  52   b , respectively. 
     The white-light image processing circuit  52   a  carries out signal processing predetermined for the white-light image on the received white-light image signal. After carrying out the predetermined signal processing, the white-light image processing circuit  52   a  sends the white-light image signal to the first image memory  53   a  and the first color gamma-correction circuit  54   a . The white-light image signal is stored in the first image memory  53   a.    
     The fluorescence-image processing circuit  52   b  carries out signal processing predetermined for a fluorescence image on the received fluorescence image signal. After carrying out the predetermined signal processing, the fluorescence-image processing circuit  52   b  sends the fluorescence image signal to the second image memory  53   b  and the second color gamma-correction circuit  54   b . The fluorescence image signal is stored in the second image memory  53   b.    
     The first color gamma-correction circuit  54   a  carries out color gamma correction on the received white-light image signal using a first color gamma coefficient. As shown in  FIG. 4 , the first color gamma coefficient is a gain which varies according to the signal level of each pixel signal, and is predetermined so that color response will approach veridicality for all pixels across the entire luminance range. 
     The second color gamma-correction circuit  54   b  carries out color gamma correction on the received fluorescence image signal using a second color gamma coefficient. As shown in  FIG. 4 , the second color gamma coefficient is a gain which varies according to the level of each pixel signal, and is predetermined so that the second gamma coefficient is lower than the first gamma coefficient especially at the low end of the signal range. 
     Once color gamma-corrected, the white-light image signal and/or the fluorescence image signal are sent to the second-signal processing circuit  55 . The second-signal processing circuit  55  carries out plural image display processing operations as required. In addition, the second-signal processing circuit  55  carries out predetermined signal processing, such as clamping, blanking, and so on, and D/A conversion. A video signal generated based on the image signal converted into analog signal is encoded by the encoder  56 , and sent to the monitor  11 . An image corresponding to the received video signal is displayed on the monitor  11 . 
     Next, an operation of the endoscope system  10  on displaying a captured subject and an image displayed on the monitor  11  are explained. The endoscope system  10  has a white-light image observation mode, a fluorescence image observation mode, and two-image observation mode where an image captured by endoscope is displayed on the monitor  11 . One among the white-light image observation mode, the fluorescence observation mode, and the two-image observation mode is selected based on an input operation to an input apparatus (not depicted) of the endoscope  30  and the endoscope processor  20 . 
     When the white-light image observation mode is selected, the white-light control signal in the high state and the exciting-light control signal in the low state are sent to the shutter driver  47   s  and the exciting-light control circuit  46  from the timing controller  22 , respectively (see the columns of “white-light control signal” and “exciting-light control signal” in  FIG. 5 ). 
     Accordingly, the light-source unit  40  supplies only the white light to the light guide  31 , and then the white light is shone to a subject. An optical image of the subject illuminated by the white light is captured, whenever the field signal is alternately switched between high and low states, and then an image signal (referred to as IS 1 , IS 2 , IS 3 , IS 4 , IS 5 , and IS 6  in the row “imaging device output” in  FIG. 5 ) is sequentially generated. 
     When the white-light image observation mode is selected, the white-light control signal in the high state is sent to the image-processing unit  50  also. As described above, the image-processing unit  50  determines that the image signals received from the AFE  33  every field period are white-light image signals. Accordingly, the received all white-light image signals are sent to the white-light image-processing circuit  52   a  via the first-signal processing circuit  51 . 
     The white-light image signal is sent to the first image memory  53   a  and the first color gamma-correction circuit  54   a  from the white-light image-processing circuit  52   a , whenever the white-light image signal is received by the white-light image-processing circuit  52   a . As described above, the white-light image signal (referred to as WS 1 , WS 2 , WS 3 , WS 4 , WS 5 , and WS 6  in the row “first image memory” in  FIG. 5 ) is stored in the first image memory  53   a . In addition, the first color gamma-correction circuit  54   a  carries out color gamma correction on the received white-light image signal. 
     The white-light image signal (referred to as WS 1 , WS 2 , WS 3 , WS 4 , WS 5 , and WS 6  in the row “image signal received by second-signal processing circuit” in  FIG. 5 ) is sent to the second-signal processing circuit  55  from the first color gamma-correction circuit  54   a . The second-signal processing circuit  55  receives an image signal only from the first color gamma-correction circuit  54   a , and then, the second-signal processing circuit  55  generates a video signal based on the received white-light image signal without carrying out plural image display processing. There is displayed a white-light image (referred to as WS 1 , WS 2 , WS 3 , WS 4 , WS 5 , and WS 6  in the row “displayed image” in  FIG. 5 ) which is an optical image of a subject illuminated by the white light on the monitor  11 . 
     When the fluorescence image observation mode is selected, the white-light control signal in the low state and the exciting-light control signal in the high state are sent to the shutter driver  47   s  and the exciting-light control circuit  46  from the timing controller  22 , respectively (see the columns of “white-light control signal” and “exciting-light control signal” in  FIG. 6 ). 
     Accordingly, the light-source unit  40  supplies only the exciting light to the light guide  31 , and then the exciting light is shone on a subject. An optical image of the subject illuminated by the exciting light is captured, whenever the field signal is alternately switched between high and low states, and then an image signal (referred to as IS 1 , IS 2 , IS 3 , IS 4 , IS 5 , and IS 6  in the row “imaging device output” in  FIG. 6 ) is sequentially generated. 
     When the fluorescence image observation mode is selected, the exciting-light control signal in the high state is sent to the image-processing unit  50  also. As described above, the image-processing unit  50  determines that image signals received from the AFE  33  every field period are fluorescence image signals. Accordingly, all the received fluorescence image signals are sent to the fluorescence-image processing circuit  52   b  via the first-signal processing circuit  51 . 
     The fluorescence image signal is sent to the second image memory  53   b  and the second color gamma-correction circuit  54   b  from the fluorescence image processing circuit  52   b , whenever the fluorescence image signal is received by the fluorescence-image processing circuit  52   b . As described above, the fluorescence image signal (referred to as FS 1 , FS 2 , FS 3 , FS 4 , FS 5 , and FS 6  in the row “second image memory” in  FIG. 6 ) is stored in the second image memory  53   b . In addition, the second color gamma-correction circuit  54   b  carries out color gamma correction on the received fluorescence image signal. 
     The fluorescence image signal (referred to as FS 1 , FS 2 , FS 3 , FS 4 , FS 5 , and FS 6  in the row “image signal received by second-signal processing circuit” in  FIG. 6 ) is sent to the second-signal processing circuit  55  from the second gamma correction circuit  54   b . The second-signal processing circuit  55  receives an image signal only from the second color gamma-correction circuit  54   b , and then, the second-signal processing circuit  55  generates a video signal based on the received fluorescence image signal without carrying out plural image display processing. There is displayed a fluorescence image (referred to as FS 1 , FS 2 , FS 3 , FS 4 , FS 5 , and FS 6  in the row “displayed image” in  FIG. 6 ) which is an optical image of a subject illuminated by the exciting light on the monitor  11 . 
     When the two-image observation mode is selected, the white-light control signal and the exciting-light control signal which are alternately switched between high and low states so as to synchronize the field signal, are sent to the shutter driver  47   s  and the exciting-light control circuit  46  from the timing controller  22 , respectively (see the columns of “field signal”, “white-light control signal”, and “exciting-light control signal” in  FIG. 7 ). 
     Accordingly, the light-source unit  40  alternately and repeatedly supplies the light guide with the white light and the exciting light synchronized with the field signal, and then the white light and the exciting light are alternately shone on a subject every field period. An optical image of the subject is captured, whenever the field signal is alternately switched between high and low states, and then an image signal (referred to as IS 1 , IS 2 , IS 3 , IS 4 , IS 5 , and IS 6  in the row “imaging device output” in  FIG. 7 ) is sequentially generated in each field period when the illumination light to the subject is altered. 
     When the two-image observation mode is selected, the white-light control signal and the exciting-light control signal which are alternately and repeatedly switched to the high state are sent to the image-processing unit  50  also. Accordingly, the image-processing unit  50  determines that the image signals (referred to as IS 1 , IS 3 , and IS 5  in the row “imaging device output”) received at the times t 1 , t 3 , and t 5  when the white light is shone on the subject, are the white-light image signals. On the other hand, the image-processing unit  50  determines that the image signals (referred to as IS 2 , IS 4 , and IS 6  in the row “imaging device output”) received at the times t 2 , t 4 , and t 6  when the exciting light is shone on the subject, are the fluorescence image signals. 
     As described above, the white-light image signals are sent to the white-light image-processing circuit  52   a  via the first-signal processing circuit  51 . On the other hand, the fluorescence image signals are sent to the fluorescence-image processing circuit  52   b  via the first-signal processing circuit  51 . 
     The determined white-light image signals, which are received by the white-light image-processing circuit  52   a  at the times t 1 , t 3 , and t 5 , are sent to and stored in the first image memory  53   a  (referred to as WS 1 , WS 3 , and WS 5  in the row “first image memory”). Simultaneously, the determined white-light image signals are sent to the first color gamma-correction circuit  54   a  (referred to as WS 1 , WS 3 , and WS 5  in the row “image signal received by second-signal processing circuit”). 
     The determined fluorescence image signals, which are received by the fluorescence-image processing circuit  52   b  at the times t 2 , t 4 , and t 6 , are sent to and stored in the second image memory  53   b  (referred to as FS 2 , FS 4 , and FS 6  in the row “second image memory”). Simultaneously, the determined fluorescence image signals are sent to the second color gamma-correction circuit  54   b  (referred to as FS 2 , FS 4 , and FS 6  in the row “image signal received by second-signal processing circuit”). 
     The white-light image signals stored in the first image memory  53   a  is read by the white-light image processing circuit  52   a  and sent to the first color gamma-correction circuit  54   a  in field periods (referred to as t 2 , t 4 , and t 6 ) following the field periods during which they were stored in the first image memory  53   a . The fluorescence image signals stored in the second image memory  53   b  is read by the fluorescence-image processing circuit  52   b  and sent to the second color gamma-correction circuit  54   b  in field periods (referred to as t 3  and t 5 ) following the field periods during which they were stored in the second image memory  53   b.    
     Accordingly, the first color gamma-correction circuit  54   a  receives the same white-light image signal in successive two-field periods (referred to as t 1  and t 2 , t 3  and t 4 , t 5  and t 6 ), carries out color gamma correction, and sends it to the second-signal processing circuit  55 . The second color gamma-correction circuit  54   b  receives the same fluorescence image signal in successive two-field periods (referred to as t 2  and t 3 , t 4  and t 5 ), carries out color gamma correction, and sends it to the second-signal processing circuit  55 . 
     The second-signal processing circuit  55  receives the white-light image signal and the fluorescence image signal from the first and second color gamma-correction circuits  54   a  and  54   b , respectively, every field period. The second-signal processing circuit  55  carries out plural image display processing and generates the video signal based on the received white-light image signal and the received fluorescence image signal. Based on the video signal, a white-light image and a fluorescence image (see the row “displayed image”) are displayed on the monitor  11 . 
     In the above embodiment, as explained in detail below, adequate color gamma correction can be carried out on a white-light image signal and a fluorescence image signal when a white-light image and a fluorescence image should be simultaneously displayed on the monitor  11 . 
     In comparing the frequency distributions of pixel signals which compose a white image signal to those of a fluorescence image signal, it is typical that a fluorescence image signal includes more pixel signals with low signal levels than a white image signal. 
     Accordingly, if the first gamma coefficient is used for color gamma-correction on a fluorescence image signal, it will overcorrect the colors at the low end of the signal level range, producing color noise in the fluorescence image. 
     In order to decrease color noise in the fluorescence image, a coefficient low enough for a fluorescence image signal in such a range, such as a second color gamma coefficient, should be used for color gamma correction. However, if the second color gamma coefficient is used for color gamma-correction on a white-light image signal, color might be faded at pixels with low luminance. 
     In a prior endoscope system, when a white-light image and a fluorescence image are simultaneously displayed, either a first color gamma coefficient or a second color gamma coefficient was used for color gamma correction on both a white-light image signal and a fluorescence image signal. Accordingly, one of two problems described above happened. On the other hand, in the above embodiment, color gamma-correction is carried out on the white-light image signal and the fluorescence image signal using the first and second color gamma coefficients, respectively. Accordingly, the above problems will not occur. 
     The endoscope processor  20  comprises the first and second image memories  53   a  and  53   b  in the above embodiment. However, the first and second image memories  53   a  and  53   b  may not be comprised. If the first and second image memories  53   a  and  53   b  are not comprised, the white-light image and the fluorescence image can not be simultaneously displayed. However, the white-light image and the fluorescence image can be individually displayed following adequate color gamma correction. 
     The first and second color gamma-correction circuits  54   a  and  54   b  carry out color gamma correction on the white-light image signal and the fluorescence image signal, in the above embodiment. However, a single color gamma-correction circuit may carry out color gamma-correction on the white-light image signal and the fluorescence image signal using the first and second color gamma coefficients, respectively. 
     For example, the same effect can be achieved by providing a coefficient memory for storing the first and second color gamma coefficients, and reading and using the first and second color gamma coefficients for color gamma correction if the single color gamma-correction circuit receives the white-light image signal and the fluorescence image signal, respectively. 
     The endoscope processor  20  may comprise a memory for storing the first and second color gamma coefficients so that the first and second color gamma-correction circuits  54   a  and  54   b  can read both of the first and second color gamma coefficients in the above embodiment. With such an endoscope processor, the processing speed of color gamma correction during illumination of a single kind of light to a subject, such as either of the white-light image observation mode and the fluorescence image observation mode, can be increased by alternately ordering the first and second color gamma-correction circuit  54   a  and  54   b  to carry out color gamma correction on image signals, which are sequentially generated every field period, using the same color gamma coefficient. For example, in the white-light image observation mode, the first and second color gamma-correction circuit  54   a  and  54   b  read the first color gamma coefficient, alternately receive white-light image signals sequentially generated by and sent from the imaging device  32 , and alternately carry out color gamma correction on the received white-light image signal using the first color gamma coefficient. 
     The white light and the exciting light can be switched in the above embodiment. However, a kind of light which can be switched is not limited to the white light and the exciting light. As long as light of differing wavelength can be switched, adequate color gamma correction for each kind of image signal can be carried out as in the above embodiment. 
     Although the embodiments of the present invention have been described herein with reference to the accompanying drawings, obviously many modifications and changes may be made by those skilled in this art without departing from the scope of the invention. 
     The present disclosure relates to subject matter contained in Japanese Patent Application No. 2007-293113 (filed on Nov. 12, 2007), which is expressly incorporated herein, by reference, in its entirety.