Abstract:
An image data processor comprising an image signal receiver, a histogram generator, a gain calculator, an amplifier, and a signal feeder, is provided. The image signal receiver receives an autofluorescence image signal. The autofluorescence image signal is generated by an imaging device when the imaging device captures an autofluorescence image. The histogram generator generates a histogram of luminance in the autofluorescence image based on the autofluorescence image signal. The gain calculator calculates a gain based on the histogram and a predetermined luminance value. The amplifier amplifies the autofluorescence image signal by the gain. And then the amplifier generates an amplified autofluorescence image signal. The signal feeder outputs the amplified autofluorescence image signal to a monitor. The monitor displays an amplified autofluorescence image.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image data processor and an electronic endoscope system which carries out a specific image process on image signals generated by an autofluorescence endoscope. 
     2. Description of the Related Art 
     It is known that an organ generates autofluorescence when the organ is illuminated by exciting light having a specific wavelength, for example ultraviolet light. It is also known that an amount of autofluorescence at, for example, a cancerous region, is less than that of a healthy area in an organ. An autofluorescence endoscope system taking advantage of the above properties has been invented. The autofluorescence endoscope system usually has a reference light source for emitting white light, an exciting light source for emitting exciting light, and a monitor. A reference image is displayed on the monitor when an object is illuminated by the white light. An autofluorescence image is displayed on the monitor when the object is illuminated by the exciting light. 
     Further, it is prohibited to irradiate strong exciting light to an organ from a medical point of view. An autofluorescence image, irradiated with a limited amount of exciting light, is too dark to observe. Accordingly, it is difficult for a user to make a diagnosis utilizing an autofluorescence image. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide an image data processor and an electronic endoscope system so that an adequately bright autofluorescence image is automatically displayed. 
     According to the present invention, an image data processor comprising an image signal receiver, a histogram generator, a gain calculator, an amplifier, and a feeder, is provided. The image signal receiver receives an autofluorescence image signal. The autofluorescence image signal is generated by an imaging device when the imaging device captures an autofluorescence image. The autofluorescence image is an optical image of an object illuminated by an exciting light. The exciting light makes an organ generate autofluorescence. The histogram generator generates an autofluorescence histogram of luminance in the autofluorescence image based on the autofluorescence image signal. The gain calculator calculates an autofluorescence gain based on the autofluorescence histogram and a predetermined luminance value for the autofluorescence image signal. The autofluorescence gain is used for amplifying the autofluorescence image signal. The amplifier amplifies the autofluorescence image signal by the autofluorescence gain. And then the amplifier generates an amplified autofluorescence image signal. The signal feeder can output the amplified autofluorescence image signal to a monitor. The monitor displays an amplified autofluorescence image. The amplified autofluorescence image corresponds to the amplified autofluorescence image signal. 
     Further preferably, the autofluorescence gain is calculated so that a maximum luminance value in the autofluorescence histogram amplified by the autofluorescence gain agrees with the predetermined luminance value. Or the autofluorescence gain is calculated so that an average luminance value in said histogram amplified by the autofluorescence gain agrees with the predetermined luminance value. 
     Further preferably, when the imaging device is mounted in an electronic endoscope, the predetermined luminance value is decided according to the properties of the electronic endoscope connected to the image data processor. 
     Further preferably, the image data processor comprises an input unit for fine manual adjustment of the autofluorescence gain calculated by the gain controller. 
    
    
     
       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 electronic endoscope system having an image data processor of an embodiment of the present invention; 
         FIG. 2  is a plan of the shutter; 
         FIG. 3  is a histogram of luminance in an autofluorescence image and an amplified autofluorescence image; 
         FIG. 4  illustrates an amplified autofluorescence image and a reference image displayed on a monitor simultaneously; 
         FIG. 5  illustrates the amplified autofluorescence image, the reference image, and the gain displayed on the monitor simultaneously; 
         FIG. 6  is a timing chart to explain a timing to illuminate reference light or exciting light, to carry out the auto gain control process, and so on; 
         FIG. 7  is a flowchart to explain the control process and the image data process; 
         FIG. 8  is a timing chart to explain a timing to illuminate reference light or exciting light, to carry out the auto gain control process, and so on in the transformed embodiment; and 
         FIG. 9  shows the control surface of the endoscope processor. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is described below with reference to the embodiment shown in the drawings. 
     In  FIG. 1 , an electronic endoscope system  10  comprises an endoscope processor  20 , an endoscope  50 , and a monitor  60 . The endoscope processor  20  is connected to the endoscope  50  and the monitor  60 . 
     A light system  21  is housed in the endoscope processor  20 . The light system  21  emits light to illuminate a required object. The light, that the light system emits, is transmitted by the light guide  51  housed in the endoscope  50 . And the required object is illuminated by the transmitted light. 
     The endoscope  50  comprises an imaging device  53 , such as a CCD, at the head end of an insert tube  52 . The imaging device  53  captures an optical image of the required object. The imaging device  53  generates image signals corresponding to the captured image. The image signal is sent to the endoscope processor  20 . The endoscope processor  20  comprises an image process system  34 . The image process system  34  carries out some predetermined signal processes for the image signals. The image process system  34  can carry out an auto gain control process, referred to as AGC process, described in detail later. The predetermined signal processes include not only the AGC process but also the usual signal processes, such as a gamma process, white balance process, and so on. The image signal, being carried out the predetermined processes, is sent to the monitor  60 . An image, corresponding to the image signal sent to the monitor  60 , is displayed on the monitor  60 . 
     The light system  21  comprises a reference light source  22 , an exciting light source  23 , condenser lens  24 , a reference light power circuit  25 , an exciting light control circuit  26 , shutter  27 , diaphragm  28 , and so on. 
     A reference light source  22  emits reference light, such as white light. An exciting light source  23  emits exciting light, such as ultraviolet light, having a specific wavelength. 
     The diaphragm  28 , shutter  27 , dichroic mirror  29 , and the condenser lens  24  are mounted in an optical path of the reference light emitted by the reference light source  22  to the incident end  51   a  of the light guide  51 . The reference light, which is almost all parallel light beams, is made incident on the incident end  51   a,  through the dichroic mirror  29  and the condenser lens  24 . The condenser lens  24  condenses the reference light for the incident end  51   a.    
     A reference light intensity is adjusted by driving the diaphragm  28 . A first motor  31   a,  controlled by the diaphragm circuit  30 , drives the diaphragm  28 . The diaphragm circuit  30  is connected to a first signal processing circuit  35   a . The first signal processing circuit  35   a  detects the luminance of the object based on the image signals generated by the imaging device  53 . The diaphragm circuit  30  calculates a driving quantity of the first motor  31   a  based on the detected luminance of the object and a preset luminance. 
     The shutter  27  is a rotary shaped shutter as shown in  FIG. 2 . Passing through or shielding the reference light is changed by driving the shutter  27 . The shutter  27  has an aperture  27   o  and a shielding plate  27   s . The aperture  27   o  is inserted into the optical path of the reference light when the reference light is controlled to pass through the optical path. The shielding plate  27   s  is inserted into the optical path of the reference light when the reference light is shielded. A second motor  31   b,  controlled by a shutter circuit  32 , drives the shutter  27 . 
     The exciting light beams emitted by the exciting light source  23  are made almost all parallel. The exciting light source  23  is mounted so that the exciting light beams, that are made almost all parallel, are incident on the incident end  51   a  after being reflected by the dichroic mirror  29 . For example, when an exciting light source  23  is set up so that an angle between the optical paths of the reference light and the exciting light is 90 degrees, the dichroic mirror  29  is mounted at an angle of 45 degrees between a plane of the dichroic mirror  29  and the optical path of the reference light. The exciting light control circuit  26  controls the switching of the exciting light source  23  on and off. 
     The shutter circuit  32  and the exciting light control circuit  26  are connected to a timing controller  40 . The timing controller  40  outputs a shutter timing signal to the shutter circuit  32 . The shutter timing signal controls an amount of time the reference light passes through the shutter  27 , and an amount of time the reference light is shielded. Further, the timing controller  40  outputs an emission timing signal to the exciting light control circuit  26 . The emission timing signal controls a timing to switch the exciting light source  23  on and off. 
     The timing controller  40  outputs the shutter timing signal and the emission timing signal so that the exciting light source  23  is switched off when the reference light is controlled to pass through the shutter  27 . The timing controller  40  outputs the shutter timing signal and the emission timing signal so that the exciting light source  23  is switched on when the reference light is shielded by the shutter  27 . Accordingly, changing the light to illuminate the required object is carried out by an operation of the timing controller  40 , the exciting light control circuit  26 , the shutter circuit  32 , the second motor  31   b,  and the shutter  27 . 
     Further, the timing controller  40  outputs a necessary timing signal for driving the imaging device  53 , to an imaging device driving circuit  41 . Further still, the timing controller  40  is connected to the image process system  34 , and the timing controller  40  outputs another timing signal, described later, to the image process system  34 . 
     Power for the reference light source  22  is supplied by the reference light power circuit  25 . The reference light power circuit  25  and the exciting light control circuit  26  are connected to a system controller  33 . The system controller  33  is connected to an input unit  57  mounted on the endoscope  50 . The input unit  57  comprises some buttons and some levers used for a user&#39;s inputting to carry out some determined functions. The reference light power circuit  25  and the exciting light control circuit  26  are started based on an input to the input unit  57 . 
     As described above, the reference light or the exciting light is incident on the incident end  51   a  of the light guide  51 . The light transmitted to the out end  51   b  of the light guide  51  illuminates a peripheral area nearby the head end of the insert tube  52  through a diffuser lens  54 . 
     An optical image of the required object illuminated by the light is captured by the imaging device  53  through an object lens  55  and an exciting light cut-off filter  56 . An optical image comprises reflected reference light components when the required object is illuminated by the reference light. The reflected reference light components of the optical image are captured by the imaging device  53 . On the other hand, an optical image comprises reflected exciting light components and autofluorescence components when the required object is illuminated by the exciting light. The reflected exciting light components of the optical image are excluded by the exciting light cut-off filter  56 . And then only the autofluorescence components of the optical image are captured by the imaging device  53 . 
     The imaging device  53  is controlled by the imaging device driving circuit  41  so that the imaging device  53  captures the optical image of one field at least while the required object is continuously illuminated by the reference light. Or the imaging device  53  is controlled by the imaging device driving circuit  41  so that the imaging device  53  captures the optical image of one field at least while the required object is illuminated by the exciting light. 
     The image process system  34  comprises the first signal processing circuit  35   a,  a second signal processing circuit  35   b,  a histogram circuit  37 , and first and second memories  39   a,    39   b.    
     The imaging device  53  is connected to the first signal processing circuit  35   a . An image signal generated by the imaging device  53  is received by the first signal processing circuit  35   a . The first signal processing circuit  35   a  carries out the predetermined signal processes, for example a white balance process, gamma correction process, and so on for the image signal. In addition, the analog image signals are converted to digital image data. 
     The first signal processing circuit  35   a  is connected to the timing controller  40 . The timing controller  40  repeatedly and reciprocally outputs a reference timing signal and an exciting timing signal. The reference timing signal is output at the same time the reference light is controlled to pass through the shutter  27 . The exciting timing signal is output at the same time the exciting light source  23  is switched on. 
     The first signal processing circuit  35   a  recognizes the image signal, generated while receiving the reference timing signal, as a reference image signal. The reference image signal corresponds to a reference image that is captured while the required object is illuminated by the reference light. On the other hand, the first signal processing circuit  35   a  recognizes the image signal, generated while receiving the exciting timing signal, as an autofluorescence image signal. The autofluorescence image signal corresponds to an autofluorescence image that is captured while the required object is illuminated by the exciting light. 
     The first signal processing circuit  35   a  is connected to the first and the second memories  39   a  and  39   b . The reference image data, corresponding to the reference image signal, is stored in the first memory  39   a . The autofluorescence image data, corresponding to the autofluorescence image signal, is stored in the second memory  39   b . The first and second memories  39   a  and  39   b  are connected to the timing controller  40 . The timing controller  40  controls the timing for storing the reference image data and the autofluorescence image data respectively in the first and second memories  39   a  and  39   b.    
     In addition, the first signal processing circuit  35   a  is also connected to the histogram circuit  37 . The autofluorescence image data is sent to the histogram circuit  37 . The histogram circuit  37  generates an autofluorescence histogram data based on the autofluorescence image data. The autofluorescence histogram data corresponds to a histogram of luminance for the autofluorescence image, hereinafter referred to as Haf (reference to as Haf in  FIG. 3 ). 
     The histogram circuit  37  is connected to the second signal processing circuit  35   b . The autofluorescence histogram data is sent to the second signal processing circuit  35   b . In addition, the second signal processing circuit  35   b  is connected to the first and second memories  39   a  and  39   b . One of the reference image data and the autofluorescence image data, or both the reference image data and the autofluorescence image data are sent to the second signal processing circuit  35   b.    
     The second signal processing circuit  35   b  carries out the AGC process. In the AGC process, an autofluorescence gain, that is an amplifying rate for brightening the autofluorescence image, is calculated based on the Haf. And then the second signal processing circuit  35   b  carries out an amplification process for the autofluorescence image data. 
     The calculation of the autofluorescence gain is carried out according to one of a max-mode and an average-mode. In the max-mode, the autofluorescence gain is calculated based on a maximum luminance of the autofluorescence image. On the other hand, the autofluorescence gain is calculated based on an average luminance of the autofluorescence image in the average-mode. A max-mode or an average-mode is selected when a user inputs an appropriate command to the input unit  57 . 
     A maximum luminance, hereinafter referred to as Lmaxaf, is detected from the Haf when the max-mode is selected. And then, the autofluorescence gain, that makes the Lmaxaf agree with a first luminance, hereinafter referred to as L 1 , is calculated by dividing the L 1  by the Lmaxaf. The L 1  is predetermined and stored in a ROM (not depicted). The L 1  may be any adequate value. It may be determined so that the autofluorescence image brightened with the autofluorescence gain, calculated with the Lmaxaf and the L 1 , does not have white blurring, like halation. For example, the L 1  is a maximum luminance value of which light can be displayed on the connected monitor  60 . 
     An average luminance, hereinafter referred to as Laveaf, is calculated from the Haf when the average-mode is selected. And then, the autofluorescence gain, that makes the Laveaf agree with a second luminance, hereinafter referred to as L 2 , is calculated by dividing the L 2  by the Laveaf. The L 2  is predetermined and stored in the ROM. The L 2  may be any adequate value. It may be determined so that the autofluorescence image brightened with the autofluorescence gain, calculated with the Laveaf and the L 2 , does not have white blurring, like halation. For example, the L 2  is a half of a maximum luminance value of which light can be displayed on the connected monitor  60 . 
     As described above, the amplification process is carried out for the autofluorescence image data after a calculation of the autofluorescence gain. In the amplification process, amplified autofluorescence image data is generated by amplifying the autofluorescence image data by the autofluorescence gain. Accordingly, an amplified autofluorescence image, corresponding to the amplified autofluorescence image data, is brighter than the autofluorescence image, as shown by a histogram of luminance for the amplified autofluorescence image (referred to as Haaf in  FIG. 3 ). 
     Further, a user can finely adjust the autofluorescence gain by an input to the input unit  57 . The amplification process is carried out by replacing the autofluorescence gain with the finely adjusted autofluorescence gain if there is an input to the input unit  57  for the fine adjustment. 
     Further, the second signal processing circuit  35   b  carries out a D/A conversion process and then the amplified autofluorescence image data, which is a digital image data, is converted to analog image signal. The second signal processing circuit  35   b  carries out some predetermined signal processes, for example a clamp process and a blanking process after the D/A conversion. Finally, an amplified autofluorescence image signal is generated. 
     The second signal processing circuit  35   b  is connected to the monitor  60 . The second signal processing circuit  35   b  outputs the amplified autofluorescence image signal to the monitor  60 . The amplified autofluorescence image is displayed over the whole display surface on the monitor  60 . 
     In addition, the second signal processing circuit  35   b  can carry out a D/A conversion process and the predetermined signal processes for the reference image data. A reference image signal, that is converted from the reference image data, is sent to the monitor  60 . And then the reference image is displayed on the monitor  60 . 
     The image displayed on the monitor  60  can be selected from the reference image and the amplified autofluorescence image when a user inputs an appropriate command to the input unit  57  (or processor surface). Or both of the images can be displayed simultaneously as shown in  FIG. 4 . In  FIG. 4 , the reference image and the amplified autofluorescence image are respectively referred to as RI, and AFI. Further, the autofluorescence gain can be displayed on the monitor  60  when a user inputs an appropriate command to the input unit  57  as shown in  FIG. 5 . 
     In the case where both of the images and the autofluorescence gain are displayed, the second signal processing circuit  35   b  carries out allocation of area to display each image, and scales down each image. The second signal processing circuit  35   b  is connected to the timing controller  40 . The allocation of the areas and the scaling down of each image are carried out based on a timing signal output from the timing controller  40 . 
     Next, timings for illuminating the reference light or exciting light, for carrying out the AGC process, and so on, are explained below using the timing chart of  FIG. 6 . 
     The timing controller  40  outputs a field signal to the exciting light control circuit  26 , the shutter circuit  32 , the first signal processing circuit  35   a,  the second signal processing circuit  35   b,  the first memory  39   a,  and the second memory  39   b . The field signal is a rectangular wave having high and low states. The high and low states change repeatedly and cyclically. 
     The shutter  27  is driven by the shutter circuit  32  so that the reference light can pass through the shutter  27  while the field signal is in the high state as shown by the timings t 1 , t 3 , and t 5  of  FIG. 6 . And during the same period, the exciting light control circuit  26  switches off the exciting light source  23 . Consequently, the reference light illuminates a required object. 
     Further, the imaging device  53  generates reference image signals, for example WL 1 , WL 3 , WL 5  of  FIG. 6 , during the same period. Further the AGC process is not carried out at the second signal processing circuit  35   b  during the same period as shown in AGC for FL of  FIG. 6 . Consequently, the predetermined signal processes without the AGC process are carried out for the reference image data, that is input to the second signal processing circuit  35   b  through the first signal processing circuit  35   a  and the first memory  39   a  during this period. 
     On the other hand, the exciting light control circuit  26  switches on the exciting light source  23  while the field signal is in the low state as shown by the timings t 2 , t 4 , and t 6  of  FIG. 6 . And during the same period, the shutter  27  is driven by the shutter circuit  32  so that the reference light can be shielded. Consequently, the exciting light illuminates a required object. 
     Further, the imaging device  53  generates autofluorescence image signals, for example FL 2 , FL 4 , and FL 6  of  FIG. 6 , during the same period. Further the AGC process is carried out in the second signal processing circuit  35   b  during the same period as shown in AGC for FL of  FIG. 6 . Consequently, the AGC process and the predetermined signal processes are carried out for the autofluorescence image data, that is input to the second signal processing circuit  35   b  through the first processing circuit  35   a  and the second memory  39   b  during this period. 
     The field signal sent to the exciting light control circuit  26  corresponds to the emission timing signal described above. And the field signal sent to the shutter circuit  32  corresponds to the shutter timing signal described above. 
     The image signal, that is sent to the first signal processing circuit  35   a  from the imaging device  53  while the field signal is in the high state, is recognized as the reference image signal. On the other hand, the image signal, that is sent to the first signal processing circuit  35   a  from the imaging device  53  while the field signal is in the low state, is recognized as the autofluorescence image signal. 
     The first memory is driven so that the first memory stores the reference image data, that is output from the first signal processing circuit  35   a  during the high state of the field signal. Consequently, the high state of the field signal corresponds to the reference timing signal described above. On the other hand, the second memory is driven so that the second memory stores the autofluorescence image data, that is output from the first signal processing circuit  35   b  during the low state of the field signal. Consequently, the low state of the field signal corresponds to the exciting timing signal described above. 
     Next, control processes and image signal processes carried out by the endoscope processor  20  are explained below using the flowchart of  FIG. 7 . 
     The control processes and the image signal processes of this embodiment start when a user inputs an appropriate command to the input unit  57  for displaying an amplified autofluorescence image on the monitor  60 . At step S 100 , the shutter timing signal is output to the shutter circuit  32  so that the shutter is driven to insert the aperture  27   o  into the optical path of the reference light. Further, the emission timing signal for switching off the exciting light is output to the exciting light control circuit  26 . Then the light to illuminate a required object is changed to the reference light by driving the shutter  27 . 
     At step S 101 , the imaging device  53  is driven to capture the optical image of the required object that is illuminated by the reference light. And then the process goes to step S 102 . At step S 102 , the predetermined signal processes, including the A/D conversion processes, are carried out for the reference image signal generated by the imaging device  53 . The reference image signal, which is analog, is converted to digital reference image data by the predetermined processes. 
     At step S 103 , the reference image data is stored in the first memory  39   a,  and then the process goes to step S 104 . At step S 104 , the emission timing signal for emitting the exciting light is output to the exciting light control circuit  26 . Then the exciting light control circuit  26  makes the exciting light source  23  emit the exciting light. Further, the shutter timing signal is output to the shutter circuit  32  so that the shutter is driven to insert the shielding plate  27   s  into the optical path of the reference light. Then the light to illuminate a required object is changed to the exciting light. 
     At step S 105 , the imaging device  53  is driven to capture the optical image of the required object that is illuminated by the exciting light. The predetermined signal processes, including the A/D conversion process, are carried out for the autofluorescence image signal generated by the imaging device  53 . Then the analog autofluorescence image signal is converted to digital autofluorescence image data by predetermined processes at step S 106 . 
     At step S 107 , the autofluorescence image data is stored in the second memory  39   b . At next step S 108 , the autofluorescence histogram data is generated based on the autofluorescence image data. After generating the autofluorescence histogram data, the process goes to step S 109 . 
     At step S 109 , the autofluorescence gain, that is the amplifying rate for brightening the autofluorescence image, is calculated based on the Haf and one of the L 1  and the L 2 . The Lmaxaf is detected from the Haf when the max-mode is selected. And then, the autofluorescence gain for the max-mode, that makes the Lmaxaf agree with the L 1 , is calculated. The Laveaf is calculated from the Haf when the average-mode is selected. And then, the autofluorescence gain for the average-mode, that makes the Laveaf agree with the L 2 , is calculated. 
     At the next step S 110 , the amplified autofluorescence image data is generated by amplifying the autofluorescence image data with the autofluorescence gain calculated at step S 109 , then the process goes to step S 111 . 
     At step S 111 , it is judged whether a plural-image-displaying mode, where the reference image is displayed with the amplified autofluorescence image, is selected or not. The process goes to step S 112  when the plural-image-display mode is selected. At step S 112 , allocation of areas to display each image, and the scaling down of each image are carried out, then the process goes to step S 113 . Or the process goes to step S 113  after carrying out the predetermined processes, excepting for the allocation and scaling down, for the amplified autofluorescence image data, when the plural-image-display mode is not selected. 
     At step S 113 , it is determined whether a user has input the appropriate command to the input unit  57  for displaying the autofluorescence gain or not. The process goes to step S 114  when there is the appropriate input at step S 113 . At step S 114 , allocation of area to display the autofluorescence gain is carried out, then the process goes to step S 115 . Or the process goes to step S 115  when there is no appropriate input. 
     At step S 115 , the amplified autofluorescence image signal or a complex image signal, corresponding to an image separately including the amplified autofluorescence image and the reference image, is output to the monitor  60 . 
     At the next step S 116 , it is judged whether the user has input an appropriate command to finish displaying the amplified autofluorescence image. When there is the appropriate input, the control processes and the image signal processes are completed. On the other hand, the process returns to step S 100  when there is no appropriate input. The processes from step S 100  to step S 116  are repeated until there is an appropriate input for finishing. 
     In the above embodiment, the autofluorescence gain is automatically calculated to be an adequate value and the autofluorescence image data is amplified by the autofluorescence gain. Consequently, it is possible to automatically make the autofluorescence image adequately bright without user&#39;s control. 
     Further in the above embodiment, it is possible to display the reference image with the amplified autofluorescence image on the monitor  60 . Consequently, a user can observe the reference image without changing between the reference image and the amplified autofluorescence image. 
     Further in the above embodiment, it is possible to finely adjust the autofluorescence gain. Consequently, a user can preferably adjust brightness of the amplified autofluorescence image even if there is some noticeable noise in the amplified autofluorescence image or the amplified autofluorescence image is still dark. 
     Further in the above embodiment, it is possible to display the autofluorescence gain on the monitor  60 . Consequently, a user can be aware of the autofluorescence gain, which can contribute to an accurate diagnosis. 
     The AGC process is carried out only for the autofluorescence image data in the above embodiment. However, it is possible to carry out additional AGC processes for the reference image data according the gain calculated based on the histogram for the reference image and to output the amplified reference image data to the monitor  60 . 
     In the transformed embodiment, the reference image data is sent to the histogram circuit  37  and reference histogram data of the reference image is generated based on the reference image data. The reference histogram data corresponds to a histogram of luminance for the reference image. The second image signal processing circuit  35   b  calculates a reference gain based on the histogram of luminance for the reference image and carries out an AGC process for the reference image. The reference gain is used for amplifying the reference image data. 
     In the transformed embodiment, the AGC process is carried out at the second signal processing circuit  35   b  while the field signal is in the high state as shown in AGC for WL of  FIG. 8 . Consequently, the AGC process and the predetermined signal processes are carried out for the reference image data, that is input to the second signal processing circuit  35   b  through the first processing circuit  35   a  and the first memory  39   a  during this period. 
     In the transformed embodiment, it is possible to make the reference image, displayed on the monitor  60 , adequately bright when the reference image is dark even if the aperture is fully opened. In addition, it is possible to control brightness of the reference image by the AGC process more quickly than by the control of light irradiation with a mechanical aperture like the diaphragm  28 . Consequently, the brightness of the reference image can be more stable. And it is easy to control the brightness of the reference image until the diaphragm  28  is stably driven. 
     The L 1  is a single predetermined value and the L 2  is a single predetermined value in the above embodiment. However, they may change according to an endoscope  50  connected to the endoscope processor  20 . For example, an endoscope has a memory to store the L 1  and the L 2 , that are predetermined according to the properties of the endoscope. And the endoscope processor  20  reads the L 1  or the L 2  stored in the memory for a gain calculation. Or the ROM stores some predetermined luminance values, that are decided according to the properties of the endoscopes expected to be connected to the endoscope processor  20 . And one of the predetermined luminance values stored in the ROM is selected as the L 1  or the L 2  according to an endoscope connected to the endoscope processor  20 . The brightness of the amplified autofluorescence image can be controlled to be suitable for a connected endoscope due to using an adequate luminance value for the L 1  or the L 2 , according to an endoscope. For example, a lower luminance value is used as the L 1  or the L 2  when a bronchus endoscope or an endoscope for the large intestines is connected to the endoscope processor  20 . This is because a gain used for the amplification process is smaller since a lumen of a bronchus or large intestines is narrow. A higher luminance value is used as the L 1  or the L 2  when an endoscope for an upper digestive tract is connected to the endoscope processor  20 . This is because a gain used for the amplification process is larger since a lumen of an upper digestive tract is wide. 
     It is described as an example in the above embodiment that the L 1  and the L 2  are respectively fixed values in the amplified autofluorescence image. However, the L 1  or the L 2  can be finely adjusted. It might not be a problem to observe the area where a user wants to look at even if a little halation appears in the amplified autofluorescence image due to diffused reflection at a mucous membrane. The amplified autofluorescence image displayed on the monitor  60  may be observable for a user even if the L 1  is finely adjusted within the decided range. For example, the L 1  or the L 2  can be finely adjusted within a range between 0% and +5%. Conversely, the L 1  or the L 2  can be finely adjusted within a range between −5% and 0%. 
     The autofluorescence gain is calculated based on the Lmaxaf or the Laveaf for an autofluorescence image captured by the imaging device for one field of image data in the above embodiment. However, it is possible to calculate the autofluorescence gain based on the average of the Lmaxafs or the Laveafs in some autofluorescence images captured by the imaging device at different times. The amount of exciting light emitted by the exciting light source  23  may swing, resulting in changing of brightness of the amplified autofluorescence image. However, the swinging of brightness of the amplified autofluorescence image can be lowered owing to using the gain calculated based on the average of the Lmaxafs or the Laveafs in some autofluorescence images captured at different timings. Further, the brightness of the amplified reference image may swing, however the swinging of brightness of the amplified reference image can be lowered in the same way. 
     The autofluorescence gain is displayed on the monitor  60  in the above embodiment. However, the autofluorescence gain can be displayed on a gain-monitor  43  mounted on the control surface of the endoscope processor  20  as shown in  FIG. 9 . 
     The above embodiment can be implemented by installing a program for the AGC process in an all purpose image data processor, which can be connected to the reference and exciting light sources. 
     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. 2005-039029 (filed on Feb. 16, 2005), which is expressly incorporated herein, by reference, in its entirety.