Patent Publication Number: US-2022225857-A1

Title: Medical control device and medical observation system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Application No. 2021-006010, filed on Jan. 18, 2021, the contents of which are incorporated by reference herein in its entirety. 
     BACKGROUND 
     The present disclosure relates to a medical control device and a medical observation system. 
     In the related art, there is known a medical observation system that irradiates a subject in a living body and the like with light using a light source device, captures light (subject image) from the subject using an image sensor, and observes the subject (see, for example, JP 2012-85790 A). 
     In the medical observation system described in JP 2012-85790 A, dimming control for controlling the light amount of illumination light to the subject in the light source device and the electronic shutter in the image sensor is executed based on the luminance level of the subject image included in the captured image generated by the image sensor. 
     SUMMARY 
       FIG. 7  is a diagram for describing a known problem. Specifically,  FIG. 7 ( a )  illustrates a luminance level of a subject image included in a captured image. In  FIG. 7 , the luminance level increases toward the right side in the drawing, and the luminance level decreases toward the left side in the drawing.  FIG. 7 ( b )  is a diagram illustrating control of an electronic shutter in an image sensor in known dimming control.  FIG. 7 ( c )  is a diagram illustrating control of a light amount of illumination light to a subject in a light source device in the known dimming control. Note that, in  FIG. 7 , a position P 1  indicated by a solid line indicates a position (hereinafter, described as a dimming stable position P 1 ) where the control of the electronic shutter and the control of the light amount are stable in a case where the subject is a specific recommended subject. 
     In the known dimming control, as illustrated in  FIG. 7 , as the luminance level of the subject image included in the captured image increases, it is common to reduce the light amount in the light source device and simultaneously narrow the electronic shutter in the image sensor. 
     In addition, in the known dimming control, in a case where the endoscope is an endoscope for abdominal cavities, the temperature and humidity are high in an abdominal cavity. Therefore, when the endoscope outside the abdominal cavity is inserted into the abdominal cavity, there is a problem that fogging occurs in an optical member such as a lens provided at the distal end of the endoscope and exposed to the outside of the endoscope. 
     According to one aspect of the present disclosure, there is provided a medical control device including: a captured image acquisition unit configured to acquire a captured image generated by an image sensor capturing a subject image introduced by an endoscope; a luminance calculation unit configured to calculate a luminance level of the subject image included in the captured image; and a dimming controller configured to control a light amount of irradiation light onto a subject and an electronic shutter of the image sensor based on the luminance level, and execute first dimming control of narrowing the electronic shutter before reducing the light amount as the luminance level increases. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a configuration of a medical observation system according to a first embodiment; 
         FIG. 2  is a block diagram illustrating a configuration of a camera head and a control device; 
         FIG. 3  is a diagram for describing a function as a type determination unit of a control unit; 
         FIG. 4  is a diagram for describing first dimming control; 
         FIG. 5  is a diagram for describing second dimming control; 
         FIG. 6  is a diagram illustrating a configuration of a medical observation system according to a second embodiment; and 
         FIG. 7  is a diagram for describing a known problem. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, modes for carrying out the present disclosure (hereinafter, embodiments) will be described with reference to the drawings. Note that the present disclosure is not limited by the embodiments described below. Furthermore, in the description of the drawings, the same units are denoted by the same reference signs. 
     First Embodiment 
     Schematic configuration of medical observation system 
       FIG. 1  is a diagram illustrating a configuration of a medical observation system  1  according to a first embodiment. 
     The medical observation system  1  is a system that is used in the medical field and captures (observes) the inside of a living body as a subject. As illustrated in  FIG. 1 , the medical observation system  1  includes an insertion unit  2 , a light source device  3 , a light guide  4 , a camera head  5 , a first transmission cable  6 , a display device  7 , a second transmission cable  8 , a control device  9 , and a third transmission cable  10 . 
     The insertion unit  2  corresponds to an endoscope according to the present disclosure. In the first embodiment, the insertion unit  2  includes a rigid endoscope. That is, the insertion unit  2  has an elongated shape that is entirely rigid or partially soft and partially rigid, and is inserted into the living body. An optical system  21  ( FIG. 1 ) that includes one or a plurality of lenses and condenses light from a subject is provided inside the insertion unit  2 . Note that, in  FIG. 1 , for convenience of description, only an optical member  211  located at the distal end of the insertion unit  2  and exposed to the outside of the insertion unit  2  in the optical system  21  is illustrated. 
     In the first embodiment, the insertion unit  2  is one of two types of insertion units (endoscopes): an insertion unit (endoscope) for abdominal cavities (for laparoscopic surgery) having a large diameter; and an insertion unit (endoscope) for otology having a small diameter. 
     The light source device  3  is connected to one end of the light guide  4 , and supplies light to irradiate the inside of the living body to the one end of the light guide  4  under the control of the control device  9 . 
     In the first embodiment, the light source device  3  is configured separately from the control device  9 , but the present disclosure is not limited to this configuration, and a configuration in which the light source device  3  is provided inside the control device  9  may be adopted. 
     One end of the light guide  4  is detachably connected to the light source device  3 , and the other end is detachably connected to the insertion unit  2 . Then, the light guide  4  transmits the light supplied from the light source device  3  from one end to the other end, and supplies the light to the insertion unit  2 . The light emitted into the living body and reflected in the living body is condensed by the optical system  21  in the insertion unit  2 . 
     The camera head  5  is detachably connected to the proximal end (eyepiece unit  22  ( FIG. 1 )) of the insertion unit  2 . Note that the camera head  5  is detachably connected to the proximal ends of both of the above two types of insertion units  2  (for abdominal cavities and otology). Then, the camera head  5  captures the light condensed by the insertion unit  2  and generates a captured image under the control of the control device  9 . 
     Note that a detailed configuration of the camera head  5  will be described in “Configuration of camera head” to be described later. 
     One end of the first transmission cable  6  is detachably connected to the control device  9  via a connector CN 1  ( FIG. 1 ), and the other end is detachably connected to the camera head  5  via a connector CN 2  ( FIG. 1 ). Then, the first transmission cable  6  transmits a captured image and the like output from the camera head  5  to the control device  9 , and transmits a control signal, a synchronization signal, a clock, power, and the like output from the control device  9  to the camera head  5 . 
     Note that, in transmission of a captured image and the like from the camera head  5  to the control device  9  via the first transmission cable  6 , the captured image and the like may be transmitted as an optical signal or may be transmitted as an electric signal. The same applies to transmission of a control signal, a synchronization signal, and a clock from the control device  9  to the camera head  5  via the first transmission cable  6 . 
     The display device  7  includes a display using liquid crystal, organic electro luminescence (EL), or the like, and displays an image based on a video signal from the control device  9  under the control of the control device  9 . 
     One end of the second transmission cable  8  is detachably connected to the display device  7 , and the other end is detachably connected to the control device  9 . Then, the second transmission cable  8  transmits the video signal processed by the control device  9  to the display device  7 . 
     The control device  9  corresponds to a medical control device according to the present disclosure. The control device  9  includes a central processing unit (CPU), a field-programmable gate array (FPGA), or the like, and integrally controls the operations of the light source device  3 , the camera head  5 , and the display device  7 . 
     Note that a detailed configuration of the control device  9  will be described in “Configuration of control device” to be described later. 
     One end of the third transmission cable  10  is detachably connected to the light source device  3 , and the other end is detachably connected to the control device  9 . Then, the third transmission cable  10  transmits the control signal from the control device  9  to the light source device  3 . 
     Configuration of Camera Head 
     Next, a configuration of the camera head  5  will be described. 
       FIG. 2  is a block diagram illustrating a configuration of the camera head  5  and the control device  9 . 
     As illustrated in  FIG. 2 , the camera head  5  includes a lens unit  51 , an imaging unit  52 , and a communication unit  53 . 
     The lens unit  51  includes one or a plurality of lenses, and forms an image of light condensed by the insertion unit  2  on an imaging surface of the imaging unit  52  (image sensor  521 ). Note that, hereinafter, for convenience of description, light from the lens unit  51  toward the image sensor  521  is described as a subject image. 
     The imaging unit  52  captures the inside of a living body under the control of the control device  9 . As illustrated in  FIG. 2 , the imaging unit  52  includes the image sensor  521  and a signal processing unit  522 . 
     The image sensor  521  includes a charge coupled device (CCD), a complementary metal oxide semiconductor (CMOS), or the like that receives a subject image and converts the subject image into an electric signal (analog signal). Then, the image sensor  521  captures a subject image to generate a captured image. 
     Under the control of the control device  9 , the signal processing unit  522  performs signal processing with respect to the captured image (analog signal) generated by the image sensor  521  and outputs the captured image (RAW signal (digital signal)). 
     For example, the signal processing unit  522  performs processing of removing reset noise, processing of multiplying an analog gain for amplifying the analog signal, and signal processing such as A/D conversion with respect to the captured image (analog signal) generated by the image sensor  521 . 
     The communication unit  53  functions as a transmitter that transmits the captured image (RAW signal (digital signal)) output from the imaging unit  52  to the control device  9  via the first transmission cable  6 . The communication unit  53  includes, for example, a high-speed serial interface that communicates a captured image with the control device  9  via the first transmission cable  6  at a transmission rate of 1 Gbps or above. 
     Configuration of Control Device 
     Next, a configuration of the control device  9  will be described with reference to  FIG. 2 . 
     As illustrated in  FIG. 2 , the control device  9  includes a communication unit  91 , a memory  92 , an observation image generation unit  93 , a control unit  94 , an input unit  95 , an output unit  96 , and a storage unit  97 . 
     The communication unit  91  functions as a receiver that receives a captured image (RAW signal (digital signal)) output from the camera head  5  (communication unit  53 ) via the first transmission cable  6 . The communication unit  91  includes, for example, a high-speed serial interface that communicates a captured image with the communication unit  53  at a transmission rate of 1 Gbps or above. That is, the communication unit  91  corresponds to a captured image acquisition unit according to the present disclosure. 
     The memory  92  includes, for example, a dynamic random access memory (DRAM) and the like. The memory  92  can temporarily store a plurality of frames of captured images sequentially output from the camera head  5  (communication unit  53 ). 
     The observation image generation unit  93  processes the captured images sequentially output from the camera head  5  (communication unit  53 ) and received by the communication unit  91  under the control of the control unit  94 . As illustrated in  FIG. 2 , the observation image generation unit  93  includes a memory controller  931 , an image processing unit  932 , and a display control unit  933 . 
     The memory controller  931  controls writing and reading of the captured image to and from the memory  92 . More specifically, the memory controller  931  sequentially writes the captured images sequentially output from the camera head  5  (communication unit  53 ) and received by the communication unit  91  in the memory  92 . In addition, the memory controller  931  reads the captured image from the memory  92  at a specific timing, and inputs the read captured image to the image processing unit  932 . 
     The image processing unit  932  executes image processing with respect to the input captured image (RAW signal (digital signal)). 
     Examples of the image processing include optical black subtraction processing, white balance adjustment processing, digital gain processing (processing of multiplying a digital gain for amplifying the digital signal with respect to the digital signal), demosaic processing, color correction matrix processing, gamma correction processing, YC processing of converting an RGB signal (captured image) into a luminance color difference signal (Y, Cb/Cr signal), and the like. 
     The display control unit  933  generates a video signal for displaying the captured image after the image processing is executed by the image processing unit  932  under the control of the control unit  94 . Then, the display control unit  933  outputs the video signal to the display device  7  via the second transmission cable  8 . 
     The control unit  94  includes, for example, a CPU, an FPGA, or the like, and outputs a control signal via the first to third transmission cables  6 ,  8 , and  10  to control the operations of the light source device  3 , the camera head  5 , and the display device  7  and to control the entire operation of the control device  9 . The control unit  94  has functions as a type determination unit, a luminance calculation unit, and a dimming control unit according to the present disclosure. Note that the functions of the type determination unit, the luminance calculation unit, and the dimming control unit will be described in “Operation of control device” to be described later. 
     The input unit  95  includes an operation device such as a mouse, a keyboard, and a touch panel, and receives a user operation by a user such as a doctor. Then, the input unit  95  outputs an operation signal corresponding to the user operation to the control unit  94 . 
     The output unit  96  includes a speaker, a printer, or the like, and outputs various types of information. 
     The storage unit  97  stores a program to be executed by the control unit  94 , information necessary for processing of the control unit  94 , or the like. 
     Operation of Control Device 
     Next, the operation of the control device  9  described above will be described. 
     Note that, for convenience of description, the function of the control unit  94  as a type determination unit, the function of the control unit  94  as a luminance calculation unit, and the function of the control unit  94  as a dimming control unit will be mainly described below. 
     Function as Type Determination Unit 
     First, the function of the control unit  94  as a type determination unit will be described. 
       FIG. 3  is a diagram for describing a function of the control unit  94  as a type determination unit. Specifically,  FIG. 3 ( a )  is a diagram illustrating an example of a captured image CI captured by the image sensor  521 .  FIG. 3 ( b )  is a diagram illustrating distribution of luminance values on a horizontal line L 5  in the captured image CI in  FIG. 3 ( a ) . 
     Here, the light (subject image) reflected in the living body and condensed inside the insertion unit  2  has a substantially circular cross section. Therefore, a subject image SI in the captured image CI has a substantially circular shape as illustrated in  FIG. 3 ( a ) . That is, the captured image CI includes a region of the subject image SI and a mask region MA (black portion in  FIG. 3 ( a ) ) other than the subject image SI. 
     The control unit  94  acquires a luminance signal (Y signal) out of the luminance color difference signal (Y, Cb/Cr signal) which is the captured image CI subjected to the YC processing by the image processing unit  932 . Then, based on the luminance signal (Y signal), the control unit  94  detects distribution of luminance values on a plurality of ( 14  in the example of  FIG. 3 ( a ) ) horizontal lines L 1  to L 14  in the captured image CI. 
     Here, in the captured image CI, the region of the subject image SI has a higher luminance value than the mask region MA. That is, for example, in the luminance distribution on the horizontal line L 5 , as illustrated in  FIG. 3 ( b ) , the luminance value increases between the two boundary points BP of the subject image SI and the mask region MA, and the luminance value decreases in the other portions. 
     Therefore, the control unit  94  compares the luminance value with a first luminance threshold SB 1  ( FIG. 3 ( b ) ), and recognizes a region where pixels having luminance values higher than the first luminance threshold SB 1  are continuously arranged as a region of the subject image SI. In addition, the control unit  94  compares the luminance value with a second luminance threshold SB 2  ( FIG. 3 ( b ) ) lower than first luminance threshold SB 1 , and recognizes a region where pixels having luminance values lower than second luminance threshold SB 2  are continuously arranged as the mask region MA. By executing the above processing on all the horizontal lines L 1  to L 14 , the control unit  94  recognizes the entire region of the subject image SI and the entire mask region MA in the captured image CI. 
     Then, the control unit  94  compares the size of the region of the subject image SI with a specific size threshold, and in a case where the size of the region of the subject image SI is equal to or larger than the size threshold, the control unit  94  determines the type of the insertion unit  2  connected to the camera head  5  to be an insertion unit (endoscope) for abdominal cavities. 
     On the other hand, in a case where the size of the region of the subject image SI is smaller than the size threshold, the control unit  94  determines the type of the insertion unit  2  connected to the camera head  5  to be an insertion unit (endoscope) for otology. 
     Function as Luminance Calculation Unit 
     Next, the function of the control unit  94  as a luminance calculation unit will be described. 
     The control unit  94  calculates a luminance level (luminance average value) in the detection region based on a luminance signal (Y signal) in the detection region that is at least a part of the entire image region of the captured image CI, of a luminance color difference signal (Y, Cb/Cr signal) that is the captured image CI subjected to the YC processing by the image processing unit  932 . 
     Function as Dimming Control Unit 
     Next, the function of the control unit  94  as a dimming control unit will be described. 
     Based on the luminance level (luminance average value) in the detection region in the captured image CI calculated as described above, the control unit  94  executes dimming control for adjusting the captured image CI to reference brightness. Here, in a case where the type of the insertion unit  2  connected to the camera head  5  is determined to be an insertion unit for abdominal cavities, the control unit  94  executes the first dimming control as the dimming control. That is, the first type according to the present disclosure is an insertion unit (endoscope) for abdominal cavities. On the other hand, in a case where the type of the insertion unit  2  connected to the camera head  5  is determined to be an insertion unit for otology, the control unit  94  executes the second dimming control as the dimming control. That is, the second type according to the present disclosure is an insertion unit (endoscope) for otology. 
     Hereinafter, the first and second dimming controls will be described in order. 
     First Dimming Control 
     First, the first dimming control will be described. 
       FIG. 4  is a diagram for describing first dimming control. Specifically,  FIG. 4 ( a )  is a diagram corresponding to  FIG. 7 ( a ) , and illustrates the luminance level in the detection region in the captured image CI calculated as described above (the luminance level of the subject image SI included in the captured image CI). In  FIG. 4 , as with  FIG. 7 , the luminance level increases toward the right in the drawing, and the luminance level decreases toward the left side in the drawing.  FIG. 4 ( b )  is a diagram illustrating control of the digital gain multiplied in the image processing (digital gain processing) by the image processing unit  932 .  FIG. 4 ( c )  is a diagram illustrating control of the analog gain multiplied by the signal processing unit  522 .  FIG. 4 ( d )  is a diagram illustrating control of the electronic shutter in the image sensor  521 .  FIG. 4 ( e )  is a diagram illustrating control of the light amount of illumination light into the living body in the light source device  3 . In  FIG. 4 , a position P 1  indicated by a broken line indicates a dimming stable position in the known dimming control illustrated in  FIG. 7 . Further, in  FIG. 4 , a position P 1 ′ indicated by a solid line indicates a dimming stable position where the dimming control is stable by the first dimming control in a case where the subject is a specific recommended subject. 
     In the first dimming control, as the luminance level of the subject image SI increases, the control unit  94  first sequentially decreases the digital gain from the maximum value to the minimum value as illustrated in  FIG. 4  ( b ). In the example of  FIG. 4 ( b ) , the maximum value of the digital gain is eight times, and the minimum value of the digital gain is one time. 
     In addition, in the process of decreasing the digital gain from the maximum value to the minimum value, the control unit  94  does not change the analog gain from the maximum value as illustrated in  FIG. 4 ( c ) . In the example of  FIG. 4 ( c ) , the maximum value of the analog gain is eight times. Then, as the luminance level of the subject image SI further increases, the control unit  94  sequentially decreases the analog gain from the maximum value to the minimum value at the same time as the timing when the digital gain is set to the minimum value. In the example of  FIG. 4 ( c ) , the minimum value of the analog gain is one time. 
     In addition, in the process of decreasing the digital gain from the maximum value to the minimum value and the process of decreasing the analog gain from the maximum value to the minimum value, the control unit  94  does not change the electronic shutter in the image sensor  521  from the minimum diaphragm as illustrated in  FIG. 4 ( d ) . In the example of  FIG. 4 ( d ) , the minimum diaphragm of the electronic shutter in the image sensor  521  is 1/60 [hour]. Then, as the luminance level of the subject image SI further increases, the control unit  94  sequentially changes the electronic shutter in the image sensor  521  from the minimum diaphragm to the maximum diaphragm at the same time as the timing when the analog gain is set to the minimum value. In the example of  FIG. 4 ( d ) , the time of the electronic shutter is increased from 1/60 [hour]. 
     In addition, when the electronic shutter in the image sensor  521  is kept at the minimum diaphragm, the control unit  94  does not change the light amount in the light source device  3  from the maximum light amount as illustrated in  FIG. 4 ( e ) . Note that, in the first dimming control, the maximum light amount is, for example, the light amount that can be emitted to the maximum (hereinafter, described as full light emission) in the light source device  3 . Then, as the luminance level of the subject image SI further increases, the control unit  94  sequentially decreases the light amount in the light source device  3  from the maximum light amount to the minimum light amount after the timing when the electronic shutter in the image sensor  521  is changed from the minimum diaphragm to the maximum diaphragm. Note that, in the first dimming control, the minimum light amount is, for example, a light amount higher than the light amount that can be emitted at the minimum in the light source device  3 . For example, as a characteristic of the light source device  3 , it is possible to decrease the light amount to 1/4000 of the full light emission, but the minimum light amount is set to 1/1000 of the full light emission. 
     As described above, in the first dimming control, as the luminance level of the subject image SI increases, the electronic shutter in the image sensor  521  is narrowed before the light amount in the light source device  3  is reduced. In addition, in the first dimming control, as the luminance level of the subject image SI increases, both the digital gain and the analog gain are decreased before the electronic shutter in the image sensor  521  is narrowed. 
     Then, in the first dimming control, as illustrated in  FIG. 4 , the dimming stable position P 1 ′ moves to the side where the luminance level of the subject image SI is high (right side in  FIG. 4 ) with respect to the dimming stable position P 1  by the known dimming control. 
     Second Dimming Control 
     Next, the second dimming control will be described. 
       FIG. 5  is a diagram for describing a second dimming control. Specifically,  FIGS. 5 ( a )  to  5  ( e ) are diagrams corresponding to  FIGS. 4 ( a )  to  4  ( e ), respectively. In  FIG. 5 , a position P 1  indicated by a broken line indicates a dimming stable position in the known dimming control illustrated in  FIG. 7 . Further, in  FIG. 5 , a position P 1 ″ indicated by a solid line indicates a dimming stable position where the dimming control is stable by the second dimming control in a case where the subject is a specific recommended subject. 
     In the second dimming control, as the luminance level of the subject image SI increases, as illustrated in  FIGS. 5 ( b )  to  5  ( d ), the control unit  94  executes control of the digital gain, control of the analog gain, and control of the electronic shutter in the image sensor  521 , similarly to the first dimming control. 
     In addition, as the luminance level of the subject image SI increases within a period in which the electronic shutter in the image sensor  521  is kept at the minimum diaphragm and is not changed (before timing at which the electronic shutter in the image sensor  521  is changed from the minimum diaphragm to the maximum diaphragm), the control unit  94  sequentially decreases the light amount in the light source device  3  from the maximum light amount to the minimum light amount as illustrated in  FIG. 5 ( e ) . In the second dimming control, the maximum light amount in the light source device  3  is, for example, ¼ of the light amount with respect to the full light emission in the light source device  3 . That is, the maximum value of the light amount in the first dimming control is larger than the maximum value of the light amount in the second dimming control. In addition, in the second dimming control, the minimum light amount in the light source device  3  is, for example, a light amount that can be emitted at the minimum in the light source device  3  ( 1/4000 of the light amount with respect to full light emission). That is, the minimum value of the light amount in the first dimming control is larger than the minimum value of the light amount in the second dimming control. Then, when the light amount in the light source device  3  is decreased to the minimum light amount after the timing at which the electronic shutter in the image sensor  521  is changed from the minimum diaphragm to the maximum diaphragm, the control unit  94  does not change the light amount from the minimum light amount even if the luminance level of the subject image SI increases. 
     As described above, in the second dimming control, as the luminance level of the subject image SI increases, the light amount in the light source device  3  is reduced before the electronic shutter in the image sensor  521  is narrowed. In addition, in the second dimming control, as the luminance level of the subject image SI increases, both the digital gain and the analog gain are decreased before the electronic shutter in the image sensor  521  is narrowed. 
     Then, in the second dimming control, as illustrated in  FIG. 5 , a dimming stable position P 1 ″ moves to the side where the luminance level of the subject image SI is low (left side in  FIG. 5 ) with respect to the dimming stable position P 1  by the known dimming control. 
     According to the first embodiment described above, the following effects are obtained. 
     In the control device  9  according to the first embodiment, in a case where the type of the insertion unit  2  connected to the camera head  5  is an insertion unit for abdominal cavities, as the luminance level of the subject image SI included in the captured image CI increases, the first dimming control of narrowing the electronic shutter is executed before the light amount is reduced. As a result, the light amount at the dimming stable position P 1 ′ can be made higher than the light amount at the dimming stable position P 1  in the known dimming control. 
     Therefore, it is possible to increase the temperature of the distal end of the insertion unit  2  and effectively suppress fogging caused in the optical member  211  (hereinafter, described as a first effect). 
     By the way, in a case where the insertion unit  2  is an insertion unit for otology, in order to avoid contact of the distal end of the insertion unit  2  having a high temperature with a portion weak to heat such as an eardrum, it is necessary to perform observation in a state in which the distal end is positioned at a position away from the observation target. 
     In the control device  9  according to the first embodiment, in a case where the type of the insertion unit  2  connected to the camera head  5  is an insertion unit for otology, as the luminance level of the subject image SI included in the captured image CI increases, the second dimming control of reducing the light amount is executed before the electronic shutter is narrowed. As a result, the light amount at the dimming stable position P 1 ′ can be made lower than the light amount at the dimming stable position P 1  in the known dimming control. 
     Therefore, the temperature of the distal end of the insertion unit  2  can be decreased, and observation can be performed in a state in which the distal end is positioned at a position close to the observation target (hereinafter, described as a second effect). 
     In particular, the maximum value of the light amount in the first dimming control is larger than the maximum value of the light amount in the second dimming control. In addition, the minimum value of the light amount in the first dimming control is larger than the minimum value of the light amount in the second dimming control. Therefore, the first and second effects described above can be suitably realized. 
     In addition, the control device  9  according to the first embodiment determines the type of the insertion unit  2  based on the region of the subject image SI included in the captured image CI. Therefore, a user such as a doctor does not need to input the type of the insertion unit  2  using the input unit  95  and the like. That is, convenience can be improved. 
     Second Embodiment 
     Next, a second embodiment will be described. 
     In the following description, the same reference signs are given to the same configurations as those of the first embodiment described above, and a detailed description will be omitted or simplified. 
       FIG. 6  is a diagram illustrating a configuration of a medical observation system  1 A according to a second embodiment. 
     In the medical observation system  1 A according to the second embodiment, as illustrated in  FIG. 6 , a scope  100  is adopted instead of the insertion unit  2 , the camera head  5 , and the first transmission cable  6  in the medical observation system  1  described in the above first embodiment. 
     The scope  100  corresponds to an endoscope according to the present disclosure. Then, in the second embodiment, the scope  100  is one of two types of scopes (endoscopes): a scope (endoscope) for abdominal cavities (for laparoscopic surgery); and a scope (endoscope) for otology. Then, the scope  100  captures a subject image partially inserted into a living body and reflected in the living body, and outputs the captured image generated by the imaging. As illustrated in  FIG. 6 , the scope  100  includes an insertion unit  2 A, an operating unit  110 , a universal cord  120 , and a connector unit  130 . 
     The insertion unit  2 A is a unit at least a part of which has flexibility and is inserted into a living body. A light guide (not illustrated), an illumination lens (not illustrated), an objective lens (not illustrated), and an imaging unit  52 A ( FIG. 6 ) are provided in the insertion unit  2 A. 
     The above light guide is routed from the insertion unit  2 A to the connector unit  130  through the operating unit  110  and the universal cord  120 . One end of the light guide is located at the distal end in the insertion unit  2 A. In addition, in a state in which the scope  100  (connector unit  130 ) is connected to the light source device  3 , the other end of the light guide is located in the light source device  3 . Note that  FIG. 6  illustrates a state in which the scope  100  and the light source device  3  are separated from each other for convenience of description, but the scope  100  is also connected to the light source device  3  in addition to the control device  9 . Then, the light guide transmits the light supplied from the light source device  3  from the other end to one end. 
     The above illumination lens faces one end of the above light guide in the insertion unit  2 A. Then, the illumination lens irradiates the inside of the living body with the light transmitted by the light guide. 
     The above objective lens is provided at the distal end in the insertion unit  2 A. Then, the objective lens forms an image of light (subject image) emitted into the living body from the above illumination lens and reflected in the living body on an image sensor  521 A. 
     As illustrated in  FIG. 6 , the imaging unit  52 A has a configuration similar to that of the imaging unit  52  described in the above first embodiment. That is, the imaging unit  52 A includes the image sensor  521 A and a signal processing unit  522 A similar to the image sensor  521  and the signal processing unit  522  forming the imaging unit  52 , respectively. 
     Here, a signal line (not illustrated) is routed from the insertion unit  2 A to the connector unit  130  through the operating unit  110  and the universal cord  120 . Then, one end of the signal line is connected to the imaging unit  52 A (signal processing unit  522 A). In addition, in a state in which the scope  100  is connected to the control device  9 , the other end of the signal line is connected to the control device  9 . Then, the signal line transmits a captured image (RAW signal (digital signal)) generated by the imaging unit  52 A, a control signal output from the control device  9 , and the like between the scope  100  and the control device  9 . 
     The operating unit  110  is connected to the proximal end in the insertion unit  2 A. Then, the operating unit  110  receives various operations with respect to the scope  100 . 
     The universal cord  120  extends from the operating unit  110  in a direction different from the extending direction of the insertion unit  2 A, and is a cord in which the above light guide, the above signal line, or the like are disposed. 
     The connector unit  130  is provided at the end of the universal cord  120 , and is detachably connected to the control device  9  and the light source device  3 . As illustrated in  FIG. 6 , the connector unit  130  is provided with a storage unit  131 . 
     The storage unit  131  stores a scope ID indicating a type (scope for abdominal cavities or scope for otology) of the scope  100 . The scope ID corresponds to type information according to the present disclosure. 
     Then, in the medical observation system  1 A according to the second embodiment, the function of the control unit  94  as the type determination unit is different from that of the first embodiment described above. 
     That is, the control unit  94  determines the type of the scope  100  connected to the control device  9  based on the scope ID stored in the storage unit  131 . Note that the function of the control unit  94  as a luminance calculation unit and the function of the control unit  94  as a dimming control unit are similar to those in the first embodiment described above. 
     Even in a case where the type of the scope  100  is determined based on the scope ID as in the present embodiment described above, the same effect as those of the first embodiment described above is obtained. 
     Other Embodiments 
     Although the embodiments for carrying out the present disclosure have been described so far, the present disclosure should not be limited only to the first and second embodiments described above. 
     In the first and second embodiments described above, both the analog gain control and the digital gain control are executed as the first and second dimming controls, but the present disclosure is not limited to this configuration, and only one of the analog gain control and the digital gain control may be executed. 
     In the first and second embodiments described above, the method for determining the type of the insertion unit  2  or the type of the scope  100  is not limited to the determination method described in the first and second embodiments described above. For example, the control unit  94  may be configured to determine the type when a user such as a doctor inputs the type of the insertion unit  2  or the type of the scope  100  using the input unit  95  and the like. 
     Note that the following configurations also belong to the technical scope of the present disclosure. 
     (1) A medical control device including: a captured image acquisition unit that acquires a captured image generated by an image sensor capturing a subject image introduced by an endoscope; a luminance calculation unit that calculates a luminance level of the subject image included in the captured image; and a dimming control unit that controls a light amount of irradiation light onto a subject and an electronic shutter of the image sensor based on the luminance level, in which the dimming control unit executes first dimming control of narrowing the electronic shutter before reducing the light amount as the luminance level increases. 
     (2) The medical control device according to (1) further including: a type determination unit that determines a type of the endoscope, in which the dimming control unit executes first dimming control in a case where the type of the endoscope is a first type, and executes second dimming control different from the first dimming control in a case where the type of the endoscope is a second type different from the first type, and the second dimming control is control in which the light amount is reduced before the electronic shutter is narrowed as the luminance level increases. 
     (3) The medical control device according to (2), in which the first type is an endoscope for an abdominal cavity, and the second type is an endoscope for otology. 
     (4) The medical control device according to (2) or (3), in which the type determination unit determines the type of the endoscope based on at least one of a region of the subject image included in the captured image and a mask region other than the subject image included in the captured image. 
     (5) The medical control device according to (2) or (3), in which the type determination unit determines the type of the endoscope based on type information that is stored in a storage unit provided in the endoscope and indicates the type of the endoscope. 
     (6) The medical control device according to any one of (2) to (5), in which a maximum value of the light amount in the first dimming control is larger than a maximum value of the light amount in the second dimming control. 
     (7) The medical control device according to any one of (2) to (6), in which a minimum value of the light amount in the first dimming control is larger than a minimum value of the light amount in the second dimming control. 
     (8) The medical control device according to any one of (1) to (7), in which the dimming control unit decreases a gain of at least one of an analog gain to be multiplied by an analog signal corresponding to the captured image and a digital gain to be multiplied by a digital signal corresponding to the captured image before narrowing the electronic shutter, as the luminance level increases. 
     (9) A medical control device including: a captured image acquisition unit that acquires a captured image generated by an image sensor capturing a subject image introduced by an endoscope; a luminance calculation unit that calculates a luminance level of the subject image included in the captured image; and a dimming control unit that controls a light amount of irradiation light onto a subject and an electronic shutter of the image sensor based on the luminance level, in which the dimming control unit executes second dimming control of reducing the light amount before narrowing the electronic shutter as the luminance level increases. 
     (10) A medical observation system including: an image sensor that captures a subject image introduced by an endoscope; a light source device that supplies irradiation light onto a subject; and a medical control device that controls operation of the image sensor and the light source device, in which the medical control device includes a captured image acquisition unit that acquires a captured image generated by the image sensor, a luminance calculation unit that calculates a luminance level of the subject image included in the captured image, and a dimming control unit that controls a light amount of the irradiation light onto the subject and an electronic shutter of the image sensor based on the luminance level, and the dimming control unit executes first dimming control of narrowing the electronic shutter before reducing the light amount as the luminance level becomes larger. 
     (11) A medical observation system including: an image sensor that captures a subject image introduced by an endoscope; a light source device that supplies irradiation light onto a subject; and a medical control device that controls operation of the image sensor and the light source device, in which the medical control device includes a captured image acquisition unit that acquires a captured image generated by the image sensor, a luminance calculation unit that calculates a luminance level of the subject image included in the captured image, and a dimming control unit that controls a light amount of the irradiation light onto the subject and an electronic shutter of the image sensor based on the luminance level, and the dimming control unit executes second dimming control of reducing the light amount before narrowing the electronic shutter as the luminance level increases. 
     With the medical control device and the medical observation system according to the present disclosure, it is possible to effectively suppress fogging caused in the optical member provided at the distal end of the endoscope. 
     Although the disclosure has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.