Patent Publication Number: US-2023136295-A1

Title: Endoscope system, control device, method of controlling light source, and computer-readable recording medium

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of International Application No. PCT/JP2020/027965, filed on Jul. 17, 2020, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The disclosure relates to an endoscope system that generates image data by applying illumination light to a subject and capturing an image, a control device, a method of controlling a light source, a computer-readable recording medium. 
     2. Related Art 
     A technique of performing stroboscopic observation on vocal cords of a subject, such as a human or an animal, that move fast by causing emission of white light intermittently has been known (for example, refer to Japanese Patent No. 6249909). In this technique, the vocal cords that move fast are observed in a stopped manner or in slow motion by sensing a frequency of vibrations of the vocal cords with a microphone, or the like, and applying a shot-pulse light that synchronizes with the frequency to the vocal cords. 
     The frequency of vocal cords around 60 to 1000 Hz in general. For this reason, in stroboscopic observation, the endoscope system applies approximately 1 to 16 pulse lights for one frame (for example, 60 hz is assumed) that the imager captures and the total exposure serves as a brightness of one frame (multi-exposure system). 
     There is, as a dimming control method in pulse light emission, a method of adjusting a Duty ratio by changing a pulse width or a light emission period and adjusting a total amount of applied light for one frame. In stroboscopic observation, because the light emission period is determined by the frequency of vibrations of vocal cords, pulse width modulation (PWM) control of adjusting a pulse width of a pulse light is performed. 
     SUMMARY 
     In some embodiments, an endoscope system includes: a light source configured to emit a pulse light; an imager configured to capture an image on a frame by frame basis; a processor configured to: determine a first Duty target value representing a Duty ratio of the pulse light to make an image that is captured in a next imaging frame have intended brightness; based on a frequency of vocal cords, determine a light emission period at a time when the pulse light is emitted; based on the first duty target value and the light emission period, determine a pulse width of the pulse light that is emitted next; and based on the light emission period and the pulse width, control the light source in light emission. As for the first Duty target value, in a case of a first pulse in the imaging frame, the first Duty target value is determined based on brightness of an image that is previously acquired, in a case of a different pulse after the first pulse in the imaging frame, an error between a second. Duty target value of a pulse light that is emitted one time before and a Duty control value representing an actual Duty ratio of the pulse light that is emitted one time before, and based on the second Duty target value and the error, the first Duty target value is determined. 
     In some embodiments, provided is a control device configured to control a light source configured to emit a pulse light. The control device includes: a processor configured to: determine a first Duty target value representing a Duty ratio of the pulse light to make an image that is captured in a next imaging frame have intended brightness; based on a frequency of vocal cords, determine a light emission period at a time when the pulse light is emitted; based on the first duty target value and the light emission period, determine a pulse width of the pulse light that is emitted next; and based on the light emission period and the pulse width, control the light source in light emission. As for the first Duty target value, in a case of a first pulse in the imaging frame, the first Duty target value is determined based on brightness of an image that is previously acquired, in a case of a different pulse after the first pulse in the imaging frame, an error between a second Duty target value of a pulse light that is emitted one time before and a Duty control value representing an actual Duty ratio of the pulse light that is emitted one time before, and based on the second Duty target value and the error, the first Duty target value is determined. 
     In some embodiments, provided is a method of controlling a light source configured to emit a pulse light. The method includes: determining a first Duty target value representing a Duty ratio of the pulse light to make an image that is captured in a next imaging frame have intended brightness; based on a frequency of vocal cords, determining a light emission period at a time when the pulse light is emitted; based on the first duty target value and the light emission period, determining a pulse width of the pulse light that is emitted next; and based on the light emission period and the pulse width, controlling the light source in light emission. As for the first Duty target value, in a case of a first pulse in the imaging frame, the first Duty target value is determined based on brightness of an image that is previously acquired, in a case of a different pulse after the first pulse in the imaging frame, an error between a second Duty target value of a pulse light that is emitted one time before and a Duty control value representing an actual Duty ratio of the pulse light that is emitted one time before, and based on the second Duty target value and the error, the first Duty target value is determined. 
     In some embodiments, provided is a non-transitory computer-readable recording medium with an executable program stored thereon. The program causes a processor to execute: determining a first Duty target value representing a Duty ratio of the pulse light to make an image that is captured in a next imaging frame have intended brightness; based on a frequency of vocal cords, determining a light emission period, at a time when the pulse light is emitted; based on the first duty target value and the light emission period, determining a pulse width of the pulse light that is emitted next; and based on the light emission period and the pulse width, controlling the light source in light emission. As for the first Duty target value, in a case of a first pulse in the imaging frame, the first Duty target value is determined based on brightness of an image that is previously acquired, in a case of a different pulse after the first pulse in the imaging frame, an error between a second Duty target value of a pulse light that is emitted one time before and a Duty control value representing an actual Duty ratio of the pulse light that is emitted one time before, and based on the second Duty target value and the error, the first Duty target value is determined. 
     The above and other features, advantages and technical and industrial significance of this disclosure will be better understood by reading the following detailed description of presently preferred embodiments of the disclosure, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram illustrating a schematic configuration of an endoscope system according to a first embodiment; 
         FIG.  2    is a block diagram illustrating functional configurations of an endoscope, a control device, and a light source device that the endoscope system according to a first embodiment includes; 
         FIG.  3    is a flowchart illustrating an overview of a process that the endoscope system according to the first embodiment executes; 
         FIG.  4    is a timing chart illustrating an overview of pulse light emission that the endoscope system according to a first embodiment executes; 
         FIG.  5    is a block diagram illustrating functional configurations of an endoscope, a control device, and a light source device that an endoscope system according to a second embodiment includes; 
         FIG.  6    is a flowchart illustrating an overview of a process that the endoscope system according to the second embodiment executes; 
         FIG.  7    is a flowchart illustrating an overview of a Duty control process in  FIG.  6   ; 
         FIG.  8    is a timing chart illustrating an overview of pulse light emission that a conventional endoscope system executes; and 
         FIG.  9    is a timing chart illustrating an overview of pulse light emission that the endoscope system according to the second embodiment executes. 
     
    
    
     DETAILED DESCRIPTION 
     Modes for carrying out the disclosure (“embodiments” below) will be described in detail below with the accompanying drawings. The following embodiments do not limit the disclosure. Each of the drawings referred to in the following description only schematically presents shapes, sizes, and a positional relationship to an extent such that the content of the disclosure is understandable. In other words, the disclosure is not limited to only the shapes, the sizes, and the positional relationship that are exemplified in each of the drawings. Furthermore, as for the illustration of the drawings, description is given with the same reference numerals being assigned to the same elements. Furthermore, an endoscope system that observes vocal cords of a living organism will be described as an example of a medical observation system according to the disclosure. As for the illustration of the drawings, description is given with the same reference numerals being assigned to the same elements. 
     First Embodiment 
     Schematic Configuration of Endoscope System 
       FIG.  1    is a schematic diagram illustrating a schematic configuration of an endoscope system according to a first embodiment. An endoscope system  1  illustrated in  FIG.  1    is used in medical fields and is a system fiat is inserted into the oral cavity and the inside of a subject that is a living organism, such as a human or an animal, (into the living organism) and that displays a captured image of the inside or the vocal cords, thereby observing the subject. Note that, in the first embodiment, a flexible endoscope system is described as the endoscope system  1 ; however, the endoscope system  1  is not limited to this and the endoscope system  1  may be, for example, a rigid endoscope system or an industrial endoscope system. 
     The endoscope system  1  illustrated in  FIG.  1    includes an endoscope  2  that is inserted into the oral cavity of a subject, that captures an image of vocal cords and the oral cavity of the subject, and that generates an imaging signal of the image of the inside of the subject; an audio input device  3  to which a sound that is made by the subject is input; a control device  4  that performs given image processing on the imaging signal that is generated by the endoscope  2  and that controls each unit of the endoscope system  1 ; a light source device  5  that supplies, to the endoscope  2 , illumination light to be emitted to the subject; and a display device  6  that displays an image (observation image) corresponding to an image signal that is generated by the control device  4  by performing the image processing. 
     First of all, the endoscope  2  will be described. The endoscope  2  includes an insertion unit  21  that is inserted into the subject, an operation unit  22  that is on the side of a proximal end of the insertion unit  21  and that an operator holds, and a universal cord  23  that extends from the operation unit  22  and that is flexible. 
     The insertion unit  21  is realized using an illumination fiber (light guide cable), an electronic cable, etc. The insertion unit  21  includes a distal end part  211  having an imaging unit that incorporates an imager that captures an image of the inside of the subject, a curve part  212  that is flexible and that consists of a plurality of curve members, and a flexible tube  213  that is flexible and that is provided on the side of a proximal end part of the curve part  212 . The distal end part  211  is provided with an illuminator that illuminates the inside of the subject via an illumination lens, an observation unit that captures an image of the inside of the subject, an opening that communicates with a treatment tool channel, and an air supply and water supply nozzle (not illustrated in the drawings). 
     The operation unit  22  includes a curve knob  221  that causes the curve part  212  to curve vertically and horizontally; a treatment tool insertion unit  222  through which a treatment tool, such as biological forceps or a laser scalpel, is inserted into a body cavity of the subject; and a plurality of switch parts  223  for operating peripherals, such as the control device  4 , the light source device  5 , an air supply device, a water supply device, and a gas supply device. The treatment tool that is inserted from the treatment tool insertion unit  222  comes out of the opening at the distal end of the insertion unit  21  via the treatment tool channel that is provided inside. 
     The universal cord  23  is configured using an illumination fiber, an electronic cable, etc. The universal cord  23  bifurcates at the proximal end and the end of a branch cord  231  that is one of the branches is a connector  232  and the proximal end of the other branch is a connector  233 . The connector  232  is detachable from the control device  4 . The connector  233  is detachable from the light source device  5 . The universal cord  23  transmits illumination light that is emitted from the light source device  5  to the distal end part  211  via the connector  232 , the operation unit  22 , and the flexible tube  213 . The universal cord  23  transmits the signal of the image that is generated by the imaging unit provided in the distal end part  211  to the control device  4 . 
     In the insertion unit  21  and the universal cord  23 , an illumination fiber  214  (refer to  FIG.  2   ) that guides the illumination light from the light source device  5  is arranged. One end of the illumination fiber  214  is positioned on a distal end face of the insertion unit  21  and the other end is positioned on a face of connection of the universal cord  23  to the light source device  5 . 
     The audio input device  3  will be described next. An audio signal (audio data) that is made from the vocal cords of the subject from is input to the audio input device  3 . A distal end of a cord  31  is connected to the audio input device  3  and a connector  311  at the proximal end is detachable from the control device  4 . The audio input device  3  outputs the input audio signal to the control device  4  via the cord  31  and the connector  311 . The audio input device  3  is configured using a microphone, an A/D conversion circuit, a gain-up circuit, etc. Note that, in the first embodiment, the audio input device  3  functions as an audio input portion. 
     The control device  4  will be described next. The control device  4  generates an image signal by performing given image processing on the imaging signal that is input from the endoscope  2  via the universal cord  23  and outputs the image signal to the display device  6 . The control device  4  controls each unit of the endoscope system  1  based on various instruction signals that are transmitted from the switch parts  223  in the operation unit  22  of the endoscope  2  via the universal cord  23 . 
     The light source device  5  will be described next. Under the control of the control device  4 , the light source device  5  emits a white light or a special light that is used for narrow band imaging (NBI) observation and infrared light observation to the endoscope  2  via the connector  232  and the universal cord  23 . Note that the light source device  5  and the control device  4  may be configured co communicate individually as illustrated in  FIG.  1    or may be configured integrally. 
     The display device  6  will be described next. The display device  6  displays an image corresponding to the image signal that is input from the control device  4  via a video cable  61 . The display device  6  displays various types of information on the endoscope system  1 . The display device  6  is configured using a display using liquid crystals or electro luminescence (EL). 
     Detailed Configuration of Endoscope, Control Device and Light Source Device 
     Detailed functional configurations of the endoscope  2 , the control device  4 , and the light source device  5  will be described next.  FIG.  2    is a block diagram illustrating the functional configurations of the endoscope  2 , the control device  4 , and the light source device  5 . 
     Configuration of Endoscope 
     First of all, the configuration of the endoscope  2  will be described. 
     The endoscope  2  includes at least an imaging unit  24 . The imaging unit  24  includes an optical system  241  and an imager  242 . 
     The optical system  241  is realized using at least one lens, etc., and forms a subject image on a light receiving surface of the imager  242 . 
     The imager  242  optically receives the subject image that is formed by the optical system  241  according to a given frame rate and outputs an imaging signal that is generated by performing photoelectric conversion to the control device  4  via a transmission cable and the connector  232  of the universal cord  23 . 
     Configuration of Control Device 
     A configuration of the control device  4  will be described next. 
     The control device  4  includes an image processor  401 , a memory  402 , an operation unit  403 , and a controller  404 . 
     Under the control of the controller  404 , the image processor  401  generates an image signal by performing the given image processing on the imaging signal that is input from the endoscope  2  and outputs the image signal to the display device  6 . The given image processing is image processing containing at least A/D conversion processing, gain adjustment processing, optical black subtraction processing, and white balance (WB) adjustment processing, and, in the case where the imager  242  has a Bayer arrangement, concurrent processing, color matrix operation processing, gamma correction processing, color reproduction processing, and edge enhancement processing. The image processor  401  is configured using a processor including a memory, such as a volatile memory or a non-volatile memory, and hardware, such as a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), and a graphics processing unit. 
     The memory  402  is realized using a volatile memory, a non-volatile memory, a frame memory, etc. The memory  402  records various programs that the endoscope system  1  executes and various types of data used during processing and an image corresponding to the image signal obtained by the image processor  401  by performing the image processing. Note that the memory  402  may be a memory card that is detachable from the control device  4 , or the like. 
     The operation unit  403  receives an input of an operation of a user and outputs a signal corresponding to the received operation co the controller  404 . The operation unit  403  is realized using a touch panel, a button, a jog dial, a switch, a foot switch, etc. 
     The controller  404  controls each unit configuring the endoscope system  1 . The controller  404  is configured using a processor that includes hardware, such as a memory, a CPU, a FPGA, an ASIC, etc. The controller  404  includes a first calculator  404   a,  a detector  404   b,  a determination unit  404   c,  a second calculator  404   d,  a third calculator  404   e,  and a light source controller  404   f.    
     The first calculator  404   a  calculates a brightness evaluation value of an image corresponding to a video signal that is input from the image processor  401  and outputs the brightness evaluation value to the second calculator  404   d.    
     The detector  404   b  detects a frequency of audio data (frequency of vocal cords) that is input from the audio input device  3  via the cord  31  and the connector  311  and outputs a result of the detection to the determination unit  404   c.  The audio data is generated from the vocal cords of the living organism (subject). 
     Based on a frequency of vibrations of the living organism that is input from the detector  404   b,  the determination unit  404   c  determines a light emission period at the time when the light source device  5  emits a pulse light and outputs the light emission period to the second calculator  404   d.    
     Based on the brightness evaluation value that is input from the first calculator  404   a,  the second calculator  404   d  calculates a Duty target value representing a Duty, ratio of the pulse light in the light source device  5  for making an image have intended brightness in the next frame of the imager  242  and outputs the Duty, target value to the third calculator  404   e.  The second calculator  404   d  calculates, as a target value, an average Duty ratio of the pulse light in the next frame of the imager  242 . 
     Based on the Duty target value that is input from the second calculator  404   d  and the light emission period that is input from the determination unit  404   c,  the third calculator  404   e  calculates a pulse width of each pulse at the time when the light source device  5  causes emission of a pulse light. The third calculator  404   e  calculates a pulse width in a step size of an integral multiple (for example, 1 μsec) of a line period of the imager  242 . The reason of this is that, for example, because of the circuit scale, or the like, such a restriction may occur. When there is such a restriction, the pulse width cannot be controlled freely (only discrete pulse widths are selectable). For this reason, the third calculator  404   e  calculates a pulse width in a step size of an integral multiple (for example, 1 μsec) of the line period of the imager  242 , thereby calculating a pulse width that enables control at brightness much closer to the target. 
     The light source controller  404   f  controls the light source device  5  based on the light emission period that is input from the determination unit  404   c  and the pulse width that is input from the third calculator  404   e.    
     Specifically, based on the light emission period that is input from the determination unit  404   c  and the pulse width that is input from the third calculator  404   e,  the light source controller  404   f  controls the pulse width at the time when a light source driver  52  of the light source device  5  to be described below emits a pulse light. 
     Configuration of Light Source Device 
     A configuration of the light source device  5  will be described next. 
     The light source device  5  includes a light source  51  and the light source driver  52 . 
     The light source  51  perform pulse light emission at a given interval based on a PWM control signal that is input from the light source driver  52 , thereby emitting a pulse light (illumination light) to the endoscope  2 . The light source  51  is realized using at least one lens and a white light emitting diode (LED) lamp, etc. As for the light source  51 , a transmission filter that transmits narrow band light of a given wavelength band (390 nm to 445 nm 530 nm to 550 nm) may be detachably provided on an optical path of the light source  51 . 
     Under the control of the light source controller  404   f,  the light source driver  52  applies a pulse current value in a given pulse width at a given interval, thereby causing the light source  51  to emit a pulse light. The light source driver  52  is realized using a drive driver circuit, etc. 
     Process Executed by Endoscope System 
     A process that is executed by the endoscope system  1  will be described next.  FIG.  3    is a flowchart illustrating an overview of a process that the endoscope system  1  executes.  FIG.  4    is a timing chart illustrating an overview of pulse light emission that the endoscope system  1  executes. In  FIG.  4   , from the top, (a) presents periods of one frame of images corresponding to a video signal that is generated by the imager  242 , (b) presents brightness evaluation values, (c) presents Duty target values of the Duty ratio, (d) presents light emission periods, and (e) presents control values of the Duty ratio. 
     As illustrated in  FIG.  3   , first of all, the first calculator  404   a  acquires an image corresponding to an image signal obtained by performing image processing on a video signal that is acquired by the image processor  401  from the imager  242  of the endoscope  2  (step S 101 ) and calculates a brightness evaluation value of the image corresponding to the image signal that is acquired from the image processor  401  (step S 102 ). 
     Subsequently, based on the brightness evaluation value that is calculated by the first calculator  404   a,  the second calculator  404   d  calculates a Duty target value representing an aimed value of the Duty ratio of the next frame of the imager  242  (step S 103 ). 
     Thereafter, the detector  404   b  detects a frequency of audio data that is input from the audio input device  3  (step S 104 ). 
     Subsequently, based on the frequency that is detected by the detector  404   b,  the determination unit  404   c  determines a light emission period of the next frame of the imager  242  (step S 105 ). 
     Thereafter, based on the Duty target value that is input from the second calculator  404   d  and the light emission period that is input from the determination unit  404   c,  the third calculator  404   e  calculates a pulse width of each pulse at the time when the light source device  5  causes emission of a pulse light (step S 106 ). Specifically, as illustrated in  FIG.  4   , the third calculator  404   e  calculates a pulse width of each pulse in the next frame of the imager  242  such that the Duty target value that is calculated by the second calculator  404   d  is enabled. In this case, the third calculator  404   e  calculates a pulse width in a step size of an integral multiple (for example, 1 μsec) of a line period of the imager  242 . The reason of this is that, for example, because of the circuit scale, or the like, such a restriction may occur. When there is such a restriction, the pulse width cannot be controlled freely (only discrete pulse widths are selectable). For this reason, by calculating a pulse width in a step size of an integral multiple (for example, 1 μsec) of the line period of the imager  242 , the third calculator  404   e  calculates a pulse width that enables control at brightness much closer to the target. For example, in the case where a Duty target value (intended value) of a Duty ratio in a period of a frame “1” is “3%” and brightness (a brightness evaluation value) in a period of a frame “3” is changed from a total exposure of “18” in the period of the frame “1” to “16”, the third calculator  404   e  calculates a pulse width of each pulse based on the light emission period such that the Duty target value is enabled. Specifically, as illustrated in  FIG.  4   , the third calculator  404   e  calculates that the number of times the Duty ratio is 3% is four times and the number of times the Duty ratio is 2% is twice in the period of the frame “3” of the imager  242 . Accordingly, the average of the Duty target values in the period of the frame “3” is “2.67%”. Thus, when the light emission period is “200 μsec”, the third calculator  404   e  calculates that the pulse width corresponding to the Duty ratio of “3%” is “6 μsec” (0.03×200 μsec). Furthermore, when the light emission period is “200 μsec”, the third calculator  404   e  calculates that the pulse width corresponding to the Duty ratio of “2%” is “4 μsec” (0.02×200 μsec). 
     Subsequently, the light source controller  404   f  outputs a PWM control signal to the light source driver  52 , thereby executing PWM control (step S 107 ). Specifically, the light source controller  404   f  causes the light source driver  52  to supply the PFM control signal in the pulse width of each pulse that is calculated by the third calculator  404   e  to the light source  51 . More specifically, as illustrated in  FIG.  4   , the light source controller  404   f  causes the light source driver  52  to supply a PWM current in the pulse width of each pulse that is calculated by the third calculator  404   e  to the light source  51 , thereby performing light emission with a duty ratio of “3%” for four times (performing light emission with a pulse width of 6 μsec for four times) and performing light emission with a duty ratio of “2%” twice (performing light emission with a pulse width of 4 μsec twice) in the period of the frame “3” of the imager  242 . In this case, as illustrated in  FIG.  4   , the light source controller  404   f  causes the light source  51  to emit light with the duty ratio varying in the period of the frame “3” of the imager  242 . Note that, in  FIG.  4   , when the average of the duty ratios in the third frame is “2.67%”, the light source controller  404   f  may change the Duty ratio or change the order of light emission. 
     Subsequently, the controller  404  determines whether light emission for the corresponding frame of the imager  242  completes (step S 108 ). When the controller  404  determines that light emission for the corresponding frame of the imager  242  completes (YES at step S 108 ), the endoscope system  1  moves to step S 109  to be described below. On the other hand, when the controller  404  determines that light emission for the corresponding frame of the imager  242  does not complete (NO at step S 108 ), the endoscope system  1  returns to step S 107  described above. 
     At step S 109 , the controller  404  determines whether an instruction signal for ending an examination on the living organism is input from the operation unit  403 . When the controller  404  determines that the instruction signal for ending the examination on the living organism is input from the operation unit  403  (YES at step S 109 ), the endoscope system  1  ends the process. On the other hand, when the controller  404  determines that the instruction signal for ending the examination on the living organism is not input from the operation unit  403  (NO at step S 109 ), the endoscope system  1  returns to step S 101  described above. 
     According to the first embodiment described above, because the light source controller  404   f  causes the light source driver  52  to supply the PNM current in the pulse width of each pulse in one frame of the imager  242 , which is calculated by the third calculator  404   e,  to the light source  51 , it is possible to obtain an observation image with brightness that is suitable for observation. 
     According to the first embodiment, because the third calculator  404   e  calculates a pulse width of each pulse in one frame based on the Duty target value representing the Duty ratio of the pulse light in the next frame of the imager  242 , which is calculated by the second calculator  404   d,  and the light emission period, which is determined by the determination unit  404   c,  even if the Duty control by the light source controller  404   f  cannot ensure a resolution sufficiently, it is possible to perform consecutive dimming control without increasing or reducing the S/N ratio. 
     Second Embodiment 
     A second embodiment will be described next. The endoscope system  1  according to the first embodiment described above calculates a pulse width based on a light emission period and a target value and an endoscope system according to the second embodiment calculates a difference (carry over error) between a target value and a control value representing a Duty ratio enabling a light source controller to control a light source, reflects the difference to the target value, and updates the target value, thereby calculating a pulse width of each pulse. A configuration of the endoscope system according to the second embodiment will be described and thereafter a process that the endoscope system according to the second embodiment executes will be described below. Note that the same reference numerals are assigned to the same elements as those of the endoscope system  1  according to the first embodiment described above and detailed description will be omitted. 
     Detailed Configuration of Control Device 
       FIG.  5    is a block diagram illustrating functional configurations of the endoscope  2 , a control device  4 A, and a light source device  5  that the endoscope system according to the second embodiment includes. An endoscope system  1 A illustrated in  FIG.  5    includes the control device  4 A instead of the control device  4  according to the endoscope system  1  according to the first embodiment described above. 
     The control device  4 A includes a controller  405  instead of the controller  404  according to the above-described first embodiment. The controller  405  includes, in addition to the elements of the controller  404  according to the above-described first embodiment, a third calculator  404   g,  a fourth calculator  404   h,  a fifth calculator  404   i,  and an update unit  404   j.    
     The third calculator  404   g  calculates a pulse width of each pulse at the time when the light source device  5  causes emission of a pulse light based on a target value that is input from the second calculator  404   d  and a light emission period that is input from the determination unit  404   c  and outputs the pulse width to the light source controller  404   f.  The third calculator  404   g  calculates a pulse width at the time when the light source  51  emits a pulse light next based on the target value that is updated by the update unit  404   j  with respect to each pulse and the light emission period that is determined by the determination unit  404   c  and outputs the pulse width to the light source controller  404   f.    
     The fourth calculator  404   h  calculates a Duty control value at the time when the light source controller  404   f  actually controls the light source  51  with respect to each pulse based on the pulse width at the time when the light source controller  404   f  causes the light source  51  to emit a pulse light and the light emission period that is determined by the determination unit  404   c  and outputs the control value of each pulse to the fifth calculator  404   i.    
     The fifth calculator  404   i  calculates an error (carry over error) between a Duty target value that is calculated by the second calculator  404   d  and the Duty control value that is calculated by the fourth calculator  404   h  with respect to each pulse and outputs the error of each pulse to the update unit  404   j.    
     Based on the error that is calculated by the fifth calculator  404   i,  the update unit  404   j  corrects (makes a reflection to) the Duty target value at the time when the light source  51  emits a pulse light next with respect to each pulse, thereby updating the Duty target value. 
     Process Executed by Endoscope System 
     A process that the endoscope system  1 A executes will be described next.  FIG.  6    is a flowchart illustrating an overview of the process that the endoscope system  1 A executes. 
     As illustrated in  FIG.  6   , first of all, the first calculator  404   a  acquires an image corresponding to an image signal obtained by performing image processing on a video signal that is acquired by the image processor  401  from the imager  242  of the endoscope  2  (step S 201 ) and calculates a brightness evaluation value of the image corresponding to the image signal that is acquired from the image processor  401  (step S 202 ). 
     Subsequently, the controller  405  performs a Duty control process for performing PWM control on the light source device  5  (step S 203 ). 
     Duty Control Process 
       FIG.  7    is a flowchart illustrating an overview of the Duty control process at step S 203  in  FIG.  6   . 
     As illustrated in  FIG.  7   , first of all, the detector  404   b  detects a frequency of vocal cords based on audio data that is input from the audio input device  3  (step S 301 ). 
     Subsequently, the determination unit  404   c  determines a light emission period of the next frame of the imager  242  based on the frequency that is detected by the detector  404   b  (step S 302 ). 
     Thereafter, the controller  405  determines whether it is the first light emission of the corresponding frame in the imager  242  (step S 303 ). When the controller  405  determines that it is the first light emission of the corresponding frame in the imager  242  (YES at step S 303 ), the endoscope system  1 A moves to step S 304  to be described below. On the other hand, when the controller  405  determines that it is not the first light emission of the corresponding frame in the imager  242  (NO at step S 303 ), the endoscope system  1 A moves to step S 306  to be described below. 
     At step S 304 , the update unit  404   j  resets the difference between a Duty target value that is calculated by the third calculator  404   g  and a Duty control value that is calculated by the fourth calculator  404   h.    
     Subsequently, based on the brightness evaluation value that is calculated by the first calculator  404   a,  the second calculator  404   d  calculates a Duty target value representing an intended value of a Duty ratio of the next frame of the imager  242  (step S 305 ). 
     Thereafter, based on a pulse width at the time when the light source controller  404   f  causes the light source  51  to emit a pulse light and the light emission period that is determined by the determination unit  404   c,  the fourth calculator  404   h  calculates a Duty control value representing a Duty ratio at the time when the light source controller  404   f  actually controls the light source  51  (step S 306 ). 
     Subsequently, based on the Duty target value that is input from the second calculator  404   d  and the light emission period that is input from the determination unit  404   c,  the third calculator  404   g  calculates a pulse width of each pulse at the time when the light source device  5  causes emission of a pulse light (step S 307 ). 
     Thereafter, the fifth calculator  404   i  calculates a difference (carry over error) between the Duty target value that is calculated by the second calculator  404   d  and the Duty control value that is calculated by the fourth calculator  404   h  (step S 308 ). 
     Subsequently, based on the error (carry over error) that is calculated by the fifth calculator  404   i,  the update unit  404   j  corrects (makes a reflection to) the Duty target value at the time when the light source  51  emits a pulse light next with respect to each pulse, thereby updating the Duty target value (step S 309 ). After step S 309 , the endoscope system  1 A returns to the main routine in  FIG.  6   . 
     Overview of Conventional Duty Control Process 
     A conventional Duty control process will be described.  FIG.  8    is a timing chart illustrating an overview of the conventional Duty control process. In  FIG.  8   , from the top, (a) presents periods of one frame captured by the imager  242 , (b) presents Duty ratios, and (c) presents pulse widths of pulse light. 
     As illustrated in  FIG.  8   , in the conventional Duty control process, to change the total exposure of brightness of an image from “18” to “17”, the target value (Duty intended value) representing a Duty ratio realized by the light source  51  for making the image have intended brightness in the next frame of the imager  242  is 3%×(17/18)=2.83%. When there is however a restriction that a control value representing a Duty ratio enabling the light source controller  404   f  to control The light source  51  is controlled only on a percent to percent basis because of the circuit scale, or the like, the Duty ratios in one frame are controlled uniformly in the conventional Duty control process. 
     For this reason, as illustrated in  FIG.  8   , in the conventional Duty control process, in the next frame of the imager  242 , the control value representing a Duty ratio is “2%” and the total exposure of brightness of the image is “12” (2%×6(sets)). Thus, in the conventional Duty control process, gain processing of 1.42 (17+12) has to be performed on a video signal that is generated by the imager  242  in order to correct the difference between the target value and the actual control value. The gain processing has an effect on the S/N of the video signal. As a result, the conventional Duty control process leads to an unnatural image. For example, the conventional Duty control method leads to an image in which the noise volume significantly increases or decreases according to fluctuations in the distance in observing a living organism. For this reason, a technique enabling consecutive dimming control without causing an adverse effect of a S/N increase or decrease the gain processing even when the Duty control cannot ensure a sufficient resolution has been required. In other words, in the second embodiment, Duty ratios in one frame are not controlled uniformly (vary per pulse) and accordingly an image suitable as an observation image is acquired. 
     Specific Example of Duty Control Process 
       FIG.  9    is a timing chart illustrating an overview of the Duty control process according to the second embodiment. In  FIG.  9   , (a) presents periods of one frame captured by the imager  242 , (b) presents Duty ratios, and (c) presents pulse widths of pulse light. 
     As illustrated in  FIG.  9   , first of all, based on a brightness evaluation value that is calculated by the first calculator  404   a,  the second calculator  404   d  calculates a Duty target value representing an aimed value of the Duty ratio of the next frame of the imager  242 . For example, in the case where a brightness evaluation value (total exposure) of a previous frame is “18”, a target value of the duty ratio is “3%”, and a brightness evaluation value (total exposure) that is calculated by the first calculator  404   a  changes to “17”, the second calculator  404   d.  calculates that a Duty target value of a current frame is “2.83%” (3%×(17/18)=2.83%). The fourth calculator  404   h  calculates that the first (first pulse) Duty control value is “3.00%” that is a value obtained by rounding the Duty control value to the closest integer. 
     Subsequently, based on the Duty target value that is input from the second calculator  404   d  and a light emission period that is input from the determination unit  404   c,  the third calculator  404   g  calculates a first (first pulse) pulse width at the time when the light source device  5  emits a pulse light. For example, when the first Duty control value is “3.00%” and the light emission period is “200 μsec”, the third calculator  404   g  calculates that the first pulse width is “6 μsec”. In this case, the third calculator  404   g  calculates a pulse width in a step size of an integral multiple (for example, 1 μsec) of a line period of the imager  242 . 
     Thereafter, the fifth calculator  404   i  calculates that the difference (carry over error) between “3.00%” that is the Duty control value calculated by the fourth calculator  404   h  and “2.83%” that is the Duty target value calculated by the second calculator  404   d  is “0.17%” (3.00%−2.83%=−0.17%). In this case, based on the difference that is calculated by the fifth calculator  404   i,  the update unit  4041  corrects (makes a reflection to) the Duty control value at the time when the light source  51  emits a pulse light next, thereby updating the second Duty control value to “2.66%” (2.83%−0.17%=2.66%). The fourth calculator  404   h  calculates that the second Duty control value is “3.00%” that is a value obtained by rounding the Duty control value to the closest integer (rounding 2.66% to 3.00%). 
     Subsequently, based on the Duty control value that is input from the fourth calculator  404   h  and the light emission period that is input from the determination unit  404   c,  the third calculator  404   g  calculates a second pulse width at the time when the light source device  5  emits a pulse light. For example, in the case where the second Duty control value is “3.00%” and the light emission period is “200 μsec”, the third calculator  404   g  calculates that the second pulse width is “6 μsec”. 
     Thereafter, the fifth calculator  404   i  calculates that the difference (carry over error) between “3.00%” that is the Duty control value calculated by the fourth calculator  404   h  and “2.66%” that is the Duty target value that is updated secondarily by the update unit  404   j  is “0.34%” (3.00%−2.66%=−0.34%). In this case, based on the difference that is calculated by the fifth calculator  404   i,  the update unit  404   j  corrects (makes a reflection to) the Duty target value at the time when the light source  51  emits a pulse light next, thereby updating the third Duty target value to “2.49%” (2.83%−0.34%=2.49%). The fourth calculator  404   h  calculates that the third Duty control value is “2.00%” that is a value obtained by rounding the Duty control value to the closest integer (rounding 2.49% to 2.00%). 
     In the case where the third Duty control value is “2.00%” and the light emission period is “200 μsec”, the third calculator  404   g  calculates that the third pulse width is “4 μsec”. 
     Thereafter, the fifth calculator  404   i  calculates that the difference (carry over error) between “2.00%” that is the Duty control value calculated by the fourth calculator  404   h  and “2.49%” that is the Duty target value that is updated thirdly by the update unit  404   j  is “+0.49%”. In this case, based on the error that is calculated by the fifth calculator  404   i,  the update unit  404   j  corrects (makes a reflection to) the Duty target value at the time when the light source  51  emits a pulse light next, thereby updating the fourth Duty target value to “3.32%” (2.83%+0.49%=3.32%). The fourth calculator  404   h  calculates than the fourth control value is “3.00%” that is a value obtained by rounding the Duty control value to the closest integer (rounding 3.32% to 3.00%). 
     In the case, where the fourth Duty control value as “3.00%” and the light emission period is “200 μsec”, the third calculator  404   g  calculates that the fourth pulse width is “6 μsec”. 
     Thereafter, the fifth calculator  404   i  calculates that the difference (carry over error) between “3.00%” that is the Duty control value calculated by the fourth calculator  404   h  and “3.32%” that is the Duty target value that is updated fourthly by the update unit  404   j  is “+0.32%”. In this case, based on the difference that is calculated by the fifth calculator  404   i,  the update unit  404   j  corrects (makes a reflection to) the Duty target value at the time when the light source  51  emits a pulse light next, thereby updating the fifth Duty target value to “3.15%” (2.83%+0.32%=3.15%). The fourth calculator  404   h  calculates that the fifth control value is “3.00%” that is a value obtained by rounding the Duty control value to the closest integer (rounding 3.15% to 3.00%). 
     In the case where the fifth Duty control value is “3.00%” and the light emission period is “200 μsec”, the third calculator  404   g  calculates that the fifth pulse width is “6 μsec”. 
     Thereafter, the fifth calculator  404   i  calculates that the difference (carry over error) between “3.00%” that is the Duty control value calculated by the fourth calculator  404   h  and “3.15%” that is the Duty target value that is updated fifthly by the update unit  404   j  is “+0.15%”. In this case, based on the difference that is calculated by the fifth calculator  404   i,  the update unit  404   j  corrects (makes a reflection to) the Duty target value at the time when the light source  51  emits a pulse light next, thereby updating the sixth target value to “2.98%” (2.83%+0.15%=2.98%). The fourth calculator  404   h  calculates that the sixth Duty control value is “3.00%” that is a value obtained by rounding the Duty control value to the closest integer (rounding 2.98% to 3.00%). 
     In the case where the sixth Duty control value is “3.00%” and the light emission period is “200 μsec”, the third calculator  404   g  calculates that the sixth pulse width is “6 μsec”. 
     As described above, the light source controller  404   f  causes emission of pulse light using five sets of 3% that is Duty ratio and a set of 2% in the next frame or the imager  242 , so that the average Duty ratio is 2.83% and the brightness evaluation value of the image can be “17”. As described above, using the difference (carry over error) that is calculated by the fifth calculator  404   i,  the update unit  404   j  corrects (makes a reflection to) the Duty target value at the time when the light source  51  emits a pulse light next with respect to each pulse, thereby updating the Duty target value and thus enabling consecutive PWM control. 
     Back to  FIG.  6   , description of step S 204  and the steps after step S 204  will be continued. 
     At step S 204 , the light source controller  404   f  executes PWM control by driving the light source driver  52  based on the pulse width of each pulse that is calculated by the third calculator  404   g.    
     Subsequently, the controller  405  determines whether light emission for the corresponding frame of the imager  242  completes (step S 205 ). When the controller  405  determines that light emission for the corresponding frame of the imager  242  completes (YES at step S 205 ), the endoscope system  1 A moves to step S 206  to be described below. On the other hand, when the controller  405  determines that light emission of the corresponding frame of the imager  242  does riot complete (NO at step S 205 ), the endoscope system  1 A returns to step S 203  described above. 
     At step S 206 , the controller  405  determines whether an instruction signal for ending an examination on a living organism is input from the operation unit  403 . When the controller  405  determines that the instruction signal for ending the examination on the living organism is input from the operation unit  403  (YES at step S 206 ), the endoscope system  1 A ends the process. On the other hand, when the controller  405  determines that the instruction signal for ending the examination on the living organism is not input from the operation unit  403  (NO at step S 206 ), the endoscope system  1 A returns to step S 201  described above. 
     According to the second embodiment described above, because the third calculator  404   g  calculates a pulse width at the time when the light source  51  emits a pulse light next based on a Duty target value that is updated by the update unit  404   j  with respect to each pulse in one frame of the imager  242  and the light emission period and an approximation to the Duty target value is made, it is possible to obtain an observation image with brightness that is suitable for observation. 
     According to the second embodiment, because the third calculator  404   g  calculates a pulse width of each pulse in one frame based on the target value that is the average Duty ratio of the pulse light in the next frame of the imager  242 , which is calculated by the second calculator  404   d,  and the light emission period, which is determined by the determination unit  404   c,  even if the Duty control by the light source controller  404   f  cannot ensure a resolution sufficiently, it is possible to perform consecutive dimming control without increasing or reducing the S/N ratio. 
     According to the second embodiment, using the difference (error) that is calculated by the fifth calculator  404   i,  the update unit  404   j  corrects (makes a reflection to) the Duty control value at the time when the light source  51  emits a pulse light next with respect to each pulse, thereby updating the Duty target value and thus enabling consecutive PWM control. 
     Other Embodiments 
     It is possible to form various embodiments by appropriately combining a plurality of elements disclosed in the endoscope systems according to the first and second embodiments of the disclosure described above. For example, some elements may be omitted from all the elements described with respect to the endoscope systems according to the first and second embodiments of the disclosure described above. Furthermore, the elements described with respect to the endoscope systems according to the first and second embodiments of the disclosure described above may be combined as appropriate. 
     In the endoscope systems according to the first and second embodiments of the disclosure, the “unit”, “-er” and “-or” described above may be read as “means”, “circuitry”, or the like. For example, the controller may be read as a control means or a control circuitry. 
     The programs that the endoscope systems according to the first and second embodiments of the disclosure are caused to execute are recorded as installable or executable file data in a computer-readable recording medium, such as a CD-ROM, a flexible disk (FD), a CD-R, a digital versatile disk (DVD), a USB medium, or a flash memory, and are provided. 
     The programs that the endoscope systems according to the first and second embodiments are caused to execute may be stored in a computer that is connected to a network, such as the Internet, and may be configured to be downloaded via the network and thus be provided. 
     In the description of the flowcharts herein, the context of the process among steps clearly specified using expressions including “first of all”, “thereafter”, and “subsequently”; however, the order of the processes necessary to implement the disclosure is not uniquely determined by those expressions. In other words, the order of processes in the flowcharts described herein is changeable within a range without inconsistency. 
     According to the disclosure, an effect that it is possible to obtain an observation image with brightness that is suitable for observation. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the disclosure in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.