Patent Publication Number: US-2018049632-A1

Title: Endoscope apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of International Application PCT/JP2015/062149, with an international filing date of Apr. 21, 2015, which is hereby incorporated by reference herein in its entirety. This application claims the benefit of International Application PCT/JP2015/062149, the content of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an endoscope apparatus. 
     BACKGROUND ART 
     There are conventionally known endoscope apparatuses in which a subject is captured a plurality of times for different exposure times, and a plurality of acquired images are composited, thereby acquiring an endoscope image in which the dynamic range has been expanded (for example, see PTL 1). 
     In an endoscope apparatus of PTL 1, a color CCD is used to capture a subject two times, namely, for a long exposure time ( 1/60 seconds) and for a short exposure time ( 1/240 seconds), and two acquired digital signals are divided into signals of three colors: R, G, and B. Next, an R signal during the short exposure time and the R signal during the long exposure time are composited, thereby generating an R signal in which the dynamic range has been expanded. The dynamic ranges of the G signal and the B signal are expanded in the same way as the R signal. Next, the R signal, the G signal, and the B signal, in which the dynamic ranges have been expanded, are used to generate a color endoscope image having a dynamic range wider than the dynamic range of the CCD. 
     In capturing performed for a short exposure time, a bright area, such as a near-point area, onto which strong illumination light is radiated is clearly captured without causing halation. In capturing performed for a long exposure time, a dark area, such as a far-point area, that illumination light is unlikely to reach is clearly captured without causing underexposed shadows. Therefore, the endoscope apparatus of PTL 1 is suitable for capturing a subject in which the difference in brightness is large, such as a tubular digestive tract. 
     CITATION LIST 
     Patent Literature 
     {PTL 1} Japanese Unexamined Patent Application, Publication No. Hei 11-234662 
     SUMMARY OF INVENTION 
     Technical Problem 
     In endoscope-image diagnosis, an observer pays attention to an inflamed portion in red, such as redness, and veins in blue. Furthermore, a method in which a blue dye, which has high contrast with respect to the color of reddish living tissue, is sprayed on an area to be diagnosed to emphasize the structure of the area to be diagnosed in the living tissue is used in some cases. In this way, endoscope images tend to have red or blue color in many cases, and information concerning red and blue is particularly important in endoscope-image diagnosis. 
     An object of the present invention is to provide an endoscope apparatus capable of acquiring an endoscope image in which the difference in color of living tissue is correctly reproduced. 
     Solution to Problem 
     An aspect of the present invention provides an endoscope apparatus including: an illumination unit that sequentially radiates illumination light of three colors of red, green, and blue onto a subject; an image acquisition unit that acquires an image by capturing the illumination light reflected at the subject; a control unit that controls the image acquisition unit so as to perform capturing in synchronization with radiation of the illumination light of the three colors from the illumination unit, thereby causing the image acquisition unit to sequentially acquire component images of three colors of red, green, and blue; a dynamic-range expanding unit that generates an expanded component image in which the dynamic range is expanded, from the component image of at least one color other than green, among the component images of the three colors acquired by the image acquisition unit; and an image generating unit that generates a colored endoscope image by compositing the expanded component image of the at least one color, which is generated by the dynamic-range expanding unit, and the component images of the other colors, wherein the control unit controls the image acquisition unit so as to capture the illumination light of the at least one color a plurality of times for different exposure times, thereby causing the image acquisition unit to acquire a plurality of component images of the at least one color; and the dynamic-range expanding unit generates the expanded component image by compositing the plurality of component images of the at least one color. 
     In the above-described aspect, the control unit may control the image acquisition unit so as to capture each of the illumination light of red and the illumination light of blue a plurality of times for different exposure times and may control the exposure times for capturing the illumination light of red and the exposure times for capturing the illumination light of blue, independently from each other. 
     The above-described aspect may further include an exposure-time setting unit that sets exposure times for next capturing of the illumination light of the at least one color performed a plurality of times, on the basis of the distribution of gradation values of the plurality of component images of the at least one color. 
     The above-described aspect may further include a region-of-interest specifying unit that specifies a region of interest in a capture range of the component image captured by the image acquisition unit, wherein the exposure-time setting unit may set exposure times for next capturing of the illumination light of the at least one color performed a plurality of times, on the basis of the distribution of gradation values in the region of interest specified by the region-of-interest specifying unit, among the plurality of component images of the at least one color. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a view showing the overall configuration of an endoscope apparatus according to a first embodiment of the present invention. 
         FIG. 2  is a front view of a color filter set in an illumination unit of the endoscope apparatus shown in  FIG. 1 . 
         FIG. 3  is a timing chart showing the timings of radiation of illumination light and exposure performed in an image acquisition device. 
         FIG. 4  is a view for explaining processing performed by a dynamic-range expanding unit and a compression unit of the endoscope apparatus shown in  FIG. 1 . 
         FIG. 5  is a flowchart showing the operation of the endoscope apparatus shown in  FIG. 1 . 
         FIG. 6  is a view showing the configuration of an image processor of an endoscope apparatus according to a second embodiment of the present invention. 
         FIG. 7  shows: an example endoscope image (at an upper part); an image signal obtained during a long exposure time, along the line A-A of the endoscope image (at a middle part); and an image signal obtained during an extended long exposure time (at a lower part). 
         FIG. 8  shows: an example endoscope image (at an upper part); an image signal obtained during a short exposure time, along the line A-A of the endoscope image (at a middle part); and an image signal obtained during a shortened short exposure time (at a lower part). 
         FIG. 9  is a graph showing the relationship between the brightness of a subject and the gradation value of an image signal obtained during a long exposure time. 
         FIG. 10  is a graph showing the relationship between the brightness of a subject and the gradation value of an image signal obtained during a short exposure time. 
         FIG. 11  is a graph showing the relationship between the number of pixels that have the maximum gradation value and an extension time for the long exposure time. 
         FIG. 12  is a graph showing the relationship between the number of pixels that have the minimum gradation value and a reduction time for the short exposure time. 
         FIG. 13  is a flowchart showing the operation of the endoscope apparatus that is provided with the image processor shown in  FIG. 6 . 
         FIG. 14  is a flowchart showing an exposure-time setting routine shown in  FIG. 13 . 
         FIG. 15  is a timing chart showing the timings of radiation of illumination light and capturing performed in the image acquisition device. 
         FIG. 16  is a view showing the configuration of a modification of the image processor shown in  FIG. 6 . 
         FIG. 17  is a flowchart showing an exposure-time setting routine in the operation of an endoscope apparatus that is provided with the image processor shown in  FIG. 16 . 
         FIG. 18  is a view showing the configuration of an image processor in an endoscope apparatus according to a third embodiment of the present invention. 
         FIG. 19  shows an example endoscope image in which a region of interest is specified. 
         FIG. 20  is a flowchart showing an exposure-time setting routine in the operation of the endoscope apparatus that is provided with the image processor shown in  FIG. 18 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     An endoscope apparatus  1  according to a first embodiment of the present invention will be described below with reference to  FIGS. 1 to 5 . 
     The endoscope apparatus  1  of this embodiment is of a frame sequential type in which illumination light of the three colors red (R), green (G), and blue (B) is sequentially radiated onto living tissue (a subject), image signals of the three colors R, G, and B are sequentially acquired, and a color endoscope image is generated from the acquired three-color image signals. 
     As shown in  FIG. 1 , the endoscope apparatus  1  is provided with: an elongated insertion portion  2  that is inserted into a living body; an illumination unit  3  that is connected to a base end of the insertion portion  2 ; and an image processor  4 . 
     The insertion portion  2  is provided with: an illumination lens  5  and an objective lens  6  that are provided on a distal end surface of the insertion portion  2 ; a condensing lens  7  that is provided on a base end surface of the insertion portion  2 ; a light guide  8  that is disposed between the illumination lens  5  and the condensing lens  7  along the longitudinal direction; and an image acquisition device (image acquisition unit)  9  that is disposed at a base end side of the objective lens  6 . 
     The condensing lens  7  focuses illumination light entering from the illumination unit  3 , on a base end surface of the light guide  8 . 
     The light guide  8  guides the illumination light incident on the base end surface thereof from the condensing lens  7  to a distal end surface thereof and emits the illumination light from the distal end surface toward the illumination lens  5 . 
     The illumination lens  5  spreads the illumination light entering from the light guide  8  to radiate it onto living tissue S. 
     The objective lens  6  images, on an imaging surface of the image acquisition device  9 , the illumination light reflected at the living tissue S and entering the objective lens  6 . 
     The image acquisition device  9  is a monochrome CCD image sensor or a monochrome CMOS image sensor. As will be described later, the image acquisition device  9  is controlled by a control unit  14  so as to perform capturing in synchronization with the radiation of illumination light L R , L G , and L B  onto the living tissue S. After the end of exposure, the image acquisition device  9  generates image signals through photoelectric conversion and sends the generated image signals to an image memory  15  (to be described later) in the image processor  4 . 
     Note that, in this embodiment, although it is assumed that the flexible insertion portion  2 , in which the image acquisition device  9  is provided at a distal end portion, is used, it is also possible to use a rigid insertion portion in which a relay optical system that relays an image formed by the objective lens  6  is provided at a base end side of the objective lens  6 . In the case of the rigid insertion portion, an image acquisition device is disposed at a base end side of the insertion portion. 
     The illumination unit  3  is provided with: a light source (for example, xenon lamp)  10  that produces white light; two condensing lenses  11  and  12  that are disposed on the output optical axis of the light source  10 ; and a color filter set  13  that is disposed between the two condensing lenses  11  and  12 . 
     The condensing lens  11  focuses light produced by the light source  10  and causes the light to enter the color filter set  13 . The condensing lens  12  focuses the light transmitted through the color filter set  13  and causes the light to enter the condensing lens  7  in the insertion portion  2 . 
     As shown in  FIG. 2 , the color filter set  13  has three-color filters  13 R,  13 G, and  13 B that are evenly arranged around a rotary shaft  13   a  that is disposed parallel to the output optical axis of the light source  10 . The R-filter  13 R transmits only R-light L R , the G-filter  13 G transmits only G-light L G , and the B-filter  13 B transmits only B-light L B . The color filter set  13  rotates about the rotary shaft  13   a , thereby causing the filters  13 R,  13 G, and  13 B to be sequentially disposed on the output optical axis and causing R-light L R , G-light L G , and B-light L B  to sequentially enter the condensing lens  7  from the color filter set  13 . 
     Here, the rotating speed of the color filter set  13  is fixed, and the three filters  13 R,  13 G, and  13 B all have the same shape and dimensions. Therefore, as shown in  FIG. 3 , from the illumination lens  5 , R-light L R , G-light L G , and B-light L B  are sequentially radiated onto the living tissue S at certain time intervals, and the irradiation times for the B-light L R , the G-light L G , and the B-light L B  per single irradiation are equal to each other. It is preferred that the rotating speed of the color filter set  13  be 30 rps or more and 60 rps or less such that the frame rate of endoscope images falls within the range from 30 fps to 60 fps, which is suitable for video. 
     The image processor  4  is provided with: the control unit  14 , which controls the image acquisition device  9 ; the image memory  15 , which temporarily holds image signals S RL , S RS , S G , S BL , and S BS  received from the image acquisition device  9 ; a dynamic-range expanding unit  16  that performs dynamic-range expansion processing on R-image signals S RL  and S RS  and B-image signals S BL  and S BS ; a compression unit  17  that compresses the gradation values of an R expanded image signal S RL +S RS  and a B expanded image signal S BL +S BS  in each of which the dynamic range has been expanded; and an image generating unit  18  that generates an endoscope image from the image signals S RL ′+S RS ′, S G , and S BL ′+S BS ′. 
     The control unit  14  obtains, from the illumination unit  3 , information of the timing of radiation of R-light L R , G-light L G , and B-light L B . The control unit  14  causes the image acquisition device  9  to perform capturing for preset exposure times T RL , T RS , T G , T BL , and T BS , on the basis of the obtained timing information, in synchronization with radiation of B-light L R , G-light L G , and B-light L B , as shown in  FIG. 3 . Accordingly, the control unit  14  causes the image acquisition device  9  to perform capturing of the R-light L R , the G-light L G , and the B-light L B  in this order during one frame period. 
     Here, the control unit  14  causes the image acquisition device  9  to perform capturing only one time for the exposure time T G , during the irradiation period for the G-light L G . Accordingly, the image acquisition device  9  acquires one G-image signal S G  during one frame period. 
     On the other hand, during the irradiation period for the R-light L R , the control unit  14  causes capturing to be performed two times for the long exposure time T RL  and for the short exposure time T RS , which is shorter than the long exposure time T RL . Accordingly, the image acquisition device  9  sequentially acquires two R-image signals S RL  and S RS  having different exposure times, during one frame period. Similarly, during the irradiation period for the B-light L B , the control unit  14  causes capturing to be performed two times for the long exposure time T BL  and for the short exposure time T BS , which is shorter than the long exposure time T BL . Accordingly, the image acquisition device  9  sequentially acquires two B-image signals S BL  and S BS  having different exposure times, during one frame period. As shown in  FIG. 4 , the image signals S RL  and S BL , which are obtained during the long exposure times, are image signals in which dark areas of the living tissue S are clearly captured at high contrast. The image signals S RS  and S BS , which are obtained during the short exposure times, are image signals in which bright areas of the living tissue S are clearly captured at high contrast. 
     The exposure times T RL , T RS , T G , T BL , and T BS  are set in the control unit  14  when an observer inputs desired values by using, for example, an input device (not shown) that is connected to the image processor  4 . Here, the exposure times T RL  and T RS  for the R-image signals S RL  and S RS  and the exposure times T BL  and T BS  for the B-image signals S BL  and S BS  can be set independently from each other. For example, when the irradiation times for the illumination light L R , L G , and L B  per single irradiation are each 15 milliseconds, the exposure time T G  is set to 15 milliseconds, the long exposure times T RL  and T BL  are each set to 10 milliseconds, and the short exposure times T RS  and T BS  are each set to 5 milliseconds. 
     The image memory  15  sequentially receives, during one frame period, the R-image signal S RL , the R-image signal S RS , the G-image signal S G , the B-image signal S BL , and the B-image signal S BS . The image memory  15  sends only the G-image signal S G , which constitutes a G-component image, to the image generating unit  18  and sends the R-image signals S RL  and S RS , which constitute an R-component image, and the B-image signals S BS  and S BL , which constitute a B-component image, to the dynamic-range expanding unit  16 . 
       FIG. 4  shows processing for the R-image signals S RL  and S RS  performed in the dynamic-range expanding unit  16  and the compression unit  17 . Although  FIG. 4  shows only the R-image signals S RL , S RS , S RL +S RS , and S RL ′+S RS ′, as example signals, the B-image signals S BL , S BS , S BL +S BS , and S BL ′+S BS ′ also have the same features. 
     When receiving two R-image signals S RL  and S RS  from the image memory  15 , the dynamic-range expanding unit  16  adds the gradation values of respective pixels in the R-image signal S RL  and the gradation values of respective pixels in the R-image signal S RS , thereby generating an R expanded image signal S RL +S RS , which constitutes an R expanded component image. Similarly, when receiving two B-image signals S BL  and S BS  from the image memory  15 , the dynamic-range expanding unit  16  adds the gradation values of respective pixels in the B-image signal S BL  and the gradation values of respective pixels in the B-image signal S BS , thereby generating a B expanded image signal S BL +S BS , which constitutes a B expanded component image. 
     The expanded image signals S RL +S RS  and S BL +S BS  have a dynamic range wider than the dynamic range of the image acquisition device  9  and have twice gradation scale of each of the image signals S RL , S RS , S G , S BL , and S BS . The dynamic-range expanding unit  16  sends the generated R expanded image signal S RL +S RS  and the generated B expanded image signal S BL +S BS  to the compression unit  17 . 
     The compression unit  17  compresses the numbers of gradations of the R expanded image signal S RL +S RS  and the B expanded image signal S BL +S BS  by half. Accordingly, the gradation scale of the R expanded image signal S RL +S RS  and the B expanded image signal S BL +S BS  become equal to the gradation scale of the G-image signal S G . The compression unit  17  sends the compressed R expanded image signal S RL ′+S RS ′ and the compressed B expanded image signal S BL ′+S BS ′ to the image generating unit  18 . 
     The image generating unit  18  performs RGB-composition on the unprocessed G-image signal S G , which is received from the image memory  15 , and the R expanded image signal S RL ′+S RS ′ and the B expanded image signal S BL ′+S BS ′, which are received from the compression unit  17 , thereby generating a colored endoscope image. The image generating unit  18  sends the generated endoscope image to a display unit  24 . 
     The display unit  24  sequentially displays received endoscope images. 
     Next, the operation of the thus-configured endoscope apparatus  1  will be described with reference to  FIG. 5 . 
     First, the exposure times T RL , T RS , T G , T BL , and T BS  are initially set by an observer, for example (Step S 1 ). Next, when the operation of the illumination unit  3  is started, B-light L R , G-light L G , and B-light L B  sequentially enter the light guide  8  in the insertion portion  2  via the condensing lenses  12  and  7 , and the R-light L R , the G-light L G , and the B-light L B  are sequentially radiated from the distal end of the insertion portion  2  toward the living tissue S, in a repeated manner (Step S 2 ). The R-light L R , the G-light L G , and the B-light L B  reflected at the living tissue S are collected by the objective lens  6  and are sequentially captured by the image acquisition device  9 , and the image signals S RL , S RS , S GL , S BL , and S BS  are sequentially acquired (Steps S 3  to S 7 ). 
     Here, during the irradiation period for the G-light L G  (YES in Step S 3 ), the control unit  14  causes the image acquisition device  9  to perform capturing only one time (Step S 4 ), thereby acquiring one G-image signal S G  (Step S 5 ). 
     On the other hand, during the irradiation period for the R-light L R  (NO in Step S 3 ), the control unit  14  causes the image acquisition device  9  to sequentially perform capturing for the long exposure time and capturing for the short exposure time (Step S 6 ), thereby acquiring two R-image signals S RL  and S RS  (Step S 7 ). Similarly, during the irradiation period for the B-light L B  (NO in Step S 3 ), the control unit  14  causes the image acquisition device  9  to sequentially perform capturing for the long exposure time and capturing for the short exposure time (Step S 6 ), thereby acquiring two B-image signals S BL  and S BS  (Step S 7 ). 
     The dynamic-range expanding unit  16  adds the two R-image signals S RL  and S RS  to each other, thereby generating an R expanded image signal S RL +S RS  in which the dynamic range is expanded (Step S 8 ). Similarly, the dynamic-range expanding unit  16  adds the two B-image signals S BL  and S BS  to each other, thereby generating a B expanded image signal S BL +S BS  in which the dynamic range is expanded (Step S 8 ). The gradation scale of the R expanded image signal S RL +S RS  and that of the B expanded image signal S BL +S BS  are compressed in the compression unit  17  (Step S 9 ), and then, the resulting signals are sent to the image generating unit  18 . 
     In the image generating unit  18 , when the G-image signal S G  is input from the image acquisition device  9  via the image memory  15 , and the R expanded image signal S RL ′+S RS ′ and the B expanded image signal S BL ′+S BS ′ are input from the compression unit  17  (YES in Step S 10 ), the three-color image signals S G , S RL ′+S RS ′, and S BL ′+S BS ′ are composited, thus generating a colored endoscope image (Step S 11 ). Generated endoscope images are sequentially displayed on the display unit  24  in the form of a moving image (Step S 12 ). 
     In this way, according to this embodiment, an endoscope image displayed on the display unit  24  is constituted by using the R expanded image signal S RL ′+S RS ′ and the B expanded image signal S BL ′+S BS ′, which have a wide dynamic range. Therefore, the endoscope image can correctly express deep red and deep blue without causing color saturation. Accordingly, red of an inflamed site and blue of a vein in the living tissue S, which are important in endoscope-image diagnosis, are correctly reproduced in the endoscope image, thus providing an advantage that an observer can observe, in the endoscope image, a slight change in red in the inflamed site and the detailed distribution of veins. 
     Furthermore, in capturing for the short exposure times T RS  and T BS , dark areas, such as far-point areas that the illumination light L R , L G , and L B  is unlikely to reach, show underexposed shadows because the image signals S RS  and S BS  have almost no gradation values and get buried in noise; however, in capturing for the long exposure times T RL  and T BL , the image signals S RL  and S BL  have sufficiently large gradation values in the dark areas. There is an advantage that the expanded image signals S RL +S RS  and S BL +S BS  are generated from these image signals S RL  and S BL , thereby making it possible to acquire an endoscope image in which the underexposed shadows are resolved. 
     Furthermore, there is an advantage that the brightness of the whole endoscope image can be ensured by the G-image signal S G , which is obtained during the longer exposure time T G  compared with the R-image signals S RL  and S RS  and the B-image signals S BL  and S BS . Furthermore, in order to acquire an endoscope image that has high color reproducibility for red and blue, as described above, only control of the image acquisition device  9  performed by the image processor  4  and processing of the image signals S RL , S RS , S G , S BL , and S BS  need to be changed from those in a conventional endoscope apparatus. Therefore, there is an advantage that an endoscope image that has high color reproducibility can be acquired without complicating the configuration and while maintaining the high resolution and high frame rate of a conventional endoscope apparatus. 
     Second Embodiment 
     Next, an endoscope apparatus according to a second embodiment of the present invention will be described with reference to  FIGS. 6 to 17 . 
     The endoscope apparatus of this embodiment differs from the endoscope apparatus  1  of the first embodiment in that feedback control is performed on the exposure times T RL , T RS , T BL , and T BS  for the next capturing of the R-light L R  and the B-light L B , on the basis of the distributions of the gradation values of the R-image signals S RL  and S RS  and the B-image signals S BL  and S BS . 
     Specifically, the endoscope apparatus of this embodiment is provided with an image processor  41  shown in  FIG. 6 , instead of the image processor  4 . The configurations other than that of the image processor  41  are the same as those in the endoscope apparatus  1  of the first embodiment shown in  FIG. 1 . 
     As shown in  FIG. 6 , the image processor  41  is further provided with a threshold processing unit  19  and an exposure-time setting unit  20 . 
     In this embodiment, the image memory  15  sends the R-image signals S RL  and S RS  and the B-image signals S BL  and S BS  to the dynamic-range expanding unit  16  and also to the threshold processing unit  19 . 
     The threshold processing unit  19  has a threshold α RL  for the gradation values of the R-image signal S RL , a threshold α RS  for the gradation values of the R-image signal S RS , a threshold α BL  for the gradation values of the B-image signal S BL , and a threshold α BS  for the gradation values of the B-image signal S BS . The thresholds α RL  and α BL  are set to the minimum gradation value 0 of the R-image signal S RL  and the B-image signal S BL , which are obtained during the long exposure times, or set to a value that is larger than the minimum gradation value and that is close to the minimum gradation value. The thresholds α BS  and α BS  are set to the maximum gradation value 255 of the R-image signal S RS  and the B-image signal S BS , which are obtained during the short exposure times, or set to a value that is smaller than the maximum gradation value and that is close to the maximum gradation value. 
     When the R-image signal S RL  is received from the image memory  15 , the threshold processing unit  19  measures, among all pixels of the R-image signal S RL , the number of pixels M R  that have gradation values equal to or less than the threshold α RL . Furthermore, when the R-image signal S RS  is received from the image memory  15 , the threshold processing unit  19  measures, among all the pixels of the R-image signal S RS , the number of pixels N R  that have gradation values equal to or greater than the threshold α RS . 
     Similarly, when the B-image signal S BL  is received from the image memory  15 , the threshold processing unit  19  measures, among all pixels of the B-image signal, the number of pixels M B  that have gradation values equal to or less than the threshold α BL . Furthermore, when the B-image signal S BS  is received from the image memory  15 , the threshold processing unit  19  measures, among all pixels of the B-image signal S BS , the number of pixels N B  that have gradation values equal to or greater than the threshold α BS . 
       FIGS. 7 and 8  each show an example endoscope image (at an upper part) and the distributions of gradation values on the line A-A in this endoscope image (at a middle part and a lower part).  FIGS. 9 and 10  show the relationships between the brightness of the living tissue S and the gradation values of the image signals S RL , S RS , S BL , and S BS . 
     As shown in  FIGS. 7 and 8 , when the living tissue S has convex portions and a concave portion, the areas of the convex portions in an endoscope image  25  become relatively bright and the area of the concave portion therein becomes relatively dark. 
     When the long exposure times T RL  and T BL  are too short, the exposure amounts of the illumination light L R  and L B  from the concave portion become too low, and thus, as shown in  FIGS. 7 and 9 , a phenomenon in which actually different darkness levels are uniformly set to the minimum gradation value 0, i.e., so-called underexposed shadows, occurs in the image signals S RL  and S BL . In this case, the numbers of pixels M R  and M B , which have gradation values equal to or less than the thresholds α RL  and α BL , are increased. On the other hand, when the short exposure times T RS  and T BS  are too long, the exposure amounts in the areas of the convex portions become too high, and thus, as shown in  FIGS. 8 and 10 , a phenomenon in which actually different brightness levels are uniformly set to the maximum gradation value 255, i.e., so-called overexposed highlights, occurs in the image signals S RS  and S BS . In this case, the numbers of pixels N R  and N B , which have gradation values equal to or greater than the thresholds α RS  and α BS , are increased. 
     A description will be given of an example case in which α RL =α BL =0, and α RS =α BS =255. Therefore, the threshold processing unit  19  measures the numbers of pixels M R  and M B  in an underexposed shadow area that has the minimum gradation value 0, and the numbers of pixels N R  and N B  in an overexposed highlight area that has the maximum gradation value 255. 
     When the number of pixels M R  is received, the exposure-time setting unit  20  calculates an extension time for the long exposure time T RL  on the basis of the number of pixels M R  and adds the calculated extension time to the current long exposure time T RL , thereby calculating a next long exposure time T RL . In calculating the extension time, for example, a first look-up table (LUT) in which numbers of pixels M R  and extension times are associated with each other in advance is used. As shown in  FIG. 11 , for example, the numbers of pixels M R  and the extension times are associated in the first LUT such that the extension time is zero when the number of pixels M R  is zero, and the extension time increases in proportion to the number of pixels M R . 
     Furthermore, when the number of pixels N R  is received, the exposure-time setting unit  20  calculates a reduction time for the short exposure time T RS  on the basis of the number of pixels N R  and subtracts the calculated reduction time from the current short exposure time T RS , thereby calculating a next short exposure time T RS . In calculating the reduction time, for example, a second LUT in which numbers of pixels N R  and reduction times are associated with each other in advance is used. As shown in  FIG. 12 , for example, the numbers of pixels N R  and the reduction times are associated in the second LUT such that the reduction time is zero when the number of pixels N R  is zero, and the reduction time increases in proportion to the number of pixels N R . 
     The exposure-time setting unit  20  calculates, when the number of pixels M B  is received, a next long exposure time T BL  on the basis of the number of pixels M B , in the same way as the long exposure time T RL , and calculates, when the number of pixels N B  is received, a next short exposure time T BS  on the basis of the number of pixels N B , in the same way as the short exposure time T RS . 
     The exposure-time setting unit  20  sends the calculated next exposure times T RL , T RS , T BL , and T BS  to the control unit  14 . 
     The control unit  14  sets the exposure times T RL , T BL , T RS , and T BS  received from the exposure-time setting unit  20  as exposure times for the next capturing of R-light L R  and B-light L B . 
     Next, the operation of the thus-configured endoscope apparatus will be described. 
     According to the endoscope apparatus of this embodiment, as shown in  FIG. 13 , after two R-image signals S RL  and S RS  are acquired in Step S 7 , the exposure times T RL  and T RS  for acquiring next R-image signals S RL  and S RS  are set on the basis of the acquired R-image signals S RL  and S RS  (Step S 13 ). 
     Specifically, as shown in  FIG. 14 , the threshold processing unit  19  measures the number of pixels M R  that have the minimum gradation value 0, among the all pixels of the R-image signal S RL , which is obtained during the long exposure time T RL  (Step S 131 ). The measured number of pixels M R  expresses the size of an underexposed shadow area in the R-image signal S RL , and, as the underexposed shadow area becomes large, the number of pixels M R  is increased. The exposure-time setting unit  20  calculates a next long exposure time T RL  such that the next long exposure time T RL  becomes longer as the number of pixels M R  is increased (Step S 132 ) and sets the calculated next long exposure time T RL  in the control unit  14  (Step S 133 ). 
     Then, during the next frame period, as shown in  FIG. 13 , capturing of R-light L R  is performed for a longer long exposure time T RL  (Step S 4 ). Accordingly, as shown in  FIGS. 7 and 9 , an image signal S RL  in which the underexposed shadows are resolved and that has contrast in the dark area is acquired. In a case in which no underexposed shadow area exists in the R-image signal S RL , and the number of pixels M R  measured in Step S 131  is zero, the current long exposure time T RL  is still calculated and set as the next long exposure time T RL . 
     Next, the threshold processing unit  19  measures the number of pixels N R  that have the maximum gradation value 255, among all pixels of the R-image signal S RS , which is obtained during the short exposure time T RS  (Step S 134 ). The measured number of pixels N R  expresses the size of an overexposed highlight area in the R-image signal S RS , and, as the overexposed highlight area becomes larger, the number of pixels N R  is increased. The exposure-time setting unit  20  calculates a next short exposure time T RS  such that the next short exposure time T RS  becomes shorter as the number of pixels N R  is increased (Step S 135 ) and sets the calculated next short exposure time T RS  in the control unit  14  (Step S 136 ). 
     Then, during the next frame period, as shown in  FIG. 13 , capturing of R-light L R  is performed for a shorter short exposure time T RS  (Step S 4 ). Accordingly, as shown in  FIGS. 8 and 10 , an image signal S RS  in which the overexposed highlights are resolved and that has contrast in the bright area is acquired. In a case in which no overexposed highlight area exists in the R-image signal S RS , and the number of pixels N R  measured in Step S 134  is zero, the current short exposure time T RS  is still calculated and set as the next short exposure time T RS . 
     In this way, according to this embodiment, in a case in which the underexposed shadows occur because the long exposure times T RL  and T BL  are insufficient for the dark area of the living tissue S, as shown in  FIG. 15 , the long exposure times T RL  and T EL  for the next capturing are extended, thereby acquiring image signals S RL  and S BL  that have clear contrast in the dark area. Furthermore, in a case in which the overexposed highlights occur because the short exposure times T RS  and T BS  are excessive for the bright area of the living tissue S, as shown in  FIG. 15 , the short exposure times T RS  and T BS  for the next capturing are shortened, thereby acquiring image signals S RS  and S BS  that have clear contrast in the bright area. 
     There is an advantage in that, by using expanded image signals S RL ′+S RS ′ and S BL ′+S BS ′ that are generated from these image signals S RL , S RS , S BL , and S BS , it is possible to acquire an endoscope image in which red and blue of the living tissue S are more correctly reproduced in both the dark area and the bright area. 
     Note that, in this embodiment, as shown in  FIG. 16 , it is also possible to further provide an input unit  21  with which an observer selects which to prioritize: the resolution of overexposed highlights or the resolution of underexposed shadows. 
     As a result of the above-described calculation of the next long exposure times T RL  and T BL  and the next short exposure times T RS  and T BS , the sum T RL +T RS  or T BL +T BS  of the long exposure time and the short exposure time can exceed the irradiation time for the illumination light L R  or L B  per single irradiation. In such a case (YES in Step S 137 ), as shown in  FIG. 17 , the exposure-time setting unit  20  sets the long exposure times T RL  and T BL  and the short exposure times T RS  and T BS  (Steps S 138  to S 143 ) according to one of the resolution of overexposed highlights and the resolution of underexposed shadows selected by using the input unit  21  (Step S 139 ). 
     If resolution of underexposed shadows is prioritized (YES in Step S 139 ), the exposure-time setting unit  20  preferentially sets the long exposure time T RL  (Step S 140 ) and sets the short exposure time T RS  to the time obtained by subtracting the long short exposure time T RL  from the irradiation time (Step S 141 ). If resolution of overexposed highlights is prioritized (NO in Step S 139 ), the exposure-time setting unit  20  preferentially sets the short exposure time T RS  (Step S 142 ) and sets the long exposure time T RL  to a time obtained by subtracting the short exposure time T RS  from the irradiation time (Step S 143 ). 
     If the sum T RL +T RS  of the long exposure time and the short exposure time is equal to or less than the irradiation time for the illumination light L R  (NO in Step S 137 ), the next exposure times T RL  and T RS  are set as in the above-described Steps S 133  and S 136  (Step S 138 ). 
     The same applies to the exposure times T BL  and T BS  for the B-image signals. 
     By doing so, in order to observe a dark area such as a concave portion in detail, the observer prioritizes the resolution of underexposed shadows, thereby making it possible to reliably observe an endoscope image  25  in which the dark area is clearly captured, and, in order to observe a bright area such as a convex portion in detail, the observer prioritizes the resolution of overexposed highlights, thereby making it possible to reliably observe an endoscope image  25  in which the bright area is clearly captured. 
     Third Embodiment 
     Next, an endoscope apparatus according to a third embodiment of the present invention will be described with reference to  FIGS. 18 to 20 . 
     The endoscope apparatus of this embodiment is obtained by modifying the endoscope apparatus of the second embodiment and differs from the endoscope apparatus of the second embodiment in that feedback control is performed on the exposure times T RL , T RS , T BL , and T BS  for the next capturing of R-light L R  and B-light L B  on the basis of the distributions of the gradation values of pixels in a region of interest, instead of all pixels in the image signals S RL , S RS , S BL , and S BS . 
     Specifically, the endoscope apparatus of this embodiment is provided with an image processor  42  shown in  FIG. 18 , instead of the image processor  4 . The configurations other than that of the image processor  42  are the same as those in the endoscope apparatus  1  of the first embodiment shown in  FIG. 1 . 
     As shown in  FIG. 18 , the image processor  42  is further provided with: a region-of-interest input unit (region-of-interest specifying unit)  22  and a position-information setting unit  23 . 
     The region-of-interest input unit  22  is, for example, a pointing device, such as a stylus pen or a mouse, with which a position can be specified on an endoscope image displayed on the display unit  24 . As shown in  FIG. 19 , the observer can specify, as a region of interest B, a desired region in the capture range of the endoscope image  25  displayed on the display unit  24 , by using the region-of-interest input unit  22 . 
     The position-information setting unit  23  obtains the position of the specified region of interest B from the region-of-interest input unit  22 , converts the obtained position into the addresses of pixels in the endoscope image  25 , and sends the addresses to the threshold processing unit  19 . 
     The threshold processing unit  19  selects, among the pixels of the R-image signals S RL  and S RS  and the B-image signals S BL  and S BS  received from the image memory  15 , the pixels in the region of interest B according to the addresses received from the position-information setting unit  23 . Next, the threshold processing unit  19  compares the gradation values of the selected pixels with the thresholds α RL , α RS , α BL  and α BS , thereby measuring the numbers of pixels M R , N R , M B  and N B . 
     The exposure-time setting unit  20  determines the next long exposure times T RL  and T BL  on the basis of the numbers of pixels M R  and M B  and calculates the next short exposure times T RS  and T BS  on the basis of the numbers of pixels N R  and N B . However, the maximum values of the numbers of pixels M R , N R , M B , and N B  vary depending on the size of the region of interest B. Therefore, the exposure-time setting unit  20  multiplies the numbers of pixels M R , N R , M B , and N B  by the ratio C B /C of the total number of pixels C B  existing in the region of interest B with respect to the total number of pixels C in the entire endoscope image, thereby obtaining correction values M R ×C B /C, N R ×C B /C, M B ×C B /C, and N B ×C B /C for the numbers of pixels M R , N R , M B , and N B . Then, the exposure-time setting unit  20  calculates the next exposure times T RL , T BL , T RS , and T BS  from the LUTs shown in  FIGS. 11 and 12  by using the obtained correction values, instead of M or N. 
     Alternatively, the exposure-time setting unit  20  may hold a plurality of LUTs corresponding to the sizes of the region of interest B, i.e., the total numbers of pixels C B . In the plurality of LUTs, the relationships between the numbers of pixels M R , N R , M B , and N B  and the extension time or the reduction time have already been corrected according to the respective total numbers C B . The exposure-time setting unit  20  can calculate the next exposure times T RL , T BL , T RS , and T BS  by selecting an appropriate LUT according to the total number C B . 
     Next, the operation of the thus-configured endoscope apparatus will be described. 
     The main routine in this embodiment is the same as the main routine in the second embodiment shown in  FIG. 13 , and the content of an exposure-time setting routine (Step S 13 ) differs from that in the second embodiment. 
     According to the endoscope apparatus of this embodiment, as in the second embodiment, after two R-image signals S RL  and S RS  are obtained in Step S 7 , the exposure times T RL  and T RS  for obtaining next R-image signals S RL  and S RS  are set in the exposure-time setting routine S 13 . 
     In the exposure-time setting routine S 13 , as shown in  FIG. 20 , first, it is determined whether the region of interest B has been specified (Step S 144 ). 
     If the region of interest B has not been specified (NO in Step S 144 ), the next exposure times T RL , T RS , T EL , and T BS  are set according to the same procedure as that in the second embodiment (Steps S 131  to S 136 ). 
     If the region of interest B has been specified (YES in Step S 144 ), the threshold processing unit  19  measures the number of pixels M R  that have the minimum gradation value 0 among the pixels that constitute the region of interest B (Step S 145 ), corrects the measured number of pixels M R  according to the total number of pixels C B  in the region of interest R (Step S 146 ), and calculates and sets the next long exposure time T RL  on the basis of the correction value M R ×C/C B  (Steps S 147  and S 148 ). Then, the threshold processing unit  19  measures the number of pixels N R  that have the maximum gradation value 255 among the pixels that constitute the region of interest B (Step S 149 ), corrects the measured number of pixels N R  according to the total number of pixels C B  in the region of interest R (Step S 150 ), and calculates and sets the next short exposure time T RS  on the basis of the correction value N R ×C/C B  (Steps S 151  and S 152 ). For B-image signals S BL  and S BS , the next long exposure time T EL  and the next short exposure time T BS  are set in Steps S 145  to S 152 , as in the R-image signals S RL  and S RS . 
     In this way, according to this embodiment, the next exposure times T RL , T RS , T BL , and T BS  are adjusted according to the presence or absence of overexposed highlights and underexposed shadows in the region of interest B, to which the observer particularly pays attention, in the endoscope image  25 . Accordingly, there is an advantage that it is possible to acquire an endoscope image  25  that has high contrast in the region of interest B, thus making it possible to more accurately observe the region of interest B. 
     In the first to third embodiments, although capturing is performed two times for different exposure times during one irradiation period for each of R-light and B-light, instead of this, capturing may be performed three times or more. In this case, it is preferred that the exposure times for three captures be different from each other. 
     Furthermore, in the first to third embodiments, although the dynamic range is expanded in both of the R-image signal and the B-image signal, instead of this, the dynamic range may be expanded in only one of the R-image signal and the B-image signal. In this case, only the image signal of which the dynamic range is to be expanded needs to be acquired a plurality of times through a plurality of captures. 
     As a result, the following aspect is read by the above described embodiment of the present invention. 
     An aspect of the present invention provides an endoscope apparatus including: an illumination unit that sequentially radiates illumination light of three colors of red, green, and blue onto a subject; an image acquisition unit that acquires an image by capturing the illumination light reflected at the subject; a control unit that controls the image acquisition unit so as to perform capturing in synchronization with radiation of the illumination light of the three colors from the illumination unit, thereby causing the image acquisition unit to sequentially acquire component images of three colors of red, green, and blue; a dynamic-range expanding unit that generates an expanded component image in which the dynamic range is expanded, from the component image of at least one color other than green, among the component images of the three colors acquired by the image acquisition unit; and an image generating unit that generates a colored endoscope image by compositing the expanded component image of the at least one color, which is generated by the dynamic-range expanding unit, and the component images of the other colors, wherein the control unit controls the image acquisition unit so as to capture the illumination light of the at least one color a plurality of times for different exposure times, thereby causing the image acquisition unit to acquire a plurality of component images of the at least one color; and the dynamic-range expanding unit generates the expanded component image by compositing the plurality of component images of the at least one color. 
     According to the aspect, in synchronization with switching among red, green, and blue of illumination light radiated onto a subject from the illumination unit, the image acquisition unit performs capturing of the subject, thereby acquiring component images of three colors, and the image generating unit generates an RGB-format endoscope image from the acquired component images of the three colors. 
     In this case, in capturing of illumination light of red or/and blue, the control unit causes the image acquisition unit to perform capturing of illumination light of the same color a plurality of times for different exposure times, thereby acquiring a plurality of component images having different brightness. 
     The dynamic-range expanding unit composites the plurality of component images of the same color having different brightness, thereby generating a red expanded component image or/and a blue expanded component image having a wider dynamic range than the dynamic range of a green component image. In this way, by using the red component image or/and the blue component image having a wide dynamic range, it is possible to acquire an endoscope image in which the difference in color, in particular, red or/and blue, in living tissue is correctly reproduced. 
     In the above-described aspect, the control unit may control the image acquisition unit so as to capture each of the illumination light of red and the illumination light of blue a plurality of times for different exposure times and may control the exposure times for capturing the illumination light of red and the exposure times for capturing the illumination light of blue, independently from each other. 
     By doing so, the dynamic ranges of the red expanded component image and the blue expanded component image are controlled independently from each other, thus making it possible to acquire an endoscope image having higher color reproducibility in living tissue. 
     The above-described aspect may further include an exposure-time setting unit that sets exposure times for next capturing of the illumination light of the at least one color performed a plurality of times, on the basis of the distribution of gradation values of the plurality of component images of the at least one color. 
     The distribution of gradation values is biased toward a minimum gradation value side when the exposure time is insufficient, and the distribution of gradation values is biased toward a maximum gradation value side when the exposure time is excessive. The exposure-time setting unit determines excess or insufficiency of the exposure time on the basis of the distribution of gradation values, sets a longer exposure time for next capturing when the exposure time is insufficient, and sets a shorter exposure time for next capturing when the exposure time is excessive. Accordingly, it is possible to acquire a component image having appropriate contrast, in the next capturing. 
     The above-described aspect may further include a region-of-interest specifying unit that specifies a region of interest in a capture range of the component image captured by the image acquisition unit, wherein the exposure-time setting unit may set exposure times for next capturing of the illumination light of the at least one color performed a plurality of times, on the basis of the distribution of gradation values in the region of interest specified by the region-of-interest specifying unit, among the plurality of component images of the at least one color. 
     By doing so, the dynamic range of a red expanded component image or/and a blue expanded component image is optimized on the basis of the color and the brightness of the region of interest. Therefore, it is possible to ensure high color reproducibility in the region of interest in the endoscope image. 
     REFERENCE SIGNS LIST 
     
         
           1  endoscope apparatus 
           2  insertion portion 
           3  illumination unit (illumination unit) 
           4 ,  41 ,  42  image processor 
           5  illumination lens 
           6  objective lens 
           7 ,  11 ,  12  condensing lens 
           8  light guide 
           9  image acquisition device (image acquisition unit) 
           10  light source 
           13  color filter set 
           14  control unit 
           15  image memory 
           16  dynamic-range expanding unit 
           17  compression unit 
           18  image generating unit 
           19  threshold processing unit 
           20  exposure-time setting unit 
           21  input unit 
           22  region-of-interest input unit (region-of-interest specifying unit) 
           23  position-information setting unit 
           24  display unit 
           25  endoscope image