Abstract:
An endoscope system has a light source system configured to emit illumination light on a target area, a scanner configured to periodically scan the illumination light over a target area at predetermined time intervals, and an imager configured to receive the illumination light reflected from the target area and to acquire in succession a sequence of image-pixel signals. Further, the endoscope system has a luminance detector that detects in succession a sequence of luminance data of the object image from the sequence of image-pixel signals; and a brightness adjuster that adjusts the brightness of an observed image on the basis of the sequence of luminance data. Then, the brightness adjuster adjusts an amount of illumination light in accordance with a scanning position of the illumination light.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an endoscope system that scans illumination light over a target to be observed, such as tissue. In particular, it relates to adjusting the brightness of an observed image. 
         [0003]    2. Description of the Related Art 
         [0004]    An endoscope system with scanning functionality is equipped with a scanning fiber such as a single mode type fiber, which is provided in an endoscope. As described in Seibel et al. (U.S. Pat. No. 6,294,775), the tip portion of the scanning fiber is held by an actuator such as a piezoelectric device, which vibrates the tip portion spirally by modulating and amplifying the fiber&#39;s vibration. Consequently, illumination light passing trough the scanning fiber is spirally scanned over an observation area. 
         [0005]    Light reflected off the observation area is entered into an image fiber, and is transmitted to a processor via the image fiber. The transmitted light is transformed to image-pixel signals by photosensors, and one frame&#39;s worth of image is generated successively. The spiral scanning is periodically carried out on the basis of a predetermined time-interval (frame rate), and one frame&#39;s worth of image-pixel signals are read from the photosensors in accordance with the frame rate. 
         [0006]    Usually, the amount of illumination light for one frame interval is nearly uniform over an entire observation area. However, if the depth from the scope tip portion to an observation area is not uniform, a part of the object image may become extremely bright and/or dark. For example, when capturing an inner surface of an organ under condition where the scope tip portion faces an axis of the organ, the center portion of the image becomes too dark, whereas the surrounding portion of the image becomes too bright so that a so-called halation occurs in the surrounding portion. On the other hand, when carrying out image processing for correcting the brightness of an observed image, image quality degrades. 
       SUMMARY OF THE INVENTION 
       [0007]    An object of the present invention is to provide an endoscope system that is capable of adjusting the brightness of an observed image without image processing. 
         [0008]    An endoscope system according to the present invention has a light source system configured to emit illumination light on a target area, a scanner configured to periodically scan the illumination light over a target area at predetermined time intervals, and an imager configured to receive the illumination light reflected from the target and to acquire in succession a sequence of image-pixel signals. 
         [0009]    For example, a light source that emits R, G, and B light as illumination light and a light source driver that controls the intensity of illumination light are provided. R, G, and B light may be emitted simultaneously. As for the scanner, a fiber driver that vibrates the tip portion of a scanning fiber in two dimensions to scan the illumination light may be provided. Reflected light may be detected by photosensors in the scope tip portion. 
         [0010]    The endoscope system has a luminance detector that detects in succession a sequence of luminance data of the object image from the sequence of image-pixel signals; and a brightness adjuster that adjusts the brightness of an observed image on the basis of the sequence of luminance data. The sequence of luminance data may be successively detected in each frame interval, and the brightness is adjusted in each frame interval on the basis of the luminance data detected in a previous frame interval. 
         [0011]    In the present invention, the brightness adjuster adjusts an amount of illumination light in accordance with a scanning position of the illumination light. Namely, an amount of illumination light in each frame interval can be changed in time-sequence, which is different from the conventional endoscope system with a scanning function. An amount of illumination light may be adjusted by controlling intensity of light, or by opening/closing a stop (diaphragm). 
         [0012]    Since an amount of illumination can be changed in each scanning position, the brightness of an observed image is adjusted in each pixel. Namely, a part of the observed image can be corrected without image processing. 
         [0013]    The brightness adjustment may be applied in accordance with various purposes other than a noise reduction. For example, the brightness adjuster may adjust the amount of illumination light incident on a scanning position corresponding to an edge portion of an observed image. 
         [0014]    Usually, the center portion of the image becomes too dark, whereas the surrounding portion of the imago becomes too bright during observation. Therefore, the brightness adjuster may increase the amount of illumination light incident on a scanning position corresponding to a pixel having a relatively low luminance level, and may decrease the amount of illumination light incident on a scanning position corresponding to a pixel having a relatively high luminance level. Specifically, the brightness adjuster may adjust the amount of illumination light so as to narrow the range of luminance level variance. In this case, the gray scale of an observed image is maintained before and after the brightness correction. 
         [0015]    Also, to make a change of the brightness from the central portion to the surrounding portion a gradual change, the brightness adjuster may increase the illumination light by a large amount as a detected luminance level of a scanning position is low within a relative low range of luminance levels, and may decrease the amount of illumination light by a large amount as a detected luminance level of a scanning position is high within a relative high range of luminance levels. For example, the brightness adjuster increases or decreases the amount of illumination light incident on each scanning position in accordance with a difference between a detected luminance level of a scanning position and a predetermined target luminance level. 
         [0016]    When the brightness of an observed image is almost proper, the brightness adjustment may not be carried out. For example, the brightness adjuster suspends an adjustment of the amount of illumination light in a subsequent scanning interval when the percentage of luminance data that fall outside of the upper or lower limit tolerance level is more than a threshold value (for example, 10%). 
         [0017]    An apparatus for adjusting the brightness of an observed image according to another aspect of the present invention has a luminance detector that detects in succession a sequence of luminance data of the object image on the basis of a sequence of image-pixel signals that are acquired in succession by illumination light reflected from the target area when periodically scanning the illumination light over the target area; a determiner that determines whether or not a brightness adjustment is necessary, on the basis of the sequence of luminance data; and a brightness adjuster that adjusts the brightness of an observed image on the basis of the sequence of luminance data when the brightness adjustment is necessary. The brightness adjuster adjusts an amount of illumination light in accordance with a scanning position or the illumination light. 
         [0018]    A computer readable medium that stores a program for adjusting the brightness of an observed image according to another aspect of the present invention has et luminance detection code segment that detects in succession a sequence of luminance data of the object image on the basis of a sequence of image-pixel signals that are acquired in succession by illumination light reflected from the target area when periodically scanning the illumination light over the target area; a determination code segment that determines whether or not a brightness adjustment is necessary, on the basis of the sequence of luminance data; and a brightness adjustment code segment that adjusts the brightness of an observed image on the basis of the sequence of luminance data when the brightness adjustment is necessary. The brightness adjustment code segment adjusts an amount of illumination light in accordance with a scanning position of the illumination light. 
         [0019]    A method for adjusting the brightness of an observed image according to another aspect of the present invention includes: a) detecting in succession a sequence of luminance data of the object image on the basis of a sequence of image-pixel signals that are acquired in succession by illumination light reflected from the target area when periodically scanning the illumination light over the target area; b) determining whether or not a brightness adjustment is necessary, on the basis of the sequence of luminance data; and c) adjusting the brightness of an observed image on the basis of the sequence of luminance data when the brightness adjustment is necessary, the adjusting comprising adjusting an amount of illumination light in accordance with a scanning position of the illumination light. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The present invention will be better understood from the description of the preferred embodiments of the invention set forth below together with the accompanying drawings, in which: 
           [0021]      FIG. 1  a block diagram of an endoscope system according to a first embodiment; 
           [0022]      FIG. 2  is an illustration of the scanning optical fiber, scanning unit, and spiral scan pattern; 
           [0023]      FIG. 3  is a timing chart of a brightness adjustment process; 
           [0024]      FIG. 4  is an illustration of an observed image displayed on a screen; 
           [0025]      FIG. 5  a flowchart of a brightness adjustment process carried out by the system controller  40 ; 
           [0026]      FIGS. 6A and 6B  illustrate a relationship between a detected luminance level in a frame interval and a target luminance level in a subsequent frame interval; 
           [0027]      FIG. 7  illustrates a brightness adjustment when an amount of illumination light exceeds a limit value; and 
           [0028]      FIG. 8  is a timing chart of a luminance level and an amount of illumination light according to the second embodiment. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0029]    Hereinafter, the preferred embodiments of the present invention are described with reference to the attached drawings. 
         [0030]      FIG. 1  is a block diagram of an endoscope system according to a first embodiment.  FIG. 2  is an illustration of the scanning optical fiber, scanning unit, and spiral scan pattern. 
         [0031]    The endoscope system is equipped with a processor  30  and an endoscope  10  that includes a scanning fiber  17  and an image fiber  14 . The single mode type of scanning fiber  17  transmits illumination light, whereas the image fiber  14  transmits light that is reflected off an observation target S such as tissue. The endoscope  10  is detachably connected to the processor  30 , and the monitor  60  is connected to the processor  30 . 
         [0032]    The processor  30  has three lasers  20 R,  20 G, and  20 B, which omit red, green, and blue light, respectively. The lasers  20 R,  20 G, and  20 B are driven by three laser drivers  22 R,  22 G, and  22 B, respectively. The simultaneously emitted red, green, and blue light is collected by half-mirror sets  24  and a collection lens  25 . Consequently, white light enters into the scanning fiber  17  and travels to the tip portion  10 T of the endoscope  10 . The light exiting from the scanning fiber  17  illuminates the target S. 
         [0033]    As shown in  FIG. 2 , a scanning unit  16  is provided in the scope tip portion  10 T. The scanning unit  16  has a cylindrical actuator  18  and scans illumination light over the target S. The optical fiber  17  passes through the axis of the actuator  18 . The fiber tip portion  17 A, which cantilevers from the actuator  18 , is supported or held by the actuator  18 . 
         [0034]    The actuator  10  fixed at the scope tip portion  10 T is, herein, a piezoelectric tubular actuator that resonates the fiber tip portion  17 A in to dimensions. Concrete speaking, the actuator  18  vibrates the fiber tip portion  17 A with respect to two axes that are perpendicular to one other, in accordance with a resonant mode. The vibration of the fiber tip portion  17 A spirally displaces the position of the fiber end surface  175  from the axial direction of the optical fiber  17 . 
         [0035]    The light emitted from the end surface  17 S of the scanning fiber  17  passes through an objective lens  19 , and reaches the target S. A pattern traced by a scanning beam, i.e., a scan line PT Perms spiral pattern (see  FIG. 2 ). Since a spiral interval AT in a radial direction is tight, the total of the observation area S is illuminated by spirally scanned light. 
         [0036]    Light reflected from the target S enters the image fiber  14  and is transmitted to the processor  30 . When the reflected light exits from the image fiber  14 , it is divided into R, G, and B light by an optical lens  26  and half-mirror sets  27 . the separated R, G, and B light then continues on to photosensors  28 R,  28 G,  28 B, respectively, which transforms the R, G, and B light to image-pixel signals corresponding to colors “R”, “G” and “B”. 
         [0037]    The generated analog image-pixel signals are converted to digital image-pixel signals by A/D converters  29 R,  29 G, and  29 B and then fed into a signal processing circuit  32 , in which a mapping process is carried out. The successively generated digital R, G, and B image-pixel signals are arrayed in accordance to the order of a spiral scanning pattern. In the mapping process, each of the digital R, G, and B image-pixel signals are associated with a corresponding scanning position, so that raster-arrayed image-pixel signals are formed. Consequently, the pixel position of each of the R, G, and B digital image-pixel signals is identified, in order, and one frame&#39;s worth of digital R, G, and B image-pixel signals are generated successively. 
         [0038]    In the signal processing circuit  32 , the generated two-dimensional image-pixel signals are subjected to various image processing, including a white balance process so that video signals are generated. The generated video signals are sent to the monitor  60  via an encoder  37 , thus an observed image is displayed on the monitor  60 . 
         [0039]    A system controller  40 , which includes a ROM unit, a RAM unit, and a CPU, controls the action of the video processor  30  and the videoscope  10  by outputting control signals to the signal processing circuit  32 , the laser driver  22 R,  22 G, and  22 B, etc. A control program is stored in the ROM unit. A timing controller  34  outputs synchronizing signals to fiber drivers  36 A,  36 B for driving the scanning unit  16 , the laser driver  22 R,  22 G, and  22 B to synchronize the vibration of the fiber tip portion  17 A with the timing of the emission of light. The system controller  40  reads data associated with a brightness adjustment process from an image memory  30  and a laser output memory  33 . 
         [0040]    The output of lasers  20 R,  20 G, and  20 B is controlled by driving signals fed from the laser drivers  22 R,  22 G, and  228 , respectively. Thus, an amount of illumination light (intensity of light) incident on a target S is adjustable. In the signal processing circuit  32 , luminance signals are generated from the digital image-pixel signals and are transmitted to the system controller  40 . The system controller  40  outputs control signals to the laser drivers  22 R,  22 G, and  22 B as to adjust an amount of illumination light in accordance with a scanning position of the illumination light. 
         [0041]      FIG. 3  is a timing chart of a brightness adjustment process.  FIG. 4  is an illustration of an observed image displayed on a screen. Hereinafter, the adjustment of an amount of illumination light is explained with reference to  FIGS. 3  and  4 . 
         [0042]    In  FIG. 3 , an amount of illumination light LM emitted from the lasers  20 R,  20 G, and  20 B is shown with the amplitudes of the fiber tip portion  17 A, and time-sequence of luminance data PL obtained in time sequence is also shown. One frame&#39;s worth of image-pixel signals are detected in an interval “RA”, which the fiber tip portion spirally vibrates within one frame interval. The fiber tip portion  17 A returns to a central position in an interval “RB” within the same frame interval. 
         [0043]    A screen shown in  FIG. 4  indicates an observed image when the scope tip portion  10 A faces the center of an internal organ such as a digestive organ. The luminance data PL in the frame interval (A) shown in  FIG. 3  is obtained based on the image shown in  FIG. 4 . Note that an amount of illumination light “LM” is herein constant in the frame (A) for ease of explanation. The value of luminance data is, for example, represented in 256 steps (e.g.: a value is in the range of 0 to 255). 
         [0044]    In the case of the image shown in  FIG. 4 , the luminance level of the luminance data PL gradually increases for screen positions that are further away from a central portion A 1  and closer to a surrounding portion A 2 . The center portion A 1  of the screen becomes extremely dark, whereas the surrounding portion A 2  becomes very bright, so that a so-called halation partially occurs within the surrounding portion A 2 . Since the scanning line forms a spiral pattern, the luminance level of an interval Q 1 , corresponding to the central portion A 1  of the image, is close to a minimum level in the sequence of the luminance data PL, whereas the luminance level of an interval Q 2 , corresponding to the image&#39;s surrounding portion A 2 , is close to a maximum level. 
         [0045]    Accordingly, while scanning illumination light in a next frame interval, the system controller  40  changes an amount of illumination light in accordance with a scanning position (spot position) in order to adjust the brightness of observed image for each scanned position, i.e., each pixel of the observed image. As shown in  FIG. 3 , in the next frame interval (B), an amount of illumination light LM is changed in each scanning position, Concretely, the amount of illumination light LM relatively increases at the center portion A 1 , and relatively decreases at the surrounding portion A 2 . 
         [0046]    Consequently, in frame interval (B), the range RR of luminance levels becomes narrow compared with that of frame interval (A). Thus, a luminance difference between a bright portion and a dark portion of the observed image does not become too large, and the luminance difference is restricted. In  FIG. 4 , an observed image according to the brightness adjustment in the frame interval (B) is shown. The brightness at both the central portion A 1  and the surrounding portion A 2  is maintained at a proper brightness. No extreme dark portion or halation exits in the observed image. This brightness adjustment is carried out in each frame interval. 
         [0047]      FIG. 5  is a flowchart of a brightness adjustment process carried out by the system controller  40 . The process is carried out during each scanning cycle (for example, 1/30 time interval).  FIGS. 6A and 6B  illustrate a relationship between a detected luminance level in a frame interval and a target luminance level in a subsequent frame interval.  FIG. 7  illustrates a brightness adjustment when an amount of illumination light exceeds a limit value. The brightness adjustment process is explained with reference to  FIGS. 5 to 7 . 
         [0048]    In Step S 101 , flags fH and fL are set to zero. The flags fH and fL are variable numbers for determining whether an amount of illumination light should be adjusted in a subsequent frame interval; each flag is set to either 0 or 1. Step  102 , a determination is made as to whether or not luminance data and illumination light data from a previous frame interval are stored in the image memory  31  and the laser output memory  33 , respectively. When the data in not stored in the image memory  31  and the laser output memory  33 , the process proceeds to Step S 109 . 
         [0049]    On the other hand, when it is determined that the data from the previous frame interval are stored in the image memory  31  and the laser output memory  33 , the process continues to Step S 103 , in which it is determined whether the flag fH or the flag fL is 1. as described later, when it is not necessary to adjust an amount of illumination light, the flags fH and fL are set to zero. When it is necessary to adjust an amount of illumination light in a subsequent frame interval, one of the flags fH and fL is set to 1. 
         [0050]    In Step S 104 , an amount of illumination light that is emitted from the lasers  20 R,  20 G, and  20 B is obtained in accordance with the following formulae. Note “L n+1 ” represents an output level in the number “n+1” frame interval, and “L n ” represents an output level in the number “n” frame interval. 
         [0000]        L   n+1   =L   n   −L   n *( Y   n   −Y   n+1 )/100  (1) 
         [0051]    (Note Y&gt;Y max /2) 
         [0000]        L   n+1   =L   n   +L   n *( Y   n   −Y   n+1 )/100  (2) 
         [0052]    (Note Y n ≦Y max /2) 
         [0053]    “Y n ” indicates a detected luminance level of each scanning position (pixel), which is detected in the number “n” frame interval. On the other hand, “Y n+1 ” indicates a target luminance level of each pixel at the number “n+1” frame interval. As shown in the following formula, “Y n+1 ” is represented as a function of “Y n ”. 
         [0000]        Y   n+1   =Y   n *(3/5)+20  (3) 
         [0054]    Hereinafter, a brightness correction represented by formulas (1) to (3) is explained in detail. The graph shown in  FIG. 6A , illustrates a relationship between “Y n ” and “Y n+1 ” in a state that the brightness adjustment is not needed. The vertical axis represents a target luminance level “Y n+1 ” in the number “n+1” frame interval. Note the value of the target luminance level is designated by a percentage (%) of a maximum luminance level. The horizontal axis represents a detected luminance level “Y n ” (also a percentage of the maximum) in the number “n” frame interval. 
         [0055]    If the brightness adjustment is not carried out in the number “n+1” frame interval, then a target luminance level “Y n+1 ” is directly set to the detected luminance level “Y n ”. Since a target luminance level “Y n+1 ” in a given pixel coincides with a corresponding detected luminance level “Y n ”, a relationship between a target luminance level “Y n+1 ” and a detected luminance level ‘Y n ” is represented by a straight line XL. Therefore, a laser output level “L n+1 ” in the number “n+1” frame interval is directly set to a laser output level from the number “n” frame interval, as can be seen from formulas (1) and (2). The output levels of the lasers  20 R,  20 G, and  20 B are the same as each other. 
         [0056]    On the other hand, when carrying out a brightness correction, the luminance level ‘Y n+1 ” in the number “n+1” frame interval is decided in accordance with formula (3). In  FIG. 6B , a straight line KL 1  is shown. The laser output level “L n+1 ” is calculated in accordance with formula (1) or (2). Based on the detected luminance level “Y n ”, it is determined whether an amount of illumination light should be increased or decreased. Concretely, when a detected luminance level “Y n ” is greater than 50% of the maximum luminance level “Y max ” the illumination light is increased by an amount determined by formula (1). When the luminance level “Y n ” is equal to or less than 50% of the maximum luminance level “Y max ”, the illumination light is decreased by an amount determined by formula (2). 
         [0057]    The laser output level “L n+1 ” is set such that an amount of illumination light becomes smaller as a detected luminance level “Y n ” is higher, and becomes larger as a detected luminance level “Y n ” is lower, as shown by the straight line KL 1 . The decreasing/increasing amount depends upon a difference between “Y n ” and “Y n+1 ”. 
         [0058]    This brightness correction narrows the range of target luminance levels. In  FIG. 6B , the range of target luminance levels changes from the range 0-100% to the range 20%-80%. Thus, an extremely dark or halation portion does not occur. The brightness correction is carried out in each scanning position or pixel. After the process in Step S 104  is carried out, the process continues to Step S 105 . 
         [0059]    In Step S 105 , it is determined whether a laser output level calculated by either formula (1) or (2) is greater than or equal to an upper limit value TL. The upper limit value TL is predetermined in accordance to the performance of the lasers  20 R,  20 G, and  20 B. When it is determined that the calculated laser output level is greater than the upper limit value TL, an actual laser output level is set to the upper limit value TL (S 106 ). 
         [0060]    In Step S 107 , it is determined whether a laser output level calculated by either formula (1) or (2) is less than or equal to a lower limit value TU. When it is determined that the calculated laser output level is less than the lower limit value TU, an actual laser output level is set to the lower limit value (S 108 ). 
         [0061]    Hereinafter, the processes of Steps S 105  to S 108  are explained in detail, In  FIG. 7 , a change in an amount of illumination light is shown. In the first frame interval, the amount of illumination light is constant. Note that the horizontal axis represents a scanning time of one frame interval. In the next frame interval the amount of illumination light is adjusted by a quantity that is determined by either formula (1) or (2), such that the amount of illumination light gradually decreases. Namely, an amount of illumination light increases the closer it is to the center of an observed image, but decreases as it becomes closer to the outer portion. 
         [0062]    If further brightness correction is necessary, an amount of illumination light in the next frame interval could increase or decreases the amount of illumination light too much at the center portion A 1  or the surrounding portion A 2  of the observed image. In this case, the calculated laser output level is either greater than the upper limit value TL or less than the lower limit value TU. Therefore, the calculated laser output level is changed or corrected to the upper limit value TL or lower limit value TU at the center portion A 1  or the surrounding portion A 2  of the observed image. 
         [0063]    After an amount of illumination light is decided for each pixel in Steps  104 - 108 , in Step S 109  the output levels of the lasers  20 R,  20 G, and  20 B are controlled in accordance to the decided amount. The amount of illumination light in a subsequent frame interval is adjusted in accordance to the vibration of the fiber tip portion  17 A. 
         [0064]    In Step S 110 , laser output levels obtained in Step S 104  are stored in the output level memory  33 . Also, a luminance level for each spot or pixel is detected during a spiral scanning procedure. The luminance level of one detected frame is stored in the image memory  31 . Herein, histogram data of the luminance levels and the laser output levels are generated and stored in the image memory  31  and the output level memory  33 , respectively. 
         [0065]    In Step S 111 , it is determined whether a percentage of luminance levels that exceed an upper limit tolerance value WU is equal or greater than 10% of one frame&#39;s worth of detected luminance levels. The predetermined upper limit tolerance value WU represents an upper level of the luminance range that does not interfere with an observation. When the percentage is found to be equal or greater than 10%, it is determined that carrying out the brightness correction in the next frame interval is necessary, and the flag fH is set to 1 (Step  112 ). On the other hand, when the percentage is found to be smaller than 10%, the brightness correction is not carried out in a next frame interval and the flag fH is set to 0 (S 113 ). 
         [0066]    In Step S 114 , it is determined whether the percentage of luminance levels that are below a lower limit tolerance value WL is equal to or greater than 10% of one frame&#39;s worth of detected luminance levels, and whether the percentage of calculated laser output levels that exceed the upper limit value TL described in Step S 105  is less than 50%. The lower limit tolerance value WL represents a limit level that does not interfere with an observation. 
         [0067]    When the percentage of luminance levels that are below a lower limit tolerance value WL is equal to or greater than 10%, and 50% or less of the calculated laser output levels exceed the limited upper value TL, it is determined that carrying out the brightness correction in the next frame interval is necessary, and the flag fL is set to 1 (Step  115 ). On the other hand, when it is determined that the percentage of luminance levels is less than 10%, or more than 50% of the calculated laser output levels exceed the limited upper value TL, it is determined that the brightness correction in the next frame interval is not carried out and the flag fL is set to 0 (S 116 ). 
         [0068]    In Step S 117 , it is determined whether an operation for terminating an observation has been performed. When it is determined that the operation has been performed, the process is terminated. 
         [0069]    In this way, in the present embodiment, the fiber tip portion  17 A vibrates spirally in earth frame interval while scanning illumination light over a subject that is observed successively. Thus, one frame&#39;s worth of luminance data is detected subsequently. When an extreme dark portion and/or halation portion, which exceeds a tolerance level, exists in an observed image (Steps S 111  and S 113 ), the brightness correction is carried out in the next frame interval (Steps S 112  and S 115 ). 
         [0070]    In the next frame interval, an amount of illumination light, namely laser output levels of the lasers  20 R,  20 G, and  20 B, are calculated on the basis of formulas (1) to (3) (Steps S 104 , S 106 , and S 108 ). A decreasing or increasing amount of illumination light is determined on the basis of a difference between a target luminance level and a detected luminance level in each pixel. Then, the lasers  20 R,  20 G, and  20 B are controlled in accordance to a scanning position or pixel such that a laser output level is decreased for a pixel having a relatively high luminance level in the previous frame interval, whereas an a laser output level is increased for a pixel having a relatively low luminance level (Step S 109 ). If the brightness correction is not necessary, a brightness adjustment process is not carried out in a subsequent frame interval (Steps S 103 , S 113 , and S 116 ). 
         [0071]    The brightness correction based on formulas (1) to (3) maintains a degree of a gray scale in an object image before and after the brightness correction. In addition, the brightness correction makes the change of the brightness from the center portion to the surrounding portion a gradual change. Thus, an operator can diagnose an issue with ease by observing an object image with a proper brightness. 
         [0072]    With reference to  FIG. 8 , the second embodiment is explained. The second embodiment is different from the first embodiment in that a brightness adjustment process is carried out to enhance an edge or contoured portion of an observed image. 
         [0073]      FIG. 8  is a timing chart of a luminance level and an amount of illumination light according to the second embodiment. Luminance data shown in  FIG. 8  include an edge portion E that corresponds to a boundary between a dark portion P and a bright portion Q. The dark portion P exists in a central portion of an observed image, whereas the bright portion Q exists in the surrounding portion. In this case, the laser output levels of the lasers  20 R,  20 G, and  20 B are controlled such that the edge portion E is emphasized in a subsequent frame interval. Concretely, a laser output level is controlled so as to cause a difference to occur substantially before and after an edge portion E as shown in an arrow ZZ. Thus, a difference between a high luminance level P′ and a low luminance level Q′ adjacent to the edge portion E is enlarged and the contours of an object image are clearly displayed. 
         [0074]    In the first and second embodiments, the brightness adjustment process is carried out in each frame interval. However, a luminance detection process and an output adjustment process may be carried out alternately over two frame intervals. In this case, for example, a luminance level is detected in the first frame interval in a state of constant illumination light, and a laser output level is adjusted in the second frame interval. 
         [0075]    The brightness correction may be carried out for an optional purpose other than a noise reduction process for eliminating halation, extreme darkness, or an edge enhancement such as described. Furthermore, a laser output level may be calculated by a method other than that shown in formulas (1) to (3). For example, a brightness adjustment process may be carried out for only pixels corresponding to a halation portion. 
         [0076]    As for the scanning procedure, a construction other than the scanning fiber may be applied. For example, illumination light may be scanned by driving an optical lens provided in the fiber. Also, a laser output level may be adjusted in each of the R, G, and B light. Furthermore, a light source system other than a laser system may be applied. 
         [0077]    The present disclosure relates to subject matter contained in Japanese Patent Application No. 2008-291846 (filed on Nov. 14, 2008), which is expressly incorporated herein, by reference, in its entirety.