Patent Publication Number: US-2009221875-A1

Title: Endoscope light source system and endoscope unit

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an endoscope light source system which supplies white light and excitation light to a single light guide with a simple configuration, and which enables both a normal endoscope and an autofluorescence endoscope to produce an acceptable white light image. 
     2. Description of the Related Art 
     There is known an autofluorescence endoscope which enables a user to observe an optical autofluorescence image from tissue by shining excitation light, such as ultraviolet light, onto the tissue. In order to transmit excitation light for illuminating a subject around a peripheral area of an insert tube, a light guide is mounted in an autofluorescence endoscope. The light guide is used for transmitting white light for illuminating a subject in order to generate a normal image. 
     A mirror is mounted which can be inserted into and removed from the optical path of the white light in order to make either white light or excitation light incident on the light guide. When the mirror is removed from the optical path, white light strikes the light guide. On the other hand, when the mirror is inserted into the optical path, the excitation light is reflected and made incident on the light guide. However, since a mechanism for moving the mirror is necessary, the structure of the light source apparatus increases in size and complexity. 
     Japanese Unexamined Patent Publications Nos. 2005-342033 and 2005-342034 propose that a dichroic mirror which reflects only the excitation light component is fixed on an optical path. In such a structure, if an autofluorescence image is desired, only excitation light reflected by the dichroic mirror is made incident on the light guide. On the other hand, if a normal image is desired, white light passing through the dichroic mirror is made incident on the light guide and the excitation light is turned off. Since a mechanism for moving a mirror is unnecessary, the structure of the light source apparatus can be smaller and simpler. 
     However, the problem arises that an acceptable white light image cannot be produced because the components of the white light in the spectral range of the excitation light are reflected by the dichroic mirror. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide an endoscope light source system that supplies white light and excitation light to a light guide, has a simple configuration, and enables both a normal endoscope and an autofluorescence endoscope to produce an acceptable white light image. 
     According to the present invention, an endoscope light source system comprising first and second light sources and an adjustment circuit, is provided. The first and second light sources respectively make first and second lights incident on an incident end of a light guide mounted in an endoscope. The first and second lights have first and second wavelength bands, respectively. The adjustment circuit adjusts the incident amounts of the first and/or second lights to satisfy a first relation if the first and second lights are simultaneously made on the incident end. The incident amounts of the first and second lights are the amounts of the first and second lights made incident on the incident end. 
     Furthermore, the first relation is determined so that the emission amounts of the first and second lights satisfy a second relation upon making the first and second lights simultaneously incident on the incident end. The emission amounts of the first and second lights are the amounts of the first and second lights emitted from an exit end of the light guide, respectively. 
     A ratio of the emission amount of the first light to the emission amount of the second light is constant in the second relation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects and advantages of the present invention will be better understood from the following description, with reference to the accompanying drawings in which: 
         FIG. 1  is a perspective view of an endoscope unit having an endoscope light source system of an embodiment of the present invention; 
         FIG. 2  is a block diagram showing the internal structure of a light-source unit; 
         FIG. 3  is a spectrograph showing the reflectance of the dichroic mirror; 
         FIG. 4  is a spectrograph showing the spectroscopic properties of the excitation light; 
         FIG. 5  is a spectrograph showing the spectroscopic properties of light emitted by the light-source unit when the white light and the excitation light are simultaneously emitted by the lamp and the laser source; 
         FIG. 6  is a graph showing the relationship between the aperture ratio and the amount of white light passing through the diaphragm; 
         FIG. 7  is a graph showing the relationship between the duty of the laser source and the amount of the excitation light emitted by the laser source; 
         FIG. 8  is a first flowchart illustrating the initialization operation for white balance carried out by the system controller; 
         FIG. 9  is a second flowchart illustrating the initialization operation for white balance carried out by the system controller; and 
         FIG. 10  is a flowchart illustrating the operation for control of light amount carried out by the system controller when a captured image is displayed using a normal endoscope; 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is described below with reference to the embodiment shown in the drawings. 
     In  FIG. 1 , an endoscope unit  10  comprises an endoscope processor  20 , an electronic endoscope  50 , and a monitor  11 . The endoscope processor  20  is connected to the electronic endoscope  30  and the monitor  11 . 
     The endoscope processor  20  emits light to illuminate a desired subject. An optical image of the illuminated subject is captured by the electronic endoscope  50 , and then the electronic endoscope  50  generates an image signal. The image signal is sent to the endoscope processor  20 . 
     The endoscope processor  20  carries out predetermined signal processing on the received image signal. The image signal, having undergone predetermined signal processing, is sent to the monitor  11 , where an image corresponding to the received image signal is displayed. 
     The endoscope processor  20  comprises a light-source unit  30 , an image-processing unit  21 , an imaging device driver  22 , a system controller  23  (the determination circuit), an input block  24  (the switch), and other components. 
     As described below, the light-source unit  30  emits light for illuminating a subject toward an incident end of a light guide  51 . In addition, as described below, the image-processing unit  21  carries out predetermined signal processing on the image signal. The imaging device driver  22  drives an imaging device  52  (detector) to capture an optical image of the subject. The system controller  21  controls the operations of all components of the endoscope unit  10 . Following the user&#39;s input to the input block  24 , various functions of the endoscope unit  10  are carried out. 
     The light-source unit  30  and a light-guide  51  mounted in the electronic endoscope  30  are optically connected by connection of the endoscope processor  20  to the electronic endoscope  50 . This connection also results in, electrical connections being made between the image-processing unit  21  and the imaging device  52  mounted in the electronic endoscope  50 , and between the imaging device driver  22  and the imaging device  52 . 
     As shown in  FIG. 2 , the light-source unit  30  comprises a lamp  31  (the first light source), a laser source  32  (the second light source), a diaphragm  33 , a rotary shutter  34 , a dichroic mirror  35 , a condenser lens  36 , a collimator lens  37 , a power supply circuit  38 , a diaphragm motor  39 , a shutter motor  40 , a light-amount control circuit  41  (the adjustment circuit), a shutter control circuit  42 , and other components. 
     The lamp  31 , such as a xenon lamp or a halogen lamp, emits white light (the first light). The diaphragm  33 , the rotary shutter  34 , a dichroic mirror  35 , and the condenser lens  36  are mounted on the optical path between the lamp  31  and the incident end of the light guide  51 . 
     The amount of the white light incident on the incident end is controlled by adjusting the aperture ratio of the diaphragm  33 . The aperture ratio of the diaphragm  33  is adjusted by the motor  39 . The movement of the motor  39  for driving the diaphragm  33  is controlled by the light-amount control circuit  41 . 
     As described later, the amount of light received by the imaging device  52  is communicated to the light-amount control circuit  41  via the system controller  23 . The light-amount control circuit  41  controls the aperture ratio on the basis of the communicated amount of light. 
     The rotary shutter  34  has an aperture area and a blocking area. When white light should be allowed to pass, the aperture area is inserted into the optical path of the white light. When white light should be blocked, the blocking area is inserted into the optical path of the white light. 
     The rotary shutter  34  is rotated by the shutter motor  40 . The passage and blocking of the white light from the light-source unit  30  are alternated by driving the shutter motor  40 . The movement of the shutter motor  40  is controlled by the shutter control circuit  42 . The shutter control circuit  42  is controlled by the system controller  23 . 
     The dichroic mirror  35  is fixed so that the angle between the surface of the dichroic mirror  35  and the optical path of the white light is 45 degrees. As shown in FIG.  3 ., the dichroic mirror  35  reflects light of a wavelength band less than or equal to a first wavelength, and passes light of a wavelength band greater than the first wavelength. Accordingly, a first light component, which is included in the white light emitted from the lamp  31  and whose wavelength band is greater than the first wavelength, passes through the dichroic mirror  35 . A second light component, which is included in the white light emitted from the lamp  31  and whose wavelength band ranges less than and equal to the first wavelength, is reflected by the dichroic mirror  35 . 
     The laser source  32  emits excitation light (second light) which makes tissue autofluoresce. The excitation light is blue, and the wavelength band of the excitation light ranges below the first wavelength, as shown in  FIG. 4 . Accordingly, the dichroic mirror  35  reflects the excitation light. The laser source  32  is fixed so that the excitation light reflected by the dichroic mirror  35  strikes the incident end of the light guide  51 . 
     The collimator lens  37  is mounted in the optical path between the laser source  32  and the dichroic mirror  35 . The collimator lens  37  collimates the excitation light emitted by the laser source  32 . 
     The white light component passing through the dichroic mirror  35  and/or the excitation light reflected by the dichroic mirror  35  is condensed by the condenser lens  36 , and is directed to the incident end of the light guide  31 . 
     The power supply circuit  38  supplies the lamp  31  with power. The system controller  23  controls the supply of power, and switches the lamp  31  on and off. 
     The laser source  32  is driven by the light-amount control circuit  41 . The amount of excitation light emitted by the laser source  32  is controlled by the light-amount control circuit  41 . As described later, the duty of the laser source  32  is adjusted according to the aperture ratio of the diaphragm  33 , and the amount of the emitted excitation light is controlled. As described later, a corresponding relation between the duty and the aperture ratio is determined on an initialization operation for white balance. 
     When a normal endoscope is connected to the endoscope processor  20 , only a white-light image can be observed. When the autofluorescence endoscope is connected to the endoscope processor  20 , either a white-light image or an autofluorescence image can be observed. In addition, a white-light image and an autofluorescence image may be simultaneously displayed, or a false color image generated by synthesizing a white-light image and an autofluorescence image may be displayed. 
     When a white-light image is to be observed, the shutter control circuit  42  orders the rotary shutter  34  to pass the white light by inserting the aperture area into the optical path, and the light-amount control circuit  41  orders the laser source  32  to emit the excitation light. As a result, the first light component and the excitation light arrive at the incident end of the light guide  51  (see  FIG. 5 ). 
     On the other hand, when an autofluorescence image is to be observed, the shutter control circuit  42  orders the rotary shutter  34  to block the white light by inserting the blocking area into the optical path, and the light-amount control circuit  41  orders the laser source  32  to emit the excitation light. As a result, the excitation light is made incident on the incident end of the light guide  51  (see  FIG. 4 ). 
     Next, the structure of the electronic endoscope  50 , an autofluorescence endoscope, is explained in detail. As shown in  FIG. 1 , the electronic endoscope  50  comprises the light guide  51 , the imaging device  52 , an exciting-light cut-off filter  53 , a diffuser lens  54 , an object lens  55 , and other components. 
     The incident end of the light guide  51  is mounted in a connector (not depicted) which connects the electronic endoscope  50  to the endoscope processor  20 . And the other end, hereinafter referred to as the exit end, is mounted at the head end of the insertion tube  56  of the electronic endoscope  50 . As described above, the first light component and/or the excitation light emitted by the light-source unit  30  arrives at the incident end of the light guide  51 . The light is then transmitted to the exit end. The light transmitted to the exit end illuminates a peripheral area near the head end of the insertion tube  56  through a diffuser lens  54 . 
     The light reflected by the subject illuminated by the first light component and/or the autofluorescence of the subject illuminated by the excitation light reaches the light-receiving surface of the imaging device  53  through the object lens  36  and the exciting-light cut-off filter  34 , and forms an optical image on the light-receiving surface. 
     The imaging device driver  22  is controlled by the system controller  23 , and transmits a driving signal to the imaging device  52 . The imaging device  52  captures an optical image on the light-receiving surface on the basis of the received driving signal, and generates an image signal. The generated image signal is transmitted to the image-processing unit  21 . 
     When the excitation light is emitted by the light-source unit  30 , the excitation light component reflected by the subject is removed from the light incident on the exciting-light cut-off filter  53  by the exciting-light cut-off filter  53 . In so doing, an optical image formed only by the autofluorescence component, autofluoresced by tissue to be observed, is captured by the imaging device  52 . 
     As described above, the endoscope processor  20  can be connected to the normal endoscope (not depicted). The normal endoscope does not comprise the exciting-light cut-off filter  53 , as compared with the autofluorescence endoscope  50 . Accordingly, when the normal endoscope is connected to the endoscope processor  20 , an optical image formed by the reflected light of a subject illuminated by the first light component and/or the autofluorescence component autofluoresced by the subject illuminated by the excitation light is captured by the imaging device  52 . 
     Next, the structure of the image-processing unit  21  is explained. The image-processing unit  21  comprises a first signal processing circuit  25  (the receiver), an image processing circuit  26 , and a second signal processing circuit  27  (see  FIG. 1 ). 
     The image signal transmitted from the imaging device is input to the first signal processing circuit  25 . The first signal processing circuit  25  digitizes the received image signal. In addition, the first signal processing circuit  25  carries out predetermined data processing, such as A/D conversion processing, YC processing, and color interpolation processing, on the image data digitized from the image signal. 
     In addition, the first signal processing circuit  25  calculates an average luminance value of light received by the entire light-receiving surface on the basis of the received image signal. Then, the first signal processing circuit  25  generates a luminance signal corresponding to the calculated average luminance value, and transmits it to the light-amount control circuit  41 . As described above, the light-amount control circuit  41  adjusts the aperture ratio of the diaphragm  33  on the basis of the received luminance signal. Furthermore, when a normal endoscope is connected to the endoscope processor  20 , the light-amount control circuit  41  adjusts the duty of the laser source  32 . 
     The image data having undergone predetermined data processing at the first signal processing circuit  25  are transmitted to the image processing circuit  26 . The image processing circuit  26  has a flash memory (not depicted), which is used as a work memory for signal processing. The image data is stored in the flash memory. 
     The image processing circuit  26  carries out color separation processing on the image data stored in the flash memory. In the color separation processing, the image data is separated into red, green, and blue data components. After color separation processing, the image processing circuit  26  carries out predetermined data processing including white balance processing on the red, green, and blue data components separately. In white balance processing, the red and blue data components are separately multiplied by the gains determined in the initialization operation for white balance. 
     The image data having undergone predetermined data processing is transmitted to the second signal processing circuit  27 . The second signal processing circuit  27  carries out predetermined data processing on the image data, such as clamp processing and blanking processing. In addition, the second signal processing circuit  27  converts the image data into an analog image signal. The image signal is transmitted to the monitor  11 , on which an image corresponding to the image signal is displayed. 
     Next, the control of the light amount emitted by the light-source unit  30  using a normal endoscope is explained. As for an autofluorescence endoscope, the control is described later. As described above, the aperture ratio of the diaphragm  33  and the duty of the laser source  32  are adjusted according to the average luminance value when a white-light image should be observed. 
     In order to compare the average luminance value, a reference value is predetermined, and reference data corresponding to the reference value is stored in a ROM (not depicted) connected to the light-amount control circuit  41  and read by the light-amount control circuit  41  when a light amount must be controlled. 
     The light-amount control circuit  41  compares the average luminance value with the reference value. If the average luminance value is less than the reference value, the diaphragm motor  39  and the laser source  32  are driven so that the aperture ratio of the diaphragm  33  and the emitted excitation light amount rise. On the other hand, if the average luminance value is more than the reference value, the diaphragm motor  39  and the laser source  32  are driven so that the aperture ratio and the amount of the emitted excitation light diminish. 
     If the aperture ratio and the amount of the emitted excitation light are adjusted in unrelated fashion, the color temperature of the light emitted from the exit end of the light guide  51  will vary. In order to keep the color temperature constant, the ratio of the amounts of excitation light to the first light component shone from the exit end, hereinafter referred to as a first ratio (the second relation), should be kept constant. 
     Optical specifications of light guide  51  for a normal endoscope may differ greatly from that of an autofluorescence endoscope. Consequently, in order to keep the first ratio constant, the ratio of the amounts of excitation light to the first light components incident on the incident end, hereinafter referred to as the second ratio, should match the ratio determined according to the kind of endoscope connected to the endoscope processor  20  (the first relation). 
     As described above, the amounts of the first light component and the excitation light are adjusted by changing the aperture ratio and the duty of the laser source  32 . As shown in  FIG. 6 , the amount of the first light component varies nonlinearly with the aperture ratio. On the other hand, the amount of the excitation light varies linearly with the duty of the laser source  32 . 
     Accordingly, in order to keep the first ratio constant, the aperture ratio and the duty should be adjusted so that the aperture ratio and the duty satisfy a specific correspondence. The specific correspondence is calculated by the initialization operation for white balance as described later. The light-source unit  30  comprises a first RAM (not depicted), and the specific correspondence is stored in the first RAM. When a white-light image is to be observed, the duty of the laser source  32  is determined according to the aperture ratio and the laser source  32  is driven at the determined duty. 
     When an autofluorescence endoscope is connected to the endoscope processor  20 , only the aperture ratio is adjusted because the amount of light does not vary even if the duty is adjusted. As described above, the autofluorescence endoscope has the exciting-light cut-off filter  53 , which removes the excitation light component. Consequently, because the excitation light components do not reach the imaging device  52 , the amount of excitation light emitted by the laser source  32  does not have to be controlled. 
     Next, the initialization operation for white balance carried out by the system controller  23  is explained using the flowcharts of  FIGS. 8 and 9 . In the initialization operation, the gains to multiply red and blue data components and the specific correspondence between the aperture ratio and the duty are determined. 
     The user is instructed to cover the head end of the insertion tube  56  with a white balance cover during the initialization operation. The white balance cover has a white interior. The initialization operation is carried out on the assumption that the head end is covered with the white balance cover. When a user inputs a command for ordering the initialization operation to the input block  24 , the system controller  23  commences the initialization operation. 
     At step S 100 , the system controller  23  orders the light-amount control circuit  41  to determine a duty of the laser source  32  to the initialization duty predetermined on manufacturing. 
     At step S 101  following step S 100 , the system controller  23  orders the laser source  32  via the light-amount control circuit  41  to emit the excitation light at the determined duty. 
     At step S 102  following step S 101 , the system controller  23  orders the imaging device  52  via the imaging device driver  22  to capture an image of the inside of the white balance cover illuminated by the excitation light. In addition, the system controller  23  orders the image processing circuit  26  to extract the blue data components from the image signal. The blue data components are extracted, and the process proceeds to step S 103 . 
     At step S 103 , the system controller  23  determines whether or not the blue data components are saturated, in other words whether or not the blue data components have reached the maximum data level representable by the image processing circuit  26 . If the blue data components are saturated, the process proceeds to step S 104 . At step S 104 , the system controller  23  orders the light-amount control circuit  41  to lower the duty of the laser source  32 . After lowering the duty, the process returns to step S 101 . Since then, steps S 101 -S 104  are repeated until the blue data components are not saturated. 
     If it is determined at step S 103  that the blue data components are not saturated, the process proceeds to step S 105 . At step S 105 , the system controller orders the light-amount control circuit  41  to store the finally determined duty as a maximum adjustable duty in the first RAM (not depicted) connected to the light-amount control circuit  41 . 
     At step S 106  following step S 105 , the system controller  23  orders the light-amount control circuit  41  to drive the diaphragm motor  39  so that the aperture ratio is 75%. In addition, the system controller  23  orders the light-amount control circuit  41  to determine the duty to the maximum adjustable duty stored at step S 105 . After adjusting the aperture ratio and the duty, the process proceeds to step S 107 . 
     At step S 107 , the system controller  23  orders the lamp  31  via the power supply circuit  38  to emit white light and orders the laser source  32  via the light amount control circuit  41  to emit excitation light at the determined duty. 
     At step S 108  following step S 107 , the system controller  23  orders the imaging device  52  via the imaging device driver  22  to capture an image of the interior of the white balance cover illuminated by the first light component and the excitation light. In addition, the system controller  23  calculates a red gain and a blue gain to multiply the red and blue data components on the basis of the captured image signal. After calculation of the gains, the process proceeds to step S 109 . 
     At step S 109 , the system controller  23  determines whether or not the blue gain is included in a permissible range predetermined on manufacturing. If the blue gain is out of the permissible range, the process proceeds to step S 108 . At step S 108 , the system controller  23  orders the light-amount control circuit  41  to lower the currently determined duty of the laser source  32 . After lowering the duty, the process returns to step S 107 . After that, steps S 107 -S 108  are repeated until the blue gain is included in the permissible range. 
     Only the blue gain is compared with the permissible range because the laser source  32  alone shines the blue light component on a subject, as explained next. If the blue light component is supplied by the lamp  31 , the calculated blue gain will be adequate. However, in the endoscope processor  20 , the amount of the blue light component in the white light shone on the subject may differ greatly from those of the red and green light components in the white light. Consequently, the calculated blue gain may be quite different than the blue gain calculated with the white light supplied by only the lamp  31  on the subject. If the white balance processing is carried out using blue gain that is far off, more blue color noise may appear in the generated image. Consequently, the range of blue gain necessary for avoiding blue color noise in the generated image is predetermined as the permissible range. 
     If it is determined at step S 109  that the blue gain is within the permissible range, the process proceeds to step S 111 . At step S 111 , the system controller  23  orders a second RAM (not depicted) connected to the image processing circuit  26  to store the red and blue gains calculated at step S 108 . After storage, the process proceeds to step S 112 . 
     At step S 112 , the system controller  23  orders the first RAM to store the present duty and the present aperture ratio which correspond to each other. After storage, the process proceeds to step S 113 . 
     At step S 113 , the system controller  23  determines whether or not the three combinations of corresponding duty and aperture ratio have been stored in the first RAM. 
     If the three combinations have not been stored, the process proceeds to step S 114 . At step S 114 , the system controller  23  orders the light-amount control circuit  41  to drive the diaphragm motor  39  so that the aperture ratio is lowered by 25%. Consequently, if the present aperture ratio is 75% and 50%, the aperture ratio is adjusted to 50% and 25%, respectively. After adjusting the aperture ratio, the process returns to step S 107 . After that, steps S 107  to S 114  are repeated until the three combinations have been stored. 
     On the other hand, if it is determined at step S 113  that the three combinations have been stored, the process proceeds to step S 115 . At step S 115 , the system controller  23  generates correspondence table data corresponding to the specific correspondence between the aperture ratio and duty on the basis of the three different duties corresponding to the aperture ratios of 75, 50, and 25%. After generating the correspondence table data, the process proceeds to step S 116 . 
     At step S 116 , the system controller  23  stores the correspondence table data in the first RAM. When the correspondence table data is stored, the initialization operation for white balance ends. 
     The above initialization operation is carried out when a normal endoscope is connected to the endoscope processor  20 . When an autofluorescence endoscope is connected to the endoscope processor  20 , an adjustment of the excitation light emitted by the laser source  32  is unnecessary because the exciting-light cut-off filter  53  is mounted. Accordingly, when an autofluorescence endoscope is connected to the endoscope processor  20 , the above initialization operation is carried out with the omission of steps S 102 , S 103 , S 104 , S 109 , S 110 , S 112 , S 113 , S 114 , S 115 , and S 116 . 
     Next, the operation for control of light amount, which is carried out by the system controller  23  when a captured image is displayed using a normal endoscope, is explained using the flowcharts of  FIG. 10 . 
     At step S 200 , the system controller  23  orders the lamp  31  and the laser source  32  via the light-amount control circuit  41  to emit the white light and the excitation light, respectively. 
     At step S 201  following step S 200 , the system controller  23  orders the imaging device  52  via the imaging device driver  22  to capture a subject illuminated by the first light component and the excitation light for generating an image signal. In addition, the system controller  23  orders the first signal processing circuit to calculate the average luminance value on the basis of the generated image signal. After calculation of the average luminance value, the process proceeds to step S 202 . 
     At step S 202 , the system controller  23  orders the light-amount control circuit  41  to calculate the difference between the average luminance value and the reference value. 
     At step S 203  following step S 202 , the system controller  23  orders the light-amount control circuit  41  to determine whether or not the absolute value of the calculated difference is less than a threshold. If the difference is less than the threshold, the operation for control of light amount ends. On the other hand, if the difference is greater than or equal to the threshold, the process proceeds to step S 204 . 
     At step S 204 , the system controller  23  orders the light-amount control circuit  41  to determine the aperture ratio of the diaphragm  33  according to the difference calculated at step S 202 . After the determination, the process proceeds to step S 205 . 
     At step S 205 , the system controller  23  orders the light-amount control circuit  41  to determine the duty of the laser source corresponding to the aperture ratio determined at step S 204  on the basis of the correspondence table data generated in the initialization operation for white balance. 
     At step S 206  following step S 205 , the system controller  23  orders the light-amount control circuit  41  to drive the diaphragm motor  39  so that the aperture ratio of the diaphragm matches the aperture ratio determined at step S 204  and to drive the laser source  32  at the duty determined at step S 205 . 
     In the above embodiment, an acceptable white image can be produced by connecting a normal endoscope to the endoscope processor by adjusting the duty of the laser source  32  according to the aperture ratio of the diaphragm. 
     In addition, in the above embodiment, a mechanism for moving the dichroic mirror  35  is unnecessary because the dichroic mirror  35  can be fixed. Accordingly, faults of light-source unit  30  can be reduced, the latency to switch light sources can be shortened, the number of parts for the light-source unit  30  can be reduced, and the manufacturing cost is reduced. 
     In addition, in the above embodiment, blue color noise can be reduced because the amounts of the first light component and the excitation light emitted by the light-source unit  30  can be separately adjustable. In general, the sensitivity of an imaging device for the blue light component may be relatively lower than for green and red. So, when a subject illuminated by white light consisting of practically the same amounts of blue, green, and red light components is imaged, relatively large blue gain is used. Accordingly, blue color noise would increase and become noticeable. On the other hand, in the above embodiment, the blue color noise will be reduced by relatively increasing the amount of excitation light emitted by the light-source unit  30 . 
     The specific correspondence between the aperture ratio of the diaphragm  33  and the duty of the laser source  32  is determined by the system controller  23  in the initialization operation for white balance in the above embodiment. However, the specific correspondence can be determined by other methods. The same effect can be achieved as long as the duty is adjusted according to the aperture ratio so that the adjusted duty and the current aperture ratio satisfy the specific correspondence. For example, the specific correspondence for each electronic endoscope can be determined on manufacturing and stored in an endoscope memory mounted in the electronic endoscope. When the endoscope is connected to the endoscope processor  20 , the light-amount control circuit  41  reads the specific correspondence and uses it for adjusting the duty. 
     The amounts of the first light component and the excitation light shone on a subject are adjusted by changing the aperture ratio of the diaphragm  33  and the duty of the laser source  32 , respectively, in the above embodiment. However, the amounts can be adjusted using any other devices for light control. As long as the amounts are controlled so that the second ratio matches a ratio determined for each endoscope, the same effect can be achieved. 
     The white light and the excitation light are simultaneously emitted by the light-source unit  30  and the amounts of color components of the received light are simultaneously detected on the initialization operation for white light in order to determine the specific correspondence, in the above embodiment. However, the white light and the excitation light can be separately emitted and the amounts of color components of the received light can be separately detected. 
     The duty of the laser source  32  is adjusted according to the aperture ratio of the diaphragm  33  for observing a white-color image in the above embodiment. However, even if the aperture ratio is adjusted according to the duty, the effect of the above embodiment can be achieved. However, the adjustment of the duty based on the aperture ratio is achieved more quickly than based on the duty. 
     The number of combinations of aperture ratio and duty actually detected on the initialization operation for white balance is three in the above embodiment. However, the number is not limited to three. In the initialization operation, by determining the adequate duties for a certain blue gain for three more different aperture ratios and by using the combinations of the aperture ratio and the duty for determining the specific correspondence, the amount of the excitation light shone on a subject is more adequately controlled. 
     The duty of the laser source  32  is adjusted when the aperture ratio is set to 75, 50, and 25% in the initialization operation for white balance in the above embodiment. However, the aperture ratio to be set is not limited to 75, 50, and 25%. The duty may be adjusted if the amount of the first light incident on the light guide  51  is kept constant and the inside of the white balance cover is captured. 
     The light-source unit  30  can simultaneously or separately emit the first light component and the blue excitation light in the above embodiment. However, the light-source unit  30  may emit at least two different kinds of light which include at least one of red, green, and blue light components. 
     Although the embodiment of the present invention has been described herein with reference to the accompanying drawings, obviously many modifications and changes may be made by those skilled in this art without departing from the scope of the invention. 
     The present disclosure relates to subject matter contained in Japanese Patent Application No. 2008-050210 (filed on Feb. 29, 2008), which is expressly incorporated herein, by reference, in its entirety.