Patent Publication Number: US-11391426-B2

Title: Light source device and light-amount adjusting method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of International Application No. PCT/JP2018/044470, filed on Dec. 4, 2018, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a light source device and a light-amount adjusting method. 
     2. Related Art 
     A technique of controlling output of respective light sources in a light source device including plural light sources that emit light having wavelengths different from one another by directly detecting leakage light from the light sources has been known (for example, WO2015/016172). 
     SUMMARY 
     In some embodiments, a light source device includes: a plurality of light sources configured to emit a plurality of kinds of light having wavelength band different from one another; a plurality of dichroic mirrors that are respectively arranged on a first optical path through which light is emitted to an outside, the plurality of dichroic mirrors having light transmission characteristics different from one another, each dichroic mirror being configured to reflect or pass light emitted from any one of the plurality of light sources; a plurality of light sensors that are respectively positioned on a plurality of second optical paths, each second optical path being a different optical path from the first optical path and being an optical path propagating light reflected on or passed through the dichroic mirror, each light sensor being configured to detect a light amount of light that propagates through the second optical path; and a controller configured to control a light amount ratio of the plurality of kinds of light based on the light amount respectively detected by the plurality of light sensors. 
     In some embodiments, a light source device includes: a first light source configured to emit light of a first wavelength band; a second light source configured to emit light of a second wavelength band; a first dichroic mirror that is arranged on a first optical path through which light is emitted to an outside of the light source device, the first dichroic mirror being configured to reflect or pass light emitted from the first light source; a second dichroic mirror that is arranged on the first optical path on a side of an emitting position of light to the outside relative to the first dichroic mirror, the second dichroic mirror having a light transmission characteristic different from the first dichroic mirror, the second dichroic mirror being configured to separate light emitted from the second light source to a component traveling to the first optical path and a component traveling to a second optical path different from the first optical path, the second dichroic mirror being configured to reflect a part of light from the first light source, which is reflected on the first dichroic mirror, toward the second optical path; and a light sensor unit that is positioned on the second optical path, the light sensor unit being configured to detect a light amount of light propagating through the second optical path. The light sensor unit includes a first light sensor to detect the light of the first wavelength band, and a second light sensor to detect the light of the second wavelength band, the first light sensor and the second light sensor are arranged at positions that are determined based on a spectral sensitivity characteristic, a maximum light amount according to a wavelength of incident light, and a light intensity distribution, the first light sensor includes at least two light sensors that are arranged at positions at which illumination intensities differ from each other in the light sensor unit, the second light sensor includes at least two light sensors that are arranged at positions at which illumination intensities differ from each other in the light sensor unit, and the light of the first wavelength band and the light of the second wavelength band are subjected to light amount detection in a dynamic range wider than a case of a single light sensor, by combining detection values of at least two light sensors. 
     In some embodiments, a light-amount adjusting method includes: emitting each of a plurality of kinds of light having wavelength band different from one another by a plurality of light sources; reflecting light emitted from a light source to a first optical path through which light is emitted to an outside, with a plurality of dichroic mirrors; passing the light emitted from the light source to a second optical path that is different from the first optical path, with the plurality of dichroic mirrors; detecting a light amount of light that has passed through the plurality of dichroic mirrors with a light sensor; and controlling a light amount ratio of the plurality of kinds of light based on the light amount detected by the light sensor, with a controller. 
     The above and other features, advantages and technical and industrial significance of this disclosure will be better understood by reading the following detailed description of presently preferred embodiments of the disclosure, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a configuration of an endoscope system that includes a light source device according to a first embodiment; 
         FIG. 2  is a diagram illustrating transmission characteristics of dichroic mirror that reflects red laser light; 
         FIG. 3  is a diagram illustrating transmission characteristics of a dichroic mirror that reflects green laser light; 
         FIG. 4  is a diagram illustrating transmission characteristics of an optical filter that passes only light of a wavelength band shorter than red light; 
         FIG. 5  is a diagram illustrating transmission characteristics of a dichroic mirror that reflects blue laser light; 
         FIG. 6  is a diagram illustrating transmission characteristics of an optical filter that passes only light of a wavelength band shorter than green light; 
         FIG. 7  is a diagram illustrating transmission characteristics of a dichroic mirror that reflects violet laser light; 
         FIG. 8  is a diagram illustrating transmission characteristics of an optical filter that passes only light of a wavelength band shorter than blue light; 
         FIG. 9  is a diagram illustrating a configuration of a light source according to a modification of the first embodiment; 
         FIG. 10  is a diagram illustrating a relation between a wavelength of light and a spectral sensitivity of a light sensor, and a relation between a wavelength of incident laser light of respective colors and a maximum light amount entering a light sensor; 
         FIG. 11  is a diagram schematically illustrating an arrangement position of a light sensor included in a light source device according to a second embodiment; 
         FIG. 12  is a diagram illustrating a configuration of a light source device according to a third embodiment; 
         FIG. 13A  is a diagram illustrating transmission characteristics of an optical filter that passes only light of a wavelength longer than a wavelength band of red light; 
         FIG. 13B  is a diagram illustrating transmission characteristics of an optical filter that passes only light of a wavelength band near green light; 
         FIG. 13C  is a diagram illustrating transmission characteristics of an optical filter that passes only light of a wavelength band near blue light; 
         FIG. 13D  is a diagram illustrating transmission characteristics of an optical filter that passes only light of a wavelength shorter than a wavelength band near violet light; 
         FIG. 14  is a diagram schematically illustrating an arrangement position of a light sensor included in a light source device according to a third embodiment; 
         FIG. 15  is a diagram illustrating a configuration of a light source device according to a fourth embodiment; 
         FIG. 16  is a diagram schematically illustrating an arrangement position of a light sensor included in the light source device according to the fourth embodiment; and 
         FIG. 17  is a diagram illustrating a configuration of a light source device according to a fifth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, forms to implement the disclosure (hereinafter, “embodiment”) will be explained with reference to the accompanying drawings. 
     First Embodiment 
       FIG. 1  is a diagram illustrating a configuration of an endoscope system that includes a light source device according to a first embodiment. An endoscope system  1  illustrated in the diagram includes an endoscope  2  that captures an in-vivo image of a subject with its distal end portion inserted into the subject, a processing device  3  that performs centralized control of an overall operation of the endoscope system  1 , a display device  4  that displays an image captured by the endoscope  2 , and a light source device  5  that generates illumination light emitted from a distal end of the endoscope  2 . 
     The endoscope  2  includes an optical system  21  that is arranged at a distal end portion of an insertion portion inserted in to the subject to gather light from a subject, an imaging device  22  that generates an electrical image signal by photoelectric converting the light gathered by the optical system  21 , a light guide  23  that propagates light generated by the light source device  5  to the distal end portion of the insertion portion, and an illumination lens  24  that is arranged on a distal end side of the light guide  23 , and that irradiates the light that has been propagated through the light guide  23  to an outside of the endoscope  2  as illumination light. The optical system  21  is constituted of one or more lenses. The imaging device  22  is constituted of, for example, a charge coupled device (CCD) image sensor, or a complementary metal oxide semiconductor (CMOS) image sensor. The light guide  23  is constituted of, for example, plural thin optical fibers bundled. 
     The processing device  3  includes an image processing unit  31 , a synchronization-signal generating unit  32 , an input unit  33 , a control unit  34 , and a storage unit  35 . The image processing unit  31  generates image data for display by subjecting an image signal received from the endoscope  2  to predetermined processing, and outputs it to the display device  4 . The synchronization-signal generating unit  32  generates a synchronization signal to synchronize operations of the endoscope  2  and the light source device  5 . The input unit  33  is constituted of a user interface, such as a switch, a button, a touch panel, a keyboard, and a mouse, and accepts an input of various kinds of signals, such as an operation instruction signal to instruct an operation of the endoscope system  1 . 
     The control unit  34  controls the operation of the endoscope  1  including the processing device  3  in a centralized manner. The control unit  34  is configured by using hardware including a general-purpose processor, such as a central processing unit (CPU), and a dedicated integrated circuit that performs a specific function, such as a field programmable gate array (FPGA), alone or in combination. 
     The storage unit  35  stores various kinds of programs to operate the endoscope system  1 , and data including various kinds of parameters necessary for the operation of the endoscope system  1 . It is constituted of a volatile memory, such as a random access memory (RAM), and a non-volatile memory, such as a read only memory (ROM). The storage unit  35  may be constituted of a computer-readable recording medium, such as an externally mountable memory card. The various kinds of programs described above can be stored in a computer-readable recording medium, such as a hard disk, a flash memory, a CD-ROM, a DVD-ROM, and a flexible disk, to be widely distributed. 
     The display device  4  displays a display image corresponding to the image data received from the image processing unit  31  of the processing device  3 . The display device  4  is constituted of a monitor, such as a liquid crystal or an organic electroluminescence (EL) monitor. 
     The light source device  5  includes four light source units  51   a  to  51   d  that generate laser light having wavelength different from one another. The light source unit  51   a  has a red semiconductor laser, the light source unit  51   b  has a green semiconductor laser, the light source unit  51   c  has a blue semiconductor laser, and the light source unit  51   d  has a violet semiconductor laser. The semiconductor laser is also called laser diode (LD). 
     On an optical path of red laser light generated by the light source unit  51   a , a dichroic mirror  52   a  is arranged. A surface of the dichroic mirror  52   a  is slanted by 45° to an incident optical path of the red laser light, and on this surface, the red laser light is reflected to change a direction of its optical path by 90°. Hereinafter, an optical path of the red laser light after reflection on the dichroic mirror  52   a  is referred to as first optical path. 
       FIG. 2  is a diagram illustrating transmission characteristics of the dichroic mirror  52   a . In  FIG. 2 , a horizontal axis is for a wavelength and a vertical axis is for a transmission rate, and bars in broken lines R, G, B, V show center wavelengths of red, green, blue, and violet, respectively. These points are same in  FIG. 3  to  FIG. 8  described later. 
     The dichroic mirror  52   a  does not pass almost all wavelength bands as indicated by a straight line  101  in  FIG. 2 . Accordingly, the dichroic mirror  52   a  reflects most, and passes a very small amount of red laser light entering from the light source unit  51   a . Hereinafter, an optical path of the red laser light that has passed through the dichroic mirror  52   a  is referred to as second optical path. On the second optical path, a neutral density (ND) filter  53   a  and a light sensor  54   a  are sequentially arranged toward downstream. The light sensor  54   a  only receives red laser light. 
     On an optical path of green laser light generated by the light source unit  51   b , a dichroic mirror  52   b  is arranged. A surface of the dichroic mirror  52   b  is slanted by 45° to an incident optical path of the green laser light, and on this surface, the green laser light is reflected to change a direction of its optical path by 90°. The surface of the dichroic mirror  52   b  is parallel to the surface of the dichroic mirror  52   a , and a reflection optical path of green laser light coincides with the first optical path. On an optical path after transmission of the green laser light through the dichroic mirror  52   b  (this is also referred to as second optical path), an optical filter  55   b , an ND filter  53   b , and a light sensor  54   b  are sequentially arranged toward downstream of the second optical path. 
       FIG. 3  is a diagram illustrating transmission characteristics of the dichroic mirror  52   b . The dichroic mirror  52   b  passes most of red laser light out of four colors of laser light as indicated by a curve  102  in  FIG. 3 , and passes little laser light of the other three colors. Accordingly, while passing most of red laser light propagated through the first optical path, the dichroic mirror  52   b  reflects most of green laser light entering from the light source unit  51   b , and multiplexes these kinds of laser light to let it propagate through the first optical path. Moreover, the dichroic mirror  52   b  reflects a very small amount of red laser light, and passes a very small amount of green laser light, to let it propagate through the second optical path. 
       FIG. 4  is a diagram illustrating transmission characteristics of the optical filter  55   b . While passing most of light having a wavelength shorter than red as indicated by a curve  201  in  FIG. 4 , the optical filter  55   b  passes little light of a wavelength band on a longer wavelength side from the wavelength band. Therefore, the optical filter  55   b  passes only green laser light out of laser light that propagates through the second optical path. As a result, the light sensor  54   b  receives only green laser light. 
     On an optical path of blue laser light generated by the light source unit  51   c , a dichroic mirror  52   c  is arranged. A surface of the dichroic mirror  52   c  is slanted by 45° to an incident optical path of the blue laser light, and on this surface, the blue laser light is reflected to change a direction of its optical path by 90°. The surface of the dichroic mirror  52   c  is parallel to the surface of the dichroic mirror  52   a , and a reflection optical path of blue laser light coincides with the first optical path. On an optical path after transmission of the blue laser light through the dichroic mirror  52   c  (this is also referred to as second optical path), an optical filter  55   c , an ND filter  53   c , and a light sensor  54   c  are sequentially arranged toward downstream of the second optical path. 
       FIG. 5  is a diagram illustrating transmission characteristics of the dichroic mirror  52   c . As indicated by a curve  103  in  FIG. 5 , the dichroic mirror  52   c  passes most of red laser light and green laser light out of laser light of four colors, and passes little blue laser light and little violet laser light. Accordingly, while passing most of red laser light and green laser light that have propagated through the first optical path, the dichroic mirror  52   c  reflects most of blue laser light entering from the light source unit  51   c , and multiplexes these kinds of laser light to let them propagate through the first optical path. Moreover, the dichroic mirror  52   c  reflects a very small amount of red laser light and green laser light, and passes a very little amount of blue laser light to let it propagate through the second optical path. 
       FIG. 6  is a diagram illustrating transmission characteristics of an optical filter  55   c . As indicated by a curve  202  in  FIG. 6 , the optical filter  55   c  passes most of light of a wavelength band shorter than green, and passes little light of a wavelength band on a long wavelength side from the wavelength band. Therefore, the optical filter  55   c  passes only blue laser light out of laser light that propagates through the second optical path. As a result, the light sensor  54   c  receives only blue laser light. 
     On an optical path of violet laser light generated by the light source device  51   d , a dichroic mirror  52   d  is arranged. A surface of the dichroic mirror  52   d  is slanted by 45° to an incident optical path of the violet laser light, and on this surface, the violet laser light is reflected to change a direction of its optical path by 90°. The surface of the dichroic mirror  52   d  is parallel to the surface of the dichroic mirror  52   a , and a reflection optical path of violet laser light coincides with the first optical path. On an optical path after transmission of the violet laser light through the dichroic mirror  52   d  (this is also referred to as second optical path), an optical filter  55   d , an ND filter  53   d , and a light sensor  54   d  are sequentially arranged toward downstream of the second optical path. 
       FIG. 7  is a diagram illustrating transmission characteristics of the dichroic mirror  52   d . As indicated by a curve  104  in  FIG. 7 , the dichroic mirror  52   d  passes laser light of three colors except violet laser light, out of laser light of four colors. Accordingly, while the dichroic mirror  52   d  passes most of red laser light, green laser light, and blue laser light that have propagated through the first optical path, the dichroic mirror  52   d  reflects most of violet laser light entering from the light source unit  51   d , and multiplexes these kinds of light to let them propagate through the first optical path. Moreover, the dichroic mirror  52   d  reflects a very small amount of red laser light, green laser light, and blue laser light, and passes a very small amount of violet laser light to let it propagate through the second optical path. 
       FIG. 8  a diagram illustrating transmission characteristics of the optical filter  55   d . The optical filter  55   d  passes most of light of a wavelength band shorter than blue, and passes little light of a wavelength band on a long wavelength side from the wavelength band as indicated by a curve  203  in  FIG. 8 . Therefore, the optical filter  55   d  passes only violet laser light out of laser light that propagates through the second optical path. As a result, the light sensor  54   d  receives only violet laser light. 
     The ND filters  53   a  to  53   d  have a function of making a light amount of laser light of the respective colors that enters the optical sensors  54   a  to  54   d  match a light receiving range of the light sensors  54   a  to  54   d.    
     The light sensors  54   a  to  54   d  are constituted of, for example, a light receiving device, such as a photodiode. Light receiving surfaces of the light sensors  54   a  to  54   d  are arranged at positions intersecting an optical path center of the second optical path perpendicularly to the optical path. Spectral sensitivity characteristics of the light sensors  54   a  to  54   d  are equal to one another. 
     The light source device  5  further includes a lens  56  that is arranged on the first optical path, and that gathers laser light of four colors propagating through the first optical path to supply to the light guide  23 , a driving unit  57  that includes a circuit to drive the light source units  51   a  to  51   d , and a control unit  58  that controls the driving unit  57  to drive such that a light amount ratio of plural kinds of laser beams respectively emitted from the light source units  51   a  to  51   d  is constant. The control unit  58  is constituted of a CPU, an FPGA, or the like. The light source device  5  and the processing device  3  may be integrated. 
     The straight line  101 , the curves  102  to  104 ,  201  to  203  are only one example, and forms thereof are not limited to the ones illustrated. 
     The light source device  5  having the above configuration supplies most of light emitted respectively from the light source units  51   a  to  51   d  to the light guide  23  of the endoscope  2  through the dichroic mirrors  52   a  to  52   d . Moreover, very small amounts out of the light emitted by the light source units  51   a  to  51   d  pass through the dichroic mirrors  52   a  to  52   d , respectively, to propagate through the second optical path, and laser light of a single color enters the respective light sensors  54   a  to  54   d.    
     The control unit  58  controls an output of the light source units  51   a  to  51   d  by driving the driving unit  57  such that respective color components of illumination light emitted by the light source device  5  maintain a predetermined light amount ratio, based on light source amounts of laser light of respective colors detected by the light sensors  54   a  to  54   d.    
     According to the first embodiment explained above, light sensors to detect laser light of respective colors are arranged on the second optical path that is different from the first optical path to emit illumination light to outside of the light source device  5  and, therefore, a light amount ratio of plural light sources can be maintained constant with high accuracy. 
     In the light amount detection disclosed in WO2015/016172 above, because direct leakage light from a light source is acquire as illumination light deviated from an optical path, it is necessary to have a configuration to deviate a part of the illumination light from the optical path on purpose, and a loss of an actual output light amount has been large. On the other hand, in the first embodiment, out of light emitted by the light source units  51   a  to  51   d , only a small amount of light that passes through a corresponding one of the dichroic mirrors  52   a  to  52   d  is used to detect a light amount. Therefore, a light amount of each laser light can be monitored without causing an unnecessary light amount loss. 
     Modification 
       FIG. 9  is a diagram illustrating a configuration of a light source according to a modification of the first embodiment. In a light source device  5 A illustrated in the diagram, a dichroic mirror  52   e  that passes most of red laser light emitted from the light source unit  51   a  is arranged on an incident optical path (the first optical path) of red laser light. The dichroic mirror  52   e  has a property that passes most of red laser light and reflects a very small amount of red laser light. 
     The red laser light that is reflected by the dichroic mirror  52   e  and propagates through the second optical path enters the light sensor  54   a  arranged on the second optical path. In the present modification, the ND filter  53   a  is not required. A configuration of the light source device  5 A excluding points explained herein is the same as the configuration of the light source device  5  described above, and an acquired effect is also similar to that of the first embodiment. 
     Second Embodiment 
     A light source device according to a second embodiment sets an arrangement position of a light sensor based on a spectral sensitivity of the light sensor. Configurations of the light source device and an endoscope system excluding this point are same as those of the first embodiment. Hereinafter, explanation will be given, assigning reference symbols identical to the components of the light source device  5  to components identical to the components of the light source device  5 . 
       FIG. 10  is a diagram illustrating a relation between a wavelength λ of light and a spectral sensitivity S(λ) of a light sensor, and a relation between a wavelength λ of incident laser light of respective colors and a maximum light amount P(λ) entering a light sensor. In  FIG. 10 , a curve  301  is a curve indicating a spectral sensitivity S(λ). Moreover, heights of bars  401 ,  402 ,  403 ,  404  indicate maximum light amount P(λ) of red laser light, green, laser light, blue laser light, and violet laser light, respectively. 
     In the case shown in  FIG. 10 , the spectral sensitivity S(λ) monotonously increases with the increase of wavelength to reach the maximum value near 80 nm in a wavelength band of about 400 nm to 800 nm, and monotonously decreases with the increase of wavelength in a wavelength band higher than that. Moreover, in the case shown in  FIG. 10 , the maximum light mounts P(λ) entering the light sensor is green, red, blue, and violet sequentially in descending order. The curve  301  is only one example, and the spectral characteristic of the light sensor is not limited thereto. 
     An intensity distribution of laser light that is emitted by a semiconductor laser constituting the light source units  51   a  to  51   d  is expressed by a Gaussian distribution in which a center is the maximum value when a distance from the center of laser light is a function. In the second embodiment, as the intensity distribution of laser light, a relative light intensity I(r) at a position r when a center of the second optical path is zero in the light receiving surface is applied. 
       FIG. 11  is a diagram schematically illustrating arrangement positions of the light sensors  54   a  to  54   d . In  FIG. 11 , a curve  501  is a curve indicating a relative light intensity I (r) according to a position r. Arrangement positions of the light sensors  54   a  to  54   d  in  FIG. 11  are arrangement position when it is viewed on a plane perpendicular to the second optical path. Diameters of four circles correspond to beam diameters of laser light. The beam diameter is a distance between two positions at which the relative light intensity is I/e 2  when the maximum value of the relative light intensity I (λ) is 1. The light sensors  54   a  to  54   d  are arranged at positions at which a product of the maximum incident light amount P(λ), the relative light intensity I(r) at the position r, and the spectral sensitivity S(λ), P(λ)×I(r)×S(λ) is uniform with one another. 
     In the light sensors  54   a  to  54   d , when distances |r| from the optical path center to the positions at which the product P(λ)×I(r)×S(λ) is constant are ra, rb, rc, and rd, respectively, these distances have a relation of ra&gt;rb&gt;rc&gt;rd=0 as illustrated in  FIG. 11 . By thus arranging the light source units  51   a  to  51   d , it is possible to irradiate laser light of illumination intensity equivalent to one another to the light sensors  54   a  to  54   d  when the outputs of the light source units  51   a  to  51   d  are maximized. 
     According to the second embodiment explained above, a light amount ratio of plural light sources can be maintained constant with high accuracy similarly to the first embodiment. 
     Moreover, according to the second embodiment, by adjusting arrangement positions of respective light sensors on an optical path, incident light amounts to respective light sensors can be optimized. 
     Furthermore, according to the second embodiment, it is possible to increase a dynamic range of a light sensor without using an ND filter. When the dynamic range of a light sensor is increased by using an ND filter, it is necessary to adjust an optical concentration of the respective ND filters to be optimized, considering a spectral sensitivity characteristic and a light amount ratio. On the other hand, according to the second embodiment, because an ND filter is not necessary, it is possible to reduce the number of parts, and is not necessary to optimize ND filters independently. Consequently, it is possible to prevent similar parts from being mistaken in combination at the time of assembling a product, and to simplify an assembly procedure. Therefore, it is possible to suppress cost necessary for manufacturing, and to provide an economical light source device. 
     Third Embodiment 
       FIG. 12  is a diagram illustrating a configuration of a light source device according to a third embodiment. A light source device  6  illustrated in the diagram includes the four light source units  51   a  to  51   d , the three dichroic mirrors  52   b  to  52   d , an ND filter  53   e , the driving unit  57 , the control unit  58 , and a light sensor unit  61 . Hereinafter, explanation will be given, assigning reference symbols identical to the components of the light source device  5  to components identical to the components of the light source device  5 . 
     The light sensor unit  61  includes four light sensors  61   a  to  61   d  that detect red laser light, green laser light, blue laser light, and violet laser light, respectively. Each of the light sensors  61   a  to  61   d  is structured by combining an optical filter that passes a wavelength band different from others, and an optical device, such as a photodiode.  FIG. 13A  to  FIG. 13D  are diagrams showing transmission characteristics of the optical filters of the respective light sensors  61   a  to  61   d . In  FIG. 13A  to  FIG. 13D , a horizontal axis is for a wavelength and a vertical axis is for a transmission rate, and bars R, G, B, V in broken lines indicate center wavelengths of red, green, blue, and violet, respectively. 
     The optical filter included in the light sensor  61   a  passes most of light of red and in a wavelength band on a long wavelength side from red, and passes little light of green, blue, and violet as indicated by a curve  601  in  FIG. 13A . Thus, the light sensor  61   a  detects a light amount of only red laser light out of laser light of four colors reflected on the dichroic mirror  52   d.    
     The optical filter included in the light sensor  61   b  passes most of light in a wavelength band near green, and passes little light of red, blue, and violet as indicated by a curve  602  in  FIG. 13B . Thus, the light sensor  61   b  detects a light amount of only green laser light out of laser light of four colors reflected on the dichroic mirror  52   d.    
     The optical filter included in the light sensor  61   c  passes most of light in a wavelength band near blue, and passes little light red, green, and violet as indicated by a curve  603  in  FIG. 13C . Thus, the light sensor  61   c  detects a light amount of only blue laser light out of laser light of four colors reflected on the dichroic mirror  52   d.    
     The optical filter included in the light sensor  61   d  passes most of light of violet and in a wavelength band on a short wavelength side from violet, and passes little light of red, green, and blue as indicated by a curve  604  in  FIG. 13D . Thus, the light sensor  61   d  detects a light amount of only violet laser light out of laser light of four colors reflected on the dichroic mirror  52   d.    
     The curves  601  to  604  are only one example, and forms thereof are not limited to the ones illustrated. 
     In the third embodiment, the light sensors  61   a  to  61   d  apply the relative light intensity I(r) at the position r when the center of the second optical path is zero in the light receiving surface as the intensity distribution of laser light similarly to the second embodiment, and the light sensors  61   a  to  61   d  are arranged at positions at which a product of the maximum incident light amount P(λ), the relative light intensity I(r) at the position r, and the spectral sensitivity S(λ), P(λ)×I(r)×S(λ) is uniform with one another. 
       FIG. 14  is a diagram schematically illustrating arrangement positions of light sensors  61   a  to  61   d . Specifically,  FIG. 14  schematically illustrates the arrangement positions of the light sensors  61   a  to  61   d  when the relation between the wavelength λ of light and the spectral sensitivity S(λ) of the light sensor is indicated by the curve  301  in  FIG. 10 , and the relation between the wavelength λ of incident laser light of each color and the maximum light amount P(λ) entering the light sensor is indicated by the bars  401  to  403  in  FIG. 10 . A curve  501  in  FIG. 14  is a curve same as that in  FIG. 11 . Moreover, the arrangement positions of the light sensors  61   a  to  61   d  illustrated in  FIG. 14  are the arrangement positions when it is viewed on a plane perpendicular to the second optical path, and a diameters of a circle corresponds to a beam diameter of laser light. In the case illustrated in  FIG. 14  also, the distance |r| from the optical path center satisfies the relation of ra&gt;rb&gt;rc&gt;rd=0 similarly to the case illustrated in  FIG. 11 . The arrangement positions of the light sensors  61   a  to  61   d  are only one example, and can be changed appropriately as long as the relation that P(λ)×I(r)×S(λ) is uniform is satisfied. 
     According to the third embodiment explained above, a light amount ratio of plural light sources can be maintained constant with high accuracy similarly to the first embodiment. 
     Moreover, according to the third embodiment, because the four light sensors  61   a  to  61   d  are arranged on the same second optical path in an optimized manner, it is possible to irradiate laser light of illumination intensity equivalent to one another to the light sensors  54   a  to  54   d  when the outputs of the light source units  51   a  to  51   d  are maximized. 
     Furthermore, according to the third embodiment, by detecting laser light of each color on the second optical path corresponding to the dichroic mirror  52   d  closest to an emitting position of illumination light to outside, it is possible to reduce an influence of characteristic variations or fluctuations of the dichroic mirror, and to further improve consistency with light to be supplied to the endoscope  2 . 
     Fourth Embodiment 
       FIG. 15  is a diagram illustrating a configuration of a light source device according to a fourth embodiment. A light source device  7  illustrated in the diagram differs in a configuration of a light sensor from the light source device  5  explained in the first embodiment. The configuration of the light source device  7  other than this point is same as the configuration of the light source device  5 . Hereinafter, explanation will be given, assigning reference symbols identical to the first embodiment to components identical to the components of the light source device  5 . 
     The light source device  7  includes four light sensor units  71   a  to  71   d . The light sensor unit  71   a  includes a light sensor  711   a  positioned at the optical path center that is a high illumination region, and a light sensor  712   a  positioned in a low illumination region apart from the optical path center by about ½ of a beam diameter. 
       FIG. 16  is a diagram schematically illustrating arrangement positions of the light sensors  711   a  and  712   a  in the light sensor unit  71   a . The curve  501  in  FIG. 16  is a curve same as that in  FIG. 11 . Moreover, the arrangement positions of the light sensors  711   a  to  712   a  illustrated in  FIG. 16  are the arrangement positions when it is viewed on a plane perpendicular to the second optical path, and a diameters of a circle corresponds to a beam diameter. 
     Arrangement positions of light sensors  711   b ,  712   b  included in the light sensor unit  71   b , arrangement positions of light sensors  711   c ,  712   c  included in the light sensor unit  71   c , arrangement positions of light sensors  711   d ,  712   d  included in the light sensor unit  71   d  are also the same. 
     According to the fourth embodiment explained above, a light amount ratio of plural light sources can be maintained constant with high accuracy similarly to the first embodiment. 
     Moreover, according to the fourth embodiment, because laser light of each color is detected in a region in which an illumination intensity is different from one another for one color, it is possible to detect light of wider dynamic range compared to the case in which a single light sensor detects laser light. Therefore, it is possible to perform adjustment of light at high resolution, and it is suitable as a light source for an endoscope. 
     Furthermore, according to the fourth embodiment, it is possible to enlarge an illumination range just by using one set of light sensor and a common ND filter, and adjustment of a light reduction ratio of an ND filter or the like is unnecessary. Therefore, it is possible to suppress cost necessary for manufacturing, and to provide an economical light source device. 
     In the fourth embodiment, the number of light sensors included in each light sensor unit may be three or more. 
     Fifth Embodiment 
       FIG. 17  is a diagram illustrating a configuration of a light source device according to a fifth embodiment. A light source device  8  illustrated in the diagram does not have the ND filters  53   a  to  53   d  and the optical filters  55   b  to  55   d  unlike the light source device  5  explained in the first embodiment, but has a correcting unit  81 . Hereinafter, for convenience of explanation, the light source units  51   a ,  51   b ,  51   c ,  51   d  are referred to as light source units  51 - 1 ,  51 - 2 ,  51 - 3 ,  51 - 4 , respectively. Moreover, the light sensors  54   a ,  54   b ,  54   c ,  54   d  are referred to as light sensors  54 - 1 ,  54 - 2 ,  54 - 3 ,  54 - 4 , respectively. To components of the light source device  8  other than these, reference symbols same as the components of the light source device  5  are assigned to be explained. 
     The correcting unit  81  corrects a light amount detected by the light sensors  54 - 1  to  54 - 4 . The correcting unit  81  stores various kinds of programs necessary for performing a correction calculation, and various kinds of data. The correcting unit  81  is constituted of hardware, such as a CPU or an FPGA, and a memory, such as a RAM and a ROM. 
     Hereinafter, the correction calculation performed by the correcting unit  81  will be explained. 
     A detection value of the light sensor  54 - i  (i=1 to 4) is s i . Because the detection value s i  has not passed through an optical filter, it is generally a detection value in a state in which plural kinds of light having wavelength bands different from one another are mixed in color. Out of this, a detection value s 1  is a detection value of only red, a detection value s 2  is a detection value in which red and green are mixed, a detection value s 3  is a detection value in which red, green and blue are mixed, and a detection value s 4  is a detection value in which red, green, blue, and violet are mixed. 
     The correcting unit  81  calculates a detection value s′ i  in a single color by correcting the detection value s i . Specifically, a detection value s′ 1  is a light amount of red laser light, a detection value s′ 2  is a light amount of green laser light, a detection value s′ 3  is a light amount of blue laser light, and a detection value s′ 4  is a light amount of violet laser light. 
     The correcting unit  81  previously stores a contribution a ij  to a light sensor  54 - j  of laser light generated by a light source unit  51 - i , to calculate the detection value s′ i . The contribution a ij  is a detection value of the light sensor  54 - j  when only the light source unit  51 - i  is activated. For example, the contribution a ij  is a detection value of the light sensor  54 - 1  when only the light source unit  51 - 1  is activated, and a 11 =1. Similarly, a 22 =a 33 =a 54 =1. Moreover, as is obvious from the configuration of the light source device  8 , a ij =0 (i&lt;j). The contribution a ij  is expresses as an element of a lower triangular matrix A below. 
     
       
         
           
             
               
                 
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     As is obvious from Eq. 1, a determinant of the matrix A is 1, and has an inverse matrix A −1 . The inverse matrix A −1  is expressed as 
     
       
         
           
             
               
                 
                   
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     A column vector s=(s 1 , s 2 , s 3 , s 4 ) T  having the detection value s i  of a single color as its element, and a column vector s′=(s′ 1 , s′ 2 , s′ 3 , s′ 4 ) T  having the detection value s′ i  of mixed colors as its element are defined herein. ( . . . ) T  signifies a transposed matrix. At this time, the column vector s and the column vector s′ have a relation of s=As′, or S′=A −1 s. 
     The correcting unit  81  corrects a light amount received by the light sensor  54 - i  out of laser light generated by the light source unit  51 - i  by calculating a column vector s′=A −1 s, using the column vector s having a detection value of the light sensor  54 - i  as its element, and the matrix A for which an element a ij  is previously stored. 
     According to the fifth embodiment explained above, a light amount ratio of plural light sources can be maintained constant with high accuracy similarly to the first embodiment. 
     Moreover, according to the fifth embodiment, because an optical filter and an ND filter are not necessary, it is possible to reduce the number of parts, to suppress cost necessary for manufacturing, and to provide an economical light source device. 
     Although a case in which the light source device  8  has four units each of the light source units and the light sensors has been explained, it is possible to calculate a light amount of a single color by performing a similar calculation by the correcting unit  81  also when the light source device has the same number of light source unit and light sensor more generally. For example, when the number of the light source unit and the light sensor is N (N is a positive integer), the column vectors s, s′ respectively have an N element, and the matrix A having the contribution a ij  as its element is to be a matrix of N rows and N columns, but the calculation performed by the correcting unit  81  is none other than s′=A −1 s. 
     Moreover, correction may be combined according to how multiplexing of light is performed in a light source device. For example, in a light source device having five units of light source units, the five light source units may be divided into a group of three units and a group of two units, to perform the correction calculation similar to the above in each group, and may synthesize light by using those. 
     Other Embodiments 
     Embodiments to implement the disclosure have so far been explained, but the disclosure is not to be limited to the first to the fifth embodiments described above. For example, the light source unit may be configured by using a light emitting diode (LED) instead of a semiconductor laser. 
     Furthermore, combinations of colors of plural light source units and an arrangement sequence in the first optical path are not limited to those of the embodiments described above. Transmission characteristics of plural dichroic mirrors and plural optical filters may also be adjusted according to the combinations of colors of the plural light source units and the arrangement sequence in the first optical path. 
     Moreover, it may be applied as a light source device of an ultrasound endoscope or an industrial endoscope, or may be applied as a light source device for purposes other than endoscopes. 
     According to the disclosure, a light amount ratio of plural light sources can be maintained constant with high accuracy. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the disclosure in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.