Abstract:
An illumination device and an observation system capable of outputting light having a continuous spectrum and high color rendering properties are provided. Employed is an illumination device including a first light source that emits first-wavelength-band light having a first wavelength band of violet color; a second light source that emits second-wavelength-band light having a second wavelength band that is broader than the first wavelength band and having a continuous spectrum; a light combining section that is composed of a dicroic mirror and that combines the first-wavelength-band light and the second-wavelength-band light; and a combination-ratio adjusting section that adjusts the combination ratio of the first-wavelength-band light and the second-wavelength-band light to be combined by the light combining section.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of International Application PCT/JP2011/078236, with an international filing date of Dec. 7, 2011, which is hereby incorporated by reference herein in its entirety. This application claims the benefit of Japanese Patent Application No. 2011-016945, the content of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an illumination device and an observation system having the same. 
     BACKGROUND ART 
     There are conventionally known illumination devices in which a plurality of light sources are provided to emit light beams having different wavelength bands from each other, and the light beams having different wavelength bands emitted from the light sources are combined by a dichroic mirror and output (for example, see PTLs 1 to 3). 
     CITATION LIST 
     Patent Literature 
     
         
         {PTL 1} Japanese Unexamined Patent Application, Publication No. 2009-86057 
         {PTL 2} Japanese Unexamined Patent Application, Publication No. 2008-292547 
         {PTL 3} Japanese Unexamined Patent Application, Publication No. 2008-176083 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     According to the illumination devices disclosed in PTLs 1 to 3, the dichroic mirror, which transmits light having a particular wavelength band and reflects light having wavelength bands other than the particular wavelength band, combines light beams having different wavelength bands; therefore, the combined light has a discontinuous spectrum. 
     Solution to Problem 
     A first aspect of the present invention provides an illumination device including: a first light source that emits first-wavelength-band light having a first wavelength band of violet color; a second light source that emits second-wavelength-band light having a second wavelength band that is broader than the first wavelength band and having a continuous spectrum; a light combining section that is composed of a dicroic mirror and that combines the first-wavelength-band light and the second-wavelength-band light; and a combination-ratio adjusting section that adjusts the combination ratio of the first-wavelength-band light and the second-wavelength-band light to be combined by the light combining section. 
     A second aspect of the present invention provides an observation system including: the above-described illumination device; and an imaging device that acquires an image of a specimen illuminated by the illumination device. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a view showing, in outline, the configuration of an illumination device according to a first embodiment of the present invention (NBI mode). 
         FIG. 2  is a graph showing a reflection characteristic of a dichroic mirror shown in  FIG. 1 . 
         FIG. 3  is a graph showing a spectral characteristic of light from a first light source shown in  FIG. 1 . 
         FIG. 4  is a graph showing a spectral characteristic of light from a second light source shown in  FIG. 1 . 
         FIG. 5  is a graph showing a spectral characteristic of the light from the second light source after being transmitted through the dichroic mirror. 
         FIG. 6  is a graph showing a spectral characteristic of light output in the NBI mode. 
         FIG. 7  is a view showing, in outline, the configuration of the illumination device shown in  FIG. 1  in a WLI mode. 
         FIG. 8  is a graph showing a spectral characteristic of light output in the WLI mode. 
         FIG. 9  is a view showing, in outline, the configuration of an illumination device according to a second embodiment of the present invention (NBI mode). 
         FIG. 10  is a graph showing a reflection characteristic of a dichroic mirror shown in  FIG. 9 . 
         FIG. 11  is a graph showing a spectral characteristic of light from a first light source shown in  FIG. 9 . 
         FIG. 12  is a graph showing a spectral characteristic of light from a second light source shown in  FIG. 9 . 
         FIG. 13  is a graph showing a spectral characteristic of light from a third light source shown in  FIG. 9 . 
         FIG. 14  is a graph showing a spectral characteristic of light output in the NBI mode. 
         FIG. 15  is a view showing, in outline, the configuration of the illumination device shown in  FIG. 9  in the WLI mode. 
         FIG. 16  is a graph showing a spectral characteristic of light from the second light source after being reflected by the dichroic mirror. 
         FIG. 17  is a graph showing a spectral characteristic of light output in the WLI mode. 
         FIG. 18  is a view showing, in outline, the configuration of an illumination device according to a third embodiment of the present invention (WLI mode). 
         FIG. 19  is a graph showing a reflection characteristic of a dichroic mirror shown in  FIG. 18 . 
         FIG. 20  is a graph showing a spectral characteristic of light from a first light source shown in  FIG. 18 . 
         FIG. 21  is a graph showing a spectral characteristic of light from a second light source shown in  FIG. 18 . 
         FIG. 22  is a graph showing a spectral characteristic of light from a third light source shown in  FIG. 18 . 
         FIG. 23  is a graph showing a spectral characteristic of light output in the WLI mode. 
         FIG. 24  is a view showing, in outline, the configuration of the illumination device shown in  FIG. 18  in the NBI mode. 
         FIG. 25  is a graph showing a spectral characteristic of light from the second light source after being transmitted through the dichroic mirror. 
         FIG. 26  is a graph showing a spectral characteristic of light output in the NBI mode. 
         FIG. 27  is a view showing, in outline, the configuration of the illumination device shown in  FIG. 18  in an MI mode. 
         FIG. 28  is a graph showing a spectral characteristic of light from the second light source after being transmitted through the dichroic mirror. 
         FIG. 29  is a graph showing a spectral characteristic of light output in the MI mode. 
         FIG. 30  is a graph showing a spectral characteristic of light superimposed in an illumination device according to a first modification. 
         FIG. 31  is a graph showing a reflection characteristic of a dichroic mirror of an illumination device according to a second modification. 
         FIG. 32  is a graph showing a spectral characteristic of light combined by the dichroic mirror of  FIG. 31 . 
         FIG. 33  is a graph showing a reflection characteristic when the stop angle of the dichroic mirror of  FIG. 31  is changed. 
         FIG. 34  is a graph showing a spectral characteristic of light combined by the dichroic mirror of  FIG. 33 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     An illumination device according to a first embodiment of the present invention will be described below with reference to the drawings. 
     An illumination device  1  of this embodiment is applied to an observation system for observing body tissue, for example, and includes, as shown in  FIG. 1 , a first light source  10  and a second light source  20  that are disposed such that their optical axes are perpendicular to each other, a dichroic mirror (light combining section)  40  that is disposed at the point of intersection of the optical axis of the first light source  10  and the optical axis of the second light source  20 , and a light guide  43  that is disposed on the optical axis of the second light source  20 . 
     The first light source  10  is an LED that emits light (first-wavelength-band light) L 1  having relatively short wavelengths, such as violet light, as shown in  FIG. 3 . 
     Lens groups  11  and  12  that convert the light L 1  from the first light source  10  into substantially collimated light are disposed on the axis of the light emitted from the first light source  10 . 
     The second light source  20  is an LED that emits light (second-wavelength-band light) L 2  having a relatively broadband and continuous spectrum, such as white light, as shown in  FIG. 4 . 
     Lens groups  21  and  22  that convert the light L 2  from the second light source  20  into substantially collimated light are disposed on the axis of the light emitted from the second light source  20 . 
     As shown in  FIG. 2 , the dichroic mirror  40  has a reflection characteristic so as to transmit light having a wavelength band from 500 nm (inclusive) to 600 nm (exclusive) and reflect light having a wavelength band up to 500 nm (exclusive) and a wavelength band from 600 nm (inclusive), for example. Note that the graphs shown in the  FIG. 2  indicate the angles of light incident on, the dichroic mirror  40 . 
     With this reflection characteristic, the dichroic mirror  40  reflects the light L 1  emitted from the first light source  10  and transmits, in the light L 2  emitted from the second light source  20 , light having a wavelength band from 500 nm (inclusive) to 600 nm (exclusive). 
     Furthermore, as shown in  FIG. 7 , the dichroic mirror  40  is moved in the direction of the optical axis of the first light source  10  by a movement mechanism (not shown) according to an illumination mode, to be described later, thereby being inserted into or removed from the optical axis of the second light source  20 . 
     The illumination device  1  of this embodiment has an NBI (narrow band imaging) mode for radiating light having particular wavelength bands and a WLI (white light imaging) mode for radiating white light. When the user selects desired one of the illumination modes with a touch panel or a switch, for example, the dichroic mirror  40  is inserted into or removed from the optical axis of the second light source  20  by the movement mechanism (not shown). 
     In the NBI mode, light having two narrow wavelength bands (for example, light having a wavelength band from 390 to 445 nm and light having a wavelength band from 530 to 550 nm) that is easily absorbed into hemoglobin in the blood is radiated, so that capillaries in a superficial portion of a mucous membrane and mucosal fine patterns can be displayed in an emphasized manner. 
     In the WLI mode, white light having a broad wavelength band is radiated, so that observation with high color rendering properties can be performed. 
     Lens groups  41  and  42  that focus the light from the dichroic mirror  40  on an inlet end of the light guide  43  are disposed between the dichroic mirror  40  and the light guide  43 . 
     The light guide  43  is a light guiding rod formed of glass, for example, and guides the light from the dichroic mirror  40  focused by the lens groups  41  and  42  to an outlet end thereof. 
     The operation of the illumination device  1 , having the above-described configuration, will be described below. 
     First, in the NBI mode, the dichroic mirror  40  is disposed on the optical axis of the second light source  20 , as shown in  FIG. 1 . 
     In this state, the first light source  10  and the second light source  20  are turned on. 
     The light L 1  having relatively short wavelengths, such as violet light, is emitted from the first light source  10 . The light L 1  emitted from the first light source  10  is converted into substantially collimated light by the lens groups  11  and  12  and is reflected to the inlet end of the light guide  43  by the dichroic mirror  40 . 
     The light L 2  having a relatively broadband and continuous spectrum, such as white light, is emitted from the second light source  20 . The light L 2  emitted from the second light source  20  is converted into substantially collimated light by the lens groups  21  and  22  and is incident on the dichroic mirror  40 . At the dichroic mirror  40 , part of the light L 2  emitted from the second light source  20  is transmitted to the inlet end of the light guide  43 , and the other part thereof is reflected, as shown in  FIG. 5 . Specifically, in the light L 2  emitted from the second light source  20 , the dichroic mirror  40  transmits light L 2 ′ having a wavelength band from 500 nm (inclusive) to 600 nm (exclusive) and reflects light having a wavelength band up to 500 nm (exclusive) and a wavelength band from 600 nm (inclusive), for example. 
     The dichroic mirror  40 , having this reflection characteristic, combines the light L 1  emitted from the first light source  10  with the light L 2 ′ having a wavelength band from 500 nm (inclusive) to 600 nm (exclusive) in the light L 2  emitted from the second light source  20 , as shown in  FIG. 6 . The thus-combined light is focused on the inlet end of the light guide  43  by the lens groups  41  and  42 , is guided by the light guide  43  to the outlet end thereof, and is output therefrom. 
     By doing so, it is possible to radiate light having two narrow wavelength bands that is easily absorbed into hemoglobin in the blood (for example, light having a wavelength band from 390 to 445 nm and light having a wavelength band from 530 to 550 nm) and to display capillaries in a superficial portion of the mucous membrane and mucosal fine patterns in an emphasized manner. 
     Next, in the WLI mode, the dichroic mirror  40  is moved in the direction of the optical axis of the first light source  10  by the movement mechanism (not shown), and is removed from the light path of the second light source  20 , as shown in  FIG. 7 . 
     In this state, the second light source  20  is turned on. Note that the first light source  10  may also be turned on because light from the first light source  10  is not guided to the inlet end of the light guide  43  in this state. 
     The light L 2  having a relatively broadband and continuous spectrum, such as white light, is emitted from the second light source  20 . The light L 2  emitted from the second light source  20  is converted into substantially collimated light by the lens groups  21  and  22 , is focused on the inlet end of the light guide  43  by the lens groups  41  and  42 , and is guided by the light guide  43  to the outlet end thereof. 
     Thus, it is possible to radiate the light L 2  emitted from the second light source  20 , i.e., white light having a broadband and continuous spectrum, as shown in  FIG. 8 , and to perform observation with high color rendering properties. 
     As described above, according to the illumination device  1  of this embodiment, the light L 1  from the first light source  10  and the light L 2  from the second light source  20  are combined by the dichroic mirror  40  and output. In this case, by moving the dichroic mirror  40  in the direction of the optical axis of the first light source  10  with the movement mechanism (not shown), it is possible to switch between and output light having different wavelength bands. By doing so, it is possible to output light by switching between the wavelength bands according to the subject to be illuminated. 
     More specifically, in order to output light having particular wavelength bands, the NBI mode is selected, and the first light source  10  and/or the second light source  20  are/is turned on with the dichroic mirror  40  disposed on the axis of the light L 2 . As a result, light having particular wavelength bands can be output. Furthermore, in order to output broad light, such as white light, the WLI mode is selected, and the second light source  20  is turned on with the dichroic mirror  40  removed from the optical axis of the second light source  20 . As a result, the light L 2  having a broadband and continuous spectrum can be output directly. 
     Second Embodiment 
     Next, an illumination device according to a second embodiment of the present invention will be described mainly with reference to  FIGS. 9 to 17 . 
     The illumination device of this embodiment differs from that of the first embodiment in that a third light source  30  is provided in addition to the first light source  10  and the second light source  20 . The differences in the illumination device of this embodiment from the first embodiment will be mainly described below, and a description of similarities will be omitted. 
     As shown in  FIG. 9 , an illumination device  2  of this embodiment includes the first light source  10 , the second light source  20 , and the third light source  30  that are disposed such that their optical axes are perpendicular to each other, the dichroic mirror (light combining section)  40  that is disposed at the point of intersection of the optical axes of the light sources, and the light guide  43  that is disposed on the optical axis of the third light source  30 . 
     The first light source  10  and the second light source  20  are disposed facing each other, and the third light source  30  is disposed with the optical axis thereof directed in a direction that is perpendicular to the optical axes of the first light source  10  and the second light source  20 . 
     The first light source  10  is an LED that emits light (first-wavelength-band light) L 1  having relatively short wavelengths, such as violet light, as shown in  FIG. 11 . 
     The lens groups  11  and  12  that convert the light L 1  from the first light source  10  into substantially collimated light are disposed on the axis of the light emitted from the first light source  10 . 
     The second light source  20  is an LED that emits light (second-wavelength-band light) L 2  having a relatively broadband and continuous spectrum, such as white light, as shown in  FIG. 12 . 
     The lens groups  21  and  22  that convert the light L 2  from the second light source  20  into substantially collimated light are disposed on the axis of the light emitted from the second light source  20 . 
     The third light source  30  is an LED that emits light (third-wavelength-band light) L 3  having intermediate wavelengths, such as green light, as shown in  FIG. 13 . The wavelength band of the light L 3  is part of the wavelength band of the light L 2  from the second light source  20 . 
     Lens groups  31  and  32  that convert the light L 3  from the third light source  30  into substantially collimated light are disposed on the axis of the light emitted from the third light source  30 . 
     As shown in  FIG. 10 , the dichroic mirror  40  has a reflection characteristic so as to transmit light having a wavelength band from 500 nm (inclusive) to 600 nm (exclusive) and reflect light having a wavelength band up to 500 nm (exclusive) and a wavelength band from 600 nm (inclusive), for example. 
     With this reflection characteristic, the dichroic mirror  40  reflects the light L 1  emitted from the first light source  10 , transmits light having a wavelength band from 500 nm (inclusive) to 600 nm (exclusive) in the light L 2  emitted from the second light source  20 , and transmits the light L 3  emitted from the third light source  30 . 
     Furthermore, as shown in  FIG. 15 , the dichroic mirror  40  is rotated by a movement mechanism (not shown) about the axis perpendicular to the optical axes of the light sources (axis vertical to the figure), according to an illumination mode, to be described later. 
     The illumination device  2  of this embodiment has the NBI (narrow band imaging) mode for radiating light having particular wavelength bands and the WLI (white light imaging) mode for radiating white light. When the user selects desired one of the illumination modes with a touch panel or a switch, for example, the dichroic mirror  40  is rotated by the movement mechanism (not shown) about the axis perpendicular to the optical axes of the light sources. 
     The lens groups  41  and  42  that focus light from the dichroic mirror  40  on the inlet end of the light guide  43  are disposed between the dichroic mirror  40  and the light guide  43 . 
     The light guide  43  is a light guiding rod formed of glass, for example, and guides the light from the dichroic mirror  40  focused by the lens groups  41  and  42  to the outlet end thereof. 
     The operation of the illumination device  2 , having the above-described configuration, will be described below. 
     First, in the NBI mode, the dichroic mirror  40  is set at an angle so as to reflect the light L 1  emitted from the first light source  10  to the inlet end of the light guide  43 , as shown in  FIG. 9 . 
     In this state, the first light source  10  and the third light source  30  are turned on. 
     The light L 1  having relatively short wavelengths, such as violet light, is emitted from the first light source  10 . The light L 1  emitted from the first light source  10  is converted into substantially collimated light by the lens groups  11  and  12  and is reflected to the inlet end of the light guide  43  by the dichroic mirror  40 . 
     The light L 3  having intermediate wavelengths, such as green light, is emitted from the third light source  30 . The light L 3  emitted from the third light source  30  is converted into substantially collimated light by the lens groups  31  and  32 , is transmitted through the dichroic mirror  40 , and enters the inlet end of the light guide  43 . 
     The dichroic mirror  40 , having this reflection characteristic, combines the light L 1  emitted from the first light source  10  with the light L 3  emitted from the third light source  30 , as shown in  FIG. 14 . The thus-combined light is focused on the inlet end of the light guide  43  by the lens groups  41  and  42 , is guided by the light guide  43  to the outlet end thereof, and is output therefrom. 
     By doing so, it is possible to radiate light having two narrow wavelength bands that is easily absorbed into hemoglobin in the blood (for example, light having a wavelength band from 390 to 445 nm and light having a wavelength band from 530 to 550 nm) and to display capillaries in a superficial portion of the mucous membrane and mucosal fine patterns in an emphasized manner. 
     Next, in the WLI mode, as shown in  FIG. 15 , the dichroic mirror  40  is rotated by the movement mechanism (not shown) about the axis perpendicular to the optical axes of the light sources and is set at an angle so as to reflect the light L 2  emitted from the second light source  20  to the inlet end of the light guide  43 . 
     In this state, the second light source  20  and the third light source  30  are turned on. 
     The light L 2  having a relatively broadband and continuous spectrum, such as white light, is emitted from the second light source  20 . The light L 2  emitted from the second light source  20  is converted into substantially collimated light by the lens groups  21  and  22  and is incident on the dichroic mirror  40 . At the dichroic mirror  40 , part of the light L 2  emitted from the second light source  20  is transmitted to the inlet end of the light guide  43 , and the other part thereof is reflected, as shown in  FIG. 16 . Specifically, in the light L 2  emitted from the second light source  20 , the dichroic mirror  40  transmits light having a wavelength band from 500 nm (inclusive) to 600 nm (exclusive) and reflects light L 2 ′ having a wavelength band up to 500 nm (exclusive) and a wavelength band from 600 nm (inclusive), for example. 
     The dichroic mirror  40 , having this reflection characteristic, combines the light L 3  emitted from the third light source  30  and the light having a wavelength band up to 500 nm (exclusive) and a wavelength band from 600 nm (inclusive) in the light L 2  emitted from the second light source  20 , as shown in  FIG. 17 . The thus-combined light is focused on the inlet end of the light guide  43  by the lens groups  41  and  42 , is guided by the light guide  43  to the outlet end thereof, and is output therefrom. 
     As a result, the light L 2 ′ having a wavelength band from 500 nm (inclusive) to 600 nm (exclusive) in the light L 2  emitted from the second light source  20  can be replaced with the light L 3  emitted from the third light source  30 , for output. It is known that, when the intensity of light having intermediate wavelengths, such as green light, is increased, the human eye perceives a significant increase in brightness. Therefore, the intensity of the light L 3  from the third light source  30  is increased, thereby making it possible to radiate bright white light having a broadband and continuous spectrum and to improve the accuracy of observation. 
     As described above, according to the illumination device  2  of this embodiment, in order to output light having particular wavelength bands, the NBI mode is selected, and the dichroic mirror  40  is rotated about the axis perpendicular to the optical axes of the light sources so as to reflect the light L 1  from the first light source  10  in the direction of the axis of combined light to be output. In this state, the first light source  10  and/or the third light source  30  are/is turned on, thus outputting light having particular wavelength bands. 
     Furthermore, in order to output broad light, such as white light, the WLI mode is selected, and the dichroic mirror  40  is rotated about the axis perpendicular to the optical axes of the light sources so as to reflect the light L 2  from the second light source  20  in the direction of the axis of combined light to be output. In this state, the second light source  20  and the third light source  30  are turned on, thus outputting bright combined light that has a broadband and continuous spectrum and that is obtained after part of the wavelength band of the light L 2  from the second light source  20  is replaced with that of the light L 3  from the third light source  30 . 
     Third Embodiment 
     Next, an illumination device according to a third embodiment of the present invention will be described mainly with reference to  FIGS. 18 to 29 . 
     The illumination device of this embodiment differs from those of the above-described embodiments in that an MI (molecular imaging) mode using IR light is provided in addition to the NBI mode and the WLI mode. The differences in the illumination device of this embodiment from those of the above-described embodiments will be mainly described below, and a description of similarities will be omitted. 
     The MI (molecular imaging) is an observation method in which the distribution and the movement of particular molecules (for example, glucose and various proteins) in the body are visualized with PET-CT etc. to dynamically capture various phenomena occurring in the body. With this observation method, the presence or absence of cancer, neurological disease, or heart disease, for example, the degree of progress thereof, and the degree of malignancy thereof can be diagnosed. 
     As shown in  FIG. 18 , an illumination device  3  of this embodiment includes the first light source  10 , the second light source  20 , and the third light source  30 , which are disposed such that their optical axes are perpendicular to each other, the dichroic mirror (light combining section)  40 , which is disposed at the point of intersection of the optical axes of the light sources, and the light guide  43  that is disposed on the optical axis of the second light source  20 . 
     The first light source  10  and the third light source  30  are disposed facing each other, and the second light source  20  is disposed with the optical axis thereof directed in a direction that is perpendicular to the optical axes of the first light source  10  and the third light source  30 . 
     The first light source  10  is an LED that emits light (first-wavelength-band light) L 1  having relatively short wavelengths, such as violet light, as shown in  FIG. 20 . 
     The lens groups  11  and  12  that convert the light L 1  from the first light source  10  into substantially collimated light are disposed on the axis of the light emitted from the first light source  10 . 
     The second light source  20  is an LED that emits light (second-wavelength-band light) L 2  having a relatively broadband and continuous spectrum, such as white light, as shown in  FIG. 21 . 
     The lens groups  21  and  22  that convert the light L 2  from the second light source  20  into substantially collimated light are disposed on the axis of the light emitted from the second light source  20 . 
     The third light source  30  is an LED that emits IR light (third-wavelength-band light) L 3 , for example, as shown in  FIG. 22 . The light L 3  has a wavelength band that is different from the light L 2  from the second light source  20 . 
     The lens groups  31  and  32  that convert the light L 3  from the third light source  30  into substantially collimated light are disposed on the axis of the light emitted from the third light source  30 . 
     As shown in  FIG. 19 , the dichroic mirror  40  has a reflection characteristic so as to transmit light having a wavelength band from 500 nm (inclusive) to 600 nm (exclusive) and reflect light having a wavelength band up to 500 nm (exclusive) and a wavelength band from 600 nm (inclusive), for example. Note that the graphs shown in the  FIG. 19  indicate the angles of light incident on the dichroic mirror  40 . 
     With this reflection characteristic, the dichroic mirror  40  reflects the light L 1  emitted from the first light source  10 , transmits light having a wavelength band from 500 nm (inclusive) to 600 nm (exclusive) in the light L 2  emitted from the second light source  20 , and transmits the light L 3  emitted from the third light source  30 . 
     Furthermore, as shown in  FIG. 24 , the dichroic mirror  40  is rotated by a movement mechanism (not shown) about the axis perpendicular to the optical axes of the light sources (axis vertical to the figure), according to an illumination mode, to be described later. 
     The illumination device  3  of this embodiment has the NBI (narrow band imaging) mode for radiating light having particular wavelength bands, the WLI (white light imaging) mode for radiating white light, and the MI (molecular imaging) mode. When the user selects desired one of the illumination modes with a touch panel or a switch, for example, the dichroic mirror  40  is rotated by the movement mechanism (not shown) about the axis perpendicular to the optical axes of the light sources. 
     The lens groups  41  and  42  that focus light from the dichroic mirror  40  on the inlet end of the light guide  43  are disposed between the dichroic mirror  40  and the light guide  43 . 
     The light guide  43  is a light guiding rod formed of glass, for example, and guides the light from the dichroic mirror  40  focused by the lens groups  41  and  42  to the outlet end thereof. 
     The operation of the illumination device  3 , having the above-described configuration, will be described below. 
     First, in the WLI mode, the dichroic mirror  40  is rotated by the movement mechanism (not shown) about the axis perpendicular to the optical axes of the light sources, as shown in  FIG. 18 , and is disposed such that the reflecting surface thereof is in the direction of the optical axis of the second light source  20 . 
     In this state, the second light source  20  is turned on. 
     The light L 2  having a relatively broadband and continuous spectrum, such as white light, is emitted from the second light source  20 . The light L 2  emitted from the second light source  20  is converted into substantially collimated light by the lens groups  21  and  22 , is focused on the inlet end of the light guide  43  by the lens groups  41  and  42 , is guided by the light guide  43  to the outlet end thereof, and is output therefrom. 
     Thus, it is possible to radiate the light L 2  emitted from the second light source  20 , i.e., white light having a broadband and continuous spectrum, as shown in  FIG. 23 , and to perform observation with high color rendering properties. 
     Next, in the NBI mode, the dichroic mirror  40  is set at an angle so as to reflect the light L 1  emitted from the first light source  10  to the inlet end of the light guide  43 , as shown in  FIG. 24 . 
     In this state, the first light source  10  and the second light source  20  are turned on. 
     The light L 1  having relatively short wavelengths, such as violet light, is emitted from the first light source  10 . The light L 1  emitted from the first light source  10  is converted into substantially collimated light by the lens groups  11  and  12  and is reflected to the inlet end of the light guide  43  by the dichroic mirror  40 . 
     The light L 2  having a relatively broadband and continuous spectrum, such as white light, is emitted from the second light source  20 . The light L 2  emitted from the second light source  20  is converted into substantially collimated light by the lens groups  21  and  22  and is incident on the dichroic mirror  40 . At the dichroic mirror  40 , part of the light L 2  emitted from the second light source  20  is transmitted to the inlet end of the light guide  43 , and the other part thereof is reflected, as shown in  FIG. 25 . Specifically, in the light L 2  emitted from the second light source  20 , the dichroic mirror  40  transmits the light L 2 ′ having a wavelength band from 500 nm (inclusive) to 600 nm (exclusive) and reflects light having a wavelength band up to 500 nm (exclusive) and a wavelength band from 600 nm (inclusive), for example. 
     The dichroic mirror  40 , having this reflection characteristic, combines the light L 1  emitted from the first light source  10  with the light L 2 ′ having a wavelength band from 500 nm (inclusive) to 600 nm (exclusive) in the light L 2  emitted from the second light source  20 , as shown in  FIG. 26 . The thus-combined light is focused on the inlet end of the light guide  43  by the lens groups  41  and  42 , is guided by the light guide  43  to the outlet end thereof, and is output therefrom. 
     By doing so, it is possible to radiate light having two narrow wavelength bands that is easily absorbed into hemoglobin in the blood (for example, light having a wavelength band from 390 to 445 nm and light having a wavelength band from 530 to 550 nm) and to display capillaries in a superficial portion of the mucous membrane and mucosal fine patterns in an emphasized manner. 
     Next, in the MI mode, the dichroic mirror  40  is set at an angle so as to reflect the light L 3  emitted from the third light source  30  to the inlet end of the light guide  43 , as shown in  FIG. 27 . 
     In this state, the second light source  20  and the third light source  30  are turned on. 
     The light L 3  having relatively short wavelengths, such as IR light, is emitted from the third light source  30 . The light L 3  emitted from the third light source  30  is converted into substantially collimated light by the lens groups  31  and  32  and is reflected by the dichroic mirror  40  to the inlet end of the light guide  43 . 
     The light L 2  having a relatively broadband and continuous spectrum, such as white light, is emitted from the second light source  20 . The light L 2  emitted from the second light source  20  is converted into substantially collimated light by the lens groups  21  and  22  and is incident on the dichroic mirror  40 . At the dichroic mirror  40 , part of the light L 2  emitted from the second light source  20  is transmitted to the inlet end of the light guide  43 , and the other part thereof is reflected, as shown in  FIG. 28 . Specifically, in the light L 2  emitted from the second light source  20 , the dichroic mirror  40  transmits the light L 2 ′ having a wavelength band from 500 nm (inclusive) to 600 nm (exclusive) and reflects light having a wavelength band up to 500 nm (exclusive) and a wavelength band from 600 nm (inclusive), for example. 
     The dichroic mirror  40 , having this reflection characteristic, combines the light L 3  emitted from the third light source  30  and the light L 2 ′ having a wavelength band from 500 nm (inclusive) to 600 nm (exclusive) in the light L 2  emitted from the second light source  20 , as shown in  FIG. 29 . The thus-combined light is focused on the inlet end of the light guide  43  by the lens groups  41  and  42 , is guided by the light guide  43  to the outlet end thereof, and is output therefrom. 
     Thus, by using the light L 3  emitted from the third light source  30  as excitation light and using the light L 2 ′ having a wavelength band from 500 nm (inclusive) to 600 nm (exclusive) in the light L 2  emitted from the second light source  20  as reference light, it is possible to observe the distribution and the movement of particular molecules (for example, glucose and various proteins) in the body and to diagnose the presence or absence of cancer, neurological disease, or heart disease, for example, the degree of progress thereof, and the degree of malignancy thereof. 
     As described above, according to the illumination device  3  of this embodiment, in order to output light having particular wavelength bands that includes the light L 1  from the first light source  10 , the NBI mode is selected, and the dichroic mirror  40  is rotated about the axis perpendicular to the optical axes of the light sources so as to reflect the light L 1  from the first light source  10  in the direction of the axis of combined light to be output. In this state, the first light source  10  and/or the second light source  20  are/is turned on, thus outputting light having desired wavelength bands. 
     Furthermore, for example, in order to output broad light, such as white light, the WLI mode is selected, and the dichroic mirror  40  is rotated about the axis perpendicular to the optical axes of the light sources such that the reflecting surface of the dichroic mirror  40 , which is a plate-like filter, is in the direction of the optical axis of the second light source  20 . In this state, the second light source  20  is turned on, thus outputting the light L 2  having a broadband and continuous spectrum directly, without being reflected by the dichroic mirror  40 . 
     Furthermore, in order to output light having particular wavelength bands that includes the light L 3  (IR light) from the third light source  30 , the MI mode is selected, and the dichroic mirror  40  is rotated about the axis perpendicular to the optical axes of the light sources so as to reflect the light L 3  from the third light source  30  in the direction of the axis of combined light to be output. In this state, the third light source  30  and/or the second light source  20  are/is turned on, thus outputting light having desired wavelength bands. 
     First Modification 
     In a first modification of the illumination device  3  of this embodiment, rotation of the dichroic mirror  40  and lighting of the light sources may be synchronized. The light L 1  emitted from the first light source  10 , the light L 2  emitted from the second light source  20 , and the light L 3  emitted from the third light source  30  have a complementary relationship in terms of their spectra. Therefore, when the light sources are sequentially turned on in synchronization with rotation of the dichroic mirror  40 , it is possible to output light having a broadband and continuous spectrum, as superimposed light, as shown in  FIG. 30 . 
     Furthermore, since the WLI mode, the NBI mode, and the MI mode can be sequentially switched at high speed, high-speed frame-sequential illumination can be performed. In this case, since images of a subject can be acquired in the three illumination modes by synchronizing the illumination modes with image acquisition, it is possible to superimpose the images obtained in the NBI mode and the MI mode on the image obtained in the WLI mode through image processing or to display the images while switching between them at high speed. 
     Second Modification 
     In a second modification of the illumination device  3  of this embodiment, the stop angle of the dichroic mirror  40  may be fine-tuned. 
     Specifically, by taking account of the incidence angle dependence (wavelength shift) of the dichroic mirror  40 , the stop angle of the dichroic mirror  40  may be set to 43° and 40°, for example, in addition to the above-described three patterns, i.e., 45°, 0°, and −45°. Thus, wavelengths in the G band to be extracted from white light (the light L 2  emitted from the second light source  20 ) can be desirably set according to the angle at which the dichroic mirror  40  is stopped. 
     For example, violet light and green light are used in both the NBI mode and an AFI mode, but the wavelengths of light to be radiated are slightly different therebetween (the center wavelength of the G band in the NBI mode is 10 nm shorter than that in the AFI mode). By changing the stop angle of the filter between the NBI mode and the AFI mode, the optimum wavelengths in the G band can be provided. 
     Furthermore, when the spectral characteristic of the dichroic mirror  40  is set as shown in  FIG. 31 , and the stop angle of the dichroic mirror  40  is changed as shown in  FIG. 33 , the wavelengths of light from the light sources can be shifted as shown in  FIG. 32  and  FIG. 34 . As a result, for example, even in order to shift the wavelengths of excitation light by several tens of nanometers depending on the type of fluorescent reagent, the stop angle of the dichroic mirror  40  can be changed to fine-tune the wavelengths of excitation light (output light), for fluoroscopy. 
     The embodiments of the present invention have been described above in detail with reference to the drawings; however, the specific configurations are not limited to the embodiments, and design changes that do not depart from the scope of the present invention are also encompassed. For example, the present invention can be applied to an embodiment obtained by appropriately combining the above-described embodiments and modifications. 
     Furthermore, in each of the embodiments, a description has been given of an example case in which the illumination device of the present invention is applied to the observation system. Example systems to which such an illumination device can be applied include an endoscope system using special light (as in infrared-light observation and drug fluorescence observation) and a microscope system for fluoroscopy. 
     Furthermore, in the embodiments, a description has been given of a case where each light source is a single separate LED; however, a multi-chip LED module in which chips having different wavelengths are packaged may be used. As a result, it is possible to increase the options for the spectrum of output light (combined light) and to obtain light of an intended band. 
     Furthermore, although a description has been given of a case where the dichroic mirror is used as a specific example of the light combining section, for example, a dichroic prism may be used. 
     REFERENCE SIGNS LIST 
     
         
           1 ,  2 ,  3  illumination device 
           10  first light source 
           11 ,  12  lens group 
           20  second light source 
           21 ,  22  lens group 
           30  third light source 
           31 ,  32  lens group 
           40  dichroic mirror (light combining section) 
           41 ,  42  lens group 
           43  light guide 
         L 1  first-wavelength-band light 
         L 2  second-wavelength-band light 
         L 3  third-wavelength-band light