Abstract:
An imaging system for use with an endoscope, including a light source which emits white light and excitation light which will produce a fluorescence response by an object under inspection, an imaging camera including separate paths for processing images produced by white light and excitation light, a selection device that causes the imaging device to operate in a white light mode or an excitation light mode, and a protective device that prevents damage to high-sensitivity imaging components from exposure to excessive light input. Fluorescent image data are separated into at least red and green color bands which are separately processed to produced a video display in which normal tissue is displayed in predetermined specific color, and abnormal tissue in one or more distinctly different colors. In one embodiment, an image color interpretation guide is provided in the form of multiple color bars which are superimposed on a single video display device with the image display. of different kinds. In another embodiment, color control is provided by adjusting the amplification of the imaging components for each of the color bands while viewing tissue known to be normal using a recursive algorithm until the ratio of the maximum values of the color separation signals fall within a predetermined range. The high-sensitivity imaging components are protected by controlling impingement of light on the imagining components, selectively controlling emission of white and excitation light from the light source, and controlling the power source for the imaging components.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This is a divisional of U.S. patent application Ser. No. 09/153,793, filed Sep. 15, 1998 in the name of Mamoru KANEKO, Hitoshi UENO, Sakae TAKEHANA, Isami HIRAO, Nobuyuki DOGUCHI, Takeshi OZAWA, Takeflimi UESUGI, Katsuichi IMAIZUMI, Yasukazu KOGEN, Makoto TOMIOKA, Tadashi HIRATA and Masahiro KAWAUCHI entitled FLUORESCENT IMAGING DEVICE. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to fluorescent imaging devices to conduct fluorescent observations by using an endoscope to irradiate an excitation light onto an area of a biological tissue to be examined with such devices being characterized by the ability to switch between fluorescent observation and a conventional reflected light observation.  
           [0004]    2. Description of the Related Art  
           [0005]    Recently, diagnostic technics have been developed using an endoscope to irradiate tissue to be studied with visible light and to detect resulting fluorescent images which are then analyzed for diagnostic purposes. These technics have been found particularly useful for diagnosing disease conditions such as cancers or tissue degeneration and for highlighting the boundary regions of the conditions under study. These technics are sometimes enhanced by also studying normal light images resulting from reflection of the irradiating visible light (usually white light).  
           [0006]    In the case of autofluorescence, i.e., the stimulated emission resulting from impingement of the excitation light onto a biological tissue, the fluorescence typically has a longer wavelength than that of the excitation light. Fluorescent substances within organisms are exemplified by collagens, NADH (nicotinamide adenine dinucleotide), FMN (flavin mononucleotide), pyridine nucleotide and the like. Recently, the relationship between such fluorescent substances and various diseases has been recognized, making it possible to diagnose cancers and the like by these fluorescences.  
           [0007]    In addition, certain fluorescent substances such as HpD (hematoporphyrin), Photofrin, ALA (δ-amino levulinic acid), and GFP (Green fluorescent protein), have been found which are selectively absorbed by cancers and thus may be used as contrast materials. In addition, certain fluorescent substances may be added to a monoclonal antibody whereby the fluorescent may be attached to affected areas by an antigen-antibody reaction.  
           [0008]    As the excitation lights, for example, lasers, mercury lamps, metal halide lamps and the like are used. For example, when a light with the wavelength of 437 nm is emitted onto a gastrointestinal tract tissue, green autofluorescence by abnormal tissues is attenuated compared to the autofluorescence of normal tissues, but red autofluorescence of abnormal tissues is not attenuated as much compared to the autofluorescence of normal tissues. A transendoscopic fluorescent observation device utilizing this principle to image the green and red fluorescent emission, and to show the existence of abnormal tissues has been disclosed in Japanese Unexamined Patent Publication No. 9-327433.  
           [0009]    Since the fluorescent images obtained in this way have very weak intensities compared to the reflected images obtained with conventional white light, photomultiplication, for example, using an image intensifier is necessary.  
           [0010]    Generally, when a blue or ultraviolet light is emitted onto biological tissue, an autofluorescence occurs within a longer wavelength band than that of the excitation light. Moreover, fluorescent spectra are different between normal tissues and abnormal tissues such as precancerous tissues, cancerous tissues, inflammatory tissues, and dysplastic tissues so that the existence of lesions and conditions of lesions can be detected based on the changes in delicate coloration of the fluorescent images.  
           [0011]    In particular, since with a blue excitation light, the intensity distribution of fluorescence stimulated near the green region, especially that of 490 nm-560 nm, is stronger in normal tissue than in diseased tissue, emissions in the green region and in the red region, e.g., wavelengths in the 620 nm-800 nm region are arithmetically processed to generate two-dimensional fluorescent images, and by these fluorescent images the discrimination between affected areas and normal areas can be achieved.  
           [0012]    Video images are produced for diagnostic observation of the autofluorescent emissions, and adjustments are made to the ratio between the video signals corresponding to the green and red fluorescent intensities to allow normal tissues to have a certain color tone. Accordingly, tissue known to be normal is first observed, and the ratios of the red and green emissions are adjusted to establish a reference color tone. Then, after the adjustment of the color tone of the normal parts, the potentially diseased tissue is observed. In this way, the normal parts are designated with a certain color tone and abnormal parts are designated with different color tones from that of the normal parts due to the attenuation of the green signal. By the differences in color tones between abnormal and normal parts, the abnormal parts can be visualized. Typically, the ratio is adjusted so that the normal tissue appears a cyanic color tone and diseased tissue appears a red color tone.  
           [0013]    Moreover, in a fluorescent observation device of Japanese Unexamined Patent Publication No. 8-557, a single light source is used both as an excitation light to conduct fluorescent observations and as a white light to conduct white light observations by insertion and removal of a filter. Endoscopes usually also include an emergency light source which permits safe removal of the instrument in case of failure of the main light source.  
           [0014]    As will be understood, when only fluorescent images are desired, there should be no illumination by white light, but only by the excitation light. Thus, switching is required so that when a white light image is to be obtained, a white light is emitted, and when a fluorescent image is to be obtained, an excitation light is emitted.  
           [0015]    Also, switching is controlled so that, when white light is emitted, the resulting image is provided only to a white image imaging device, and when the excitation light is emitted, the fluorescent image is provided only to the high-sensitivity fluorescent imaging device. However, with conventional fluorescent imaging devices, since the endoscope is out of the body when power is applied, if the device is accidently set in its fluorescent observation mode, ambient light may impinge on the fluorescent imaging device. Then, an excess of light enters the image intensifier, and overprint at the high-sensitivity imaging plane of the image intensifier occurs, resulting in its breakdown.  
           [0016]    Also with the fluorescent observation device of said Japanese Unexamined Patent Publication No. 8-557, in the case of lamp failure during fluorescent observation, the emergency light provides insufficient luminous energy to excite the tissue sufficiently, making it difficult to observe fluorescence. In addition, even with the emergency light, if the filter for excitation light generation is carelessly removed from in front of the emergency light, the image intensifier will be burnt.  
           [0017]    Moreover, since the delicate variations in coloration of fluorescent images are subjectively visualized by the operator, the lack of fixed discrimination standards makes it difficult to compare of findings by different users, and at different facilities such as hospitals.  
           [0018]    Also in the conventional example in Japanese Unexamined Patent Publication No. 9-327433, since adjustment of color tone for normal parts is performed on the individual judgment of the user, the absence of fixed calibration standards renders objective diagnosis by color tone difficult.  
         SUMMARY OF THE INVENTION  
         [0019]    One object of the present invention is to provide a fluorescent imaging device which protects a fluorescent image high-sensitivity imaging measure even under a transitional condition such as at the power input.  
           [0020]    Another object of the present invention is to provide a fluorescent imaging device which prevents damage to the high-sensitivity camera if the normal emitting lamp fails during a fluorescent observation and is replaced by the emergency light.  
           [0021]    Still another object of the present invention is to provide a fluorescent imaging device which objectively discriminates against delicate changes in coloration of fluorescent images so that an operator can easily visualize the existence of lesions and conditions of the lesions.  
           [0022]    A further object of the present invention is to provide a fluorescent imaging device which adjusts the color tone of normal tissues to a desired tone by conducting a simple operation during the observation of the normal tissue, while displaying the color tone of abnormal tissue in contrast with the color tone of the normal tissues.  
           [0023]    The fluorescent imaging device of the present invention has a light source, which selectively switches between an excitation light and a white light, introduces the light into a light guide, and then emits the light onto the tissue being inspected; a high-sensitivity fluorescent device for fluorescent images; a white image imaging device for white light images; a device which couples the fluorescent image to the fluorescent imaging device, a device which prevents overprint on the high-sensitivity imaging plane of the fluorescent imaging device, a visible image generation device which generates an electric signal output from the fluorescent imaging device, and a separate visible image generation device which generates an electric signal output from the white light image imaging device.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    A first embodiment of the present invention is illustrated in drawings FIGS.  1 - 4  where:  
         [0025]    [0025]FIG. 1 is a structural schematic of an endoscopic fluorescent imaging device;  
         [0026]    [0026]FIG. 2 is a front elevation which illustrates one structural example of the rotary filter;  
         [0027]    [0027]FIG. 3 is an enlarged section of the structure around the movable mirror;  
         [0028]    [0028]FIG. 4 is a table which illustrates the relationship between switching conditions of each device and imaging conditions of the camera;  
         [0029]    A second embodiment of the present invention is illustrated in drawing FIGS.  5 - 8  where:  
         [0030]    [0030]FIG. 5 is a structural schematic of an endoscopic fluorescent imaging device;  
         [0031]    [0031]FIG. 6 is a front elevation which illustrates structure of the rotary filter that is not illustrated in FIG. 5;  
         [0032]    [0032]FIG. 7 is a front elevation which illustrates an RGB rotary filter;  
         [0033]    [0033]FIG. 8 is a table which illustrates the relationship between switching conditions of each device and shutter conditions of the camera;  
         [0034]    A third embodiment of the present invention is illustrated in drawing FIGS. 9 and 10 where:  
         [0035]    [0035]FIG. 9 is a front elevation of another rotary filter;  
         [0036]    [0036]FIG. 10 is a structural schematic of the fluorescent observation device;  
         [0037]    A fourth embodiment of the present invention is illustrated in drawing FIGS.  11 - 14  where:  
         [0038]    [0038]FIG. 11 is a structural schematic of the fluorescent image device of the fourth embodiment;  
         [0039]    [0039]FIG. 12 is a spectrum atlas of the fluorescences emitted from normal and abnormal tissues;  
         [0040]    [0040]FIG. 13 is a color distribution diagram showing the relationship in coloration between the normal part and the lesion parts in fluorescent color observation images;  
         [0041]    [0041]FIG. 14 is a schematic diagram that illustrates one structural example of the color index;  
         [0042]    A fifth embodiment of the present invention is illustrated in drawing FIGS.  15 - 17  where:  
         [0043]    [0043]FIG. 15 is a structural schematic of the structure of the fluorescent image device of the fifth embodiment;  
         [0044]    [0044]FIG. 16A is a histogram showing the frequency of the green image signal level;  
         [0045]    [0045]FIG. 16B is a histogram showing the frequency of the red image signal level;  
         [0046]    and  
         [0047]    [0047]FIG. 17 is a flowchart which illustrates the operations to set normal tissues to a certain tone.  
         [0048]    [0048]FIG. 18 is a schematic diagram of a sixth embodiment of the present invention.  
         [0049]    [0049]FIG. 19 is a schematic diagram of a seventh embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0050]    The first embodiment of the present invention will now be described with particular reference to FIGS.  1 - 4 .  
         [0051]    The fluorescent imaging device of this embodiment has an imaging camera including a white light imaging device and a high-sensitivity fluorescent imaging device, a device to couple a power source to the camera, and an overprinting prevention device to protect the high-sensitivity imaging plane by controlling a movable mirror on an optical path so that under the imaging condition where the power source is ON, imaging by the fluorescent imaging device is prevented.  
         [0052]    As shown in FIG. 1, the fluorescent imaging device  1  comprises an optical endoscope  2  which is inserted into the body under examination, a light source  3  which supplies an illumination light to the endoscope  2 , an imaging camera  4 , including an integrated imaging device which can be removably attached to the endoscope  2 , a control center  5  which conducts signal conditioning for the imaging device in the camera  4 , and a monitor  6  which provides a visible image for diagnostic observation.  
         [0053]    A switch  7  on camera  4  is provided to operate a control circuit  8  in control center  5 . Control circuit  8  controls the operation of light source  3  and camera  4  to provide the desired light source, to prevent impingement of white light on the fluorescent image detectors, and, by means of a switching arrangement  42 , to provide either a signal representing either the fluorescent image or the white light image to monitor  6 .  
         [0054]    Endoscope  2  has a slender insertion part  11 , an operation part  12  at the back end of the insertion part, an eyepiece part  13  at the back end of the operation part  12 , and a light guide cable  14  which extends from the operation part  12 . A connector  15  removably couples light source  3  to the end of light guide cable  14 .  
         [0055]    A light guide  16  which functions to conduct the incoming white light or excitation light, is inserted in insertion part  11 , operation part  12 , and light guide cable  14 . By installing connector  15  onto light source device  3 , the white light or the excitation light is provided from the light source device  3  to light guide  16 .  
         [0056]    As an illumination light source  2  such as a metal halide lamp or the like is provided in light source device  3 . The white light emitted from this lamp  21  passes through a rotary filter  23  which is rotated by a stepping motor  22 , and then is supplied to the light admittance end of the light guide  16  through a condensing lens  24 .  
         [0057]    As shown in FIG. 2, rotary filter  23  is disk shaped, and includes a first circular aperture  25 A and a second circular aperture  25 B. A clear glass insert  25  may be provided in aperture  25 A if desired. Second aperture  25 B is fitted with a blue filter  26  which passes the excitation light with a wavelength in a narrow-band of the blue region, preferably about 400-450 nm. When aperture  25 A is positioned in front of light source  21  white light is supplied into light guide  16 , and when the blue filter  26  is positioned in front of light source  21  (as shown in FIG. 2), blue light for fluorescent observation is supplied into the light guide  16 .  
         [0058]    The rotational position of the stepping motor  22  is controlled by control circuit  8 . Moreover, a small opening  27  is formed at the circumference of rotary filter  23 , and a photo coupler  28  is disposed so that it spans across the circumference. When the photo coupler  28  detects the hole  27 , it provides a position detection signal indicating that the blue filter  26  is positioned on the optical path.  
         [0059]    As shown in FIG. 1, photo coupler  28  includes a light source and a light detector (not shown) disposed on opposite sides of the rotary filter  23 . When filter  26  is aligned with light source  21 , hole  27  is positioned between the light source and the detector of photo coupler  28 , allowing light to pass through from the light source to the detector. The light detector is coupled to control circuit  8 .  
         [0060]    A power switch (not shown) is provided for light source  3 . When this switch is turned ON, power is supplied to lamp  21  and to stepping motor  22 , which starts to rotate filter  23 .  
         [0061]    The light which is transferred by light guide  16  is emitted onto the tissue under examination, such as an organ of a body cavity, through the illumination lens  31  which provides an illumination window at the tip part  29  of the insertion part  11 .  
         [0062]    An objective lens  32  which provides an observation window is installed near to lens  31 . This focuses an image, either reflected white light or a fluorescent image resulting from the excitation light, on an image plane at the tip of the image guide  33 . The image which is formed on the tip plane of image guide  33  is transferred onto the back end plane  33 A of the image guide  33 . A magnified view of the image provided by light guide  33  is available through an eyepiece lens  34  of an eyepiece part  13  which is positioned adjacent to the back end plane of the image guide  33 . This image may be viewed by the naked eye when camera  4  is not attached to endoscope  2 .  
         [0063]    When the camera  4  is mounted onto the eyepiece part  13 , the image-forming lens  37  within the camera is disposed opposite to eyepiece lens  34 .  
         [0064]    On the resulting optical path, within camera  4 , is a movable mirror  38 , which is movable between the position shown in FIG. 1 and the position  38 A shown in outline. When it is in the position shown, light focused by lens  37  is reflected to impinge on a second fixed mirror  39 , so that the light which is reflected from movable mirror  38  is also reflected by mirror  39 , to form an image on the imaging plane on a first charge coupled device (CCD)  40  which services the white light imaging device.  
         [0065]    The optical image which impinges on CCD  40  is converted to an electrical signal and is coupled to a first camera control unit (CCU)  41 . This converts the input electrical signal into video signal for display on monitor  6  through a switching arrangement  42  when the tissue is being examined under white illumination. Movable mirror  38  is driven by a driver controlled by control circuit  8 . For the white light observation mode, mirror  38  is in the position shown in solid lines and the light focused by lenses  34  and  37  is coupled to white light image imaging device  40 . For the fluorescent observation mode, a control signal sent from control part  8  to driver  43  causes movable mirror  38  to be set in the position shown by dotted lines. Then, light which goes through lenses  34  and  37  is coupled to fluorescent imaging device  44 .  
         [0066]    The position of movable mirror  38  is detected by a photoreflector  45 . As shown in FIG. 3, the luminous element  46   a  and the light detector  46   b  which form photoreflector  45  are disposed opposite to the plane of, for example, the proximal end of movable mirror  38 . Thus, when mirror  38  is in the position shown with the solid line, the output signal of light detector  46   b  is provided as the second mode signal (see FIG. 1) to control circuit  8 .  
         [0067]    Fluorescent imaging device  44  comprises a dichroic mirror  48  which is inclined at  45  degrees on the optical path. Dichroic mirror  48  selectively reflects red light but transmits the rest of the visible spectrum.  
         [0068]    The light transmitted by dichroic mirror  48  then passes through a green filter  49  which selectively transmits the light with green wavelengths to an image intensifier (I.I.)  50 . The green light is amplified by I.I.  50  to form an image on an imaging plane  51  of the CCU for fluorescence use  56 .  
         [0069]    The light which is reflected by dichroic mirror  48  is further reflected at a mirror  52  and then, passes through a red filter  53  which selectively transmits light with red wavelengths, to an I.I. 54 . The red light then is amplified at I.I.  54  to form an image on the imaging plane  55  of the CCU for fluorescent use  57 .  
         [0070]    The outputs of CCU  56  and  57  are converted into a video display signal by an image processing device  58 . The video signal is coupled to monitor  6  through the switching arrangement  42  described below.  
         [0071]    Switching arrangement  42  is controlled by the control circuit  8  in conjunction with switch  7  which allows the operator to select a mode of operation, i.e., white light imaging fluorescent imaging or simultaneous white light and fluorescent imaging.  
         [0072]    As previously noted, power for camera  4  is supplied by control center  5 . When the power is first turned ON, the control circuit  8  goes into operation ahead of other parts. Specifically, the control circuit  8  confirms that the power is present prior to operating a relay which supplies power to the other circuits.  
         [0073]    Thereafter, control circuit  8  determines the position of movable mirror  38  at its initial condition. If in error  38  is found to be in the position shown by the solid lines and the camera  4  is not operation, control circuit  8  operates driver  43  to set mirror  43  in the position shown by the dotted lines. In this way, even if a transitional condition exists (e.g., if light source  3  is ON and filler  23  is set to emit white light, when the power source of the control center  5  is turned ON), white light will not impinge on fluorescent imaging device  44 .  
         [0074]    In addition, when the power for control center  5  is turned OFF, a shut down operation is initiated on which driver  43  is disabled. This again prevents white light from impinging on the fluorescent imaging device  4 .  
         [0075]    Also, control circuit  8  monitors the ON/OFF condition of the power for lamp  21 . When the power for light source  3  is turned ON, after a delay to allow the power source to reach a stable state, control circuit  8  operates stepping motor  22  with reference to a feedback signal provided by photo coupler  28  and also operates movable mirror  38  through driver  43 . During the start-up delay, control circuit  8  retains mirror  38  in the position shown by the solid lines. Thereafter stepping motor  22  and driver  43  are operated to select the desired color for the light supplied by filter  23  and the optical path for light collected by image guide  33 , in accordance with the position of switch  7 .  
         [0076]    When the power for light source device  3  is turned OFF, the control circuit  8  immediately disables driver  43 , which sets movable mirror  38  into the position shown by the solid lines. This prevents admittance of light to the fluorescent imaging device  44 , and thereby prevents damage to I.I.s  50  and  54 .  
         [0077]    Now, the operation of this first embodiment will be explained. The sequence of operation for camera  4  from the condition in which the power for light source  3  and control center  5  are both OFF to the condition in which both are turned ON is illustrated with reference to FIG. 4.  
         [0078]    When the power for both light source  3  and control center  5  is OFF, neither the white light nor fluorescent light is emitted into the camera  4 , and operating power is not supplied to the imaging devices of camera  4 . Therefore, the camera  4  is in the “inoperative” condition as shown in FIG. 4.  
         [0079]    When the power for light source  3  is turned ON, but the power for control center  5  is still OFF, although the white light or the fluorescent light is ready to be emitted into the camera  4 , operating power is not supplied to the imaging devices and camera  4  is still in the “inoperative” condition as shown in FIG. 4.  
         [0080]    If the power for light source  3  is OFF but the power for control center  5  is turned ON, at first, neither reflected white light nor stimulated fluorescent light is coupled to the camera  4 . Nevertheless, since operating power is supplied to both of the imaging devices, movable mirror  38  is set at the position shown by the solid lines, and the camera  4  is set to operate in the “white light mode,” as indicated in FIG. 4.  
         [0081]    If power is supplied to both light source device  3  and control center  5 , during the start up delay, movable mirror  38  is set at the solid line position so that even through both of imaging devices are energized, camera  4  is still in “the white light mode.”  
         [0082]    After the start-up delay, if white light imaging is selected by switch  7 , the control circuit  8  controls the rotation of the stepping motor  22  and positions a clear glass insert  25  in the optical path of lamp  21 , and confirms the position of filter  23  by detection of a signal from photo coupler  28 . If the level of the detection signal indicates that the detector in photo coupler  28  is not energized, control circuit  8  keeps the movable mirror at the solid line position.  
         [0083]    Then, white light from lamp  21  is transmitted through clear glass insert  25  in aperture  25   a  of filter  23 , and passes light guide  16  to illumination lens  31  to illuminate the tissue under examination.  
         [0084]    The light which is reflected from the tissue under examination is focused at the tip plane of the image guide  33  by the objective lens  32 , is transferred onto the back end plane of the image guide  33 , is reflected at the movable mirror  38  and is then imaged at the CCD for white use  40 .  
         [0085]    The output signal of this CCD for white use  40  undergoes signal conditioning at the CCU for white use  41  and is converted into a picture signal, which is displayed as a white light image on the monitor  6  through switching arrangement  42 .  
         [0086]    On the other hand, if the fluorescent imaging mode is selected by switch  7 , the control circuit  8  controls the rotation of the stepping motor  22  and positions the blue filter  26  on the optical path, while confirming the position by the detection signal of the photo coupler  28 . When the detection signal indicates that the blue filter is in the proper position, control circuit  8  operates driver  43  to switch movable mirror  38  to the dotted line position, thereby enabling the “fluorescent mode.” 
         [0087]    With the blue filter in front of lamp  21 , only light components with blue wavelengths are transmitted through light guide  16 , to illuminate the tissue under examination.  
         [0088]    The fluorescence generated by the blue excitation light is focused onto the tip plane of image guide  33  by objective lens  32 , is transferred onto the back end plane of the image guide  33 , and then impinges on dichroic mirror  48  within the camera  4 . The light transmitted by dichroic mirror  48  passes through green filter  49 , is amplified by I.I.  50 , and imaged at the CCD for fluorescence use  51 .  
         [0089]    On the other hand, the light reflected by dichroic mirror  48  is further reflected by mirror  52 , passes through red filter  53 , is amplified by I.I.  54  and is imaged at the CCD for fluorescence use  55 .  
         [0090]    The output signals of CCDs for fluorescent use  51  and  55  undergo signal conditioning at the CCUs for fluorescent use  56  and  57 , respectively, and are converted into picture signals. Then, image processing such as adjustment of intensity of the images, image component registration and the like is performed by image processing device  58 , and, both images are superimposed with different colors and displayed as a fluorescent image on the monitor  6  through the switching arrangement  42 .  
         [0091]    If combined white light and fluorescent imaging is selected by switch  7 , the control circuit  8  rotates the stepping motor  22  at a constant speed. Then as shown in FIG. 2, when the detection signal of the photocoupler  28  indicates that the blue filter  26  is disposed on the optical path, control circuit  8  operates driver  43  to switch the position of the movable mirror  38  from the solid line to the position to the dotted line position, and conducts the fluorescent imaging as described above, and then stores the fluorescent image in a memory circuit (not shown) within image processing device  58 .  
         [0092]    When blue filter  26  is rotated away from lamp  21  by stepping motor  22 , the control circuit  8  disables driver  43 , and movable mirror  38  moves from the dotted line position to the solid line position to permit white light imaging. The resulting white light image is stored in a memory circuit (not shown) in the CCU for white use  41 .  
         [0093]    Thereafter filter  23  rotates further, and clear glass  25  withdraws from the optical path. When the blue filter  26  is again positioned in the optical path, as indicated by the detection signal of the photocoupler  28 , the control circuit  8  operates driver  43  to switch the position of the movable mirror  38  back to the dotted line position. In this way, the movable mirror  38  is switched into the fluorescent imaging condition and conducts the fluorescent imaging of the next frame, and then stores the fluorescent image in memory within the image processing device  58 . In this way, both images of each frame, namely the white light image and the fluorescent image are sequentially obtained and stored into the memory.  
         [0094]    By operating switching arrangement  42  alternatively with appropriate time intervals, control circuit  8  allows the white light image and the fluorescent images to be alternatively displayed on monitor  6 .  
         [0095]    Alternatively, by shifting the timing between reading the memory of the CCU for white use  41  and reading the memory of the image processing device  58 , both images may be displayed simultaneously on monitor  6 .  
         [0096]    Thus, according to this embodiment, before a certain operation mode is set, such as during the start-up delay excessive light is prevented from impinging on fluorescent imaging device  44  to protect the high-sensitivity image plane from overprinting, with a consequent breakdown of the I.I.s  50  and  54  caused by the admittance of excessive light.  
         [0097]    Even during a transitional condition of switching from the fluorescent imaging mode to the white light imaging mode, the imaging circuits are switched before the condition of the light source device  3  is shifted from the emitting of the excitation light to the radiation of the white light.  
         [0098]    In addition, when switching from the white light imaging mode to the fluorescent imaging, the imaging circuits are switched after the light source  3  is switched from white light to the excitation light, so a breakdown of the I.I.s  50  and  54  caused by admittance of excessive light into the fluorescent imaging device  44  is prevented.  
         [0099]    Although photocoupler  28  detects that the blue filter  26  is positioned on the optical path as shown in FIG. 2, a second photocoupler may be provided to detect that the clear glass insert  25  is positioned on the optical path. Thus, by detecting signals from these two photocouplers, the rotary operation of the stepping motor  22  and the operation of the movable mirror  38  can be controlled with greater certainty.  
         [0100]    Moreover, although in this embodiment, where movable mirror  38  is positioned on the optical path when the power source of control center  5  is turned ON, the admittance of the light to the fluorescent imaging device  44  is prevented so that the damage to the I.I.s  50  and  54  that would be caused by the admittance of an excessive light or the like, is prevented. Also, the fluorescent imaging device  44  may be in the non-imaging condition by controlling the operating power source to the I.I.s  50  and  54 , to provide further protection.  
         [0101]    For example, when the switch for the power source of the control center  5  is turned to be ON, the control circuit  8  may detect the condition of the light source  3  so when light source  3  is turned ON, unless the blue filter  26  is set on the optical path, as indicated by the output of the photocoupler  28 , operating power would not be supplied to the I.I.s  50  and  54 .  
         [0102]    In this case, for example, when the power source for control center  5  is turned ON but the power for light source  3  is kept OFF, the non-imaging condition is established, but even if the power for light source  3  is turned ON, only if the blue filter  26  is disposed on the optical path, is operating power supplied to the I.I.s  50  and  54 .  
         [0103]    Moreover, when rotary filter  23  is rotated to shift clear glass insert  25  into position on the optical path, until it is actually positioned on the optical path, as indicated by the detection signal from the photocoupler  28 , the non-imaging condition is not established where the operating power source is supplied to the I.I.s  50  and  54 . By establishing this non-imaging condition, the breakdown due to the excessive admittance of the light into the I.I.s  50  and  54  can be prevented, which breakdown could possibly occur when the fluorescent imaging mode is still maintained during the switching operation. When switching from the white imaging mode to the fluorescent imaging mode condition, the breakdown of the I.I.s  50  and  54  can similarly be prevented.  
         [0104]    Instead of controlling the operation power source to the I.I.s  50  and  54 , by decreasing the sensitivities of the I.I.s  50  and  54 , a condition may be set where even if light having an intensity that far exceeds that of fluorescence enters the fluorescent imaging device  44 , no overprint can occur and breakdown is prevented.  
         [0105]    In addition, provision may be made to control the light source  3  which emits the excitation light and the white light to be the initial condition where the excitation light is secured to be emitted when, for example, the power source of the light source device is turned to be ON. In this way, even with a camera which is intended only for fluorescent imaging, and has no provision to protect the fluorescent imaging device  44 , damage which might result if the power for the camera is turned ON before the camera is set to the proper condition for use can be prevented.  
         [0106]    The second embodiment of the present invention is illustrated with reference to FIGS.  5  to  8 .  
         [0107]    In this embodiment, the endoscope is an electronic endoscope which integrates a white light image imaging device at its tip, where a fluorescent image introducing part of a fluorescent imaging device is inserted into a forceps channel of this electronic endoscope, a fluorescent image which is introduced through his fluorescent image introducing part is imaged at the fluorescent imaging part, and then the signals are processed in, for example, a CCU for fluorescent use within a control center so that a white light image and a fluorescent image are to be designated on a monitor.  
         [0108]    As shown in FIG. 5, the fluorescent imaging device  61  of this embodiment is composed of an electronic endoscope  62 , a light source  63 , a CCU for white use  64 , a fluorescent observation device  65 , control center  66 , and a monitor  67 .  
         [0109]    Unlike the endoscope  2  of FIG. 1, in electronic endoscope  62 , the CCD for white use  68  is disposed at the imaging position of the object lens  32 . Therefore, the electronic endoscope  62  does not have the image guide  33  and the eyepiece part  13 . Also, CCD for white use  68  in this embodiment does not generate an image of reflected white light per se, but rather a synthesis of red, green and blue component color images within the visible region, as an equivalent of a white light image.  
         [0110]    The signal conductor  68   a  connected to CCD for white use  68  passes from insertion part  11  through a light guide cable  14 , and is connected to the CCU for white use  64  through an additional cable  64   a  which is connected to cable  68   a  through a suitable  64   b.    
         [0111]    Also, a forceps channel  71  is provided. A light tube  72  is positioned in forceps channel  71 , as described in more detail below.  
         [0112]    Light source device  63  includes a rotating filter  74  (FIG. 7) positioned on the optical path between rotary filter  23  and lamp  21 . Rotary filter  74  is driven by a motor  75 .  
         [0113]    As shown in FIG. 6, the structure of filter  23  is the same as that in FIG. 2, and the output signal from photocoupler  28  is provided to a control circuit  77  within a control center  66 .  
         [0114]    As shown in FIG. 7, filter  74  comprises a red filter component  76  R, a green filter component  76  G, and a blue filter component  76  B as indicated by two-headed arrow  74   a , filter  74  and the motor  75  are movably mounted so that filter  74  may be shifted out of the optical path of lamp  21 . A suitable driver mechanism  78  operated by control circuit  74  is provided for this purpose.  
         [0115]    Positioned within light tube  72  is an image guide  81 . A lens  82  is located at the back end of image guide  81 . A lens  82  is optically coupled to an imaging lens  83 . A movable shutter  84  is disposed front of lens  83 . Control center  66  includes a control circuit  77 .  
         [0116]    Among the functions of control circuit  77  are to operate movable shutter  84 . In the position shown in FIG. 5 by solid lines, shutter  84  permits light to pass from lens  83  to the dichroic mirror  48 . In the position indicated by the dotted lines, shutter  84  blocks light passing through lens  83  from reaching mirror  48 . Mirror  48  is part of fluorescent imaging device  44  which is the same as that included in the first embodiment described in connection with FIG. 1.  
         [0117]    The output signal of the CCDs  51  and  55  are provided to a common CCU for fluorescent use  85  within control center  66 , which generates a fluorescent picture signal. This signal is coupled by switching arrangement  68  to a monitor  67 . A picture signal from the CCU for white use  64  is also coupled by switching arrangement  68  to monitor  67 .  
         [0118]    A switch  87  is provided on the camera  73  to select the mode of operation. Based on the selection made by switch  87 , circuit  77  controls the operation of shutter  84 , driver  78  and switching arrangement  86  as described in more detail below. Control center  66  also provides operating power to camera  73  and CCU for white use  64 .  
         [0119]    In the second embodiment, essentially the same control functions are provided as in the first embodiment. For example, shutter  84  is normally in a closed condition. When a driving signal is provided by control circuit  77 , the shutter is moved to an open condition. In addition, control circuit  77  monitors the ON/OFF condition of the power for light source  63 . When the power source is turned ON, circuit  77  maintains shutter  84  in its closed condition during a start-up delay interval. When the power for light source  63  is turned OFF, circuit  77  immediately switches shutter  84  into its closed condition. Further description of the structural features is omitted in the interest of brevity.  
         [0120]    The operational states for this embodiment are illustrated with reference to FIG. 8.  
         [0121]    When the power sources for light source  63  and control center  66  are both OFF, the light source  63  does not generate a white light or an excitation light, and since the driving signal is not applied by control circuit  77  to shutter  84 , shutter  84  remains in the “closed” position, as indicated in FIG. 8.  
         [0122]    When the power source of the light source  63  is OFF but the power source for control center  66  is ON, at first, control circuit  77  keeps the shutter  84  in its closed condition. Therefore the shutter  84  is in the “closed” condition, as indicated in FIG. 8. Shutter  84  also remains closed during the start-up delay period when the power sources of both the light source  63  and control center  66  are turned on.  
         [0123]    After the start-up delay period, if the white light imaging mode is selected by operation of switch  87 , the control circuit  77  controls the rotation of the stepping motor  22  to position clear glass insert  25  on the optical path for lamp  21  and confirms the position by means of the detection signal of the photocoupler  28 .  
         [0124]    In response, the detection signal corresponding to the positioning of clear insert  25  in the path of lamp  21  shutter remains in the “closed” condition. Also, the control circuit  77  control the drive mechanism  78  to position the RGB rotary filter  74  on the optical path of lamp  21 , and operates monitor  75  to rotate filter  74 . As a result, recurring sequences of red, green, and blue light pulses pass through clear glass insert  25  of rotary filter  23  and are coupled to light guide  16  through lens  24 . The light pulses are emitted onto the tissue under examination through the illumination lens  31 . The light pulses which are reflected from the illuminated tissue are imaged onto the CCD for white use  68  by means of the object lens  32  as previously described in connection with the first embodiment.  
         [0125]    The output signal from CCD for white use  68  undergoes signal conditioning at the CCU for white use  64  to convert it into a picture signal which is then displayed on the monitor  67  through switching arrangement  86 .  
         [0126]    When the fluorescent imaging mode is selected by means of switch  87 , the control circuit  77  controls drive mechanism  78  to shift motor  75  and filter  74  out of the optical path of lamp  21 , and operates stepping motor  22  to position blue filter  26  in the optical path. The proper positioning of blue filter  26  is confirmed by the detection signal from photocoupler  28 , as previously described. When the control circuit  77  receives the detection signal indicating the blue filter  26  is properly positioned, it drives shutter  84  to the “open,” position, as indicated in FIG. 8.  
         [0127]    With filter  26  in place, only light with the blue wavelength is supplied into the light guide  16 , to provide the excitation light for illuminating the tissue under examination.  
         [0128]    The fluorescence generated by the excitation light is imaged at the tip plane of the image guide  81  by the lens  82 , and is transferred through the image guide  81 , to dichroic mirror  48  within the camera  73 . As the light which is transmitted by dichroic mirror  48  passes through green filter  49  and then, after the light-amplification by the I.I.  50 , is imaged with the CCD for fluorescent use  5   1 .  
         [0129]    On the other hand, the light which is reflected by the dichroic mirror  48  is further reflected by mirror  52 , and passes through red filter  53 . After light-amplification by I.I.  54 , the red light is imaged at the CCD for fluorescent use  55 .  
         [0130]    The output signals of the CCDs for fluorescent use  51  and  55  undergo signal conditioning at the CCU for fluorescent use  85  and are converted into picture signals for display as fluorescent images on the monitor  67  under control of switching arrangement  86 .  
         [0131]    If the operating mode is switched from fluorescent to white light imaging, control circuit  77  first closes shutter  84 , operates drive mechanism  78  to shift the RGB rotary filter  74  into operating position in front of lamp  21 , and operates monitor  22  to set the clear glass insert  25  of the rotary filter  23  in the optical path of lamp  21 .  
         [0132]    Then, the RGB rotary filter  74  which is rotated by the motor  75 , images the white light image as described above and the white light image is displayed on monitor  67 .  
         [0133]    Thus, according to this embodiment, before an operation mode is set, for example, during start-up, shutter  84  remains in its closed condition to prevent excessive light from being admitted to the fluorescent imaging device  44 , with a consequent damage to the I.I.s  50  and  54 .  
         [0134]    When switching from fluorescent to white light imaging, shutter  84  is set to be closed before light source  63  is switched from emitting excitation light to emitting of white light. Similarly, when switching from white light to fluorescent imaging, shutter  84  is opened only after light source  63  is switched from emitting of white light to emitting of excitation blue light, again preventing admittance of an excessive light into the fluorescent imaging device  44 .  
         [0135]    A third embodiment of the present invention is illustrated in FIGS. 9 and 10.  
         [0136]    As shown in FIG. 9, the rotary filter  23   a  differs from the rotary filter  23  of the first and second embodiments, in that an emergency light  91  is also provided.  
         [0137]    As shown in FIG. 10, light source  63   a  is provided with rotary filter  23   a  and a burnt lamp detector  92 . The latter detects if lamp  21  becomes inoperable, e.g., if it burns out, by monitoring the lamp output.  
         [0138]    The detection signal from burnt lamp detector  92  is coupled to a control circuit  77  by a conductor  92   a . If the light output of lamp  21  falls below a predetermined level as indicated by the signal on conductor  92   a , control circuit  77  operates to active emergency light  91 . Also, a warning device  93  is activated to warn an operator that the lamp  21  is inoperative.  
         [0139]    In particular, if lamp  21  becomes inoperative during use, the burnt lamp detector signals control circuit  77  and warning device  93 , which emits a signal by means of a buzzer, warning light or the like.  
         [0140]    If this happens with the system in the fluorescent mode, control circuit  77  responds to the detection signal on conductor  92   a  to close shutter  84  (see FIG. 10) so that light transmitted through imaging lens  83  does not impinge on dichroic mirror  48 .  
         [0141]    Next, the control circuit  77  drives motor  22  to rotate the rotary filter  23  a so that the light from the emergency light  91  is introduced into light guide  16 . Then, once emergency light  91  is aligned with the light admittance end of light guide  16 , control circuit  77  turns on emergency light  91  to secure a field of view.  
         [0142]    Emergency light  91  may be activated before or at the same time as motor  22 , as long as it is after shutter  84  has been closed. Alternatively, instead of closing shutter  84 , the sensitivities of the I.I.s  50  and  54  may be reduced.  
         [0143]    On the other hand, if white light is being used, when lamp  21  becomes inoperative, the control circuit  77  responds to the detection signal on conductor  92   a  to maintain shutter  84  in the closed position, and drives motor  22  to move rotary filter  23   a  into a position such that light from the emergency light  91  may be coupled into light guide  16 . Then, when the rotation of filter  23   a  is completed, and emergency light  91  is positioned properly, control circuit  77  activates emergency light  91 . Alternatively, emergency lamp  91  may be activated while motor  22  is still in motion.  
         [0144]    Thus, in this third embodiment, whether lamp  21  becomes inoperative during the course of fluorescent observation or white light observation, the emergency light is not turned on until the image intensifiers have been protected.  
         [0145]    In addition, since when the emergency light  91  is lighted, the blue filter  26  of the rotary filter  23   a  is always out of the optical path, sufficient illumination is provided for safe removal of endoscope  61 .  
         [0146]    Alternatively, in the third embodiment, provision may be for the operator to activate the emergency lighting sequence described above, for example, by means of a switch or the like.  
         [0147]    Moreover, emergency light  91  may be, for example, a light emitting diode which emits a light with a wavelength within the detection band of the fluorescent imaging device. Therefore, if the lamp  21  becomes inoperative in the course of a fluorescent observation, the rotary filter  23   a  is rotated so that light from the light emitting diode is coupled to the light guide  16 . On the other hand, if the lamp  2  becomes inoperative in the course of a white light observation, the shutter  84  which has been positioned to block the optical path of imaging lens  83  may be opened while the rotary filter  23   a  is being repositioned, after which light form the light emitting diode is coupled to the light guide  16 .  
         [0148]    In this way, imaging using the light emitting diode emergency light source may continue, and miniaturization, electric power-saving and reduction in costs of the illumination for emergency use can be achieved.  
         [0149]    A fourth embodiment of the present invention which provides only fluorescent imaging is illustrated in FIGS.  11 - 14 .  
         [0150]    As shown in FIG. 11, the fluorescent imaging device  100  comprises a light source  101 , an endoscope  102 , an imaging subsystem or camera  103 , an image processing subsystem  104 , a monitor  105 , and a color reference subsystem  106 .  
         [0151]    Light source  101  includes a lamp  101   a  which may be a metal halide lamp, a mercury lamp, or the like to provide a source of white light. A blue filter  101   b  is disposed in the optical paths of lamp  101  a to generate an excitation light within the blue region, for example between about 400 nm and 450 nm.  
         [0152]    Endoscope  102  has a slender insertion part  102   a  which is designed for insertion into the organism being examined, and an illumination system comprising a light guide  107   a  which transfers the excitation light from light source  101  to an illumination window  107   b  at the tip of insertion part  102   a . Endoscope  102  also includes an observation optical system comprising an observation window  108   a  which couples a fluorescent image from the tissue under examination to an image guide  108   b .  
         [0153]    Image guide  108  terminates in an eyepiece section  102   a . A lens  108   c  focuses the light output of light guide  108   b  for visual observation, or connection to camera  103 .  
         [0154]    Camera  103  is removably connected to eyepiece  102   b . Camera  103  includes a dichroic mirror  110  which divides the fluorescent image from eyepiece lens  108   c  into a transmitted portion  110   a  and a reflected portion  110   b . A first band pass filter  110  which transmits a wavelength band λ 1  is positioned to intercept transmitted light portion  110   a  from dichroic mirror  110 . A second mirror  113  is positioned to intercept reflected light portion  110   d  and to reflect it in turn through a second band pass filter  112  having wavelength passband λ 2 . A first image intensifier  114  amplifies the light transmitted by filter  111  and a second image intensifier  115  amplifies the light transmitted by filter  112 . Image intensifiers  114  and  115  respectively provide outputs to CCD&#39;s  116  and  117 .  
         [0155]    Image processing circuit  104  and a color reference subsystem  106  provide visual data for display on monitor  105 . Image processing circuit  104  converts the red and green image signals generated by CCD&#39;s  116  and  117  into a fluorescence image display signal. Color reference subsystem  106  includes a color reference generator  106   a  which generates a color discrimination scale display signal and a superimposing circuit  106   b , which combines the fluorescence image display signal and the color discrimination scale display signal into a composite video signal for display on monitor  105 . The display includes a portion  105   a  representing the tissue under examination and a color discrimination scale  105   b . As explained below, color discrimination scale  105   b  provides a reference for objective identification of diseased tissues in accordance with color tone variations in the tissue image display  105   a.    
         [0156]    The operation of fluorescent imaging device  100  will now be illustrated.  
         [0157]    The excitation light λ 0  within the blue region is generated by lamp  101   a  of the light source  101  and is then introduced into the light guide  107   a  of the endoscope  102 . The excitation light λ 0  passes through light guide  107   a  and then is emitted through illumination window  107   b  toward the tissue under observation. The fluorescent image stimulated by the excitation light is transferred through the observation window  108   a  and the image guide  108   b  to the eyepiece part  102   b  at the operator side and then is emitted into the camera  103 .  
         [0158]    The fluorescent image which is emitted into the camera  103  is partially transmitted and partially reflected by dichroic mirror  110 . The transmitted portion  111   a  passes through first bandpass filter  111 ; and after being amplified at the first image intensifier  114 , is imaged at CCD  116  to undergo photoelectric conversion to an electric signal.  
         [0159]    The reflected portion  110   b  of the fluorescent image is again reflected by the mirror  113  and then passes through second bandpass filter  112 , and after being amplified at the second image intensifier  115 , is imaged at CCD  117  to undergo photoelectric conversion to an electric signal.  
         [0160]    As will be understood, the electric signals produced by CCDs  116  and  117  represent single-color fluorescent light images with different color tones. These are connected into the image processing circuit  1   04   a  which arithmetically processes the two input signals to generate the fluorescence image display signal.  
         [0161]    As shown in FIG. 12, the fluorescence within the visible region which is stimulated by the excitation light shows an intensity distribution in a longer wavelength band than that of the excitation light λ 0  which is emitted from the light source device  101 . Normal tissue shows a strong fluorescent intensity within the range near to the green region λ 1 , especially of 490 nm through 560 nm, while for abnormal tissue such as that of cancer or the like, the fluorescent intensity is relatively weaker in this band. On the hand, the fluorescent intensity of abnormal tissue the red region λ 2 , especially within the range of about 620 nm through about 800 nm, though attenuated compared to the intensity of normal tissue, is attenuated to a much lesser degree relative to normal tissue than in the green band λ 1 . Accordingly, it is possible to utilize the differences in relative intensity between normal and abnormal tissue in the red and green bands to discriminate between and normal and abnormal tissue.  
         [0162]    Therefore, the fluorescences which exist near green region λ 1  and red region λ 2  may be converted by image processing circuit  104  into a single fluorescence image display signal from which the condition of tissue may be observed, by viewing the tissue image display  105   a  on the screen of monitor  105 .  
         [0163]    To make it easy to discriminate visibly between normal and abnormal tissue, an image of the green region λ 1  is displayed as a cyan video signal and an image of the red region λ 2  is displayed as a red video signal.  
         [0164]    Then, as shown in FIG. 13, when the tissue image  105   a  is displayed on monitor  105  with cyan and red, normal tissues are visualized as cyan and cancer lesions as dark red. A dysplasia, which is a precancerous lesion, is visualized as a lighter red.  
         [0165]    The value of the difference or the ratio of the λ 1  and λ 2  image signals may be obtained from image processing circuit  104  as the fluorescence image display signal, the color of which corresponds to the value of the difference or the ratio.  
         [0166]    Referring still to FIG. 11, a color reference generator  106   a , which may be of any conventional or desired design, combines a signal representing a cyan color and a signal representing a red color in various ratios to generate the color indication signal data representing the colors for a color discrimination scale  105   b  on monitor  105 .  
         [0167]    In this embodiment, as shown in FIG. 14, the coloration discrimination scale  105   b  comprises four distinct bands  200   a - 200   d , respectively providing cyan, white, bright red and dark red reference colors. Color discrimination scale  105   b  is displayed on the monitor  105  by superimposing circuit  106   b  along with the tissue image  105   a.    
         [0168]    Therefore, an operator can make an objective discrimination of subtle coloration differences in the tissue image display by comparing the coloration of image  105   a  with the color reference bands  200   a - 200   d  in color discrimination scale  105   b , and then can diagnose abnormal conditions such as the existence of a lesion, the extent of the lesion, and the like is an objective manner. In other words, a common discrimination standard can be provided which is independent of differences between operators and also in facilities such as hospitals and the like.  
         [0169]    Although two single colors are used in this embodiment to form the tissue image  105   a , many single colors may be mixed. Also, the color discrimination scale  105   b  is not limited to four color reference bands. Therefore, by displaying the tissue image while increasing the number of color reference bands as well as the relative brightness of individual colors, changes in appearance of the image due to the brightness of the fluorescent image can be confirmed. Moreover, the color discrimination scale  105   b  may be moved by means of superimposing circuit  106   b  to position it adjacent to a particular portion of tissue image  105   a  (or may even overlie other parts of image  105   a ) which makes color comparisons easer and more reliable. This may be done in any conventional or desired manner, as by use of a scale positioner  205  such as mouse or other manual input device.  
         [0170]    A fifth embodiment of the present invention is illustrated in FIGS.  15 - 17 .  
         [0171]    As shown in FIG. 15, a fluorescent imaging device  100   a  of this embodiment includes light source  101 , an endoscope  102  and a camera  103  which are the same as the corresponding components of the fourth embodiment, and an image processing circuit  104  described in detail below.  
         [0172]    Light source  101  includes, by way of example, a wide-band lamp  101   a  and a blue filter  101   b  which passes only blue and ultraviolet light to light guide  107   a  in endoscope  102 . An illumination window  107   b  directs the excitation light onto the tissue under observation.  
         [0173]    A fluorescent image generated in response to the excitation light is transferred through an observation window  108   a  and an image guide  108   b  in endoscope  102  to an eyepiece  102   b , and is then coupled to camera  103  through the eyepiece lens  108   c.    
         [0174]    Image processing circuit  104  comprises first and second CCU&#39;s  143  and  146 , respectively coupled to the outputs of first and second CCD&#39;s  116  and  117  in camera  103 . First and second analog to digital (A/D) converters  144  and  147  are respectively connected to the outputs of CCU&#39;s  143  and  146 . These, in turn, provide input signals for respective lookup tables (LUT&#39;s)  145  and  148 . LUT  145  corrects the output of A/D converter  144  in accordance with the response characteristics of I.I.  114  and CCD  116  in camera  103 , and LUT  148  adjusts the output of A/D converter  147  to match the response characteristics of the second I.I.  115  and the second CCD  117  in camera  103  to the typical characteristics of human vision. A video processor  149  generates tissue image display signals from the corrected data generated by LUTs  145  and  148  for display on monitor  105 .  
         [0175]    Image processing circuit  104  computes the maximum values of the brightness levels of the raw color image signals  152  and  154  provided by CCD&#39;s  116  and  117  and adjusts these signals to produce an output video display on monitor  105  in which normal tissue is displayed in a predetermined reference color. This is accomplished by a computation circuit  141 , a control circuit  142 , and a color tone adjustment switch  150 .  
         [0176]    Computation circuit  141  computes the frequencies of the brightness levels (histograms) of the image signals from LUTS  145  and  148 , and control circuit  142  obtains the peak ratios of the distributions of the histograms for the green and red signals obtained by computation circuit  141 , and also adjusts and controls the amplification ratios of I.I.s  114  and  115  so that the peak ratios are at the frequencies corresponding to the color tones of normal tissues. Image processing circuit  104  further includes a color tone adjustment switch  150  which initiates the process of adjustment of the amplification ratio.  
         [0177]    In operation, camera  103  functions as described in connection with the fourth embodiment to produce image signals on conductors  152  and  154  respectively representing the green and red components of the fluorescent image.  
         [0178]    The green fluorescent image signal on conductor  152  is processed by the first CCU  143  and then is converted into a digital signal by A/D converter  144 . Then this digital data is corrected to match the response characteristics of human vision by the first LUT  145  where the correction data which are fitted to the response characteristics of the first I.  114  and the first CCD  116  are recorded.  
         [0179]    The red fluorescent image signal appearing on conductor  154  is processed by CCU  146  and then is converted into a digital signal by A/D converter  147 . This digital data is corrected to match the response characteristics of human vision by the second LUT  148  where the correction data which are fitted to the response characteristic of the second I.I.  115  and the second CCD  117  are recorded.  
         [0180]    The corrected digital signals are used to generate a so-called pseudo color image signal in video processor  149 . This is displayed as an image representing the tissue under examination on the screen of the monitor  105 . The color tone of the tissue image corresponds to the ratio of the digital data for the green and red fluorescent images produced by LUTs  145  and  148  respectively. In order words, for a normal tissue characterized by a green image component which is larger than the red component, the image is displayed with a cyan color tone, and in the case of an abnormal tissues such as cancer tissues where the red component is larger than green components, the image is displayed with a red color tone.  
         [0181]    However, if the gain or amplification of the second I.I.  115  which amplifies the red fluorescence is relatively higher than the gain of first I.I.  114  which amplifies the green fluorescence, normal tissue will be displayed with a whitish cyan coloration and an abnormal tissue will be displayed with a much more reddish coloration. On the other hand, if the gain of second I.I.  115  is much lower than that of first I.I.  11   4 , normal tissue will be displayed with a much greater cyanic color tone and abnormal tissue will be displayed with a darker color.  
         [0182]    To calibrate the system so normal tissue is displayed in the desired cyan color, the operator pushes color tone adjustment switch  150  while observing normal tissue.  
         [0183]    This automatically starts the color tone adjustment process. In general, the gain of the second I.I.  115  which amplifies the red fluorescence is related to the gain of I.I.  114  which amplifies the green fluorescence according to the relationship:  
           R ( G )= aG   2   +bG+c   (1)  
         [0184]    Here, R is the gain of the second I.I.  115 , G is the gain of the first I.I.  114 . The coefficients (a) and (b) and additive term (c) are constants.  
         [0185]    Coefficient (a) corrects for non-linearity of the individual gain characteristics of the I.I.s  114  and  115 , and coefficient (b) corrects the relative gains of the I.I.s  114  and  115 . Constant term (c) is an offset value.  
         [0186]    By adjusting the value of (b), adjustment of the color tone for normal tissue can be achieved.  
         [0187]    To simplify the illustration of the process of the color tone adjustment which is described hereinafter, the values of (a) and (c) are set to be 0, i.e. that there is no non-linearity or offset. Referring to FIGS. 16A, 16B and  17 , to begin the process, color tone adjustment switch  150  is pushed as described above during the observation of normal tissues. At Step S 1 , computation circuit  141  operates control circuit  142  to set the gain of I.I.&#39;s  114  and  115  to be equal, i.e., b=1 in equation (1). Using these gain values, the green and red fluorescent image signals  152  and  154  are processed by CCU&#39;s  143  and  146 , A/D converters  144  and  147 , and LUT&#39;s  145  and  148  as described above, and the resulting image signals provided by LUT&#39;s  145  and  148  are provided to computation circuit  141 . The process then shifts to Step S 2 .  
         [0188]    At Step S 2 , as shown in FIG. 16A and FIG. 16B, the histograms of the red and green image signals are calculated. The process then shifts to Step S 3  where from the histogram of the individual colors, namely those of red and green, the maximum values for green H G  and red H R  are computed. The process then shifts to Step  4 , where the ratio R=(H G /H R ) of the maximum value for green, H G  and the maximum value for red, H R  is obtained.  
         [0189]    The process then shifts to Step S 5  where the value of the ratio R which is obtained in Step S 4  is compared with a first reference value R 1 . If R&lt;R 1  the process shifts to Step S 6  where the value of the term b is increased by, for example 0.1 and the treatment from Step S 2  is repeated again using LUT values for relative gain represented by b=1.1. Therefore, until R&gt;R 1 , the steps from Step S 2  to Step S 5  are repeatedly performed. When the value of the ratio R becomes larger than the R 1 , the process shifts to Step S 7 .  
         [0190]    Here, the value of R for which R&gt;R 1  is compared to a second reference value R 2 . If R&gt;R 2 , the process shifts to Step S 8  where the value of the term b is reduced by, for example, 0.1, and steps S 2 -S 5  and S 7  are repeated.  
         [0191]    The process continues as described until the value of R falls within the range R 2 &gt;R&gt;R 1 . When this condition is satisfied, the process terminates.  
         [0192]    For the above-described computation process it is found that the values of R 1  and R 2  should be set to a relatively wide value compared to the changes in the value by 0.1 in Step S 8 .  
         [0193]    As a result of the above-described enhancement process, normal tissues are displayed on monitor  105  in an easily recognized cyan color, and abnormal tissues are displayed in an equally recognizable dark red color tone. As a result, an operator can objectively discriminate between normal and abnormal tissues and easily identify lesions, cancerous and precancerous tissues, etc.  
         [0194]    [0194]FIG. 18 illustrates a sixth embodiment of the present invention which in effect combines the features of the first and fourth embodiments. For this embodiment, camera  4  (not shown) which is identical to that shown in FIG. 1, provides a white image output signal from CCD  40  and fluorescent image output signals from CCDs  51  and  55 . These are provided as inputs to control center  5 ′. This includes CCU for white use  41  which receives as its input the white image signal from CCD  40 , and image processing circuit  104   a  which receives its inputs from CCDs  51  and  55  (see FIG. 11). CCU for white use  41  provides an input to switching arrangement  42 , a second input to which is provided by superimposing circuit  106   b . This in turn receives its inputs from image processing circuit  104   a , color reference generator  106   a  and positioning device  205 , all of which function in the manner of the correspondingly numbered elements described in connection with FIG. 11.  
         [0195]    Control center  5 ′ also includes a control circuit  8  and a start switch  7 . Control circuit  8  provides a control output to switching arrangement  42  to select whether the white light image provided by CCU for white use  41  or the fluorescent image provided by superimposing circuit  106   b  is coupled to a monitor  6 . Control circuit  8  receives additional inputs and provides additional outputs as described in connection with FIG. 1.  
         [0196]    [0196]FIG. 19 shows a seventh embodiment of the present invention which essentially combines the features of the first and the fifth embodiments. Here, camera  4  (not shown) and monitor  6  function essentially in the same manner as described in connection with FIG. 1, and control center  5 ″ performs a combination of the functions of control center  5  of FIG. 1 and image processing circuit  104  of FIG. 15.  
         [0197]    Specifically, control center  5 ″ includes CCU for white use  41  which processes the white light image signal provided by CCD  40 , CCU  143 , A/D converter  144 , and Look Up Table (LUT)  145  which process the green color band component signal of the fluorescent image, and CCU  146 , a/d converter  147  and LUT  148  which process the red color component signal of the fluorescent image. The outputs of LUTS  145  and  148  are coupled to video processor  149 . All of these elements function in the same manner as the correspondingly numbered elements described in connection with FIG. 15. Video processor  149  provides a second signal input to switching arrangement  42 . A control signal input for switching arrangement  42  is provided by control circuit  8  which selects between the white light video signal and the fluorescent light video signal in the manner described in connection with FIG. 1.  
         [0198]    Control center  5 ″ also includes computation circuit  141 , image intensifier control circuit  142  and calibration start switch  150  all of which function in the manner described with respect to the like numbered elements in FIG. 15.  
         [0199]    As previously indicated, the sixth and seventh embodiments respectively illustrated in FIGS. 18 and 19 function in the same manner as the embodiment of FIG. 1 to select between display of a white light image or a fluorescent image and to protect the high sensitivity imaging circuits used to process the fluorescent image color band data. The sixth embodiment operates as described in connection with FIGS. 11 through 14 to provide an enhanced fluorescent image display and a color reference display for simultaneous presentation on the monitor, while the seventh embodiment operates in the manner described in connection with FIGS. 15 through 17 to provide color and light level correction for the video display of the fluorescent image.  
         [0200]    Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will be apparent to those skilled in the art. It is intended, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.