Abstract:
Illumination light supplied from an illumination light supplying unit that can selectively supply a plurality kinds of illumination lights whose wavelengths fall in different regions is supplied to an object. A signal picking up an image of the object is received by a programmable circuit unit that is programmably constructed based on circuit data, and then subjected to signal processing. A circuit data holding unit holds a plurality kinds of circuit data to be used for the programmable circuit unit. A control unit selects circuit data, which is used for the programmable circuit unit, from among all the circuit data items held in the circuit data holding unit corresponding to illumination light supplied from the illumination light supplying unit.

Description:
[0001]    This application claims the benefit of Japanese Application No. 2003-185715 filed on Jun. 27, 2003, the contents of which are incorporated by this reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to an image processing device enabling observation under a plurality kinds of observation light.  
           [0004]    2. Description of the Related Art  
           [0005]    Electronic endoscopes are designed to be inserted into body cavities and the like and widely used to observe the alimentary tract including the esophagus, stomach, small intestine, and large intestine or the trachea such as lungs. The electronic endoscope is also adapted to various kinds of treatment or cure that are performed using a treatment instrument passed through a treatment instrument channel.  
           [0006]    For example, in a field-sequential endoscope system, light emanating from a light source unit is passed through an optical filter in order to sequentially irradiate red, green, blue light and the like to an object. A monochrome image pickup device receives the lights reflected from the object. A processor (signal processing unit) performs signal processing on an output signal of the image pickup device. Eventually, a color image is displayed on a display device.  
           [0007]    The signal processing to be performed in the processor includes color enhancement that is intended to help discover a lesion. In the color enhancement, a color is enhanced using an amount of hemoglobin contained in the mucosa of a living body as a criterion. This helps distinguish a normal mucosa from an abnormal mucosa on the basis of a difference in color.  
           [0008]    Moreover, when an endoscope is used for diagnosis, normal observation is performed in order to display a color image, which depicts an object in the same manner as the object is seen with the naked eyes, on a monitor. In addition, self-fluorescent observation that utilizes light resulting from self-fluorescence of a living-body tissue is prevailing. In the self-fluorescent observation, the spectral characteristic of self-fluorescent light deriving from fluorescence of a living-body tissue caused with excitation light whose wavelengths range from the infrared region of the electromagnetic spectrum to the blue region thereof varies depending on whether the living-body tissue is a normal mucosa or a tumor.  
           [0009]    Diagnosis is performed by utilizing the fact that the spectral characteristic of self-fluorescent light varies depending on whether a living-body tissue is a normal mucosa or a tumor.  
           [0010]    An image represented by self-fluorescent light and an image represented by light reflected from a living-body tissue are assigned different colors and displayed on a monitor. Consequently, a lesion can be clearly identified based on a difference in color from a normal region.  
           [0011]    Moreover, as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2002-95635, narrow-band light observation (or narrow-band imaging (NBI)) is adopted as observation under light whose wavelengths fall within a narrower band than the wavelengths of normal observation light. In the narrow-band light observation, the vessels in the mucosal membrane can be observed with a higher contrast.  
           [0012]    Since the narrow-band light observation is observation under narrow-band light, an image whose color tone is different from the color tone of a normal endoscopic image is displayed. A color conversion circuit is therefore incorporated in a processor in order to adjust colors. After the color tone is converted into a color tone more helpful in distinguishing a lesion, the image is displayed on the monitor.  
           [0013]    Moreover, infrared observation that is observation under near infrared light is popular. During infrared observation, a chemical agent called indocyanine green (ICG) to which near infrared light is absorbed is injected into the vessel. Consequently, the vascular kinetics in a submucous deep region that is not visualized by normal observation can be observed. Even during the infrared observation, if color enhancement is performed using an amount of ICG contained in the mucosa as a criterion, the vessels can be observed with a higher contrast.  
           [0014]    The facilities for performing the foregoing normal observation, fluorescent observation, narrow-band light observation, and infrared observation may be implemented in one system by employing a lighting unit capable of switching illumination lights.  
           [0015]    In order to reduce the scale of the circuitry of an endoscope system, as disclosed in Japanese Unexamined Patent Application Publication No. 5-277065 (Japanese Patent No. 3382973), a programmable logic element is included for each of charge-coupled devices (CCDs) incorporated in endoscopes to be connected. Thus, the same circuit is used in common in different facilities.  
         SUMMARY OF THE INVENTION  
         [0016]    The present invention is an image processing device for performing image processing on an image pickup signal that represents the picked-up image of an object. The image processing device comprises:  
           [0017]    a programmable circuit unit that performs signal processing on the image pickup signal using a circuit which is programmably constructed based on selected circuit data;  
           [0018]    a circuit data holding unit that holds a plurality kinds of circuit data; and  
           [0019]    a control unit that controls such that a second circuit is programmably constructed by selecting, based on a directive signal that directs switching, a second circuit data that is different from first circuit data used in the first circuit, from among the circuit data items held in the circuit data holding unit. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 to FIG. 12 are concerned with a first embodiment of the present invention; FIG. 1 shows the overall configuration of an endoscope system including the first embodiment of the present invention;  
         [0021]    [0021]FIG. 2 is an explanatory diagram showing a band switching filter;  
         [0022]    [0022]FIG. 3 is an explanatory diagram showing a rotary filter panel;  
         [0023]    [0023]FIG. 4 shows the properties of transmittance exhibited by a normal/fluorescent observation filter and an infrared observation filter respectively;  
         [0024]    [0024]FIG. 5 shows the property of transmittance exhibited by a narrow-band light observation filter;  
         [0025]    [0025]FIG. 6 shows the properties of transmittance exhibited by red, green, and blue filters;  
         [0026]    [0026]FIG. 7 shows the properties of transmittance exhibited by an excitation filter, a G′ filter, and an R′ filter;  
         [0027]    [0027]FIG. 8 shows the property of transmittance exhibited by an excitation cut filter;  
         [0028]    [0028]FIG. 9 is a flowchart describing control actions to be performed in order to switch filters in the first embodiment;  
         [0029]    [0029]FIG. 10 shows the configuration of a color enhancement circuit constructed using a field-programmable gate array (FPGA);  
         [0030]    [0030]FIG. 11 shows the configuration of a noise cancellation circuit constructed using the FPGA;  
         [0031]    [0031]FIG. 12 shows the configuration of a color conversion circuit constructed using the FPGA; and  
         [0032]    [0032]FIG. 13 is a flowchart describing control actions to be performed in order to switch filters in a second embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]    Referring to the drawings, embodiments of the present invention will be described below.  
       First Embodiment  
       [0034]    Referring to FIG. 1 to FIG. 12, a first embodiment of the present invention will be described below.  
         [0035]    An object of the present embodiment is to provide an image processing device and an electronic endoscope system which make it possible to perform different kinds of signal processing corresponding to a plurality kinds of observation light (observation modes) using small-scale circuitry and possible to display a view image in the form of a motion picture even during construction of a programmable logic element (construction of a circuit). To begin with, the configuration of the present embodiment will be described below.  
         [0036]    As shown in FIG. 1, an electronic endoscope system  1  including the first embodiment of the present invention comprises: an electronic endoscope (hereinafter, an endoscope)  2  that is inserted into a body cavity in order to pick up an image of an object  8  such as a lesion in the body cavity; a light source unit  3  to or from which the endoscope  2  is freely coupled or uncoupled and which generates illumination light for observation; a processor  4  to or from which the endoscope is freely coupled or uncoupled and which performs signal processing or the like on an image signal representing the picked-up image; a monitor  5  which is connected to the processor  4 , to which a video signal is transmitted from the processor  4 , and on which an image represented by the video signal is displayed; a keyboard  6  connected to the processor  4  and used to enter data or commands; and a footswitch  7 .  
         [0037]    The endoscope  2  comprises: an elongated insertional unit  11  that is inserted into a body cavity; an operating unit  12  disposed at the back end of the insertional unit  11 ; and a universal cord  13  extending from the operating unit  12 .  
         [0038]    A light guide fiber  14  by which illumination light is transmitted is run through the insertional unit  11  of the endoscope  2 . The back side portion of the light guide fiber  14  is passed through the universal cord  13 . A light guide connector  15  is attached to the back end of the light guide fiber  14 . A user couples the light guide connector  15  freely detachably to the light source unit  3 . Illumination light emanating from the light source unit  3  is introduced to the incidence end surface of the light guide connector  15 .  
         [0039]    The illumination light transmitted to the light guide fiber  14  is irradiated from a distal end surface (emission end surface), which is attached to an illumination window formed in the distal section of the insertional unit  11 , through an illumination lens  16  to the object  8  in the body cavity.  
         [0040]    An objective lens  17  is attached to an observation window (image pickup window) adjoining the illumination window. A high-sensitivity solid-state image pickup device, or more particularly, a charge-coupled device (CCD)  18  is disposed at the image-forming position of the objective lens  17 . The CCD  18  photoelectrically coverts the picked-up image formed on the image pickup surface of the CCD  18 . The CCD  18  can be connected to or disconnected from the processor  4  by a connector formed at an end of a signal line.  
         [0041]    An excitation light cut filter  19  is disposed in front of the image pickup surface of the CCD  18 . The excitation light cut filter  19  cuts out excitation light that is used for fluorescent observation, so that feeble fluorescent light can be introduced into the CCD  18 .  
         [0042]    Moreover, the operating unit  12  of the endoscope  2  includes a filter selection switch  20  that is used to direct switching of illumination lights to be achieved through switching of filters. A directive signal produced by the filter selection switch  20  is transmitted to a CPU  45 .  
         [0043]    The light source unit  3  comprises: a lamp  21  that is formed with a xenon lamp or the like and that radiates light whose wavelength ranges from the infrared region to the visible region; a band switching filter  22  that is disposed on the path of illumination light emanating from the lamp  21  and that limits transmitted wavelength; a motor  23  used to switch the band switching filter  22 ; a rotary filter panel  24  including filters that pass light of different wavelength band; a motor  25  used to rotate or drive the rotary filter panel  24 ; a motor  26  used to move the rotary filter panel  24  in a direction A perpendicular to the illumination optical axis; and a condenser lens  27  that concentrates light transmitted by the rotary filter panel  24  and has the light incident on the end surface of the light guide connector  15 .  
         [0044]    In this case, the motor  25  for rotating the rotary filter panel  24  has a rack  28  attached thereto. A pinion  29  that is meshed with the rack  28  is attached to the rotation shaft of the motor  26 . By rotating or driving the motor  26 , the motor  25  and rotary filter panel  24  are moved in the direction A perpendicular to the illumination optical axis.  
         [0045]    Moreover, a normal light/special light switching switch  31  and a special light selection switch  32  are located at a position on, for example, a front panel of the light source unit  3  at which a user can easily manipulate the switches. Herein, the special light signifies any of fluorescent observation, narrow-band light observation, and infrared observation.  
         [0046]    The processor  4  is designed such that a video signal will orderly flow into: a preprocessing circuit  34  that performs preprocessing on an image pickup signal received from the CCD  18 ; an A/D conversion circuit  35 ; a color balance correction circuit  36 ; a first simultaneous memory  37 ; a delay circuit  38  or a field programmable gate array (FPGA)  39  that is programmably constructs a circuit; a selector  40  that selects either of the signals received from the delay circuit  38  and FPGA  39 ; a gamma correction circuit  41 , a structure enhancement circuit  42  that is formed with a spatial filter circuit; a second simultaneous memory  43 ; and a D/A conversion circuit  44 .  
         [0047]    The CPU  45  that is responsible for control and the like and incorporated in the processor  4  is electrically connected to external equipment such as the endoscope  2 , keyboard  6 , footswitch  7 , and light source unit  3 , and also electrically connected to internal circuits such as a color tone designation switch  33 , a load control circuit  46  and the like that are incorporated in the processor  4 .  
         [0048]    The FPGA  39  that programmably constructs a circuit has an internal memory, as well as a gate circuit, that performs logic operations. The FPGA  39  may include lookup tables or an image memory.  
         [0049]    The load control circuit  46  is connected to be able to designate an address in a data ROM  47  in which data to be loaded into the FPGA  39  is stored. An output terminal of the data ROM  47  is connected to a data load pin in a connector included in the FPGA  39 .  
         [0050]    A total of four circuit data items corresponding to observation modes for normal observation, fluorescent observation, narrow-band light observation, and infrared observation (normal observation mode circuit data  47   a , fluorescent observation mode circuit data  47   b , narrow-band light observation mode circuit data  47   c , and infrared observation mode circuit data  47   d ) are stored in the data ROM  47 . Selected circuit data is loaded into the FPGA  39 , whereby a circuit described by the circuit data is constructed.  
         [0051]    The keyboard  6  includes, in addition to normal keys, four keys for use in designating normal observation, fluorescent observation, narrow-band light observation, or infrared observation, that is, a normal observation designation key  6   a , a fluorescent observation designation key  6   b , a narrow-band light observation designation key  6   c , and an infrared observation designation key  6   d . Moreover, the footswitch  7  includes switches  7   a  and  7   b  equivalent to the normal light/special light switching switch  31  and special light selection switch  32  disposed in the light source unit  3 .  
         [0052]    As shown in FIG. 2, the band switching filter  22  includes a normal/fluorescent observation filter  48 , an infrared observation filter  49 , and a narrow-band light observation filter  50 .  
         [0053]    [0053]FIG. 4 and FIG. 5 show the spectral characteristics of lights transmitted by the filters. Namely, FIG. 4 shows the property of transmittance  48   a  exhibited by the normal/fluorescent observation filter  48  and the property of transmittance  49   a  exhibited by the infrared observation filter  49 . FIG. 5 shows the properties of transmittance  50   a ,  50   b , and  50   c  exhibited by the narrow-band light observation filter  50 .  
         [0054]    The property of transmittance  48   a  of the normal/fluorescent observation filter  48  is such that the normal/fluorescent observation filter  48  transmits light whose wavelength ranges from 400 nm to 660 nm. The property of transmittance of the infrared observation filter  49  is such that the infrared observation filter  49  transmits light whose wavelengths range from 790 nm to 980 nm.  
         [0055]    As shown in FIG. 5, the narrow-band light observation filter  50  exhibits three peaks of the properties of transmittance  50   a ,  50   b , and  50   c  which transmit three respective lights belonging to three discrete wavelength bands. Namely, the narrow-band light observation filter  50  exhibits the properties of transmittance  50   a ,  50   b , and  50   c  of transmitting respective lights whose wavelengths range from 400 nm to 430 nm, from 530 nm to 560 nm, or from 600 nm to 630 nm.  
         [0056]    The outer circumference of the rotary filter panel  24  includes, as shown in FIG. 3, an R filter  51 , a G filter  52 , and a B filter  53  that transmit respective lights whose wavelengths fall within the red, green, or blue region.  
         [0057]    The inner circumference of the rotary filter panel  24  includes a G′ filter  54  that transmits light whose wavelength ranges from 540 nm to 560 nm, an excitation filter  55  that transmits excitation light whose wavelength ranges from 390 nm to 450 nm, and an R′ filter  56  that transmits light whose wavelength ranges from 600 nm to 620 nm.  
         [0058]    [0058]FIG. 6 and FIG. 7 show the spectral characteristics of lights transmitted by the outer and inner-circumference filters of the rotary filter panel  24 . Namely, FIG. 6 shows the properties of transmittance  51   a ,  52   a , and  53   a  exhibited by the R filter  51 , G filter  52 , and B filter  53  respectively.  
         [0059]    As shown in FIG. 6, the outer-circumference filters  51 ,  52 , and  53  have the properties of partially transmitting light whose wavelength falls within not only the visible region but also the near infrared region.  
         [0060]    More specifically, the properties of transmittance  51   a  and  52   a  of the R filter  51  and G filter  52  are such that the R filter  51  and G filter  52  transmit light whose wavelength ranges from 750 nm to 820 nm. The property of transmittance  53   a  of the B filter  53  is such that the B filter  53  transmits light whose wavelength is equal to or larger than 900 nm.  
         [0061]    The excitation light cut filter  19  exhibits, as shown in FIG. 8, the property of transmittance  19   a  of intercepting light whose wavelength is equal to or smaller than 450 nm. The wavelength band transmitted by the excitation light cut filter  19  does not overlap the wavelength band transmitted by the excitation filter  55 .  
         [0062]    According to the present embodiment, the light source unit  3  introduces illumination light, which is associated with a selected one of the plurality kinds of observation modes, to the light guide fiber  14  included in the endoscope  2 .  
         [0063]    Moreover, according to the present embodiment, whichever of the observation modes is selected, once circuit data to be read from the data ROM  47  is determined (the load control circuit  46  controls loading), a circuit that performs required signal processing on a signal representing an image picked up in the observation mode can be programmably constructed by the FPGA  39 . This process is controlled by the CPU  45 . Moreover, a control program describing the process is stored in a memory  45   a  incorporated in, for example, the CPU  45 .  
         [0064]    According to the present embodiment, owing to the foregoing constituent feature, the FPGA  39  is used in common to construct a color enhancement circuit for, for example, normal observation or infrared observation or to construct a noise cancellation circuit for fluorescent observation. Thus, required signal processing is achieved despite the small-scale circuitry.  
         [0065]    Moreover, according to the present embodiment, in addition to the FPGA  39 , the delay circuit  38  is provided in combination to construct a bypass circuit that causes, for example, motion picture data to bypass the FPGA  39  and that thus enables display of a motion picture.  
         [0066]    The selector  40  can select either a circuit programmably constructed by the FPGA  39  or the bypass circuit. When construction of a circuit constructed by the FPGA  39  is under way, motion picture data is temporarily routed to the bypass circuit. Consequently, even when construction of a circuit by the FPGA  39  is under way, a motion picture can be displayed. This leads to improved user-friendliness.  
         [0067]    Next, the operation of the present embodiment will be described below.  
         [0068]    Light with which an object is illuminated is radiated from the lamp  21  included in the light source unit  3 . The light radiated from the lamp  21  passes through the band switching filter  22  and rotary filter panel  24 . Thereafter, the light is converged by the condenser lens  27  and introduced into the light guide fiber  14  included in the endoscope  2 .  
         [0069]    The band switching filter  22  is driven to rotate by the motor  23  in response to a filter selection directive signal sent from the CPU  45 . During normal observation or fluorescent observation, the normal/fluorescent observation filter  48  is inserted into the path of illumination light. During narrow-band light observation, the narrow-band light observation filter  50  is inserted into the path of illumination light. During infrared observation, the infrared observation filter  49  is inserted into the path of illumination light.  
         [0070]    During normal observation, narrow-band light observation, or infrared observation, any of the outer-circumference filters of the rotary filter panel  24  is inserted into the optical axis of illumination light. The rotary filter panel  24  is driven to rotate at a predetermined speed by the motor  25 , whereby the R filter  51 , G filter  52 , and B filter  53  are sequentially inserted into a light path.  
         [0071]    Owing to the combined use of the rotary filter panel  24  and band switching filter  22 , red, green, and blue lights are transmitted during normal observation. Lights whose wavelength ranges from 400 nm to 430 nm, 530 nm to 560 nm, or 600 nm to 630 nm is transmitted during narrow-band light observation. Lights whose wavelength ranges from 790 nm to 820 nm or 900 nm to 980 nm is transmitted during infrared observation.  
         [0072]    In order to acquire an image formed by feeble fluorescent light with a long exposure period, the motor  25  is rotated at a slower speed during fluorescent observation than in the other observation modes. Moreover, during fluorescent observation, the rotary filter panel  24  is moved in a direction A perpendicular to the path of illumination light by the motor  26  in response to a filter selection directive signal sent from the CPU  45 . Consequently, any of the inner-circumference filters is inserted into the path of illumination light.  
         [0073]    When the inner-circumference filters are sequentially inserted, lights whose wavelength ranges from 540 nm to 560 nm, 390 nm to 450, or 600 nm to 620 nm is sequentially emitted from the light source unit  3 . The light whose wavelengths range from 390 nm to 450 nm is excitation light for use in exciting self-fluorescence of a living-body tissue.  
         [0074]    Light incident on the light guide fiber  14  included in the endoscope  2  is irradiated to the object  8  such as the alimentary tract through the illumination window formed in the distal section of the endoscope  2 . Light scattered, reflected, or radiated from the object  8  forms an image on the CCD  18  by the objective lens  17  disposed on the observation window formed in the distal section of the endoscope  2 , and the image is photoelectrically converted to be picked up.  
         [0075]    The excitation light cut filter  19  is located in front of the CCD  18 , and works to intercept excitation light whose wavelength ranges from 390 nm to 450 nm and to extract fluorescent light. The CCD  18  is driven by a CCD drive circuit, which is not shown, synchronously with the rotation of the rotary filter panel  24 . Image signals representing respective illumination lights that have passed through the R filter  51 , G filter  52 , and B filter  53  of the rotary filter panel  24  are sequentially received by the processor  4 .  
         [0076]    The image signals received by the processor  4  are first received by the preprocessing circuit  34 . The preprocessing circuit  34  performs correlation double sampling (CDS) or the like, and the image signals are taken out.  
         [0077]    The signals sent from the preprocessing circuit  34  are converted from an analog form into a digital form by the A/D conversion circuit  35 . The signals sent from the A/D conversion circuit  35  are received by the color balance correction circuit  36 .  
         [0078]    The color balance correction circuit  36  adjusts the amplification factors for the signals such that when an object image serving as a reference is picked up, the object image will be displayed on the monitor  5  in predetermined colors. Thereafter, the resultant signals are temporarily stored in the first simultaneous memory  37 . Image data items sequentially stored in the first simultaneous memory  37  are read simultaneously with one another, whereby the simultaneity of the field-sequential images is performed.  
         [0079]    The image data resulting from the synchronization is received by the delay circuit  38  and FPGA  39 . The delay circuit  38  is formed with a memory and used to match the timing of a received signal with the timing of a signal that passes through the FPGA  39 , that is, to delay the received signal by the same time interval as the time interval required for the signal sent to the FPGA  39  to pass through the FPGA  39 .  
         [0080]    The output signal of the delay circuit  38  and that of the FPGA  39  are received by the selector  40 . The selector  40  selects either the signal received from the delay circuit  38  or the signal received from the FPGA  39 , and changes the simultaneous signal into field-sequential signals. The resultant field-sequential signals are then transmitted to the gamma correction circuit  41 .  
         [0081]    Herein, the simultaneous signal is changed to the field-sequential signals so as to reduce the scales of the gamma correction circuit  41  and structure enhancement circuit  42 . Specifically, when the selector  40  selects the signal received from the first simultaneous memory  37 , the selector  40  time-sequentially reads the components of the signal so as to thus frame-sequence the received signals.  
         [0082]    The gamma correction circuit  41  performs conversion processing for the purpose of correcting the gamma characteristic of an image to be displayed on the monitor  5 . Moreover, the structure enhancement circuit  42  performs signal processing to enhance the contour of an image. Thereafter, the signals having undergone the enhancement are received by the second simultaneous memory  43 .  
         [0083]    The simultaneity of signals is performed again in the second simultaneous memory  43 . A resultant signal is converted into an analog form by the D/A conversion circuit  44 , and transmitted to the monitor  5 . An image picked up by the CCD  18  is then displayed on the display surface of the monitor  5 .  
         [0084]    When a user manipulates any of the filter selection switch  20  on the endoscope  2 , the keyboard  6 , the footswitch  7 , and the switches  31  and  32  on the light source unit  3 , switching of observation lights (although lights actually switched are illumination lights, since observation lights are switched accordingly, the expression “switching of observation lights” is adopted) is achieved.  
         [0085]    The keyboard  6  includes the keys  6   a  to  6 D that are associated with normal observation, fluorescent observation, narrow-band light observation, and infrared observation. A user selects a desired observation mode by directly manipulating any of the keys, whereby an associated directive signal is received by the CPU  45 . In response to the directive signal, the CPU  45  switches observation lights.  
         [0086]    When the filter selection switch  20  on the endoscope  2  is pressed, every time a user presses the switch  20 , the CPU  45  sequentially switches observation lights in the order of normal observation, fluorescent observation, narrow-band light observation, infrared observation, normal observation, etc.  
         [0087]    When the normal light/special light switching switch  31  on the light source unit  3  (or the equivalent switch  7   a  on the footswitch  7 ) is pressed, if the mode attained before the switch is pressed is any of the fluorescent observation, narrow-band light observation, and infrared observation modes, the CPU  45  will change the mode to the normal observation mode.  
         [0088]    If the mode attained before the switch is pressed is the normal observation mode, the CPU  45  will change the mode to any of the fluorescent observation, narrow-band light observation, and infrared observation modes. In this case, to whichever of the fluorescent observation, narrow-band light observation, and infrared observation modes the normal observation mode is switched can be determined using the special light selection switch  32 .  
         [0089]    Every time a user presses the special light selection switch  32 , three special light observation modes are sequentially switched over in the order of fluorescent observation, narrow-band light observation, infrared observation, fluorescent observation, etc. Thus, the user can select any of the three special light observation modes.  
         [0090]    If the mode attained before the special light selection switch  32  is pressed is the normal observation mode, the CPU  45  does not switch observation lights. An indicator LED that is not shown informs a user of the fact that the special light observation modes have been changed. If the mode attained before the special light selection switch  32  is pressed is not the normal observation mode, the CPU  45  actually switches observation lights.  
         [0091]    [0091]FIG. 9 describes the details of a process of switching observation lights that are executed by the CPU  45  according to a control method described in a control program stored in the memory  45   a.    
         [0092]    The image processing control method according to which the CPU  45  executes the control process as described in FIG. 9 will be briefed below.  
         [0093]    If a state of currently selected illumination light and a state of a circuit constructed with the FPGA  39  associated with the light and being in operation is changed to another state directed to perform image processing according to the switching direction, the CPU  45  controls the FPGA  39  to construct a new circuit having a different image processing capability.  
         [0094]    While construction of a new circuit is under way, the CPU  45  transmits a tentative image (a monochrome motion picture in the present embodiment). Thus, an image is displayed on the monitor  5  without fail. When construction of a new circuit through the FPGA  39  is completed, the CPU  45  transmits an image produced by the constructed new circuit to the display means. Owing to this control process, even when construction of a circuit through the FPGA  39  is under way, an event that no image is displayed can be prevented. A description will be made in conjunction with FIG. 9 below.  
         [0095]    When the CPU  45  receives a switching directive entered by a user, the CPU  45  controls the second simultaneous memory  43  to assign the same signal (green color signal) to all the channels for red, green, and blue signals to be transmitted to the monitor  5 . Consequently, a monochrome image is transmitted to the monitor (see step S 1 ).  
         [0096]    At the next step S 2 , the CPU  45  causes the selector  40  to select an image signal received from the delay circuit  38 , and prevents an image signal from passing through the FPGA  39 . Namely, the CPU  45  selects the image signal having passed through the delay circuit  38 .  
         [0097]    At the next step S 3 , the CPU  45  directs the load control circuit  46  to load data into the FPGA for the purpose of reconfiguring of the FPGA (see step S 4 ), and thus reconfigures the FPGA  39 .  
         [0098]    The load control circuit  46  designates an address in the data ROM  47  associated with observation light, and reads circuit data, which is loaded into the FPGA  39 , from the data ROM  47 . Consequently, the circuit data read from the data ROM  47  corresponding to the observation light is loaded into the FPGA  39 .  
         [0099]    The CPU  45  judges from the action of the load control circuit  46  whether reconfuguring is completed (step S 5 ). Specifically, the CPU  45  waits until a signal confirming that loading of data is completed is sent from the FPGA  39 . Moreover, the CPU  45  waits for completion of switching of filters at step S 6 . After switching of filters is completed, the CPU  45  causes the selector  40  to select an output of the FPGA  39  at step S 7 .  
         [0100]    When the CPU  45  judges whether switching of filters is completed, the CPU  45  may judge from a switching completion signal received from the light source unit  3 . Otherwise, the CPU  45  may judge from an output signal of a timer that is set to a time longer than the time required to complete switching of filters.  
         [0101]    At the next step S 8 , the CPU  45  controls the second simultaneous memory  43  such that a color image will be retransmitted.  
         [0102]    According to the present embodiment, the bypass circuit that causes a signal to bypass the FPGA  39  into which data is being loaded is included in order to transmit a tentative image through the bypass circuit. Thus, even when reprogramming of the FPGA  39  is under way, a motion picture is displayed for continuous observation.  
         [0103]    An image resulting from image processing performed by the FPGA  39  cannot be displayed for a short period of time during which an image signal is passing through the bypass circuit. However, during the period, an image whose color tone is not normal is displayed because filters are being switched. Therefore, no particular problem occurs.  
         [0104]    Moreover, according to the present embodiment, a motion picture can be seen. Therefore, the movement of the endoscope  2  can be checked even during switching of filters. User-friendliness is ensured. Moreover, by using a monochrome image during switching of filters, a disorder in the color tone of an image caused by the switching of filters can be made indiscernible.  
         [0105]    [0105]FIG. 10 shows an example of the configuration of a color enhancement circuit  60  constructed in the FPGA  39  during normal observation or infrared observation.  
         [0106]    Based on an input image signal (more particularly, a signal Rin, Gin, or Bin), an amount-of-pigment calculation circuit  61  included in the color enhancement circuit  60  calculates an amount of pigment. The amount-of-pigment calculation circuit  61  calculates an amount of hemoglobin from each pixel value during normal observation, or calculates an amount of ICG from each pixel value during infrared observation.  
         [0107]    The amount of hemoglobin, IHb, is calculated according to the following expression:  
           IHb =log( R / G )  
         [0108]    The amount of ICG, IIcg, is calculated according to the following expression:  
           IIcg =log( B / R )  
         [0109]    Moreover, the amount-of-pigment calculation circuit  61  not only calculates the amount of pigment (amount of hemoglobin or ICG) from each pixel value but also calculates an average amount of pigment (an average of amounts of pigments) from the values of pixels constituting one image frame.  
         [0110]    The amounts of pigment and the average amount of pigment calculated by the amount-of-pigment calculation circuit  61  are received by an enhancement coefficient calculation circuit  62 . The enhancement coefficient calculation circuit  62  calculates an enhancement coefficient on the basis of a difference between each amount of pigment and the average amount of pigment.  
         [0111]    For normal observation (when an amount of hemoglobin, IHb, is calculated), the enhancement coefficient α for each pixel is calculated according to the following expression:  
         α= IHb−Ave ( IHb )  
         [0112]    where Ave(IHb) denotes an average of amounts of hemoglobin as described later. For infrared observation (when an amount of ICG is calculated), IIcg is substituted for IHb in the above expression.  
         [0113]    Ave(IHb) in the above expression denotes an average of amounts of hemoglobin calculated from the values of the respective pixels constituting one image frame. Herein, a difference between an amount of pigment calculated from each pixel value and the average of amounts of pigment calculated from the values of the respective pixels constituting one frame is calculated. Consequently, even an image whose color distribution is distorted can be enhanced effectively.  
         [0114]    Moreover, the signal Rin, Gin, or Bin serving as an input image signal is delayed by a delay circuit  63   a ,  63   b , or  63   c . Thereafter, the signal is received by a lookup table (LUT)  64   a ,  64   b , or  64   c.    
         [0115]    In other words, the image signal having the timing thereof adjusted while passing through the delay circuit  63   a ,  63   b , or  63   c , the enhancement coefficient α, and a color tone designation level specified by the CPU  45  are respectively received by the lookup table  64   a ,  64   b , or  64   c . Consequently, color enhancement is performed on each pixel on the basis of the received data and an amount of pigment.  
         [0116]    The color enhancement is achieved according to the following expression:  
           R out= R in× e×p ( h×kR ×α)  
           G out= G in× e×p ( h×kG ×α)  
           B out= B in× e×p ( h×kB ×α)  
         [0117]    where Rin, Gin, or Bin denotes an input image signal of red, green, or blue respectively. Rout, Gout, or Bout denotes an output image signal of red, green, or blue respectively. KR, kG, or kB denotes a coefficient which is determined with the absorbency of pigment into each color and whose value varies depending on whether an amount of hemoglobin is calculated or an amount of ICG is calculated.  
         [0118]    h denotes a coefficient indicating the degree of enhancement. The h value is determined by means of the CPU  45  according to a designation level entered using the color tone designation switch  33 .  
         [0119]    When the color enhancement is performed based on an amount of hemoglobin, an image expressing an apparent increase in the amount of hemoglobin is produced. When the color enhancement is performed based on an amount of ICG, an image expressing an apparent increase in the amount of ICG is produced.  
         [0120]    The circuitry is the same irrespective of whether normal observation or infrared observation is performed. However, the absorbency of pigment is different from light to light. Therefore, the data items of the lookup tables cannot be used in common between normal observation and infrared observation.  
         [0121]    According to the present embodiment, even when normal observation and infrared observation are switched, the FPGA  39  is reconfigured in order to construct required lookup tables  64   a ,  64   b , and  64   c . Therefore, the scale of circuitry can be minimized. Moreover, since the lookup tables  64   a ,  64   b , and  64   c  are incorporated in the FPGA  39 , compared with a ROM is disposed outside, fast access is enabled.  
         [0122]    [0122]FIG. 11 shows an example of the configuration of a noise cancellation circuit  70  constructed in the FPGA  39  during fluorescent observation.  
         [0123]    The noise cancellation circuit  70  includes a  3 × 3  median filter. owing to delay circuits  71  (more particularly,  71   a  to  71   f ) for delaying a signal by a time equivalent to the cycle of one clock and owing to line memories  72  (more particularly,  72   a  and  72   b ) for delaying a signal by a time, which is required to treat pixels constituting almost one line, the values of nine pixels around a pixel concerned are received by a median selection circuit  73 .  
         [0124]    For example, an input image signal Rin is received by the median selection circuit  73  as it is. Moreover, the signal Rin is delayed by a time required to treat one pixel by the delay circuit  71   a , and then received by the median selection circuit  73 . The signal delayed by the time required to treat one pixel is further delayed by the time required to treat one pixel by the delay circuit  71   b , and received by the median selection circuit  73 .  
         [0125]    Likewise, the input signal Rin is delayed by the time required to treat pixels constituting almost one line by the line memory  72   a , and then received by the median selection circuit  73 . The signal is further delayed by the time required to treat one pixel, and a time required to treat two pixels by the delay circuits  71   a  and  71   b  respectively, and then received by the median selection circuit  73 .  
         [0126]    The signal delayed by the time required to treat pixels constituting almost one line by the line memory  72   a  is delayed by the time required to treat pixels constituting almost one line by the line memory  72   b , and then received by the median selection circuit  73 . Moreover, the signal is delayed by the time required to treat one pixel, and the time required to treat two pixels by the delay circuits  71   a  and  71   b  respectively, and then received by the median selection circuit  73 .  
         [0127]    The median selection circuit  73  selects a pixel whose value is a median of the values of nine neighboring pixels, and transmits it. FIG. 11 shows only the circuit elements that treat the red signal. The same applies to the green and blue signals. FIG. 12 shows the configuration of a color conversion circuit  80  constructed in the FPGA  39  during narrow-band light observation.  
         [0128]    In the color conversion circuit  80 , an input image signal Rin, Gin, or Bin passes through a lookup table  81   a ,  81   b , or  81   c , and undergoes an arithmetic operation performed by a matrix circuit  82 . The resultant signal is then transmitted via a lookup table  83   a ,  83   b , or  83   c.    
         [0129]    The arithmetic operation performed by the matrix circuit  82  is expressed as follows:  
           R out= a 1· R in+ b 1· G in+ c 1· B in  
           G out= a 2· R in+ b 2· G in+ c 2· B in  
           B out= a 3· R in+ b 3· G in+ c 3· B in  
         [0130]    Where a, b, and c denote coefficients and a plurality of sets of values of coefficients are stored in a memory that is not shown. Any of the sets of values is selected based on a color tone designation level entered using the color tone designation switch  33  by means of the CPU  45 . As this, the color tone designation switch  33  is used to designate a color enhancement level for normal observation or infrared observation and also used to select the values of the coefficients that are employed in the arithmetic operation performed by the matrix circuit during narrow-band light observation.  
         [0131]    The designated levels are stored in the CPU  45  in association with the respective observation modes. When observation modes are switched, a stored level is designated accordingly. The lookup tables  81  and  83  are used for adjustment or used to compress or convert colors, which are not supported by the monitor  5 , into colors that can be displayed.  
         [0132]    According to the present embodiment, the FPGA  39  is employed. Any other programmable logic device (PLD) will do.  
         [0133]    Moreover, observation lights to be switched are not limited to self-fluorescent light, narrow-band light, and infrared light. Alternatively, fluorescent light caused by a chemical agent that can be administered to a human body or light having undergone the Raman effect will do.  
         [0134]    Moreover, the present invention is not limited to the field-sequential type endoscope system  1 , but may be adapted to a simultaneous type endoscope system.  
         [0135]    Moreover, as described in Japanese Unexamined Patent Application Publication No. 10-210324, the degree of color enhancement to be performed during normal observation or infrared observation may be adjusted based on the magnitude of a feature of an image.  
         [0136]    Moreover, the FPGA  39 , load control circuit  46 , and data ROM  47  may be mounted on an independent substrate other than a substrate on which the other circuit elements are mounted. The substrate may be able to be inserted into or pulled out of the processor  4  (or may be attachable to or detachable from the processor  4 ), whereby a facility for performing image processing such as color conversion can be provided for a user as an extension facility of the processor  4 .  
         [0137]    Moreover, preferably, a user can assign various facilities to the switches disposed on the endoscope  2  or footswitch  7 .  
         [0138]    The present embodiment provides advantages described below.  
         [0139]    According to the present embodiment, a programmable logic element is reconfigured during switching of filters along with switching of observation lights. Different kinds of signal processing associated with a plurality kinds of observation lights can be achieved despite a small scale of circuitry.  
         [0140]    Moreover, according to the present embodiment, while a logic element is being configured to perform image processing, a signal is transmitted via a bypass circuit that causes the signal to bypass the logic element. Consequently, even when configuring of the programmable logic element is under way, a view image can be seen in the form of a motion picture.  
       Second Embodiment  
       [0141]    Next, referring to FIG. 13, a second embodiment of the present invention will be described below.  
         [0142]    An object of the present embodiment is to provide an image processing unit that can perform different kinds of signal processing associated with a plurality kinds of observation lights despite a small scale of circuitry and that enables display of a view image in the form of a still image even during configuring of a programmable logic element.  
         [0143]    The present embodiment has the same hardware configuration as the first embodiment. However, the control programs stored in the memory  45   a  of the CPU  45  are different from those employed in the first embodiment. According to the present embodiment, as described below, while a circuit is constructed using the FPGA  39  or when observation lights are switched, the CPU  45  extends control so as to display a still image.  
         [0144]    Next, the operation of the present embodiment will be described below.  
         [0145]    The present embodiment is different from the first embodiment in a point of switching of observation lights.  
         [0146]    [0146]FIG. 13 describes control actions to be performed by the CPU  45  for the purpose of switching of observation lights.  
         [0147]    First, when switching of observation lights is directed, the CPU  45  controls the second simultaneous memory  43  at step S 11  so as to inhibit writing of data in the second simultaneous memory  43 . Thus, image data written immediately before writing is inhibited is repeatedly read out, that is, a frozen still image is transmitted.  
         [0148]    At step S 12 , the CPU  45  transmits a filter selection directive signal to each of the motor  23  and motor  26  so as to switch to predetermined observation light.  
         [0149]    At this time, at step S 13 , the CPU  45  directs the load control circuit  46  to reconfigure the FPGA. Thus, the FPGA  39  is reconfigured.  
         [0150]    The load control circuit  46  designates an address in the data ROM  46  associated with the observation light, and reads circuit data that is to be loaded into the FPGA  39 . Consequently, the circuit data associated with the observation light and read from the data ROM  47  is loaded into the FPGA.  
         [0151]    At step S 14 , the CPU  45  judges from a load completion signal received from the load control circuit  46  whether reconfiguring has been completed.  
         [0152]    After the CPU  45  receives the load completion signal, the CPU  45  judges at step S 15  whether switching of filters has been completed. After switching of filters is completed, the CPU  45  controls the second simultaneous memory  43  to cease the freeze mode. Thus, a motion picture is retransmitted (step S 16 ).  
         [0153]    As mentioned above, according to the present embodiment, an image is frozen during loading of data into the FPGA  39 . Consequently, even during configuring of the FPGA  39 , a color still image can be viewed for observation. A disorder in an image deriving from switching of filters does not occur.  
         [0154]    Moreover, during reconfiguring of the FPGA  39 , a motion picture cannot be viewed. However, since reconfiguring of the FPGA  39  may be performed during switching of filters, an examination time will be saved owing to the reconfiguring of the FPGA  39 .  
         [0155]    The present embodiment provides advantages described below.  
         [0156]    According to the present embodiment, a programmable logic element is reconfigured during switching of filters along with switching of observation lights. Different kinds of signal processing associated with the plurality kinds of observation lights respectively can be achieved despite a small scale of circuitry.  
         [0157]    Moreover, according to the present embodiment, during reconfiguring of a logic element that performs image signal processing, an image is frozen in a stage that is located closer to an output stage than to the logic element. Consequently, even during reconfiguring of the programmable logic element, a still image devoid of a disorder can be viewed for observation.  
         [0158]    As described above, according to the present invention, different kinds of signal processing associated with a plurality kinds of observation lights respectively can be achieved despite a small scale of circuitry.  
         [0159]    The preferred embodiments of the present invention have been described with reference to the accompanying drawings. It should be understood that the present invention is not limited to the precise embodiments but a person skilled in the art can make various changes or modifications without departing from the spirit or scope of the invention defined in the appended claims.