Abstract:
There is provided a confocal scanning endoscope system, which is provided with a light source that emits light, a scanning unit that deflects the light emitted by the light source so that the light scans on a subject in two dimensions, an objective optical system that directs the light deflected by the scanning unit to the subject, an extraction unit that extracts only part of the light returning from a convergence point at which the light is converged by the objective optical system on an object side, an image formation unit configured to form an image based on the part of the light extracted by the extraction unit, and a display area adjustment unit configured to measure Encircled Energy of the part of the light extracted by the extraction unit and to adjust a display area of the image based on the measured Encircled Energy.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a confocal scanning endoscope system having a confocal observation function. 
     Confocal scanning microscope systems capable of providing highly magnified high resolution images have been used. The confocal scanning microscope system is configured to emit a laser beam scanning on a sample in two dimensions. An operator of the confocal scanning microscope system observes the sample through a highly magnified high resolution observation image formed by the confocal scanning microscope system by illuminating the sample with the laser beam and receiving the laser beam from the sample through a pinhole positioned at a point conjugate with the sample. Examples of such a confocal scanning microscope system are disclosed in Japanese Patent Provisional Publications Nos. 2000-275528A and 2004-138947A. 
     The above mentioned confocal microscope system requires an operator to remove part of tissue in a body cavity of a subject using an endoscope so that the removed part of the tissue can be observed using the confocal scanning microscope system. That is, the confocal observation requires the operator to conduct troublesome work in which removal of tissue and the observation of the tissue have to be conducted using separate apparatuses. 
     For this reason, an endoscope system having the above mentioned function of the confocal scanning microscope system is desired. Such an endoscope system (hereafter, referred to as a confocal scanning endoscope system) is able to reduce a load on the operator during observations. Because the confocal scanning endoscope system is configured to move light in two dimensions on tissue, there is a case where the light passes through a peripheral region of a confocal optical system provided in the confocal scanning endoscope system. Because the peripheral region of the confocal optical system causes aberrations in a larger amount than a central region of the confocal optical system, light-gathering power of the confocal optical system decreases in the peripheral region. Therefore, resolution and brightness of a part of an image formed in the confocal scanning endoscope optical system by the light passing through the peripheral region of the confocal optical system decrease, and thereby a peripheral part of an observation image may become blurred. 
     Using the confocal scanning endoscope system, the operator attempts to find abnormality of tissue, for example, based on change of a shape of a cell while observing the observation image displayed on a monitor of the confocal scanning endoscope system. If a blurred part appears on the observation image due to the above mentioned factors specific to the confocal scanning endoscope system, the diagnosis by the operator may be adversely affected. 
     As in the case of a normal endoscope system, the confocal scanning endoscope system is required to reduce the diameter of an insertion tube which is inserted into a body cavity of a subject by downsizing the confocal optical system so that a load on the subject during the diagnosis can be reduced. 
     However, downsizing an optical system leads to increasing the amount of off-axial aberration. In other words, downsizing the confocal optical system for decreasing the diameter of the insertion tube, the blurred part in the observation image may become larger. Further, if the confocal optical system has positional errors, such as decentering, the blurred part appears on the observation image in a shape asymmetric about an optical axis of the confocal optical system. Such deterioration of the observation image may also adversely affect the diagnosis by the operator. 
     SUMMARY OF THE INVENTION 
     The present invention is advantageous in that it provides a confocal scanning endoscope system capable of providing an observation image from which a blurred part caused by factors specific to the confocal scanning endoscope system is removed. 
     According to an aspect of the invention, there is provided a confocal scanning endoscope system, which is provided with a light source that emits light, a scanning unit that deflects the light emitted by the light source so that the light scans on a subject in two dimensions, an objective optical system that directs the light deflected by the scanning unit to the subject, an extraction unit that extracts only part of the light returning from a convergence point at which the light is converged by the objective optical system on an object side, an image formation unit configured to form an image based on the part of the light extracted by the extraction unit, and a display area adjustment unit configured to measure Encircled Energy of the part of the light extracted by the extraction unit and to adjust a display area of the image based on the measured Encircled Energy. 
     By adjusting the display area based on the measured Encircled Energy, it is possible to form the image based on the light passing through a part of the objective optical system having high resolution. In other words, it is possible to display a high quality image for observations not having a blurred region caused by factors specific to the confocal scanning endoscope system. Such a configuration enables an operator to conduct endoscopic observations. 
     In at least one aspect, the display area adjustment unit adjusts the display area such that the Encircled Energy is larger than or equal to a predetermined level in the display area in accordance with a relationship between the Encircled Energy and lateral resolution of the objective optical system. 
     With this configuration, it is possible to form the image based on the light passing through a part of the objective optical system having a resolution higher than or equal to a predetermined level. 
     In at least one aspect, the display area adjustment unit adjusts the display area such that the Encircled Energy in the display area is larger than or equal to −5 dB with respect to the Encircled Energy defined for the part of the light proceeding along an optical axis of the objective optical system. 
     In at least one aspect, the display area adjustment unit adjusts the display area such that the Encircled Energy in the display area is larger than or equal to −3 dB with respect to the Encircled Energy defined for the part of the light proceeding along the optical axis of the objective optical system. In this case, lateral resolution of the objective optical system defined in the display area with respect to lateral resolution of the objective optical system at a center of the image is 0.7. 
     In at least one aspect, the Encircled Energy is measured by the display adjustment unit in a condition where the subject illuminated with the light is a sample formed of homogeneous material. 
     In at least one aspect, the display area adjustment unit generates information concerning the adjusted display area and provides the information for the image formation unit. In this case, the image formation unit forms the image further based on the information concerning the adjusted display area. 
     In at least one aspect, the display area adjustment unit generates information concerning the adjusted display area and provides the information for the scanning unit. In this case, the scanning unit deflects the light based on the information concerning the adjusted display area so that the light is deflected in a range corresponding to the adjusted display area. 
     In at least one aspect, the confocal scanning endoscope system further includes an endoscope comprising an flexible insertion tube in which the light source, the scanning unit, the objective optical system and the extraction unit are accommodated, a processor comprising the image formation unit, and a display unit on which the image formed by the image formation unit is displayed. In this case, the endoscope and the display unit are connected to the processor, and the display area adjustment unit is detachably attached to the processor. 
     In at least one aspect, the endoscope includes an optical fiber provided in the flexible insertion tube. In this case, a facet of the optical fiber at a tip portion of the flexible insertion tube serves as the light source. 
     In at least one aspect, the scanning unit deflects the light by moving the facet of the optical fiber in a plane with which an optical axis of the objective optical system perpendicularly intersects. 
     In at least one aspect, the facet of the optical fiber is located at a position conjugate with the subject with respect to the objective optical system. In this case, the facet serves as the extraction unit. 
     In at least one aspect, the Encircled Energy is proportional to a square of lateral resolution of the objective optical system. 
     According to another aspect of the invention, there is provided a confocal scanning endoscope system, which is provided with a light source that emits light, a scanning unit that deflects the light emitted by the light source so that the light scans on a subject in two dimensions, an objective optical system that directs the light deflected by the scanning unit to the subject, an extraction unit that extracts only part of the light returning from a convergence point at which the light is converged by the objective optical system on an object side, and an image formation unit configured to form an image based on the part of the light extracted by the extraction unit. Encircled Energy of the part of the light extracted by the extraction unit is larger than or equal to a predetermined level in an entire region of the image formed by the image formation unit. 
     With this configuration, it is possible to form the image based on the light passing through a part of the objective optical system having high resolution. In other words, it is possible to display a high quality image for observations not having a blurred region caused by factors specific to the confocal scanning endoscope system. Such a configuration enables an operator to conduct endoscopic observations. 
     In at least one aspect, the Encircled Energy in the entire region of the image is larger than or equal to −5 dB with respect to the Encircled Energy defined for the part of the light proceeding along an optical axis of the objective optical system. 
     In at least one aspect, the confocal scanning endoscope system further comprises a display unit on which the image formed by the image formation unit is displayed. 
     According to another aspect of the invention, there is provided an adjustment method for an image formed by a confocal scanning endoscope system comprising a scanning unit that deflects light emitted by a light source so that the light scans on a subject in two dimensions, an extraction unit that extracts only part of the light returning from a convergence point at which the light is converged by an objective optical system on an object side, and an image formation unit configured to form an image based on the part of the light extracted by the extraction unit. The method includes measuring Encircled Energy of the part of the light extracted by the extraction unit, and adjusting a display area of the image based on the measured Encircled Energy. 
     With this configuration, it is possible to form the image based on the light passing through a part of the objective optical system having high resolution. In other words, it is possible to display a high quality image for observations not having a blurred region caused by factors specific to the confocal scanning endoscope system. Such a configuration enables an operator to conduct endoscopic observations. 
     In at least one aspect, the measuring of Encircled Energy is conducted in a condition where the subject illuminated with the light is a sample formed of homogeneous material. 
     In at least one aspect, the display area of the image is adjusted by masking a part of the image formed by the image formation unit. 
     In at least one aspect, the display area of the image is adjusted by causing the scanning unit to adjust a scanning range of the light. 
    
    
     
       BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
         FIG. 1  is a block diagram of a confocal scanning endoscope system according an embodiment of the invention. 
         FIG. 2  is a cross section of a tip portion of a flexible insertion tube of the confocal scanning endoscope system illustrating an internal structure of the tip portion. 
         FIG. 3  is a graph illustrating a relationship between Encircled Energy and lateral resolution of an objective optical system of the confocal scanning endoscope system. 
         FIG. 4  is a flowchart illustrating a display area adjustment process performed under control of a display area adjustment device in the confocal scanning endoscope system. 
         FIG. 5  illustrates an example of a graph of contour lines representing the distribution of the Encircled Energy. 
         FIG. 6  illustrates the distribution of the Encircled Energy on which a recangular frame representing the display area is overlayed. 
         FIG. 7  illustrates the distribution of the Encircled Energy and the rectangular frame representing the display area when components in a confocal observation unit in the confocal scanning endoscope system have positional errors. 
         FIG. 8  illustrates the distribution of the Encircled Energy in which an octagonal frame representing the display area is overlayed. 
         FIG. 9  illustrates the distribution of the Encircled Energy and the octagonal frame representing the display area when components in the confocal observation unit in the confocal scanning endoscope system have positional errors. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, an embodiment according to the invention is described with reference to the accompanying drawings. 
       FIG. 1  is a block diagram of a confocal scanning endoscope system (hereafter, simply referred to as a confocal endoscope system)  500  according an embodiment of the invention. The confocal endoscope system  500  includes an electronic endoscope  100  having a flexible insertion tube  10  to be inserted into a body cavity to obtain an image of tissue in the body cavity, a processor  200  to which the electronic endoscope  100  is connected, a processor  300 , a display area adjustment device  400  connected to the processor  200 , and monitors  200 M and  300 M respectively connected to the processors  200  and  300 . On the monitors  200 M and  300 M, images output by the processors  200  and  300  are displayed, respectively. 
     The electronic endoscope  100  has a confocal observation function of obtaining information concerning an image of tissue in a body cavity through use of a confocal optical system as well as a function of imaging tissue in a body cavity through use of an image pick-up device, such as a CCD. As shown in  FIG. 1 , the electronic endoscope  100  includes the flexible insertion tube  10  having flexibility, a tip portion  11  of the flexible insertion tube  10 , an insertion hole  12  into which various types of treatment instruments, such as forceps, can be inserted, a holding part  13  to be held by an operator for operation of the electronic endoscope  100 , an operation unit  14  having various type of buttons and levers to be operated by the operator, a cable  15  to be connected to the processor  200 , and a cable  16  to be connected to the processor  300 . 
     The processor  200  is used for the confocal observation. The processor  200  includes an image forming and processing unit  210  configured to form and process images to be output to the monitor  200 M, and a light source unit  220 . The processor  300  is used for normal observation. The processor  300  includes an image processing unit (not shown) configured to from and process images to be output to the monitor  300 M. 
       FIG. 2  is a cross section of the tip portion  11  of the flexible insertion tube  10  illustrating an internal structure of the tip portion  11 . As shown in  FIG. 2 , in the tip portion  11 , a confocal observation unit  50  and a normal observation unit  90  are provided. The confocal observation unit  50  includes a single mode optical fiber (hereafter, simply referred to as an optical fiber)  20 , an objective optical system  30 , a cover glass  31 , and piezoelectric elements  40 A and  40 B. These components of the confocal observation unit  50  are held in a cylindrical frame  50   a . The cylindrical frame  50   a  is held in a cylindrical metallic pipe  60  having a diameter slightly larger than that of the cylindrical frame  50   a  so as to be slidable in the cylindrical metallic pipe  60 . 
     In  FIG. 2 , an axis equal to an optical axis of the objective optical system  30  is defined as a Z-axis. Axes which are orthogonal to each other and are orthogonal to the Z-axis are defined as X and Y axes. The X and Y axes define an X-Y plane with which the z-axis perpendicularly intersects. The optical fiber  20  is provided between the light source unit  220  and the objective optical system  30 , and serves to guide light between the objective optical system  30  and the processor  200 . The piezoelectric elements  40 A and  40 B are located in the vicinity of a facet  21  of the optical fiber  20 . The piezoelectric elements  40 A and  40 B are positioned such that displacement directions thereof are orthogonal to each other (i.e., X and Y directions). When applied a voltage, each of the piezoelectric elements  40 A and  40 B presses and moves a tip portion near the facet  21  of the optical fiber  20  in the X or Y direction. According to movement of the tip portion of the optical fiber  21  in a direction perpendicular to the optical axis of the objective optical system  30  caused by the piezoelectric elements  40 A and  40 B, a light beam emerging from the facet  21  of the optical fiber  21  scans on a surface of the tissue S in two dimensions. 
     Between an outer wall  51  of the cylindrical frame  50   a  and an inner wall  61  of the cylindrical metallic pipe  60 , a coil spring  70  and shape-memory alloy  80  are provided. Each of the outer wall  51  and the inner wall  61  is in parallel with the X-Y plane. The shape-memory alloy  80  deforms when an external force acts thereon at ambient temperatures, and contracts to a memorized shape when heated to a temperature higher than or equal to a predetermined temperature. If temperature increases from a state shown in  FIG. 2  (i.e., if the shape-memory alloy  80  is heated), the shape-memory alloy  80  contracts in the Z-direction. In the state shown in  FIG. 2 , the coil spring  70  is in a state of being compressed with respect to its natural length. Therefore, in the state shown in  FIG. 2 , the coil spring  70  presses the cylindrical frame  50  toward the front side (i.e. toward the tip portion of the flexible insertion tube  50 ). 
     When heated by an applied voltage, the shape-memory alloy  80  contracts. The strength of the contractile force of the shape-memory alloy  80  is larger than the pressing force of the coil spring  70 . Therefore, when the shape-memory  80  is heated, the cylindrical frame  50  slides toward the rear side (i.e., an opposite side with respect to the cover glass  31 ). In this case, a light convergence point at which the light passed through the objective optical system  30  converges shifts in the Z-direction. Consequently, scanning of the light in the Z-direction can be achieved. 
     Hereafter, an image forming process performed through the confocal observation unit  50  is explained. The optical fiber  20  has the function of guiding light from the light source unit  220  and emitting light from the facet  21 . In this case, the facet  21  of the optical fiber  20  serves as a secondary point source. In the following, a region on a plane which is parallel with the X-Y plane and in which the facet  21  is moved by the effect of the piezoelectric elements  40 A and  40 B is referred to as “a sweeping plane”. 
     The light emitted form the facet  21  passes through the objective optical system  30  and the cover glass  31  contacting the tissue S, and is converged on the tissue S. The light reflected from the tissue S passes through the cover glass  31  and the objective optical system  30  and then returns to the facet  21 . To receive light from the tissue S, the objective optical system  30  and the optical fiber  20  are located such that the facet  21  is positioned at the front focal point of the objective optical system  30 . In other words, the objective optical system  30  and the optical fiber  20  are located such that, to the facet  21  situated at a certain point in the sweeping plane, only light reflecting from the tissue S at a converging point which is conjugate with the facet  21  enters. Therefore, the facet  21  has a light extraction function of extracting only light which converged on the tissue S. 
     The returning light entering the facet  21  is guided to the processor  200  through the optical fiber  20 . Part of the returning light is guided to the image forming and processing unit  210 , for example, by a fiber coupler. The image forming and processing unit  210  forms a point image from the returning light, locates the point image at a position corresponding to the scanned position on the tissue, and continues such operations so that an image (a still image) corresponding to one frame can be formed. Further, the image forming and processing unit  210  executes predetermined image processing on the formed image. Then, the processed image is output to the monitor  200 M. The operator conduct diagnosis on the tissue S while observing the highly magnified, high resolution image of the tissue S displayed on the monitor  200 M. 
     The normal observation unit  90  includes an objective optical system through which white light from the processor  300  is emitted toward the tissue S, and an image pick-up device (not shown). In the normal observation, light from the processor  300  illuminates the tissue S. The light reflected from the tissue S is received by the image pick-up device in the normal observation unit  90 . The image pick-up device generates an image signal corresponding to an image formed thereon, and transmits the image signal to the processor  300 . The processor executes predetermined image processing on the image signal, and outputs the processed image signal to the monitor  300 M. Then, the image of the tissue S is displayed on the monitor  300 M. 
     Adjustment of a display area for the confocal observation will now be described in detail. Conventionally, resolution test of an objective optical system is conducted using a resolution chart on which a plurality of striped patterns are arranged. However, if such an resolution test is conducted to test resolution of the confocal observation unit  50  of the confocal endoscope system  500  in which one frame image is formed as a group of point images obtained by the light scanning on the tissue S, the resolution test needs to be conducted for each of the point images. As described below, in this embodiment, the adjustment of a display area for the confocal observation is conducted without requiring such a conventional resolution test. 
     If a beam spot formed on the tissue S by the objective optical system  30  of the confocal observation unit  50  is not suitably converged (i.e., the beam spot is diffused), Encircled Energy of the light reflecting from the tissue S decreases depending on the degree of diffusion of the beam spot. The Encircled Energy means the intensity of part of light reflecting from an illuminated sample (i.e., the tissue S) incident on the facet  21 . That is, the Encircled Energy means the intensity of light extracted by the facet  21 . 
     In general, an aberration becomes larger at a point farther from an optical axis of an objective optical system (i.e., lateral resolution of an objective optical system decreases as the distance form an optical axis of the objective optical system increases). The lateral resolution means the resolution of the objective optical system  30  defined in the X-Y plane. The resolution in the longitudinal direction (i.e., the Z-axis direction) can also be defined for the confocal observation unit  50  because as described above the confocal observation unit  50  is able to move along the longitudinal direction (the Z-axis direction). However, in the following, attention is focused on the lateral resolution. 
     On the tissue S, light passed through a peripheral region of the objective optical system  30  forms a beam spot which is diffused more largely than a beam spot formed by paraxial light of the objective optical system  30 . Therefore, the Encircled Energy of the light passed through the peripheral region of the objective optical system  30  becomes smaller in comparison with the paraxial light.  FIG. 3  is a graph illustrating a relationship between the Encircled Energy and the lateral resolution of the objective optical system  30 . In  FIG. 3 , a horizontal axis represents a lateral resolution ratio, and the vertical axis represents a Encircled Energy ratio. The lateral resolution ratio means a value of the lateral resolution defined with respect to a reference resolution having a value of 1 (i.e., the maximum lateral resolution defined at the center of the objective optical system  30 ). The Encircled Energy ratio means a value of the Encircled Energy defined with respect to the maximum Encircled Energy of light obtained on the facet  21 . As shown in  FIG. 3 , the Encircled Energy and the lateral resolution of the objective optical system  30  have a relationship. 
     More specifically, because the lateral resolution can be divided into two components defined in two directions orthogonal to each other, and the Encircled Energy is figured out as intensity of light per unit area, the square of the lateral resolution is proportional to the Encircled Energy. 
     Based on the above mentioned consideration, the display area for the confocal observation is adjusted by measuring the Encircled Energy in the sweeping area and determining an area in the sweeping area having the Encircled Energy larger than a certain level. An image formed within the display area is a bright high resolution image. 
       FIG. 4  is a flowchart illustrating a display area adjustment process performed under control of the display area adjustment device  400  in the confocal endoscope system  500 . The display area adjustment process may be initiated when the display area adjustment device  400  is attached to the processor  200  or when an instruction for initiation of the display area adjustment is inputted to the processor  200  to which the display area adjustment device  400  is connected. 
     As shown in  FIG. 4 , first, current settings concerning the display area adjustment are initialized (step S 1 ). The step S 1  may be omitted if the initialization of settings concerning the display area adjustment is conducted before shipment. Next, in step S 3 , a reference Encircled Energy is set to the display area adjustment device  400 . The reference Encircled Energy may be inputted to the display area adjustment device  400  from an external device each time the display area adjustment process is conducted or may be stored in advance in the display area adjustment device  400 . For example, the Encircled Energy ratio of −5 dB (corresponding to the lateral resolution of approximately 0.56) is defined as the reference Encircled Energy. The light amount corresponding to the Encircled Energy ratio of −5 dB is approximately one-third of the light amount corresponding to the maximum Encircled Energy ratio. 
     Next, in step S 5 , the Encircled Energy is measured. The Encircled Energy measurement is conducted by illuminating a reference sample Sref and imaging the reference sample Sref through the confocal observation unit  50 . The reference sample Sref is a homogeneous material. For example, water-soluble paint or an aqueous solution in which fluorescent material is solved may be used as the reference sample Sref. 
     In this embodiment, the Encircled Energy is measured as the intensity of light extracted by the facet  21  and received by the image forming and processing unit  210  through the optical fiber  20  because loss of light occurring by passing through the optical fiber  20  before reaching the image forming and processing unit  210  is almost zero and therefore the Encircled Energy on the facet  21  can be regarded as the intensity of light received by the image forming and processing unit  210 . A measurement result is then stored in a memory in the display area adjustment device  400  (step S 7 ). 
     After the facet  21  has moved in the entire sweeping plane (i.e., the scanning for generating one frame image of the reference sample Sref is finished), the display area adjustment device  400  executes a predetermined calculation on the results of measurements of the Encircled Energy stored therein to obtain distribution of Encircled Energy on the sweeping plane. The display area adjustment device  400  displays an image representing the distribution of the Encircled Energy by contour lines on the monitor  200 M.  FIG. 5  illustrates an example of a graph of the contour lines representing the distribution of the Encircled Energy. In  FIG. 5  (and in the following similar drawings), X and Y axes correspond to the X-axis direction and the Y-axis direction in  FIG. 2 . Scales on the X and Y axes represent the distance (shift amount) from the origin. 
     In  FIG. 5 , positional errors of the components (e.g., the objective optical system  30 ) in the confocal observation unit  50  are neglected. Therefore, the intensity of the Encircled Energy is symmetrical about the origin (i.e., the initial position (initial state) at which the facet  21  is situate when no piezoelectric force of the piezoelectric elements  40 A and  40  B is applied to the facet  21 ). As can be seen from  FIG. 5 , the Encircled Energy decreases as the distance from the origin increases. When the facet  21  is in the initial state and the components in the confocal observation unit  50  have no positional errors, the central axis of the facet  21  (i.e., an optical axis of the optical fiber  20 ) coincides with the optical axis of the objective optical system  30 . 
     After the contour lines shown in  FIG. 5  are displayed, the display area adjustment device  400  determines an area (i.e., the display area) satisfying a condition where in the area the Encircled Energy is larger or equal to the reference Encircled Energy, and displays a frame suitably shaped to represent the display area (step S 11 ). In general, an image displayed on the monitor  200 M has a rectangular shape. Therefore, the frame of the display area may be formed in a rectangular shape as shown in  FIG. 6 . In  FIG. 6 , the rectangular frame indicated by a heavy line represents the display area. 
     After the step S 11  is processed, information concerning the display area satisfying the condition where in the display area the Encircled Energy is larger than or equal to the reference Encircled Energy is registered in the processor  200  (step S 13 ). 
       FIG. 7  illustrates an example of a graph of the contour lines representing the distribution of the Encircled Energy and the frame representing the display area when the components in the confocal observation unit  50  have the positional errors. In  FIG. 7 , the distribution of the Encircled Energy is not symmetrical about the origin due to the positional errors, such as decentering. However, the frame appropriately represents the display area in which the Encircled Energy is larger than or equal to the reference Encircled Energy. 
     After the above mentioned display adjustment process in which the information concerning the display area is registered in the processor  200  is executed, the confocal endoscope system  500  forms an image corresponding to the display area. More specifically, in the image forming and processing unit  210  which obtains one frame image using the light transmitted from the confocal observation unit  50  via the optical fiber  20 , a peripheral region of the one frame image is cut out or masked so that a part of the obtained one frame image corresponding to the display area is displayed on the monitor  200 M in accordance with the information concerning the display area. Alternatively or additionally, a movement range of the facet  21  may be limited to a range corresponding to the display area by driving the piezoelectric elements  40 A and  40 B in accordance with the information concerning the display area so that an image corresponding to the display area can be formed using the light from the confocal observation unit  50  in the image forming and processing unit  210 . 
     As described above, once the display area adjustment process is executed and the information concerning the display area is registered, the confocal endoscope system  500  becomes able to display a high resolution image suitable for the confocal observation. Therefore, once the information concerning the display area is registered in the processor  200 , the display area adjustment device  400  is not required. Therefore, the display area adjustment device  400  may be configured to detachably attached to the processor  200  or the monitor  200 M. In this case, the confocal endoscope system  500  may be configured not to have the display area adjustment device  400  as a standard component. Consequently, cost reduction and downsizing of the confocal endoscope system  50  can be achieved. In this case, the display adjustment device  400  may be connected to the confocal endoscope system  500  only when the initial setting before shipment is conducted. Alternatively, the display area adjustment device may be given only to a maintenance person so that the display area adjustment can be executed in periodical maintenance services by the maintenance person. 
     Although the present invention has been described in considerable detail with reference to certain preferred embodiments thereof, other embodiments are possible. 
     Although in the above mentioned embodiment the frame representing the display area has a rectangular shape, the frame may be formed to have a shape different from the rectangular shape. A shape resembling a rectangle may be used as the shape of the frame representing the display area. For example, an octagonal shape formed by cutting out the corners of the rectangular shape may be used as the shape of the frame representing the display area. 
     In the above mentioned embodiment, the Encircled Energy ratio of −5 dB (i.e., the lateral resolution ratio of 0.56) is used as the reference Encircled Energy. The Encircled Energy ratio of −5 dB corresponds to a minimum level required to display an image having sufficiently high resolution and brightness for endoscopic observations on the monitor  200 M. For example, for displaying an image having higher resolution and brightness, the Encircled Energy ratio of −3 dB (corresponding to the lateral resolution of approximately 0.70) may be used as the reference Encircled Energy. 
       FIGS. 8 and 9  illustrate examples of graphs of contour lines representing the distribution of the Encircled Energy and frames representing the display area calculated through the steps S 11  of the display area adjustment process. In each of the graphs shown in  FIGS. 8 and 9 , the frame is formed in an octagonal shape. As can be seen from the comparison between the graphs shown in  FIGS. 6 to 9 , when the relatively high reference Encircled Energy is employed, the image in the display area has high Encircled Energy (i.e., a high resolution) although in this case the size of the display area reduces. 
     The confocal endoscope system  500  described in the above mentioned embodiment has both the functions of the confocal observation and the normal observation. However, the confocal endoscope system  500  may be configured to only have the confocal observation function. In this case, the normal observation unit  90  and the processor  300  can be omitted. 
     This application claims priority of Japanese Patent Application No. P2005-231433, filed on Aug. 10, 2005. The entire subject matter of the application is incorporated herein by reference.