Abstract:
An ocular examination system includes an illumination system illuminating a region of an eye fundus including a target blood vessel and adjusting the intensity of light illuminating the region, an image pickup device receiving light scattered from the region and producing signals in response to receiving the scattered light from the region, a control system receiving the signals and computing the diameter of the target blood vessel based on the signals, and a tracking system performing an automatic tracking operation on the target blood vessel based on the signals. The control system controls the illumination system to increase the intensity of the illumination applied to the region when the automatic tracking operation and the target-blood-vessel-diameter computation are simultaneously performed above the level of the illumination applied to the region when the automatic tracking operation is performed while the target-blood-vessel-diameter computation is not performed.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention generally relates to an ocular examination system for examining the blood vessels of an eye fundus.  
           [0003]    2. Description of the Related Art  
           [0004]    The principle of a measurement operation performed by a retinal blood flow meter, which is an example of an ocular examination system, is discussed below. A laser beam having a wavelength λ is applied to a subject blood vessel of a patient&#39;s eye, and the light scattered and reflected by the blood vessel is received by a photodetector. Then, an interference signal of a Doppler shift component, i.e., the light scattered and reflected by the blood moving in a blood vessel and the light scattered and reflected by a stationary blood vessel wall is detected. Upon analyzing the frequency of the interference signal, the blood velocity is determined. More specifically, the blood velocity, i.e., the maximum V max , is determined according to the following equation: 
           V max ={λ/(n·α)}·||Δf max1 |−|Δf max2 ||/cos β  (1) 
           [0005]    wherein Δf max1  and Δf max2  indicate the maximum frequency shifts calculated from the received-light signals received by two photodetectors; λ represents the wavelength of the laser light; n designates the index of refraction of a portion of the eye to be examined; α indicates the angle between the two light-detecting optical axes within the eye; and β represents the angle between the plane formed by the two light-detecting optical axes and the velocity vector of the blood flow.  
           [0006]    By measuring the blood velocity from the two directions as discussed above, contributions due to the directions of incidence of the measuring beams are canceled, thereby making it possible to measure the velocity of blood at a certain portion on the eye fundus. By matching the line of intersection between the plane formed by the two light-detecting optical axes and the eye fundus to the angle β, β becomes 0 degrees, thereby measuring the true maximum velocity.  
           [0007]    According to the retinal blood flow meter for measuring the shape of a blood vessel or the blood velocity in a blood vessel in a particular portion of the eye fundus by utilizing the laser beam, it is necessary that a beam of measuring light is precisely applied to a subject portion for a predetermined period. In actuality, however, it is difficult to precisely keep applying the measuring light to the subject portion due to involuntary eye movement. In order to solve this problem, an ocular system provided with a tracking function for detecting the position of a blood vessel and moving the measuring light to the subject portion in accordance with involuntary eye movement in real time is disclosed in Japanese Patent Laid-open Nos. 63-288133 and 6-503733 (by PCT).  
           [0008]    In the above-described ocular system, the tracking operation is performed as follows. A linear charge-coupled device (CCD) is used for applying a beam of tracking light to an eye fundus and for receiving light reflected by the eye fundus and producing a blood-vessel-image signal. Then, the waveform of the blood-vessel-image signal is processed so as to calculate the amount of movement of the blood-vessel image from a tracking reference position. Tracking light emitted from a tracking illumination light source and measuring light are applied to an eye fundus via a pupil conjugate mirror, and the linear CCD is used for receiving the light reflected by the eye fundus so as to process the waveform of the blood-vessel-image signal. A system for calculating a blood-vessel diameter by using a blood-vessel-image signal output from a linear CCD is disclosed in Japanese Patent Laid-open No. 7-31596.  
           [0009]    However, the above-described ocular examination systems have to perform a tracking operation while following the fast eye movement, and thus, a sufficient accumulation time of the linear CCD cannot be ensured. Accordingly, in order to obtain a blood-vessel image having a sufficient amplitude for performing the tracking operation and for calculating the blood-vessel diameter, an image intensifier is used for performing light amplification. However, with an excessively high amplification factor, the signal-to-noise (S/N) ratio of the blood-vessel-image signal from the linear CCD is reduced, thereby resulting in the lowered measurement precision of the blood-vessel diameter.  
           [0010]    This problem may be solved by removing unwanted high-frequency noise components by performing filtering processing. However, since various diameters of blood vessels are measured, the feature points for calculating the blood-vessel diameter may inconveniently be removed together with the noise components depending on the setting of the cut-off frequency. Alternatively, the intensity of tracking light applied to a patient&#39;s eye fundus may be uniformly increased so as to reduce the amplification factor of the image intensifier, thereby suppressing noise components. In the aforementioned ocular examination systems, however, the maximum permissible exposure (MPE), which is the maximum permissible laser energy to be applied to a patient&#39;s eye, is set by the American National Standard Institute (ANSI) for the safety of the patients. Accordingly, measuring light is applied to a patient&#39;s eye without exceeding the MPE, resulting in a limitation on the number of measurements for the same subject portion of the eye.  
         SUMMARY OF THE INVENTION  
         [0011]    Accordingly, a major object of the present invention is to improve a conventional ocular examination system. More specifically, it is an object of the present invention to achieve high-precision ocular examination by applying light to an eye fundus at a suitable intensity level.  
           [0012]    It is another object of the present invention to achieve high-precision ocular examination without restricting the number of measurements for the same subject portion of the eye.  
           [0013]    In order to achieve the above objects, according to the present invention, there is provided an ocular examination system comprising an illumination system configured to illuminate a region of an eye fundus of an eye including a target blood vessel, the illumination system being configured to adjust the intensity of light illuminating the region, an image pickup device positioned and configured to receive light scattered from the region illuminated by the illumination system and to produce signals in response to receiving the scattered light from the region, a control system connected to the illumination system and the image pickup device so as to receive the signals produced by the image pickup device and configured to compute the diameter of the target blood vessel based on the signals from the image pickup device, and a tracking system connected to the control system and configured to perform an automatic tracking operation on the target blood vessel based on the signals from the image pickup device. The control system is configured to control the tracking system so that the tracking system performs the automatic tracking operation simultaneous with the control system computing the diameter of the target blood vessel and so that the tracking system performs the automatic tracking operation when the control system does not computer the diameter of the target blood vessel. The control system is also configured to control the illumination system to change the intensity of the illumination applied to the region when the automatic tracking operation and the target-blood-vessel-diameter computation are simultaneously performed above the level of the illumination applied to the region when the automatic tracking operation is performed while the control system does not compute the target-blood-vessel diameter.  
           [0014]    According to another aspect, the present invention that achieves these objectives relates to an ocular examination system comprising illumination means for illuminating a region of an eye fundus of an eye including a target blood vessel and for adjusting the intensity of light illuminating the region, image pickup means for receiving light scattered from the region illuminated by the illumination means and for producing signals in response to receiving the scattered light from the region, control means for computing the diameter of the target blood vessel based on the signals from the image pickup means, and tracking means for performing an automatic tracking operation on the target blood vessel based on the signals from the image pickup means. The control means comprises means for controlling the tracking means so that the tracking means performs the automatic tracking operation simultaneous with the control means computing the diameter of the target blood vessel and so that the tracking means performs the automatic tracking operation when the control means does not compute the diameter of the target blood vessel. The control means also comprises means for controlling the illumination means to change the intensity of the illumination applied to the region when the automatic tracking operation and the target-blood-vessel-diameter computation are simultaneously performed above the level of the illumination applied to the region when the automatic tracking operation is performed while the control means does not compute the target-blood-vessel diameter.  
           [0015]    Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a schematic diagram illustrating an embodiment of the present invention;  
         [0017]    [0017]FIG. 2 is a front view illustrating a neutral density (ND) filter unit;  
         [0018]    [0018]FIG. 3 illustrates an observation eye fundus image;  
         [0019]    [0019]FIG. 4 illustrates beams of light formed on the pupil of a patient&#39;s eye;  
         [0020]    [0020]FIG. 5 is a flow chart illustrating the operation of the first embodiment;  
         [0021]    FIGS.  6 A- 6 D illustrate the waveforms of blood-vessel-image signals;  
         [0022]    FIGS.  7 A- 7 D illustrate the waveforms of blood-vessel-image signals;  
         [0023]    [0023]FIG. 8 is a front view illustrating an ND filter unit of a modification made to the first embodiment; and  
         [0024]    [0024]FIG. 9 is a flow chart illustrating the operation of the modification using the ND filter unit shown in FIG. 8. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    [0025]FIG. 1 is a block diagram illustrating a retinal flow meter according to an embodiment of the present invention. On an illumination optical path from an observation light source  1 , such as a tungsten lamp, which emits white light, to an objective lens  2 , which faces a patient&#39;s eye E, a transmitting-type liquid crystal panel  3  for displaying a fixation point, which is in an optically conjugate position with an eye fundus Ea of the patient&#39;s eye E and which is movable along the optical path, a relay lens  4 , an apertured mirror  5 , and a band-pass mirror  6 , which transmits yellow-range wavelength light and reflects most of the other ranges of light, are sequentially disposed. Behind the apertured mirror  5 , an eye fundus observation system, which is formed of an image forming lens  7 , which is movable along the optical path, and an eyepiece  8 , is sequentially disposed before an operator&#39;s eye “e”.  
         [0026]    On the optical path in the diversion direction of the band-pass mirror  6 , an image rotator  9  and a galvanometric mirror  10  having both sides polished and having a rotational axis perpendicular to the plane of the drawing are provided. A relay lens  11 , which is movable along the optical path is disposed in the reflecting direction of a lower reflecting surface  10   a  of the galvanometric mirror  10 . A lens  12  and a focusing unit  13 , which is movable along the optical path, are disposed in the reflecting direction of an upper reflecting surface  10   b  of the galvanometric mirror  10 . The galvanometric mirror  10  has a slit at the lower portion of the above-described rotational axis. The front focal plane of the lens  12  is in a conjugate position with a pupil Ep of the patient&#39;s eye E, and the galvanometric mirror  10  is placed on the front focal plane of the lens  12 .  
         [0027]    Behind the galvanometric mirror  10 , a lens  14  and a concave mirror  15  are disposed, which form a relay optical system for guiding a beam of light that passes through the slit of the galvanometric mirror  10  without being reflected at the lower reflecting surface  10   a  to the upper reflecting surface  10   b.    
         [0028]    In the focusing unit  13 , a dichroic mirror  16  is disposed on the same optical path as the lens  12 , and a mask plate  17 , having a rectangular diaphragm, and a mirror  18  are provided on the optical path in the reflecting direction of the dichroic mirror  16 . A lens  19  is disposed on the optical path in the transmitting direction of the dichroic mirror  16 . The focusing unit  13  can be integrally moved.  
         [0029]    On the optical path in the direction of incidence of the lens  19 , a collimated, coherent measurement light source  20 , such as a laser diode which emits, for example, red light, is disposed. On the optical path in the direction of incidence of the mirror  18 , a tracking light source  21 , such as a helium neon laser which emits high luminance light, for example, green light, different from other light sources, and a neutral density (ND) filter unit  22 , for controlling the intensity of the tracking light source  21 , are disposed.  
         [0030]    The ND filter unit  22  is formed of a plurality of ND filters whose transmittance is different, and is connected to a motor  23 . By operating the motor  23 , the ND filter to be inserted into the optical path is selected, thereby making it possible to control the intensity (quantity) of light to be applied to the patient&#39;s eye E from the tracking light source  21 . FIG. 2 is a front view of the ND filter unit  22  provided with two ND filters A and B.  
         [0031]    On the optical path of the reflecting direction of the lower reflecting surface  10   a  of the galvanometric mirror  10 , the relay lens  11 , a dichroic mirror  24 , a magnifying lens  25 , an image intensifier  26 , which is an example of an optical amplifier, and a linear CCD  27  are sequentially disposed so as to form a blood-vessel detection system. Photomultipliers  28  and  29  are located in the reflecting direction of the dichroic mirror  24  so as to form a measurement light-receiving system. Although all the optical paths are shown on the same plane for ease of representation, the reflecting direction of the dichroic mirror  24  is perpendicular to the plane of the drawing.  
         [0032]    A system controller  30  is provided for controlling the overall blood flow meter and also for performing calculations for measurements. The system controller  30  is electrically connected to an operation device  31  actuated by an operator, an output device  32 , such as a display device or a printer for displaying or printing a measurement result, a storage device  33 , outputs of the photomultipliers  28  and  29 , an output of the image intensifier  26 , and an output of the linear CCD  27 . An output of the system controller  30  is electrically connected to a mirror controller  34  for controlling the galvanometric mirror  10  and to a filter controller  35  for controlling the transmittance of the ND filters. An output of the system controller  30  is also electrically connected to the image intensifier  26  so as to change the amplification factor of the image intensifier  26 . The system controller  30  integrates a processor for executing a program for calculating the blood-flow velocity by analyzing frequencies of the signals generated by the photomultipliers  28  and  29 . The program executed by the processor of the system controller  30  also instructs the system controller to perform the steps illustrated in FIGS. 5 and 9 and discussed below. The program is stored in the system controller  30 , but it is within the scope of the invention to store this program in any computer-readable medium, such as on floppy disc, a CD-ROM, a hard drive, a ROM, etc. The above-described liquid crystal panel  3 , the image forming lens  7 , the focusing unit  13 , and the relay lens  11  are movable in the direction of the optical axis simultaneously in response to operating a focusing knob (not shown) so that the eye fundus Ea of the patient&#39;s eye E is constantly and optically conjugate with the liquid crystal panel  3 , the eye fundus of the operator&#39;s eye “e”, the mask plate  17 , and the light-receiving surface of the image intensifier  26 .  
         [0033]    Light emitted from each light source is applied to the patient&#39;s eye E, and the light reflected and scattered by the patient&#39;s eye E is guided to the operator, the photomultipliers  28  and  29 , or the light-receiving surface of the image intensifier  26 . More specifically, white light emitted from the observation light source  1  illuminates the liquid crystal panel  3  from behind, passes through the relay lens  4 , and is reflected by the apertured mirror  5 . The band-pass mirror  6  transmits only yellow-range wavelength light, which then passes through the objective lens  2  and temporarily forms an image on the pupil Ep of the patient&#39;s eye E as an eye fundus illumination light image I, as seen in FIG. 4. Then, the light almost uniformly illuminates the eye fundus Ea. Simultaneously, a fixation target F is displayed on the liquid crystal panel  3 . The fixation target F is projected on the eye fundus Ea of the patient&#39;s eye E by illumination light, and is presented to the patient&#39;s eye E as a fixation target image F′, as shown in FIG. 3.  
         [0034]    The light reflected by the eye fundus Ea returns via the same optical path, and is extracted as a beam of light O for observing the eye fundus Ea from the pupil Ep, as shown in FIG. 4. The light O then passes through the opening at the center of the apertured mirror  5  and the image forming lens  7 , and an eye fundus image Ea′ can be observed by the operator&#39;s eye “e” via the eyepiece lens  8 , as seen in FIG. 3. The operator performs alignment of the flow meter while observing the eye fundus image Ea′ via the eyepiece lens  8 .  
         [0035]    The collimated measuring light emitted from the measurement light source  20  passes through the lens  19  and the dichroic mirror  16 . The tracking light emitted from the tracking light source  21  is reflected by the mirror  18  and is shaped into a desired configuration by the mask plate  17 . Then, the tracking light is reflected by the dichroic mirror  16  and is then superimposed on the above-described measuring light. In this case, the measuring light forms a spot image which is in conjugate position with the opening at the center of the mask plate  17  by the lens  19 . The superimposed measuring light and the tracking light pass through the lens  12  and are reflected by the upper reflecting surface  10   b  of the galvanometric mirror  10 . After passing through the lens  14 , the measuring light and the tracking light are reflected by the concave mirror  15 , pass through the lens  14  once again, and then return to the galvanometric mirror  10 .  
         [0036]    The galvanometric mirror  10  is in a position conjugate with the pupil Ep of the patient&#39;s eye E. The concave mirror  15  and the lens  14  are concentrically disposed on the optical path, and are provided with a relay function of forming an image on the galvanometric mirror  10  with −1 times magnification. Accordingly, the measuring light and the tracking light reflected on the upper reflecting surface  10   b  of the galvanometric mirror  10  return to the slit of the galvanometric mirror  10  and are then directed to the image rotator  9  without being reflected by the galvanometric mirror  10 . The measuring light and the tracking light pass through the image rotator  9 , are deflected to the objective lens  2  by the band-pass mirror  6 , and illuminate the eye fundus Ea of the patient&#39;s eye E via the objective lens  2 .  
         [0037]    In this manner, the measuring light and the tracking light are reflected on the upper reflecting surface  10   b  of the galvanometric mirror  10 , and are again incident on the galvanometric mirror  10  later while being eccentric from the optical axis of the objective lens  2 . Then, the measuring light and the tracking light form a spot image P 2  or P 2 ′ on the pupil Ep, as shown in FIG. 4, and illuminate the eye fundus Ea in a spot-like shape. Also, in FIG. 4, a region M surrounded by a one-dot chain line denotes the image of the lower reflecting surface  10   a  of the galvanometric mirror  10 , O denotes the position of the beam of light for observing the eye fundus generated by the observation light source  1  and represents an image of the opening portion of the apertured mirror  5 , and V denotes the position of the measuring light received from the target blood vessel, and represents an image of the effective portions of the upper and lower reflecting surfaces  10   b  and  10   a  of the galvanometric mirror  10 .  
         [0038]    The measuring light and the tracking light scattered and reflected by the eye fundus Ea are condensed by the objective lens  2 , and most of the beams are reflected by the band-pass mirror  6 . The reflected light then passes through the image rotator  9  and is reflected on the lower reflecting surface  10   a  of the galvanometric mirror  10 . After passing through the relay lens  11 , the light is separated into the measuring light and the tracking light by the dichroic mirror  24 . The tracking light passes through the dichroic mirror  24 , and is magnified by the magnifying lens  25  at a greater scale than the eye fundus image Ea′ formed by the eye fundus observation optical system, thereby forming a blood-vessel image Ev′ on the photoelectric surface of the image intensifier  26 , as shown in FIG. 3. After being intensified, the blood-vessel image Ev′ is captured on the linear CCD  27 .  
         [0039]    Meanwhile, the measuring light is reflected by the dichroic mirror  24 , and is received by the photomultipliers  28  and  29 , which generate received-light signals in response to receiving the measuring light. The received-light signals are output from the photomultipliers  28  and  29  and are input into the system controller  30 , and based on the received-light signals, the processor of the system controller  30  determines the blood velocity of the blood vessel of the eye fundus Ea by analyzing the frequencies of the received-light signals according to the foregoing equation (1).  
         [0040]    The light scattered and reflected by the eye fundus Ea, which is condensed by the objective lens  2  and passes through the band-pass mirror  6 , is partially directed to the operator&#39;s eye “e” via the same optical path as the light scattered and reflected by the eye fundus Ea from the light from the observation light source  1 , and can be observed by the operator as a tracking target image T shown in FIG. 3 and a measuring light image together with the eye fundus image Ea′.  
         [0041]    [0041]FIG. 5 is a flow chart illustrating the control operation performed by the system controller  30 . The operator first actuates an operation stick (not shown) so as to align the optical axis of the patient&#39;s eye E with the optical axis of the objective lens  2 . The operator then brings the eye fundus Ea into focus by operating the above-described focusing knob (not shown) while observing the eye fundus image Ea′. Then, as discussed above, the fixation target F on the liquid crystal panel  3  comes into an optically conjugate position with the eye fundus Ea, and is presented to the patient&#39;s eye E. Accordingly, when the patient fixes his/her viewpoint on the fixation target image F′ shown in FIG. 3, the operator is able to observe the eye fundus image Ea′ shown in FIG. 3. Then, the operator moves the fixation target F by actuating the operation device  31  so as to guide the patient&#39;s eye E. As a result, a first subject portion can be fixed substantially at the center of the viewfield of the objective lens  8 .  
         [0042]    Then, the operator applies the tracking light to the eye fundus Ea by actuating the operation device  31 . The operator also actuates a rotation operation knob (not shown) so that the tracking target image T is perpendicular to a first subject blood vessel, and controls the angle of the galvanometric mirror  10  so as to apply the measuring light to the first subject blood vessel.  
         [0043]    As discussed above, after applying the tracking light to the blood vessel Ev, an image is formed on the photoelectric surface of the image intensifier  26  as the blood-vessel image Ev′. After being amplified, the blood-vessel image Ev′ is then captured on the linear CCD  27 , and is output as a blood-vessel-image signal. Then, the control operation of the system controller  30  shown in FIG. 5 is started. In step Si, the system controller  30  first determines whether the tracking light is input. If the outcome of step Si is no, step Si is repeated until the tracking light is input. If the result of step Si is yes, the process proceeds to step S 2 . In step S 2 , the system controller  30  instructs the filter controller  35  to select the ND filter A having low transmittance so as to insert it into the optical path of the tracking light source  21 . In response to this instruction, the filter controller  35  drives the motor  23  to insert the ND filter A into the optical path.  
         [0044]    After determining the subject portion, the operator actuates the operation device  31  once again to input an instruction to start the tracking operation. Then, a signal output from the linear CCD  27  is input into the system controller  30 . The system controller  30  then determines in step S 3  whether an instruction to start the tracking operation is input. If the outcome of step S 3  is no, the system controller  30  waits until the instruction is input. When it is found in step S 3  that the tracking starting instruction is input, the process proceeds to step S 4  in which the system controller  30  determines the amplification factor of the image intensifier  26  so that a suitable blood-vessel image Ev′ can be captured by the linear CCD  27 . Then, in step S 5 , based on the blood-vessel image Ev′ captured by the linear CCD  27 , the system controller  30  calculates data indicating the amount of movement of the blood-vessel image Ev′, and outputs the blood-vessel image Ev′ and the data indicating the amount of movement to the mirror controller  34 . In response, the mirror controller  34  drives the galvanometric mirror  10  so as to compensate for the movement of the blood-vessel image Ev′, thereby performing the tracking operation on the subject blood vessel.  
         [0045]    After the tracking operation is stabilized to produce a stable and stationary blood-vessel image, the operator actuates the operation device  31  once again to input an instruction to start the blood-velocity the measurement operation. The system controller  30  then determines in step S 6  whether the measurement starting instruction is input. If the result of step S 6  is no, the system controller  30  waits until the instruction is input. When the instruction is input in step S 6 , the process proceeds to step S 7 . In step S 7 , the system controller  30  instructs the filter controller  35  to select the ND filter B having high transmittance so as to insert it into the optical path. In response to the instruction, the filter controller  35  drives the motor  23  to place the ND filter B into the optical path, and in step S 8 , the system controller  30  resets the amplification factor of the image intensifier  26  so as to prevent saturation of the blood-vessel image captured by and output from the linear CCD  27 .  
         [0046]    In this case, the intensity of the reflection from the eye fundus Ea is increased due to the high transmittance of the ND filter B input into the optical path. Thus, the system controller  30  reduces the amplification factor of the image intensifier  26 . Subsequently, in step S 9 , the system controller  30  drives the measurement light source  20  to emit measuring light so as to receive signals representing the blood-vessel image from the linear CCD  27  and the Doppler signal from the photomultipliers  28  and  29 .  
         [0047]    The system controller  30  determines in step S 10  whether the input of the blood-vessel image and the Doppler signal into the system controller  30  is completed. If the outcome of step S 10  is no, the system controller  30  waits until the input of the data is completed. If the result of step S 10  is yes, the process proceeds to step S 11  in which the system controller  30  turns off the measurement light source  20 . Then, in step S 12 , the system controller  30  instructs the filter controller  35  to insert the light-shielding portion without an ND filter into the optical path. In response to this instruction, the filter controller  35  drives the motor  23  to block light from tracking light source  21  from falling on mirror  18 .  
         [0048]    Accordingly, the application of the tracking light to the patient&#39;s eye E is stopped. Then, in step S 13 , the tracking operation is completed. In step S 14 , the system controller  30  calculates the blood-vessel diameter and the blood velocity, and also determines the blood-flow amount from the blood-vessel diameter and the blood velocity, thereby completing the measurement operation.  
         [0049]    After applying the tracking light to the blood vessel Ev, as shown in FIG. 3, the blood-vessel image Ev′ is formed on the photoelectric surface of the image intensifier  26 . After being amplified, the blood-vessel image Ev′ is captured on the linear CCD  27 . When the ND filter B having high transmittance is placed in the optical path of the tracking light source  21 , which reduces the gain of the image intensifier  26 , a blood-vessel-image signal having a good S/N ratio, such as that indicated by FIG. 6A, is output from the linear CCD  27 . In FIGS.  6 A- 6 D and  7 A- 7 D the horizontal axis represents time and the vertical axis represents amplitude. On the other hand, when the ND filter A having low transmittance is placed in the optical path, which increases the gain of the image intensifier  26 , a blood-vessel-image signal having a poor S/N ratio, such as that indicated by FIG. 6C, is output from the linear CCD  27 , where the great thickness of the graph represents a poor S/N ratio.  
         [0050]    The blood-vessel-image signals indicated by FIGS. 6A and 6C are subjected by the system controller  30  to filtering processing without impairing the information on the blood vessel, and the resulting signals are indicated by FIGS. 6B and 6D, respectively. From these signal waveforms, the darkest point Min 1  is detected and is calculated as information indicating the central position of the blood vessel. Then, the system controller  30  determines the difference D or D′ between the blood-vessel central position and the tracking central position to which the measuring light is applied as the amount of movement of the blood vessel Ev.  
         [0051]    During the tracking operation, it is only necessary to detect the information of the blood-vessel central position from the blood-vessel-image signal. Accordingly, FIGS. 6B and 6D show that the amounts of movement D and D′ become almost the same without being influenced by the transmittance of the ND filter.  
         [0052]    When calculating the blood-vessel diameter, in addition to the information of the blood-vessel central position Min 1 , as shown in FIGS.  7 A- 7 D, the boundary points Max 1  and Max 2  between the blood vessels and peripheral tissue are detected by the system controller  30 . From these three points, the half-width of (Min 1 -Max 1 ) and (Min 1  Max 2 ) is calculated by the system controller  30 , and is then corrected by the system controller  30  by using magnification of the flow meter, the refractive index of the eyeball, and the visual axis length of the ocular examination system, thereby determining the blood-vessel diameter.  
         [0053]    As discussed above, when the ND filter B having high transmittance is located in the optical path of the tracking light source  21 , which reduces the gain of the image intensifier  26 , a blood-vessel-image signal having a good S/N ratio, such as that indicated by FIG. 7A, is obtained. In contrast, when the ND filter A having low transmittance is located on the optical path of the tracking light source  21 , which increases the gain of the image intensifier  26 , a blood-vessel image having a poor S/N ratio is output, such as that indicated by FIG. 7C.  
         [0054]    As in the tracking operation, the blood-vessel images indicated by FIGS. 7A and 7C are subjected to filtering processing by the system controller  30 , and the resulting signals are represented in FIGS. 7B and 7D. From these signal waveforms, the half-widths Dia and Dia′, which are required for determining the blood-vessel diameter, are calculated by the system controller  30  according to the above-described technique. Then, as indicated by FIGS. 7B and 7D, Dia′ becomes greater than Dia due to the influence of noise. That is, the measurement values of the blood-vessel diameter and the blood velocity disadvantageously vary according to the transmittance of the ND filter.  
         [0055]    In this embodiment, two types of ND filers A and B are used for the ND filter unit  22 . However, three types of ND filters A, B, and C whose transmittance is different, as shown in FIG. 8, or more types of ND filters, may be used, thereby making it possible to determine the blood-vessel diameter with a smaller error. Additionally, a suitable combination of the transmittance of the ND filter and the amplification factor of the image intensifier  26  is selected so that the intensify of-the tracking light applied to the eye fundus Ea is minimized, thereby increasing the number of measurements.  
         [0056]    In a modification to the first embodiment, the three types of ND filters shown in FIG. 8 may be used, and the transmittance of the ND filter may be increased in the order of the following three states 1, 2, and 3 so as to progressively increase the intensity of light applied to the eye fundus Ea:  
         [0057]    1. the alignment state before starting the tracking operation;  
         [0058]    2. the tracking state; and  
         [0059]    3. the measurement state in which the blood-vessel diameter is measured while performing the tracking operation.  
         [0060]    In state 1, the tracking light is used only for enabling the operator to perform alignment of the flow meter so that the measuring light can be applied to the subject blood vessel, and signal processing using a blood-vessel image output from the linear CCD  27  is not performed. Accordingly, the brightness of the tracking light applied to the eye fundus Ea is sufficient to enable the operator to visually check the eye fundus Ea. Thus, the intensity of the light can be set to a minimal level.  
         [0061]    In state 2, the operator has to detect the positional information of the subject blood vessel by using the blood-vessel image output from the linear CCD  27 . Accordingly, the intensity of the tracking light applied to the eye fundus Ea must be raised to a greater level than that set in state 1.  
         [0062]    In state 3, the intensity of the tracking light must be further increased to a greater level than that set in state 2.  
         [0063]    The operation performed by the system controller  30  when using the ND filter unit  22  shown in FIG. 8 according to the above-described modification is discussed below with reference to the flow chart of FIG. 9. The operator actuates the operation device  31 . Then, in step S 1 , the system controller  30  first determines whether tracking light is input. Then, in step S 2 , the system controller  30  instructs the filter controller  35  to select the ND filter A having the lowest transmittance so as to insert it into the optical path of the tracking light source  21 . In response to this instruction, the filter controller  35  drives the motor  23  to insert the ND filter A into the optical path of the tracking light source  21 . This state is equivalent to the alignment state 1 before starting the tracking operation.  
         [0064]    After determining the subject portion of the eye to be examined, the operator actuates the operation device  31  to input a tracking starting instruction, and a signal output from the linear CCD  27  is input into the system controller  30 . Then, the system controller  30  determines in step S 3  whether a tracking starting instruction is input. If it has not been input, the system controller  30  awaits for its input. If the outcome of step S 3  is yes, the process proceeds to step S 4 . In step S 4 , the system controller  30  instructs the filter controller  35  to select the ND filter B having the second lowest transmittance so as to insert it into the optical path. In response to this instruction, the filter controller  35  drives the motor  23  to insert the ND filter B into the optical path of the tracking light source  21 .  
         [0065]    Then, in step S 5 , the system controller  30  sets the amplification factor of the image intensifier  26  so that suitable blood-vessel image Ev′ can be captured on the linear CCD  27 . In step S 6 , based on the blood-vessel image Ev′, the system controller  30  calculates data indicating the amount of movement of the blood-vessel image Ev′, and outputs the blood-vessel image Ev′ and the information of the amount of movement to the mirror controller  34 . The mirror controller  34  then drives the galvanometric mirror  10  to compensate for the amount of movement so that the blood-vessel image is stationary on the photomultipliers  28  and  29  and on the CCD  27 , thereby tracking the blood vessel. This state is equivalent to the tracking state 2.  
         [0066]    After the tracking operation is performed and the location of the blood-vessel image is stabilized, the system controller  30  actuates the operation device  31  once again to input a measurement starting instruction. The system controller  30  then determines in step S 7  whether a measurement starting instruction is input. If the result of step S 7  is yes, the process proceeds to step S 8 . In step S 8 , the system controller  30  instructs the filter controller  35  to select the ND filter C having the highest transmittance to insert it into the optical path. In response to this instruction, the filter controller  35  drives the motor  23  to insert the ND filter C into the optical path of the tracking light source  21 .  
         [0067]    When the ND filter C is inserted into the optical path, in step S 9 , the system controller  30  resets the amplification factor of the image intensifier  26  so as to prevent saturation of a blood-vessel image captured by and output from the linear CCD  27 . The system controller  30  receives the blood-vessel image and the data of the amount of movement required for calculating the blood-vessel diameter from the linear CCD  27 . In this case, with the high transmittance of the ND filter disposed on the optical path, the intensity of the reflection from the eye fundus Ea increases. Accordingly, the amplification factor of the image intensifier  26  should be decreased. This state is equivalent to the measurement state 3 in which the blood-vessel diameter is measured while performing the tracking operation.  
         [0068]    The system controller  30  then determines in step S 10  whether the input of the blood-vessel image and the data of the amount of movement is completed. If not, the system controller  30  awaits completion of the input of this data. If the outcome of step S 10  is yes, the process proceeds to step S 11 . In step S 11 , the system controller  30  instructs the filter controller  35  to select the ND filter B having the second low transmittance once again so as to insert it into the optical path. In response to this instruction, the filter controller  35  drives the motor  23  to insert the ND filter B into the optical path of the tracking light source  21 .  
         [0069]    In step S 12 , the system controller  30  resets the amplification factor of the image intensifier  26  so that a suitable blood-vessel image Ev′ can be captured on the linear CCD  27 . Subsequently, in step S 13 , the system controller  30  turns on the measurement light source  21  so that the photomultipliers  28  and  29  receive measuring light and generate received-light signals that are input into the system controller  30  as measurement data. This state is equivalent to the tracking state 2.  
         [0070]    The system controller  30  then determines in step S 14  whether the input of the data from the photomultipliers  28  and  29  and the linear CCD  27  is completed. If not, the system controller  30  awaits completion of the input of this data. If the result of step S 14  is yes, the process proceeds to step S 15  in which the system controller  30  instructs the turning off of the measurement light source  20 . Then, in step S 16 , the system controller  30  instructs the filter controller  35  to insert the light-shielding portion without an ND filter into the optical path of the tracking light source  21 . In response to this instruction, the filter controller  35  drives the motor  23  to insert the light-shielding portion into the optical path of the tracking light source  21 . Accordingly, the tracking light is no longer applied to the patient&#39;s eye E. Then, in step S 17 , the tracking operation is completed. In step S 18 , the system controller  30  calculates the blood-vessel diameter and the blood velocity, and determines the blood-flow amount based on the blood-vessel diameter and the blood velocity. Then, the measurement operation is completed.  
         [0071]    In the above-described embodiment and modification, the input into the system controller  30  of the data from the photomultipliers  28  and  29  and the input of the data from the linear CCD  27  are not performed simultaneously, and the transmittance of the ND filter is changed for each input operation. However, both data can be input into the system controller  30  simultaneously.  
         [0072]    In the aforementioned embodiment and modification, the intensity of the laser light applied to the eye fundus Ea is controlled by the ND filter. However, the intensity of the laser light itself output from the laser light source may be electrically controlled.  
         [0073]    While the present invention has been described with reference to what are presently considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.