Abstract:
A stimulus light from a stimulus light source is projected to the retina of an ocular fundus under examination to stimulate the retina in localized fashion to generate a bioelectric signal from the retina. The stimulus light source is mounted on a movable block, and an operating lever is used to move the stimulus light source to change the stimulus light on the retina. The size of the stimulus light can also be varied using a panel having a plurality of openings with different diameters for allowing the stimulus light to pass through. This assures a reliable local ERG examination because the bioelectrical signal can be produced in a wider area of the retina. The operating lever is further fixed by inserting a cap to keep the projected stimulus light unchanged. This allows the burden on the examiner to be reduced remarkably, assuring a reliable local ERG examination.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an ocular light stimulus apparatus, and more particularly relates to an ocular light stimulus apparatus for providing localized light stimulus to a retina of an ocular fundus under examination by stimulus light from a stimulus light source to perform biological examination through use of a bioelectrical signal from the retina. 
         [0003]    2. Description of the Prior Art 
         [0004]    Conventional known methods used primarily for ophthalmic examination include not only capturing a fundus image and performing fundus examination, but also electroretinogram (ERG) examination in which stimulus light is radiated to a retina, an action potential generated in the retina is measured, and an electroretinogram is created to perform an ophthalmic biological examination. 
         [0005]    In an ERG examination, a background light is needed for illuminating the background onto which the stimulus light is radiated, and the quality of the ERG examination is determined by the proper combination of background light intensity and stimulus light intensity. Japanese Journal of Ophthalmology, Vol. 92, No. 9 (Sep. 10, 1988, 5-(1423) to 11-(1429)) describes the effects obtained from an ERG examination (local ERG) when stimulus light is radiated in localized fashion to a point on the macula of the fundus using visible background light. 
         [0006]    Japanese Examined Patent Application Publication No. 1987-16090 discloses a light stimulus apparatus in which stimulus light is radiated to an ocular fundus under examination using a fundus camera, and electrical information obtained through the light stimulus is displayed as an electroretinogram. Japanese Examined Patent Application Publication No. 1987-20809 proposes a technique for observing which portions are stimulated in what manner by local ERG stimulus light while observing a wide range of the fundus by infrared light. 
         [0007]    Furthermore, Japanese Examined Patent Application Publication No. 1992-19852 discloses that an ocular fundus under examination is observed without dilation, and a beam of light for observing and imaging the eye under examination is used as stimulus light. Japanese Laid-open Patent Application No. 2005-323815 discloses that a light stimulus apparatus for radiating stimulus light to a fundus is attached as a unit to a fundus camera to perform ERG examination. Japanese Laid-open Patent Application No. 2006-42952 discloses that, to perform local ERG examination, the entire retinal area is irradiated by white light using a white-light-emitting diode with this illumination used as a background, and a stimulus light spot is radiated from a high-luminance light-emitting diode while observing the fundus by infrared light. 
         [0008]    In an ophthalmic examination apparatus, a light source (fixation lamp) used for ophthalmic examination is moved on a plane perpendicular to the optical axis (Japanese Laid-open Patent Application No. 1987-144635 and Japanese Patent No. 2736780). 
         [0009]    In the conventional techniques, the position of the stimulus light source or the size of the stimulus light is not varied when the local ERG examination is performed, and as a result, drawbacks occur in that a bioelectric signal is not obtained from an arbitrary position of the retina, and a reliable local ERG examination is difficult to perform. 
         [0010]    When the stimulus light projection position is varied in the local ERG examination, drawbacks also occur in that the projection position is not easily maintained at a prescribed position, and a reliable bioelectric signal is difficult to obtain from the retina. 
         [0011]    It is therefore an object of the invention is to provide an ocular light stimulus apparatus capable of performing a reliable local ERG examination with a simple structure. 
       SUMMARY OF THE INVENTION 
       [0012]    An ocular light stimulus apparatus according to the invention is adapted for use in providing localized light stimulus to a retina of an ocular fundus under examination to perform biological examination through use of a bioelectrical signal from the retina. The ocular light stimulus apparatus comprises a stimulus light source for emitting stimulus light to provide the localized light stimulus to the fundus under examination; a movement mechanism for moving the stimulus light source to change a projection position of the stimulus light on the fundus; and a stimulus light size changing mechanism for changing a size of the stimulus light from the stimulus light source. 
         [0013]    In the invention, the position of the stimulus light source and the size of the stimulus light can be varied, so that a bioelectrical signal can be obtained from the various areas of the retina of the eye under examination, thus assuring a reliable local biological examination. 
         [0014]    Furthermore, the ocular light stimulus apparatus comprises a stimulus light source for emitting stimulus light to provide the localized light stimulus to the fundus under examination; a movement mechanism for moving the stimulus light source to change a projection position of the stimulus light on the fundus; an operating member operated to act on the movement mechanism to change the projection position of the stimulus light; and a fixing member for fixing the operating member. 
         [0015]    In the invention, it is possible to fix the operating member that is operated so as to move the stimulus light source, so that the movement of the projected stimulus light can be locked. This allows the burden on the examiner to be alleviated and a reliable local ERG examination to be performed. 
         [0016]    Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and following detailed description of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a schematic structural view showing the overall structure of an ophthalmic light stimulus apparatus of the present invention; 
           [0018]      FIG. 2  is a sectional view showing the detailed structure of a stimulus light source unit; 
           [0019]      FIG. 3  is a front view showing an indicator disk shown in  FIG. 2 ; 
           [0020]      FIG. 4   a  is a longitudinal sectional view showing the stimulus light source unit when an operating lever is in the center position; 
           [0021]      FIG. 4   b  is a longitudinal sectional view showing the stimulus light source unit when the operating lever is moved to the lower position; 
           [0022]      FIG. 5  is a sectional view along line A-A′ in  FIG. 4   a;    
           [0023]      FIG. 6  is an illustrative view showing the movement of a movable block of the stimulus light source unit; 
           [0024]      FIG. 7  is a front view showing an indicator panel; 
           [0025]      FIG. 8  is a screen view of a monitor for displaying the alignment state; 
           [0026]      FIG. 9  is a perspective view showing a cap inserted into the stimulus light source unit to fix the operating lever; 
           [0027]      FIG. 10  is a longitudinal sectional view corresponding to  FIG. 4   a  when the cap is inserted; and 
           [0028]      FIG. 11  is an illustrative view showing the structure for turning on the fixation lamp on the optical axis when the cap is inserted. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0029]    The present invention will now be described in detail with reference to the embodiments shown in the attached drawings. 
         [0030]      FIG. 1  shows an ocular light stimulus apparatus that comprises a light stimulus unit  10  and a stimulus light source unit  50 . In  FIG. 1 , the reference symbol R indicates the position conjugate with a fundus  1   a  of an eye  1  under examination, and the reference symbol P indicates the position conjugate with the anterior ocular segment  1   b  (particularly the pupil) thereof. 
         [0031]    The light stimulus unit  10  is provided with an illumination optical system for illuminating the fundus of the eye under examination, and an imaging optical system for imaging the illuminated fundus. In the illumination optical system, light emitted from a halogen lamp or other illumination light source  11 , and light reflected by a concave mirror  12  are converted to infrared light via a visible-cutting infrared-transmitting filter  13  and directed into a diffusing plate  15  that diffuses the transmitted infrared light. A ring slit  16  is disposed in the conjugate position P with the anterior ocular segment (pupil)  1   b  of the eye  1  under examination. The illumination light from the ring slit  16  passes through a lens  17  and a black spot plate  18  for removing reflection of an objective lens  22 , as well as through a half mirror  19  and a relay lens  20 . After the illumination light is reflected by a total reflection mirror  21  having an aperture in the center thereof, the light is directed through the objective lens  22  into the fundus  1   a  from the anterior ocular segment  1   b  of the eye  1  to illuminate the fundus  1   a  thereof by infrared light. 
         [0032]    The reflected light from the fundus  1   a  is received via the objective lens  22 , and passes through the aperture in the total reflection mirror  21 , a photographic stop  31  disposed in the position P conjugate with the anterior ocular segment, a focus lens  32 , and an imaging lens  33 . The light is then reflected by a half mirror  34  and directed into a half mirror  36  via a field stop  35  disposed in the position R conjugate with the fundus  1   a.  The infrared light that has passed through the half mirror  36  is reflected by a mirror  38 , passes through an imaging lens  37 , and impinges on an imaging device  40  disposed in the fundus conjugate position R. The imaging device  40  is composed of an infrared CCD or the like sensitive in the infrared and visible light regions, and produces a signal that is inputted to a monitor  41 . 
         [0033]    A stimulus light source unit  50  is attached to the light stimulus unit  10 . The unit  50  includes a stimulus light source  51  composed of a light-emitting diode or the like for emitting visible light. The stimulus light source  51  is fixed inside a cavity  52   a  formed in a movable block  52 , as shown in detail in  FIG. 2 . Protrusions  52   b,    52   c  are formed on the movable block  52 , and the protrusions  52   b,    52   c  are fitted into a circumferential groove  53   a  formed in a housing  53  of the light source unit  50 . The movable block  52  is supported by a spherical body  54  that is attached to one end of an operating lever  57 . The lever  57  is provided with a spherical body  55  that is supported by a bearing  56  fixed to the housing  53 . The lever  57  can be operated to move the movable block  52  in a plane x-y perpendicular to the optical axis via the spherical bodies  54  and  55 . 
         [0034]    An indicator disk  60  is attached to the movable block  52  via a shaft  61  mounted on the movable block  52 . A plurality of openings  60   a,    60   b,    60   c  having different diameters is formed in the indicator disk  60  as shown in  FIG. 3 . The indicator disk  60  can be rotated about the shaft  61  manually through the use of a lever  62  fixed to the indicator disk  60 , or through the use of a motor  63  fixed to the movable block  52  to bring any of the openings into a position in which it faces the stimulus light source  51  and a diffusing plate  58 . The operation of the lever  62  allows the spot diameter of the stimulus light to be varied and the operation of the lever  57  allows the stimulus light to be positioned in the x-y plane. 
         [0035]    The stimulus light source unit  50  is attached to the light stimulus unit  10  via a cylindrical housing  59  fixed to the housing  53  so that the indicator disk  60  is disposed in the position R conjugate with the fundus, as shown in  FIG. 1 . 
         [0036]    Operating a switch  46   a  provided to a joystick  46  causes the stimulus light source  51  to be turned on by a control unit  84  to emit stimulus light, which impinges on the diffusing plate  58  via a lens  70 . The stimulus light diffused by the diffusing plate  58  is set to a prescribed spot size by the openings  60   a  through  60   c  of the indicator disk  60 . The stimulus light then passes through a variable-power lens  47   a  ( 47   b ) and impinges on the half mirror  36 . The visible-light reflected by the half mirror  36  is projected as stimulus light to the fundus  1   a  from the pupil  1   b  of the eye under examination via the half mirror  34 , the lenses  33 ,  32 , the aperture of total reflection mirror  21 , and the objective lens  22 . 
         [0037]    An ERG electrode  86  is attached to the eye  1  under examination. A signal from the electrode  86  is inputted to a control computer (personal computer)  80  provided with a display device  81  and a storage device  82 . The electroretinogram created in the computer  80  is displayed by the display device  81  or stored by the storage device  82 . 
         [0038]    In order for a projection index (stimulus light) from the stimulus light source  51  to be observed and displayed on the monitor  41 , the visible light from the stimulus light transmitted by the half mirror  36  is reflected by the mirror  38  via a mirror  91 , a lens  94 ′, a prism  92 , and an infrared-transmitting visible-reflecting mirror  93 , and directed into the imaging device  40 . 
         [0039]    To prevent the visible light from the stimulus light source  51  that is divided and reflected by the half mirror  36  from being reflected by the surface of the imaging lens  33  and returned as reflected light into the imaging device  40 , a filter  90  for reflecting visible light and transmitting infrared light is inserted between the half mirror  36  and the infrared-transmitting visible-reflecting mirror  93 . Since the filter  90  has infrared-transmitting characteristics, the observation light is directed into the imaging device  40  without being cut by the filter  90 . 
         [0040]    A light source  94  for working distance (hereinafter referred to as the WD light source) is provided for alignment. The WD light source  94  is composed of a light-emitting diode for emitting both visible light (white light) and infrared light, for example. The beam of light from the WD light source  94  is directed via an optical fiber  95  to the vicinity of the center of the aperture of the total reflection mirror  21  to form a working distance indicator. The working distance indicator is projected by the objective lens  22  to the cornea of the eye  1  under examination. When the working distance is correct, the beam of light frontally reflected by the cornea of the eye  1  is substantially afocal. 
         [0041]    The WD light source  94  is also used as a light source for the background light used for the local ERG examination. The light beam from the WD light source  94  is projected as background light to the fundus  1   a  via the anterior ocular segment  1   b  to illuminate the fundus  1   a  by visible light. The light intensity of the WD light source  94  at this time is set to a different intensity than the alignment light intensity for adjusting the working distance. 
         [0042]    The illumination optical system is provided with a focusing light source  30  composed of an infrared LED for producing a focusing beam. The infrared light from the light source  30  is incident on the fundus  1   a  as a focus dot via the half mirror  19 . The position of the focus dot on the fundus is changed according to the movement of the focus lens  32 , and the examiner can bring the fundus into focus by observing the focus dot. In the initial stage of alignment, an anterior ocular segment lens  42  is inserted, and the examiner can therefore confirm image of the anterior ocular segment  1   b  of the eye  1  under examination on the monitor  41 . 
         [0043]    During the alignment or focusing operation, an internal fixation lamp  43  is turned on, and the examiner can make certain of the alignment or focusing operation by calling the attention of the examinee to the fixation lamp. 
         [0044]    The computer (control unit)  80  can set various measurement conditions in order to perform the local ERG examination. The measurement conditions include the intensity (amount of light) of the background light from the WD light source  94 , the intensity (amount of light) of the stimulus light from the stimulus light source  51 , the wavelength components of the background light and the stimulus light, the spot diameter (positions of the openings  60   a  through  60   c  of the indicator disk  60 ) of the stimulus light, the radiation time (lamp-on time) of the stimulus light, the number of radiations of the stimulus light, the blink period (when blinking is performed even in the alignment state) of the stimulus light, the position of the fixation lamp  43 , and so on. 
         [0045]    A relay section  83  is provided between the light stimulus apparatus and the computer  80  in order to synchronize retinal stimulus with the measurement condition set by the computer  80 . The relay section  83  may be provided within the light stimulus apparatus. 
         [0046]    The procedure by which the local ERG examination is performed in such a configuration will be described hereinafter. 
         [0047]    A measurement condition such as described above is set prior to the examination. When the illumination light source  11  is turned on, the fundus  1   a  is illuminated by infrared light. The anterior ocular segment lens  42  is inserted, and an image of the anterior ocular segment is observed to perform alignment by the anterior ocular segment as needed. The anterior ocular segment lens  42  is then retracted from the optical path, and the WD light source  94  is turned on to perform alignment for adjustment of the relative position between the eye under examination and the light stimulus apparatus. 
         [0048]    The light from the WD light source  94  is radiated via the optical fiber  95  to the cornea of the eye  1  as alignment light for adjusting the working distance. Radiation of visible light of white light components is basically unnecessary during alignment, but when visible background light is suddenly radiated at the start of measurement, a stimulus that can be considered strong is imparted to the examinee. This may cause the initial data to be unstable. For this reason, visible light is preferably radiated prior to measurement at an intensity that is equivalent to that at the time of measurement, or visible light of the weakest intensity is radiated that is necessary for the stable initial data to be obtained at the time of measurement. Since the WD light source  94  thus radiates visible light and infrared light even during alignment, the infrared light component of the reflected light from the cornea passes through the half mirror  36 , and is received by the imaging device  40 . This allows the examiner to observe the corneal reflected light through the use of the monitor  41 . When the working distance is correct, the beam of light frontally reflected by the cornea is substantially afocal. Therefore, alignment (adjustment of the working distance) of the optical axis direction (Z direction) is performed so that the corneal reflection image can be observed as a spot. On the other hand, alignment of the directions (X, Y directions) perpendicular to the optical axis is performed using the joystick  46  so that the corneal reflection image is substantially at the center of the fundus image. 
         [0049]    The infrared focus dot from the focusing light source  30  enters the fundus  1   a  via the half mirror  19 , and the focus dot position changes according to the movement of the focus lens  32 . Therefore, the examiner can bring the eye under examination into focus by observing the focus dot on the monitor  41 . During the alignment or focusing operation, the internal fixation lamp  43  is turned on, and the examiner can make certain of the alignment or focusing operation by calling the attention of the examinee to the fixation lamp. 
         [0050]    When the alignment and focusing operations are completed, the switch  46   a  is operated, and the local ERG examination is initiated. At this time, the background light and stimulus light from the WD light source  94  and the stimulus light source  51  are set to the desired brightness and/or wavelength component in accordance with the measurement conditions transmitted via the relay section  83 . 
         [0051]    The set measurement conditions and the like are displayed by the display device  81 , but cannot be displayed by pressing a specific button, if the displaying may disturb the alignment. 
         [0052]    At the time of the local ERG examination, infrared light is unnecessary, but it is possible for misalignment to occur due to poor fixation of the eye under examination during examination, and infrared light is therefore radiated during examination as well so that the working distance can be confirmed. 
         [0053]    The stimulus light obtained from the stimulus light source  51  is turned on for a prescribed time set as a measurement condition. The stimulus light is thus radiated as visible light via the half mirror  36  to the fundus  1   a  that is irradiated with background light. The radiation position of the stimulus light can be changed by operating the lever  57 , and the spot size of the stimulus light can be changed by selecting any of the openings  60   a  through  60   c  by rotating the indicator disk  60  through the use of the motor  63  or by operating the lever  62 . In addition, the variation range can be widened, and the balance between the spot size and the intensity (amount of light) of the stimulus light can be varied through joint use of variable-power lenses  47   a,    47   b.    
         [0054]    When the stimulus light is radiated to the fundus, an action potential is generated in the retina. The ERG electrode  86  detects the potential generated, which is inputted to the computer  80  to create an electroretinogram. 
         [0055]    The stimulus light is also radiated in advance at the time of alignment so that the examiner can distinguish the location being stimulated when alignment is completed. At this time, the stimulus light is blinked at a prescribed blinking period that is set as a measurement condition, so as not to be mistaken for the focus dot, the working dot, or the like. When the stimulus light is difficult to recognize at the prescribed blinking period set as the measurement condition, a blinking period for alignment is set that is easier to recognize. The beam of the stimulus light transmitted by the half mirror  36  is reflected by the mirror  38  via the mirror  91 , the prism  92 , the lens  94 ′, and the infrared-transmitting visible-reflecting mirror  93 , and directed into the imaging device  40 . The examiner can therefore recognize the portion of the eye under examination to which the stimulus light is being radiated. 
         [0056]    The intensity (brightness) of the stimulus light also depends on the intensity of the background light, so that the intensity of the stimulus light is set to an intensity whereby an action potential is generated in the retina. The stimulus light is radiated during alignment as well. Since the stimulus light is used at the time of alignment merely to determine the position to which the stimulus light is being radiated, the stimulus light is set to an intensity weaker than the intensity thereof during the local ERG examination. 
         [0057]    When the stimulus light is radiated the number of times set as the measurement conditions, the local ERG examination is completed. The electroretinogram created by the computer  80  is displayed by the display device  81  and stored by the storage device  82 . 
         [0058]    In the alignment state during the local ERG examination, the stimulus light is blinked at the prescribed blinking period as described above, and the position of the stimulus light during measurement can be confirmed in advance without mistaking the stimulus light for the working dot or the like. The examiner can therefore modify the appropriate alignment while observing the fundus of the examinee. When the examinee is significantly misaligned, the local ERG examination can be stopped by pressing and holding the switch  46   a  once more. 
         [0059]    The stimulus light source  51  is used to generate the stimulus light in the embodiment described above, but a liquid crystal display device (LCD) may also be disposed in the fundus conjugate position R instead of the stimulus light source  51  to perform the local ERG examination. Since the liquid crystal display device is connected to the computer  80  via the control unit  84  and the relay section  83 , an indicator can be displayed by a display device (not shown). The displayed indicator is projected as stimulus light to the fundus of the eye under examination. 
         [0060]      FIGS. 4   a  and  4   b  show another embodiment of the stimulus light source unit. 
         [0061]    A stimulus light source unit  100  shown in  FIG. 4  has a housing  104  fixed to the light stimulus unit  10 . A light-emitting diode or other stimulus light source  101  for emitting the same visible light as the stimulus light source  51  is fixed to the side of a movable block  102  facing the unit  10  via an attachment panel  103  in the stimulus light source unit  100 . The side of the movable block  102  opposite the unit  10  forms a disk having an external peripheral part  102   a,  an intermediate part  102   b  having a smaller width than the external peripheral part  102   a,  and a center part  102   c  that has a circular concave part  102   d  (see also  FIG. 5 ). The external peripheral part  102   a  of the movable block  102  is engaged in a circumferential groove  104   a  formed within the housing  104  of the stimulus light source unit  100 , and the concave part  102   d  of the center part  102   c  serves as a bearing support for supporting a spherical body  105  inserted therein so as to allow the spherical body  105  to rotate. 
         [0062]    A bearing  107  for providing support so that a spherical body  106  can rotate is fixed to the housing  104  via a middle ring  108 . The spherical bodies  105 ,  106  are pinned to the distal end part of a rod  110  of an operating lever  111 , and the movable block  102  can move in the x-y plane perpendicular to the optical axis  130  of the stimulus light projection optical system through the use of the operating lever  111  that functions as an operating member via the spherical bodies  105 ,  106 . Slits  105   a,    106   a  are formed in the spherical bodies  105 ,  106 , respectively. A stop pin  112  fixed to the center part  102   c  of the movable block  102 , and a stop pin  113  fixed to the bearing  107  are engaged in the slits  105   a,    106   a,  respectively. This allows the movable block  102  to be prevented from rotating about the optical axis  130 . 
         [0063]    An electric motor  120  composed of a stepping motor or the like is attached to the side of the movable block  102  opposite the operating lever  111  so that the motor shaft  120   a  is parallel to the optical axis  130 . As shown in  FIG. 7 , an indicator panel  121  in which a plurality of openings  121   a,    121   b,    121   c  having different diameters is formed is attached to the motor shaft  120   a  in the x-y plane perpendicular to the optical axis  130 . The indicator panel  121  is disposed in the same fundus conjugate position as the indicator disk  60  shown in  FIG. 3 , and rotates about the motor shaft  120   a.  The centers of the openings  121   a,    121   b,    121   c  of the indicator panel  121  are at equal distances from the motor shaft  120   a.  The spot size of the stimulus light can be varied by rotating the indicator panel  121  in the x-y plane perpendicular to the optical axis  130  and moving any of the openings to the position facing the stimulus light source  101 . A diffusing plate  122  for diffusing the stimulus light is attached to a portion of the movable block  102  between the indicator panel  121  and the stimulus light source  101 . Since the indicator panel  121  is attached to the movable block  102  via the electric motor  120 , stimulus lights having different spot sizes can be placed in a prescribed position in the x-y plane perpendicular to the optical axis  130  by operating the operating lever  111 . 
         [0064]    In the stimulus light source unit  100  configured as described above, the movable block  102  and the stimulus light source  101  and indicator panel  121  fixed thereto move in the x and y directions on the x-y plane perpendicular to the optical axis  130  about the spherical body  106  when the examiner manually moves the operating lever  111  in the x-y direction. 
         [0065]    For example, when the operating lever  111  is moved from the state shown in  FIG. 4   a  to the state shown in  FIG. 4   b  to the lower limit in the y direction, the movable block  102  moves upward accordingly, and the position of the stimulus light source  101  moves as shown in  FIG. 6  from the position P 1  of the state shown in  FIG. 4   a  to the position P 2  of the state shown in  FIG. 4   b.  By moving the operating lever  111  in the x-y direction, the movable block  102  can be arbitrarily moved within the range defined by the circumferential groove  104   a  of the housing  104 . For example, the stimulus light source  101  can be placed in the position P 3 , as indicated by the dashed line in  FIG. 6 . 
         [0066]    Any of the openings  121   a,    121   b,    121   c  of the indicator panel  121  can be moved to the position facing the stimulus light source  101  by rotating the indicator panel  121  about the motor shaft  120   a.  The size of the stimulus light can thus be changed by passing the stimulus light through the selected opening. 
         [0067]    Such an arrangement allows the position of the stimulus light source  101  in the x-y plane perpendicular to the optical axis  130  as well as the spot size of the stimulus light to be varied in a wide range, and a satisfactory action potential and a reliable bioelectric signal can be generated from a wide range of the retina in the same manner as in the stimulus light source  50  shown in  FIG. 2 . 
         [0068]    Since the size of the stimulus light can be known by the rotation position of the electric motor  120 , the information of the rotation angle of the motor can be transmitted to the control unit  84 , and the size of the stimulus light can be displayed on the monitor  41  as an angle of view on the retina, as indicated by the reference symbol  41   d  in  FIG. 8 . In  FIG. 8 , the reference symbol  41   a  indicates an alignment image,  41   b  indicates operation mode information,  41   c  indicates fixation lamp information,  41   e  indicates left/right eye information, and  41   f  indicates setting information for the background light, the stimulus light, and the stimulus light time. 
         [0069]    In the stimulus light source unit  100  shown in  FIG. 4 , the indicator panel  121  is rotated merely by the electric motor  120 , and a manual lever  62  for manual rotation is not provided, as in the stimulus light source unit  50  shown in  FIG. 2 . Therefore, it is possible to overcome the drawback in which the lever is erroneously operated during measurement, and the stimulus light size is changed. 
         [0070]    Since the size information of the stimulus light is obtained from the motor rotation information, the size information can be displayed on the monitor, and the size of stimulus light being projected on the fundus during measurement can easily be known. 
         [0071]    The movable block  102  must be prevented from rotating about the optical axis  130  because the electric motor  120  is mounted on the movable block  102  and the power cord to the electric motor should be prevented from being drawn into the stimulus light source unit  100 . For this reason, the stop pins  112 ,  113  are provided as described above. In particular, the stop pin  113  is fixed to the bearing  107  that is attached so as to be unable to rotate with respect to the axis perpendicular to the x-y plane in the stimulus light source unit  100 . Therefore, the movable block  102  and the stimulus light source  101  and indicator panel  121  attached to the movable block  102  can be prevented from rotating about the optical axis  130  or the axis parallel to the optical axis  130 . 
         [0072]    In the embodiment described above, the operating lever  111  is operated in order to change the projection position of the stimulus light onto the fundus. However, when the projection position of the stimulus light is determined and ERG measurement is performed, the examiner must maintain the operating lever in a constant position during measurement and make the projection position invariable. This creates the problem of increased burden on the examiner. 
         [0073]    Therefore, as shown in  FIGS. 9 and 10 , the operating lever  111  is fixed to the main body of the stimulus light source unit  100  by a cap  140  that functions as a lock member or a fixing member. The distal end of the cap  140  forms a screw part  140   a  and a through-hole  140   b  is formed in the center thereof to lock the operating lever  111 . 
         [0074]    Since the inside diameter of the through-hole  140   b  of the cap  140  is slightly larger than the outside diameter of the operating lever  111 , the operating lever  111  is inserted into the through-hole  140   b  of the cap  140 , and the distal end of the cap  140  is fitted into a concave part  107   b  formed in the bearing part of the stimulus light source unit  100 . This allows the movement of the operating lever  111  within the stimulus light source unit  100  to be locked to fix the projection position of the stimulus light. The cap  140  can be securely attached to the stimulus light source unit  100  by screwing together the screw part  140   a  of the cap  140  and the screw part  107   a  formed in the concave part of the bearing  107 . Alternatively, the operating lever  111  can also be fixed by providing a concave part or a convex part to the cap  140 , providing a convex part or concave part for fitting therewith to the operating lever  111 , and fitting together the convex and concave parts of the cap  140  and operating lever  111 . 
         [0075]    As shown in  FIG. 4  or  FIG. 10 , the operating lever  111  is fixed when the rod  110  of the operating lever  111  is horizontal, and the stimulus light source  101  is positioned on the optical axis  130  of the stimulus light projection optical system. In this position, the stimulus light is referably designed so as to be projected onto the macula of the fundus to stimulate the macula. In order to achieve this, a microswitch  141  for detecting insertion of the cap  140  is attached inside the bearing  107 . When the cap  140  is inserted in the stimulus light source unit  100 , the microswitch  141  is activated to produce the ON signal that is inputted to the control unit  84 , as shown in  FIG. 11 . The control unit  84  then turns on one fixation lamp  43   a  among the plurality of internal fixation lamps  43 . Specifically, the internal fixation lamps  43  are disposed on a plane perpendicular to the optical axis  130 ′ of an optical system that is branched from the stimulus light projection optical system by the mirror  34 , as shown in  FIGS. 1 and 11 . 
         [0076]    When the cap  140  is inserted, the control unit  84  turns on a fixation lamp  43   a  positioned on the optical axis  130 ′. The projection optical system is configured so that the stimulus light is projected on the macula of the fundus when the examinee looks fixedly at the fixation lamp that is turned on. 
         [0077]    Since the stimulus light source  101  and the selected internal fixation lamp  43   a  are each on the optical axis  130  when the macula is stimulated, the ERG examination can be performed without problems in the same position of the operating lever  111  regardless of whether the eye under examination changes to the left or right eye. 
         [0078]    During macular stimulation, which is the most frequently performed operation in the ERG examination, the examiner can let go off the operating lever and perform the measurement because the operating lever can be fixed and immobilized. This allows the burden on the examiner to be reduced remarkably. 
         [0079]    When the cap  140  is fitted to fix the operating lever  111 , the operating lever  111  can be accommodated in the through-hole of the cap  140 . This makes it possible to protect the operating lever during transporting of the apparatus.