Abstract:
An ophthalmic photographic apparatus determines whether an eye under examination is the left eye or the right eye. In accordance with the determination, a corresponding fixation target is presented to a main unit, and a reference mark corresponding to the eye under examination, which is stored in a memory, is displayed on a monitor. An alignment mark projected onto and reflected from the anterior eye portion and the reference mark are displayed on the monitor, and hence alignment appropriate for the left and right eyes can be performed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to ophthalmic photographic apparatuses for use in ophthalmic hospitals. 
     2. Description of the Related Art 
     In order to align a subject&#39;s eye (eye under examination) and a photographic apparatus, a fundus camera projects marks so that an image reflected from the cornea can be formed substantially conjugate to the fundus of the eye. The image reflected from the cornea and an image of the fundus are displayed on a monitor and are observed. The fundus camera is positioned so that the image reflected from the cornea can be in focus and be displayed symmetrically relative to the center of the monitor. 
     In the foregoing known example, the following problems occur: 
     (1) In general, fundus photography in group examination is performed by aligning the approximate central point between the optic disk (discus nervi optici) and the macula lutea to be at the center of the photographic screen. Since the field angles of the optic disk and the macula lutea are separated from each other by approximately 18 degrees, this photographic method captures images with a tilt of approximately 9 degrees. 
     The eyeball optical system has rotation symmetry with respect to the optical axis between the approximate center of the pupil and the macula lutea. Capturing an image eccentrically means that the image is captured while decentering the eyeball. As a result, one side of the image tends to become flared. When a visual field diaphragm is reduced to avoid flare, the photographic field angle becomes narrow. 
     (2) In the known method, images are captured from an eccentric position relative to the center of the pupil. When capturing an image of a subject&#39;s eye having the pupil with a small diameter, the fundus of the eye cannot be illuminated uniformly. 
     SUMMARY OF THE INVENTION 
     In order to solve the foregoing problems, it is an object of the present invention to provide an ophthalmic photographic apparatus for appropriately displaying fixation targets corresponding to a subject&#39;s left and right eyes under examination. 
     In order to achieve the foregoing objects, according to the present invention, an ophthalmic photographic apparatus is provided including a mark projecting unit for projecting a mark onto a cornea of an eye under examination; an observation unit for observing a reflected image of the mark and an image of a fundus of the eye; a left-right detecting unit for detecting whether the eye under examination is the left eye or the right eye; and a control unit for displaying, on the observation unit, the reference mark for aligning the reflected image of the mark at different positions corresponding to the left and right eyes in accordance with information detected by the left-right detecting unit. 
     As described above, an ophthalmic photographic apparatus according to the present invention includes a determination unit for determining whether a subject&#39;s eye (eye under examination) is the left eye or the right eye, and reference marks for alignment are presented at eccentric positions in accordance with the determination. Thus, the following advantages can be achieved: 
     (a) Since flare-free, satisfactory images can be obtained, the image capturing and reading efficiency can be improved; 
     (b) With tolerance to flare, images can be simultaneously taken at wide field angles, and the diagnosis efficiency can be further improved; and 
     (c) Since images can be taken from the center of the pupil, satisfactory images of the fundus of a subject&#39;s eye having a small-diameter pupil can be captured. 
     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 
     FIG. 1 is an external perspective view of a fundus camera according to an embodiment of the present invention. 
     FIG. 2 is a schematic diagram of the optical system and the electrical system. 
     FIG. 3 is schematically illustrates a position detector. 
     FIG. 4 schematically illustrates the arrangement of an apertured mirror, fibers, and light sources. 
     FIGS. 5A and 5B are illustrations of images of the fundus and fixation targets displayed on a television monitor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will become clear from the following description of the preferred embodiment with reference to the accompanying drawings. FIG. 1 is an external view of a fundus camera. On a base  1  containing a circuit board and electrical components, a stage  2  is mounted in a horizontally movable manner. On the stage  2 , a main unit  3  is mounted in a vertically movable manner. On the base  1 , a chinrest  4  for holding the chin of a subject S and a headrest  5  are provided. An examiner who wants to take a photograph operates a joystick  6 , which is provided on the stage  2 , to adjust the positional relationship between the main unit  3  and the subject&#39;s eye E. 
     FIG. 2 is a diagram of the optical system and the electrical system in the fundus camera  3 . In front of the subject&#39;s eye E, an objective lens  11 , an apertured mirror  12 , a photographic diaphragm  13  disposed in the aperture of the apertured mirror  12 , a focus lens  14  which is movable along the optical axis, a photographic lens  15 , and an optical-path branching prism  16  are sequentially disposed. In the reflecting direction of the optical-path branching prism  16 , a lens  17 , a mirror  18 , a lens  19 , a color separation prism  20 , and image pickup elements  21   r ,  21   b , and  21   g  are provided. The color separation prism  20  directs infrared light and red light toward the image pickup element  21   r , blue light toward the image pickup element  21   b , and green light toward the image pickup element  21   g , thus forming a fundus photographing unit. 
     In the transmission direction of the optical-path branching prism  16 , a liquid crystal display (LCD)  22  functioning as an internal fixation target and a backlight unit  23  are disposed to form an internal fixation target presenting unit. 
     In the incident direction of the apertured mirror  12 , a relay lens  24 , a diaphragm  25  having a ring aperture, a stroboscopic light source  26  for emitting flashes, a wavelength selection filter  27  for blocking visible light and transmitting infrared light, which can be freely inserted into and removed from the optical path, a condenser lens  28 , and an observation light source  29 , such as a halogen lamp, for emitting fixed light including visible light and infrared light, are disposed. These components, together with the apertured mirror  12  and the objective lens  11 , form a fundus illuminator. 
     On both sides of the photographic diaphragm  13 , end faces of fibers  30   a  and  30   b  are disposed. Light sources  31   a  and  31   b  are disposed on an end face opposite to the photographic diaphragm  13 . The end face at which the photographic diaphragm  13  is provided functions as a mark for adjusting the working distance. The mark is disposed at a position at which an image reflected from the cornea can be substantially conjugate to the fundus Er when the distance between the objective lens  11  and the cornea of the subject&#39;s eye E is appropriate. 
     The outputs of the image pickup elements  21   r ,  21   b , and  21   g  are connected to an image control circuit  42  via a signal amplifier circuit  41  for amplifying electrical signals. A television monitor  43 , an image recorder  44 , and a controller  45  with a memory  45   a  are connected to the image control circuit  42 . The image recorder  44  is a drive for writing images to and reading images from a recording medium D, such as an MO (magneto-optical disk), an MD (mini disk), a DVD-RAM (digital versatile disk-random access memory), a VCR (videocassette recorder) tape, or a hard disk, which can store and maintain information without an external power supply. The LCD  22 , the observation light source  29 , a photographic switch  36 , an operation unit  47  for specifying the position at which the fixation target is presented, a stroboscopic flash controller  48  for controlling the stroboscopic light source  26 , a light control unit  49  for controlling the light intensity of the observation light, the light control unit  49  including switches  49   a  and  49   b  and a display unit  49   c , and a detector switch  50  for detecting the position of the main unit  3 , are connected to the controller  45 . 
     In the LCD  22 , cells which can control the transmission and blocking of light are arranged in the form of matrix. The LCD  22  functions as a fixation target when the backlight unit  23  is observed through a light transmitting portion of the LCD  22 . The backlight unit  23  includes a plurality of LEDs (light emitting diodes). The position of the light transmitting portion of the LCD  22 , that is, the fixation target, is controlled by the operation unit  47  for specifying the position at which the fixation target is presented. In other words, the examiner who wants to take a photograph operates four switches  47   a ,  47   b ,  47   c , and  47   d  of the operation unit  47  to lead the subject&#39;s visual axis to an arbitrary position. The light transmitting portion of the LCD  22  is presented at a position where the center of the line between the optical disk and the macula lutea becomes substantially the center of the screen when the subject&#39;s eye E is fixed on the fixation target. 
     It is assumed that the subject&#39;s eye E under examination is the left eye and a photograph is taken using the fundus camera. The examiner who wants to take a photograph asks the subject S to sit in front of the fundus camera and to position his or her chin on the chinrest  4  and his or her forehead against the headrest  5 . The examiner operates the joystick  6  to position the main unit  3  in front of the subject&#39;s eye E. 
     FIG. 3 shows a position detector. The height of an upper portion  1   a  of the base  1  differs in the horizontal direction. Due to the difference in height, the detector switch  50  is turned on and off. Thus, it is possible to detect the position of the main unit  3 , that is, it is possible to detect whether the main unit is at the left or at the light, and hence it can be detected whether the subject&#39;s eye E is the left eye or the right eye. In other words, when the controller  45  detects, for example, that the main unit  3  is at the position of the left eye using the detector switch  50 , the controller  45  presents a fixation target for capturing an image of the left eye. 
     When the examiner observes the subject&#39;s eye E, a light beam emitted from the observation light source  29  is focused by the condenser lens  28 , and the wavelength selection filter  27  disposed in the optical path only transmits infrared light. The transmitted light passes through the stroboscopic light source  26 , the diaphragm  25 , and the relay lens  24 , and the light is reflected to the left by the peripheral mirror portion of the apertured mirror  12 . The reflected light passes through the objective lens  11  and the pupil Ep of the subject&#39;s eye E and illuminates the fundus Er. 
     An image of the fundus Er, which is illuminated by infrared light, again passes through the objective lens  11 , the photographic diaphragm  13 , the focus lens  14 , and the photographic lens  15  and is reflected upward by the optical-path branching prism  16 . The reflected image passes through the lens  17  and is reflected to the left by the mirror  18 , and the reflected image enters the color separation prism  20  through the lens  19 . The image is formed at the image pickup element  21   r  for the red and infrared light and is converted into an electrical signal. The signal passes through the signal amplifier circuit  41  and is amplified by a predetermined factor. The amplified signal is input to the image control circuit  42  and displayed on the television monitor  43 . 
     FIG. 4 illustrates the apertured mirror  12  viewed from the objective lens  11 . Light emitted from the light sources  31   a  and  31   b  enters the fibers  30   a  and  30   b , and the end face at which the photographic diaphragm  13  is provided is illuminated with this light. An image of this end face is formed by the objective lens  11  substantially at the midpoint between the apex of the cornea Ec of the subject&#39;s eye E and the center of the corneal curvature. If the working distance between the subject&#39;s eye E and the objective lens  11  is appropriate, an image of the marks reflected from the cornea, which is formed by the fibers  30   a  and  30   b , is formed substantially conjugate to the fundus Er. In other words, the image reflected from the cornea passes through the objective lens  11 , the photographic diaphragm  13 , the focus lens  14 , and the photographic lens  15  and is reflected upward by the optical-path branching prism  16 . The reflected image is formed in the vicinity of the lens  17 , and the image is reflected to the left by the mirror  18 . Similar to the image of the fundus, this image reflected from the cornea is formed at the image pickup element  21   r  through the lens  19 . 
     FIG. 5A shows an image of the fundus (fundus image) Er′ in which reflected images Ra and Rb, formed by the cornea Ec, of the marks are displayed on the television monitor  43 . The controller  45  displays reference marks LMa and LMb for alignment, which are stored in the memory  45   a  and which are used to take a photograph of the left eye on the television monitor  43 . The center of the reference marks LMa and LMb is decentered to the right with respect to the center of the fundus image Er′ by approximately six degrees. Having observed the fundus image Er′ and the images reflected from the cornea Ra and Rb, the examiner operates the joystick  6  so that the reflected images Ra and Rb are placed inside the reference marks LMa and LMb and the contrast of the reflected images Ra and Rb is maximized. After confirming that the photographic range, positions, and focusing are satisfactory, the examiner operates the photographic switch  36  and captures a still image. 
     The controller  45  detects that the photographic switch  36  is operated and emits light from the stroboscopic light source  26  via the stroboscopic flash controller  48 . The light emitted from the stroboscopic light source  26  passes through the ring aperture of the diaphragm  25  and the relay lens  24  and is reflected to the left by the peripheral mirror portion of the apertured mirror  12 . The reflected light passes through the objective lens  11  and the pupil Ep of the subject&#39;s eye E, and the fundus Er is illuminated with this light. 
     The illuminated fundus image Er′ again passes through the objective lens  11 , the photographic diaphragm  13 , the focus lens  14 , and the photographic lens  15 , and the image Er′ is reflected upward by the optical-path branching prism  16 , which is disposed in the optical path. The reflected image Er′ passes through the lens  17  and is reflected to the left by the mirror  18 . The image Er′ passes through the lens  19  and enters the color separation prism  20 . The image is formed at the image pickup elements  21   r ,  21   b , and  21   gb  and converted into an electrical signal. The signal is amplified by the signal amplifier circuit  41  by a predetermined factor, and the amplified signal is input to the image control circuit  42 . Using the amplified signal and presentation position information for the fixation target, the image recorder  44  records a color fundus image in the recording medium D, and the recorded image is displayed on the television monitor  43 . 
     In order to capture an image when the subject&#39;s eye E under examination is the right eye, the examiner operates the joystick  6  and moves the main unit  3  including the optical system to be in front of the right eye. Accordingly, the detector switch  50  for detecting whether the left eye or the right eye is to be examined is turned off. Having detected that the detector switch  50  has been turned off, the controller  45  presents a fixation target for the right eye and displays reference marks RMa and RMb for right-eye alignment, which are stored in the memory  45   a , on the television monitor  43 . As shown in FIG. 5B, the center of the reference marks RMa and RMb is decentered to the left with respect to the center of the fundus image Er by approximately six degrees. 
     As described above, the fundus Er is illuminated with infrared light emitted from the observation light source  29 . The illuminated fundus image Er′ and the reflected images of the marks Ra and Rb, which are formed by illuminating the cornea Ec by the light sources  31   a  and  31   b , are displayed on the television monitor  43 . As described above with reference to FIG. 5B, the examiner operates the joystick  6  to position the main unit  3  in accordance with the subject&#39;s eye E so that the reflected images Ra and Rb can be symmetrically placed inside the reference marks RMa and RMb and the contrast of the reflected images Ra and Rb can be maximized. 
     After confirming that the photographic range, positions, and focusing are satisfactory, the examiner operates the photographic switch  36  and captures a still image. Having detected that the photographic switch  36  is operated, the controller  45  controls the stroboscopic light source  26  to emit light so that the fundus Er can be illuminated with visible light. The illuminated fundus image Er′ again passes through the objective lens  11 , the photographic diaphragm  13 , the focus lens  14 , and the photographic lens  15  and is reflected upward by the optical-path branching prism  16 . The reflected light passes through the lens  17  and is reflected to the left by the mirror  18 . The reflected light enters the color separation prism  20 , and an image is formed at the image pickup elements  21   r ,  21   b , and  21   b . The image is converted into an electrical signal, and the electrical signal is amplified by the signal amplifier circuit  41  by a predetermined factor. The amplified signal passes through the image control circuit  42  and is recorded by the image recorder  44  as a color fundus image in the recording medium D. The image is displayed on the television monitor  44 , and the image capturing process is terminated. 
     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.