Abstract:
An apparatus ( 50 ) for obtaining an image of the eye, has a light source ( 114 ) for providing an incident illumination and an apertured mirror ( 104 ) for directing at least a portion of the incident illumination along an optical axis. A curved objective mirror ( 102 ) directs the incident illumination received along the optical axis toward the retina of the eye and directs image-bearing light reflected from the retina back along the optical axis. The apertured mirror ( 104 ) transmits the image-bearing light reflected from the retina toward a sensor ( 108 ) for obtaining an image of the retina thereby.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention generally relates to electronic imaging apparatus for fundus imaging and more particularly relates to an improved fundus imaging apparatus using a curved mirror objective for forming an image of the eye.  
       BACKGROUND OF THE INVENTION  
       [0002]     Fundus camera imaging is acknowledged to be an important diagnostic tool for detection of various conditions affecting the eye, including diabetic retinopathy and macular degeneration. Various embodiments of fundus imaging apparatus are disclosed, for example in U.S. Pat. No. 5,713,047 (Kohayakawa); U.S. Pat. No. 5,943,116 (Zeimer); U.S. Pat. No. 5,572,266 (Ohtsuka); U.S. Pat. No. 4,838,680 (Nunokawa); U.S. Pat. No. 6,546,198 (Ohtsuka); U.S. Pat. No. 6,636,696 (Saito); U.S. Pat. No. 4,247,176 (Ito); U.S. Pat. No. 5,742,374 (Nanjo et al.); and U.S. Pat. No. 6,296,358 (Cornsweet et al).  
         [0003]     While these patents attest to continuous improvements in fundus camera design, there are still significant hurdles to obtaining good quality images from these devices. Fundus cameras must solve the fairly difficult problem of simultaneously illuminating the retina through the pupil and obtaining the retinal image, with both illumination and image-bearing light traveling along substantially the same optical path. One particularly troublesome problem relates to stray light caused by unwanted reflection from the lens surface of the patient&#39;s eye itself as well as from optical surfaces within the camera apparatus. Unless its level is controlled, this unwanted reflected light can degrade image contrast and overall image quality.  
         [0004]     This problem is most readily illustrated by an overview of the operation of the illumination subsystem in a conventional fundus imaging apparatus. Referring to  FIG. 1 , there is shown a fundus imaging apparatus  10  in which a conventional illumination section  12  is used. The patient&#39;s eye E is positioned along an optical axis O using an alignment subsystem (not shown in  FIG. 1 ). Illumination section  12  directs light either from an observation light source  14  and a lens  16  or from an image capture light source  18  and a lens  20  as controlled by control logic circuitry for fundus imaging apparatus  10  (not shown in  FIG. 1 ). A dichroic mirror  22  directs light from the appropriate source through a ring-slit diaphragm  24  and a lens  26 , to an apertured mirror  28 . Apertured mirror  28  directs the illumination light along axis O and through an objective lens  42  toward the pupil for illuminating the retina of eye E. Depending on the use of fundus imaging apparatus  10  at any one time, either observation light source  14  or image capture light source  18  are activated. Observation light source  14  is typically infrared (IR) light, to which eye E is insensitive. Image capture light source  18 , on the other hand, may be a high-brightness source such as a xenon lamp, for example. Depending on the application, image capture light source  18  may be pulsed or strobed.  
         [0005]     Ring-slit diaphragm  24  has the characteristic functional arrangement shown in  FIG. 2 . Light is transmitted through an inner ring  30  and is blocked at a middle section  32  and at an outer section  34 . As is shown in the received illumination ring of  FIG. 3 , inner ring  30  is directed into a pupil  36  of the patient as a ring  40  of illumination. To obtain the retinal image, apertured mirror  28  ( FIG. 1 ) has an aperture suitably centered about optical axis O to allow light that has been reflected from the retina of eye E and directed through lenses  42  and  44  to reach a sensor  46 , such as a CCD.  
         [0006]     The high-level block diagram of  FIG. 1  thus gives an overview of illumination section  12  that applies for conventional fundus imaging apparatus. There have been numerous methods disclosed for optimizing the performance of illumination section  12 , including components arranged to prevent stray reflected light from the cornea of eye E and from optical surfaces from being directed back toward sensor  46 . Referring to the schematic block diagram of  FIG. 1 , three basic approaches have been followed in order to reduce or eliminate stray light from these sources: 
        (i) Using a pair of crossed polarizers. Using this approach, a first polarizer  600  is placed in the illumination path, prior to apertured mirror  28 . A second polarizer  602  is then positioned in the image path, following apertured mirror  28 . With reference to  FIG. 1 , first polarizer  600  and second polarizer  602  are positioned as shown at phantom locations. The polarizers  600  and  602  are cross-aligned so that the light reflected back from the lens surfaces can be blocked by polarizer  602 .     There are two key problems with this method. The first problem relates to the needed high power lamp when using this strategy. Because only that portion of light having the proper polarization is transmitted through polarizer  600 , more light is needed from image capture light source  18 . The use of second polarizer  602  further blocks the useful light reflected from the retina by 50%. As a result, the power of light source  18  must be about 4 times as high as would be necessary without polarizers  600  and  602 . The second problem relates to the nature of light reflected from the cornea. Since this light can be depolarized, particularly due to the large incident angle, second polarizer  602  will be less effective in blocking unwanted stray light.     (ii) In the illumination path, blocking reflected light which, otherwise, will reflect from the lens surface and reach the sensor  46 . This solution, however, reduces uniformity of the desired light reflected from the retina, particularly noticeable when attempting to obtain retinal images from near-sighted patients.     (iii) Separating illumination and imaging optical paths. A beamsplitter can be placed in front of objective lens  42  to effect this separation. However, this type of solution requires additional light power in order to obtain suitable reflected light from the retina and necessitates a longer working distance for objective lens  42 .        
 
         [0011]     Reflective optics have been used in display apparatus that require a highly compact optical arrangement. For example, U.S. Pat. No. 5,889,625 (Chen et al.) discloses an optical arrangement that directs an image-bearing light to a human observer using a curved mirror as part of a head-mounted device (HMD). Similarly, U.S. Pat. No. 5,499,139 (Chen et al.) discloses a helmet-mounted optical apparatus for providing a wide-field image to a pilot, where the optical apparatus employs a curved mirror and compensation for image aberration. However, while mirrors have been used effectively in display applications of this type, the use of curved mirrors in a system that must simultaneously illuminate and capture an image is understandably much more difficult.  
         [0012]     Mirrors have also been utilized in some more complex ophthalmological cameras for imaging internal structures of the eye. For example, U.S. Pat. No. 5,847,805 (Kohayakawa et al.) discloses an apparatus for scanning a pair of beams into the eye using a combination of rotary polygon scanning mirror and a galvanometric mirror. Similarly, U.S. Pat. No. 6,585,374 (Matsumoto) discloses various embodiments using a movable concave mirror mounted on a rotation axis for imaging different portions of the eye from different rotated positions. The apparatus of U.S. Pat. Nos. 5,847,805 and 6,585,374 are relatively costly, high-end ophthalmological imaging devices that require added movable components in the optical path in order to obtain multiple images for diagnosis.  
         [0013]     There is a need for inexpensive fundus imaging cameras where scanning operation is not needed. This less complex type of camera is designed for use in physician&#39;s offices and is used for first-level screening for diabetic retinopathy, for example. With such an apparatus, a single retinal image from each eye is all that is needed for screening.  
         [0014]     In summary, there is a need for a lower cost optical system in a fundus imaging camera that reduces stray light from lens surface reflection without significantly increasing the needed illumination brightness and without adversely affecting image quality.  
       SUMMARY OF THE INVENTION  
       [0015]     Briefly, according to one aspect of the present invention an apparatus for obtaining an image of the retina of the eye comprises: 
        a) a light source for providing an incident illumination;     b) an apertured mirror for directing at least a portion of the incident illumination along an optical axis;     c) a curved objective mirror for directing the incident illumination received along the optical axis toward the retina of the eye and for directing image-bearing light reflected from the retina back along the optical axis;     wherein the apertured mirror transmits the image-bearing light reflected from the retina toward a sensor; and     the sensor obtaining an image of the retina thereby.        
 
         [0021]     It is a feature of the present invention that it provides a fundus imaging apparatus with a mirror that acts as the objective lens.  
         [0022]     It is an advantage of the present invention that it minimizes stray reflected light from the surfaces of lenses as well as from other optical components in the imaging path.  
         [0023]     These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the present invention, it is believed that the invention will be better understood from the following description when taken in conjunction with the accompanying drawings, wherein:  
         [0025]      FIG. 1  is a schematic block diagram showing a conventional arrangement of illumination and imaging optics in a fundus imaging apparatus;  
         [0026]      FIG. 2  is a plan view of a ring-slit diaphragm used in a conventional fundus imaging apparatus;  
         [0027]      FIG. 3  is a plan view representation of the ring of illumination applied to the pupil of a patient in a conventional apparatus;  
         [0028]      FIG. 4  is a schematic block diagram showing the overall arrangement of imaging components in a fundus imaging apparatus of the present invention;  
         [0029]      FIG. 5  is a diagram showing the path of imaging light in one embodiment;  
         [0030]      FIG. 6  is a diagram showing the path of imaging light in an alternate embodiment;  
         [0031]      FIG. 7  is a schematic block diagram showing the overall arrangement of imaging components in a fundus imaging apparatus of an alternate embodiment;  
         [0032]      FIG. 8A  is a diagram showing structure and focal points for an ellipse;  
         [0033]      FIG. 8B  is a cross-sectional diagram showing behavior of an elliptical mirror;  
         [0034]      FIG. 9A  is a diagram showing structure and focal points for an hyperbola; and  
         [0035]      FIG. 9B  is a cross-sectional diagram showing behavior of a mirror having hyperbolic shape. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]     The present description is directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.  
         [0037]     Referring to  FIG. 4 , there is shown a block diagram of illumination and imaging components of a fundus imaging apparatus  50  in a first embodiment of the present invention. An imaging light source  114  provides imaging illumination through lenses  124  and  112  and through a ring-slit diaphragm  122  to an apertured mirror  104 . This illumination is then directed into eye E by a curved mirror  102 . Curved mirror  102  is off-axis and serves as the objective lens in this embodiment. This arrangement eliminates back-reflection from the objective lens, such as from lens  42  in  FIG. 1 . Curved mirror  102  is toroidal in one embodiment, rather than spherical, to minimize third-order astigmatism, commonly introduced by off-axis mirrors. A concave elliptical mirror could also be advantageous.  
         [0038]     In order to minimize third-order astigmatism, the principal radii in x- and y-directions, R x  and R y  respectively, must meet the Coddington equations, as follows:  
                 1   t     +     1     t   ′         =     2       R   y     ⁢   cos   ⁢           ⁢   θ               (   1   )                   1   s     +     1     s   ′         =       2   ⁢   cos   ⁢           ⁢   θ       R   x               (   2   )             
 
 where t, s, t′, and s′ are the distance along the rays from the astigmatism focal surface to the focus from the object and image distance, respectively, and θ is the angle of the mirror with respect to the optical axis. 
 
         [0039]     With the eye properly aligned, the light reflected from the retina is substantially at infinity; thus, values  
         1   t     ⁢           ⁢   and   ⁢           ⁢     1   s         
 
 are zero. Thus: 
 
 R   x   =R   y (cos θ) 2  
 
         [0040]     Ring-slit diaphragm  122 , apertured mirror  104 , and the cornea C of eye E are optically conjugate. The ring of illumination is sizeable enough so that light reflected back from the cornea is blocked by apertured mirror  104  and by a stop aperture  126 . Only the image light is directed toward a sensor  108  by a camera lens  106 .  
         [0041]     The basic arrangement of  FIG. 4  can be implemented in a number of ways. For example, referring to  FIG. 5 , there is shown a ray diagram of camera lens  106  in one embodiment. This design utilizes a symmetric refractive lens. In order to minimize aberration from curved mirror  102  such as coma and astigmatism, sensor  108  is tilted relative to lens  106 .  
         [0042]     The alternate arrangement of  FIG. 6  shows a ray diagram of camera lens  106  in another embodiment. Here, lens  106  is decentered to compensate for mirror aberration. With such an arrangement, it can be difficult to correct for distortion; however, digital techniques can be employed to correct for distortion in fundus imaging apparatus  50 .  
         [0043]     Referring to  FIG. 7 , there is shown another embodiment of fundus imaging apparatus  50  in which a pair of mirrors is utilized to minimize aberration. A second curved mirror  116  is used to direct illumination from apertured mirror  104  and to direct image-bearing light through apertured mirror  104  to sensor  108 .  
         [0044]     The arrangement of  FIG. 7  can be advantageous for reducing distortion, where mirrors  102  and  116  are carefully selected. In one embodiment, a combination is used in which mirror  102  is an ellipsoid and second curved mirror  116  is hyperboloid. For an ellipsoid shape in Cartesian x,y,z space, the basic equation is as follows:  
                     x   2     +     y   2         a   2       +       z   2       b   2         =   1           (   3   )             
 
 and a 2 −b 2 =c 2 . Parameters a, b, and c are as represented in the diagram of  FIG. 8A  for an ellipse  200 . Ellipse  200  has two focal points FE 1  and FE 2 . An ellipsoid shape is a type of quadric shape generated by rotation of the ellipse about the axis between its focal points, the major axis. 
 
         [0045]      FIG. 8B  shows the behavior of an elliptical mirror  202 , that is, a mirror having ellipsoid shape. Light emanating from one focal point FE 1  is reflected toward the other focal point FE 2 .  
         [0046]     For a hyperboloid shape in Cartesian x,y,z space, the basic equation is as follows:  
                     x   2     +     y   2         a   2       -       z   2       b   2         =   1           (   4   )             
 
 and a 2 +b 2 =c 2 . Parameters a, b, and c for an hyperbola  300  are as represented in the diagram of  FIG. 9A . Hyperbola  300  has two focal points, FH 1  and FH 2 . A hyperboloid shape is a type of quadric shape that can be generated by rotation of the hyperbola about the axis between its focal points. 
 
         [0047]      FIG. 9B  shows the behavior of a hyperbolic mirror  302 , that is, a mirror having a substantially hyperboloid shape. Light rays that are directed toward one focal point FH 2  are reflected toward the other focal point FH 1 .  
         [0048]      FIG. 7  takes advantage of the behavior of hyperboloid and ellipsoid mirrors to reflect light from one focal point to another focal point. In operation, then, ellipsoid mirror  102  reflects light from the retina of eye E from one of its focal points FE 1  (at or near the retina) toward its other focal point FE 2 , which is a focal point FH 1  shared with hyperboloid second mirror  116 . Second curved mirror  116  reflects this light to its other focal point FH 2  near apertured mirror  104 .  
         [0049]     By using an ellipsoid/hyperboloid combination, distortion of curved mirror  102  is at least partially corrected by second curved mirror  116 . Mirror  116  also acts as a folding mirror, allowing a more compact imaging system. For this combination of ellipsoid/hyperboloid mirrors  102  and  116  respectively, the ideal arrangement is to have focal point FE 1  at or very near the lens of eye E. Focal points FE 2  and FH 2  should be substantially concentric. Focal point FH 2  should be at the aperture of apertured mirror  104 . Of course, perfect positioning would be difficult; some slight tolerance for positioning error would be necessary.  
         [0050]     The use of curved mirror  102  as the objective optical component eliminates one cause of possible stray reflection (that is, from the surface of an objective lens  42  in  FIG. 1 ) and provides an optical mechanism for preventing unwanted reflected light from the imaging path, while transmitting the desired reflected light that bears the retinal image.  
         [0051]     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention as described above, and as noted in the appended claims, by a person of ordinary skill in the art without departing from the scope of the invention. For example, curved mirror  102  may be moved along the optical axis O in order to achieve better focus. Any of a number of different types of light sources could be used for observation, focus, and imaging.  
         [0052]     Thus, what is provided is a fundus imaging apparatus using a curved mirror objective for forming an image of the eye.  
       PARTS LIST  
       [0000]    
       
           10  fundus imaging apparatus  
           12  illumination section  
           14  observation light source  
           16  lens  
           18  image capture light source  
           20  lens  
           22  dichroic mirror  
           24  ring-slit diaphragm  
           26  lens  
           28  apertured mirror  
           30  inner ring  
           32  middle section  
           34  outer section  
           36  pupil  
           40  ring  
           42  lens  
           44  lens  
           46  sensor  
           50  fundus imaging apparatus  
           102  curved mirror  
           104  apertured mirror  
           106  lens  
           108  sensor  
           112  lens  
           114  light source  
           116  second curved mirror  
           122  ring-slit diaphragm  
           124  lens  
           126  stop aperture  
           200  ellipse  
           202  elliptical mirror  
           300  hyperboloid  
           302  hyperbolic mirror  
           600  polarizer  
           602  polarizer