Abstract:
The invention is directed to an optical system for a fundus camera in which lens pairs are tilted relative to the imaging beam path to prevent flare. This tilting is carried out in two planes and these planes are preferably oriented perpendicular to one another.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation application of application Ser. No. 10/552,756 filed Jun. 20, 2006 now U.S. Pat. No. 7,500,753, which claims priority of International Application No. PCT/EP2004/002893, filed Mar. 19, 2004, and German Application No. 103 16 416.2, filed Apr. 10, 2003, the complete disclosures of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     a) Field of the Invention 
     The invention is directed to an optical system for a fundus camera which serves to image the ocular fundus. When imaging the ocular fundus with a camera of this type, flare generally results at the cornea and at surfaces of the optical system and has a degrading effect on the image quality. 
     In its basic construction, a fundus camera comprises a multiple-stage optical system. An opthalmoscope lens generates an intermediate image that is imaged by a sequential system (principal objective) on a film or a CCD matrix. The opthalmoscope lens is also a component part of the illumination. 
     b) Description of the Related Art 
     A particular problem in observing and photographing the fundus is posed by flare at the cornea and at the surfaces of the opthalmoscope lens because the light which is reflected by the retina and which carries the relevant image information is substantially less intense than the light that is reflected before entering the eye. Troublesome corneal flare is normally prevented by dividing the pupil of the eye. For this purpose, the opthalmoscope lens images an illumination ring in the pupil of the eye. The illumination rays reflected at the cornea miss the observation aperture. Only the area within the illumination ring is used for observation. 
     There are substantially two known concepts for suppressing flare at the opthalmoscope lens. 
     DE-OS 35 19 442 describes an optical system in which components of light that could enter the observation aperture by reflection at the opthalmoscope lens or cornea are masked out by “black-dot plates” which are arranged at a suitable location in the beam path and which are coated in a defined manner with light-absorbing layers. This method of suppressing reflections has come to be known as “anti-flare dot objective”. 
     The proximity of the anti-flare dot to the field diaphragm is a disadvantage in this design. The absorption of individual light components can become visible as an inhomogeneous illumination of the ocular fundus. Ring-shaped shadows occur which degrade the image and accordingly impede evaluation by the ophthalmologist. 
     Another solution is described in U.S. Pat. No. 4,730,910. This solution does away with masking of determined light components within the illumination optics. Instead of the opthalmoscope lens, a multiple-lens objective is used whose lenses are tilted relative to one another in such a way that the direct flares at the glass-air interfaces do not enter the observation aperture. For this purpose, the optical axes of the lenses lie in a plane together with the optical axis of the observation beam path. This solution requires a considerable expenditure on mechanical mounting and has considerable problems with correcting imaging errors. 
     A system of the type mentioned above exhibits a clear difference between imaging scales in the meridional and sagittal section. Anamorphotic effects can be observed in object imaging as well as in pupil imaging (illumination). The images are distorted and there is no similarity of imaging. Further, the system exhibits coma and astigmatism when imaging the axial point and the imaging errors in the field are not symmetric with respect to rotation. 
     OBJECT AND SUMMARY OF THE INVENTION 
     It is the primary object of the invention to overcome the disadvantages of the prior art and to provide an optical system for a fundus camera which effectively eliminates flare at the optical surfaces from the imaging beam path and ensures extensive imaging fidelity. 
     This object is met by an optical system preferably for a fundus camera, which has a substantially coaxial illumination beam path and imaging beam path, and comprises a lens system of at least four lenses. At least two lenses are tilted with respect to their optical axes relative to the illumination beam path and imaging beam path. The optical axes of the lenses and the optical axis of the illumination beam path and imaging beam path lie in a plane. At least two additional lenses are tilted with respect to their optical axes relative to the illumination beam path and imaging beam path. The optical axes of the two additional lenses and the optical axis of the illumination beam path and imaging beam path lie in a second plane which intersects the first plane substantially along the optical axis of the illumination beam path and imaging beam path. 
     The tilting of the lenses in two planes extending perpendicular to one another brings about matching of the imaging scales for two sections extending perpendicular to one another. The images are less distorted and similarity of imaging is achieved. Further, the axial astigmatism can be corrected in this way and the rotational symmetry of the errors in the field can be reproduced approximately. 
     A preferred further development of the invention comprises four collecting lenses which are divided into two lens pairs. The lenses of the first lens pair are tilted in a first plane. The lenses of the second lens pair are tilted in a second plane, these two planes extending perpendicular to one another. It is particularly advantageous when the tilting angle and displacements of the lenses are selected in such a way that the area in the vicinity of the optical axes of the lenses is not penetrated by the illuminating bundle. 
     This system enables very good matching of the imaging scales for two sections extending perpendicular to one another. 
     The invention will be described more fully in the following with reference to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  shows a side view of the optical system according to the invention; 
         FIG. 2  shows a top view of the optical system according to the invention; and 
         FIG. 3  is a schematic view of the residual distortion. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a schematic side view showing the optical system according to the invention, wherein parts that are not substantial to the invention, such as the patient&#39;s eye, light sources, photographic device and observation system, are not shown for the sake of simplicity. 
     Four lenses  1 ,  2 ,  3 ,  4  are so arranged along the optical axis  5  that defines the illumination beam path and imaging beam path of the fundus camera that the lenses  1  and  2  are tilted relative to the drawing plane  6  of  FIG. 1 , while lenses  3  and  4  are tilted relative to the optical axis  5  within the drawing plane  6  so that their optical axes  7  and  8  likewise lie in the drawing plane  6 . 
       FIG. 2  shows a top view of the optical system. The drawing plane  9  is accordingly perpendicular to the drawing plane  6  of  FIG. 1 . The lenses  1  and  2  are tilted within the drawing plane  9  such that their optical axes  10  and  11  lie in the drawing plane  9 . The lenses  3  and  4  are accordingly tilted relative to the drawing plane  9 . 
     The lenses are arranged in such a way that the illumination bundle of the imaging beam path and illumination beam path with the optical axis  5  penetrates none of the lenses  1 ,  2 ,  3 ,  4  in their central area, that is, the optical axes  10 ,  11 ,  7 ,  8  of the lenses  1 ,  2 ,  3 ,  4  lie outside the bundle beams shown schematically in  FIGS. 1 and 2 . 
     The optical data of a preferred realization of the invention are listed in the following table. Designations with the suffix v pertain to the front sides of the lenses  1 ,  2 ,  3 ,  4  and designations with suffix h pertain to the back sides of the lenses  1 ,  2 ,  3 ,  4 . 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
               
               
                   
                 Radius of curvature 
                 Distance to the next 
                 Medium following 
               
               
                 No. 
                 [mm] 
                 surface [mm] 
                 the surface 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Obj. 
                 infinity 
                 39.42477 
                 air 
               
               
                 1v 
                 −76.12524 
                 16.63675 
                 LaK8 
               
               
                 1h 
                 −50.66730 
                 −2.16757 
                 air 
               
               
                 2v 
                 −197.58358 
                 29.93724 
                 LaK8 
               
               
                 2h 
                 −89.73436 
                 7.92140 
                 air 
               
               
                 3v 
                 580.99844 
                 26.79594 
                 LaK8 
               
               
                 3h 
                 −260.25600 
                 −9.88489 
                 air 
               
               
                 4v 
                 169.89283 
                 20.11881 
                 LaK8 
               
               
                 4h 
                 −1636.24830 
                 32.91953 
                 air 
               
               
                   
                 infinity 
                 145.13560 
                 air 
               
               
                   
               
             
          
         
       
     
     The following table contains the values for the displacements and tilting of the lenses  1 ,  2 ,  3 ,  4  relative to the optical axis  5  that are necessary for realizing this embodiment example. 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                   
                 Placement of 
               
               
                   
                 Displacement [mm] 
                 Rotation [°] 
                 center of rotation 
               
             
          
           
               
                 Surface 
                   
                 x- 
                 y- 
                 around x- 
                 around y- 
                 in z-direction 
               
               
                 No. 
                 Type of decentering 
                 direction 
                 direction 
                 axis 
                 axis 
                 [mm] 
               
               
                   
               
             
          
           
               
                 1v 
                 1. Displacement, 2. 
                 10.1400 
                 1.0882 
                 0.000 
                 29.760 
                 −23.8578 
               
               
                   
                 rotation 
               
               
                 1h 
                 1. Displacement, 2. 
                 10.1400 
                 1.0882 
                 0.000 
                 29.760 
                 −40.4946 
               
               
                   
                 rotation 
               
               
                 2v 
                 1. Displacement, 2. 
                 −6.5758 
                 1.0882 
                 0.000 
                 −18.959 
                 −93.6604 
               
               
                   
                 rotation 
               
               
                 2h 
                 1. Displacement, 2. 
                 −6.5758 
                 1.0882 
                 0.000 
                 −18.959 
                 −123.5976 
               
               
                   
                 rotation 
               
               
                 3v 
                 1. Displacement, 2. 
                 15.5328 
                 −34.7519 
                 5.988 
                 0.000 
                 −202.8670 
               
               
                   
                 rotation 
               
               
                 3h 
                 1. Displacement, 2. 
                 15.5328 
                 −34.7519 
                 5.988 
                 0.000 
                 −229.6629 
               
               
                   
                 rotation 
               
               
                 4v 
                 1. Displacement, 2. 
                 15.5328 
                 −82.9383 
                 14.841 
                 0.000 
                 465.0016 
               
               
                   
                 rotation 
               
               
                 4h 
                 1. Displacement, 2. 
                 15.5328 
                 −82.9383 
                 14.841 
                 0.000 
                 444.8823 
               
               
                   
                 rotation 
               
               
                   
                 1. Displacement, 2. 
                 3.7576 
                 −5.0827 
                 2.447 
                 4.089 
                 0.0000 
               
               
                   
                 rotation, rotated 
               
               
                   
                 coordinates also 
               
               
                   
                 apply for the 
               
               
                   
                 subsequent surfaces 
               
               
                   
               
             
          
         
       
     
     The distortions resulting with this system are shown in  FIG. 3 . It can be seen that the imaging scales extensively correspond in the x-direction and y-direction. The comparison grid (straight lines) has the same extension in the x-direction and y-direction. Therefore, the requirements for similarity of imaging are ensured to a very great extent. 
     Further, the optical system according to the invention is characterized in that the errors at the edge of field are distinctly symmetric with respect to rotation. This makes it possible to compensate for these errors through a rotationally symmetric sequential system. 
     The following table juxtaposes the essential data of an optical system according to the prior art (U.S. Pat. No. 4,730,910=system A) and according to the invention (system B). 
     
       
         
               
               
               
             
               
               
               
             
               
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                 System A 
                 System B 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Pupil Imaging 
                   
                   
               
               
                 Length of system [mm] 
                 60 
                 104 
               
               
                 Focal length f′ [mm] 
                 31.5 
                 57.3 
               
             
          
           
               
                 Pupil imaging scale 
                 β px   
                 −2.19 
                 −2.5 
               
               
                   
                 β py   
                 −2.46 
                 −2.5 
               
             
          
           
               
                 Imaging length l′ p  [mm] 
                 171.6 
                 304 
               
               
                 Working distance [mm] 
                 24.2 
                 38.1 
               
               
                 Field angle illumination [°] 
                 23.5 
                 26 
               
               
                 Average spot radius (axis) [mm] 
                 0.9974 
                 0.2349 
               
               
                 Average spot radius (field) [mm] 
                 1.2532 
                 0.4334 
               
               
                 Petzval curvature r p  [mm] 
                 −71.9424 
                 −93.4579 
               
               
                 Distortion 
                 2.64% 
                 0.74% 
               
               
                 Longitudinal color aberration S e -S C′  [mm] 
                 −3.13 
                 −5.76 
               
               
                 Longitudinal color aberration S e -S F′  [mm] 
                 3.08 
                 5.66 
               
               
                 Object imaging 
               
               
                 Field angle observation [°] 
                 22.5 
                 25 
               
               
                 Average spot radius (axis) [mm] 
                 0.0119 
                 0.0020 
               
               
                 Average spot radius (field) [mm] 
                 0.0575 
                 0.0158 
               
               
                 Distortion 
                 −13.01% 
                 −9.43% 
               
               
                 Longitudinal color aberration S e -S C′  [μm] 
                 −17 
                 −7.6 
               
               
                 Longitudinal color aberration S e -S F′  [μm] 
                 18 
                 8.9 
               
               
                   
               
             
          
         
       
     
     A significant improvement over the prior art is achieved in all of the relevant parameters for imaging quality. 
     In the realization according to the invention, it is possible to replace the lenses shown in  FIGS. 1 and 2  with corresponding lens segments which encompass at least the area of the penetration of the illumination beam path and the imaging beam path. 
     The invention is not limited to the embodiment example shown herein. In certain cases, it may also be advantageous when the two tilting planes are not perpendicular to one another. 
     In order to improve the optical imaging characteristics, it can be advantageous to provide at least one of the lenses with an aspheric surface. It is also possible to realize the invention by using diffractive optical elements instead of one or more lenses. 
     The principle of the invention is also applicable in other fields in which flare must be suppressed at optical surfaces. 
     While the foregoing description and drawings represent the present invention, it will be obvious to those skilled in the art that various changes may be made therein without departing from the true spirit and scope of the present invention.