Abstract:
A testing and calibrating device for optical eye length measurement devices comprising two plano-convex lenses which are arranged in the illumination beam path so as to be oppositely oriented. A neutral filter with a defined transmission is located between these two plano-convex lenses.

Description:
BACKGROUND OF THE INVENTION 
     a) Field of the Invention 
     The present invention relates to a testing and calibrating device for optical eye length measurement devices. 
     b) Description of the Related Art 
     A spherical transparent test body which is preferably made of glass, has coatings and a refractive index of approximately 2 is known as a testing and calibrating device for optical eye length measurement instruments (EP-A 509903, U.S. Pat. No. 5,347,327, U.S. Pat. No. 5,347,328) from DE 19504465 A1 by the present Applicant. The coating, chiefly of the back of the test body, involves a costly technique and difficult technology. 
     OBJECT AND SUMMARY OF THE INVENTION 
     Therefore, the primary object of the present invention is a test body for optical eye length measurement devices that is simple to produce but is universally applicable. 
     In accordance with the invention, a testing and calibrating device for optical eye length measurement devices comprises two plano-convex lenses which are arranged in the illumination beam path so as to be oppositely oriented. A neutral filter with a defined transmission is located between these two plano-convex lenses. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. I shows a schematic view of the test body. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Relative to FIG.  1 : 
     The test body comprises three individual parts in the direction of illumination of the eye in an eye length measurement device according to the prior art cited above: 
     a first piano-convex lens PL 1  which can be approximately or exactly hemispherical 
     a plane plate PP made of gray filter or neutral density filter material 
     a second plano-convex lens PL 2 . 
     The parallel illumination light BL is bundled through the first lens PL 1  and focussed on the back of the second lens PL 2 . The accuracy of this test ball intended for axial length measurement is determined by the thickness values matching the refractive indices and by the two radii. 
     The cementing of the individual parts is carried out by edge alignment, for which reason the edge cylinders are fabricated in a highly precise register. 
     In order to prevent reflections, principally at the plane plate, the entire arrangement is utilized at an angle (advantageously between 10 and 20 degrees). The input surface radius is set at around 7 mm analogous to the corneal surface radius of the human eye. 
     The thickness of the first lens is determined in an advantageous manner with respect to manufacturing technique for hemispherical expansion. The thickness of the neutral density filter plate is calculated for the actual oblique beam path through the plane plate (15-degree beam inclination in our example), wherein the forward path and return path are to be added, namely, in such a way that the desired value for the total reflectivity is achieved for the entire system after reflecting at the rear radius and exiting at the first radius (around 7 mm), the transmission at the neutral density filter plate in the forward and return path multiplied by the reflection at the rear radius approximately determines the total reflectivity at 
     
       
         Transmission  T =0.23 dforward+dretum ×(( n −1)/( n +1)) 2 , 
       
     
     where 0.23 is the net transmittance, the value for the utilized NG 3  neutral density filter glass for 1 mm glass path length, n is the index of refraction of the rear lens PL 2 , and dforward and dreturn are the thicknesses traversed for the forward and return path (they are approximately equal). 
     The approximate transmission was determined for the useful wavelength (780 nm) at 8.2×10 −5 . 
     The thickness of the rear lens must be determined in such a way that the intersection for the main beam of the utilized bundle diameter is located on the rear radius. 
     The rear radius is determined by adding the respective wavelengths of the main beam through the front lens, neutral density filter and rear lens in relation to air. 
     The entire arrangement can be centered after cementing such that it is suitably received in a mechanical mount so that it is used at an angle such that reflections are eliminated at the neutral density filter plate. 
     An arrangement dimensioned in this way reflects the incident beam bundle in itself, even with small angular changes in the position of the test ball, without disturbing secondary reflections, as would be had in a comparable test ball with the refractive index  2  for this wavelength with a reflectivity of the above-indicated value for the rear ball surface which could only be produced, if at all, with an expensive special coating. 
     By using another neutral density filter or varying its thickness at the expense of the rear lens thickness, another test ball can be dimensioned effortlessly. Accordingly, test balls which simulate different turbidity values due to eye cataract can be advantageously produced. 
     Preferably, a ball is used which simulates an eye without turbidity and without defective vision (T=approximately 10 −4 ) and a ball with T=10 −9  which corresponds to an eye with high turbidity due to cataract. 
     These two balls are used specifically for testing the functionality of the axial length measuring instrument (particularly the internal electronic amplification). In must be possible to process large and small measurement signals in a corresponding manner. Further, the balls can be utilized for training in the operation of the device and for regular monitoring of the constancy of measurements. The front surface of the balls is designed in such a way that the corneal radius measurement function can also be tested (training and quality assurance). 
     By varying the tilt of the beam path, an accurate degree of absorption can also be advantageously adjusted because a tilt by an amount of approximately five degrees means a transmission correction of about 20-25%, since the beam length is changed in the neutral density filter. 
     
       
         
               
               
             
               
               
               
               
             
               
               
             
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Test ball for normal eye: (τ = 10 −4 ) 
               
             
          
           
               
                   
                 front hemisphere 
                 plane plate PP 
                 rear hemisphere 
               
               
                   
                 PL1 
                   
                 PL2 
               
               
                   
                 radius of curvature: 
               
               
                   
                 R1 = 7.182 
                   
                 R2 = 9.172 
               
               
                   
                 Glass type: SLF6 
                 NG3 
                 SFL6 
               
               
                   
                 thickness: d1 = 7.182 
                 d2 = 2.1 
                 d3 = 6.54 
               
             
          
           
               
                   
                 Test ball, weak for high cataract: (τ = 10 −9 ) 
               
             
          
           
               
                   
                 PL1 
                 PP 
                 PL2 
               
               
                   
                 radius of curvature: 
               
               
                   
                 R1 = 7.182 
                   
                 R2 = 9.172 
               
               
                   
                 Glass type: SLF6 
                 NG10 
                 SFL6 
               
               
                   
                 thickness: d1 = 7.182 
                 d2 = 2.3 
                 d3 = 6.3 
               
               
                   
                   
               
             
          
         
       
     
     While the foregoing description and drawings represent the preferred embodiments of the present invention, it will be obvious to one skilled in the art that various changes and modifications may be made therein without departing from the true spirit and scope of the present invention.