Abstract:
Two problems arise when measuring length at the eye by short-coherence interferometry. First, the measurement focus and coherence window usually do not coincide. Second, the scanning process along the eye axis is time-consumin g. Both result in poor signal quality and inaccurate measurements. The present application is directed to a short-coherence interferometer in which a right-angle mirror and focusing optics jointly carry out a periodic back-and-forth movement in such a way that the measurement beam focus which is generated by the focusing optics and imaged on the eye by relay optics is moved synchronously with the coherence window from the cornea along the optic axis of the eye to the fovea centralis. Further, different path lengths are generated in the measurement beam path and reference beam path by means of a plurality of reflectors, so that the scanning process is limited to distances which are smaller than the optical length of the eye. The present invention is advantageously implemented using on a fiber-optic interferometer. According to the invention, the reference interferometer arm and measurement interferometer arm are combined with the arms of a fiber-optic interferometer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority of International Application No. PCT/EP2005/001164, filed Feb. 4, 2005, Austrian Application No. 179/2004, filed Feb. 6, 2004 and Austrian Application No. 719/2004, filed Apr. 27, 2004, the complete disclosures of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   a) Field of the Invention 
   The present application is directed to a measurement method for diagnostic opthalmology, particularly measurement arrangements for measuring partial distance lengths at the eye in connection with cataract surgery and refractive eye surgery. 
   b) Description of the Related Art 
   In cataract surgery and refractive eye surgery, a determined refractive power of the eye is obtained or achieved through suitable selection of the refractive power of the intraocular lens to be implanted. For this purpose, it is necessary to measure the eye&#39;s initial state of refractive power and also, as the case may be, after operating, to measure the final refractive state. Keratometers are used for measuring the corneal curvature, and acoustic or optical length measurement methods are used for determining the axial partial distances of the eye. 
   At present, it is already very common to determine axial eye length by optical short-coherence interferometry which, compared with the formerly prevalent ultrasound methods, has the advantage of contactless, highly precise operation. In short-coherence interferometry, interference patterns occur only when the object and reference mirror are situated at the same optical distance from the beamsplitter up to the coherence length l C  or, in other words, when the object structure in question is located in the “coherence window.” In order to measure distances of object structures using the conventional time-domain method, these object structures are moved into the coherence window successively in time through monitored displacement of a mirror in the reference beam or measurement beam. Therefore, the measuring accuracy is defined by the coherence length l C  of the measurement light bundle which depends not only on wavelength λ but also chiefly on its spectral width Δλ (strictly speaking, the shape of the spectrum also plays a part):
 
l c ˜λ 2 /Δλ.  (1)
 
   In opthalmologic short-coherence interferometry, the coherence window usually has a length (=l C ) of several micrometers. 
   In contrast to conventional optical short-coherence interferometry in which the reference mirror of the interferometer traverses or “scans” the entire distance to be measured, special methods have been developed for opthalmologic measurement of eye length. In spite of distances at the eye of up to about 30 millimeters, these methods make it possible to measure these distances even in living and, therefore, unstable objects. One of the alternatives for solving this problem in short-coherence interferometric measurement of the distance of unstable structures which are separated in depth is to use the so-called dual-beam method. This method is described in Laid Open Application DE 3201801A1. In this instance, the cornea and the other eye structure, e.g., the fundus, which is separated from the cornea with respect to depth are illuminated by a dual measurement beam. This dual measurement beam is formed by two output beams of a Michelson interferometer having different path lengths. Using a diffractive lens, this measurement beam is focused on the cornea and on the fundus simultaneously. The Michelson interferometer is adjusted to the distance between the cornea and fundus. An interferometer mirror scan distance of a few millimeters is sufficient for this purpose. This adjustment is determined by the interference patterns of short-coherence light which occur in this way. Since only the distance between the cornea and fundus is decisive in this instance, requirements respecting interferometric stability are satisfied in an ideal manner; measurement is not impeded by movements of the eye. 
   Also, in the method described in Patent Application WO 01/38820A1, the two object areas which are at a distance from one another with respect to depth are illuminated by a dual measurement beam. In this case, another measurement beam is initially reflected out of the measurement beam illuminating the measured object in front of the measured object by means of a beamsplitter and, after traversing an indirect path in which additional refractive optics can also be arranged for focusing, is reflected into the original measurement beam again. This method reduces the interferometer scan distance to a smaller value than the distance to be measured. With correspondingly fast scanning, this method can likewise meet the requirements for interferometric stability. 
   However, the methods mentioned above have the disadvantage that the measurement light simultaneously illuminates two or more object areas at a distance from one another. The light that is not used for measurement generates unwanted background and noise. Further, it is difficult to implement focusing of the measurement light on the respective measurement location in this way; these problems are severe in particularly when there is a plurality of object areas at a distance from one another. But modern opthalmologic length measurement at the eye requires measurement of more than one length, namely, distances such as the anterior chamber depth, cornea thickness and eye lens thickness in addition to the eye length. 
   OBJECT AND SUMMARY OF THE INVENTION 
   Therefore, it is the primary object of the invention to provide short-coherence interferometers for measuring partial distances of the eye which focus the measurement beam on the respective coherence window and which, further, reduce the required interferometer mirror scan distances to distances that are less than the distances that must be measured. 
   According to the invention, this object is met by a short-coherence interferometer in which a right-angle mirror and focusing optics jointly carry out a periodic back-and-forth movement in such a way that the measurement beam focus which is generated by the focusing optics and imaged on the eye by relay optics is moved synchronously with the coherence window from the cornea along the optic axis of the eye to the fovea centralis, and, further, different path lengths are generated in the measurement beam path and reference beam path by means of a plurality of reflectors. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
       FIG. 1  illustrates in schematic form the geometric optics of the depth scan at the eyes; 
       FIG. 2  illustrates in schematic form the course of the focus position from the corneal vertex into the interior of the eye; 
       FIG. 3  illustrates in schematic form a first arrangement of the present invention; 
       FIG. 4  illustrates in schematic form a second arrangement of the present invention; 
       FIG. 5  illustrates in schematic form a third arrangement of the present invention; 
       FIG. 6  illustrates in schematic form a simplified interferometer construction in accordance with the present invention; 
       FIG. 7  shows an example in schematic form of a specific arrangement of the system shown in  FIG. 4 ; 
       FIG. 8  shows an arrangement in schematic form in accordance with the invention where the beam path differs from that shown in  FIG. 7 ; 
       FIG. 9  shows another beam path according to the invention in which the moving right-angle mirror of the interferometer reference arm and the moving focusing optics of the interferometer measurement arm are mounted on separate scanner plates; 
       FIG. 10  illustrates that a dispersive lens can also be used as focusing optics and the corresponding beam path of the interferometer measurement arm is shown in schematic form; 
       FIG. 11  illustrates a further embodiment of the invention in schematic form; and 
       FIG. 12  shows a schematic arrangement in accordance with the invention where the dispersion compensation can be carried out by a wedge arrangement. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  illustrates the geometric optics of the depth scan at the eye:  101  designates the moving focus of the measurement beam of a short-coherence interferometer which moves along the distance of length S indicated by the double-arrow  102 . The focus  101  is imaged on the eye  104  by means of relay optics  103 . The condition for short-coherence interferometry in this instance is that the image  101 ′ of the focus  101  must scan the entire length of the eye  104  from the corneal vertex  105  to the fovea centralis  106  for a movement of the focus  101  by the optical length S=L of the eye (in Gullstrand&#39;s eye model, L is roughly equal to 33 mm). Accordingly, point  107  must be imaged at the end of this movement in point  106 . This is achieved for optics with a focal length f when the corneal vertex is situated at a distance of 
                   b   0     =       f   ⁡     (     f   +   L     )       L             (   2   )               
from the relay optics  103 . For example, when using optics with a focal length f=33 nun, b=66 mm in Gullstrand&#39;s eye model.
 
   Further, the measurement beam must be focused inside the coherence window on the object for a good short-coherence interferometric signal. While the arrangement described in  FIG. 1  ensures that when the coherence window at the cornea coincides with the focus  101 ′ it also coincides at the fovea centralis with the focus of the measurement beam in point  106 , this is not completely guaranteed along the intervening distance. This can be seen by following the course of the focus position from the corneal vertex  201  ( FIG. 2 ) into the interior of the eye. Because of the refraction of light at the cornea  211 , the focus of a light bundle  212  focused on a point  210  behind the cornea  211  is displaced by a shorter distance (t) than the light bundle itself ( ) and therefore than the coherence window. The new focus does not lie in point  210  but rather in  213 , see  FIG. 2 . Further, the coherence window does not lie at  210 , but rather at optical distance n G ·T from the vertex, where n G =group index. 
   This divergence of the coherence window and measurement bundle focus is important particular when taking measurements in the anterior segment of the eye because the latter contains structures (back of the cornea, front and back of the eye lens) that should be measured with high precision. This divergence can be reduced for the anterior segment of the eye by ensuring the initial coincidence of the focus  210  and coherence window not—as is customary—for points outside of the eye, that is, e.g., for the corneal vertex, but for a point approximately in the center of the anterior chamber, e.g., 3 mm behind the vertex. This is achieved by a corresponding positioning of the reflector for the coherence window of the cornea. For this purpose, the arm length of the interferometer is adjusted by means of the adjusting table  358 ,  558 , or  758 . 
   A first arrangement according to the invention is shown in  FIG. 3 . In this instance, a short-coherence light source  301 , for example, a superluminescent diode, delivers a light beam  302  which is partially coherent temporally and, as far as possible, fully coherent spatially and which illuminates the beamsplitter  304  by means of optics  303 . The beamsplitter  304  divides the light beam  302  into measurement beam  315  and reference beam  305 . The reference beam  305  is reflected in direction of the beamsplitter  308  by the reference mirror  306 . In this way, it passes through the two dispersion compensation prisms  307  and  307 ′ two times. After passing through the beamsplitter  308 , the reference beam  305  strikes the optics  310  and is focused on the photodetector  311  by the latter. 
   The measurement beam  315  reflected by the beamsplitter  304  (at left in the drawing) is reflected by the right-angle mirror  316  by 90° out of its original direction and strikes the reflector mirror  317  which is formed in this instance as a mirrored rear surface of a plane plate  318 ′. Of course, other mirrors can also be used for this purpose, for example, surface mirrors with the reflector side facing the incident beam. The reflected measurement beam  315 ′ is directed again to the beamsplitter  304  by the right-angle mirror  316 , passes through the beamsplitter  304  and also beamsplitter  308  in a straight line and is focused by the focusing optics  319  as a measurement beam  334  in the focus  320 . As is shown in  FIG. 3 , for example, the focus  320  is located at twice the object-side focal length of the relay optics  321 . The relay optics  321  image the focus  320  on the eye  323 . As is shown in  FIG. 3 , for example, the focus  320  is imaged at twice the image-side focal length of the relay optics  321  in point  322 . Point  322  is located on the cornea of an eye  323  whose length L is measured. 
   The beamsplitters  304  and  308  can be constructed as polarizing beamsplitters to prevent unwanted reflections and to optimize the beam intensities. By rotating a linear polarizer  330 , the splitting ratio of the reference beam intensity to the measurement beam intensity can then be optimized in such a way that an optimal signal-to-noise ratio is obtained at the photodetector  311 . Further, a quarter-wave plate  331  can be arranged after the beamsplitter  304  in the measurement beam  315  at 45° to the polarization direction. The reflector  317  is then illuminated by a circularly polarized light bundle  315  which is circularly polarized in the opposite direction after reflection as light bundle  315 ′ and then, after a further pass through the quarter-wave plate  331 , is linearly polarized again, namely, orthogonal to the original polarization direction. This beam  332  therefore traverses the polarizing beamsplitters  304  and  308  without reflection losses and strikes another quarter-wave plate  33  arranged at 45°, where the measurement beam  334  is circularly polarized again and is focused by the focusing optics  319  in focus  320  and again, by the relay optics  321 , in focus  322 . The corneal vertex reflects the light bundle  324  which is now circularly polarized in the opposite direction. This returning measurement light bundle  324  is again linearly polarized orthogonal to the polarization direction of the bundle  332  by the quarter-wave plate  333  and is therefore reflected by 100% in direction of the optics  310  by the polarizing beamsplitter  308  and is focused by the latter on the photodetector  311 . In this way, output losses in the measurement beam are prevented to a great degree. 
   After being reflected at the reference reflection prism  306 , the linearly polarized reference beam  305  traverses the dispersion compensators  307  and  307 ′ and the beamsplitter  308  and is likewise focused on the photodetector  311  by the optics  310 . The size of the mutually interfering components from the measurement beam and reference beam can be adjusted by the linear polarizer  326 . The linear polarizer  326  is oriented in such a way that an optimal signal-to-noise ratio occurs at the photodetector  311 . 
   The short-coherence interferometer described above is constructed on a plate  300 . The right-angle mirror  316  and focusing optics  319  are located on a scanner plate  335  ( 335 ′) which is movable in direction of the interferometer axis  340 , shown in dash-dot lines, by distance S. The scanner plate  335  can be the moving slide of a voice coil scanner, e.g., manufactured by the firm Physik Instrumente, or of an ultrasound scanning table or other corresponding device whose base plate  356  is fastened to the bottom plate  300 . On the other hand, components  301 ,  302 ,  303 ,  304 ,  331 ,  308 ,  333 ,  326 ,  310 , and  311  are located on a carrier plate  357  which is fixedly connected to the bottom plate  300  and spans the scanner plate  335 . 
   The reflection prism  306  is mounted on a displacement table  358 . Its position can be adapted in such a way that the coherence window is located at the corneal vertex of the eye  323  in the configuration of the scanner plate  335  of the interferometer indicated in  FIG. 3  by solid lines. When the scanner plate  335 —on which the right-angle mirror  316  and focusing optics  319  are mounted—moves in direction of the interferometer axis  340 , shown in dash-dot lines, by distance S, the measurement beam path to the left of the beamsplitter  304  in the drawing is reduced by 2 times S. As a result, the coherence window is displaced by distance S from the corneal vertex into the eye. In order to measure eye length, the coherence window can be displaced to the right by the entire optical eye length L, and this distance can be delimited based on the short-coherence interference patterns that occurs between the light reflected by the fondus  325  and the reference light. Alternatively, as was stated above, the position of the reflection prism  306  can be adapted in such a way by means of the displacement table  358  that the focus for a point roughly in the center of the anterior chamber of the eye is situated in the middle of the coherence window. An adjustment of this kind is particularly important when precise measurements of the anterior chamber geometry must be carried out. 
   The accuracy of short-coherence interferometric measurements is impaired by dispersion in the interferometer arms. To achieve the greatest possible accuracy which is approximately on the order of the coherence length l C , the dispersion in the two interferometer arms must be identical as far as possible. This is referred to as dispersion compensation. The dispersion caused by the component parts of the interferometer can be achieved by correspondingly selected thicknesses of the mirror plate  318  or by additional plane plates ( 350 ,  350 ′). The object-dependent dispersion can be compensated in the reference arm by mutual displacement (double-arrows  351  and  351 ′) of two wedge plates ( 307 ,  307 ′). 
   A semitransparent mirror  362  can be arranged in the measurement beam in order to observe the position of the test subject&#39;s eye  323  relative to the measurement beam. This observation can be carried out directly ( 363 ), by means of an eyepiece  364 , or by means of a television camera  365 . For this purpose, it may be advisable to additionally illuminate the test subject&#39;s eye  323  with an incoherent light source  366 . The image  370  of a reticle  371  reflected on the test subject&#39;s cornea by a semitransparent mirror  372  can also be used for precise positioning of the test subject&#39;s eye on the interferometer axis  340 . 
   The methods described above have the disadvantage that the scanner plate  335  must be displaced by the entire eye length L, which is time-consuming. 
   In another construction of the invention, another reflector  517  is arranged in front of reflector  317  as is shown in  FIG. 4 . This plane mirror is located at a known distance D from the reflector  317 . A portion of the measurement light beam  315  is already reflected at this mirror. This portion of the measurement light beam  315 ′ already has a coherence window that is offset toward the right-hand side by the optical length D relative to the measurement light reflected at the reflector  317 . When the interferometer has been adjusted in such a way that the measurement light reflected by the reflector  317  (associated with the cornea) generates short-coherence interference with the reference light after reflection at the cornea, a displacement of the scanner plate  335  by distance S=L−D is already sufficient to allow short-coherence interference between the light bundle that is directed to the ocular fundus from the reflector  517  (associated with the fundus) and reflected thereat and the reference light. The eye length L is given by the distance L−D measured by short-coherence interferometry and the known distance D. The distance between the corneal vertex and the relay optics  321  must then equal 
   
     
       
         
           
             
               
                 b 
                 = 
                 
                   f 
                   + 
                   
                     
                       f 
                       2 
                     
                     
                       L 
                       - 
                       D 
                     
                   
                 
               
             
             
               
                 ( 
                 3 
                 ) 
               
             
           
         
       
     
   
   The modifications described in relation to the arrangement according to  FIG. 3  for preventing unwanted reflections, for dispersion compensation, and for observation of the position of the test subject&#39;s eye can also be implemented in the beam path according to  FIG. 4  in an analogous manner. Further, the position of the reflector  317  and the roof prism  306  can be adapted to the actual partial length distances of the eye by means of the displacement table  558  and  358  through axial displacement (double-arrow  559 ) in such a way that the required mechanical scan process is reduced to a few millimeters by the scanner plate  335  of the short-coherence interferometer. Accordingly, not only is the measurement process at the eye made considerably faster, but the use of inexpensive scanners is also made possible. 
   Further, in addition to eye length, the different intraocular partial distances can also be measured. For this purpose, additional reflectors can be arranged so as to be staggered with respect to depth, e.g., a reflector  519 , so that its position roughly corresponds to the position of the front surface of the lens. The distance between the cornea and eye lens can then also be measured based on corresponding short-coherence interference. 
   The thickness of the carrier plates of the reflectors ( 318 ,  518 ,  519 ) can be adapted in a corresponding manner to the required dispersion compensation. 
   In the arrangements described thus far, all of the measurement beams associated with the different object depths are still at the photodetector simultaneously because the reflectors  517  and  519  must be semitransparent. This reduces the signal quality and can lead to confusion between signals. This problem is prevented by the further development of the arrangement according to the invention which is described in the following. 
   In the arrangement according to  FIG. 5 , depending upon the position of the scanner plate  335 , the measurement beam  315  which is reflected by 90° from its original direction by the right-angle mirror  316  impinges on a plurality of reflectors  617  (with plate  618 ) and  619  (with plate  620 ) which are staggered not only in depth but also laterally. As a result, only one measurement beam takes part in the short-coherence interferometric measurement. In this instance, all reflectors can be fully mirrored. All of the measurements that are separated with respect to depth are now carried out separately optically. Also, the scanner plate  335  need only be moved by a distance of about S=L−D for measuring the eye length L. Here also, the distance of the corneal vertex from the relay optics  321  must equal 
   
     
       
         
           b 
           = 
           
             f 
             + 
             
               
                 
                   f 
                   2 
                 
                 
                   L 
                   - 
                   D 
                 
               
               . 
             
           
         
       
     
   
     FIG. 6  shows a simplified interferometer construction. In this instance, the right-angle mirror  316  and focusing optics  319  are mounted next to one another on the scanner plate  335  which is moved back and forth (double-arrow  560 ) periodically in direction of the optical axis  340  between the position  335  shown in solid lines and the position  355 ′ shown in dashed lines. The rest of the reference numbers correspond to those in the preceding drawings. In this instance, the carrier plate  357  is not required. 
   Also, all of the other devices discussed with reference to  FIGS. 3 ,  4  and  5 , e.g., for preventing unwanted reflections by polarization-optical steps and the dispersion compensation, can be carried out for the arrangement according to  FIG. 6 . In case of defective vision, collecting or dispersive auxiliary optics  339  for compensating defective vision can be placed in front of the eye  323 . 
   With regard to fiber-optic implementation of the arrangements according to the invention, an advantageous implementation of the present invention is carried out based on a fiber-optic interferometer. A reference interferometer arm and measurement interferometer arm which are designed in accordance with the invention are combined with the arms of a fiber-optic Michelson interferometer. Advantages result because the central interferometer structure with the fiber-optic beamsplitters is compact, stable against vibrations, and operates reliably. There are various ways to design the fiber-optic Michelson interferometer particularly on the detector side.  FIG. 7  shows (in the box  777  in dash-dot lines) a fiber-optic interferometer with balanced detection according to the prior art. Balanced detection compensates for mode noise in the broadband light sources that are required for this purpose, which enables a signal-to-noise gain of up to 20 dB. The invention is not directed to this fiber-optic interferometer or to the associated signal processing. Other fiber-optic interferometers can also be used, for example, those employing fiber-optic circulators. Rather, the invention is directed to the opto-mechanical structure and implementation of the optical beam paths coupled to the fiber interferometer outputs  703  and  743 . Outputs  703  and  743  can use different fiber-optic interferometers. Also, the signal processing can be carried out in different ways. For example, the electric output signal can reproduce the direct time sequence of the interference term or it can be demodulated so that only the envelope appears at the output. 
   Essential to the fiber-optic Michelson interferometer is a fiber coupler  700  which distributes the light coming from the short-coherence light source  760  to the interferometer measurement arm  701  and the interferometer reference arm  702 . The light bundle  704  exiting the fiber at  703  is collimated by the optics  705  of a fiber collimator and is directed by the mirror  706  in direction of the optical axis  740  of the short-coherence interferometer. To facilitate adjustment, the reflector  706  is mounted in a holder  707  which is rotatable around two axes lying in its mirror plane. The holder  707  is fixedly mounted on the plate  708  which is in turn fixedly connected to the bottom plate  799  and spans the scanner plate  725 . The light bundle  704  reflected by the mirror  706  in direction of the optical axis  740  is focused by the focusing optics  709  in the focus  710 . The focus  710  is imaged by the relay optics  711  in point  712 . The point  712  is located on the cornea of an eye  723  whose length L is measured. 
   The focusing optics  709  are mounted on the scanner plate  725  by means of a holder  724 . The scanner plate  725  can be the moving slide of a voice coil scanner, e.g., manufactured by the firm Physik Instrumente, or of an ultrasound scanning table or other corresponding device whose base plate  726  is fastened to the bottom plate  799 . During the measurement, the scanner plate  725  is moved back and forth periodically along the optical axis  740  by distance S between the position  725  shown in continuous lines and the position  727  shown in dashed lines. When the scanner plate is located in the position shown in dashed lines, the focus of the light bundle  704 —for example, in the configuration shown in FIG.  7 —is in focal point  728  of the relay optics  711 , and the light bundle  35  is directed to the eye  723  as a parallel collimated light bundle  730 . The eye focuses this light bundle on its fundus. In case of defective vision, collecting or dispersive auxiliary optics  739  for compensating defective vision can be placed in front of the eye  723 . 
   The reference light bundle  744  exiting at  743  from the light-conducting fiber  702  is collimated by the optics  745  of a fiber collimator and, after passing a dispersion compensation device comprising, for example, two wedge plates ( 746 ,  746 ′), is directed by the roof mirror  747  to the right-angle mirror  748  and from the latter in direction of the optical axis  740  of the short-coherence interferometer. The light bundle  744  strikes the moving right-angle mirror  749  which directs the light bundle  744  to the reflector  750 . The reference light bundle  744  is reflected by this mirror in itself and travels back into the light fiber  702 . 
   When the scanner plate  725  moves along the optical axis  740  in direction of the position shown in dashed lines, the reference light bundle  744  is directed to the reflectors  750 ,  751  and  752  successively by the right-angle mirror  749 . Additional reflectors can be arranged in a staggered manner with respect to depth and laterally. Further, plane plates  753  can also be arranged for dispersion compensation. Accordingly, reference light distances of different lengths are realized, for example, in order to measure positions of other structures such as the inner corneal surface, the front surface of the lens, and the back surface of the lens in addition to the position of the fundus and corneal vertex. 
     FIG. 7  shows an example in which the distance between the reflectors  750  and  752  (which are associated with the cornea and fundus) is equal to distance D. As was already described in connection with the arrangement according to  FIG. 4 , the distance to be traveled by the scanner plate  725  for measuring eye length is reduced by D. Only a movement by distance S=L−D is required for measuring the length L of an eye. In fact, this distance could even be close to zero: the scanner plate  725  would actually only have to be moved by the coherence length l C  in order to detect the interference. However, because of the scattering of actual eye lengths, which is quite extensive, the scanner plate  725  would have to be moved by a distance approximately equal to this scattering, that is, by several (x) millimeters. In any case, the distance L to be scanned, which is otherwise quite long, can be reduced to a few millimeters. The reflectors  750 ,  751 ,  752  must be correspondingly wide, namely, equal to the width of the beam  744  plus x millimeters. Further adaptation is possible by adjusting the reference beam length by means of the reflection prism  747  which is mounted on an adjusting device  758  which is displaceable in the directions indicated by the double-arrows  754 . 
   According to the invention, as is indicated above, the measurement light bundle  704  in the positions  625  and  727  of the scanner plate is focused once on the cornea and once on the fundus. As was already mentioned with reference to  FIG. 3 , in order that the measurement beam  704  is focused once on the cornea (scanner plate position indicated by solid lines) and once at the other end of the movement (scanner position indicated by dashed lines), the distance of the vertex of the front surface of the cornea from the relay optics  711  must be equal to 
             b   =     f   ⁡     (     1   +     f     L   -   D         )         ,         
where f is the focal length of the relay optics  711 . In case the coherence window is initially (scanner plate position  725 ) situated at the cornea, it will be situated at the fundus at the conclusion of the displacement of the scanner plate (scanner plate position  727 ). For example, when S=f then b=2f, for a focal length of the relay optics  711  of f=50 mm, then b=100 mm.
 
   It should be mentioned that the actual mechanical scan area for the scanner plate  725  is selected so as to be somewhat larger than S, for example, in order to achieve an approximately constant speed in the actual measurement area, which facilitates the subsequent electronic signal processing. 
   Further, the position of the reflection prism  747  can be adapted by means of the displacement table  758  in such a way that the focus of the measurement beam on the eye for a point approximately in the center of the anterior chamber also lies in the center of the coherence window. An adjustment of this kind is particularly important when the anterior chamber geometry must be precisely measured. 
   In this instance, as was already mentioned in connection with the reflector  519  in  FIG. 4 , the anterior chamber depth, for example, or other partial distances of the eye can also be measured by means of an additional reflector  751 . 
   A superluminescent diode or other short-coherence light source outfitted with a pigtail fiber can be used as short-coherence light source  760 . Its radiation must be coupled into the light-conducting fiber  761  by couplers corresponding to the prior art. When using a pigtail superluminescent diode, the radiation can be coupled directly from the pigtail into the first fiber coupler  762 . This fiber coupler  762  couples the light wave into the fiber  763  which distributes the light wave in the 50:50 coupler (3 dB coupler)  700  to measurement fiber  701  and reference light fiber  702 . Fiber-loop polarization controllers  765  can be used to adjust the polarization state in the two interferometer arms. The electric outputs  770  and  771  of the two photodetectors  772  and  773  are applied to the inputs of a differential amplifier  774  whose output signal is bandpass-filtered and demodulated, for example. 
   As was already mentioned in connection with the arrangement described with reference to  FIG. 3 , a semitransparent mirror  362  can also be arranged in the measurement beam in this instance in order to observe the position of the test subject&#39;s eye  723  relative to the measurement beam. Observation can then be carried out directly ( 363 ), by means of an eyepiece  364 , or by means of a television camera  365 . In this instance, it can also be useful to additionally illuminate the test subject&#39;s eye  323  by an incoherent light source  366 . Further, the image  370  of a reticle  371  which is reflected on the test subject&#39;s cornea by a semitransparent mirror  372  can also be used for precise positioning of the test subject&#39;s eye on the interferometer axis  740 . 
   Three boxes  778 ,  779  and  780  (in dashed lines) are shown in  FIG. 7 . The components in these boxes form functional groups which can be set up separately to a great extent. The beam path indicated in  FIG. 8  differs from that shown in  FIG. 7  first in that the moving right-angle mirror  749  associated with the reference beam path and the focusing optics  709  generating the focus  710  in the measurement beam path are arranged next to one another on the scanner plate. Accordingly, the plate  708  is dispensed with, and all of the components located thereon can be mounted directly on the bottom plate  799 , which not only simplifies the construction in its entirety, but also makes it more stable. 
   Finally,  FIG. 9  shows another beam path according to the invention in which the moving right-angle mirror  749  of the interferometer reference arm and the moving focusing optics  709  of the interferometer measurement arm are mounted on separate scanner plates  925  and  925 ′. These separate scanner plates can be electrically synchronized. However, they can also be operated with different scanning distances. It need only be ensured that S≧L−D and the distance of the vertex of the front surface of the cornea from the relay optics  711  equals 
             b   =     f   ⁡     (     1   +     f     L   -   D         )         ,         
where f is the focal length of the relay optics  711 . With the parameters otherwise remaining the same, the opthalmologic short-coherence interferometer can easily be fitted to other ophthalmologic devices and measurement devices through suitable selection of the focal length f of the relay optics  711 . Instead of the interferometer reference arm shown here, a so-called rapid scan optical delay line, as is described in K. F. Kwong, et al., 400-Hz mechanical scanning optical delay line, Optics Letters 18 (1993) pp. 558-560, can be combined, as reference arm, with the interferometer measurement arm according to  FIG. 9 .
 
   The modifications described in connection with the arrangement shown in  FIG. 3  for preventing unwanted reflections, for dispersion compensation, and for observation of the position of the test subject&#39;s eye can also be realized in an analogous manner in the beam path according to  FIGS. 7 ,  8  and  9 . Further, using the displacement table  758 , the position of the reflector  752  and the roof prism  747  can be adapted by axial displacement (double-arrow  754 ) to the actual partial distance lengths of the eye in such a way that the required mechanical scan process of the short-coherence interferometer is reduced to a few millimeters. This not only makes the measurement process at the eye considerably faster, but also make it possible to use inexpensive scanners. 
   Finally, it should also be noted that a dispersive lens ( 709 ′) can also be used as focusing optics ( 103 ,  319 ,  709 ).  FIG. 10  shows a corresponding beam path of the interferometer measurement arm. The virtual focus  728  now occurs in place of the real focus  728 . 
     FIG. 11  shows another arrangement according to the invention. This arrangement makes it possible to use scanners with smaller scan travel by folding the reference beam. In the arrangement shown in the drawing, the reference beam runs back and forth three times between the reflectors  1001 ,  1002 ,  1003 , and  1004 . This reduces the scanner travel by a factor of 3. A further reduction in the required scan travel can be achieved by additional reflectors. 
   The short-coherence light source  301 , e.g., a superluminescent diode, emits a light beam  302  which is partially coherent temporally and, as far as possible, fully coherent spatially and which illuminates the beamsplitter  304  through beamsplitter  308  by means of optics  303 . The beamsplitter  304  divides the light beam  302  into measurement beam  315  and reference beam  305 . The reference beam  305  is optically folded by a series of mirrors and reflectors. After traversing the dispersion compensation prisms  307  and  307 ′, the reference beam  305  is initially directed by the right-angle mirror  1001  to the retroreflector mirror  1002  and from the latter back to the reflector mirror  1003  which reflects the reference beam further to the right-angle mirror  1004 . Finally, the right-angle mirror  1004  directs the reference beam  305  to the reference mirror  1005  with the mirror surface  1006 . The reference beam that is reflected at the latter runs back through the optical folding and is directed from the beamsplitter  308  and optics  310  to the photodetector  311 . 
   The measurement beam  315  transmitted by the beamsplitter  304  (at right in the drawing) is focused on the focusing optics  1013  by the retroreflector  1010  via the right-angle mirrors  1011  and  1012  and is then focused in focus  320  by the focusing optics  1013 . The relay optics  321  image the focus  320  on the eye  323  at point  322 . Point  322  is located on the cornea of the eye  232  whose length is to be measured, for example. 
   Optics  1013 ,  1002 , and  1004  are mounted on a scanner plate  1335  which is movable in direction of the interferometer axis  340  shown in dash-dot lines. The scanner plate  1335  can be the moving slide of a voice coil scanner—made by the company Physik Instrumente, for example—or of an ultrasound piezo-scanning table or another corresponding device. 
   Further, an auxiliary laser  1014  is provided for adjustment purposes, e.g., a helium-neon laser, whose beam  1015  is reflected in via the beamsplitter  304 . The plane plate  1016  serves for dispersion compensation for measurements in the depth of the eye  323 . 
   The dual-beam arrangement can also be used for all arrangements with the known advantage that eye movements of the test subject can be compensated in a corresponding manner. 
   As is shown in the example in  FIG. 12 , the dispersion compensation can also be carried out by a wedge arrangement or prism arrangement  746  which is traversed depending on the displacement position of the scanning table. The beam deflections (not shown) which occur in this way must be taken into account in orienting the rest of the components (e.g., reference mirror). 
   Another advantageous arrangement consists in providing additional means which make it possible to carry out deliberate orientation of the optical axis of the device relative to the optical axis of the eye or visual axis, for example, by means of a method such as that described in PCT Application WO 2002/065899 A2, whose entire disclosure is hereby referenced, or through correspondingly adjustable beam deflecting elements (prisms, wedges) in the device itself. 
   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.